[19] Koinuma H, Nagata H, Tsukahara T, Gonda S, Yoshimoto M. Ceramic layer epitaxy by pulsed laser deposition in an ultrahigh vacuum system. Applied Physics Letters. 1991;58:2027-2029

ety. 2006;1:845-848

Protocols. 2009;4:155-173

ety. 2008: pp. 3594–3597

2005;26:3609-3614

mental Neurology. 2007;205:347-359

ical Engineering. 2006;8:425-454

cell characterization. Sensors. 2013;13:9029-9046

in Solid State Physics (Springer). 2003;43:505-517

Cu.(100). Physical Review B. 1999;59:1196-1207

Surface Science Reports. 2004;52:163-218

7084-7094 Advance article

Materials. Hoboken, New Jersey: John Wiley & Sons, Inc.; 2006

## Acknowledgements

D. Nieto thanks to the Consellería de Cultura, Spain for his support under the Galician Program for Research Innovation and Growth (2011–2015) (I2C Plan). This work has been supported by the Xunta de Galicia under contract Agrupación estratéxica 2015–AEFIS AGRUP2015/11 (PC034).

## Author details

Angel Luis Aragón Beloso1 , María del Carmen Bao Varela1 , Alejandro Fernández Rodríguez2 , Gerard O'connor3 , Eliseo Pérez Trigo4 , Antonio Pazos Álvarez4 and Daniel Nieto García1,3\*

\*Address all correspondence to<EMAIL_ADDRESS>
1 Photonics4life Group, University of Santiago de Compostela, Santiago de Compostela, Spain

2 Superconducting Materials and Large Scale Nanostructures, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Barcelona, Spain

3 NCLA/Inspire Labs, School of Physics, National University of Ireland, Galway, Ireland

4 Experimental High Energy Physics Group, University of Santiago de Compostela, Santiago de Compostela, Spain

## References

Annual International Conference of the IEEE Engineering in Medicine and Biology Society. 2006;1:845-848

[6] Tandon N, Cannizaro C, Chao P, Maidhoe R, Marsano A, Au H, Radisic M, Vunjak-Novakovic MG. Electrical stimulation systems for cardiac tissue engineering. Nature Protocols. 2009;4:155-173

isolated from the cell culture by using a thin soda-lime glass, solving biocompatibility issues

D. Nieto thanks to the Consellería de Cultura, Spain for his support under the Galician Program for Research Innovation and Growth (2011–2015) (I2C Plan). This work has been supported by the Xunta de Galicia under contract Agrupación estratéxica 2015–AEFIS AGRUP2015/11 (PC034).

, María del Carmen Bao Varela1

1 Photonics4life Group, University of Santiago de Compostela, Santiago de Compostela,

3 NCLA/Inspire Labs, School of Physics, National University of Ireland, Galway, Ireland

2 Superconducting Materials and Large Scale Nanostructures, Institut de Ciència de Materials

4 Experimental High Energy Physics Group, University of Santiago de Compostela, Santiago

[1] Hankey GJ, Pomeroy VM, King LM, Pollock A, Baily-Hallam A, Langhorne P. Electrostimulation for promoting recovery of movement or functional ability after stroke. Stroke.

[2] Dehail P, Duclos C, Barat M. Electrical stimulation and muscle strengthening. Annales de

[3] Boussetta N, Abeldelmalek S, Aloui K, Souissi N. The effect of neuromuscular electrical stimulation on muscle strength, functional capacity and body composition in haemodialysis

[4] Archer S, Li T, Tudor A, Britland S, Morgan H. Cell reactions to dielectrophoretic manipulation. Biochemical and Biophysical Research Communications. 1999;257:687-698

[5] Tandon N, Cannizzaro C, Figallo E, Voldman J, Vunjak-Novakovic G. Characterization of electrical stimulation electrodes for cardiac tissue engineering. Conference Proceedings:

Readaptation et de Medicine Physique. 2008;51:441-451

patients. Biological Rhythm Research. 2017;48:157-174

, Alejandro Fernández Rodríguez2

, Antonio Pazos Álvarez4 and Daniel Nieto García1,3\*

,

between the material of the tracks and the cell culture.

, Eliseo Pérez Trigo4

\*Address all correspondence to<EMAIL_ADDRESS>
de Barcelona (ICMAB-CSIC), Barcelona, Spain

Acknowledgements

240 Laser Ablation - From Fundamentals to Applications

Author details

Gerard O'connor3

de Compostela, Spain

2006;37:2441-2442

References

Spain

Angel Luis Aragón Beloso1

[20] Stafe M, Marcu A, Puscas N. Pulsed Laser Ablation of Solids, Springer Series in Surface Sciences. Vol. 53. Berllin Heidelberg: Springer-Verlag; 2014

**Chapter 12**

Provisional chapter

**In Vitro Evaluation of Laser-Induced Periodic Surface**

Structures on New Zirconia/Tantalum Biocermet for

In Vitro Evaluation of Laser-Induced Periodic Surface

DOI: 10.5772/intechopen.70820

**Structures on New Zirconia/Tantalum Biocermet for**

This study investigates the biological response of zirconia/tantalum biocermet materials with laser-induced periodic surface structures (LIPSS) generated using a femtosecond laser working at 1030 nm wavelength. LIPSS were formed by laser radiation slightly above the applied threshold fluence. LIPSS features were characterized using techniques such as atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). LIPSS were generated in this study by applying femtosecond pulses with 500 fs pulse duration at a high-repetition rate to smooth-polished zirconia/tantalum biocermet surfaces, with an original roughness value of 3.8 0.2 and 3.1 0.2 nm, respectively. We have demonstrated in vitro that LIPSS are an efficient option to increase osteoblastic differentiation of human bone marrow mesenchymal stem cells (hBMSCs) in ZrO2:Ta biocermets. LIPSS created increase cell metabolism statistically (best values in 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay) and decrease inflammatory response to the material (IL-6 and TNF-alpha values). Extracellular matrix production (ECM) is produced in more quantity and cells differentiate to osteoblast easily. These differences are seen from the beginning until the endpoint (day 20).

Keywords: laser-induced periodic surface structures, LIPSS, hard-tissue replacement,

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Hard-Tissue Replacement**

Alberto Jorge-Mora, Naroa Imaz,

Nekane Frutos, Ana Alonso, Carlota García Santiago, Rodolfo Gómez-Vaamonde,

Jesús Pino-Minguez,

Jose Bartolomé,

Abstract

Hard-Tissue Replacement

Ana Alonso, Carlota García Santiago,

http://dx.doi.org/10.5772/intechopen.70820

zirconia/tantalum biocermet

Gerard O'connor and Daniel Nieto

Alberto Jorge-Mora, Naroa Imaz, Nekane Frutos,

Rodolfo Gómez-Vaamonde, Jesús Pino-Minguez, Jose Bartolomé, Gerard O'connor and Daniel Nieto

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Provisional chapter

## **In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard-Tissue Replacement** Structures on New Zirconia/Tantalum Biocermet for Hard-Tissue Replacement

In Vitro Evaluation of Laser-Induced Periodic Surface

DOI: 10.5772/intechopen.70820

Alberto Jorge-Mora, Naroa Imaz, Nekane Frutos, Ana Alonso, Carlota García Santiago, Rodolfo Gómez-Vaamonde, Jesús Pino-Minguez, Jose Bartolomé, Gerard O'connor and Daniel Nieto Nekane Frutos, Ana Alonso, Carlota García Santiago, Rodolfo Gómez-Vaamonde, Jesús Pino-Minguez, Jose Bartolomé,

Additional information is available at the end of the chapter Gerard O'connor and Daniel Nieto

http://dx.doi.org/10.5772/intechopen.70820 Additional information is available at the end of the chapter

Alberto Jorge-Mora, Naroa Imaz,

#### Abstract

[20] Stafe M, Marcu A, Puscas N. Pulsed Laser Ablation of Solids, Springer Series in Surface

[21] Warrender JM, Aziz MJ. Kinetic energy effects on morphology evolution during pulsed laser deposition of metal-on-insulator films. Physical Review B. 2007;75:085433

[22] Madsen NR, Gamaly EG, Rode AV, Luther-Davies B. Cluster formation through the action of a single picosecond laser pulse. Journal of Physics: Conference Series. 2007;59:762-768

[23] Rosales A, Castañeda-Guzmán R, de Ita A, Sánchez-Aké C, Pérez-Ruiz SJ. Detection of zinc blende phase by the pulsed laser photoacoustic technique in ZnO thin films deposited via pulsed laser deposition. Materials Science in Semiconductor Processing. 2015;34:93-98 [24] Kudyakova VS, Shishkin RA, Elagin AA, Baranov MV, Beketov AR. Aluminium nitride cubic modifications synthesis methods and its features. Review. Journal of the European

[25] Lowndes D, Geohegan DB, Puretzky AA, Norton DP, Rouleau CM. Synthesis of novel

[26] Li G, Wang W, Yang W, Wang H. Epitaxial growth of group III-nitride films by pulsed laser deposition and their use in the development of LED devices. Surface Science Reports.

[27] Jelinek M. Functional planar thin film optical waveguide lasers. Laser Physics Letters.

[28] Wei X, Zhao R, Shao M, Xu X, Huang J. Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate. Nanoscale Research Letters.

[29] Matthews J, Lee SY, Wellstood FC, Gilbertson AF, Moore GE, Chatraphorn S. Multi channel high-T/sub c/scanning SQUID microscope. IEEE Transactions on Applied Super-

[30] Hontsu S, Sakatani T, Fujimaki A, Nishikawa H, Nakamori M, Kawai T. Mechanically tunable high-temperature superconducting microwave filter with large shift of resonant

[31] Hedge MS. Epitaxial oxide thin films by pulsed laser deposition: Retrospect and prospect.

[32] Ji Y, Qin C, Niu H, Sun L, Jin Z, Bai X. Electrochemical and electrochromic behaviors of polyanilinegraphene oxide composites on the glass substrate/Ag nano-film electrodes prepared by vertical target pulsed laser deposition. Dyes and Pigments. 2015;117:72-82

frequency. Japanese Journal of Applied Physics. 2001;40:L1148

Journal of Chemical Sciences. 2001;113:445-458

thin-film materials by pulsed laser deposition. Science. 1996;273:898-903

Sciences. Vol. 53. Berllin Heidelberg: Springer-Verlag; 2014

Ceramic Society. 2017;37:1143-1156

242 Laser Ablation - From Fundamentals to Applications

2015;70:380-423

2012;9(2):91-99

2013;8:112-120

conductivity. 2003;13:219-222

This study investigates the biological response of zirconia/tantalum biocermet materials with laser-induced periodic surface structures (LIPSS) generated using a femtosecond laser working at 1030 nm wavelength. LIPSS were formed by laser radiation slightly above the applied threshold fluence. LIPSS features were characterized using techniques such as atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). LIPSS were generated in this study by applying femtosecond pulses with 500 fs pulse duration at a high-repetition rate to smooth-polished zirconia/tantalum biocermet surfaces, with an original roughness value of 3.8 0.2 and 3.1 0.2 nm, respectively. We have demonstrated in vitro that LIPSS are an efficient option to increase osteoblastic differentiation of human bone marrow mesenchymal stem cells (hBMSCs) in ZrO2:Ta biocermets. LIPSS created increase cell metabolism statistically (best values in 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay) and decrease inflammatory response to the material (IL-6 and TNF-alpha values). Extracellular matrix production (ECM) is produced in more quantity and cells differentiate to osteoblast easily. These differences are seen from the beginning until the endpoint (day 20).

Keywords: laser-induced periodic surface structures, LIPSS, hard-tissue replacement, zirconia/tantalum biocermet

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Orthopaedic surgery has experienced a big development, thanks to the evolution of the materials used, which made it possible to perform procedures, such as total hip replacement with feasibility. These procedures are common in most orthopaedic departments and the survival of these implants is critical to prevent loosening and the need for revision arthroplasty with a cost of more than 15,000 euros in total hip or knee replacement. The ideal surface to interact with bone has not been created. We can find promising results in some materials, for example, hydroxyapatite plasma coating creates early and strong integration, but it seems that this treatment experiments long-term reviews due to the loosening of the coat [1]. Multiple techniques have been proposed to increase osteointegration in metals. Probably the most accepted and used are the creation of a plasma coating with hydroxyapatite in some situations and the texturization of the metal in the microscale [2–6].

2. Materials and methods

length and 2 mm diameter).

2.2. Microstructure and surface characterization

10 different locations over 3 samples of each type.

an atomic force microscope (Agilent 5500).

scope (Zeiss AxioVert A.1) and Zen software.

distribution in an area of 8 8 cm2

2.3. Laser processing

deposited onto their surface using a KSV Instruments Cam 200 device.

We have previously described the manufacturing process of the composite, were ceramic and metal powders are mixed to create the composite [21]. The samples used in this work are discs of 20 mm diameter and 2 mm thickness were cut and machined from cylindrical bars (5 mm

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard…

http://dx.doi.org/10.5772/intechopen.70820

245

The microstructure of samples was measured on surfaces polished down to 1 mm using a scanning electron microscopy (SEM, Phenom G2). Roughness measurements were performed by using SEM and 3D roughness reconstruction application (Phenom TM Pro Suite). The average surface roughness (Ra) was determined by measuring the surface roughness at

Morphological observation of the surfaces was undertaken by means of a Carl Zeiss Ultra Plus field emission scanning electron microscope (FE-SEM). The composition was analysed by an Oxford instruments INCA X-sight X-ray electron probe microanalyzer (EDX).The wettability of the studied materials was assessed by measuring the contact angle of deionized water drops

Laser-induced periodic surface structures are fabricated using a fluence above the damage threshold fluence for the zirconia/tantalum biocermet. Chemical modification was analyzed using a scanning electron microscope (Hitachi S2600N). The topography was measured using

During the fabrication process, we used an optical Nikon MM-400 microscope to visually inspect the samples; surface roughness and topography was measured using a SENSOFAR 2300 Plμ confocal microscope. Biological observations were performed using a Zeiss Micro-

ZrO2-Ta surfaces were exposed to multiple incident laser shots in air at a repetition rate of 100 kHz at various pulse energies ranging from 2 to 6 mJ. LIPSS were fabricated using a Yb:KYW laser system (Amplitude Systemes S-pulse HP) with a pulse duration of approximately 500 fs and a wavelength of 1030 nm. The beam delivery process for fabricating the LIPSS can be described as follows: the beam is focused on the top of the zirconia/tantalum biocermet surfaces using a flat-field lens (focal length = 100 mm). This kind of lens ensures a homogeneous light

Gaussian profile. The spatial profile of the laser was Gaussian in nature with a nominal M2 value

of <1.2. The laser set-up for texturing ZrO2-Ta surfaces with LIPSS is shown in Figure 1.

. The beam spot size (21 μm) was determined at 1/e2 of the

2.1. Material

Laser processing has gained significant importance in the medical industry and is an integral part of manufacturing, for example, welding endoscopes, drilling holes in hypodermic needles and verification laser marking [2, 3]. Lasers offer high precision and repeatability, high speed and quality, accuracy, cost efficiency, minimal thermal input and noncontact [7–10]. In particular, femtosecond lasers are ideal for surface structuring because they have a minimal heat affected zone, there is rapid heating and cooling and no laser plasma interaction [11–16]. Femtosecond laser-induced periodic surface structures (LIPSS), known as "ripples," have been fabricated on material surfaces such as metals [17], polymers [18], glass, dielectric and semiconductors [19, 20]. There are a number of applications of LIPSS such as increasing the surface area and surface energy, altering the hydrophilic or hydrophobic performance of a materials' surface, improving coating adhesion, optics and tribology [21, 22]. These characteristics are of great interest for biomedical applications, in particular for hard-tissue replacement [23–25].

This study investigates the bioresponse of zirconia/tantalum biocermets (ZrO2:Ta) with laser-induced periodic surface structures (LIPSS) produced by femtosecond laser pulses. Experiments were carried out using a Yb:KYW chirped-pulse-regenerative amplification laser system (Amplitude Systemes S-pulse HP) that delivered laser pulses with a duration of approximately 500 fs at wavelength of 1030 nm. We investigate the physical and biological response of LIPSS created using a femtosecond laser on an alloy surface. This includes determining the role of incubation and nanostructures, the physical characterization of LIPSS and the biological response of LIPSS. ZrO2:Ta surfaces were exposed to multiple incident laser shots in air at a repetition rate of 100 kHz at various pulse energies ranging from 2 to 6 mJ. Periodic surface nanostructures were formed by exposure to laser radiation slightly above the applied threshold fluence and observed using techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM). Section 2 introduces materials and methods. In Section 3, we describe the LIPPS fabrication procedure and results. Section 4 is devoted to biological results and Section 5 to conclusions.

## 2. Materials and methods

#### 2.1. Material

1. Introduction

244 Laser Ablation - From Fundamentals to Applications

metal in the microscale [2–6].

[23–25].

to conclusions.

Orthopaedic surgery has experienced a big development, thanks to the evolution of the materials used, which made it possible to perform procedures, such as total hip replacement with feasibility. These procedures are common in most orthopaedic departments and the survival of these implants is critical to prevent loosening and the need for revision arthroplasty with a cost of more than 15,000 euros in total hip or knee replacement. The ideal surface to interact with bone has not been created. We can find promising results in some materials, for example, hydroxyapatite plasma coating creates early and strong integration, but it seems that this treatment experiments long-term reviews due to the loosening of the coat [1]. Multiple techniques have been proposed to increase osteointegration in metals. Probably the most accepted and used are the creation of a plasma coating with hydroxyapatite in some situations and the texturization of the

Laser processing has gained significant importance in the medical industry and is an integral part of manufacturing, for example, welding endoscopes, drilling holes in hypodermic needles and verification laser marking [2, 3]. Lasers offer high precision and repeatability, high speed and quality, accuracy, cost efficiency, minimal thermal input and noncontact [7–10]. In particular, femtosecond lasers are ideal for surface structuring because they have a minimal heat affected zone, there is rapid heating and cooling and no laser plasma interaction [11–16]. Femtosecond laser-induced periodic surface structures (LIPSS), known as "ripples," have been fabricated on material surfaces such as metals [17], polymers [18], glass, dielectric and semiconductors [19, 20]. There are a number of applications of LIPSS such as increasing the surface area and surface energy, altering the hydrophilic or hydrophobic performance of a materials' surface, improving coating adhesion, optics and tribology [21, 22]. These characteristics are of great interest for biomedical applications, in particular for hard-tissue replacement

This study investigates the bioresponse of zirconia/tantalum biocermets (ZrO2:Ta) with laser-induced periodic surface structures (LIPSS) produced by femtosecond laser pulses. Experiments were carried out using a Yb:KYW chirped-pulse-regenerative amplification laser system (Amplitude Systemes S-pulse HP) that delivered laser pulses with a duration of approximately 500 fs at wavelength of 1030 nm. We investigate the physical and biological response of LIPSS created using a femtosecond laser on an alloy surface. This includes determining the role of incubation and nanostructures, the physical characterization of LIPSS and the biological response of LIPSS. ZrO2:Ta surfaces were exposed to multiple incident laser shots in air at a repetition rate of 100 kHz at various pulse energies ranging from 2 to 6 mJ. Periodic surface nanostructures were formed by exposure to laser radiation slightly above the applied threshold fluence and observed using techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM). Section 2 introduces materials and methods. In Section 3, we describe the LIPPS fabrication procedure and results. Section 4 is devoted to biological results and Section 5 We have previously described the manufacturing process of the composite, were ceramic and metal powders are mixed to create the composite [21]. The samples used in this work are discs of 20 mm diameter and 2 mm thickness were cut and machined from cylindrical bars (5 mm length and 2 mm diameter).

#### 2.2. Microstructure and surface characterization

The microstructure of samples was measured on surfaces polished down to 1 mm using a scanning electron microscopy (SEM, Phenom G2). Roughness measurements were performed by using SEM and 3D roughness reconstruction application (Phenom TM Pro Suite). The average surface roughness (Ra) was determined by measuring the surface roughness at 10 different locations over 3 samples of each type.

Morphological observation of the surfaces was undertaken by means of a Carl Zeiss Ultra Plus field emission scanning electron microscope (FE-SEM). The composition was analysed by an Oxford instruments INCA X-sight X-ray electron probe microanalyzer (EDX).The wettability of the studied materials was assessed by measuring the contact angle of deionized water drops deposited onto their surface using a KSV Instruments Cam 200 device.

Laser-induced periodic surface structures are fabricated using a fluence above the damage threshold fluence for the zirconia/tantalum biocermet. Chemical modification was analyzed using a scanning electron microscope (Hitachi S2600N). The topography was measured using an atomic force microscope (Agilent 5500).

During the fabrication process, we used an optical Nikon MM-400 microscope to visually inspect the samples; surface roughness and topography was measured using a SENSOFAR 2300 Plμ confocal microscope. Biological observations were performed using a Zeiss Microscope (Zeiss AxioVert A.1) and Zen software.

#### 2.3. Laser processing

ZrO2-Ta surfaces were exposed to multiple incident laser shots in air at a repetition rate of 100 kHz at various pulse energies ranging from 2 to 6 mJ. LIPSS were fabricated using a Yb:KYW laser system (Amplitude Systemes S-pulse HP) with a pulse duration of approximately 500 fs and a wavelength of 1030 nm. The beam delivery process for fabricating the LIPSS can be described as follows: the beam is focused on the top of the zirconia/tantalum biocermet surfaces using a flat-field lens (focal length = 100 mm). This kind of lens ensures a homogeneous light distribution in an area of 8 8 cm2 . The beam spot size (21 μm) was determined at 1/e2 of the Gaussian profile. The spatial profile of the laser was Gaussian in nature with a nominal M2 value of <1.2. The laser set-up for texturing ZrO2-Ta surfaces with LIPSS is shown in Figure 1.

Afterwards, we used a Ficoll-Histopaque (Sigma-Aldrich) gradient to isolate MSC. Then, we cultured the MSC to expand them under sterile conditions until we had obtained a concentration of 25,000 cells/cm2 as previously reported. We confirmed the desired cell concentration

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard…

http://dx.doi.org/10.5772/intechopen.70820

247

The survival and proliferation of hBMSCs on discs were examined with a MTT assay after 5, 10, 15 and 20 days of culture (groups of 6 discs). The MTT assays were carried out as per manufacturer's protocol (Sigma-Aldrich). Briefly, hBMSCs were seeded on the sample discs (both surfaces were tested at the same time to diminish variability) at a density of 25 103 cells per well in a 24 well plate. The medium was removed periodically at each time point, MTT solution was added and cells were incubated overnight. Next step was to remove the MTT solution and dissolve the purple formazan crystals in 100 mL of dimethyl sulphoxide (DMSO) by shaking the plate for 15 min (50 mL solution of each well was added into a new 24-well plate). An automated plate reader (PerkinElmer) was used to quantify the OD value by measuring the absorption at 570 nm. Afterwards, we proceed to clean the discs to use them for remaining biological assays. We proceed to clean all surfaces chemically (to destroy all biological tissues without modifying the surface) and treated them with an ultrasonic bath for effective cleaning and then autoclaved

To assess the amount of inflammatory cytokine concentration, both IL-6 and TNF-alpha ELISA kits (eBioscience) were performed according to the manufacturer's indications. Kits were read

Formation of an ECM implies the deposition of collagen (the main protein of the ECM) on the biomaterial surface. Collagen concentration was assessed by means of a total protein assay

Alkaline phosphatase (ALP) protein was assessed every 5 days. Samples were lysed using Cell Lysis Buffer (Cell Signalling), collected, and spun down at 14,000 g for 10 min at 4C. The supernatant was then collected to measure ALP protein levels via LabAssay ALP (Wako); protein levels were normalized by measuring total protein content via a BCA Protein Assay

Osteocalcin and osteopontin levels were assessed to gain an insight of the mesenchymal cell fate determination. Increased values of osteocalcin and osteopontin are associated with osteoblastic differentiation. Supernatants were collected at different time points and measured by

We performed a Kolmogorov-Smirnov test to confirm that values have a normal distribution. After we used a T-student test to compare rough and smooth surfaces, we accepted a p value

The laser set-up for fabricating the laser-induced periodic surface structures (LIPSS) on ZrO2: Ta surfaces consists of a Yb:KYW chirped-pulse-regenerative amplification laser system

on a spectrophotometer at the wavelength indicated by the manufacturer.

with a Neubauer chamber.

(Selecta Autotester) at 121C for 30 min.

(Pierce Labs USA).

kit (Thermo Fisher Scientific).

ELISA kits (Thermo Fisher).

3. Experimental results

lower than 0.05 as statistically significant.

Figure 1. (a) Laser set-up and (b) beam delivery system for generating the LIPSS; (c) schematic of LIPSS fabrication process and (d) femtosecond laser characteristics.

#### 2.4. Biological assay

We created 50 discs of the cermet. Each disc had a diameter of 6.75 mm ( 0.02 mm) and a thickness of 1.25 mm ( 0.02 mm). Once all the discs have been obtained, we proceed to polish those discs to prepare and to apply further treatments. We maintained 25 unaltered discs as a control group. In the remaining 25, we apply a laser treatment, which is explained later in detail, to create regular structures or roughness in the metallic structures of the cermet at disc surfaces. The samples used were treated with an ultrasonic bath for effective cleaning and then autoclaved (Selecta Autotester) at 121C for 30 min. We used one disc from each group to culture mesenchymal stem cells (MSC) for 48 h to compare under direct vision cell behaviour.

After Institutional Review Board approval, and after giving consent, we obtained mesenchymal stem cells (MSC) from iliac crest of patients who were operated elective, with no previous disease and with an age between 18 and 50 years old. Under spinal anaesthesia, during the main orthopaedic procedure, we performed a puncture with a trocar in the iliac crest to aspirate bone marrow mesenchymal stem cells. We used a low-pressure technique with heparin and saline to prevent haemolysis and the formation of clots.

Afterwards, we used a Ficoll-Histopaque (Sigma-Aldrich) gradient to isolate MSC. Then, we cultured the MSC to expand them under sterile conditions until we had obtained a concentration of 25,000 cells/cm2 as previously reported. We confirmed the desired cell concentration with a Neubauer chamber.

The survival and proliferation of hBMSCs on discs were examined with a MTT assay after 5, 10, 15 and 20 days of culture (groups of 6 discs). The MTT assays were carried out as per manufacturer's protocol (Sigma-Aldrich). Briefly, hBMSCs were seeded on the sample discs (both surfaces were tested at the same time to diminish variability) at a density of 25 103 cells per well in a 24 well plate. The medium was removed periodically at each time point, MTT solution was added and cells were incubated overnight. Next step was to remove the MTT solution and dissolve the purple formazan crystals in 100 mL of dimethyl sulphoxide (DMSO) by shaking the plate for 15 min (50 mL solution of each well was added into a new 24-well plate). An automated plate reader (PerkinElmer) was used to quantify the OD value by measuring the absorption at 570 nm.

Afterwards, we proceed to clean the discs to use them for remaining biological assays. We proceed to clean all surfaces chemically (to destroy all biological tissues without modifying the surface) and treated them with an ultrasonic bath for effective cleaning and then autoclaved (Selecta Autotester) at 121C for 30 min.

To assess the amount of inflammatory cytokine concentration, both IL-6 and TNF-alpha ELISA kits (eBioscience) were performed according to the manufacturer's indications. Kits were read on a spectrophotometer at the wavelength indicated by the manufacturer.

Formation of an ECM implies the deposition of collagen (the main protein of the ECM) on the biomaterial surface. Collagen concentration was assessed by means of a total protein assay (Pierce Labs USA).

Alkaline phosphatase (ALP) protein was assessed every 5 days. Samples were lysed using Cell Lysis Buffer (Cell Signalling), collected, and spun down at 14,000 g for 10 min at 4C. The supernatant was then collected to measure ALP protein levels via LabAssay ALP (Wako); protein levels were normalized by measuring total protein content via a BCA Protein Assay kit (Thermo Fisher Scientific).

Osteocalcin and osteopontin levels were assessed to gain an insight of the mesenchymal cell fate determination. Increased values of osteocalcin and osteopontin are associated with osteoblastic differentiation. Supernatants were collected at different time points and measured by ELISA kits (Thermo Fisher).

We performed a Kolmogorov-Smirnov test to confirm that values have a normal distribution. After we used a T-student test to compare rough and smooth surfaces, we accepted a p value lower than 0.05 as statistically significant.

## 3. Experimental results

2.4. Biological assay

process and (d) femtosecond laser characteristics.

246 Laser Ablation - From Fundamentals to Applications

prevent haemolysis and the formation of clots.

We created 50 discs of the cermet. Each disc had a diameter of 6.75 mm ( 0.02 mm) and a thickness of 1.25 mm ( 0.02 mm). Once all the discs have been obtained, we proceed to polish those discs to prepare and to apply further treatments. We maintained 25 unaltered discs as a control group. In the remaining 25, we apply a laser treatment, which is explained later in detail, to create regular structures or roughness in the metallic structures of the cermet at disc surfaces. The samples used were treated with an ultrasonic bath for effective cleaning and then autoclaved (Selecta Autotester) at 121C for 30 min. We used one disc from each group to culture mesenchymal stem cells (MSC) for 48 h to compare under direct vision cell behaviour. After Institutional Review Board approval, and after giving consent, we obtained mesenchymal stem cells (MSC) from iliac crest of patients who were operated elective, with no previous disease and with an age between 18 and 50 years old. Under spinal anaesthesia, during the main orthopaedic procedure, we performed a puncture with a trocar in the iliac crest to aspirate bone marrow mesenchymal stem cells. We used a low-pressure technique with heparin and saline to

Figure 1. (a) Laser set-up and (b) beam delivery system for generating the LIPSS; (c) schematic of LIPSS fabrication

The laser set-up for fabricating the laser-induced periodic surface structures (LIPSS) on ZrO2: Ta surfaces consists of a Yb:KYW chirped-pulse-regenerative amplification laser system (Amplitude Systemes S-pulse HP). The laser set-up used for fabricating the LIPSS is described in Section 2.3. Figure 1 shows the laser set-up for fabricating the LIPSS on ZrO2:Ta samples.

The laser system for fully texturing materials with LIPSS is shown in Figure 1c. For fabricating the LIPSS, the incident direction of the laser was orthogonal to the sample plane. Each sample was fully textured by combining an aligned array of spots at a laser wavelength of 1030 nm. The irradiation conditions for both materials were applied fluence of 0.30 J/cm2 , laser spot of 21 μm, wavelength of 1030 nm, repetition rate of 100 kHz, pulse width of 500 fs and sample scan speed of 200 mm/s; the sample was fully textured using highly over-lapped (95%) pulses. This method is very effective in accelerating the processing times while the obtained results are similar to conventional pulsed-laser treatments for creating LIPSS. Fluence was calculated using Eqs. (2) and (3). The applied threshold fluence for one pulse was previously calculated and it was found that with increasing number of laser pulses, the threshold fluence decreases [26].

The calculations for determining applied threshold fluence (φth) and (φ0) are obtained according to the method of Liu et al. [27]. The spatial fluence, (φr), for a Gaussian beam is given by:

$$
\phi(r) = \phi\_0 e^{-2r^2/\omega\_0^2} \tag{1}
$$

where φ0 is the peak fluence in the beam, r is the distance from the centre of the beam and ω0 is the Gaussian spot radius (1/e2 ). The maximum fluence and the pulse energy, Ep, are related by:

$$\phi\_0 = \frac{2E\_p}{\pi \omega\_0^2} \tag{2}$$

wavelength of the incident laser beam. The period of ripples is estimated to be 760 � 48 nm and the depth is 251 � 7 nm when exposed to a 1030 nm, and the roughness average is 54 � 2 nm. The depth of LIPSS depends on various parameters such as absorption coefficient, thermal diffusion and radiative cooling of the material. The effective penetration depth (α�1) is proportional to the laser wavelength. After femtosecond laser exposure, the alloy changes its optical penetration depth, as the material become amorphous. After multiple-pulse exposure which incorporates rapid heating and cooling cycles, the zirconia/tantalum material could evolve toward a metallic-

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard…

http://dx.doi.org/10.5772/intechopen.70820

249

Wettability results assessed in aqueous medium are presented in Figure 4. Since liquid ZrO2:Ta interfacial tension is not easy to measure directly, the contact angle of a deionized water droplet on the studied cermets was used to indicate surface wettability. The equilibrium contact angle

where σSV, σSL and σLV are the interfacial tensions between solid-gas, solid-liquid and liquidgas phases, respectively. Our results indicate that the as-cast cermet shows high wettability with a contact angle value of 81.2�. On the other hand, laser-treated ZrO2:Ta alloy presents significantly higher contact angle (113.5�), that is, it shows a more hydrophobic character.

σLV cosθeq ¼ σSV � σSL (4)

(θeq) of a liquid drop on an ideal solid surface is defined by Young's equation [29]:

like glass which would favor deeper surface structures [28].

Figure 2. FE-SEM image of the ZrO2:Ta cermet used for performing experiments.

The peak fluence is related to the diameter of the ablated spot

$$D^2 = 2\omega\_0^2 \text{ln}\left(\frac{\phi\_0}{\phi\_{th}}\right) \tag{3}$$

where D2 is the maximum diameter of the damaged region zone. It is possible to determine the beam radius using the value for ω0 from the plot of D2 versus the logarithm of the pulse energy. Once ω0 is calculated, fluence values can then be found using Eq. (3). By plotting D2 versus the natural log of the applied laser fluence and extrapolating the D2 line to zero, φth can be calculated.

Figure 2 shows a polished ZrO2:Ta sample used for generating ripples that exhibits a roughness average of 3.7 � 0.2 nm.

LIPPS were fabricated on ZrO2:Ta samples illustrated on Figure 2 by irradiation with a p-polarized femtosecond laser. The laser parameters used for generating the LIPSS were repetition rate of 100 kHz, pulse width of 500 fs, sample scan speed of 200 mm/s and 50 passes of highly over-lapped (95%). In Figure 3, we can see the typical fabricated LIPSS or ripples observed in the scanning electron microscopy images. Periodical micropatterns were generated in ZrO2:Ta using the abovementioned laser parameters at 200 mw laser power.

As shown in Figure 3, periodic structures were generated on tantalum while zirconia shows irregular structures at surface. The period of the surface ripples is significantly smaller than the In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard… http://dx.doi.org/10.5772/intechopen.70820 249

Figure 2. FE-SEM image of the ZrO2:Ta cermet used for performing experiments.

(Amplitude Systemes S-pulse HP). The laser set-up used for fabricating the LIPSS is described in Section 2.3. Figure 1 shows the laser set-up for fabricating the LIPSS on ZrO2:Ta samples. The laser system for fully texturing materials with LIPSS is shown in Figure 1c. For fabricating the LIPSS, the incident direction of the laser was orthogonal to the sample plane. Each sample was fully textured by combining an aligned array of spots at a laser wavelength of 1030 nm. The

wavelength of 1030 nm, repetition rate of 100 kHz, pulse width of 500 fs and sample scan speed of 200 mm/s; the sample was fully textured using highly over-lapped (95%) pulses. This method is very effective in accelerating the processing times while the obtained results are similar to conventional pulsed-laser treatments for creating LIPSS. Fluence was calculated using Eqs. (2) and (3). The applied threshold fluence for one pulse was previously calculated and it was found

The calculations for determining applied threshold fluence (φth) and (φ0) are obtained according to the method of Liu et al. [27]. The spatial fluence, (φr), for a Gaussian beam is given by:

where φ0 is the peak fluence in the beam, r is the distance from the centre of the beam and ω0 is

φ0 ¼ 2Ep πω2 0

where D2 is the maximum diameter of the damaged region zone. It is possible to determine the beam radius using the value for ω0 from the plot of D2 versus the logarithm of the pulse energy. Once ω0 is calculated, fluence values can then be found using Eq. (3). By plotting D2 versus the natural log of the applied laser fluence and extrapolating the D2 line to zero, φth can be calculated.

Figure 2 shows a polished ZrO2:Ta sample used for generating ripples that exhibits a rough-

LIPPS were fabricated on ZrO2:Ta samples illustrated on Figure 2 by irradiation with a p-polarized femtosecond laser. The laser parameters used for generating the LIPSS were repetition rate of 100 kHz, pulse width of 500 fs, sample scan speed of 200 mm/s and 50 passes of highly over-lapped (95%). In Figure 3, we can see the typical fabricated LIPSS or ripples observed in the scanning electron microscopy images. Periodical micropatterns were gener-

As shown in Figure 3, periodic structures were generated on tantalum while zirconia shows irregular structures at surface. The period of the surface ripples is significantly smaller than the

ated in ZrO2:Ta using the abovementioned laser parameters at 200 mw laser power.

0ln φ0 φth

D2 ¼ 2ω2

�2r2=ω2

). The maximum fluence and the pulse energy, Ep, are related by:

φð Þ¼ r φ0e

, laser spot of 21 μm,

(2)

(3)

0 (1)

irradiation conditions for both materials were applied fluence of 0.30 J/cm2

that with increasing number of laser pulses, the threshold fluence decreases [26].

The peak fluence is related to the diameter of the ablated spot

the Gaussian spot radius (1/e2

248 Laser Ablation - From Fundamentals to Applications

ness average of 3.7 � 0.2 nm.

wavelength of the incident laser beam. The period of ripples is estimated to be 760 � 48 nm and the depth is 251 � 7 nm when exposed to a 1030 nm, and the roughness average is 54 � 2 nm. The depth of LIPSS depends on various parameters such as absorption coefficient, thermal diffusion and radiative cooling of the material. The effective penetration depth (α�1) is proportional to the laser wavelength. After femtosecond laser exposure, the alloy changes its optical penetration depth, as the material become amorphous. After multiple-pulse exposure which incorporates rapid heating and cooling cycles, the zirconia/tantalum material could evolve toward a metalliclike glass which would favor deeper surface structures [28].

Wettability results assessed in aqueous medium are presented in Figure 4. Since liquid ZrO2:Ta interfacial tension is not easy to measure directly, the contact angle of a deionized water droplet on the studied cermets was used to indicate surface wettability. The equilibrium contact angle (θeq) of a liquid drop on an ideal solid surface is defined by Young's equation [29]:

$$
\sigma \text{LV} \cos \theta \text{eq} = \sigma S V - \sigma S L \tag{4}
$$

where σSV, σSL and σLV are the interfacial tensions between solid-gas, solid-liquid and liquidgas phases, respectively. Our results indicate that the as-cast cermet shows high wettability with a contact angle value of 81.2�. On the other hand, laser-treated ZrO2:Ta alloy presents significantly higher contact angle (113.5�), that is, it shows a more hydrophobic character.

nanopattern may be responsible for the observed hydrophobicity increase, which could also be influenced by the intrinsic change in the surface chemistry with the exposure to air after the

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard…

http://dx.doi.org/10.5772/intechopen.70820

251

Cell adhesion and several cellular functions are strongly influenced by the surface properties of a material. It has been proved that surface chemistry, energy, topography and wettability are key factors that affect protein adsorption and blood coagulation, which may have an impact on the adhesion, migration and differentiation of osteogenic cells, playing an important role in implants' osteointegration [31, 32]. In the studied cases, an increase in the protein adsorption and osteoblast differentiation parameters for laser-treated surfaces was observed, in spite of their lower wetting properties. The higher specific surface area offered by the fabricated nanostructures can play a decisive role in providing more adsorption sites for bioactive mole-

On the biological side, one of the main challenges in this study was to investigate the cells' behaviour when cultured over nanometric structures (LIPPS) to increase osteoblastic differentiation. After fabricating laser-induced periodic surface structures, we studied the behaviour of

MTT Mean 0.711 0.722 0.784 0.767 0.868 0.917 0.943 0.980

IL-6 Mean 0.018 0.019 0.026 0.023 0.031 0.031 0.032 0.030

ECM Mean 0.033 0.029 0.044 0.055 0.325 0.452 0.333 0.425

AF Mean 0.544 1.037 0.580 1.123 0.626 1.224 0.679 1.317

OSC Mean 0.539 0.639 0.575 0.624 0.622 0.778 0.601 0.787

OSP Mean 0.716 0.838 0.765 0.825 0.837 0.831 0.922 0.884

Table 1. Mean value for each assay and time point with the standard deviation.

Mean 0.018 0.018 0.023 0.023 0.024 0.024 0.045 0.044

SMOOTH 5 LIPSS 5 SMOOTH 10 LIPSS 10 SMOOTH 15 LIPSS 15 SMOOTH 20 LIPSS 20

0.046 0.053 0.067 0.051 0.061 0.119 0.079 0.085

0.002 0.002 0.004 0.003 0.003 0.003 0.002 0.003

0.001 0.002 0.003 0.002 0.002 0.002 0.003 0.007

0.026 0.015 0.028 0.031 0.044 0.034 0.040 0.052

0.038 0.198 0.026 0.145 0.047 0.073 0.033 0.132

0.052 0.043 0.049 0.069 0.063 0.069 0.049 0.050

0.057 0.075 0.077 0.050 0.130 0.123 0.059 0.076

cules and proteins, mitigating the effect of surface hydrophobicity [33].

laser fabrication process [30].

4. Biological results

Std. deviation

Std. deviation

Std. deviation

Std. deviation

Std. deviation

Std. deviation

Std. deviation

TNFalpha

Figure 3. FE-SEM images of the fabricated micropatterned structures on ZrO2:Ta (Ta: Tantalum; Zr: Zirconia) at different magnifications.

Figure 4. Photographs of as-deposited water droplets onto the surface of (a) as-cast and (b) ZrO2-Ta with ripples.

Considering γlv constant for a certain liquid in a particular environment, the observed variability in wettability values could be ascribed to variations in γsv and γsl, caused by the hierarchical structures fabricated on ZrO2:Ta surface by the applied laser process. LIPSS nanopattern may be responsible for the observed hydrophobicity increase, which could also be influenced by the intrinsic change in the surface chemistry with the exposure to air after the laser fabrication process [30].

Cell adhesion and several cellular functions are strongly influenced by the surface properties of a material. It has been proved that surface chemistry, energy, topography and wettability are key factors that affect protein adsorption and blood coagulation, which may have an impact on the adhesion, migration and differentiation of osteogenic cells, playing an important role in implants' osteointegration [31, 32]. In the studied cases, an increase in the protein adsorption and osteoblast differentiation parameters for laser-treated surfaces was observed, in spite of their lower wetting properties. The higher specific surface area offered by the fabricated nanostructures can play a decisive role in providing more adsorption sites for bioactive molecules and proteins, mitigating the effect of surface hydrophobicity [33].

## 4. Biological results

Considering γlv constant for a certain liquid in a particular environment, the observed variability in wettability values could be ascribed to variations in γsv and γsl, caused by the hierarchical structures fabricated on ZrO2:Ta surface by the applied laser process. LIPSS

Figure 3. FE-SEM images of the fabricated micropatterned structures on ZrO2:Ta (Ta: Tantalum; Zr: Zirconia) at different

magnifications.

250 Laser Ablation - From Fundamentals to Applications

Figure 4. Photographs of as-deposited water droplets onto the surface of (a) as-cast and (b) ZrO2-Ta with ripples.

On the biological side, one of the main challenges in this study was to investigate the cells' behaviour when cultured over nanometric structures (LIPPS) to increase osteoblastic differentiation. After fabricating laser-induced periodic surface structures, we studied the behaviour of

Table 1. Mean value for each assay and time point with the standard deviation.

bone marrow mesenchymal stem cells compared to polished surfaces. We cultured hBMSCs in a concentration of 25,000 cells/cm2 in the surfaces created for 20 days. We determined MTT, TNF-alpha, IL-6, collagen production, alkaline phosphatase, osteopontin and osteocalcin for every 5 days until day 20. Table 1 summarizes the average of all measurements for each assay.

(lower right corner). Figure 8 shows the images obtained on smooth surfaces, which showed a

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard…

http://dx.doi.org/10.5772/intechopen.70820

253

nonorganized mess of cells without polarization.

Figure 6. Data obtained for ECM assay with the standard deviation.

Figure 7. Confocal image of a treated surface.

MTT values in both surface treatments were similar (Figure 5), without reaching the statistical significance at any time point of control. Inflammatory response of both materials described by IL-6 and TNF-alpha values was also similar, with no difference between surfaces. We found very low values for both surfaces. Figure 5 shows the data obtained for MTT assay with the standard deviation.

We found a significant difference in ECM production (Figure 6) and osteoblast differentiation parameters (FA, OSC and OSP). ECM production was statistically increased in LIPSS surfaces at days 15 and 20. Alkaline phosphatase activity was increased at all time point controls. Osteocalcin levels were also elevated at every control, but at day 10 there was no statistical difference. Osteopontin values were also superior to control in every control, but only reached statistical difference at day 5. Figure 6 shows the data obtained for ECM assay with the standard deviation.

Confocal microscopic image (Figure 7) showed a polarization of cells in the metal area (where LIPSS are generated). In the surface where the ceramic is present, we can see a disordered pattern seen in most of the smooth surfaces.

We can see a transition between LIPSS and the ceramic surface. In the upper left corner, we can see a polarization of the long axis of the cell, which is not seen in the ceramic surface

Figure 5. Data obtained for MTT assay with the standard deviation.

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard… http://dx.doi.org/10.5772/intechopen.70820 253

(lower right corner). Figure 8 shows the images obtained on smooth surfaces, which showed a nonorganized mess of cells without polarization.

Figure 6. Data obtained for ECM assay with the standard deviation.

bone marrow mesenchymal stem cells compared to polished surfaces. We cultured hBMSCs in a concentration of 25,000 cells/cm2 in the surfaces created for 20 days. We determined MTT, TNF-alpha, IL-6, collagen production, alkaline phosphatase, osteopontin and osteocalcin for every 5 days until day 20. Table 1 summarizes the average of all measurements for each assay. MTT values in both surface treatments were similar (Figure 5), without reaching the statistical significance at any time point of control. Inflammatory response of both materials described by IL-6 and TNF-alpha values was also similar, with no difference between surfaces. We found very low values for both surfaces. Figure 5 shows the data obtained for MTT assay with the

We found a significant difference in ECM production (Figure 6) and osteoblast differentiation parameters (FA, OSC and OSP). ECM production was statistically increased in LIPSS surfaces at days 15 and 20. Alkaline phosphatase activity was increased at all time point controls. Osteocalcin levels were also elevated at every control, but at day 10 there was no statistical difference. Osteopontin values were also superior to control in every control, but only reached statistical difference at day 5. Figure 6 shows the data obtained for ECM assay with the standard deviation. Confocal microscopic image (Figure 7) showed a polarization of cells in the metal area (where LIPSS are generated). In the surface where the ceramic is present, we can see a disordered

We can see a transition between LIPSS and the ceramic surface. In the upper left corner, we can see a polarization of the long axis of the cell, which is not seen in the ceramic surface

standard deviation.

252 Laser Ablation - From Fundamentals to Applications

pattern seen in most of the smooth surfaces.

Figure 5. Data obtained for MTT assay with the standard deviation.

Figure 7. Confocal image of a treated surface.

Acknowledgements

Author details

Daniel Nieto7,8\*

Compostela, Spain

References

Alberto Jorge-Mora1,2, Naroa Imaz3

Científicas (CSIC), Madrid, Spain

\*Address all correspondence to<EMAIL_ADDRESS>
D. Nieto is grateful to the Consellería de Cultura, Spain for the support under the Galician

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard…

, Nekane Frutos3

1 SERGAS (Galician Health Service), Universidad de Santiago de Compostela, Santiago de

6 Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones

[1] Arifin A, Sulong AB, Muhamad N, Syarif J, Ramli MI. Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials using powder metal-

[2] Coathup MJ, Blunn GW, Mirhosseini N, Erskine K, Liu Z, Garrod DR, et al. Controlled laser texturing of titanium results in reliable osteointegration. Journal of Orthopaedic Research. 2016

[3] Kurella A, Dahotre NB. Review paper: Surface modification for bioimplants: The role of laser surface engineering. Journal of Biomaterials Applications. 2005;20(1):5-50

[4] Rokkum M, Reigstad A, Johansson CB. HA particles can be released from well-fixed HAcoated stems: histopathology of biopsies from 20 hips 2–8 years after implantation. Acta

7 NCLA / Inspire Labs, School of Physics, National University of Ireland, Galway, Ireland 8 Photonics4Life Research Group, Departamento de Física Aplicada, Facultad de Física,

, Ana Alonso2

, Carlota García Santiago4

http://dx.doi.org/10.5772/intechopen.70820

, Gerard O'connor6 and

,

255

Program for Research Innovation and Growth (2011–2015) (I2C Plan).

Rodolfo Gómez-Vaamonde1,2, Jesús Pino-Minguez1,5, Jose Bartolomé6

2 IDIS (Musculoskeletal Pathology Group), Santiago de Compostela, Spain

Universidade de Santiago de Compostela, Santiago de Compostela, Spain

lurgy: A review. Materials and Design. 2014;55:165-175

Orthopaedica Scandinavica. 2002;73(3):298-306

4 Department of Pharmacology, Universidade de Santiago de Compostela, Spain 5 IDIS (NEIRID Group), Orthopaedic Department, Santiago de Compostela, Spain

3 Surface Engineering Division, IK4-CIDETEC, San Sebastián, Spain

Figure 8. Confocal image of the smooth surface.

## 5. Conclusions

We have demonstrated in vitro that LIPSS are an efficient option to increase osteoblastic differentiation of hBMSCs in ZrO2:Ta biocermets. When we create a laser-induced periodic surface with a femtosecond laser, we found that proliferation and inflammatory response are not increased when compared to a smooth surface. We know that LIPSS in metals decreases hydrophobicity compared to smooth surfaces, but in cermet this situation is not seen. We believe that we found a dual phenomenon. In the metal area of the cermet, the contact angle is decreased, but in the ceramic area, the transformation phase seen with this laser treatment increases the contact angle and the hydrophobicity. The optimization of the laser treatment probably will improve the wettability minimizing the transformation phase.

We can see an increase in the ECM in treated surfaces at days 15 and 20. These values add more significance to the values obtained for osteoblastic differentiation.

With values of proliferation similar in both materials, we found an increase in osteoblastic differentiation in LIPSS created over ZrO2:Ta compared to smooth surfaces. This increase is demonstrated by higher values in the studied assays of alkaline phosphatase, osteocalcin and osteopontin production at every control point. This behaviour is seen in metals such as titanium and tantalum. This situation is similar to previous studies that demonstrate a more biological activity in LIPSS surfaces compared to smooth ones.

We can conclude that creating laser-induced periodic surfaces is a promising and cheap modification to increase osteoblastic response to a material, without adding chemicals or new materials. Optimization of the laser fluence and parameters will increase this effect, minimizing the transformation phenomenon in the ceramic area, but creating LIPSS in the metal islands.

## Acknowledgements

D. Nieto is grateful to the Consellería de Cultura, Spain for the support under the Galician Program for Research Innovation and Growth (2011–2015) (I2C Plan).

## Author details

Alberto Jorge-Mora1,2, Naroa Imaz3 , Nekane Frutos3 , Ana Alonso2 , Carlota García Santiago4 , Rodolfo Gómez-Vaamonde1,2, Jesús Pino-Minguez1,5, Jose Bartolomé6 , Gerard O'connor6 and Daniel Nieto7,8\*

\*Address all correspondence to<EMAIL_ADDRESS>
1 SERGAS (Galician Health Service), Universidad de Santiago de Compostela, Santiago de Compostela, Spain

6 Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain

7 NCLA / Inspire Labs, School of Physics, National University of Ireland, Galway, Ireland

8 Photonics4Life Research Group, Departamento de Física Aplicada, Facultad de Física, Universidade de Santiago de Compostela, Santiago de Compostela, Spain

## References

5. Conclusions

Figure 8. Confocal image of the smooth surface.

254 Laser Ablation - From Fundamentals to Applications

We have demonstrated in vitro that LIPSS are an efficient option to increase osteoblastic differentiation of hBMSCs in ZrO2:Ta biocermets. When we create a laser-induced periodic surface with a femtosecond laser, we found that proliferation and inflammatory response are not increased when compared to a smooth surface. We know that LIPSS in metals decreases hydrophobicity compared to smooth surfaces, but in cermet this situation is not seen. We believe that we found a dual phenomenon. In the metal area of the cermet, the contact angle is decreased, but in the ceramic area, the transformation phase seen with this laser treatment increases the contact angle and the hydrophobicity. The optimization of the laser treatment

We can see an increase in the ECM in treated surfaces at days 15 and 20. These values add

With values of proliferation similar in both materials, we found an increase in osteoblastic differentiation in LIPSS created over ZrO2:Ta compared to smooth surfaces. This increase is demonstrated by higher values in the studied assays of alkaline phosphatase, osteocalcin and osteopontin production at every control point. This behaviour is seen in metals such as titanium and tantalum. This situation is similar to previous studies that demonstrate a more

We can conclude that creating laser-induced periodic surfaces is a promising and cheap modification to increase osteoblastic response to a material, without adding chemicals or new materials. Optimization of the laser fluence and parameters will increase this effect, minimizing the trans-

formation phenomenon in the ceramic area, but creating LIPSS in the metal islands.

probably will improve the wettability minimizing the transformation phase.

more significance to the values obtained for osteoblastic differentiation.

biological activity in LIPSS surfaces compared to smooth ones.

[5] Han CD, Shin KY, Lee HH, Park KK, Yang IH, Lee WS. The results of long-term follow-up of total hip arthroplasty using hydroxyapatite-coated cups. Hip & Pelvis. 2015;27(4):209-215

[20] Hohm S, Herzlieb M, Rosenfeld A, Kruger J, Bonse J. Formation of laserinduced periodic surface structures on fused silica upon two-color double-pulseirradiation. Applied Phys-

In Vitro Evaluation of Laser-Induced Periodic Surface Structures on New Zirconia/Tantalum Biocermet for Hard…

http://dx.doi.org/10.5772/intechopen.70820

257

[21] Bartolome JF, Moya JS, Couceiro R, Gutierrez-Gonzalez CF, Guiti F, Martinez-Insua A. In vitro and in vivo evaluation of a new zirconia/niobium biocermet for hard tissue replace-

[22] Daminelli G, Krüger J, Kautek W. Femtosecond laser interaction with silicon under water

[23] Suchanek W, Yoshimura M. Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants. Journal of Materials Research. 1998;13:94-117

[24] Karageorgiou V, Kaplan D. Porosity of 3D biomaterial scaffolds and osteogenesis. Bio-

[25] Vasconcellos LMRd, Oliveira MVd, Graça MLdA, Vasconcellos LGOd, Carvalho YR, Cairo CAA. Porous titanium scaffolds produced by powder metallurgy for biomedical

[26] Nieto D, Arines J, O'connor GM, flores-Arias MT. Single-pulse laser ablation threshold of borosilicate, fused silica, sapphire, and soda-lime glass for pulse widths of 500 fs, 10 ps,

[27] Liu X-L, Cheng W, Petrarca M, Polynkin P. Measurements of fluence profiles in femtosec-

[28] McDaniel C, Gladkovskaya O, Flanagan A, Rochev Y, O'Connor GM. In vitro study onthe response of RAW264.7 and MS-5 fibroblast cells on laser-induced periodic surfaces-

[30] Martínez-Calderón M, Rodríguez A, Dias A, Morant-Miñana MC, Gómez-Aranzadi M, Olaizola SM. Femtosecond laser fabrication of highly hydrophobic stainless steel surface with hierarchical structures fabricated by combining ordered microstructures and LIPSS.

[31] Wennerberg A, Jimbo R, Stübinger S, Obrecht M, Dard M, Berner S. Nanostructures and hydrophilicity influence osseointegration – A biomechanical study in the rabbit tibia.

[32] Kenar H, Akman E, Kacar E, Demir A, Park H, Abdul-Khaliq H, AktaCs KE. Femtosecond laser treatment of 316L improves its surface nanoroughness and carbon content and promotes osseointegration: An in vitro evaluation. Colloids and Surfaces B. 2013;108:305-312

[33] Kopf BS, Ruch S, Berner S, Spencer ND, Maniura-Weber K. The role of nanostructures and hydrophilicity in osseointegration: In-vitro protein-adsorption and blood-interaction studies. Journal of Biomedical Materials Research Part A. 2015;2015(103A):2661-2672

ics Letters. 2013;103

ment. Biomaterials. 2016;76:313-320

materials. 2005;26:5474-5491

confinement. Thin Solid Films. 2004;467:334-341

applications. Materials Research. 2008;11:275-280

ond laser filaments in air. Optics Letters. 2016;41(20):4751-4754

tructures for stainless steel alloys. RSC Advances. 2015;5:42548-42558

[29] Young T. Philosophical Transactions of the Royal Society of London. 1805;95:65

20 ns. Applied Optics. 2015;54(29):8596-8601

Applied Surface Science. 2015;374:81-89

Clinical Oral Implants Research. 2014;25:1041

[20] Hohm S, Herzlieb M, Rosenfeld A, Kruger J, Bonse J. Formation of laserinduced periodic surface structures on fused silica upon two-color double-pulseirradiation. Applied Physics Letters. 2013;103

[5] Han CD, Shin KY, Lee HH, Park KK, Yang IH, Lee WS. The results of long-term follow-up of total hip arthroplasty using hydroxyapatite-coated cups. Hip & Pelvis. 2015;27(4):209-215

[6] Martinez-Calderon M, Manso-Silvan M, Rodriguez A, Gomez-Aranzadi M, Garcia-Ruiz JP, Olaizola SM, et al. Surface micro- and nano-texturing of stainless steel by femtosecond

[7] Nieto D, Delgado T, Flores-Arias MT. Fabrication of microchannels on soda-lime glass substrates with a Nd:YVO4 laser. Optics and Lasers in Engineering. 2014;63:11-18

[8] Nieto D, Couceiro R, Aymerich M, Lopez-Lopez R, Abal M, Flores-Arias MT. A laserbased technology for fabricating a soda-lime glass based microfluidic device for circulat-

[9] Chen M-F, Chen Y-P, Hsiao W-T, Gu Z-P. Laser direct write patterning technique of

[10] Nieto D, McGlynn P, de la Fuente M, Lopez-Lopez R, O'connor GM Laser microfabrication of a microheater chip for cell culture outside a cell incubator Colloids Surf B Biointerfaces.

[11] Mannion PT, Magee J, Coyne E, O'Connor GM, Glynn TJ. The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-

[12] Hwang TY, Vorobyev AY, Guo CL. Ultrafast dynamics of femtosecond laser-induced

[13] Hwang TY, Guo CL. Femtosecond laser-induced blazed periodic grooves on metals.

[14] Bonse J, Kruger J, Hohm S, Rosenfeld A. Femtosecond laser-induced periodic surface

[15] Gurevich EL, Gurevich SV. Laser induced periodic surface structures induced by surface

[16] Sedao X, Maurice C, Garrelie F, Colombier JP, Reynaud S, Quey R, Blanc G, Pigeon F. Electron backscatter diffraction characterization of laser-induced periodic surface struc-

[17] Huynh TTD, Petit A, Semmar N. Picosecond laser induced periodic surface structure on

[18] Perez S, Rebollar E, Oujja M, Martin M, Castillejo M. Laser-induced periodic surface structuring of biopolymers. Applied Physics A: Materials Science & Processing. 2013;

[19] Kumar B, Soni RK. Pulsed-laser-induced photochemical growth of the periodic surface

plasmons coupled via roughness. Applied Surface Science. 2014;302:118-123

tures on nickel surface. Applied Surface Science. 2014;302:114-117

structure on InP. Semiconductor Science and Technology. 2009;24

copper thin films. Applied Surface Science. 2014;302:109-113

machining of common metals in air. Applied Surface Science. 2004;233:275-287

nanostructure formation on metals. Applied Physics Letters. 2009;95

ing tumour cell capture. Colloids and Surfaces B: Biointerfaces. 2015;134:363-369

laser for the control of cell migration. Scientific Reports. 2016;6:36296

indium tin oxide film. Thin Solid Films. 2007;515:8515-8518

2017;154:263-269

256 Laser Ablation - From Fundamentals to Applications

110:683-690

Optics Letters. 2011;36:2575-2577

structures. Journal of Laser Applications. 2012;24

**Chapter 13**

**Provisional chapter**

**Laser Ablation of Energetic Materials**

**Laser Ablation of Energetic Materials**

Ruiqi Shen, Lizhi Wu, Wei Zhang and Haonan Zhang

DOI: 10.5772/intechopen.71892

The initiation of explosives by laser is a new initiation method. Compared with traditional initiation methods, laser initiation has the characteristics of high reliability and high safety. It can be used as one of the alternative technologies for future initiation device. A microscopic understanding of the complex physical and chemical processes involved in the reaction process is essential for laser initiation. Shock initiation technology of laser-driven flyer was studied. Several typical laser-driven flyer targets researches were introduced. Some significant characteristics including velocity and impact stress of flyers were tested via photonic Doppler velocimetry and polyvinylidene fluoride pressure sensor, respectively. Some types of flyers including Al and Cu single-layer flyers and CuO/Cu, CuO/Al, and CuO/Al/ Cu multilayer flyers with relatively high velocities were used to initiate PETN explosive. In order to give a better understanding of the mechanism of laser interaction with typical energetic materials (RDX, HMX, TNT, and HNS), a time of flight mass spectrometer (TOFMS) was used to detect the positive ions and the negative ions were produced in the laser-induced dissociation processes. The influences of laser wavelength, the laser fluence, and the delay time of the decomposition process have been studied as well. The results

may throw some light on the laser interaction mechanism of energetic materials.

**Keywords:** laser ablation, PDV, laser-driven flyer, TOFMS, nitramines, aromatic nitro

Laser ablation of energetic materials, such as pyrotechnics, explosives, and propellants, is different from other materials because of the interaction with chemical reaction and laser beam. The interaction processes between energetic material and laser beam are accompanied with thermal physics process irradiated by laser (photo-thermal, melting, and gasification),

Ruiqi Shen, Lizhi Wu, Wei Zhang and

http://dx.doi.org/10.5772/intechopen.71892

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Haonan Zhang

**Abstract**

compounds

**1. Introduction**

**Chapter 13**

**Provisional chapter**

## **Laser Ablation of Energetic Materials Laser Ablation of Energetic Materials**

Ruiqi Shen, Lizhi Wu, Wei Zhang and

Haonan Zhang Ruiqi Shen, Lizhi Wu, Wei Zhang and Haonan Zhang

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71892

#### **Abstract**

The initiation of explosives by laser is a new initiation method. Compared with traditional initiation methods, laser initiation has the characteristics of high reliability and high safety. It can be used as one of the alternative technologies for future initiation device. A microscopic understanding of the complex physical and chemical processes involved in the reaction process is essential for laser initiation. Shock initiation technology of laser-driven flyer was studied. Several typical laser-driven flyer targets researches were introduced. Some significant characteristics including velocity and impact stress of flyers were tested via photonic Doppler velocimetry and polyvinylidene fluoride pressure sensor, respectively. Some types of flyers including Al and Cu single-layer flyers and CuO/Cu, CuO/Al, and CuO/Al/ Cu multilayer flyers with relatively high velocities were used to initiate PETN explosive. In order to give a better understanding of the mechanism of laser interaction with typical energetic materials (RDX, HMX, TNT, and HNS), a time of flight mass spectrometer (TOFMS) was used to detect the positive ions and the negative ions were produced in the laser-induced dissociation processes. The influences of laser wavelength, the laser fluence, and the delay time of the decomposition process have been studied as well. The results may throw some light on the laser interaction mechanism of energetic materials.

DOI: 10.5772/intechopen.71892

**Keywords:** laser ablation, PDV, laser-driven flyer, TOFMS, nitramines, aromatic nitro compounds

## **1. Introduction**

Laser ablation of energetic materials, such as pyrotechnics, explosives, and propellants, is different from other materials because of the interaction with chemical reaction and laser beam. The interaction processes between energetic material and laser beam are accompanied with thermal physics process irradiated by laser (photo-thermal, melting, and gasification),

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

**Figure 1.** Interaction mechanism between laser beam and energetic materials.

chemical reaction in condensed phase and gaseous phase, generation of plasma, and lasersupported combustion (LSC) and laser-supported detonation (LSD) depending on laser power and material characteristics, seen in **Figure 1**.

surface of the metal film, so that the film substance is heated and evaporated. Then, the plasmas formed at the interface between substrate and metal absorb the laser energy continuously to heat up and expand. Next, the rest part of metal film is released out to form high-velocity flyer. Compared with conventional high-velocity driving methods including gas cannon, detonation, and electromagnetic driving, laser-driven flyer shows advantages of greatly enhancing the velocity of flyer (up to tens of km/s) and impact stress (up to TPa order), owning simple structure and low cost. Therefore, laser-driven technology is widely used in high-pressure physics, space science, material science**,** and research of rapid detonation for explosives [1].

Laser Ablation of Energetic Materials http://dx.doi.org/10.5772/intechopen.71892 261

Laser-driven flyer impact initiation technology is an application of laser-driven flyer, which is the process of initiating high energy explosives with high-velocity flyer driven by laser. As shown in **Figure 3**, the process of laser-driven flyer initiating explosive can be divided into

**a.** The laser beam penetrates into the transparent substrate and interacts with the film on the back of substrate so that a part of film material is ablated instantaneously to form plasmas,

**b.** The plasmas with high temperature and high pressure can only expand in the direction of the film material due to the confinement of substrate so that the remaining nonablated films are sheared down to form high-velocity flyer, which is accelerated in accelerating

**c.** The flying flyer at high velocity impacts on explosive surface and causes detonation to

It is the precise time control, rapid response, strong electromagnetic interference resistance, and low sensitivity that makes laser-driven flyer impact initiation technology as a good poten-

At present, the flying target with transparent substrate is mainly used in laser-driven flyer impact initiation technology due to its great performance of enhancing the absorption of laser energy by plasma and increasing the impact momentum of flyer. Usually, BK7, K9**,** or

three main steps:

chamber immediately,

**2.1. Laser-driven flyer target**

which can absorb the laser energy unceasingly,

**Figure 2.** Two modes of laser-driven flyers: (a) inward driving (b) linear driving.

initiate explosive (e.g. HNS and PETN).

tial for the application to modern fuze system.

Laser ablation of energetic materials is different from other materials' in chemical reaction, in which the chemical reactions in condensed and gaseous phase can induce ignition, combustion, and/or detonation. The research results show the reaction irradiated by laser is attributed to the category of thermochemistry at low power (<103 W/mm2 ), but it is a complex reaction including laser ablation and laser plasma at high power (>103 W/mm2 ).

## **2. Shock initiation technology of laser-driven flyer**

As a type of driving energy, laser is able to ablate the target to generate substances with high temperature and pressure to propel objects up to high velocity. There exist mainly two modes of laser-driven flyers including inward driving and linear driving as shown in **Figure 2**.

**Figure 2(a)** shows an inward driving structure with free outward spherical surface as target. After being ablated by intense laser (with intensity of light over 1012 W·cm−2), the target will be disaggregated as plasmas with high temperature, high pressure, and high density to promote the rest part of target shell to be a flyer with high velocity. Immediately, the high-velocity flyer will cause implosion of deuterium and tritium medium inside the shell to achieve compressed fusion ignition (CFI). In linear driving structure shown in **Figure 2(b)**, the incident surface is covered by the transparent substrate or the optical fiber. If the intensity of incident laser is not that high (109 –1011 W·cm−2), the transparent substrate can block the expansion of plasmas temporarily to enhance the impact stress and velocity of flyer (the remainder of target plate after ablation). This chapter mainly focuses on laser-driven flyer technology based on transparent substrate, which is gradually becoming a significant dynamic high pressure loading technology.

The process of laser-driven flyer is very complex and can be roughly divided into several steps as follows. First, the laser beam penetrates into the transparent substrate and irradiates on the

**Figure 2.** Two modes of laser-driven flyers: (a) inward driving (b) linear driving.

surface of the metal film, so that the film substance is heated and evaporated. Then, the plasmas formed at the interface between substrate and metal absorb the laser energy continuously to heat up and expand. Next, the rest part of metal film is released out to form high-velocity flyer.

Compared with conventional high-velocity driving methods including gas cannon, detonation, and electromagnetic driving, laser-driven flyer shows advantages of greatly enhancing the velocity of flyer (up to tens of km/s) and impact stress (up to TPa order), owning simple structure and low cost. Therefore, laser-driven technology is widely used in high-pressure physics, space science, material science**,** and research of rapid detonation for explosives [1].

Laser-driven flyer impact initiation technology is an application of laser-driven flyer, which is the process of initiating high energy explosives with high-velocity flyer driven by laser. As shown in **Figure 3**, the process of laser-driven flyer initiating explosive can be divided into three main steps:

It is the precise time control, rapid response, strong electromagnetic interference resistance, and low sensitivity that makes laser-driven flyer impact initiation technology as a good potential for the application to modern fuze system.

#### **2.1. Laser-driven flyer target**

chemical reaction in condensed phase and gaseous phase, generation of plasma, and lasersupported combustion (LSC) and laser-supported detonation (LSD) depending on laser

Laser ablation of energetic materials is different from other materials' in chemical reaction, in which the chemical reactions in condensed and gaseous phase can induce ignition, combustion, and/or detonation. The research results show the reaction irradiated by laser is attributed

As a type of driving energy, laser is able to ablate the target to generate substances with high temperature and pressure to propel objects up to high velocity. There exist mainly two modes of laser-driven flyers including inward driving and linear driving as shown in **Figure 2**.

**Figure 2(a)** shows an inward driving structure with free outward spherical surface as target. After being ablated by intense laser (with intensity of light over 1012 W·cm−2), the target will be disaggregated as plasmas with high temperature, high pressure, and high density to promote the rest part of target shell to be a flyer with high velocity. Immediately, the high-velocity flyer will cause implosion of deuterium and tritium medium inside the shell to achieve compressed fusion ignition (CFI). In linear driving structure shown in **Figure 2(b)**, the incident surface is covered by the transparent substrate or the optical fiber. If the intensity of incident laser is not

porarily to enhance the impact stress and velocity of flyer (the remainder of target plate after ablation). This chapter mainly focuses on laser-driven flyer technology based on transparent substrate, which is gradually becoming a significant dynamic high pressure loading technology. The process of laser-driven flyer is very complex and can be roughly divided into several steps as follows. First, the laser beam penetrates into the transparent substrate and irradiates on the

–1011 W·cm−2), the transparent substrate can block the expansion of plasmas tem-

W/mm2

W/mm2 ).

), but it is a complex reaction

power and material characteristics, seen in **Figure 1**.

260 Laser Ablation - From Fundamentals to Applications

**Figure 1.** Interaction mechanism between laser beam and energetic materials.

to the category of thermochemistry at low power (<103

that high (109

including laser ablation and laser plasma at high power (>103

**2. Shock initiation technology of laser-driven flyer**

At present, the flying target with transparent substrate is mainly used in laser-driven flyer impact initiation technology due to its great performance of enhancing the absorption of laser energy by plasma and increasing the impact momentum of flyer. Usually, BK7, K9**,** or

**Figure 3.** Process of laser-driven flyer impact initiation.

sapphire glass with high transmittance of the corresponding wavelength are adopted as the transparent substrate.

In general, thickness of the film deposited on the transparent substrate is typically a few microns to tens of microns and the diameter is about 1 mm. Paisley et al. [2] found that the impact stress of flyer must be higher than the rarefaction wave's reflected from the free surface of flyer during plasma-driving laser ablation. Only under this condition the flyer continue to be accelerated to reach the final velocity. In addition, the laser pulse width must be 2.5 times larger than round-trip time of sound in the flyer in order to ensure that the flyer will not be fully impacted, ablated, and chipped during the acceleration.

$$R = T / \left[ \because 2.5 \left( 2t / \text{C}\_{\text{i}} \right) \right], \text{that is} \\ R = \frac{0.2 \left( \text{TC}\_{\text{i}} \right)}{t} \tag{1}$$

Multilayer flyer is developed according to the formation process of laser-driven flyer. Generally, multilayer flyer owns three-layer structure, as shown in **Figure 4(b)**. The main function of ablation layer is to absorb laser energy and be ablated to form plasma with high temperature and high pressure to drive the flyer. Thermal insulation layer is to restrain the formation and diffusion of plasma in order to reduce the ablation of flyer and ensure its integrity. Sometimes multilayer flyer can be divided into four layers. An absorption layer is added between ablation layer and transparent substrate to improve the absorption of laser energy.

Several typical flyer targets are listed in **Table 1** in detail. In laser-driven flyer impact initiation technology, the research on flyer has been developed from single-metal flyer to multilayer flyer. According to **Table 1**, the main materials of single-layer flyer are Al and Cu. As for multilayer

are commonly good choices for thermal insulation materials. Al, Ti, and Cu are usually

In the study of laser-driven flyer impact initiation technology, the detonation of explosive depends mainly on the impact stress of flyer and pulse width of impact stress. The impact stress of flyer depends on the velocity, structure, and material of flyer while the pulse width

S. A. Sheffield [3] Al **— — — —**

**Ablation layer**

Al **— — —** BK7

O3 **— —**

O3 Al **—**

O3 Ge, C, Ti, Al **—**

O3 Al C

O3 Al CuO

, TiO2 C, Mg, Ge, Al, Ti

O3 Ge, Ti, Zn

Al, Mg **—**

C, Ti

**Insulation layer**

D. L. Paisley [4] Al, Cu **— — —**

D. B. Stahl [7] Al **—** C **—**

O3 , MgF2 ,

O3

ZnS

Al, Cu Al2

Al, Ti Al2

O3 , MgF2

Laser Ablation of Energetic Materials http://dx.doi.org/10.5772/intechopen.71892 263

**Absorption layer**

, ZnS, and

**Transparent substrate**

glass

or

or

K9 glass

Sapphire glass

flyer, the ablation layer materials are mostly C, Al, Mg, Ge, Ti, Zn, etc. Al2

**layer**

*2.1.2. Typical flyer targets*

used to prepare flyer layers.

**2.2. Characteristics of laser-driven flyer**

**Structure Author Flyer**

Multilayer flyer D. L. Paisley [5, 6] Al, Cu Al2

D. J. Hatt and J. A. Waschl [10]

M. D. Bowden and S. L. Knowles [14]

**Table 1.** Typical laser-driven flyer targets researched.

W. M. Trott [8, 9] Al Al2

J. L. Labaste [11] Al Al2

D. L. Paisley [12, 13] Al Al2

L. Wu [15] Al Al2

H. R. Brierley [16] Al Al2

TiO2

Single-layer flyer

In Eq. (1), the dimensionless quantity *R* must be greater than 1, *T* (s) is the laser pulse width, *t* (mm) is the thickness of flyer, *C1* (mm/s) is the radial sound velocity of flyer.

#### *2.1.1. Structure of flyer target*

The structure of flyer target can be divided into two types: single-layer flyer and multilayer flyer. **Figure 4(a)** shows the structure of single-layer flyer. As we can see, single-layer flyer is prepared by depositing metal film on the transparent substrate. After the interaction between laser and metal film, the plasma with high temperature and high pressure produced through ablation will drive the nonablated film and shear it out to form a flyer.

**Figure 4.** Structure of (a) single-layer flyer and (b) multilayer flyer.

Multilayer flyer is developed according to the formation process of laser-driven flyer. Generally, multilayer flyer owns three-layer structure, as shown in **Figure 4(b)**. The main function of ablation layer is to absorb laser energy and be ablated to form plasma with high temperature and high pressure to drive the flyer. Thermal insulation layer is to restrain the formation and diffusion of plasma in order to reduce the ablation of flyer and ensure its integrity. Sometimes multilayer flyer can be divided into four layers. An absorption layer is added between ablation layer and transparent substrate to improve the absorption of laser energy.

#### *2.1.2. Typical flyer targets*

sapphire glass with high transmittance of the corresponding wavelength are adopted as the

In general, thickness of the film deposited on the transparent substrate is typically a few microns to tens of microns and the diameter is about 1 mm. Paisley et al. [2] found that the impact stress of flyer must be higher than the rarefaction wave's reflected from the free surface of flyer during plasma-driving laser ablation. Only under this condition the flyer continue to be accelerated to reach the final velocity. In addition, the laser pulse width must be 2.5 times larger than round-trip time of sound in the flyer in order to ensure that the flyer will

*R* = *T*/[~2.5 ( 2*t*/*C*1) ], that is *R* = 0.2 ( *TC*1) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *t* (1)

In Eq. (1), the dimensionless quantity *R* must be greater than 1, *T* (s) is the laser pulse width,

The structure of flyer target can be divided into two types: single-layer flyer and multilayer flyer. **Figure 4(a)** shows the structure of single-layer flyer. As we can see, single-layer flyer is prepared by depositing metal film on the transparent substrate. After the interaction between laser and metal film, the plasma with high temperature and high pressure produced through

(mm/s) is the radial sound velocity of flyer.

not be fully impacted, ablated, and chipped during the acceleration.

ablation will drive the nonablated film and shear it out to form a flyer.

**Figure 4.** Structure of (a) single-layer flyer and (b) multilayer flyer.

transparent substrate.

**Figure 3.** Process of laser-driven flyer impact initiation.

262 Laser Ablation - From Fundamentals to Applications

*t* (mm) is the thickness of flyer, *C1*

*2.1.1. Structure of flyer target*

Several typical flyer targets are listed in **Table 1** in detail. In laser-driven flyer impact initiation technology, the research on flyer has been developed from single-metal flyer to multilayer flyer.

According to **Table 1**, the main materials of single-layer flyer are Al and Cu. As for multilayer flyer, the ablation layer materials are mostly C, Al, Mg, Ge, Ti, Zn, etc. Al2 O3 , MgF2 , ZnS, and TiO2 are commonly good choices for thermal insulation materials. Al, Ti, and Cu are usually used to prepare flyer layers.

#### **2.2. Characteristics of laser-driven flyer**

In the study of laser-driven flyer impact initiation technology, the detonation of explosive depends mainly on the impact stress of flyer and pulse width of impact stress. The impact stress of flyer depends on the velocity, structure, and material of flyer while the pulse width

**Table 1.** Typical laser-driven flyer targets researched.

of impact stress depends on the planarity and integrity of flyer. Therefore, it is necessary to select flyer with high velocity, strong impact stress, and good planarity and integrity to develop laser-driven flyer impact initiation technology.

#### *2.2.1. Velocity of flyer*

#### *<IP_ADDRESS>. Measurement of velocity*

The velocity of flyers can be measured via photonic Doppler velocimetry (PDV). **Figure 5** shows a schematic view of PDV. It is mainly based upon two physical processes: Doppler effect and optical mixing, that is, heterodyning.

**Figure 6** shows schematic view of the measuring setup of flyer velocity. Velocity information of flyer is collected and recorded by PDV measurement system and time history of flyer is obtained by computer program analysis.

of Al/Al2

Al2 O3 O3

*2.2.2. Impact stress of flyer*

impact stress of flyer.

*<IP_ADDRESS>. Measurement of impact stress*

absorption layer increases the velocity of C/Al/Al2

**Figure 7.** Final velocities of four flyers in different laser energy.

**Figure 6.** Schematic of measuring setup of flyer velocity.

/Al multilayer flyer is lower than that of Al/Al2

/Al multilayer flyer lower than that of Al single-layer flyer. The addition of C

O3

/Al multilayer flyer at low laser energy.

Laser Ablation of Energetic Materials http://dx.doi.org/10.5772/intechopen.71892 265

/Al multilayer flyer.

O3

However, C layer is easy to be broken down at high laser energy, which reduces the restraining effect of substrate and accelerating chamber to plasma. As a result, the velocity of C/Al/

The impact stress can be measured by a polyvinylidene fluoride (PVDF) pressure sensor (shown in **Figure 8**), and the current mode of PVDF pressure sensor is used to measure the

#### *<IP_ADDRESS>. Regularity of velocity*

According to the research of Wu [15] in Nanjing University of Science and Technology, four types of flyers including Al/Al2 O3 /Al multilayer, C/Al/Al2 O3 /Al multilayer, Mg/Al/Al2 O3 /Al multilayer and Al single-layer were tested with the laser energy of 36.4–195.1 mJ by PDV measurement system. The result is indicated in **Figure 7**.

According to **Figure 7**, the analysis to result are as follows. At the same laser energy, Mg/Al/ Al2 O3 /Al multilayer flyer owns the maximum velocity, which is little bit better than Al singlelayer flyer. The velocity of C/Al/Al2 O3 /Al multilayer flyer is higher than that of Al/Al2 O3 /Al multilayer flyer when the laser energy is less than 150 mJ. Within the range of 150–195.1 mJ of laser energy, the velocity of C/Al/Al2 O3 /Al multilayer flyer is comparable to that of Al/Al2 O3 / Al multilayer flyer. As the ablation layer, Mg layer can greatly improve the velocity of flyer. The addition of Al2 O3 insulation layer increases the mass of flyer, which makes the velocity

**Figure 5.** Schematic of PDV.

**Figure 6.** Schematic of measuring setup of flyer velocity.

of Al/Al2 O3 /Al multilayer flyer lower than that of Al single-layer flyer. The addition of C absorption layer increases the velocity of C/Al/Al2 O3 /Al multilayer flyer at low laser energy. However, C layer is easy to be broken down at high laser energy, which reduces the restraining effect of substrate and accelerating chamber to plasma. As a result, the velocity of C/Al/ Al2 O3 /Al multilayer flyer is lower than that of Al/Al2 O3 /Al multilayer flyer.

#### *2.2.2. Impact stress of flyer*

#### *<IP_ADDRESS>. Measurement of impact stress*

The impact stress can be measured by a polyvinylidene fluoride (PVDF) pressure sensor (shown in **Figure 8**), and the current mode of PVDF pressure sensor is used to measure the impact stress of flyer.

**Figure 7.** Final velocities of four flyers in different laser energy.

**Figure 5.** Schematic of PDV.

of impact stress depends on the planarity and integrity of flyer. Therefore, it is necessary to select flyer with high velocity, strong impact stress, and good planarity and integrity to

The velocity of flyers can be measured via photonic Doppler velocimetry (PDV). **Figure 5** shows a schematic view of PDV. It is mainly based upon two physical processes: Doppler

**Figure 6** shows schematic view of the measuring setup of flyer velocity. Velocity information of flyer is collected and recorded by PDV measurement system and time history of flyer is

According to the research of Wu [15] in Nanjing University of Science and Technology, four

/Al multilayer, C/Al/Al2

multilayer and Al single-layer were tested with the laser energy of 36.4–195.1 mJ by PDV

According to **Figure 7**, the analysis to result are as follows. At the same laser energy, Mg/Al/

multilayer flyer when the laser energy is less than 150 mJ. Within the range of 150–195.1 mJ of

Al multilayer flyer. As the ablation layer, Mg layer can greatly improve the velocity of flyer.

/Al multilayer flyer owns the maximum velocity, which is little bit better than Al single-

O3

/Al multilayer flyer is higher than that of Al/Al2

insulation layer increases the mass of flyer, which makes the velocity

/Al multilayer flyer is comparable to that of Al/Al2

/Al multilayer, Mg/Al/Al2

O3 /Al

O3 /Al

> O3 /

O3

O3

O3

measurement system. The result is indicated in **Figure 7**.

develop laser-driven flyer impact initiation technology.

effect and optical mixing, that is, heterodyning.

obtained by computer program analysis.

*2.2.1. Velocity of flyer*

*<IP_ADDRESS>. Measurement of velocity*

264 Laser Ablation - From Fundamentals to Applications

*<IP_ADDRESS>. Regularity of velocity*

Al2 O3

types of flyers including Al/Al2

layer flyer. The velocity of C/Al/Al2

laser energy, the velocity of C/Al/Al2

O3

The addition of Al2

**Figure 8.** Structure of PVDF sensor.

#### *<IP_ADDRESS>. Regularity of impact stress*

**Figure 9(a)** shows the impact stress of six types of flyers at 175, 212, 248, 282, 315, and 362 mJ laser energy, respectively. With the increase of laser energy, the impact stresses of flyer tend to rise up gradually. At the same laser energy, the impact stress of Al single-layer flyer is lower than Al/ Al2 O3 /Al multilayer flyer. When the laser energy is not more than 248 mJ, the impact stress of CuO/ Al/Al2 O3 /Al multilayer flyer is comparable to that of CuO/Al2 O3 /Al multilayer flyer. As the energy continues to increase, the impact stress of former will exceed that of the latter. In most cases, the impact stress of C/Mg/Al/Al2 O3 /Al multilayer flyer is greater than C/Al/Al2 O3 /Al multilayer flyer.

In 1969, Walker and Wasley [17] proposed the criterion for impact initiation of heterogeneous explosive

$$E\_{\mathbb{C}} = P^2 \mathfrak{T} \tag{2}$$

Value of *P2*

the maximum value of *P2*

*2.3.1. Theory of initiating explosives*

*2.3.2. Measuring setup for impact initiation*

[15]. The results are shown in **Table 2**.

*2.3.3. Initiation of PETN*

*τ* for six types of flyers are shown in **Figure 9(b)**. The value of *P2*

Impact initiation of agglomerate explosive is a significant initiation method besides thermal initiation. Due to the press fitting of solid agglomerate explosive, voids are retained around the crystal grains with the degree of porosity of 1–4%. After the shock wave entering explosive, the air voids and bubbles are compressed adiabatically under impact. The specific heat of gas is lower than that of the explosive crystal. Therefore, the temperature of bubble is

The schematic view of measuring setup for impact initiation is shown in **Figure 10**. The laser focused by lens irradiates on flyer target and ablates a part of the film to form plasma. Immediately, the remaining part of the film is sheared along the edge of accelerating chamber and is driven out at high velocity to form a flyer. Then, the flyer is accelerated in accelerating

Five types of flyers including Al and Cu single-layer flyers and CuO/Cu, CuO/Al and CuO/ Al/Cu multilayer flyers with relatively high velocities were used to initiate PETN explosive

Compared to the results in **Table 2**, it is easy to find that Al single-layer flyer shows the best performance of initiation in the same laser energies, which succeeded in initiating as long as the laser energy was higher than 161 mJ. CuO/Cu multilayer flyer owned four successful detonations as well. Besides, the success rate of initiation for other flyers is arranged from high to low, followed by Cu single-layer flyer, CuO/Al and CuO/Al/Cu multilayer flyers. The results indicate

chamber and hits explosive cylinder causing the explosive to be struck and detonated.

*τ,* which means the best ability to initiate explosive.

flyer's ability to initiate explosive. It is obvious from **Figure 10** that *P2*

higher than that of the crystal, which will cause the appearance of hot spots.

higher than that of single-layer flyers. Especially, C/Mg/Al/Al2

**2.3. Characteristics of laser-driven flyer impact initiation**

**Figure 10.** Schematic view of measuring setup for impact initiation.

*τ* indicates the

*τ* of multilayer flyers is

/Al multilayer flyer obtains

Laser Ablation of Energetic Materials http://dx.doi.org/10.5772/intechopen.71892 267

O3

In Eq. (2), *P* is the shock wave pressure entering into explosive, *τ* is the time that shock wave travels back and forth in flyer, and *Ec* is a constant determined by the specific explosive. Only when the value of *P2 τ* is higher than that of *Ec* , the explosive can be initiated. Ignoring the area effect of impact initiation, the criterion for it can be written as

$$P^2 \tau \ge \mathcal{E}\_{\mathcal{C}} \tag{3}$$

**Figure 9.** Impact stresses (a) and value of *P2 τ* for flyers at different laser energy (b).

**Figure 10.** Schematic view of measuring setup for impact initiation.

Value of *P2 τ* for six types of flyers are shown in **Figure 9(b)**. The value of *P2 τ* indicates the flyer's ability to initiate explosive. It is obvious from **Figure 10** that *P2 τ* of multilayer flyers is higher than that of single-layer flyers. Especially, C/Mg/Al/Al2 O3 /Al multilayer flyer obtains the maximum value of *P2 τ,* which means the best ability to initiate explosive.

#### **2.3. Characteristics of laser-driven flyer impact initiation**

#### *2.3.1. Theory of initiating explosives*

*<IP_ADDRESS>. Regularity of impact stress*

266 Laser Ablation - From Fundamentals to Applications

**Figure 8.** Structure of PVDF sensor.

impact stress of C/Mg/Al/Al2

when the value of *P2*

travels back and forth in flyer, and *Ec*

**Figure 9.** Impact stresses (a) and value of *P2*

Al2 O3

Al/Al2 O3

explosive

**Figure 9(a)** shows the impact stress of six types of flyers at 175, 212, 248, 282, 315, and 362 mJ laser energy, respectively. With the increase of laser energy, the impact stresses of flyer tend to rise up gradually. At the same laser energy, the impact stress of Al single-layer flyer is lower than Al/

continues to increase, the impact stress of former will exceed that of the latter. In most cases, the

In 1969, Walker and Wasley [17] proposed the criterion for impact initiation of heterogeneous

*EC* = *P*2 *τ* (2)

In Eq. (2), *P* is the shock wave pressure entering into explosive, *τ* is the time that shock wave

*P*2 *τ* ≥ *EC* (3)

*τ* for flyers at different laser energy (b).

/Al multilayer flyer is comparable to that of CuO/Al2

*τ* is higher than that of *Ec*

effect of impact initiation, the criterion for it can be written as

O3

/Al multilayer flyer. When the laser energy is not more than 248 mJ, the impact stress of CuO/

/Al multilayer flyer is greater than C/Al/Al2

O3

/Al multilayer flyer. As the energy

/Al multilayer flyer.

O3

is a constant determined by the specific explosive. Only

, the explosive can be initiated. Ignoring the area

Impact initiation of agglomerate explosive is a significant initiation method besides thermal initiation. Due to the press fitting of solid agglomerate explosive, voids are retained around the crystal grains with the degree of porosity of 1–4%. After the shock wave entering explosive, the air voids and bubbles are compressed adiabatically under impact. The specific heat of gas is lower than that of the explosive crystal. Therefore, the temperature of bubble is higher than that of the crystal, which will cause the appearance of hot spots.

#### *2.3.2. Measuring setup for impact initiation*

The schematic view of measuring setup for impact initiation is shown in **Figure 10**. The laser focused by lens irradiates on flyer target and ablates a part of the film to form plasma. Immediately, the remaining part of the film is sheared along the edge of accelerating chamber and is driven out at high velocity to form a flyer. Then, the flyer is accelerated in accelerating chamber and hits explosive cylinder causing the explosive to be struck and detonated.

#### *2.3.3. Initiation of PETN*

Five types of flyers including Al and Cu single-layer flyers and CuO/Cu, CuO/Al and CuO/ Al/Cu multilayer flyers with relatively high velocities were used to initiate PETN explosive [15]. The results are shown in **Table 2**.

Compared to the results in **Table 2**, it is easy to find that Al single-layer flyer shows the best performance of initiation in the same laser energies, which succeeded in initiating as long as the laser energy was higher than 161 mJ. CuO/Cu multilayer flyer owned four successful detonations as well. Besides, the success rate of initiation for other flyers is arranged from high to low, followed by Cu single-layer flyer, CuO/Al and CuO/Al/Cu multilayer flyers. The results indicate

in chemical reactions, or undergo internal conversion back to the ground state of the starting molecule. A microscopic understanding of the complex physical and chemical processes involved in the combustion and decomposition of energetic material is essential for the development of predictive detonation and reliable model for performance, stability, and hazard analysis of explosives [19]. Many investigations have been conducted concerning the mechanism of decomposi-

In order to ensure the reliability and safety of laser initiation system, it is necessary to reveal the microscopic mechanism of the reaction of energetic materials under laser irradiation. Spectroscopic method is an effective means to reveal the mechanism of laser-induced dissociation of energetic materials [23]. In the present work, time of flight mass spectrometry (TOFMS) was employed to study the ion fragments produced by explosives after laser excitation with the purpose of understanding the effect of different experimental parameters on the

The explosive samples were pressed into an aluminum holder (6 mm diameter and 11 mm depth) using a pressing tool to ensure a flat surface and a consistent filling density with a pressure of 80 MPa. The apparatus used in this study was a homemade reflection time of flight mass spectrometer. The time of flight mass spectrometer consists of four parts: ion source, ion acceleration zone, flight area, reflector and detector. The detector is made from a pair of microchannel plates (MCPs) [24]. It was combined with a vacuum chamber, pumping down to 4 × 10−4 Pa with two turbomolecular pumps. The output from a Q-switched Nd:YAG laser (LOTIS TII, LS-2147, 450 mJ@532 nm/15 ns, 800 mJ@ 1064 nm/15 ns) was focused to the sample surface by a quartz focus lens with focus length of 500 mm) and spot size of about 0.8 mm2

The laser fluence was measured by an energy meter (Ophir, Model 30A). After ionization, the laser generated ion fragments were first driven by the initial kinetic energy into the acceleration zone, and then accelerated by a pulsed electric field, and finally detected by the MCPs [25]. The ions were mass analyzed by its flight time. In order to reduce the noise, the weak signal measurement technique with multiple signal superposition was adopted. The TOF signals were accumulated 128 times by a digital oscilloscope (Tektronix, DPO7140) to improve the signal-to-noise ratio. The trigger of each instrument was controlled by a pulse delay generator (University of Science and Technology of China, GH024). **Figure 11** shows the schematic

Typical negative and positive ion mass spectra of RDX obtained by 532 nm laser dissociation

with the highest intensity in the negative-ion spectrum are at m/z = 46 and corresponds to

C(N-NO2

fragment. Other relatively strong peaks appear at m/e = 26, 106, and 134 and can be

)2

for positive ions with delay time of 50 μs. For negative mode, the peak

.

for negative ions with delay time of

Laser Ablation of Energetic Materials http://dx.doi.org/10.5772/intechopen.71892 269

, respectively. Finally, some low intensity

tion of explosives under laser irradiation [20–22].

resultant mass spectra and laser-induced reaction.

diagram of the experimental setup [18].

assigned to CN, (CNC)C(NN)2, and H2

*3.2.1. RDX*

the NO2

50 μs and 5.2 J/cm2

**3.2. Laser-induced TOF mass spectroscopy of nitramines**

are shown in **Figure 12**. The laser energy was 4.8 J/cm2

**3.1. Detection of laser-induced decomposition products**

**Table 2.** Initiation results of PETN for different types of flyers.

that a certain range of laser energy is necessary in the process of laser-driven flyer impact initiation, which can ensure enough velocity and impact stress of flyer.

#### **2.4. Analysis of laser-driven flyer initiating explosives**

There are two main factors affecting the impact initiation of flyer including velocity and shape of flyer, which are indispensable. Al flyer shows the best performance in impact initiation while CuO/Al and CuO/Al/CuO multilayer flyers show the worst performance. It is analyzed that the oxidation reaction between Al and CuO will release a lot of energy to melt or vaporize some part of flyer and change the shape of flyer, which can reduce the impact effect of it and result in the decrease of flyer velocity. The velocity of Cu flyer is higher than Al's, but its impact initiation ability is not as good as Al flyer. This is because Cu flyer is more brittle than Al flyer and is more likely to be broken during flight, which will affect the initiation effect. A large number of experimental results show that a certain range of laser energy is needed to drive the flyer. Thus, the velocity and impact stress can be high enough to initiate the explosives successfully.

## **3. Laser-induced time of flight mass spectroscopy of typical energetic materials**

Laser-material interaction is essentially based on thermal and photonic processes, where as a consequence of laser irradiation, different phenomena occur on a microscopic or macroscopic scale [18]. It will populate an excited state that can either couple to a reaction pathway, resulting in chemical reactions, or undergo internal conversion back to the ground state of the starting molecule. A microscopic understanding of the complex physical and chemical processes involved in the combustion and decomposition of energetic material is essential for the development of predictive detonation and reliable model for performance, stability, and hazard analysis of explosives [19]. Many investigations have been conducted concerning the mechanism of decomposition of explosives under laser irradiation [20–22].

In order to ensure the reliability and safety of laser initiation system, it is necessary to reveal the microscopic mechanism of the reaction of energetic materials under laser irradiation. Spectroscopic method is an effective means to reveal the mechanism of laser-induced dissociation of energetic materials [23]. In the present work, time of flight mass spectrometry (TOFMS) was employed to study the ion fragments produced by explosives after laser excitation with the purpose of understanding the effect of different experimental parameters on the resultant mass spectra and laser-induced reaction.

#### **3.1. Detection of laser-induced decomposition products**

The explosive samples were pressed into an aluminum holder (6 mm diameter and 11 mm depth) using a pressing tool to ensure a flat surface and a consistent filling density with a pressure of 80 MPa. The apparatus used in this study was a homemade reflection time of flight mass spectrometer. The time of flight mass spectrometer consists of four parts: ion source, ion acceleration zone, flight area, reflector and detector. The detector is made from a pair of microchannel plates (MCPs) [24]. It was combined with a vacuum chamber, pumping down to 4 × 10−4 Pa with two turbomolecular pumps. The output from a Q-switched Nd:YAG laser (LOTIS TII, LS-2147, 450 mJ@532 nm/15 ns, 800 mJ@ 1064 nm/15 ns) was focused to the sample surface by a quartz focus lens with focus length of 500 mm) and spot size of about 0.8 mm2 . The laser fluence was measured by an energy meter (Ophir, Model 30A). After ionization, the laser generated ion fragments were first driven by the initial kinetic energy into the acceleration zone, and then accelerated by a pulsed electric field, and finally detected by the MCPs [25]. The ions were mass analyzed by its flight time. In order to reduce the noise, the weak signal measurement technique with multiple signal superposition was adopted. The TOF signals were accumulated 128 times by a digital oscilloscope (Tektronix, DPO7140) to improve the signal-to-noise ratio. The trigger of each instrument was controlled by a pulse delay generator (University of Science and Technology of China, GH024). **Figure 11** shows the schematic diagram of the experimental setup [18].

#### **3.2. Laser-induced TOF mass spectroscopy of nitramines**

#### *3.2.1. RDX*

that a certain range of laser energy is necessary in the process of laser-driven flyer impact initia-

**Structure of flyer Laser energy/mJ 67 109 161 203 264 336.7 375.7** Cu Flyer velocity/m·s−1 1965 2402 2597 3389 3389 4020 3065

Al Flyer velocity/m·s−1 3162 3433 4145 4324 4761 5194 5000

CuO/Cu Flyer velocity/m·s−1 2105 2500 2649 2909 3149 3252 3305

CuO/Al Flyer velocity/m·s−1 3478 3773 4597 4878 5031 4848 5673

CuO/Al/Cu Flyer velocity/m·s−1 2352 2507 2702 3100 3265 3791 3625

*τ*/×1010 Pa·s 0.2 0.2 1.3 6.3 10.4 6.3 3.0 Detonation × × × √ √ √ ×

*τ*/×1010 Pa·s 0.2 0.9 5.4 5.6 9.4 7.6 6.6 Detonation × × √ √ √ √ √

*τ*/×1010 Pa·s 2.1 1.5 2.6 5.8 6.8 5.4 7.1 Detonation × × × √ √ √ √

*τ*/×1010 Pa·s 0.6 2.4 3.2 3.7 11.0 8.7 4.9 Detonation × × × × √ √ ×

*τ*/×1010 Pa·s 0.9 1.1 1.9 3.3 5.1 2.0 2.0 Detonation × × × × √ × ×

There are two main factors affecting the impact initiation of flyer including velocity and shape of flyer, which are indispensable. Al flyer shows the best performance in impact initiation while CuO/Al and CuO/Al/CuO multilayer flyers show the worst performance. It is analyzed that the oxidation reaction between Al and CuO will release a lot of energy to melt or vaporize some part of flyer and change the shape of flyer, which can reduce the impact effect of it and result in the decrease of flyer velocity. The velocity of Cu flyer is higher than Al's, but its impact initiation ability is not as good as Al flyer. This is because Cu flyer is more brittle than Al flyer and is more likely to be broken during flight, which will affect the initiation effect. A large number of experimental results show that a certain range of laser energy is needed to drive the flyer. Thus, the velocity and impact stress can be high enough to initiate the explosives successfully.

**3. Laser-induced time of flight mass spectroscopy of typical energetic**

Laser-material interaction is essentially based on thermal and photonic processes, where as a consequence of laser irradiation, different phenomena occur on a microscopic or macroscopic scale [18]. It will populate an excited state that can either couple to a reaction pathway, resulting

tion, which can ensure enough velocity and impact stress of flyer.

**2.4. Analysis of laser-driven flyer initiating explosives**

**Table 2.** Initiation results of PETN for different types of flyers.

*P2*

268 Laser Ablation - From Fundamentals to Applications

*P2*

*P2*

*P2*

*P2*

**materials**

Typical negative and positive ion mass spectra of RDX obtained by 532 nm laser dissociation are shown in **Figure 12**. The laser energy was 4.8 J/cm2 for negative ions with delay time of 50 μs and 5.2 J/cm2 for positive ions with delay time of 50 μs. For negative mode, the peak with the highest intensity in the negative-ion spectrum are at m/z = 46 and corresponds to the NO2 fragment. Other relatively strong peaks appear at m/e = 26, 106, and 134 and can be assigned to CN, (CNC)C(NN)2, and H2 C(N-NO2 )2 , respectively. Finally, some low intensity

Typical negative and positive ion mass spectra of RDX obtained by 1064 nm laser dissociation

negative ion mass spectrum under 1064 nm is similar to that under 532 nm. The difference lies in the intensity of the major peak. From the negative ions mass spectrum, it can be found

Different with the results at 532 nm, two small new peaks produce at *m/z* = 98 and 100, which

) 3 N2

with low intensity can be observed at *m/z* = 42, 64, and 84 are most likely the CNO or (CH2

A series of peaks are observed at *m/z* = 40, 42, 43, 44, 46, and 47, and can be ascribed to CN2

O or CO2

The thermal decomposition results of the isotopically labeled RDX confirmed that the formula

most likely N-methylformamide. Some small peaks can be observed at *m/z* = 57, 58, 59, and 60,

or O, H3

Typical negative and positive ion mass spectra of HMX obtained by 532 nm laser dissociation

**Figure 13.** Typical TOF mass spectra of negative ions (left) and positive ions (right) for RDX produced by 1064 nm laser

NNO2

, (CH2

with the highest intensity can be found at *m/z* = 30, corresponding to the NO or CH2

that the highest peak with the appears at *m/z* = 106 and corresponds to CH2

(NN) and (CH2

H2 , NO2

The second strong peak appears at *m/z* = 18, and can be ascribed to H2

N, NCOH, N2

topically labeled results also showed that *m/z* = 59 was corresponding to CH3

, and (CH)2

for positive ions with delay time of 70 μs. The position of the peaks of

fragments, respectively. In the positive ions mass spectrum, the peak

NO, NO2

NCHO and which might be formamide. Likewise, the iso-

)NNO2, and H2

or CH2

) or N(COH)N, CNO2

O, C2 , C2 H3

for positive ions with delay time of 70 μs. It can be seen from the negative

, respectively. Some other peaks can be found at *m/z* = 16, 19, 24, 27,

, respectively [18, 25].

for negative ions with delay time of

(NO). The relative strong peaks at *m/z* = 26,

) 2

O or NH4

or (COH)NH(CH2

for negative ions with delay time of

, or CHN, N2

C(N-NO2

(NO2 ) 2

Laser Ablation of Energetic Materials http://dx.doi.org/10.5772/intechopen.71892

. Moreover, the peaks

and HONO, respectively.

fragment.

271

) 2 N

or

O species.

, respectively.

NHCHO and was

or CO or C2

), CHNO2

H4 or

are shown in **Figure 13**. The laser energy was 34.9 J/cm2

) 3 N2

> ) 2

N)3

)N or (CH2

these ions can be assigned to (CO)NH(CH2

28, 74, 79, and 86, which may be from CH4

, (HC)N(CN)2

are shown in **Figure 14**. The laser energy was 8.6 J/cm2

for the ion signal at *m/z* = 45 was H2

, or NNO2

80 μs and 49.4 J/cm2

can be assigned to (CH2

N, N(CH2

NO2

NNO2

N, CH2

75 μs and 7.8 J/cm2

*3.2.2. HMX*

ablation.

or CH2 N2

C2 H2

and CH2

CH2

46, 88 and 134 may be CN or C2

and (CH2

**Figure 11.** Schematic diagram of the TOFMS system.

peaks are observed at m/z = 42,60,84,88,93,130, and 176, possible assignments for major peaks are N(CH2 )N, N-NO2 , (H2 CN)3 ,(CH2 )2 N–NO2 , (CH)(NCNC)(NN), (CH2 )3 N(N-NO2 )N and (CH2 )3 (N-NO2 )2 N, respectively. For positive mode, positive ions have three series, m/z = 17 can be attributed to OH. m/z = 26, 27, 28, 29, 30, and 32 ions can be classified as CN, HCN, CO/N2 /H2 CN, HCO, NO, and O2 , respectively. The third series with m/z of 42, 44, and 47, and their corresponding attributions are C2 H4N/ CH2 N2 /N (H2 C) N, N2 O/CH2 NO, and HONO, respectively [18, 26].

**Figure 12.** Typical TOF mass spectra of negative ions (left) and positive ions (right) for RDX produced by 532 nm laser ablation.

Typical negative and positive ion mass spectra of RDX obtained by 1064 nm laser dissociation are shown in **Figure 13**. The laser energy was 34.9 J/cm2 for negative ions with delay time of 80 μs and 49.4 J/cm2 for positive ions with delay time of 70 μs. The position of the peaks of negative ion mass spectrum under 1064 nm is similar to that under 532 nm. The difference lies in the intensity of the major peak. From the negative ions mass spectrum, it can be found that the highest peak with the appears at *m/z* = 106 and corresponds to CH2 (NO2 ) 2 fragment. Different with the results at 532 nm, two small new peaks produce at *m/z* = 98 and 100, which can be assigned to (CH2 ) 3 N2 (NN) and (CH2 ) 3 N2 (NO). The relative strong peaks at *m/z* = 26, 46, 88 and 134 may be CN or C2 H2 , NO2 , (CH2 )NNO2, and H2 C(N-NO2 ) 2 . Moreover, the peaks with low intensity can be observed at *m/z* = 42, 64, and 84 are most likely the CNO or (CH2 ) 2 N or CH2 N2 and (CH2 N)3 fragments, respectively. In the positive ions mass spectrum, the peak with the highest intensity can be found at *m/z* = 30, corresponding to the NO or CH2 O species. The second strong peak appears at *m/z* = 18, and can be ascribed to H2 O or NH4 , respectively. A series of peaks are observed at *m/z* = 40, 42, 43, 44, 46, and 47, and can be ascribed to CN2 or C2 H2 N, N(CH2 )N or (CH2 ) 2 N, NCOH, N2 O or CO2 or CH2 NO, NO2 and HONO, respectively. The thermal decomposition results of the isotopically labeled RDX confirmed that the formula for the ion signal at *m/z* = 45 was H2 NCHO and which might be formamide. Likewise, the isotopically labeled results also showed that *m/z* = 59 was corresponding to CH3 NHCHO and was most likely N-methylformamide. Some small peaks can be observed at *m/z* = 57, 58, 59, and 60, these ions can be assigned to (CO)NH(CH2 ) or N(COH)N, CNO2 or (COH)NH(CH2 ), CHNO2 and CH2 NO2 , or NNO2 , respectively. Some other peaks can be found at *m/z* = 16, 19, 24, 27, 28, 74, 79, and 86, which may be from CH4 or O, H3 O, C2 , C2 H3 , or CHN, N2 or CO or C2 H4 or CH2 N, CH2 NNO2 , (HC)N(CN)2 , and (CH)2 NNO2 , respectively [18, 25].

#### *3.2.2. HMX*

peaks are observed at m/z = 42,60,84,88,93,130, and 176, possible assignments for major peaks

can be attributed to OH. m/z = 26, 27, 28, 29, 30, and 32 ions can be classified as CN, HCN,

**Figure 12.** Typical TOF mass spectra of negative ions (left) and positive ions (right) for RDX produced by 532 nm laser

H4N/ CH2

, (CH)(NCNC)(NN), (CH2

, respectively. The third series with m/z of 42, 44, and 47, and

C) N, N2

N, respectively. For positive mode, positive ions have three series, m/z = 17

N2 /N (H2 )3

O/CH2

N(N-NO2

NO, and HONO,

)N and

are N(CH2

/H2

respectively [18, 26].

(CH2 )3 (N-NO2 )2

CO/N2

ablation.

)N, N-NO2

, (H2

CN, HCO, NO, and O2

**Figure 11.** Schematic diagram of the TOFMS system.

270 Laser Ablation - From Fundamentals to Applications

their corresponding attributions are C2

CN)3

,(CH2 )2 N–NO2

> Typical negative and positive ion mass spectra of HMX obtained by 532 nm laser dissociation are shown in **Figure 14**. The laser energy was 8.6 J/cm2 for negative ions with delay time of 75 μs and 7.8 J/cm2 for positive ions with delay time of 70 μs. It can be seen from the negative

**Figure 13.** Typical TOF mass spectra of negative ions (left) and positive ions (right) for RDX produced by 1064 nm laser ablation.

Typical negative and positive ion mass spectra of HMX obtained by 1064 nm laser dissocia-

ion peaks of HMX at 1064 nm is similar to that of 532 nm laser, and all of them have more

generated at *m/z* of 184. The distribution of positive ions is almost the same as that of 532 nm. However, there are differences in the intensity for the ions, among which the relative intensity

tion of ions indicates that the decomposition mechanism of HMX under the irradiation of two

Typical negative and positive ion mass spectra of TNT obtained by 532 nm laser dissociation

gest peak appeared at *m/z* = 26, which might correspond to CN. Two relatively strong peaks

of small peaks were observed at *m/z* = 50, 66, 77, 137, 152, 167, 197, 212, 225, and 238, these

. The mass charge ratio of C6

larger than TNT (*m/z* = 227), it can be conjectured that TNT may get some atoms to form larger fragments. For the positive ions, the dominant peaks were appeared at *m/z* = 18, 19, 24,

**Figure 16.** Typical TOF mass spectra of negative ions (left) and positive ions (right) for TNT produced by 532 nm laser

, CH3

(NO2 )2

tive intensity of *m/z* of 60 and 46 increased significantly and a new ion peak CNN(CNNO2

for positive ions with delay time of 70 μs. The distribution of negative

N2 /C2 H2

), and 134 (CH2

for positive ions with delay time of 70 μs. For the negative ions, the stron-

CHCOCHCNO2

N2 /C2 H2

O, H3

CCNO2

H2

O, C2

for negative ions with delay time

Laser Ablation of Energetic Materials http://dx.doi.org/10.5772/intechopen.71892

for negative ions with delay time of

O/CNO and NO2

)3

, C6 H2 (NO2 )3 , C6 H2 CH

, HCN, CO/N2

CHCH(NO2

), 60(CH2

), except that the rela-

NO2 / 273

) 2 is

. A series

N, NO,

CHCOCHCOCCO,

/CH2

(*m/z* = 238) is

O/CNO), 46(NO2

(NNO2 ) 2

N) reduced significantly. The similarity of the distribu-

tion are shown in **Figure 15**. The laser energy was 45.0 J/cm2

obvious ion peaks at *m/z* of 17(OH), 26(CN), 42 (CH2

), 106 (N(NO2

are shown in **Figure 16**. The laser energy was 24.6 J/cm2

appeared at *m/z* = 42 and 46, which might be due to CH2

(CHCO)2

27, 28, 30, and 40, these ions could be correspond to H2

)3

CHCH(NO2

)2 /CH2

**3.3. Laser-induced TOF mass spectroscopy of aromatic nitro compounds**

ions could be ascribed to CHCCHC, CCHCOCH, CHCHCHCHCHC, CH3

CCNO2

/C2 H2

of 80 μs and 41.4 J/cm2

), 88((CH2

)2 NNO2

), 27(HCN), and 40(CN2

kinds of laser wavelengths is similar [18, 24].

N2 O2

of 24 (C2

*3.3.1. TNT*

(CHCO)2

(NO2 )3

ablation.

70 μs and 25.8 J/cm2

CCNO2

, and C6

, CH3

H2

**Figure 14.** Typical TOF mass spectra of negative ions (left) and positive ions (right) for HMX produced by 532 nm laser ablation.

ion spectrum that the ion distribution of HMX has some similarity with that of RDX. HMX has three strong ion peaks with *m/z* of 26, 46, and 106. According to the molecular structure and decomposition properties of HMX, it can be deduced that these three peaks are CN2 and NO2 (NO2 ) 2 /N(NO2 ) 2 , respectively. There are some relatively weak peaks at *m/z* of 17, 42, 60, 88, 134, and 188, which correspond to OH, CH2 N2 /C2 H2 O/CNO, CH2 NO2 /N2 O2 , (CH2 )2 NNO2 , CH2 (NNO2 )2 , and CNN(CH2 NNO2 )2 , respectively. The distribution of positive ions is obviously different from that of negative ions, which is mainly distributed in three regions: *m/z* of 18 and 19 were in the first region, which may be due to H2 O and H3 O, respectively. The *m/z* of the second distribution region is 24–32, the possible attributions are C2 (*m/z* = 24), C2 H(*m/z* = 25), CN(*m/z* = 26), HCN(*m/z* = 27), CO/N2 /H2 CN (*m/z* = 28), HCO (*m/z* = 29), NO/CH2 O (*m/z* = 30), and O2 (*m/z* = 32). The *m/z* of the third distribution region is 40–47, which can be assigned to CN2 /C2 H2 N (*m/z* = 40), CH2 N2 /C2 H2 O/CNO (*m/z* = 42), N2 O/CO2 /CH2 NO(*m/z* = 44), HN2 O (*m/z* = 45), NO2 (*m/z* = 46), and HONO (*m/z* = 47), respectively [18].

**Figure 15.** Typical TOF mass spectra of negative ions (left) and positive ions (right) for HMX produced by 1064 nm laser ablation.