**Meet the editor**

Dr. Muhammad Akhyar Farrukh is an associate professor and founding director of Nano-Chemistry Lab. at GC University Lahore, Pakistan. He was selected by UNE-SCO and participated as a "Representative of Pakistan" in founding WAYS in Morocco. He was awarded "Young Chemist Award" by IUPAC in Italy in 2007 and selected as "Young Scientist" by TWAS in Egypt and IAP in Germany

in 2010. He was selected as "Young Scientist" by IAP/World Economic Forum in China in 2010. He was selected as "Young Researcher" in 2013 by the Council for Lindau Nobel Laureate Meetings. He was awarded IUPAC-2015 *Award for Chemists* as an outstanding chemist from developing countries. He has *published* over *90 papers* in international/national reputed journals, published/edited *20 books*, issued/filed *07 US patents*, presented *44 papers* in international/national conferences, and given *24 plenary/invited lectures* in *24 countries up to 2016*.

## Contents

Chapter 7 **Multifunctional Polymer Nanocomposites Based on Thermoplastic Polyesters 123** Sandra Paszkiewicz

Chapter 8 **Nanocomposites of Carbon Nanotubes and Semiconductor Nanocrystals as Advanced Functional Material with Novel Optoelectronic Properties 145** Rima Paul and Apurba Krishna Mitra

## Preface

Chapter 8 **Nanocomposites of Carbon Nanotubes and Semiconductor**

**Optoelectronic Properties 145** Rima Paul and Apurba Krishna Mitra

**VI** Contents

**Nanocrystals as Advanced Functional Material with Novel**

Nano means "small," it is equal to one billionth of a meter, and generally the nanometer scale runs from 1 to 100 nm while discussing the applications of nanomaterials. Nanomateri‐ als have very high potency and emerge with large applications piercing through all the dis‐ cipline of knowledge, leading to industrial and technological growth. Nanotechnology is expected to have an impact on nearly every industry including food, medical devices, chem‐ ical coatings, personal health, catalysis, sensors, water purification, computer games, solar cells, water splitting, etc.

Nanotechnology is a multidisciplinary science that has its roots in fields such as colloidal science, device physics, and biomedical and supramolecular chemistry. The scope of the top‐ ic is quite large; however, the objective of this book is to cover maximum main areas focus‐ ing on synthesis techniques, characterization of nanomaterials with various techniques like thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray powder dif‐ fraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared spectro‐ scopy (FTIR), etc., and their multiple applications.

This book is divided into two sections with "Non-carbon Compounds" and "Carbon Com‐ pounds." The synthesis, characterization, and applications of metal, metal oxides, and metal hydroxide nanoparticles and composites are covered in the section "Noncarbon Com‐ pounds," while the section "Carbon Compounds" focuses on the carbon nanotubes, graph‐ ite oxide, graphene oxide, etc.

I would like to thank the Publishing Process Manager Ms. Andrea Koric for her cooperation throughout the process of publication of this book.

> **Dr. Muhammad Akhyar Farrukh** Nano-Chemistry Laboratory GC University Lahore Pakistan

## **Non-Carbon Compounds**

#### **Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2 Nanoparticles for Automotive Applications Provisional chapter Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2 Nanoparticles for Automotive Applications**

S. Sivasankaran and Abdulaziz S. Alaboodi S. Sivasankaran and Abdulaziz S. Alaboodi

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

http://dx.doi.org/10.5772/65947

#### **Abstract**

The main aims of the present chapter are to: learn synthesis procedure of AA 6061‐x wt.% TiO2 nanocomposites (x = 0, 2, 4, 6, 8, 10 and 12 wt.%) by mechanical alloying (MA); inves‐ tigate structural characterization of manufactured nanocomposite powders using X‐ray line profile analysis, scanning electron microscope (SEM) and transmission electron microscope (TEM); examine consolidation method and mechanical behavior in terms of sintered density, Vickers hardness and compressive stress‐strain behavior; study the improvement of ductility in nanocomposites; and simulate the mechanical behavior using ANSYS. Here, the synthesized nanocomposites via MA were consolidated using conventional uniaxial die compaction; then, the green compacts were sintered at differ‐ ent temperatures. TEM microstructures of as‐milled powder samples showed the matrix crystallite sizes ranging from 45 to 75 nm, which depended on the amount of reinforce‐ ment. A remarkable decrease in matrix powder particles size with the function of rein‐ forcement was observed due to the ceramic nano TiO2 particles acted as milling agent. The sintered nanocomposites yielded maximum strength of 1.126 GPa. The study of tri‐ modeled composite and its mechanical behavior revealed the possibility of achieving improvements in ductility and toughness for nanocomposites. The simulated mechanical behavior results using finite element method were good agreement with experimental results.

**Keywords:** mechanical alloying, nanocrystallite matrix, nanoparticles, characterization, mechanical behavior

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, 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,

#### **1. Introduction**

Production of suitable materials to make our lives as comfortable, scientific development is playing a major role nowadays. Materials science and engineering is establishing the appro‐ priate relationship between atomic structure and physical properties of materials which would design a novel material to attain improved properties [1]. The best performances of most of the components can be achieved in the present scenario by nanocomposites. The combined properties of metallic and ceramic materials compared with monolithic alloys can be achieved by metal matrix composites (MMCs) as it possesses high tensile strength, fatigue resistance, wear resistance and high elastic modulus. Several methods, namely, powder metallurgy (P/M), stir casting, pressure infiltration etc., have been developed for manufacturing of MMCs. In aerospace, space, automotive and structural parts, particulate lightweight aluminum‐based nanocrystallite matrix reinforced with nanoparticles have outstanding properties [2] as it has an excellent combination of mechanical and physical properties [3]. Amid all Al alloys, Al 6061 possesses outstanding formability besides its high strength and excellent corrosion resistance [4, 5]. The superior performance of nanocompos‐ ites can be obtained by uniform, homogeneous and embedding of nanoparticles over the nanocrystallite matrix [6]. Agglomeration/clustering of very fine/nanoreinforcement par‐ ticles over metal matrix would deteriorates the mechanical properties [7]. However, it can be avoided by mechanical alloying (MA) which is one of the solid‐state powder metallurgy (P/M) process [2]. Further, uniform distribution and embedding of nanoreinforcement over the matrix can be achieved which would improve the mechanical performances of materials. The mechanical alloying is one of the severe plastic deformation (SPD) technique. During mechanical alloying (MA), high strain is given on the material and the structural refinement such as crystallite size reduction, lattice strain increment, changes in dislocation density, lattice parameter variation etc., would occur by shear and fracture of powder phase mix‐ tures. By this process, nanostructured/nanocrystallite materials can be manufactured [2]. Conventional cold uniaxial die compaction process is one of simple and economic one of consolidation of powder materials into bulk‐shaped products followed by sintering under controlled atmosphere. Various authors have worked on P/M aluminum‐based metal matrix composites reinforced by ceramic particles such as graphite, silicon carbide particulate (SiCp), aluminum oxide (Al2 O3 ), titanium carbide (TiC), vanadium carbide (VC), aluminum nitride (AlN), boron carbide (B4 C), silicon nitride (Si3 N4 ), titanium boride (TiB2 ), aluminum boride (AlB2 ), zirconium carbide (ZrC) and magnesium boride (MgB2 ) which have been suc‐ cessfully dispersed and investigated via mechanical alloying/mechanical milling. There is no detailed study concerning TiO2 powders used as a reinforcement in Al‐based MMCs. Titania (TiO2 ) has outstanding properties, namely, good wear and corrosion resistance, low coefficient of thermal expansion, high thermal shock resistance, excellent catalytic perfor‐ mance and thermal stability at high temperatures [8, 9]. The material selected as AA 6061‐ TiO2 particulate MMCs is mainly proposed for automotive and aircraft parts. For instance, it can be suitable for automotive engine pistons and connecting rod, bicycle hub, bike frames, valves and valve parts, brake pistons, hydraulic pistons, marine fittings, electrical fittings, contactors, aircraft fittings and couplings.

#### **2. Synthesis and consolidation procedure of nanocomposites**

#### **2.1. Nanocomposite powders preparation**

**1. Introduction**

4 Functionalized Nanomaterials

(SiCp), aluminum oxide (Al2

boride (AlB2

Titania (TiO2

TiO2

nitride (AlN), boron carbide (B4

no detailed study concerning TiO2

contactors, aircraft fittings and couplings.

O3

C), silicon nitride (Si3

cessfully dispersed and investigated via mechanical alloying/mechanical milling. There is

coefficient of thermal expansion, high thermal shock resistance, excellent catalytic perfor‐ mance and thermal stability at high temperatures [8, 9]. The material selected as AA 6061‐

 particulate MMCs is mainly proposed for automotive and aircraft parts. For instance, it can be suitable for automotive engine pistons and connecting rod, bicycle hub, bike frames, valves and valve parts, brake pistons, hydraulic pistons, marine fittings, electrical fittings,

), zirconium carbide (ZrC) and magnesium boride (MgB2

), titanium carbide (TiC), vanadium carbide (VC), aluminum

powders used as a reinforcement in Al‐based MMCs.

), titanium boride (TiB2

), aluminum

) which have been suc‐

N4

) has outstanding properties, namely, good wear and corrosion resistance, low

Production of suitable materials to make our lives as comfortable, scientific development is playing a major role nowadays. Materials science and engineering is establishing the appro‐ priate relationship between atomic structure and physical properties of materials which would design a novel material to attain improved properties [1]. The best performances of most of the components can be achieved in the present scenario by nanocomposites. The combined properties of metallic and ceramic materials compared with monolithic alloys can be achieved by metal matrix composites (MMCs) as it possesses high tensile strength, fatigue resistance, wear resistance and high elastic modulus. Several methods, namely, powder metallurgy (P/M), stir casting, pressure infiltration etc., have been developed for manufacturing of MMCs. In aerospace, space, automotive and structural parts, particulate lightweight aluminum‐based nanocrystallite matrix reinforced with nanoparticles have outstanding properties [2] as it has an excellent combination of mechanical and physical properties [3]. Amid all Al alloys, Al 6061 possesses outstanding formability besides its high strength and excellent corrosion resistance [4, 5]. The superior performance of nanocompos‐ ites can be obtained by uniform, homogeneous and embedding of nanoparticles over the nanocrystallite matrix [6]. Agglomeration/clustering of very fine/nanoreinforcement par‐ ticles over metal matrix would deteriorates the mechanical properties [7]. However, it can be avoided by mechanical alloying (MA) which is one of the solid‐state powder metallurgy (P/M) process [2]. Further, uniform distribution and embedding of nanoreinforcement over the matrix can be achieved which would improve the mechanical performances of materials. The mechanical alloying is one of the severe plastic deformation (SPD) technique. During mechanical alloying (MA), high strain is given on the material and the structural refinement such as crystallite size reduction, lattice strain increment, changes in dislocation density, lattice parameter variation etc., would occur by shear and fracture of powder phase mix‐ tures. By this process, nanostructured/nanocrystallite materials can be manufactured [2]. Conventional cold uniaxial die compaction process is one of simple and economic one of consolidation of powder materials into bulk‐shaped products followed by sintering under controlled atmosphere. Various authors have worked on P/M aluminum‐based metal matrix composites reinforced by ceramic particles such as graphite, silicon carbide particulate

In the present investigation, the AA 6061 Al alloy reinforced with different weight percentage of TiO2 particulate nanocomposite powders was prepared by MA (high‐energy ball milling) method. The AA 6061 Al alloy matrix composition and powder size of each elemental pow‐ der used in this study are given in **Table 1**. **Figure 1(a)** and **(c)** shows the morphologies of as‐received Al and TiO2 powder particles, respectively, using secondary electron image (SEI) of scanning electron micrograph (SE‐SEM). From **Figure 1(a)** and **(c)**, it can be observed that the Al matrix powder particles were in irregular flake‐like shape and TiO2 particles were in clustered tetragonal shape. **Figure 1(b)** and **(d)** shows the X‐ray diffraction patterns of as‐ received Al and TiO2 particles, respectively, which indicated the well‐crystalline nature of the powders. The as‐received TiO2 particles are in anatase formed and tetragonal structure with average clustered particles size of <1 μm which was measured by laser scattering system tech‐ nique. The prepared nanocomposites contained 0, 2, 4, 6, 8, 10 and 12 wt.% of TiO2 powder particles were completely embedded in the soft alloy matrix using the planetary type high‐ energy ball mill (Insmart systems Ltd., Hyderabad, India) (**Figure 2a**). Nine highly hardened stainless steel balls of 20 mm diameter (33.5 g mass of each ball, totally, 301.5 g) together with 30 g mass of AA 6061100‐x–x wt.% TiO2 (x=0–12 with a step of 2 wt.%) powder mixture under toluene medium as a process control agent (PCA) were poured in a hardened stainless steel vial. The ball‐to‐powder ratio (BPR) was set approximately 10:1. The plate (i.e., sun wheel) speed of mill was set to 100 rpm (bowl/vial speed, 280 rpm). The milling was carried out in wet medium using toluene (C6 H5 CH3 ) of sulfur free (Ranbaxy, India) to prevent undue oxida‐ tion, agglomeration of powders, balls and vial with the powder [10]. The composite powders containing the different wt.% of TiO2 were milled up to 40 h. The milling was carried out up to 20 min and then alternated with 20 min of cooling to avoid significant temperature rise. The

**Table 1.** Chemical composition and powder size used to make AA 6061 Al alloy matrix.

**Figure 1.** The morphology of as‐received powders: (a) Al and (c) TiO2 , XRD patterns of as‐received powders: (b) Al and (d) TiO2 .

**Figure 2.** (a) High‐energy planetary ball mill and (b) schematic diagram showing the synthesis of nanocomposite powders.

schematic diagram of synthesizing the nanocomposite by high‐energy ball milling method is also shown in **Figure 2b**.

#### **2.2. Synthesis of trimodeled composite powders**

In order to evaluate the improvement of ductility while maintaining high strength and tough‐ ness for nanostructured AA 6061‐TiO2 composite materials, trimodeled AA 6061‐TiO2 nano‐ composite powders (three phases consisting of nanocrystallite matrix, nano ceramic and coarse crystallite matrix) were synthesized by the same planetary high‐energy ball milling. For instance, first, the nanostructured composite powder of AA 6061‐12 wt.% TiO2 was syn‐ thesized by 40 h of MA using the same ball milling parameters mentioned in Section 2.1 (con‐ sists of nanocrystallite matrix and nano‐TiO2 ceramic). Then, these nanocomposite powders were mechanically blended with 0, 5, 10, 15, 20, 25 and 30 wt.% coarse grains (CG) elemental powders corresponding to AA 6061 alloy matrix composition in the same planetary ball mill. Here, the BPR of 1:1 at 120 rpm for 2 h was carried out.

#### **2.3. Consolidation of composite powders**

A hydraulic press (**Figure 3a**) capacity of 40 tons made by Insmart systems, Hyderabad, India, was used to do uniaxial compaction. The cold uniaxial compaction is an important process in P/M to attain near net shape fabrication of engineering components prior to sintering [11, 12]. First, the precursor powders obtained from high‐energy ball milling were cold compacted using conventional cold uniaxial pressing in a rigid double end compaction type with com‐ paction pressure of 500 MPa [13, 14]. The obtained dimensions of compacted samples called green pellets were 10 mm in diameter and 6–7 mm thickness [15]. The die was made of high‐ carbon high‐chromium hardened and tempered steel (inner and outer diameters of 10 and 60 mm), and the two punches (10 mm diameter) were made of the same material as die to consolidate the powders from top to bottom simultaneously. Zinc stearate [Zn (C18H35O2 )2 ] was used as lubricant to minimize the friction between powder particles and die wall during compaction process. **Figure 3b** shows the schematic design of the double end compaction die.

For investigating the sintering behavior, the consolidated nanocomposites of AA 6061100‐x–x wt.% TiO2 (x=0, 4, 8 and 12 wt.%) of the green pellets compacted at 500 MPa were degassed first at 350°C for 60 min [16]. Then, the degassed samples were sintered for 120 min in the temperature range of 400, 475, 550 and 625°C under reducing atmosphere [17] in a mechanical pusher furnace as applicable to P/M industries. The schematic diagram of mechanical pusher furnace is shown in **Figure 4**.

#### **2.4. Consolidation of trimodeled composite powders**

**Figure 2.** (a) High‐energy planetary ball mill and (b) schematic diagram showing the synthesis of nanocomposite powders.

, XRD patterns of as‐received powders: (b) Al and

**Figure 1.** The morphology of as‐received powders: (a) Al and (c) TiO2

(d) TiO2 .

6 Functionalized Nanomaterials

The synthesized trimodeled AA 6061‐12 TiO2 nanocomposite consisting of 0, 5, 10, 15, 20, 25 and 30 wt.% CG 6061 alloy (obtained from elemental powders by mixing) matrix powders was first dried and stress recovered at 343 K under N2 atmosphere (mass flow rate of 63 /h and dew point temperature of <‐40°C) [17]. The stress recovering of trimodeled composite powders was carried out to avoid the unwanted oxide phase at elevated temperature during sintering. These stress recovered powders were then consolidated by cold uniaxial compaction at 500 MPa. The post‐compacts were degassed and then sintered at 823 K for 6 h under N2 atmosphere [17].

**Figure 3.** (a) Hydraulic press and (b) schematic diagram of conventional cold uniaxial compaction die process (double end compaction type).

**Figure 4.** Schematic diagram of mechanical pusher furnace.

#### **3. Structural characterization methods**

#### **3.1. Geometric characterization techniques**

The transmission electron microscope (TEM) and X‐ray diffraction (XRD) can be effec‐ tively used for investigating the structural characterization of fine powders, polycrystalline materials, amorphous materials and thin films. These two techniques are the commanding qualitative characterization techniques used in various areas of materials science [18, 19]. The structural characterization in terms of crystallites size, lattice strain, crystal structure, crystallinity, composition and strain variance can be determined/investigated using these instruments [20]. However, the morphology of powders such as powder shape, size etc., and detailed microstructure of compacted and sintered samples can be studied qualitatively using scanning electron microscope (SEM).

#### **3.2. X‐ray diffraction (XRD)**

The XRD patterns are used to identify the structure and the different phases present in the materials. The phase constitution of milled powders, post‐compacts, post‐sintered and post‐ deformed preforms are evaluated by XRD using CuKα radiation. The used XRD equipment is made by D/MAX ULTIMA III, Rigaku Corporation, Japan, operating at 30 mA and 40 kV. The set 2°/min scanning speed and 20–100° scanning range were used here. It is well known that the average crystallite size and lattice strain or microstrain or lattice distortions owing to the presence of lattice defects which commonly occurred in MA products can be easily measured by XRD peaks of line profile analysis [20]. During MA with respect to milling time and per‐ centage of reinforcements in the matrix, XRD peak broadening occurs which depends upon crystallite size and strain present in the nanocomposites/nanomaterials. Therefore, nanostruc‐ tured materials/nanocomposites have a large volume of grain boundaries when compared to bulk materials [21]. If a material is not having any strain mean, Scherer's formula can be used to calculate crystallite size (Simplest method) [22]. Williamson–Hall (W–H) [23] and Warren–Averbach (W–A) [24] analyses are other two analytical methods which are gener‐ ally used to measure the strain and the crystallite size of nanomaterials/nanocomposites. The W‐A method is based on Fourier analysis of peak intensities in which Fourier coefficients are used to separate the crystallite size and strain contribution from XRD peak profiles. However, this approach needs peak intensities at high diffraction angles. But, it is difficult to use this method when higher angle reflections are weak [25]. On the other hand, the W‐H method is used to investigate the same information at lower diffraction angle. Also, it is easy and suit‐ able for lattice strain and crystallite size calculation of cubic crystals. Therefore, reduction in crystallite size and rise in microstrain can be found out using W‐H analysis of Eq. (1).

$$
\beta\_{hl}\cos\Theta\_{hll} = \left(\frac{\text{K}\lambda}{t}\right) + 4\varepsilon\sin\Theta\_{hll}.\tag{1}
$$

Here, K is the shape factor of 0.9, λ is the X‐ray wavelength of 1.5406 Å, θhkl is the Bragg angle in degree and t is the effective crystallite size in nm and ε is the lattice strain. The X‐ray line profile breadth, *β*hkl, after corrected instrumental broadening as a full width at half‐maximum (FWHM), was calculated on each reflection of 2θ. For constructing a linear plot of *β*hklcosθ against 4sinθhkl for determining crystallite size and lattice strain, X‐ray diffraction of first five reflections (1 1 1), (2 0 0), (3 1 1), (2 2 2) and (4 0 0) was used. Then, crystallite size (t) was obtained from the intercept c (i.e., c=kλ/t) and the strain (ε) from the slope (i.e., m=ε).

#### **3.3. Scanning electron microscope (SEM)**

**3. Structural characterization methods**

**Figure 4.** Schematic diagram of mechanical pusher furnace.

The transmission electron microscope (TEM) and X‐ray diffraction (XRD) can be effec‐ tively used for investigating the structural characterization of fine powders, polycrystalline materials, amorphous materials and thin films.  (a) Hydraulic press and (b) schematic diagram of conventional cold uniaxial compaction die process (double

**3.1. Geometric characterization techniques**

end compaction type).

8 Functionalized Nanomaterials

The milled powder samples were analyzed for powder morphological evaluation by scan‐ ning electron microscopy (SEM) using HITACHI S 3000 H operating at 8.5 mA and 20 kV. Before examining the powder samples in SEM, sputtering was done on the surface of the powders to conduct charge away from the surface. The microstructure of sintered preforms (after polishing and etching) was examined by F E I Quanta FEG 200—high‐resolution scan‐ ning electron microscope (HR‐SEM) with EDAX operating at 30–50 kV. In order to identify and confirm the elements shown in the SEM micrographs, energy dispersion spectrometer (EDS) was used. During analysis, both secondary electron image (SEI) mode and back‐scat‐ tered electron image (BSEI) mode were operated.

#### **3.4. Transmission electron microscope (TEM)**

To measure the matrix crystallite size exactly, and identifying the embedding and distribu‐ tion of nano level reinforcement in the matrix grain, both dark and bright field image of trans‐ mission electron microscope (TEM) can be used. Selected area of diffraction (SAD) patterns is also used to confirm the *NC* nature of the samples. Here, PHILIPS CM 12 TEM was used with spot EDAX facility. For powder samples, first the ball‐milled powder particles were under suspension in ethanol (volatile liquid) and then a drop of this suspension was allowed to dry on a thin carbon foil supported by a conventional microscope grid leaving the powder par‐ ticles ready for the observation. For sintered solid samples, manual polishing and mechani‐ cal grinding were performed until the sample thickness <100 Âμm was achieved. Following mechanical grinding, the samples were then dimpled to sample thickness of approximately 50 Âμm. Finally, ion milling was carried out on the dimpled samples using an ion miller of Edwards Model E306A.

#### **4. Mechanical testing methods**

#### **4.1. Density measurement**

The density of the post‐compacts and sintered preforms was estimated precisely by Archimedes principle [26] using electronic density balance. This principle is based on first measuring weight of sample in air column and then weighed in water column. Then, the ratio of weight of sample in air column and apparent loss of weight (difference between weight of air column and water column) give the specific gravity (SG) of post‐compacts/sintered preforms. Lastly, the density of post‐compact/sintered density can be determined by multi‐ plying the SG and density of water. Here, paraffin treatment was performed over the samples to avoid infiltration of water into the sample [27]. The calculated error in the density treatment was conformed within 1%. Three independent experimental readings were used and aver‐ aged for investigation. In addition, theoretical density of sintered composites with different percentage of reinforcements was determined using the rule of mixture.

#### **4.2. Hardness measurement**

Mechanical properties in terms of strength can be analyzed using hardness measurements. The hardness of sintered preforms was measured using PC‐based Ratnakar Vickers tes‐ ter. Here, the hardness was tested at a load of 1 kg. At least 15 measurements were taken in various places of polished samples for each sample. Then the average was taken for investigation.

#### **4.3. Simple compression test**

Before examining the powder samples in SEM, sputtering was done on the surface of the powders to conduct charge away from the surface. The microstructure of sintered preforms (after polishing and etching) was examined by F E I Quanta FEG 200—high‐resolution scan‐ ning electron microscope (HR‐SEM) with EDAX operating at 30–50 kV. In order to identify and confirm the elements shown in the SEM micrographs, energy dispersion spectrometer (EDS) was used. During analysis, both secondary electron image (SEI) mode and back‐scat‐

To measure the matrix crystallite size exactly, and identifying the embedding and distribu‐ tion of nano level reinforcement in the matrix grain, both dark and bright field image of trans‐ mission electron microscope (TEM) can be used. Selected area of diffraction (SAD) patterns is also used to confirm the *NC* nature of the samples. Here, PHILIPS CM 12 TEM was used with spot EDAX facility. For powder samples, first the ball‐milled powder particles were under suspension in ethanol (volatile liquid) and then a drop of this suspension was allowed to dry on a thin carbon foil supported by a conventional microscope grid leaving the powder par‐ ticles ready for the observation. For sintered solid samples, manual polishing and mechani‐ cal grinding were performed until the sample thickness <100 Âμm was achieved. Following mechanical grinding, the samples were then dimpled to sample thickness of approximately 50 Âμm. Finally, ion milling was carried out on the dimpled samples using an ion miller of

The density of the post‐compacts and sintered preforms was estimated precisely by Archimedes principle [26] using electronic density balance. This principle is based on first measuring weight of sample in air column and then weighed in water column. Then, the ratio of weight of sample in air column and apparent loss of weight (difference between weight of air column and water column) give the specific gravity (SG) of post‐compacts/sintered preforms. Lastly, the density of post‐compact/sintered density can be determined by multi‐ plying the SG and density of water. Here, paraffin treatment was performed over the samples to avoid infiltration of water into the sample [27]. The calculated error in the density treatment was conformed within 1%. Three independent experimental readings were used and aver‐ aged for investigation. In addition, theoretical density of sintered composites with different

Mechanical properties in terms of strength can be analyzed using hardness measurements. The hardness of sintered preforms was measured using PC‐based Ratnakar Vickers tes‐ ter. Here, the hardness was tested at a load of 1 kg. At least 15 measurements were taken

percentage of reinforcements was determined using the rule of mixture.

tered electron image (BSEI) mode were operated.

**3.4. Transmission electron microscope (TEM)**

Edwards Model E306A.

10 Functionalized Nanomaterials

**4.1. Density measurement**

**4.2. Hardness measurement**

**4. Mechanical testing methods**

The uniaxial simple compression test [28] was done using servo‐controlled universal testing machine with strain rate of 10‐3 s‐1 at room temperature to determine the maximum compressive strength. For compression test, the sample size of 5 mm×5 mm×10 mm was used as per ASTM standard [29]. The samples were cut from sintered one by wire‐cut electrical discharge machine; then polished by abrasive sheet of SiC paper to 1200 grit; further graphite lubricant was applied at the top and bottom surface of samples to minimize friction effect. Here, three samples were used in each composition, and the average was used for the interpretation.

#### **5. Results and discussion**

#### **5.1. Morphology of nanocomposite powders as function reinforcement**

The main purpose of studying the powder surface morphological examination is to inves‐ tigate the powder particle shape changes while manufacturing of AA 6061100‐x–x wt.% TiO2 , x=0, 2, 4, 6, 8, 10 and 12 wt.% particulate nanocomposite powders produced by MA. The homogeneous distribution of reinforcement particles over the nanocrystallite matrix, embed‐ ding of reinforcement particles inside the nanocrystallite matrix, presence of any agglomera‐ tion or clustering of reinforcement particles with the nanocrystallite matrix, powder particles shape and particle size can be investigated through the ball‐milled powder particles as it influences the mechanical properties. The powder morphology can be evaluated qualita‐ tively by secondary electron image (SEI) of SEM and back‐scattered electron image (BSEI) of SEM. **Figure 5** shows the influence of reinforcement addition in the nanocrystallite matrix powders after 40‐h MA. It can be clearly seen from **Figure 5** that the size of the particle/ agglomerate decreased with the function of reinforcement considerably. However, almost spherical‐shaped and equiaxed particles were obtained in all the system with the same mill‐ ing condition (40 h). These results implied the attainment of equilibrium state [30–32] after 40‐h MA. The morphological results showed that the addition of TiO2 particles in the matrix produced the powder morphology size reduction of matrix steadily. This was attributed to TiO2 ceramic particles also acted as milling agent. Further, it was observed here that the embedding and dispersing of nano‐sized TiO2 particles of around 119 nm with the soft Al alloy matrix powder for AA 6061‐12 wt.% TiO2 nanocomposite (**Figure 5h**). The morphologi‐ cal results showed that the addition of TiO2 particles favors to refining of matrix particles during MA. On the other hand, despite the ductile nature of the Al matrix, the formation of large agglomerate size (**Figure 5a**) was diminished with the presence of TiO2 particles. This is due to the fact that the increasing percentage of TiO2 particles in the soft matrix decreases the ductility so as to increase the amount fracture in the matrix particles before impacts of matrix particles that cause lamination. These results inferenced here that more amount of work hardening occurred in higher amount of reinforcement. The decreasing of particle/ agglomerate size of the matrix with carrying reinforcement was also observed by several

**Figure 5.** The morphology of AA 6061100‐x–x wt.% TiO2 nanocrystallite/nanocomposite powder after 40‐h MA: (a) 0%, (b) 2%, (c) 4%, (d) 6%, (e) 8%, (f) 10%, (g) 12% and (h) BSEI of magnified view of (g) shows the embedding of TiO2 particles on the matrix.

Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 13

**Figure 6.** (a) EDAX mapping of AA 6061‐10 wt.% TiO2 nanocomposite powder after 40‐h MA, red, green and blue indicates Al, O and Ti elements, respectively and (b) the corresponding EDAX spectrum.

researchers related to Al‐based MMCs processed via MA/MM [33–36]. **Figure 6** shows the X‐ ray mapping of 40‐h powders which confirms the uniform TiO2 particles within the Al matrix (Red: Al, Green: O and Blue: Ti).

#### **5.2. Effect of percentage of reinforcement on powder morphology of nanocomposite powders**

**Figure 7** shows the influences of the addition of TiO2 ceramic particles on nanocrystallite AA 6061 matrix particles size. It was observed here that the intensity of fracture mecha‐ nism increased and dominated the soft ductile matrix particle‐particle cold welding as the percentage of TiO2 ceramic particles increased in the soft Al alloy matrix. These resulted the formation of very fine particles/agglomerates in the case of 12 wt.% TiO2 ‐reinfroced nano‐ composites (**Figures 5g** and **7**) when compared to unreinforced nanocrystallite alloy powder (**Figures 5a** and **7**) during the same milling condition (40 h). The increasing of hard ceramic particles would accelerate the fracturing process which was also reported elsewhere [37]. It was attributed to importing more collisions on matrix powders which meant high‐velocity energy acted over the matrix powders. Thus, it was clear here that the tendency for fracturing was higher than the cold welding for longer milling time of 40 h. In general, after reaching the steady state of longer milling (40 h here) under wet milling, particle‐particle cold weld‐ ing on soft ductile alloy matrix would occur. Consequently, the particle/agglomerate size would start to increase (**Figure 5a**) when compared to un‐milled matrix powder (**Figure 1a**). This was attributed to higher cold welding tendency overcoming the fracture process [38]. However, almost equiaxed with spherical‐shaped and refined microstructure were obtained. Further, the rate of soft alloy matrix particle‐particle getting cold‐welded started to decrease as the percentage of TiO2 ceramic particles increased. This was attributed to more domination of fracturing tendency of TiO2 particles. Therefore, the powder/agglomerate size started to decrease in the case of higher reinforced nanocomposites (AA 6061‐12 wt.% TiO2 , **Figures 5g** and **7**). Similar kind of results was observed by Fogagnolo et al. 2003 [39] during ball milling

**Figure 5.** The morphology of AA 6061100‐x–x wt.% TiO2

on the matrix.

12 Functionalized Nanomaterials

2%, (c) 4%, (d) 6%, (e) 8%, (f) 10%, (g) 12% and (h) BSEI of magnified view of (g) shows the embedding of TiO2

nanocrystallite/nanocomposite powder after 40‐h MA: (a) 0%, (b)

particles

**Figure 7.** Influences of TiO2 ceramic particles on nanocrystallite AA 6061 matrix particles/agglomerate size after 40‐h MA.

of Al‐AlN system. Also, it can be explained in another way that the strength increased as the percentage of reinforcement particles increased according to Orowan strengthening mechanism.

The particle size of reinforcement phase decreased in which the clustered average particle size of TiO2 of around 1 μm (**Figure 1c**) was reduced to individual TiO2 particles of 119 nm (**Figure 5h**). The TiO2 particle size after 40‐h MA of AA 6061‐12 wt.% TiO2 nanocomposite powders was checked using bright and dark field image of transmission electron micro‐ scope (TEM) (**Figure 8**). Using several bright and dark field images of TEM, the average TiO2 particle size of around 120 nm was obtained (50 particles were counted), which coherent with the results obtained from back‐scattered electron image of SEM morphology. The pink color arrow head indicates the TiO2 particles. Hence, the result of the higher percentage of TiO2 phase in the soft matrix produced smaller particle/agglomerate size of the nanocom‐ posite powders (40‐h milling) (**Figure 5g**) [39–41]. The addition of TiO2 powder particles had great influence on the morphological characteristics of the fabricated nanocomposite powders via MA.

Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 15

**Figure 8.** TEM image of AA 6061‐12 wt.% TiO2 nanocomposite powder: (a) bright field image and (b) dark field image.

#### **5.3. Structural evaluation of mechanically alloyed nanocomposite powders**

The process of manufacturing of MMCs powders through MA causes morphological and structural changes. Severe plastic deformation of the powder particles during MA can lead to grain refining, variation in the crystallite size, accumulation of internal stress, density of dislocation and variation of the lattice parameter [42]. The objective of this study is to report the structural characteristics of MAed powders (40 h) as the function of reinforcement (0, 2, 4, 6, 8, 10 and 12 wt.% TiO2 ) using XRD, TEM and EDS.

#### *5.3.1. XRD analysis*

of Al‐AlN system. Also, it can be explained in another way that the strength increased as the percentage of reinforcement particles increased according to Orowan strengthening

ceramic particles on nanocrystallite AA 6061 matrix particles/agglomerate size after 40‐h

The particle size of reinforcement phase decreased in which the clustered average particle

powders was checked using bright and dark field image of transmission electron micro‐ scope (TEM) (**Figure 8**). Using several bright and dark field images of TEM, the average TiO2 particle size of around 120 nm was obtained (50 particles were counted), which coherent with the results obtained from back‐scattered electron image of SEM morphology. The pink

particle size after 40‐h MA of AA 6061‐12 wt.% TiO2

phase in the soft matrix produced smaller particle/agglomerate size of the nanocom‐

had great influence on the morphological characteristics of the fabricated nanocomposite

particles. Hence, the result of the higher percentage of

particles of 119 nm

nanocomposite

powder particles

of around 1 μm (**Figure 1c**) was reduced to individual TiO2

posite powders (40‐h milling) (**Figure 5g**) [39–41]. The addition of TiO2

mechanism.

MA.

size of TiO2

TiO2

(**Figure 5h**). The TiO2

**Figure 7.** Influences of TiO2

14 Functionalized Nanomaterials

powders via MA.

color arrow head indicates the TiO2

**Figure 9** shows the XRD patterns of Al 6061–x wt.% TiO2 , (x=0, 2, 4, 6, 8, 10 and 12 wt.%) nanocomposites powders after 40‐h MA. This figure clearly shows that the Al peak width, for example, at (1 1 1) plane, increased with the percentage of reinforcement. This was due to which TiO2 particles were also acting as a milling agent which accelerated the milling process. The inset of **Figure 9** shows that the case of 12 wt.% TiO2 nanocomposite powders more broad‐ ened Al peak at (1 1 1) plane occurred than unreinforced nanocrystallite powders. It indicates that more spatial coherent length confinement occurred in higher reinforced nanocomposite [43]. Also, it can be noted from the inset of **Figure 9** that shifting of Bragg's angle occurred in the case of 12 wt.% TiO2 particulate nanocomposite powders. These results show that more distortion occurred in higher reinforced nanocomposite powder than unreinforced nanocrys‐ tallite powder during the same milling time. Further, the shifting of Bragg's angle indicates the changes in the lattice parameter value with respect to percentage of reinforcement during 40‐h MA. The increasing of peak broadening of matrix phase by the addition of reinforcement ceramic phase can be clearly seen from **Figure 10**. The structural characteristics such as crys‐ tallite size, lattice microstrain, dislocation density, lattice parameter and the unit cell volume of Al as the function of reinforcement after 40‐h milling are reported in **Table 2**. Owing to the characteristics of high‐energy ball milling, large amount of structural defects in terms of dislocations would occur due to severe plastic deformation [44]. Sever plastic deformation (SPD) of high‐energy milling brings about a deformed lattice with high dislocation density during early stage of milling. These indicate the amount of dislocations pile up near the GBs

**Figure 9.** XRD patterns of AA 6061100‐x–x wt.% TiO2 (x=0, 2, 4, 6, 8, 10 and 12%) nanocrystallite/nanocomposite powder after 40 h of high‐energy ball milling. Inset shows shift in Bragg's angle.

will be increased. When prolonged milling, sometimes the GBs might have turned to be soft or relaxed. On this time, the amount of dislocations pileup near the GBs will be decreased. Hence, for MAed powders, dislocations are the main defects of which dislocation density (*ρ*D) can be determined as [45]:

$$\rho\_D = 2\sqrt{3}\frac{<\varepsilon\_{rms}>}{tb} \tag{2}$$

where <εr.m.s> is RMS lattice strain which is determined as:

 $<\varepsilon\_{r,ms}>$  =  $\left(\frac{2}{\pi}\right)^{1/2}\left(\frac{\Delta d}{d\_{0\,\text{(sh)}}}\right)$

*Δd* = *d*(*hkl*) − *d*0(*hkl*) , where *d*(hkl) and *d*0(hkl) are the calculated and observed interplanar spacing, *t* is the crystallite size in *m* and *b* is the burgers vector of dislocation in *m* which is equal to *a*/√ \_\_ 2 for the FCC structure, *a* is the lattice parameter in *m*. The actual lattice parameter was Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 17

**Figure 10.** XRD patterns of AA 6061100‐x–x wt.% TiO2 , x=0, 4, 8, and 12%, composite powder after 40 h of high‐energy ball milling.

**Table 2.** Structural characterization of AA 6061100‐x –x wt.% TiO2 composite powder after 40‐h milling, x = 0, 2, 4, 6, 8, 10 and 12%.

obtained as described by Cullity [22], by constructing the linear plot between the calculated lattice parameter for each Bragg's angle and the corresponding value of cos2 θ/sinθ.

#### *5.3.2. TEM analysis*

will be increased. When prolonged milling, sometimes the GBs might have turned to be soft or relaxed. On this time, the amount of dislocations pileup near the GBs will be decreased. Hence, for MAed powders, dislocations are the main defects of which dislocation density (*ρ*D)

> \_\_ 3 < *εr*.*m*.*s* > \_\_\_\_\_\_\_\_\_\_

*π*) 1/2 (\_*Δd d*0 (*hkl*)

*t* is the crystallite size in *m* and *b* is the burgers vector of dislocation in *m* which is equal to

2 for the FCC structure, *a* is the lattice parameter in *m*. The actual lattice parameter was

, where *d*(hkl) and *d*0(hkl) are the calculated and observed interplanar spacing,

*tb* , (2)

(x=0, 2, 4, 6, 8, 10 and 12%) nanocrystallite/nanocomposite powder

), (3)

can be determined as [45]:

16 Functionalized Nanomaterials

*Δd* = *d*(*hkl*) − *d*0(*hkl*)

*a*/√ \_\_

*ρD* = 2 √

**Figure 9.** XRD patterns of AA 6061100‐x–x wt.% TiO2

< *εr*.*m*.*s* > = (\_\_2

where <εr.m.s> is RMS lattice strain which is determined as:

after 40 h of high‐energy ball milling. Inset shows shift in Bragg's angle.

The TEM micrographs of AA 6061100‐x–x wt.% TiO2 (x=0, 4, 8 and 12 wt.%) nanocomposite powder is shown in **Figure 11a**–**l**. The matrix crystallite size was calculated based on several bright and dark field images (**Figure 11**), and minimum of more than 200 grains was counted

**Figure 11.** TEM micrographs of as‐milled nanocomposite powders: (a) bright field image (BFI) of 0% TiO2 , (b) SAD pattern of 0% TiO2 , (c) EDAX analysis of 0% TiO2 , (d) BFI of 4% TiO2 , (e) dark field image (DFI) of 4% TiO2 (inset shows the SAD), (f) EDAX analysis of 4% TiO2 , (g) BFI of 8% TiO2 , (h) DFI of 8% TiO2 (inset shows the SAD), (i) EDAX analysis of 8% TiO2 , (j) BFI of 12% TiO2 , (k) DFI of 12% TiO2 (inset shows the SAD) and (l) EDAX analysis of 12% TiO2 . Note: single arrow represents TiO2 particle.

for each composition to get the average grain size. The calculated matrix crystallite size for 0, 4, 8 and 12 wt.% TiO2 nanocomposite powders were to be around 69, 63, 54 and 49 nm, respectively. From the selected area of diffraction (SAD), the continuous sharp ring pattern was observed in all the system which confirmed the nanocrystalline nature of the Al matrix in as‐milled (MA, 40 h) condition [34]. Single arrow in the bright field images of **Figure 11** indicates the TiO2 particle embedded in the matrix. Further, the corresponding EDAX analy‐ sis was also performed and tabulated in the respective figures. The presence and embedding Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 19

**Figure 12.** Variation in crystallite size, lattice parameter and solid solution of TiO2 for AA 6061100‐x–x wt.% TiO2 (x=0, 2, 4, 6, 8, 10 and 12 wt.%) nanocomposite powder as a function of reinforcement.

of TiO2 particles in the matrix were confirmed by EDAX spectrum in which the TiO2 spectrum was observed to increase with percentage of reinforcement.

#### *5.3.3. Effect of percentage of reinforcement on structural changes*

**Figure 11.** TEM micrographs of as‐milled nanocomposite powders: (a) bright field image (BFI) of 0% TiO2

, (g) BFI of 8% TiO2

, (d) BFI of 4% TiO2

for each composition to get the average grain size. The calculated matrix crystallite size for

respectively. From the selected area of diffraction (SAD), the continuous sharp ring pattern was observed in all the system which confirmed the nanocrystalline nature of the Al matrix in as‐milled (MA, 40 h) condition [34]. Single arrow in the bright field images of **Figure 11**

sis was also performed and tabulated in the respective figures. The presence and embedding

, (h) DFI of 8% TiO2

(inset shows the SAD) and (l) EDAX analysis of 12% TiO2

nanocomposite powders were to be around 69, 63, 54 and 49 nm,

particle embedded in the matrix. Further, the corresponding EDAX analy‐

, (c) EDAX analysis of 0% TiO2

, (k) DFI of 12% TiO2

pattern of 0% TiO2

arrow represents TiO2

18 Functionalized Nanomaterials

indicates the TiO2

of 8% TiO2

the SAD), (f) EDAX analysis of 4% TiO2

, (j) BFI of 12% TiO2

0, 4, 8 and 12 wt.% TiO2

particle.

, (b) SAD

(inset shows

. Note: single

, (e) dark field image (DFI) of 4% TiO2

(inset shows the SAD), (i) EDAX analysis

The XRD results from **Figure 10** corroborate that an increase in the amount of TiO2 in the soft matrix results in a finer crystallite size. The crystallite size variation with percentage of reinforcement is shown in **Figure 12**. The crystallite size of soft matrix was around 65, 60, 57, 53, 50, 48 and 46 nm for 0, 2, 4, 6, 8, 10 and 12 wt.% of TiO2 particulate composite powder, respectively, after 40‐h MA. As the weight percentage of hard TiO2 content increased, the ductility of the alloy matrix particles decreased, resulting in the dominance of fragmenta‐ tion which led to a finer grain size in the case of higher TiO2 content. The estimated lattice parameter (**Table 2** and **Figure 12**) of Al matrix for all reinforced nanocomposite powder was a lower value than the lattice parameter of pure FCC‐Al at room temperature (4.0496 Å). This indicates that distortions occurred in the Al lattice and decreasing of lattice parameter with percentage of reinforcement meaning that ceramic phase addition influenced the lattice parameter considerably [46]. The decreased lattice parameter with percentage of reinforce‐ ment was due to the balancing between the dissolution of Ti and O atoms in which O atoms dissolution dominated at the higher reinforcement particulate composite powders. Because, the atomic radius of O atom is lower than the atomic radius of Al atom and the atomic radius of Ti atom is higher than Al. Therefore, the O atom can easily dissolve in the Al lattice which decreased the lattice parameter. However, numerically, the deviation of lattice parameter value with percentage of reinforcement was very little. This can be checked by calculating the percentage of solid solution of TiO2 particles from XRD results. The percentage of solid solution of TiO2 increased with the percentage of reinforcement. Numerically, this value is equivalent to zero.

#### **5.4. Sintering behavior of nanocomposites**

The excellent mechanical properties can be expected from P/M processed composites when the second phase particles (here TiO2 ceramic particles) is homogeneously distributed in the matrix. Generally, in conventional metal matrix composites (MMCs), matrix‐to‐reinforcement particle size ratio would affect the mechanical performances of MMCs. If matrix‐to‐particle size ratio is greater than unity mean, clustering/agglomeration of reinforcement particles over the matrix would occur which decrease the mechanical properties. This is owing to the fact that reinforcement clusters/agglomerates cannot transfer tensile and shear stresses as it form interparticulate boundaries that act cracks [46]. Therefore, the main objectives of the present section are to investigate whether there is any effect on matrix‐to‐reinforcement particle size ratio (here, all the samples are greater than unity) as here the reinforcement particles size is around 119 nm, percentage of reinforcement and grain refinement on hardness and densifica‐ tion behavior of nanocomposites.

#### *5.4.1. Influence of reinforcement content on sintering of nanocomposites*

**Figure 13a** shows the densification in terms of percentage of theoretical density of nanocom‐ posite sintered over the temperature range 400–625°C with a step size of 75°C. The percentage of theoretical density sintered nanocomposite at 400°C was 80.25, 81.50, 83.00 and 84.50% for 0, 4, 8 and 12 wt.% TiO2 particles, respectively, which varied from 80 to 84%. It is interest‐ ing to note that the percentage of theoretical density was increased from 80% (unreinforced *NC*/*UFG* alloy) to 84% (12 wt.% reinforced nanocomposite) with the function of reinforce‐ ment. The increasing percentage of theoretical density with the function of reinforcement was attributed to abrupt changes in the powders morphology size reduction. The observed rate of increased percentage theoretical density was marginally greater up to 4% TiO2, and then, it increased considerably up to 12 wt.% TiO2 . This was attributed to a powder matrix particle size reduction, uniform distribution and complete embedded/dispersed TiO2 particles on the soft alloy matrix. The sintering response with function of reinforcement and function of tem‐ perature can be evaluated by a contour graph which is shown in **Figure 13b**. From **Figure 13b**, the percentage of theoretical density curves increased steadily first over both the percentage of reinforcement and sintering temperature up to 500°C. Beyond the sintering temperature of 550°C, irrespective of the reinforcement, the percentage of theoretical density increased slightly. One can notice that the percentage of theoretical density increased with the function of reinforcement. The highest percentage of theoretical density can be achieved when the per‐ centage of reinforcement is >10 wt.% between the temperature range from 500 to 550°C. The highest percentage of theoretical density achieved on this region was attributed to powder morphology size reduction and homogeneous with embedded nano‐sized Titania particles in the matrix [47]. Further, at the lower value of percentage of reinforcement (<2 wt.%), the percentage of theoretical density can be achieved only around 80%. This worsened sintering Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 21

**Figure 13.** (a) Densification of AA 6061100‐x–x wt.% of TiO2 , x=0,4, 8 and 12 wt.% nanocomposites, (b) contour graph of sintering behavior in terms of % theoretical density.

behavior was attributed to very large agglomerate size with almost spherical morphology of the matrix after 40‐h milling (**Figure 5a**).

#### *5.4.2. Structural evolution of sintered nanocomposites*

the percentage of solid solution of TiO2

**5.4. Sintering behavior of nanocomposites**

the second phase particles (here TiO2

tion behavior of nanocomposites.

it increased considerably up to 12 wt.% TiO2

0, 4, 8 and 12 wt.% TiO2

*5.4.1. Influence of reinforcement content on sintering of nanocomposites*

solution of TiO2

equivalent to zero.

20 Functionalized Nanomaterials

particles from XRD results. The percentage of solid

ceramic particles) is homogeneously distributed in the

increased with the percentage of reinforcement. Numerically, this value is

The excellent mechanical properties can be expected from P/M processed composites when

matrix. Generally, in conventional metal matrix composites (MMCs), matrix‐to‐reinforcement particle size ratio would affect the mechanical performances of MMCs. If matrix‐to‐particle size ratio is greater than unity mean, clustering/agglomeration of reinforcement particles over the matrix would occur which decrease the mechanical properties. This is owing to the fact that reinforcement clusters/agglomerates cannot transfer tensile and shear stresses as it form interparticulate boundaries that act cracks [46]. Therefore, the main objectives of the present section are to investigate whether there is any effect on matrix‐to‐reinforcement particle size ratio (here, all the samples are greater than unity) as here the reinforcement particles size is around 119 nm, percentage of reinforcement and grain refinement on hardness and densifica‐

**Figure 13a** shows the densification in terms of percentage of theoretical density of nanocom‐ posite sintered over the temperature range 400–625°C with a step size of 75°C. The percentage of theoretical density sintered nanocomposite at 400°C was 80.25, 81.50, 83.00 and 84.50% for

ing to note that the percentage of theoretical density was increased from 80% (unreinforced *NC*/*UFG* alloy) to 84% (12 wt.% reinforced nanocomposite) with the function of reinforce‐ ment. The increasing percentage of theoretical density with the function of reinforcement was attributed to abrupt changes in the powders morphology size reduction. The observed rate of increased percentage theoretical density was marginally greater up to 4% TiO2, and then,

soft alloy matrix. The sintering response with function of reinforcement and function of tem‐ perature can be evaluated by a contour graph which is shown in **Figure 13b**. From **Figure 13b**, the percentage of theoretical density curves increased steadily first over both the percentage of reinforcement and sintering temperature up to 500°C. Beyond the sintering temperature of 550°C, irrespective of the reinforcement, the percentage of theoretical density increased slightly. One can notice that the percentage of theoretical density increased with the function of reinforcement. The highest percentage of theoretical density can be achieved when the per‐ centage of reinforcement is >10 wt.% between the temperature range from 500 to 550°C. The highest percentage of theoretical density achieved on this region was attributed to powder morphology size reduction and homogeneous with embedded nano‐sized Titania particles in the matrix [47]. Further, at the lower value of percentage of reinforcement (<2 wt.%), the percentage of theoretical density can be achieved only around 80%. This worsened sintering

size reduction, uniform distribution and complete embedded/dispersed TiO2

particles, respectively, which varied from 80 to 84%. It is interest‐

. This was attributed to a powder matrix particle

particles on the

The XRD patterns of 0, 4, 8 and 12 wt.% reinforced particulate nanocomposite sintered at 550°C for 2 h are shown in **Figure 14**. The observed phases are mainly α‐Al and TiO2 . The crystallite size of α‐Al phase for these sintered nanocomposites was calculated by standard Williamson Hall analysis. The calculated crystallite size after sintering at 550°C is 308, 265, 238 and 205 nm for 0, 4, 8 and 12 wt.% reinforced nanocomposite, respectively. The measured crystallite sizes were in ultrafine level. To confirm the crystallite size in ultrafine level, AA 6061–12 wt.% TiO2 sintered at 550°C sample was examined using TEM analysis. **Figure 15a** shows the bright field image of 12 wt.% reinforced nanocomposite. From **Figure 15a**, the observed crystallite size of α‐Al matrix was almost equiaxed. Further, nanometer‐sized Titania particles were uniformly distributed and embedded in the α‐Al matrix. **Figure 15b** shows the corresponding selected area of diffraction (SAD) patterns which produced a ring pattern. These results confirmed that the individual grains of α‐Al matrix were separated by high‐angle grain boundaries and have a random orientation with neighboring grains. Also, the SAD ring pattern reveals that α‐Al grains are in ultra‐fine level. Further, the average grain size of the α‐Al matrix is found to be around 225±15 nm based on 250 grains counted from the bright field images of TEM microstructure.

**Figure 16a** shows the crystallite size of AA 6061100‐x–x wt.% TiO2 (x=0, 4, 8 and 12%) nano‐ composites in as‐milled and as‐sintered at 550°C. Metal matrix composites are all immisci‐ ble systems in which the diffusion (migration of atoms) of one kind into the other is usually restricted because crystallite growth would occur based on diffusion. Therefore, MMCs, in such immiscible systems very small crystal growth is expected on heating [48]. This is

**Figure 14.** XRD patterns of AA 6061100‐x–x wt.% TiO2 , x=0, 4, 8 and 12 wt.% nanocomposite sintered at 550 °C for 2 h.

**Figure 15.** (a) TEM bright field image of AA 6061‐12 wt.% TiO2 nanocomposite sintered at 550°C; (b) the corresponding SAD ring pattern indicating *UFG* nature of matrix.

clearly evident from **Figure 16a**, wherein the growth of crystallite size of α‐Al was restricted significantly after sintering at 550°C for higher percentage of reinforcement. It is to be noted here that an increase in the TiO2 content of the nanocomposites, the nano‐sized Titania (∼119 nm) particles pinned the matrix grain boundary and prevented the grain growth dur‐

Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 23

**Figure 16.** (a) Crystallite size as function of reinforcement in as‐milled and as‐sintered at 550°C condition and (b) effect of composition on Vickers hardness of AA 6061100‐x–x wt.% TiO2 (x=0, 2, 4, 6, 8, 10 and 12) bulk micro and nanocomposites (sintered at 625°C).

ing sintering. An increase in the TiO2 content reduced the probability of Al‐Al contacts and increased the probability of Al‐TiO2 interfaces and hence reduced their tendency for crystallite growth due to the immiscible nature of the soft Al alloy matrix with hard TiO2 particles [49].

#### *5.4.3. Influence of reinforcement content on hardness of sintered nanocomposites*

clearly evident from **Figure 16a**, wherein the growth of crystallite size of α‐Al was restricted significantly after sintering at 550°C for higher percentage of reinforcement. It is to be noted

(∼119 nm) particles pinned the matrix grain boundary and prevented the grain growth dur‐

content of the nanocomposites, the nano‐sized Titania

nanocomposite sintered at 550°C; (b) the corresponding

, x=0, 4, 8 and 12 wt.% nanocomposite sintered at 550 °C for 2 h.

here that an increase in the TiO2

SAD ring pattern indicating *UFG* nature of matrix.

**Figure 15.** (a) TEM bright field image of AA 6061‐12 wt.% TiO2

**Figure 14.** XRD patterns of AA 6061100‐x–x wt.% TiO2

22 Functionalized Nanomaterials

The measured Vickers hardness values as function of sintering temperature is given in **Table 3**. From **Table 3**, it was observed that the Vickers hardness values increased steadily first up to 4% TiO2 and then increased abruptly as the percentage of reinforcement increased after 40‐h milling. The highest value of hardness for AA 6061‐12 wt.% TiO2 nano‐ composites was obtained after sintering at 625°C. This was attributed to crystallite refine‐ ment occurred in the structure, very fine particle size of matrix and embedding of larger quantity of TiO2 particles in the matrix [46]. **Figure 16b** shows the variation in hardness of the nanocomposites sintered at 625°C as a function of percentage of TiO2 particles. The hardness varied from 564 to 1126 MPa to the corresponding increase of TiO2 content. This higher value was attributed to their finer grain size and the presence of nano‐sized TiO2 particles as dispersoid in the matrix and embedded in the soft matrix. Very high hard‐ ness value was obtained at higher reinforcement [46], which was due to very fine par‐ ticle size distribution, crystallite refinement, highly embedded TiO2 ceramic particles in the matrix and improved dislocation density at the grain boundary. The hardness of AA 6061‐12 wt.% TiO2 nanocomposites was 1.41–2.11 times higher than that of unreinforced AA 6061 unreinforced alloy. For instance, the observed size of the hardness indentation for 0, 4, 8 and 12 wt.% TiO2 nanocomposite sintered at 550°C was around 0.185, 0.179, 0.161 and 0.146 μm, respectively. The matrix crystallite sizes based on peak profile analyses were 308, 265, 238 and 205 nm (**Figure 14**) for 0, 4, 8 and 12 wt.% TiO2 nanocomposite, respec‐

**Table 3.** Vickers hardness as a function of sintering temperature.

tively. These results indicated that as the crystallite size decreases, the hardness impres‐ sion was also decreases steadily due to more refinement occurs in the structure with the reinforcement content in the same milling condition. It can be concluded here that there was no possibility of clusters/agglomerates formation in these nanocomposites; the matrix‐ to‐reinforcement particle size ratio was insignificant in Vickers hardness. To conform this one, the back‐scattered electron images (BSEI) of 2, 6 and 8 wt.% TiO2 ‐reinforced par‐ ticulate sintered nanocomposite at 550°C is shown in **Figure 17a**–**f**. In this, left side of **Figure 17a**, **c** and **e** shows the microstructure of sintered preform at low‐magnification (400 ×) value. The corresponding magnified view is shown on right side of **Figure 17b**, **d** and **f** for 2, 6 and 8 wt.% reinforced composite, respectively. Even at high magnifica‐ tion itself in all preforms, there was no clustering observed in the present manufactured nanocomposite. In **Figure 17**, double arrow represents the distribution of reinforcement particles on the matrix. Hence, better mechanical properties can be expected in the case of nanocomposite which meant the effect of matrix‐to‐reinforcement particle size ratio on the present nanocomposites was almost nil.

#### **5.5. Mechanical behavior of nanocomposites**

#### *5.5.1. Compressive stress‐strain curves of nanocomposites*

The mechanical behavior in terms of simple uniaxial compressive stress‐strain curves at room temperature (303 K) of AA 6061‐x wt.% TiO2 (x=0, 4, 8 and 12 wt.%) nanocomposites is shown in **Figure 18**. From **Figure 18**, it was observed that the compressive stress flow curve was started to increase drastically when the amount of reinforcement of TiO2 ceramic particles increases in the nanocrystallite matrix. Maximum ultimate compressive strength of 442 MPa was obtained in AA 6061‐12 wt.% TiO2 nanocomposite which was 1.6 times higher than AA 6061 nanocrystallite alloy. The observed ultimate compressive strengths were 275, 295, 365 and 442 MPa for 0, 4, 8 and 12 wt.% TiO2 ‐reinforced nanocomposites, respectively. The cor‐ responding decreased compressive strains were 0.076, 0.065, 0.052 and 0.039 for 0, 4, 8 and 12 wt.% TiO2 ‐reinforced nanocomposites, respectively. The increase in value of compressive strength and decrease in strain with the function of nano‐TiO2 particles were attributed to Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 25

tively. These results indicated that as the crystallite size decreases, the hardness impres‐ sion was also decreases steadily due to more refinement occurs in the structure with the reinforcement content in the same milling condition. It can be concluded here that there was no possibility of clusters/agglomerates formation in these nanocomposites; the matrix‐ to‐reinforcement particle size ratio was insignificant in Vickers hardness. To conform this

ticulate sintered nanocomposite at 550°C is shown in **Figure 17a**–**f**. In this, left side of **Figure 17a**, **c** and **e** shows the microstructure of sintered preform at low‐magnification (400 ×) value. The corresponding magnified view is shown on right side of **Figure 17b**, **d** and **f** for 2, 6 and 8 wt.% reinforced composite, respectively. Even at high magnifica‐ tion itself in all preforms, there was no clustering observed in the present manufactured nanocomposite. In **Figure 17**, double arrow represents the distribution of reinforcement particles on the matrix. Hence, better mechanical properties can be expected in the case of nanocomposite which meant the effect of matrix‐to‐reinforcement particle size ratio on the

The mechanical behavior in terms of simple uniaxial compressive stress‐strain curves at room

in **Figure 18**. From **Figure 18**, it was observed that the compressive stress flow curve was

increases in the nanocrystallite matrix. Maximum ultimate compressive strength of 442 MPa

6061 nanocrystallite alloy. The observed ultimate compressive strengths were 275, 295, 365

responding decreased compressive strains were 0.076, 0.065, 0.052 and 0.039 for 0, 4, 8 and

‐reinforced nanocomposites, respectively. The increase in value of compressive

started to increase drastically when the amount of reinforcement of TiO2

strength and decrease in strain with the function of nano‐TiO2

(x=0, 4, 8 and 12 wt.%) nanocomposites is shown

nanocomposite which was 1.6 times higher than AA

‐reinforced nanocomposites, respectively. The cor‐

‐reinforced par‐

ceramic particles

particles were attributed to

one, the back‐scattered electron images (BSEI) of 2, 6 and 8 wt.% TiO2

**Composition Vickers hardness, HV1.0 (MPa) (function of sintering temperature)**

6061 Al alloy 488.14±35.6 497.91±50.5 540.78±54.8 564.60±57.2 6061 Al alloy+2% TiO2 506.24±42.3 525.82±32.8 561.18±24.9 588.36±43.7 6061 Al alloy+4% TiO2 533.22±53.9 548.77±54.7 576.93±57.9 606.35±60.8 6061 Al alloy+6% TiO2 570.15±52.4 602.48±37.2 645.31±46.7 710.80±24.6 6061 Al alloy+8% TiO2 608.73±62.4 648.66±65 712.54±72.0 810.47±81.5 6061 Al alloy+10% TiO2 621.35±42.8 724.89±27.5 790.35±53.7 972.64±34.3 6061 Al alloy+12% TiO2 628.90±63.9 793.10±81 866.09±86.9 1126.12±115

**400°C 475°C 550°C 625°C**

present nanocomposites was almost nil.

**5.5. Mechanical behavior of nanocomposites**

temperature (303 K) of AA 6061‐x wt.% TiO2

was obtained in AA 6061‐12 wt.% TiO2

12 wt.% TiO2

and 442 MPa for 0, 4, 8 and 12 wt.% TiO2

*5.5.1. Compressive stress‐strain curves of nanocomposites*

**Table 3.** Vickers hardness as a function of sintering temperature.

24 Functionalized Nanomaterials

**Figure 17.** SEM/BSEI of sintered nanocomposite AA 6061100‐x–x wt.% TiO2 at 550°C. Left side of (a), (c) and (e) shows 2, 6 and 12 wt.% TiO2 nanocomposite. Right side of (b), (d) and (f) shows magnified view of corresponding sintered preforms. Note: double arrow represents the distribution of TiO2 particles.

**Figure 18.** Compressive stress‐strain curves of experimental and finite element analysis of: (a) AA 6061 alloy, (b) AA 6061‐4TiO2 , (c) AA 6061‐8TiO2 , (d) AA 6061‐12TiO2 nanocomposites.

crystallite refinement in the matrix, uniform distribution and embedding of reinforcements over the matrix. Due to this, effective load transfer between matrix and reinforcement had occurred.

#### *5.5.2. Finite element simulation of compressive stress‐strain curves of nanocomposites*

Finite element simulations were also carried out using simple uniaxial compressive stress‐ strain data. In the simulation, 3D solid model of 5 mm×5 mm×10 mm as per ASTM stan‐ dard specimen was used; one end of model was set as fixed support, and the other end was used to give compressive load. Before simulations, the material properties such as density, Young's modulus value, Poisson's ratio and ultimate compressive strength were given as input in ANSYS workbench software. **Figure 18** in dashed line shows the finite element results. Excellent agreement between experimental results and finite element results was observed. **Figure 19a** and **b** shows the stress distribution at ultimate compressive strength of AA 6061 and AA 6061‐12 wt.% TiO2 nanocomposites, respectively. It can be observed from **Figure 19** that the stress concentration at the bottom of developed/simulated model was more in AA 6061‐12 wt.% TiO2 nanocomposite when compared to AA 6061 nano‐ crystallite alloy. This was attributed to more crystallite refinement, uniform distribution of nano TiO2 particle and more amount of embedding of TiO2 ceramic particles over the matrix.

#### *5.5.3. Compressive stress‐strain curves of trimodeled nanocomposites*

crystallite refinement in the matrix, uniform distribution and embedding of reinforcements over the matrix. Due to this, effective load transfer between matrix and reinforcement had

**Figure 18.** Compressive stress‐strain curves of experimental and finite element analysis of: (a) AA 6061 alloy, (b) AA

nanocomposites.

Finite element simulations were also carried out using simple uniaxial compressive stress‐ strain data. In the simulation, 3D solid model of 5 mm×5 mm×10 mm as per ASTM stan‐ dard specimen was used; one end of model was set as fixed support, and the other end was used to give compressive load. Before simulations, the material properties such as density, Young's modulus value, Poisson's ratio and ultimate compressive strength were given as input in ANSYS workbench software. **Figure 18** in dashed line shows the finite element results. Excellent agreement between experimental results and finite element results was observed. **Figure 19a** and **b** shows the stress distribution at ultimate compressive strength

from **Figure 19** that the stress concentration at the bottom of developed/simulated model

crystallite alloy. This was attributed to more crystallite refinement, uniform distribution

particle and more amount of embedding of TiO2

nanocomposites, respectively. It can be observed

ceramic particles over the

nanocomposite when compared to AA 6061 nano‐

*5.5.2. Finite element simulation of compressive stress‐strain curves of nanocomposites*

, (d) AA 6061‐12TiO2

occurred.

6061‐4TiO2

26 Functionalized Nanomaterials

, (c) AA 6061‐8TiO2

of nano TiO2

matrix.

of AA 6061 and AA 6061‐12 wt.% TiO2

was more in AA 6061‐12 wt.% TiO2

It was observed from **Figure 18** that the manufactured nanocomposites exhibited highest com‐ pressive strength. However, it showed poor in toughness and ductility. Therefore, in order to improve toughness and ductility by sacrificing the strength slightly, trimodel‐based nanocom‐ posites were successfully fabricated. These trimodel nanocomposites were consisting of nano‐ crystallite matrix (after 40‐h MA), nano TiO2 particles (∼119 nm) and coarse crystallite matrix. It was expected that the addition of coarse crystallite matrix in the nanostructured materials would enhance the toughness and ductility. As an example, AA 6061‐12 wt.% TiO2 nanocom‐ posites (after 40‐h MA) were taken here for investigation. **Figure 20** shows the compressive stress‐strain curves of AA 6061‐12 wt.% TiO2 ‐ y wt.% CG nanocomposites (y=0, 5, 10, 15, 20, 25 and 30) of both experiment (**Figure 20a**) and finite element (**Figure 20b**). Inspection of AA 6061‐12 wt.% TiO2 nanocomposite curves showed that there was a poor strain (poor in ductil‐ ity) of around 0.039 mm/mm only; however, it exhibited an ultimate compressive strength of 442 MPa. The poor value of strain (ductility) for nanostructured alloy/composites via Ma was attributed to MA processing which causes residual moisture during cold consolidation led to pores after sintering [50], segregation of impurities created by milling media (i.e., balls and vial material) [51] and the dispersoid‐matrix interface de‐cohesion [52]. The measured ultimate compressive strength for 0, 5, 10, 15, 20, 25 and 30% CG nanocomposites were 442, 449, 454, 474, 461, 438 and 427 MPa, respectively. These results revealed that the ultimate com‐ pressive strength was started to increase up to 15 wt.% CG matrix in AA 6061‐12 wt.% TiO2 nanocomposites, and then, it started to decrease from 20 to 30 wt.% CG matrix in the nanocom‐ posites. Indeed, as the CG matrix increases in the nanocomposites, it tries to accommodate the dislocation mobility. Further, the CG tries to arrest the crack propagation that usually occurs in nanocomposites as it is subjected either tensile force or compressive force. Around 7% of increased ultimate compressive strength was observed in AA 6061‐12 wt.% TiO2 ‐15 wt.% CG nanocomposites when compared to AA 6061‐12 wt.% TiO2 nanocomposite. The corresponding compressive strain was 0.039, 0.050, 0.057 and 0.071 mm/mm for 0, 5, 10 and 15 CG nanocom‐ posites, respectively. This was attributed to enhanced densification by increasing soft phase and uniform distribution of CG matrix in nanocomposite structures. However, the observed ultimate compressive strength started to decrease when the CG matrix addition was beyond 15%. This was expected to the domination of softer CG matrix and coalescences of CG par‐ ticles which might have decreased the effective load transfer rate. In contrast, the ductility was improved in a better manner as the large number of CG matrix phase in the nanocomposites might have effectively delayed the crack initiation in nanocomposites during deformation. The compressive ductility for 30% CG trimodel nanocomposite was around 520>0% CG nano‐ composites. To conform the uniform distribution and coalescence of CG matrix in nanocom‐ posite, the microstructures of AA 6061‐12 TiO2 ‐ y wt.% CG (y=0, 10, 20 and 30) nanocomposites were taken and the same is shown in **Figure 20a**–**d**. From the microstructures, two different regions, namely, gray regions and white regions were seen. The gray region characterizes ultra‐fine grain (UFG) matrix phase embedded with nano‐TiO2 ceramic particles, whereas the white region characterizes CG matrix phase. From **Figure 21b**–**d**, it was very clear that a good homogeneous distribution of CG matrix phase was observed in UFG matrix. Further, it can be observed that as the percentage of CG matrix phase increases, the size of CG domain also increases. Due to this, the dislocations move a significant distance without crystallite bound‐ ary interruption when trimodel material is deformed [53, 54]. Therefore, it was expected to

**Figure 19.** Stress distribution at ultimate compressive strength of simulated model of: (a) AA 6061 nanocrystalline alloy, (b) AA 6061‐12 wt.% TiO2 nanocomposite.

Structural Characterization and Mechanical Behavior of Al 6061 Nanostructured Matrix Reinforced with TiO2... http://dx.doi.org/10.5772/65947 29

**Figure 20.** Compressive stress‐strain curves of AA 6061‐12 wt.% TiO2 – y wt.% CG nanocomposites (y=0, 5, 10, 15, 20, 25 and 30): (a) experimental results and (b) finite element results.

**Figure 21.** Trimodel microstructures of as‐sintered AA 6061‐12 wt.% TiO2 composites containing x wt.% *CG* matrix: (a) x=0%, (b) x=10%, (c) x=20% and (d) x=30%. The gray regions represent *UFG* matrix reinforced with nano Titania, the bright regions represent *CG* matrix.

**Figure 19.** Stress distribution at ultimate compressive strength of simulated model of: (a) AA 6061 nanocrystalline alloy,

(b) AA 6061‐12 wt.% TiO2

28 Functionalized Nanomaterials

nanocomposite.

have improvement in ductility of nanocomposite material by incorporating CG matrix. The size of CG matrix phase was not increased up to 15% CG‐blended nanocomposites. The measured average CG size was less than 45 μm up to 15% CG‐blended composites due to non‐coalescence of individual CG matrix particles. Conversely, the average size of CG matrix phase started to increase when CG matrix content increases beyond 15% (**Figure 21c** and **d**). From **Figure 20d**, AA 6061‐12 wt. TiO2‐30% CG nanocomposite is clearly showing a very large size of CG matrix phases due to coalescence of individual CG matrix particles. Witkin et al. [55] have also observed the same behavior while studying the bimodal crystallite size of Al‐Mg alloy (0, 15 and 30% CG matrix phase) prepared by cryomilling and blending. Due to this, a good ductility was observed by slight decrement of compressive strength.

#### **6. Summary and conclusions**

A new nanocomposite of AA 6061 Al alloy reinforced with different weight percentage of titania particles was developed and was successfully synthesized and investigated for its mechanical behavior. It was found that the matrix powder particle size, shape and its morphol‐ ogy, reinforcement particle size and its distribution on the matrix, amount of reinforcement, crystallite size of the matrix and type of microstructures (bimodel and/or trimodel) were reflected and influenced the performance of the fabricated light weight‐based nanocomposite which is suitable for automotive and structural applications.

#### **6.1. Powder surface morphology**

The powder morphology of nanocomposite powders with the function of reinforcement was investigated. It was experimentally found that the incorporation of hard ceramic rein‐ forcements in the nanocomposite powders exhibited drastic changes on the morphological characteristics of MMCs. The produced nanocomposite powders morphology showed that all powder particles regardless of the reinforcement were almost in spherical‐shaped and equiaxed particles which indicated the attainment of steady‐state condition during 40‐h MA. On the other hand, the irregular flake‐like morphology of as‐received soft matrix powder particles was eliminated after 40‐h MA and changed to a shape near the equiaxed. Further, nano‐sized TiO2 particles are completely embedded in soft matrix of nanocomposite powder after 40 MA. There is no way of forming of reinforcement clustering in nanocomposite pow‐ der via MA during the time taken (40 h) for the present investigation.

#### **6.2. Structural evaluation**

The structural characteristics of ball‐milled mechanically alloyed powders were investi‐ gated and reported. These results confirmed the occurrence of structural refinement in nanocomposites with the function of reinforcement through mechanical alloying, and hence, decreased crystallite size was observed. The MA time selected for this study produced the crystallite size of around 46 and 65 nm for 12 wt.% reinforced particulate nanocomposite and unreinforced nanocrystallite powders, respectively. The nanocrystalline nature of the matrix powder particle and aggregation of nanocomposite powder particle was confirmed by TEM analysis.

#### **6.3. Mechanical strength and sintering behavior**

have improvement in ductility of nanocomposite material by incorporating CG matrix. The size of CG matrix phase was not increased up to 15% CG‐blended nanocomposites. The measured average CG size was less than 45 μm up to 15% CG‐blended composites due to non‐coalescence of individual CG matrix particles. Conversely, the average size of CG matrix phase started to increase when CG matrix content increases beyond 15% (**Figure 21c** and **d**). From **Figure 20d**, AA 6061‐12 wt. TiO2‐30% CG nanocomposite is clearly showing a very large size of CG matrix phases due to coalescence of individual CG matrix particles. Witkin et al. [55] have also observed the same behavior while studying the bimodal crystallite size of Al‐Mg alloy (0, 15 and 30% CG matrix phase) prepared by cryomilling and blending. Due to this, a

A new nanocomposite of AA 6061 Al alloy reinforced with different weight percentage of titania particles was developed and was successfully synthesized and investigated for its mechanical behavior. It was found that the matrix powder particle size, shape and its morphol‐ ogy, reinforcement particle size and its distribution on the matrix, amount of reinforcement, crystallite size of the matrix and type of microstructures (bimodel and/or trimodel) were reflected and influenced the performance of the fabricated light weight‐based nanocomposite

The powder morphology of nanocomposite powders with the function of reinforcement was investigated. It was experimentally found that the incorporation of hard ceramic rein‐ forcements in the nanocomposite powders exhibited drastic changes on the morphological characteristics of MMCs. The produced nanocomposite powders morphology showed that all powder particles regardless of the reinforcement were almost in spherical‐shaped and equiaxed particles which indicated the attainment of steady‐state condition during 40‐h MA. On the other hand, the irregular flake‐like morphology of as‐received soft matrix powder particles was eliminated after 40‐h MA and changed to a shape near the equiaxed. Further,

after 40 MA. There is no way of forming of reinforcement clustering in nanocomposite pow‐

The structural characteristics of ball‐milled mechanically alloyed powders were investi‐ gated and reported. These results confirmed the occurrence of structural refinement in nanocomposites with the function of reinforcement through mechanical alloying, and hence, decreased crystallite size was observed. The MA time selected for this study produced the crystallite size of around 46 and 65 nm for 12 wt.% reinforced particulate nanocomposite and unreinforced nanocrystallite powders, respectively. The nanocrystalline nature of the matrix powder particle and aggregation of nanocomposite powder particle was confirmed by TEM

der via MA during the time taken (40 h) for the present investigation.

particles are completely embedded in soft matrix of nanocomposite powder

good ductility was observed by slight decrement of compressive strength.

which is suitable for automotive and structural applications.

**6. Summary and conclusions**

30 Functionalized Nanomaterials

**6.1. Powder surface morphology**

nano‐sized TiO2

analysis.

**6.2. Structural evaluation**

Nanostructured AA 6061 Al alloy reinforced with different weight percentage of TiO2 particles processed via MA followed by cold consolidation and sintering yielded maximum strength (hardness) of around 1.126 GPa of matrix grain size of 200–300 nm (*UFG*) embedded with nano‐ sized Titania (∼119 nm). This was attributed to structural refinement contributed from various strengthening mechanisms such as grain size, dislocation, solid solution and dispersion strength‐ ening. Here, the matrix to reinforcement particle size ratio of nanocomposite was greater than unity and even very higher value. However, excellent behavior in terms of densification, sinter‐ ability and Vickers hardness was obtained. These results indicated that the effect of matrix to reinforcement particle size ratio was insignificant in the case of nanocomposite because there was no possibility of reinforcement clusters while manufacturing of MMCs via MA.

#### **6.4. Compressive stress-strain behavior**

The compressive stress‐strain curves of nanocomposites were started to increase drasti‐ cally with the function of reinforcement which indicated the enhancement of compres‐ sive strength while it exhibited the decreased value of ductility with the function of reinforcement. These results were attributed to more crystallite refinement and complete embedding of titania ceramic particles in the matrix. A trimodel grain size distribution for enhancing the ductility and toughness of high strength AA 6061–TiO2 nanocomposites was fabricated successfully and the uniaxial compressive deformation behavior through simple compression test at room temperature was studied, analyzed and reported. These results explained that the addition of coarse crystallite powder particles in the nanostructured materials would enhance the ductility, delay the plastic instability and enhance toughness. In other words, the addition of CG matrix phase decelerates the micro‐crack nucleation cum proliferation in nanostructured phase during deformation. Therefore, the ductility in nanocomposite materials can be improved which would be suitable for manufacturing of automotive and structural components. Here, the 30% CG‐incorporated nanocompos‐ ite exhibited improved ductility of around 500% more than that of 0% CG incorporated nanocomposite.

#### **Author details**

S. Sivasankaran\* and Abdulaziz S. Alaboodi

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Mechanical Engineering, College of Engineering, Qassim University, Kingdom of Saudi Arabia

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#### **Fungal-Derived Nanoparticles as Novel Antimicrobial and Anticancer Agents Fungal-Derived Nanoparticles as Novel Antimicrobial and Anticancer Agents**

Muhammad Waseem and Muhammad Atif Nisar Muhammad Waseem and Muhammad Atif Nisar

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

http://dx.doi.org/10.5772/66922

#### **Abstract**

In order to control microbial resistance against commonly used antibiotics, it is indispensable to develop novel and efficient antimicrobial agents. For this purpose, metallic nanoparticles (mainly inorganic) with their antimicrobial activites represent an effective solution for this global problem. However, synthesis of nanoparticles involves the use of expensive, poisonous and dangerous chemicals responsible for different biological and environmental hazards. This fact increases the necessity of developing environmentfriendly procedure by means of green synthesis (using plants) and extra-biological methods (using microbes such as bacteria and fungi). More recently, metallic nanoparticles, derived from fungal sources, have demonstrated their potential not only as a new-generation antimicrobial agents but also as anticancer agents. Therefore, this chapter is aimed to explore the various nanoparticles producing fungi with ultimate objective of elucidating the possible (i) mechanism of biosynthesis of metallic NPs by various fungi and (ii) mode of action of these mycosynthesized NPs on bacterial cell. This chapter would certainly increase our knowledge about interaction of nanoparticles with bacterial cell for their use in health biotechnology.

**Keywords:** nanoparticles, silver, myconanosynthesis, fungi, cancer, antibacterial activities

#### **1. Introduction**

In the field of nanotechnology and nanomaterial, more specifically, nanoparticles are extremely important because of their unique optical, physicochemical, and biological facets. Though their biological synthesis is still at early stages and many material scientists around the globe are working on the production of nanoparticles from different sources including plants, animals, microbes, and metallic compounds like gold, silver, platinum, etc. Various studies on biosyn-

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

thesis of nanoparticles have been carried out using a wide array of microorganisms such as algae, bacteria, actinomycetes, fungi, yeasts, and viruses [1, 2]. Among the many possible bioresources, biologically active products from fungi and yeast represent excellent scaffolds for this purpose. Since fungi and yeast are very effective secretors of extracellular enzymes and number of species grow fast and therefore culturing and keeping them in the laboratory is very simple [3]. They are able to produce metal nanoparticles and nanostructure via reducing enzyme extracellularly [3, 4]. Not only the harvesting of extracellularly synthesized nanoparticles from fungi is easy and inexpensive [2, 5], they can also be manipulated by controlling the pH, temperature, substrate concentration (metal ions), and reaction time [6, 7]. Therefore, the biosynthesis could be ideally used for large-scale production of nanoparticles for several industrial applications [3]. Importantly, fungi have also secreted fairly large amount of proteins and secondary metabolites extracellularly, and hence, the fungal biomass could reduce the metal ions more easily leading to the rapid formation of nanoparticles [8]. Because of these advances, the myco-based extracellular synthesis method is often considered as a better resource for higher productivity of nanoparticles [4].

On the other hand, increasing incidence of microbial resistance to clinically approved classes of antibiotics has emerged in recent years and is a major health problem, requiring to develop novel and effective antimicrobial agents [4, 9–11]. Owing to their antibacterial activities, metallic nanoparticles (mainly silver and gold) represent a novel and an effective solution for overcoming bacterial resistance. In general, nanoparticles are divided into two groups, i.e., organic and inorganic, where the later one being significant biomedical agents [12]. These silver and gold nanoparticles, derived from microbial sources, have demonstrated their potential not only as a new generation antimicrobial [13] agents against a broad spectrum of Gram-positive and Gram-negative bacteria including multidrug-resistant human pathogens, but also as anticancer agents [3]. They have also significantly enhanced the bactericidal activity when used in combination with standard antibiotics [14]. However, synthesis of nanoparticles involves the use of expensive, poisonous, and dangerous chemicals responsible for different biological and environmental hazards. This fact increases the necessity of developing environment-friendly procedure by means of green synthesis (using plants) and extra biological methods (using microbes such as bacteria and fungi) [15]. Therefore, as a part of our continuing search to identify microorganisms with the potential to synthesize nanoparticles with amazing biological properties, interest has spurred on microbes [11], especially fungi. Indeed a few fungal species around the globe have been reported to secrete nanoparticles. Therefore, the present chapter is aimed to provide an overview of nanoparticles producing fungi, with the ultimate objective of producing effective antimicrobial agents.

#### **2. Biosynthesis of nanoparticles by fungi**

In the last decade, biomineralization has been developed as an emerging and attractive technique for the synthesis of metallic nanoparticles. In which microbial cells are preferably being used for the synthesis of nanosized material involving the oxidation/reduction of metallic ions (like silver, gold, platinum, etc.) giving rise to nanostructures and nanoparticles [16]. This green synthesis of metallic nanoparticles undergo three necessary steps (**Figure 1**) that are (i) choice of appropriate solvent medium, (ii) choice of appropriate reducing agent (environment friendly), and (iii) stabilizing agents or substances for silver nanoparticles stability [17]. Generally, this reduction is carried out by means of microbial enzymes (**Figure 1**), present in microbes, and considered to be responsible for synthesis of nanoparticles by those organisms [18, 19].

For this purpose, various fungi have been used as novel and assuring resources for manufacturing of nanoparticles extracellularly as well as intracellularly, such as *Aspergillus, Pencillium, Fusarium*, and *Verticillium*. Though synthesis of metallic nanoparticles (NPs) having defined size, shape, and composition by fungi are major challenges in this field [20, 21]. In general, fungal enzymes with reducing capabilities are mostly responsible for reduction of metal ions (**Figure 1**) to their corresponding nanoparticle(s) as described earlier [22]. We will discuss biosynthesis of NPs from various fungi one by one.

#### **2.1. Biosynthesis of nanoparticles by** *Fusarium*

thesis of nanoparticles have been carried out using a wide array of microorganisms such as algae, bacteria, actinomycetes, fungi, yeasts, and viruses [1, 2]. Among the many possible bioresources, biologically active products from fungi and yeast represent excellent scaffolds for this purpose. Since fungi and yeast are very effective secretors of extracellular enzymes and number of species grow fast and therefore culturing and keeping them in the laboratory is very simple [3]. They are able to produce metal nanoparticles and nanostructure via reducing enzyme extracellularly [3, 4]. Not only the harvesting of extracellularly synthesized nanoparticles from fungi is easy and inexpensive [2, 5], they can also be manipulated by controlling the pH, temperature, substrate concentration (metal ions), and reaction time [6, 7]. Therefore, the biosynthesis could be ideally used for large-scale production of nanoparticles for several industrial applications [3]. Importantly, fungi have also secreted fairly large amount of proteins and secondary metabolites extracellularly, and hence, the fungal biomass could reduce the metal ions more easily leading to the rapid formation of nanoparticles [8]. Because of these advances, the myco-based extracellular synthesis method is often considered as a better

On the other hand, increasing incidence of microbial resistance to clinically approved classes of antibiotics has emerged in recent years and is a major health problem, requiring to develop novel and effective antimicrobial agents [4, 9–11]. Owing to their antibacterial activities, metallic nanoparticles (mainly silver and gold) represent a novel and an effective solution for overcoming bacterial resistance. In general, nanoparticles are divided into two groups, i.e., organic and inorganic, where the later one being significant biomedical agents [12]. These silver and gold nanoparticles, derived from microbial sources, have demonstrated their potential not only as a new generation antimicrobial [13] agents against a broad spectrum of Gram-positive and Gram-negative bacteria including multidrug-resistant human pathogens, but also as anticancer agents [3]. They have also significantly enhanced the bactericidal activity when used in combination with standard antibiotics [14]. However, synthesis of nanoparticles involves the use of expensive, poisonous, and dangerous chemicals responsible for different biological and environmental hazards. This fact increases the necessity of developing environment-friendly procedure by means of green synthesis (using plants) and extra biological methods (using microbes such as bacteria and fungi) [15]. Therefore, as a part of our continuing search to identify microorganisms with the potential to synthesize nanoparticles with amazing biological properties, interest has spurred on microbes [11], especially fungi. Indeed a few fungal species around the globe have been reported to secrete nanoparticles. Therefore, the present chapter is aimed to provide an overview of nanoparticles producing fungi, with the ultimate

In the last decade, biomineralization has been developed as an emerging and attractive technique for the synthesis of metallic nanoparticles. In which microbial cells are preferably being used for the synthesis of nanosized material involving the oxidation/reduction of metallic ions (like silver, gold, platinum, etc.) giving rise to nanostructures and nanoparticles [16].

resource for higher productivity of nanoparticles [4].

38 Functionalized Nanomaterials

objective of producing effective antimicrobial agents.

**2. Biosynthesis of nanoparticles by fungi**

Several fungal strains belonging to genera *Fusarium* have been studied for the production of metallic nanoparticles. Dias and coworkers [23] have screened different *Fusarium* species to be used as potential candidate(s) for biosynthesis of silver NPs. They have obtained the tiniest size silver-nanoparticles (Ag-NPs) by *Fusarium oxysporum* through extracellular reduction of Ag+ ions in aqueous medium [24–26]. Generally, two methods were used to evaluate the synthesis of silver nanoparticles by *Fusarium* species involving (i) silver reduction method that involve the fungus biomass in conical flask containing AgNO3 solution and distilled water. In other method, fungal filtrate were obtained by keeping fungal biomass in distilled water for 72 h at 28°C that was further challenged with AgNO3 solution (10−3 M). In this study, several strains of *Fusarium oxysporum* were exposed to AgNO3 solution (10−3 M) leading to the reduction of silver ions and thereby formation of silver hydrosol. The silver nanoparticles produced

**Figure 1.** Intracellular and extracellular biosynthesis of metallic nanoparticles (NPs) by fungi.

were in range of 20–50 nm in diameter as a result of nitrate-dependent reductase and shuttle quinone extracellular activities [27]. Similar study was also performed using similar fungal species, i.e., *Fusarium oxysporum* by Mohammadian et al. [28] who observed that synthesis of silver nanoparticles was extremely rapid even at ambient conditions that can compete with chemical synthesis, highly stable even for months in darkness (without light). To date, various *Fusarium* species have been reported to produce metallic nanoparticles for silver [13, 29–33], zirconia [34, 35], platinum NPs intra- and extracellularly [13, 32].

#### **2.2. Biosynthesis of nanoparticles by** *Penicillium*

Certain filamentous fungi such as *Penicillium* were also investigated for the synthesis of metallic nanoparticles. For this purpose, various studies have been carried out using different species of *Penicillium* including (but not limited to) *P. brevicompactum* [36], *P. fellutanum* [37], *Penicillium* sp. [38], *P. citrinum* [39], *P. fellutanum* [36], and *P. purpurogenum* [40]. Nanoparticles produced by *Penicillium* spp. are relatively more stable at neutral pH (>8.0) due to repulsion as they possess zeta potential (negative) [41]. In another study, where *P. fellutanum* was challenged with silver ions, it could reduce metallic ions in 10 min, when it came in contact with fungus filtrate [37]. Presence of single protein having molecular weight of about 70 kDa was observed in gel electrophoreses. The authors supposed that nitrate reductase enzymes could possibly be responsible for the reduction of silver ions (**Figure 1**). In addition, the whole reduction process could be manipulated by controlling concentration and exposure time to Ag+ ions, pH, and temperature [37, 42]. Although exact mechanism commanding the Ag+ ions reduction is not yet comprehended, however, authors have assumed that reducing agents are secreted by fungi as a response to stress posed by metal ions [40, 42]. Moreover, these silver NPs were uniformly distributed over fungal mycelium (*Penicillium* sp.) suggesting that silver NPs were bound with cells of fungal surface. This was further confirmed by FTIR spectroscopy. In which it was described that both carbonyl groups from amino acid residues and peptides of protein(s) had strong affinities for binding with Ag+ ions in order to form protective covering around silver NPs to avoid their clustering [36].

#### **2.3. Biosynthesis of nanoparticles by** *Aspergillus*

Biosynthesis of NPs from species belonging to genera *Aspergillus* is more desirable and economically important due to its occurrence in natural habitat along with easy culturing on growth media [43]. In the same study, five species belonging to genus *Aspergillus*, i.e., *A. nidulans, A. flavus, A. terreus, A. fumigatus*, and *A. niger*, were synthesized, in which *A. terreus* was proved to be more potent NPs producer among all and was further investigated. It was observed that silver NPs were highly stable even after 4 months due to the presence of stabilizing agents mostly "protein" that would allow functionalizing the NPs with other biological molecules [44]. Their proper functionalizing is an important factor for appropriate antimicrobial activities of these metallic nanoparticles (NPs). In addition, FTIR analyses revealed amide linkages between amino acid residues and NPs. Most active functional groups involved in the reduction of silver ions being the hydroxyl, carboxyl, and carbonyl groups [10, 43, 45–48]. Depending on the biological system used, different sizes and shapes of NPs had been reported ranging from 5 to 45 nm [49] with spherical NPs being the dominant [43].

In another study, AgNP were synthesized using aqueous supernatant of *A. niger*. Synthesized silver NPs were mainly round shape ranging in size from 1 to 20 nm [50]. Various studies have shown the mycosynthesis of silver nanoparticles extracellularly through the reduction of Ag+ ions using different species belonging to genus *Aspergillus*, such as *A. niger* [51], *A. flavus* [52], *A. terreus* [53], *A. clavatus* [54, 55], *A. fumigatus* [56], and *A. tamari* [57]. Majority of these NPs were in the range of 1–20 nm with spherical shape.

#### **2.4. Biosynthesis of nanoparticles by other fungi**

were in range of 20–50 nm in diameter as a result of nitrate-dependent reductase and shuttle quinone extracellular activities [27]. Similar study was also performed using similar fungal species, i.e., *Fusarium oxysporum* by Mohammadian et al. [28] who observed that synthesis of silver nanoparticles was extremely rapid even at ambient conditions that can compete with chemical synthesis, highly stable even for months in darkness (without light). To date, various *Fusarium* species have been reported to produce metallic nanoparticles for silver [13, 29–33],

Certain filamentous fungi such as *Penicillium* were also investigated for the synthesis of metallic nanoparticles. For this purpose, various studies have been carried out using different species of *Penicillium* including (but not limited to) *P. brevicompactum* [36], *P. fellutanum* [37], *Penicillium* sp. [38], *P. citrinum* [39], *P. fellutanum* [36], and *P. purpurogenum* [40]. Nanoparticles produced by *Penicillium* spp. are relatively more stable at neutral pH (>8.0) due to repulsion as they possess zeta potential (negative) [41]. In another study, where *P. fellutanum* was challenged with silver ions, it could reduce metallic ions in 10 min, when it came in contact with fungus filtrate [37]. Presence of single protein having molecular weight of about 70 kDa was observed in gel electrophoreses. The authors supposed that nitrate reductase enzymes could possibly be responsible for the reduction of silver ions (**Figure 1**). In addition, the whole reduction process could be manipulated by controlling concentration and exposure time to

ions, pH, and temperature [37, 42]. Although exact mechanism commanding the Ag+

reduction is not yet comprehended, however, authors have assumed that reducing agents are secreted by fungi as a response to stress posed by metal ions [40, 42]. Moreover, these silver NPs were uniformly distributed over fungal mycelium (*Penicillium* sp.) suggesting that silver NPs were bound with cells of fungal surface. This was further confirmed by FTIR spectroscopy. In which it was described that both carbonyl groups from amino acid residues and pep-

Biosynthesis of NPs from species belonging to genera *Aspergillus* is more desirable and economically important due to its occurrence in natural habitat along with easy culturing on growth media [43]. In the same study, five species belonging to genus *Aspergillus*, i.e., *A. nidulans, A. flavus, A. terreus, A. fumigatus*, and *A. niger*, were synthesized, in which *A. terreus* was proved to be more potent NPs producer among all and was further investigated. It was observed that silver NPs were highly stable even after 4 months due to the presence of stabilizing agents mostly "protein" that would allow functionalizing the NPs with other biological molecules [44]. Their proper functionalizing is an important factor for appropriate antimicrobial activities of these metallic nanoparticles (NPs). In addition, FTIR analyses revealed amide linkages between amino acid residues and NPs. Most active functional groups involved in the reduction of silver ions being the hydroxyl, carboxyl, and carbonyl groups [10, 43, 45–48]. Depending on the biological system used, different sizes and shapes of NPs had been reported

ions

ions in order to form protective

zirconia [34, 35], platinum NPs intra- and extracellularly [13, 32].

tides of protein(s) had strong affinities for binding with Ag+

ranging from 5 to 45 nm [49] with spherical NPs being the dominant [43].

covering around silver NPs to avoid their clustering [36].

**2.3. Biosynthesis of nanoparticles by** *Aspergillus*

**2.2. Biosynthesis of nanoparticles by** *Penicillium*

40 Functionalized Nanomaterials

Ag+

Various other fungal genera were also exploited for the synthesis of silver nanoparticles. Metallic NPs can be synthesized using fungal biomass and cell-free supernatant. Among eukaryotes, yeast was repeatedly explored for the formation of metallic nanoparticles. Since then, many studies were carried out in order to evaluate the fungal potential as potent producer of silver nanoparticles. For example, cadmium nanoparticles (CdS) were synthesized intracellularly by *Schizosaccharomyces pombe* and *Candida glabrata* when challenged with Cd+2 ions [42, 58–60]. The authors proposed that these NPs were covered with phytochelatin [42]. Similarly, other studies have also reported the biosynthesis of metallic nanoparticles using fungi such as Toluropsis sp. for Pb nanocrystals [60] and for Cu NPs using *Humicola* sp. [61]. Formation of gold nanoparticles was described in yeast Yarrowia lipolytica NCIM 3589 that were highly stable [62, 63]. As described earlier, shape and size of NPs mainly rely on the reaction conditions like temperature, pH, metal concentrations, and reaction time [16].

However, first example of intracellular mycosynthesis of silver and gold NPs was demonstrated using *Verticillium sp*. biomass, challenged with aqueous silver ions [64, 65]. Although the mechanism of biosynthesis was not clearly understood, however, it was assumed that metal ions (silver or gold) either (i) adsorbed on cell surface or enter into cytoplasm via diffusion. In both cases, these metallic ions were reduced by enzymes present on the cellular membrane and/or cytoplasm [64, 65]. Similarly, different species belonging to genus *Trichoderma* and *Phoma* were also screened for the synthesis of metallic NPs with dimensions ranging from 8 to 60 nm [66, 67] for the selection of potential species as a new synthesizer of silver NPs.

#### **3. Application of mycosynthesized nanoparticles in biomedics**

Mycosynthesized nanoparticles are of extreme importance because of their unique optical, physicochemical and biological facets. These nanosized particles are continuously being used in different fields including electronics [68], catalytic processes [69], optical devices [12], sensor technology [70], biological labeling [71], and may suppress the expression of proteins associated with adenosine triphosphate production [72], agriculture, pharmacology, and environmental monitoring [73]. More recently they are being used as novel antimicrobial and anticancer agents [61, 74–82]. Antimicrobial mediators can be either synthetic and/or partially modified natural compounds [82, 83]. Type of antimicrobials agent(s) may vary according to targeted pathogens, e.g., antibacterial (for bacteria), antifungal (for fungi), and antiviral (for virus). Similarly, their mechanism of action on microbial cell varies according to the nature of the antimicrobial agents and pathogens. Main targets for antimicrobial compounds could be

**Figure 2.** Possible mechanism of action of silver NPs on bacterial cell.

cell wall (cephalosporins, bacitracin, penicillins, etc.) and protein synthesis, DNA replication (tetracyclines, chloramphenicol, lincosamides, etc.) and metabolism (intermediary) processes (valinomycin, Gramicidin A, etc.), and DNA-synthesis (rifampicin, sulfonamides, quinolones, etc.) as described in **Figure 2** [82]. However, abusive use of antimicrobials against various pathogens has evolved into the development of highly adapted and resistant microbes. In addition, these resistant microbes are now being spread globally, challenging the treatment to common infections, and posing significant health threat around the globe. Multidrug-resistant bacteria, such as methicillin-resistant *Staphylococcus aureus* (MRSA) and vancomycin-resistant enterococci, have emerged as a symbol of global health hazard during the last decade.

#### **4. Antibacterial activity of mycosynthesized nanoparticles**

Uses of nanoparticles in medicine, particularly in the treatment of bacterial infections, have proved them as effective and novel healing tools against microorganisms. Though for centuries, natural silver has already been considered as one of the most safe and nontoxic inorganic antimicrobial agents [84]. Even silver nanoparticles have also exhibited broad-spectrum antimicrobial properties due to its unique physiochemical and biological characteristics [85]. Recently, silver NPs have shown excellent antimicrobial efficacy against certain bacterial pathogens such as *Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Staphylococcus epidermis, Leuconostoc mesenteroides, Klebsiella pneumonia*, and *K. mobilis* [84]. Reason for their higher antimicrobial efficacy lies with higher surface area-to-volume ratio of AgNPs that enables them to bring large portion of Ag atoms in contact with the environment (i.e., microbial cell). However, mechanism of antibacterial activity of silver NPs has not been completely understood [84–88].

Silver nanoparticles have shown inhibitory effect on growth in both Gram-positive and Gram-negative bacteria. As described earlier, larger surface area of silver nanoparticles are believed to be key player in interaction with bacterial cell wall and would have resulted in more antibacterial activity. Although mechanism of action of silver NPs on bacterial cell is debatable; however, assumed mechanism include (i) adherence of AgNPs with bacterial cell wall causing membrane lysis, ultimately releasing cellular contents leading to destruction of cell structure and death [89–91], (ii) interaction of Ag ions with free thiol groups (−SH) and disulfide bridges (-S-S-) of cellular enzymes and membrane of bacteria to downregulate or inactivate the cellular protein synthesis by interfering the translation [72, 91, 92], (iii) blocking DNA replication [93], and (iv) inhibition of signal transduction by dephosphorylation of peptide substrates on tyrosine residues (**Figure 2**) [91]. Furthermore, inhibitory effect of AgNPs varied for different bacterium, e.g., Gram-positive bacteria were more susceptible to silver nanoparticles as compared to Gram-negative one [84]. The authors have found larger inhibition zone (diameter) in case of *S. aureus* than that of *E. coli* suggesting that variation in inhibitory response could be attributed to difference in cell wall and capsular composition, S-layer thickness, cellular metabolism and its byproducts, or a combination of these [84].

#### **5. Anticancer potential of mycosynthesized nanoparticles**

cell wall (cephalosporins, bacitracin, penicillins, etc.) and protein synthesis, DNA replication (tetracyclines, chloramphenicol, lincosamides, etc.) and metabolism (intermediary) processes (valinomycin, Gramicidin A, etc.), and DNA-synthesis (rifampicin, sulfonamides, quinolones, etc.) as described in **Figure 2** [82]. However, abusive use of antimicrobials against various pathogens has evolved into the development of highly adapted and resistant microbes. In addition, these resistant microbes are now being spread globally, challenging the treatment to common infections, and posing significant health threat around the globe. Multidrug-resistant bacteria, such as methicillin-resistant *Staphylococcus aureus* (MRSA) and vancomycin-resistant

enterococci, have emerged as a symbol of global health hazard during the last decade.

Uses of nanoparticles in medicine, particularly in the treatment of bacterial infections, have proved them as effective and novel healing tools against microorganisms. Though for centuries, natural silver has already been considered as one of the most safe and nontoxic inorganic antimicrobial agents [84]. Even silver nanoparticles have also exhibited broad-spectrum antimicrobial properties due to its unique physiochemical and biological characteristics [85]. Recently, silver NPs have shown excellent antimicrobial efficacy against certain bacterial pathogens such as *Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Staphylococcus epidermis, Leuconostoc mesenteroides, Klebsiella pneumonia*, and *K. mobilis* [84]. Reason for their higher antimicrobial efficacy lies with higher surface area-to-volume ratio of AgNPs that enables

**4. Antibacterial activity of mycosynthesized nanoparticles**

**Figure 2.** Possible mechanism of action of silver NPs on bacterial cell.

42 Functionalized Nanomaterials

About 10 million people are being diagnosed with cancer annually, with higher mortality rate around the globe [94]. It is generally developed due to disturbance in various physiological processes like cell signaling and apoptosis [94]. In this disease, continuous growth of cancer cell due to resistance to wide range of anticancer agents has become the major challenge to pharmacist while treating the disease [95, 96]. Certain factors such as insufficient drug concentrations approaching tumor cells, intolerable cytotoxicity, nonspecific distribution of anticancer agents, and poor monitoring of drug responses particularly in developing countries has made cancer incurable [97, 98].

Recently, the emerging field of nano-biomedicine has highlighted the possibility of metallic NPs to be used for disease diagnosis and cancer treatment in humans [94]. Among various metallic NPs, silver nanoparticles are considered as novel and promising nanoproducts that can be exploited in the field of nanomedicine due to their unique and distinctive properties. Although antibacterial activity of Ag nanoparticles is well established, however, their anticancer activities have recently been reported against various cancerous cell lines [94, 99], and mechanism of action of AgNPs are still under evaluation and there are many ambiguities that should be addressed in future research.

It is well established that silver nanoparticles when interacting with various biomolecules, such as DNA, proteins, and carbohydrates, may cause cell death due to apoptosis or cytotoxicity probably through reactive oxygen species (ROS) [32, 94]. Various factors influencing cytotoxicity of silver nanoparticles include dosage, contact time, and size of particles. For example, in case of human epidermoid larynx (Hep-2) cell line, cellular damage was dose dependent, causing cell death through ROS [96, 100, 101]. In another study, silver nanoparticles have shown cytotoxic effect through loss of mitochondrial integrity and activation of caspase-cascade exhibiting the apoptotic effect leading to cell death. In a recent study, AgNPs from plant extract, inhibited proliferation of human colon cancer cell line HCT15 while suppressing its cellular growth, inhibiting the G0 /G1 -phase, interfering DNA synthesis, and ultimately causing cell death through apoptosis [102]. Another important criterion for efficient anticancer drug is its ability to induce apoptosis cancer cell since they generally escape from programed cell death [103]. In recent studies it has been shown that silver nanoparticles possess this property [104]. The authors have treated the tumor-bearing mice (*in vivo*) with AgNPs that have reduced the tumor weight and increased its life span. The hematologic studies revealed reduced white blood cell (WBC) and platelet count in diseased mice as compared to control (disease free) suggesting that silver NPs were relatively nontoxic and did not produce any change in hematologic parameters and controlled the WBC that were important constituent of immune system of body. The authors further proposed that silver NPs could be used as potent therapeutic agents in order to delay the tumor progression in DLA cell lines through growth suppression and cytotoxic effects that increased vascular permeability leading to tumor cell death that could be linked to caspase enzyme activation. However, mode of action of silver NPs due to which they inhibit cell proliferation and viability causing tumor cell death has not been clearly understood.

#### **6. Conclusion**