glass forming batches for the canasite - and potassium fluorrichterite - containing glass - ceramic precursor glasses are easily melted and can be shaped into articles of widely - varying geometries using such conventional glass forming processes as drawing , precision rolling , pressing , and spinning . the properties of the resultant glass - ceramics rendering them eminently suitable for high performance rigid disk substrates include : ( 1 ) high body strengths and fracture toughnesses as reflected in moduli of rupture ranging about 15 , 000 - 40 , 000 psi and k ic values between about 3 - 5 mpa √ m . that combination of properties allows the fabrication of very thin , rigid disks , i . e ., ≈ 0 . 5 - 0 . 6 mm in thickness . ( 2 ) an elastic modulus similar to that of aluminum metal such that the material is dimensionally stable at all thicknesses and rotational velocities . ( 3 ) a hard surface hardness ( knoop hardness & gt ; 500 in contrast with & lt ; 100 for aluminum ) which imparts high scratch resistance . ( 4 ) precision ( flat and smooth ) surfaces are readily available for tight , consistent control of properties . ( 5 ) a thermal expansion that can be carefully controlled through composition between about 75 - 125 × 10 - 7 /° c . over the temperature range 25 °- 300 ° c ., the higher value approaching that of stainless steel . ( 7 ) a body with an inherently textured surface of the desired roughness ( r a = 1 . 5 - 5 nm ) for enhanced magnetics and lower flying height . feature ( 7 ) is of special value inasmuch as no texturizing operation is required , either mechanical or chemical , to bring the surface in condition for application of the magnetic medium . this factor permits a major cost savings to be enjoyed over the conventional procedures necessary for placing the surface of aluminum or glass in condition to receive the magnetic medium . furthermore , we have found that the surface texture or &# 34 ; roughness &# 34 ; of the inventive glass - ceramics can be tailored to yield a r a value within the range of about 1 - 6 nm by means of minor adjustments in composition , especially through changes in levels of fluoride . of course , it will be appreciated that the surface texture can also be modified through mechanical or chemical means , e . g ., by grinding and polishing or chemical etching . two tests have been devised to determine the utility of a material as a rigid disk substrate . a start / stop test measures the &# 34 ; stiction &# 34 ; coefficient as the disk is started and stopped repeatedly over tens of thousands of cycles . a take - off and landing velocity test measures two parameters ; viz ., the rotational velocity of the disk when the head lifts off the surface , and the distance the head slides on the surface of the disk before it lifts off . those two parameters are clearly related to the acceleration of the disk as it begins to rotate . in the start / stop test , aluminum disks typically exceed the industry - specified &# 34 ; stiction &# 34 ; coefficient limit of 0 . 5 at 30 , 000 cycles . in contrast , disks utilizing substrates prepared from compositions of the three abovedescribed , chain silicate systems of glass - ceramics can exceed 100 , 000 cycles without reaching the &# 34 ; stiction &# 34 ; coefficient of 0 . 5 . in one start / stop test utilizing a canasite glass - ceramic substrate , the &# 34 ; stiction &# 34 ; coefficient of 0 . 5 was not reached in over 200 , 000 cycles . it is also worthy of note that inspection of the magnetic heads employed in the start / stop tests using that glass - ceramic substrate indicated that no more material was transferred to the heads after 200 , 000 cycles than the amount which was transferred thereto from the aluminum disks after only 30 , 000 cycles . in the take - off and landing velocity tests , aluminum disks customarily exhibit a sliding distance of about 57 inches (≈ 145 cm ), whereas disks prepared from the three above - described , chain silicate glass - ceramics can demonstrate a sliding distance of less than 12 inches (≈ 30 cm ), with the above - described canasite glass - ceramic substrate exhibiting a sliding distance of no more than about 7 inches (≈ 18 cm ). the table below records three glass compositions , expressed in terms of parts by weight on the oxide basis , which , when crystallized , illustrate the three chain silicate glass - ceramic systems operable in forming substrates complying with the above - described vital criteria . inasmuch as it is not known with which cation ( s ) the fluoride is combined , it is merely tabulated in terms of fluoride , in accordance with conventional glass and glass - ceramic analysis practice . furthermore , because the sum of the individual constituents in each glass closely approximates 100 , for all practical purposes the reported values in the table may be deemed to represent weight percent . the actual batch ingredients can comprise any materials , either oxides or other compounds , which , when melted together , will be converted into the desired oxides in the proper proportions . for example , caco 3 and mgco 3 can serve as the batch materials for cao and mgo , respectively . the table also recites the crystal phase ( s ) generated in the glasses upon heat treatment to a glass - ceramic , along with values , where measured , of coefficient of thermal expansion [ exp . coef .] ( 25 °- 300 ° c .) expressed in terms of x 10 - 7 /° c ., modulus of rupture [ mor ] expressed in terms of psi , k ic expressed in terms of mpa √ m , knoop hardness , and surface roughness [ r a ] expressed in terms of nanometers . table______________________________________ 1 2 3sio . sub . 2 57 . 3 65 . 6 67 . 6al . sub . 2 o . sub . 3 2 . 0 2 . 0 1 . 6mgo -- 14 . 8 14 . 1cao 20 . 2 6 . 2 4 . 6na . sub . 2 o 8 . 0 3 . 0 3 . 2k . sub . 2 o 8 . 8 4 . 2 4 . 7li . sub . 2 o -- 0 . 8 0 . 88bao -- 0 . 25 0 . 22p . sub . 2 o . sub . 5 -- 0 . 75 1 . 2f 6 . 3 4 . 2 3 . 3 potassium potassium fluorrichterite canasite fluorrichterite cristobalitecrystals minor caf . sub . 2 minor mica minor micaexp . coef . 120 75 120mor 40 , 000 25 , 000 20 , 000k . sub . ic 5 . 0 -- 3 . 0knoop & gt ; 500 & gt ; 500 & gt ; 500r . sub . a 1 - 6 1 - 6 1 - 6______________________________________ based upon the overall combination of physical properties , glass - ceramics wherein canasite constitutes the predominant crystal phase ( illustrated in example 1 above ) comprise the most preferred substrate material . in the take - off and landing velocity tests , aluminum disks customarily exhibit a sliding distance of about 57 inches (≈ 145 cm ), whereas disks prepared from the three above - described , chain silicate glass - ceramics can demonstrate a sliding distance of less than 12 inches (≈ 30 cm ), with the above - described canasite glass - ceramic substrate exhibiting a sliding distance of no more than about 7 inches (≈ 18 cm ). the table below records three glass compositions , expressed in terms of parts by weight on the oxide basis , which , when crystallized , illustrate the three chain silicate glass - ceramic systems operable in forming substrates complying with the above - described vital criteria . inasmuch as it is not known with which cation ( s ) the fluoride is combined , it is merely tabulated in terms of fluoride , in accordance with conventional glass and glass - ceramic analysis practice . furthermore , because the sum of the individual constituents in each glass closely approximates 100 , for all practical purposes the reported values in the table may be deemed to represent weight percent . the actual batch ingredients can comprise any materials , either oxides or other compounds , which , when melted together , will be converted into the desired oxides in the proper proportions . for example , caco 3 and mgco 3 can serve as the batch materials for cao and mgo , respectively . the table also recites the crystal phase ( s ) generated in the glasses upon heat treatment to a glass - ceramic , along with values , where measured , of coefficient of thermal expansion [ exp . coef .] ( 25 °- 300 ° c .) expressed in terms of x 10 - 7 /° c ., modulus of rupture [ mor ] expressed in terms of psi , k ic expressed in terms of mpa √ m , knoop hardness , and surface roughness [ r a ] expressed in terms of nanometers . table______________________________________ 1 2 3sio . sub . 2 57 . 3 65 . 6 67 . 6al . sub . 2 o . sub . 3 2 . 0 2 . 0 1 . 6mgo -- 14 . 8 14 . 1cao 20 . 2 6 . 2 4 . 6na . sub . 2 o 8 . 0 3 . 0 3 . 2k . sub . 2 o 8 . 8 4 . 2 4 . 7li . sub . 2 o -- 0 . 8 0 . 88bao -- 0 . 25 0 . 22p . sub . 2 o . sub . 5 -- 0 . 75 1 . 2f 6 . 3 4 . 2 3 . 3 potassium potassium fluorrichterite canasite fluorrichterite cristobalitecrystals minor caf . sub . 2 minor mica minor micaexp . coef . 120 75 120mor 40 , 000 25 , 000 20 , 000k . sub . ic 5 . 0 -- 3 . 0knoop & gt ; 500 & gt ; 500 & gt ; 500r . sub . a 1 - 6 1 - 6 1 - 6______________________________________ based upon the overall combination of physical properties , glass - ceramics wherein canasite constitutes the predominant crystal phase ( illustrated in example 1 above ) comprise the most preferred substrate material .