SCRATCH-RESISTANT GLASS

Abstract: Abrasion resistant glass, which consists essentially of the following components: SiO 2 75.5 to 85.5 (wt%), RO 1 to 8 (wt%), R' 2 O 10 to 23.5 (wt%), RO+R' 2 O 11 to 24.5 (wt%), Al 2 O 3 0 to 5 (wt%), RO/R' 2 O (weight ratio) at most 0.5, provided that R is at least one member selected from Mg and Ca, and R' is at least one member selected from Li, Na and K, and of which the density measured at room temperature is at most 2.41 g/cc.

Description: The present invention relates to glass, of which the abrasion resistance is very high. Glass is essentially a brittle material. Particularly, its strength decreases by fine surface defects which form during its use, and if a force is exerted to such defects, cracks will spread from these fine defects, leading to breakage. Therefore, in order to increase the durability of glass, attempts have been made for a long time to improve abrasion resistance of the surface by incorporating a hard coating on the surface, for example. However, such a technique is not an attempt to essentially overcome the brittleness of glass itself. Accordingly, there is a problem that once the surface coating is removed by abrasion or deterioration, for example, its effects will no longer be obtained. On the other hand, sodalime silica glass which is presently most widely employed, usually has the following composition, although there has been some change historically: SiO 2 66 to 75 wt%, MgO 0 to 5 wt%, CaO 7 to 12 wt%, Na 2 O 12 to 20 wt%, K 2 O 0 to 3 wt%, Al 2 O 3 0 to 4 wt%. Not so much research has been done on the relationship between the composition and the hardness with respect to sodalime silica glass. According to general common knowledge, if the silica content is increased, the hardness will increase; if CaO is substituted for Na 2 O, the network bonding will increase to improve the strength; an addition of Al 2 O 3 serves to increase the strength and the hardness as well. Further, abrasion hardness corresponding to scratchability of the surface, is important for determining the practical strength of glass, but it does not necessarily have a good correlation with the Vickers hardness. This is believed to be attributable to the fact that the abrasion resistance is not only a factor relating to the hardness, but also susceptible to an influence of the frictional heat or falling off of glass fragments from a test piece due to formation and spreading of fine cracks. In glass of sodalime silicate type, if the component ratio of silica is increased simply to increase the hardness in accordance with the conventional knowledge, it tends to be hardly meltable, whereby its production by a melting method with good productivity, will be impossible. Glass having a relatively high silica component ratio, is disclosed in the specification of US-Patent 3,811,853. However, such glass is limited to an application for decomposable containers, and it does not have adequate water resistance to be used as glass of general use. An object of the present invention is to overcome the brittleness of glass itself thereby to prevent formation of abrasion marks by an external force during its practical use and to provide glass which is hardly breakable and which is relatively easily meltable in spite of high abrasion resistance. Another object is to provide glass which has, in addition to the above properties, durability (particularly water resistance) comparable to conventional sodalime silicate glass. The present invention provides an abrasion resistant glass, which consists essentially of the following components SiO 2 more than 79 to 85.5 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 5.7 (wt%), RO 1.5 to 6.7(wt%), Na 2 O 10 to lerss than 19 (wt%), K 2 O 0 to less than 9 (wt%), Li 2 O 0 to less than 9 (wt%), R' 2 O 10 to less than 19 (wt%), Al 2 O 3 1 to 5 (wt%), RO+R' 2 O 11 to 24.5 (wt%), RO/R' 2 O at most 0.5, (weight ratio) provided that R is at least one member selected from Mg and Ca, and R' is at least one member selected from Li, Na and K, and of which the density measured at room temperature is at most 2.41 g/ cm 3 . Furthermore, the present invention provides an abrasion resistant glass, which consists essentially of the following components SiO 2 75 to 79 (wt%), MgO 1 to 7.3 (wt%), CaO 0.5 to 6.8 (wt%), RO 2.7 to 7.8 (wt%), Na 2 O 10.7 to 17 (wt%), K 2 O 0 to 10 (wt%), Li 2 O 0 to 10 (wt%) R' 2 O 10.7 to 19.6 (wt%), Al 2 O 3 1 to 5 (wt%), RO + R' 2 O 11 to 24.5 (wt%), RO/ R' 2 O 0.2 to 0.5, (weight ratio) provided that R is at least one member selected from Mg and Ca, and R' is at least one member selected from Li, Na and K, and of which the density measured at room temperature is at most 2.41 g/ cm 3 . Moreover, the present invention provides an abrasion resistant glass, which consists essentially of the following components: SiO 2 75.5 to 79 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 6.8 (wt%), RO 1 to 7.8 (wt%), Na 2 O 5 to 20 (wt%), K 2 O 0 to 10 (wt%), Li 2 O 0 to 10 (wt%) K 2 O + Li 2 O 2.5 to 11 (wt%), R' 2 O 10.7 to 22.5 (wt%), Al 2 O 3 1 to 5 (wt%), RO + R' 2 O 11 to 24.5 (wt%), RO/ R' 2 O at most 0.5, (weight ratio) provided that R is at least one member selected from Mg and Ca, and R' is at least one member selected from Li, Na and K, and of which the density measured at room temperature is at most 2.41 g/ cm 3 . Figure 1 is a schematic view for explaining the definition of the brittleness in the present invention. Figure 2 is a graph showing the relation between the brittleness index value B and the abrasion wear by sand blasting. The present inventors have studied in detail the relationship between the composition and the abrasion resistance with respect to various sodalime silica glasses and as a result, have found that within the above-mentioned range of the glass composition, glass is made to have a density of at most 2.41 g/cm 3 (the density of usual sodalime silica glass being at a level of from 2.49 to 2.52 g/ cm 3 ), whereby it becomes hardly scratchable as compared with conventional sodalime silica glass. Namely, the present invention is based on the finding that in sodalime silica glass, density is a factor which is closely related to abrasion resistance, its control is possible within a certain compositional range, and a glass satisfying the specific compositional range and the density, has durability and is relatively easily meltable, and it is also excellent in abrasion resistance. Namely, as shown in the Examples given hereinafter, by optimizing the composition, it is possible to obtain glass which is readily meltable and of which the abrasion mark-forming load is higher by at least 5 times than usual sodalime silica glass, and the practical strength is substantially improved. Now, the compositional range of the glass of the present invention will be described. SiO 2 is the main component for forming the network structure of glass. If this is too small, the amount of non-crosslinking oxygen relatively increases, whereby the network structure will be weak, and the density will increase to facilitate propagation of cracks, whereby the strength itself will be low. On the other hand, if it is too much, the meltability tends to be poor, whereby it will be difficult to obtain uniform glass by melting method. In the present invention, it is from 75.5 to 85.5 wt% based on the entire glass. With a view to securing adequate abrasion resistance, it is preferably at least 76.5 wt%. Further, it is particularly preferably at most 82 wt%, more preferably at most 79 wt%, whereby production may be carried out by a melting method by means of a large scale installation such as burner heating. RO (wherein R is at least one member selected from Mg and Ca) is a component essential for improvement of the meltability and improvement of chemical durability. It is required to be at least 1 wt% in order to impart chemical durability such as alkaline resistance, and it is required to be at most 8 wt% in order to obtain adequate abrasion resistance. As of R, there is such a tendency that the more the content of an element having a smaller element or atomic number, the better the improvement in abrasion resistance. From such a viewpoint, as R, it is preferred to increase Mg rather than Ca. Accordingly, it is preferred that MgO is an essential component. Specifically, it is preferred that MgO is from 1 to 8 wt%, and CaO is from 0 to 7 wt%. Preferably, RO is at most 7.5 wt% and at least 1.5 wt%. Specifically, MgO is from 1 to 7.5 wt%, and CaO is from 0 to 6.5 wt%. In the present specification and claims, "0 to" means that there will be a composition containing no such component. R' 2 O (wherein R' is at least one member selected from Li, Na and K) is required to be at least 10 wt% to improve the meltability and required to be at most 23.5 wt% to avoid crystallization and to maintain the chemical resistance. To further improve the meltability, it is preferably at least 11.5 wt%. Further, to avoid crystallization and to further improve the chemical resistance, it is preferably at the most 22.5 wt%. Specifically, it is preferred that Na 2 O is from 10 to 22.5 wt%, K 2 O is from 0 to 10 wt%, and Li 2 O is from 0 to 10 wt%. In an application where chemical resistance is particularly required, R' 2 O is preferably at most 21.5 wt% in a total amount, and specifically, it is preferred that Na 2 O is from 10 to 21.5 wt%, K 2 O is from 0 to 10 wt%, and Li 2 O is from 0 to 10 wt%. The total amount of RO and R' 2 O is from 11 to 24.5 wt%. If RO+R' 2 O is less than 11 wt%, the meltability tends to be poor, and if it exceeds 24.5 wt%, it tends to be difficult to obtain high abrasion resistance. To obtain high abrasion resistance, it is preferably at most 23.5 wt%, more preferably at most 22.5 wt%. The compositional ratio of RO to R' 2 O is substantially influential over the abrasion resistance. Specific functions of the respective components have not yet been understood in detail, but as is evident from the Examples given hereinafter, in the present invention, the abrasion resistance is improved by an addition of R' 2 O rather than RO. At least this result does not agree with the conventional knowledge about strength. From this viewpoint, in the present invention, it is necessary that the weight ratio of RO/R' 2 O is at the most 0.5, preferably at most 0.45. Further, in a case where the total amount of RO and R' 2 O is restricted within the preferred range, the respective ranges of RO and R' 2 O may then be restricted to bring the weight ratio of RO/R' 2 O to the predetermined value. By an addition of a small amount of Al 2 O 3 , the chemical durability of glass can be remarkably improved and uniform glass can be produced. To suppress phase separation, it is very preferable to incorporate it in an amount of at least 1 wt% of the glass. Further, excessive addition tends to bring about deterioration of abrasion resistance, and accordingly, it is at most 5 wt%, preferably at most 4 wt%. In the present invention, "essentially" means that the main components comprising SiO 2 , Al 2 O 3 , RO and R' 2 O constitute at least 96 wt% of the entire glass. As other components in very small amounts, Fe, Ni, Se, Co, Ce, etc. may be incorporated for the purpose of homogenizing the entire glass, coloring, or controlling the infrared transmittance and ultraviolet transmittance. Further, to facilitate the production of more homogeneous glass, a known refining agent may be incorporated. As such a refining agent, SO 3 , Cl, As 2 O 3 , or Sb 2 O 3 , for example, may be mentioned. Further, ZnO may be substituted for MgO or CaO suitably within a range not to impair the effects of the present invention. Further, SrO, BaO or B 2 O 3 , for example, may be incorporated within a range not to impair the effects of the present invention and in an amount less than 0.5 wt% based on the entire glass. Of the glass of the present invention, the density measured at room temperature is at the most 2.41 g/cc. The present inventors have discovered that in sodalime silicate type glass, the density has a very close relation to abrasion resistance. According to the knowledge obtained by the present inventors, the abrasion resistance becomes extremely high, when the density measured at room temperature becomes at most 2.41 g/cc within the above-mentioned compositional range. In the present invention, the brittleness index value B proposed by Lawn et al. (B. R. Lawn and D. B. Marshall, J. Am. Ceram. Soc., 62[7-8]347-350(1979)) is used as an index for brittleness (abrasion resistance) of glass. Here, the brittleness index value B is defined by the formula (1) wherein Hv is the Vickers hardness of the material and Kc is the fracture toughness value: B=Hv/Kc A substantial problem in a case where this brittleness index is applied to glass, is that the fracture toughness value Kc can not accurately be evaluated. Therefore, as a result of a study of several methods, the present inventors have found that the brittleness can quantitatively be evaluated from the relation between the size of the impression of the indenter remaining on the glass surface when the Vickers indenter is indented against the surface and the length of cracks formed from the four corners of the impression. This relation is defined by the formula (2), wherein P is the indentation load of the Vickers indenter, and a and c are, as shown in Figure 1, the diagonal length of the Vickers impression and the length of the cracks formed from the four corners (the entire length of the two symmetrical cracks inclusive of the impression of the indenter), respectively: c/a=0.0056B 2/3 p 1/6 Brittleness of glasses can readily be evaluated by using the formula (2) and the sizes of Vickers impressions formed on the surfaces of various glasses. Figure 2 shows a relation between the brittleness index value B thus measured and the abrasion wear by sand blasting. The brittleness index value B and the abrasion wear by sand blasting are in a very strong correlation, and it is evident that the brittleness index value is a very good index for the abrasion resistance. In the following specification, the brittleness index value B obtained by the formula (2) will be used. From the foregoing, specifically, preferred glass in the present invention consists essentially of the following composition, of which the density measured at room temperature is at most 2.41 g/cc. Here, the upper limit values for MgO, CaO, and RO are values obtained from the values calculated automatically by the total amount and ratio of RO and R' 2 O by taking into account the fractions of 0.05 and over as 0.1 and disregarding the rest. SiO 2 75.5 to 85.5 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 6.8 (wt%), RO 1 to 7.8 (wt%), Na 2 O 10 to 22.5 (wt%), K 2 O 0 to 10 (wt%), Li 2 O 0 to 10 (wt%), R' 2 O 10 to 22.5 (wt%), Al 2 O 3 1 to 5 (wt%). Especially when the SiO 2 content exceeds 79 wt%, high abrasion resistance and excellent water resistance can be obtained simultaneously essentially within the following compositional range. Here, the upper limit values for MgO, CaO and RO are values obtained from the values calculated automatically by the total amount and ratio of RO and R' 2 O by taking into account the fractions of 0.05 and over as 0.1 and disregarding the rest. SiO 2 more than 79 to 85.5 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 5.7 (wt%), RO 1 to 6.7 (wt%), Na 2 O 10 to less than 19 (wt%), K 2 O 0 to less than 9 (wt%), Li 2 O 0 to less than 9 (wt%), R' 2 O 10 to less than 19 (wt%), Al 2 O 3 1 to 5 (wt%). On one hand, with the above composition, the melting property is poor as compared with a case where the SiO 2 content is at most 79 wt%, whereby the composition is required to be melted at a relatively high temperature. It is more preferred that melting can be conducted at a relatively low temperature, and at the same time, adequate water resistance and abrasion resistance can be obtained. From this viewpoint, one of the preferred compositions consists essentially of the following. Here, the lower limit values for R' 2 O, Na 2 O and RO and the upper limit values for MgO, CaO and RO, are values obtained from the values calculated automatically by the total amount and ratio of RO and R' 2 O by taking into account the fractions of 0.05 and over as 0.1 and disregarding the rest. SiO 2 75 to 79 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 6.8 (wt%), RO 2.7 to 7.8 (wt%), Na 2 O 10.7 to 17 (wt%), K 2 O 0 to 10 (wt%), Li 2 O 0 to 10 (wt%), R' 2 O 10.7 to 19.6 (wt%), Al 2 O 3 1 to 5 (wt%), RO/R' 2 O (weight ratio) 0.2 to 0.5. If Na 2 O exceeds 17 wt%, the water resistance tends to be low. Specifically, the eluted amount of sodium when the glass is exposed to hot water tends to increase. Further, if the weight ratio of RO/R' 2 O is less than 0.2, the ratio of RO to R' 2 O relatively decreases, and again the water resistance tends to be low. To obtain better water resistance, it is very effective to incorporate at least 0.5 wt% of K 2 O by substituting K 2 O for Na 2 O, although this is not essentially required. It is also effective to incorporate at least 0.5 wt% of CaO by substituting CaO for MgO. Another preferred composition which is meltable at a relatively low temperature and whereby adequate water resistance and abrasion resistance can be obtained, consists essentially of the following. Here, the lower limit values for R' 2 O and the upper limit values for MgO, CaO and RO, are values obtained from the values calculated automatically by the total amount and ratio of RO and R' 2 O by taking into account the fractions of 0.05 and over as 0.1 and disregarding the rest. SiO 2 75.5 to 79 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 6.8 (wt%), RO 1 to 7.8 (wt%), Na 2 O 5 to 20 (wt%), K 2 O 0 to 10 (wt%), Li 2 O 0 to 10 (wt%), K 2 O+Li 2 O 2.5 to 11 (wt%), R' 2 O 10.7 to 22.5 (wt%), Al 2 O 3 1 to 5 (wt%). In this composition, if K 2 O+Li 2 O is less than 2.5 wt%, the water resistance tends to be low. To improve the water resistance, it is effective to incorporate at least 0.5 wt% of CaO by substituting CaO for MgO, like in the above case. The glass of the present invention thus obtained is typically the one of which the brittleness index value B is at most 6,000 m -1/2 , preferably at most 5,800 m -1/2 . Further, the temperature at which the viscosity becomes 100 poises, is preferably not higher than 1,800°C, more preferably not higher than 1,680°C. The glass of the present invention has an appropriate melting property, and accordingly various preparation methods may be employed. Namely, various starting materials may be blended by a conventional method to obtain a predetermined composition, which is then heated to from 1,450 to 1,650°C for melting. Then, this molten glass is refined, then formed into a predetermined shape, followed by gradual cooling to obtain a commercial product. In the following table, 63 types of glass compositions are disclosed which were studied as glasses of the present invention (Examples 1 to 48) and of Comparative Examples (Examples 49 to 63). To obtain each composition, 200 g of starting material powder was put into a platinum crucible and heat-melted in the atmosphere at from 1,450 to 1,650°C for 4 hours with stirring. In a Comparative Example (Example 63) wherein SiO 2 was 88%, the melting property was poor, whereby uniform glass was not obtained. Except for this case, substantially uniform glass was obtained in all cases. Glass of each composition uniformly melted, was cast into a carbon mold to form a plate having a size of about 10 × 10 cm and a thickness of 5 mm, followed by cooling. The obtained glass was annealed at a temperature of from 490 to 570°C to remove any strain, followed by cutting and polishing to obtain a test sample having a size of 2 × 2 cm and a thickness of 4 mm. In the table, the density of the glass, the brittleness index value and the temperature at which the viscosity becomes 100 poise (T(°C) at logη = 2) and the temperature at which the viscosity becomes 10,000 poise (T(°C) at logη = 4), as indices for the melting property, are shown. The brittleness index value was measured as follows. A sample polished to a mirror surface was further heated to a temperature slightly higher than the strain point temperature of the glass (100°C/hr), maintained (3 hr) and then gradually cooled (60°C/hr) to remove the surface residual stress due to polishing. The brittleness was measured in such a state that the surface heat and processing strain had been substantially completely removed. For the measurement of brittleness, a Vickers hardness tester was used. By the same apparatus, same impressions were formed on the glass surface, and from the average length of cracks formed from the four corners of each impression and the indentation load of the Vickers indenter, the brittleness index value B was calculated by the formulas (1) and (2). The indentation load was 5 kgf. The density ρ was calculated from the dry weight and the weight in water of the sample by an Archimedes method. For the measurement, a high precision weighing machine capable of measuring up to µ g, was used, and an effective number was calculated up to five figures and the last figure was rounded to take a number of four figures. Further, with respect to some of the samples, the abrasion wear by sand blasting was also indicated. The abrasion wear was determined in such a manner that sand blasting was applied to a circular region of the material with a diameter of 2 cm under a pressure of 1 kgf/cm 2 for two minutes, whereupon the abrasion depth D was calculated from the weight reduction of the material. With respect to the temperature at which the viscosity becomes 100 poise (T at logη = 2) and the temperature at which the viscosity becomes 10,000 poise (T at logη = 4), those identified with symbol * were obtained by simulation by means of a computer. In all the Examples of the present invention shown in the table, the brittleness index value B is at most 6,000 m -1/2 , indicating high abrasion resistance. Further, it is evident that the density and the brittleness of glass are in a good correlation, and as the density decreases, the brittleness index value also decreases, i.e. abrasion marks tend to be hardly formed. In Examples 1 to 5, Na 2 O is from 10.7 to 17 wt%, and RO/R' 2 O is from 0.2 to 0.5. Further, in Examples 6 to 33, at least either one of K 2 O or Li 2 O is incorporated, and K 2 O+Li 2 O is adjusted to be from 2.5 to 11 wt%. Further, in Examples 34 to 48, SiO 2 exceeds 79 wt%. Further, Examples 1 to 3 indicate that with the glass of the present invention, the abrasion resistance can be improved by substituting MgO for CaO. This indicates that in a case where the SiO 2 content is decreased to improve the melting property, it is advisable to employ MgO rather than CaO, as the RO component. On the other hand, Examples 49 to 64 are examples outside the scope of the present invention. Especially, Example 50, 51 and 58 to 63 are examples wherein the abrasion resistance was inadequate, although a relatively large amount of SiO 2 was incorporated as in the present invention. Namely, it is evident that even if the SiO 2 content is simply increased, the remarkable improvement in the abrasion resistance as in the present invention may not sometimes be observed. Examples 54 to 56 show the effects of substituting SiO 2 by Al 2 O 3 which is commonly regarded as contributing to improvement in hardness. In this case, it is evident that Al 2 O 3 does not necessarily contribute to improvement in hardness. Examples 58, 62 and 63 are examples wherein the abrasion resistance became low, since the density was large, although the compositions themselves were within the range of the present invention. Further, when a Vickers indenter was pressed against the glass of Example 3, the crack-forming load (the load at which cracks formed from the four corners of the indentation impression for the first time, as the load was gradually increased) was 1,000 gf. With usual sodalime glass, the crack-forming load obtained by the same method, is 200 gf (brittleness index value B: about 6,800 m -1/2 ). Thus, it is evident that by improving the brittleness, the crack-forming load was increased up to five times. Separately, with respect to some glasses, as a index for judging the water resistance, eluted amounts of Na 2 O were measured in the unit of µ g/cm 2 in such a manner that a sample of 50 mm x 50 mm was polished on both sides (the side surface was a surface cut by a diamond saw), and then immersed in distilled water at 99°C for 3 hours. If this eluted amount of Na 2 O is less than 5 µ g/cm 2 , such glass is considered to have sufficient water resistance. From comparison of Examples 2 and 5, it is evident that an addition of K 2 O is remarkably effective for improving the water resistance. INDUSTRIAL APPLICABILITY The glass of the present invention can be produced by a melting method with good productivity, and by using it, it is possible to reduce for example, a decrease in strength or deterioration in transparency due to the surface defects during its practical use, whereby the useful life of the commercial product can be substantially prolonged, and its reliability can be substantially improved. Further, the glass of the present invention has a low density and thus is effective for reducing the weight of a product employing it. The glass of the present invention is essentially hardly scratchable and thus has high durability without applying any particular treatment to the surface. Further, it is excellent also in the melting property, it can be used in a wide range. Especially, it is very suitable for use as a sheet glass. In a case where it is to be used for bottles, it may be adjusted to a viscosity suitable for blow molding within the compositional range of the present invention, and by suitably adding a colorant component to shield ultraviolet rays, it is also possible to obtain a colored glass bottle such as a bottle for beer. A bottle made of the glass of the present invention is hardly scratchable, free from deterioration in transparency and hardly breakable, and it is suitable for use repeatedly, and further it is possible to reduce the weight of the bottle itself. Further, the glass of the present invention is suitable as a cover glass for a watch, for which expensive sapphire glass or the like is presently used, from the characteristics of being light and readily scratchable. Further, in many cases, fracture of glass is caused by small abrasion marks. Accordingly, the glass of the present invention, which is hardly scratchable, is hardly breakable. Accordingly, it is suitable for e.g. mirrors or covers for illumination equipments, which are dangerous in case of fracture. Further, the glass of the present invention is suitable as glass for a water tank. A water tank is not only very dangerous in that when broken, water contained will flow out all at once together with the broken glass, but also has a problem that the transparency will be impaired when abrasion marks are formed by stones put therein or by other instruments or the like. Accordingly, the glass of the present invention which is hardly scratchable and hardly breakable, is suitable. Further, it is suitable also as cover glasses for meters used outdoors, such as meters for electricity, gasses and city water, of which the transparency is likely to decrease by abrasion marks and thus the visibility is likely to deteriorate. Further, when glass is placed on the entire surface of a desk or on a part thereof, the glass may have abrasion marks due to cutting operation by a cutter, repeated contact with a mouse of a computer and various other factors, which tend to lead to deterioration of the appearance or to fracture. The abrasion resistant glass of the present invention is suitable for use for such an application. Further, by virtue of light weight as compared with usual glass, it can more easily be used in daily life. The glass of the present invention is suitable also for a bar code reader or a window material for a copying machine, which is normally scratchable by contact with various foreign matters. Further, the glass of the present invention is suitable as a glass for various display substrates such as liquid crystal or plasma display substrates, whereby even a small number of fine abrasion marks will be regarded as a defect of the product, and as cover glasses thereof. It is thereby possible to prevent formation of abrasion marks during washing of the glass or during the transportation to a next step, whereby the yield of the products can be improved. Further, for a hard disk apparatus to be used for a memory apparatus of a computer, an aluminum disk is usually employed. Whereas, a glass disk has a small thermal expansion coefficient as compared with aluminum and undergoes little property change as a function of temperature, and thus, it is employed in a case where high precision is required. The glass having the composition of the present invention is suitable also as glass for such a hard disk, whereby a trouble in memory data due to abrasion marks is less likely to occur, and fracture is less likely. Further, in an application to bearings or the like, of which abrasion marks on the surface are substantially influential over their performance, a high performance can be maintained for a long period of time by using the glass of the present invention which is hardly scratchable. Further, it is useful also as a packing material for a heat exchanger or a rectification tower, by processing it into glass beads or other shapes. In such an application, abrasion marks are less likely to form, and fracture is also less likely to result, due to contact with the container or glass materials to one another. Accordingly, the glass of the present invention is suitable in that the initial performance can be maintained. Further, glass balloons prepared by foaming fine glass particles of the present invention containing a foaming component such as SO 3 , or glass fibers obtainable by withdrawing the glass of the present invention through pin holes of platinum, are particularly useful as a filler for resins by virtue of their characteristic of being light and hardly breakable.

Claims: An abrasion resistant glass which consists essentially of the following components SiO 2 more than 79 to 85.5 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 5.7 (wt%), RO 1.5 to 6.7(wt%), Na 2 O 10 to lerss than 19 (wt%), K 2 O 0 to less than 9 (wt%), Li 2 O 0 to less than 9 (wt%), R' 2 O 10 to less than 19 (wt%), Al 2 O 3 1 to 5 (wt%), RO+R' 2 O 11 to 24.5 (wt%), RO/R' 2 O at most 0.5, (weight ratio) provided that R is at least one member selected from Mg and Ca, and R' is at least one member selected from Li, Na and K, and of which the density measured at room temperature is at most 2.41 g/ cm 3 . An abrasion resistant glass which consists essentially of the following components SiO 2 75 to 79 (wt%), MgO 1 to 7.8 (wt%), CaO 0.5 to 6.8 (wt%), RO 2.7 to 7.8 (wt%), Na 2 O 10.7 to 17 (wt%), K 2 O 0 to 10 (wt%), Li 2 O 0 to 10 (wt%) R' 2 O 10.7 to 19.6 (wt%), Al 2 O 3 1 to 5 (wt%), RO + R' 2 O 11 to 24.5 (wt%), RO/ R' 2 O 0.2 to 0.5, (weight ratio) provided that R is at least one member selected from Mg and Ca, and R' is at least one member selected from Li, Na and K, and of which the density measured at room temperature is at most 2.41 g/ cm 3 . An abrasion resistant glass which consists essentially of the following components SiO 2 75.5 to 79 (wt%), MgO 1 to 7.8 (wt%), CaO 0 to 6.8 (wt%), RO 1 to 7.8 (wt%), Na 2 O 5 to 20 (wt%), K 2 O 0 to 10 (wt%), Li 2 O 0 to 10 (wt%) K 2 O + Li 2 O 2.5 to 11 (wt%), R' 2 O 10.7 to 22.5 (wt%), Al 2 O 3 1 to 5 (wt%), RO + R' 2 O 11 to 24.5 (wt%), RO/ R' 2 O at most 0.5, (weight ratio) provided that R is at least one member selected from Mg and Ca, and R' is at least one member selected from Li, Na and K, and of which the density measured at room temperature is at most 2.41 g/ cm 3 . The abrasion resistant glass according to any one of claims 1 to 3, of which the brittleness index value B is at most 6,000 m -1/2 , where B = Hv/Kc, wherein Hv is the Vickers hardness, and Kc is the fracture toughness value. The abrasion resistant glass according to any one of claims 1 to 4, of which the temperature at which the viscosity becomes 100 poise, is not higher than 1,800°C.

Inventors: ITO SETURO, KASE JUNICHIRO, NAKAO YASUMASA, SEHGAL JEETENDRA, TAKADA AKIRA, TAKAHASHI HIDEO, ITO, SETURO, KASE, JUNICHIRO, NAKAO, YASUMASA, SEHGAL, JEETENDRA, TAKADA, AKIRA, TAKAHASHI, HIDEO, ITO, Seturo Asahi Glass Company Ltd., KASE, Junichiro Asahi Glass Company Ltd., NAKAO, Yasumasa Asahi Glass Company Ltd., SEHGAL, Jeetendra Asahi Glass Company Ltd., TAKADA, Akira Asahi Glass Company Ltd., TAKAHASHI, Hideo Asahi Glass Company Ltd.
Applicants: ASAHI GLASS CO LTD, ASAHI GLASS COMPANY LTD.
Application year: 1996
Main classification: C03C3
IPC codes: C03C3