High-hardness silica glass and method of producing same

Abstract: High-hardness silica glass having transparency substantially on the same level as usual silica glass and a Vickers hardness of 12-20 GPa is produced by covering silica glass (4) with powder (5) and treating the silica glass at a temperature of 1400°-2500°C and pressure of 10-300 MPa in an atmosphere of an inert gas or a nitrogen gas.

Description: The present invention relates to a high-hardness silica glass and a method of producing it, and more particularly to high-hardness silica glass as transparent as usual silica glass and having a Vickers hardness of 12 to 20 GPa useful for optical parts such as optical elements, optical communication elements, etc., structural materials such as fibrous materials added to composite materials, and window glasses for watches, aircrafts, autoclaves and other high-pressure containers. Pure silica glass is highly excellent in transmittancy and also has a relatively high hardness (Vickers hardness of 5.8 to 6.9 GPa). The addition of some additives to further increase its hardness, however, usually leads to the deterioration of transmittancy thereof. Other types of glass provided with improved hardness are reportedly crystallized glass, oxinitride glass, chemically reinforced glass, etc. Among them, what has the highest hardness is the oxinitride glass which contains nitrogen in the glass structure to increase its hardness. Why the glass is provided with higher hardness by introducing nitrogen into the glass structure is not necessarily clear, but it may be presumed that the formation of Si-N bonds in the glass structure makes the glass denser. The oxinitride glass may be produced by various methods as shown below: (1) Blowing an NH₃ gas into a molten glass heated at high temperatures. (2) Treating porous glass with an NH₃ gas at high temperatures. (3) Using a nitride starting material and melting it at high temperatures. (4) Using a nitride starting material and melting it at high temperatures and pressure. Actually, however, any of the above methods can hardly provide glass having excellent transmittancy. Some types of oxinitride glass having high transmittancy and hardness which have been reported so far are shown in Table 1. Composition N(at%) Vickers Hardness (GPa) Y-Al-Si-O-N 5 11.67 La-Si-O-N 18 11.96 As shown in Table 1, considerable improvement in hardness has been achieved, but further improvement in hardness is desired for glass used in severe applications. In addition, strong demand exists to provide a method of easily producing glass having high transmittancy and hardness. DE-A-2 049 731 discloses a tubular silica glass vessel having an interior layer comprising silicon nitride. A silicon nitride layer is formed by introducing a gas stream consisting of a mixture of NH₃ and nitrogen for at least 30 hours into the glass vessel at a temperature of 800 to 1000°C. Under these conditions, S-NH bonds are formed in the silica glass which are liable to the reduction of the Vickers hardness and the decrease in transparency. The Vickers hardness of the glass is not mentioned in DE-A-2 049 731. An object of the present invention is to provide glass having excellent transmittancy and hardness without suffering from the above problems. Another object of the present invention is to provide a method of producing such glass. These objects are achieved by the silica glass and the method of producing such glass according to the claims. As a result of intense research in view of the above objects, the inventors have found that glass with high hardness and transmittancy can be obtained by treating silica glass surrounded by carbon or nitride powder at high temperatures and pressure. Thus, the high-hardness silica glass according to the present invention has transparency substantially on the same level as silica glass and a Vickers hardness of 12 to 20GPa. The method of producing a high-hardness silica glass according to the present invention comprises covering silica glass with powder and treating the silica glass at a temperature of 1400° to 2500°C and pressure of 10 to 300MPa in an atmosphere of an inert gas or a nitrogen gas. The invention is described in detail in connection with the drawings in which: Fig.1 is a schematic cross-sectional view showing a hot press apparatus for producing high-hardness silica glass according to the present invention; and Fig.2 is a schematic cross-sectional view showing a crucible for hot isostatic press for producing high-hardness silica glass according to the present invention. In order to provide high-hardness silica glass having transparency substantially on the same level as silica glass and a Vickers hardness of 12 to 20 GPa, a block of silica glass inherently having relatively high hardness is used as a starting material, and it is surrounded by powder composed of at least one component selected from carbon and nitrides such as silicon nitride and boron nitride, and treated at a temperature of 1400 to 2500°C and pressure of 10 to 300MPa. The silica glass usable in the present invention is not restricted in a production method, a shape and a size. However, from the aspect of easiness of handling, it is preferable that the silica glass is in advance formed substantially in the same shape and size as those of a final product which is usually as big as 20 cm or so. For instance, the silica glass of 10mm in thickness itself has as high transmittance as 92% or more when measured with a wavelength of 200 to 2000 nm. It is extremely important that the silica glass is surrounded by powder when it is treated at high temperatures and pressure. The above powder is preferably made of carbon or nitride compounds such as silicon nitride, boron nitride, etc. These kinds of powder may be used alone or in combination. The existance of the powder around the silica glass makes it possible to produce high-hardness silica glass while retaining its initial shape completely, because the powder surrounding the silica glass serves to prevent the silica glass from being deformed by flowing even when it becomes flowable at high treatment temperatures. The powder used in the present invention preferably has a particle size of about 50µm to 5mm. When it is smaller than 50µm, the powder is sintered too strongly, making it difficult to remove the treated silica glass. On the other hand, when it is larger than 5mm, it is difficult to keep the initial shape of the silica glass which becomes flowable by a high-temperature treatment. When the powder has a particle size in the range of about 50µm to 5mm, it is partially sintered, which is desirable not only for keeping the shape of the silica glass being treated but also for ensuring the easy removal of the completed high-hardness silica glass. The silica glass surrounded by the above powder is treated at a temperature of 1400 to 2500°C and pressure of 10 to 300MPa. The treatment temperature should be 1400°C or higher to provide the silica glass being treated with fluidity, but when it exceeds 2500°C, SiO₂ tends to sublimate vigorously. The preferred temperature is 1500 to 2000°C. With respect to the treatment pressure, it may vary depending upon the size of the silica glass block being treated, but it should be 10MPa or more so that compression stress remains in the resulting silica glass. However, when the pressure exceeds 300MPa, the powder surrounding the silica glass is strongly sintered, making difficult the removal of the resulting high-hardness silica glass. Therefore, it should be 10 to 300MPa. The preferred treatment pressure is 150 to 200 MPa. The production of high-hardness silica glass under the above-mentioned conditions may be carried out by hot pressing or hot isostatic pressing. Referring to Fig.1, a hot press apparatus comprises a lower die 1 having a cavity 2 and an upper die 3 made of carbon and slidably received in the cavity 2. The silica glass 4 formed in the shape of a final product is placed in the cavity 2 while being embedded in the powder 5. In a state that the silica glass 4 is surrounded by the powder 5, the upper die 3 is lowered to press the silica glass 4 together with the powder 5 while heating them in an atmosphere of an inert gas or a nitrogen gas. If the silica glass is subjected to hot isostatic pressing (HIP), it is contained in a carbon crucible as shown in Fig. 2. Specifically, the silica glass 10 is embedded in the powder 11 contained in the carbon crucible 12, and the carbon crucible 12 is placed in a hot isostatic press apparatus (not shown). The treatment conditions in HIP are essentially the same as in hot pressing. Incidentally, in the case of hot pressing, the silica glass can be treated as if it is subjected to HIP; in other words, so-called pseudo-HIP effects can be obtained, because the die is filled with the powder surrounding the silica glass. And when the treatment is carried out at temperatures of 1700°C or higher under high pressure, HIP is more preferable than hot pressing because the powder tends to be strongly sintered in hot pressing. In the case of HIP, the filled powder is scarcely sintered, so that it is extremely easy to remove the resulting high-hardness silica glass. The reasons why high-hardness silica glass having excellent transmittancy can be produced according to the present invention are considered as follows: by using a silica glass block inherently having relatively high transmittancy as a starting material, surrounding it with at least one kind of powder selected from carbon powder and nitride powder made of silicon nitride or boron nitride and treating it in an atmosphere of an inert gas or a nitrogen gas at a temperature of 1400 to 2500°C and pressure of 10 to 300MPa, (a) Compression stress remains in the silica glass, thereby increasing the hardness of the silica glass; (b) A nitrogen gas is diffused into the silica glass, thereby forming a structure similar to oxinitride having Si-N bonds so that the hardness is increased; (c) Part of nitride powder is diffused into the silica glass, thereby forming a structure similar to oxinitride having Si-N bonds so that the hardness is increased; (d) Carbon powder serves to reduce the oxygen in Si-O bonds of the silica glass, which is then substituted by nitrogen in the N₂atmosphere, thereby forming a structure similar to oxinitride having Si-N bonds so that the hardness is increased; and (e) The synergistic effects of an nitrogen gas and nitride powder serve to form a structure similar to oxinitride having Si-N bonds, thereby increasing the hardness. Because the compression stress remaining in the silica glass is not so large and because the similar structure to oxinitride is introduced in an extremely small proportion, they do not substantially affect the transmittancy of the silica glass. Therefore, the treated silica glass has an increased hardness while retaining excellent transmittancy inherently possessed by the starting silica glass. Incidentally, since some compression stress remains in the high-hardness silica glass having excellent transmittancy which is produced according to the present invention, the high-hardness silica glass has relatively poor machinability, and also accurate measurement of hardness is difficult due to the fact that cracking tends to take place in the course of measuring hardness. This problem can be solved by a heat treatment which is usually applied to silica glass; such a heat treatment can provide the high-hardness silica glass of the present invention with good machinability while retaining high hardness. It should be noted, however, that this heat treatment is not indispensable and that it may be omitted if the silica glass is in advance formed in the shape of a final product. The present invention will be explained in further detail by means of the following Examples without intention of restricting the scope of the present invention. Example 1 A silica glass block of 70mm in diameter and 70mm in height was placed in the cavity of a hot press die shown in Fig.1, with BN powder of 50 to 100µm in particle size surrounding the silica glass block. The hot pressing was conducted at various temperatures shown in Table 2 below under pressure of 10MPa for 1 hour. Incidentally, temperature increase and decrease in the hot pressing were conducted at a rate of 200°C/hr. The resulting silica glass thus treated was heat-treated in the atmosphere at 1150°C for 1 hour, and its Vickers hardness was measured by a Vickers hardness tester under a load of 25g according to JIS Z 2244. Temp.(°C) Hardness(GPa) Powder Atmosphere - 6.88 - - 1500 12.36 BN Argon 1600 12.67 BN Argon 1700 12.85 BN Argon All of the resulting silica glass specimens permitted a light of 0.16 to 4.8µm in wavelength to pass therethrough substantially in the same transmittance as that of usual silica glass. Example 2 A cubic silica glass block of 200mm in each side was placed in the cavity of a carbon crucible for HIP as shown in Fig.2 with Si₃N₄ powder of 0.1 to 0.5mm in particle size surrounding the glass block. The HIP treatment was conducted under various temperature and pressure conditions for 1 hour. Incidentally, temperature increase and decrease in HIP were at a rate of 200°C/hr. Each of the resulting silica glass was heat-treated at 1150°C for 1 hour in the atmosphere. Hardness was measured on each silica glass by a Vickers hardness tester under a load of 25g. The results are shown in Table 3. Temp. (°C) Pressure (MPa) Hardness (GPa) Powder Atmosphere 1500 120 12.47 Si₃N₄ Argon " 180 12.65 Si₃N₄ Argon 1700 120 13.44 SI₃N₄ Argon " 180 13.73 Si₃N₄ Argon 1900 120 13.94 Si₃N₄ Argon " 180 14.31 Si₃N₄ Argon All of the above silica glass specimens permitted a light of 0.16 to 4.8µm in wavelength to pass therethrough substantially in the same transmittance as that of usual silica glass. Example 3 A silica glass block of 70mm in diameter and 70mm in height was placed in the cavity of a hot press die shown in Fig.1, with carbon powder of 50 to 100µm in particle size surrounding the silica glass block. The hot pressing was conducted at various temperatures shown in Table 4 below under pressure of 10MPa for 1 hour. Incidentally, temperature increase and decrease in the hot pressing were at a rate of 200°C/hr. The resulting silica glass thus treated was heat-treated in the atmosphere at 1150°C for 1 hour, and its Vickers hardness was measured by a Vickers hardness tester under a load of 25g. Temp.(°C) Hardness(GPa) Powder Atmosphere 1500 15.31 Carbon Nitrogen 1600 15.47 Carbon Nitrogen 1700 15.63 Carbon Nitrogen All of the resulting silica glass specimens permitted a light of 0.16 to 4.8µm in wavelength to pass therethrough substantially in the same transmittance as that of usual silica glass. Example 4 A cubic silica glass block of 200mm in each side was placed in the cavity of a carbon crucible for HIP as shown in Fig.2 with carbon powder of 0.1 to 0.5mm in particle size surrounding the glass block. The HIP treatment was conducted under various temperature and pressure conditions for 1 hour. Incidentally, temperature increase and decrease in HIP were at a rate of 200°C/hr. Each of the resulting silica glass was heat-treated at 1150°C for 1 hour in the atmosphere. Hardness was measured on each silica glass by a Vickers hardness tester under a load of 25g. The results are shown in Table 5. Temp. (°C) Pressure (MPa) Hardness (GPa) Powder Atmosphere 1500 120 15.57 Carbon Nitrogen " 180 15.72 Carbon Nitrogen 1700 120 16.03 Carbon Nitrogen " 180 16.48 Carbon Nitrogen 1900 120 16.88 Carbon Nitrogen " 180 17.12 Carbon Nitrogen All of the above silica glass specimens permitted a light of 0.16 to 4.8µm in wavelength to pass therethrough substantially in the same transmittance as that of usual silica glass. Example 5 A silica glass block of 70mm in diameter and 70mm in height was placed in the cavity of a hot press die shown in Fig.1, with BN powder of 50 to 100µm in particle size surrounding the silica glass block. The hot pressing was conducted at various temperatures shown in Table 6 below under pressure of 10MPa for 1 hour. Incidentally, temperature increase and decrease in the hot pressing were at a rate of 200°C/hr. The resulting silica glass thus treated was heat-treated in the atmosphere at 1150°C for 1 hour, and its Vickers hardness was measured by a Vickers hardness tester under a load of 25g. Temp.(°C) Hardness(GPa) Powder Atmosphere 1500 16.34 BN Nitrogen 1600 16.52 BN Nitrogen 1700 16.73 BN Nitrogen All of the resulting silica glass specimens permitted a light of 0.16 to 4.8µm in wavelength to pass therethrough substantially in the same transmittance as that of usual silica glass. Example 6 A cubic silica glass block of 200mm in each side was placed in the cavity of a carbon crucible for HIP as shown in Fig.2 with Si₃N₄ powder of 0.1 to 0.5mm in particle size surrounding the glass block. The HIP treatment was conducted under various temperature and pressure conditions for 1 hour. Incidentally, temperature increase and decrease in HIP were at a rate of 200°C/hr. Each of the resulting silica glass was heat-treated at 1150°C for 1 hour in the atmosphere. Hardness was measured on each silica glass by a Vickers hardness tester under a load of 25g. The results are shown in Table 7. Temp. (°C) Pressure (MPa) Hardness (GPa) Powder Atmosphere 1500 120 16.60 Si₃N₄ Nitrogen " 180 16.68 Si₃N₄ Nitrogen 1700 120 16.73 SI₃N₄ Nitrogen " 180 16.82 Si₃N₄ Nitrogen 1900 120 17.04 Si₃N₄ Nitrogen " 180 17.23 Si₃N₄ Nitrogen All of the resulting silica glass specimens permitted a light of 0.16 to 4.8µm in wavelength to pass therethrough substantially in the same transmittance as that of usual silica glass. As explained above, by forming silica glass substantially in the shape of a final product, surrounding it with at least one kind of powder selected from carbon powder and nitride powder made of silicon nitride or boron nitride and treating it in an atmosphere of an inert gas or a nitrogen gas at a temperature of 1400 to 2500°C and pressure of 10 to 300MPa, high-hardness silica glass having excellent transmittancy can easily be obtained within a short period of time while retaining its initial shape.

Claims: High-hardness silica glass having transparency in the visible and ultraviolet range and a Vickers hardness of 12 to 20 GPa. A method of producing high-hardness silica glass according to claim 1, said method comprising covering silica glass with nitride powder and/or carbon powder and treating said silica glass at a temperature of 1400 to 2500°C and a pressure of 10 to 300 MPa in an atmosphere of an inert gas or a nitrogen gas, with the proviso that at least one source of nitrogen in the form of said nitride powder and/or said nitrogen atmosphere is present. The method according to claim 2, wherein said nitride powder is made of silicon nitride or boron nitride. The method according to claim 2 or 3, wherein said inert gas is selected from rare gases. The method according to claims 2 to 4, wherein said silica glass covered by said powder is hot-pressed or hot isostatic-pressed.

Inventors: KONDO SHINICHI, NISHIZAWA KEIICHIRO, SEGAWA HIDEAKI, KONDO, SHINICHI, NISHIZAWA, KEIICHIRO, SEGAWA, HIDEAKI
Applicants: TOSOH CORP, TOSOH CORPORATION
Application year: 1988
Main classification: C03C23
IPC codes: C03C23, C03B32, C03C17