SINTERED NON-OXIDE CERAMIC COMPOSITE AND METHOD FOR PREPARING THE SAME

Abstract: A sintered non-oxide ceramic composite high in density, high-temperature strength and toughness and having option­ ally and widely changeable electrical conductivity is obtained by compacting and sintering a mixed powder comprising 5-­ 95% by weight of at least one titanium carbide, nitride or car­ bonitride, 0.1-5% by weight (based on silicon carbide) of boron or a boron compound in terms of boron, 0.1-5% by weight (based on silicon carbide) of carbon or a carbon com­ pound in terms of residual carbon and the balance being sub­ stantially silicon carbide.

Description: This invention relates to a sintered non-oxide ceramic composite which has high density, good high-temperature strength and high toughness and electrical conductivity which can be optionally and widely changed and a method for preparing the same. Silicon carbide (SiC) is one of the promising materials as ceramics which are high in hardness, excellent in wear resistance and oxidation resistance, small in the thermal coefficient of expansion and chemically highly stable. Further, silicon carbide high density sintered bodies are superior in high-temperature strength and thermal shock resistance and are expected to be used for engine parts, gas turbines, etc. as structural materials along with silicon nitridae (Si₃N₄). Furthermore, titanium carbide · nitride such as titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), etc. are also known as ceramics high in hardness and excellent in wear resistance, high in electrical conductivity and high in the melting point. High density sintered bodies of titanium carbide · nitride are known to have good hight-temperature strength and undergo plastic deformation at high temperatures, though inferior in oxidation resistance at high temperatures and are also expected to be applied as high-temperature structural materials like silicon carbide. However, ceramic materials including SiC sintered bodies are generally brittle materials and low in toughness and are readily broken due to surface or internal micro-defects or abrupt stress concentration. Thus, they are poor in reliability. Moreover, ceramic materials are generally high in hardness and, except for titanium carbide · nitride and the like, are difficultly workable materials because of low electrical conductivity and inferior electrical discharge workability. This problem in workability is one cause for the delayed commercialization of ceramics. In order to improve the toughness of ceramic materials, for example, zirconia (ZrO₂), research has been done to improve the toughness by partial stabilization of zirconia by addition of yttrium oxide (Y₂O₃), magnesium oxide (MgO) or calcium oxide (CaO), and the product has been known as partially stabilized zirconia (PSZ). With reference to non-oxide ceramics including SiC, substantially little research has been done for the improvement of their toughness, but according to "J. Am. Ceram. Soc." Vol. 67, No. 8 (1984), pages 571-574, to silicon carbide (SiC) were added titanium carbide (TiC) and aluminum (Al) and carbon (C) as sintering aids to improve the fracture toughness (K IC ) of the resulting sintered body. On the other hand, for improving electrical conductivity of silicon carbide (SiC), "Journal of the Japanese Ceramic Society", Vol. 93, No. 3 (1985), pages 123-129 discloses that to SiC is added titanium carbide (TiC), titanium nitride (TiN), zirconium carbide (ZrC), tungsten carbide (WC), zirconium boride (ZrB₂), titanium boride (TiB₂), hafnium boride (HfB₂), niobium boride (NbB₂), tantalum boride (TaB₂) or the like in an amount of 50% by volume and is further added 2% by weight of Al₂O₃ as a sintering aid for SiC and then the mixture is subjected to hot press sintering to sharply reduce the resistivity of the sintered body. Furthermore, JP-A-196770/82 discloses the addition to SiC of 0.5-30% by volume of carbides, nitrides, borides and/or oxides of elements of Groups IVa, Va and VIa of the periodic table and aluminum carbide (Al₄C₃), thereby obtaining sintered bodies improved in electrical conductivity and superior in electric discharge workability. However, this publication makes no mention of sintering aids for SiC. It is difficult to sinter SiC alone owing to its markedly high covalent bonding. Therefore, the production of sintered composites having desirable characteristics such as high density, high strength, etc. becomes possible only with addition of a specific sintering aid to SiC. When aluminum (Al) or aluminum compounds are used as a sintering aid for SiC as reported in the above "J. Am. Ceram. Soc. " and "Journal of the Japanese Ceramic Society", sintered bodies of high density can be obtained, but the high-temperature strength at 1200-1500°C is extremely deteriorated. According to JP-A-112669/85, the growth of crystal grains of a SiC sintered body is prevented by adding 0.1-1.0% by weight of TiN to SiC with addition of boron or boron compounds and carbon or carbon compounds as a sintering aid. However, the addition of TiN in such small amount of 0.1-1.0% by weight cannot afford sufficient increase of fracture toughness and besides, remarkable improvement of electrical conductivity cannot be expected. It is an object of this invention to provide a sintered non-oxide ceramic body with high density, excellent high-temperature strength, high toughness and electrical conductivity capable of being widely changed. Another object of this invention is to provide a method for production of such a sintered non-oxide ceramic body. The sintered non-oxide ceramic body of this invention is produced by combining SiC with at least one of carbides, nitrides and carbonitrides of titanium such as TiC, TiN and TiCN together with simultaneous addition of boron or boron compounds and carbon or carbon compounds which are sintering aids and sintering the mixture. That is, this invention relates to a sintered non-oxide ceramic composite obtained by compacting and sintering a mixed powder comprising 5-95% by weight, preferably 10-90% by weight, more preferably 30-70% by weight of at least one carbide, nitride and/or carbonitride of titanium, 0.1-5% by weight (based on silicon carbide and in terms of boron) of boron or a boron compound, 0.1-5% by weight (based on silicon carbide and in terms of residual carbon) of carbon or a carbon compound and the balance being essentially silicon carbide. The density of the sintered body of this invention is preferably at least 85% of theoretical value. The sintered composite of this invention has an electrical conductivity which can be optionally chosen between its lower level and higher level by changing the blending ratio of SiC and titanium carbide · nitride and which further possesses both excellent high-temperature strength and high toughness. The mechanism, how the toughness and electrical conductivity of the sintered SiC-titanium carbide · nitride composite of this invention are improved has not yet been elucidated, but it is supposed as follows: With reference to the toughness, in the sintered ceramic composite, SiC and titanium carbide · nitride have a micro-structure in which the respective particles are dispersed even after sintering and crack propagation from fracture origin is deflected, resulting in improvement of strength and toughness. With reference to the electrical conductivity, it is supposed that the titanium carbide · nitride particles itch are superior in electrical conductivity are homogeneously dispersed in the ceramic composite. They contact each other to form a network of electrical conduction and improve the conductivity. Furthermore, sintered composites of high density can be obtained by joint addition of boron or boron compounds and carbon or carbon compounds as sintering aids. When the sintering aid for SiC is not added, sintered bodies of high density can hardly be obtained and it becomes difficult to make ceramics of high toughness and high-temperature strength which are intended by this invention. Boron or boron compounds and carbon or carbon compounds added as sintering aids are considered to have the following purpose: Carbon or carbon compounds reduce and remove the silicon oxide (SiO₂) film formed on SiC particles to increase the surface energy of the SiC particles and activate them. On the other hand, boron (boron in boron compounds) diffuses into the activated SiC particle surface to decrease the surface energy of SiC particles, whereby the surface diffusion of the SiC particles is inhibited and the diffusion and boundary diffusion of the SiC particles are promoted to accelerate the densification of SiC. Aluminum (Al) and aluminum compounds have also been known as sintering aids for SiC. However, when aluminum or aluminum compounds are used as a sintering aid for SiC, a sintered body of high density can be obtained, but there is formed mainly an Al₂O₃-SiO₂ glass phase at the grain boundaries in the sintered body, which deteriorates the high-temperature strength at 1200-1500°C. Sintered non-oxide ceramic bodies of high density, good high-temperature strength and high toughness and electrical conductivity can be produced only by combining SiC with titanium carbides, nitrides or carbonitrides such as TiC, TiN and TiCN with simultaneous addition of boron or boron compounds and carbon or carbon compounds and the problems of low toughness and low electrical conductivity common to ceramic materials have been solved. The SiC powders used in this invention include those of β-type or α-type in crystalline form or mixed powders of them or amorphous SiC powders and are chemically highly pure ultrafine powders of normally 10 µm or less, preferably 1 µm or less (submicron) in average particle size. Carbides, nitrides and carbonitrides (these may be called merely "carbide · nitride") of titanium which are combined with SiC may be any of TiC, TiN, and mixed powders of them or carbonitride (TiCN). Their amount is 5-95%, preferably 10-90%, most preferably 30-70% by weight of the total amount. These titanium carbide · nitrides are chemically highly pure ultrafine powders of normally 10 µm or less, preferably about 1 µm in average particle size. When the amount of the titanium carbide · nitride is less than 5% by weight, an effect of powder dispersion with SiC powders is not seen and a remarkable increase of toughness cannot be expected. When the amount is more than 95% by weight, the effect of powder dispersion is small and the oxidation resistance at high temperatures is deteriorated. As sintering aids for SiC, 0.1-5%, preferably 0.3-3% by weight of boron or boron compounds in terms of boron and based on the amount of SiC and 0.1-5%, and/or preferably 0.3-3% by weight of carbon or carbon compounds in terms of carbon and based on the amount of SiC are simultaneously added. As boron or boron compounds, there may be used, alone or in combination of two or more, metallic boron (B) or boron compounds such as boron carbide (B₄C), boron nitride (BN), boron oxide (B₂O₃) and the like. When the amount of boron or boron compounds is less than 0.1% by weight of SiC in terms of boron, a dense sintered body can hardly be obtained and when it is more than 5% by weight, the preferred characteristics of the sintered SiC-titanium/carbide · nitride composite are damaged. As carbon or carbon compounds, there may be used, alone or in combination of two or more, carbon black, acetylene black and the like and furthermore organic materials such as phenolic resins, polyvinyl alcohol, corn starch, sugars, coal tar pitch and the like which provide residual carbon upon sintering. When the amount of carbon or carbon compounds is less than 0.1% by weight of SiC in terms of residual carbon, a dense sintered body can hardly be obtained and when it is more than 5% by weight, the preferred characteristics of the sintered SiC-titanium carbide · nitride composite are damaged. As explained above, it becomes possible to produce a sintered non-oxide ceramic composite having preferred characteristics such as high density, high-temperature strength, toughness and electrical conductivity by combining SiC wits titanium carbide · nitride such as TiC, TiN and TiCN with simultaneously adding boron or boron compounds and carbon or carbon compounds as sintering aids for SiC. It is not easy to accurately determine the theoretical density of such composite ceramics as of this invention, but in order to obtain the above-mentioned preferred characteristics, it is necessary that the sintered body has a density of at least 85% of the theoretical value, especially preferably at least 90% of the theoretical value. For sintering, there may be employed any of the known sintering methods such as pressureless sintering, hot press sintering, hot isostatic pressure sintering (HIP), etc. This invention will be further explained by the following examples. Example 1 To SiC powder (β-type crystal of 0.3 µm in average particle size and at least 98% in purity) was added TiC powder (1.2 µm in average particle size and at least 99% in purity) at the weight ratios as shown in Table 1 and these were well mixed in a ball mill. Then, the mixture was subjected to hot press sintering at 2150°C and under a pressure of 40 MPa for 2 hours in an argon atmosphere. B₄C as a boron compound was added in an amount of 1% by weight of SiC in terms of boron and carbon black as a carbon compound was also added in an amount of 1% by weight of SiC. The thus obtained sintered body was worked into a test piece of 3 mm × 4 mm × 38 mm according to JIS R1601 and its density was measured by the water displacement method. Thereafter, the test piece was subjected to the three-point bending test at room temperature and a high temperature of 1500°C in argon gas. The fracture toughness of the sintered body was evaluated by the IM (Indentation Microfracture) method and SEPB (Single Edge Pre-Crack Beam) method which comprises introducing pre-cracks into the sintered body by pop-in and obtaining K IC . Table 1 shows the relative densities (in percent of the theoretical value), the average three-point bending strengths at room temperature and a high temperature (1500°C) and the K IC values of the sintered bodies of the respective compositions. It is recognized from Table 1 that the strength at room temperature and high temperatures is improved by combining SiC with TiC. Especially the strength reaches a maximum when the addition amount of TiC is 5-30% by weight and the strength is not deteriorated even at a high temperature of 1500°C. It is also clear that K IC is markedly improved when SiC is combined with TiC as compared with monolithic SiC and monolithic TiC and the K IC reaches a maximum with the addition of TiC in an amount of about 50% by weight. Example 2 β-type SiC of 0.3 µm in average particle size and 98% or more in purity which was similar to that used in Example 1 was well mixed in a ball mill with TiC powder (average particle size: 1.2 µm and purity: 99% or more) and TiN powder (average particle size: 1.2 µm and purity: 99% or more) at the weight ratios as shown in Table 2 and the mixture was subjected to hot press sintering at 2150°C for 2 hours under 40 MPa in an argon inert atmosphere. As in Example 1, B₄C as a boron compound was added in an amount of 1% by weight of SiC in terms of boron and carbon black as a carbon compound in an amount of 1% by weight of SiC. The toughness of the resulting sintered bodies was evaluated by measuring the fracture toughness K IC in accordance with the SEPB method as in Example 1. Table 2 shows the relative densities (in percent of the theoretical value) and the K IC values of the sintered bodies of the respective composition. It is clear from Table 2 that when SiC is combined with titanium carbide · nitride, the fracture toughness K IC is markedly improved as compared with monolithic SiC, monolithic TiC and monolithic TiN. Example 3 The electrical conductivity was measured for the sintered bodies made in Examples 1 and 2. Table 3 shows the electrical conductivities of the sintered ceramic composites of the respective compositions. Table 3 indicates that the electrical conductivity is conspicuously improved by combining SiC with TiC and TiN as compared with monolithic SiC. Example 4 To β-type SiC of 0.3 µm in average particle size and 98% or more in purity which was the same as used in Example 1 was added 50% by weight of TiC powder (average particle size: 1.2 µm and purity: 99% or more). B₄C and carbon black were further added at the weight ratios as shown in Table 4. These powders were well mixed in a ball mill and the resulting mixture was subjected to hot press sintering at 2150°C, for 2 hours and under 40 MPa in an argon atmosphere. The strength and toughness of thus obtained sintered bodies were evaluated in the same manner as in Example 1. Table 4 shows the relative density (in percent of the theoretical value, the three-point bending strength and the K IC of the sintered bodies of the respective addition amounts of B₄C and C. It is seen from Table 4 that sintered bodies having high density and excellent characteristics can be obtained by adding suitable amounts of B₄C and C. Comparative Example In order to examine the effects of boron or boron compounds and carbon or carbon compounds as sintering aids for SiC, measurements were conducted on SiC and TiC composite ceramics where no sintering aid was added and where aluminum oxide (Al₂O₃: average particle size: 0.2 µm and purity: 99.9% or more) was added in an amount of 2% by weight of the total weight as a sintering aid. Powders were prepared at the weight ratios as shown in Table 5 and well mixed in a ball mill in the same manner as in Example 1. The resulting mixture was subjected to hot pressing at 2150°C, for 2 hours, under 40 MPa in an argon inert atmosphere. The sintered bodies were formed to test pieces and subjected to the bending test in the same manner as in Example 1. Table 5 shows the relative density (in percent of the theoretical value) and the results of the three-point bending test at room temperature and a high temperature of 1500°C of the sintered bodies of the respective compositions. It is clear from Table 5 that when no sintering aid was used in the SiC-TiC ceramic system, sintered bodies of high density can hardly be obtained and the strength is very low and when Al₂O₃ was used as a sintering aid, sintered bodies of high density can be obtained, but the high-temperature strength at 1500°C is conspicuously deteriorated. According to this invention, sintered nonoxide ceramic bodies high in density and excellent in high-temperature strength, toughness and electrical conductivity can be obtained by combining silicon carbide with at least one carbide, nitride or carbonitride of titanium with simultaneous addition of boron or a boron compound and carbon or a carbon compound. The sintered non-oxide ceramic composites of this invention are free from the problems of low toughness and low reliability common to ceramic materials and besides, electric discharge works can be widely applied.

Claims: 1. A sintered non-oxide ceramic composite high in density, high-temperature strength and toughness which is produced by compacting and sintering a mixed powder which comprises 5-95% by weight of at least one carbide, nitride or carbonitride of titanium, 0.1-5% by weight of boron or a boron compound, based on the weight of the silicon carbide and in terms of boron, 0.1-5% by weight of carbon or a carbon compound, based on the weight of the silicon carbide and in terms of residual carbon, and the balance being substantially silicon carbide. 2. A sintered body according to claim 1, wherein the amount of the carbide, nitride and/or carbonitride of titanium is 10-90% by weight. 3. A sintered body according to claim 2, wherein the amount of the carbide, nitride and/or carbonitride of titanium is 30-70% by weight. 4. A sintered body according to claim 1, which has a density of at least 85% of the theoretical value. 5. A sintered body according to claim 1, wherein the amount of boron or boron compound is 0.3-3% by weight of the silicon carbide and in terms of boron and that of carbon or carbon compound is 0.3-3% by weight of the silicon carbide and in terms of residual carbon. 6. A sintered body according to claim 1, wherein the silicon carbide powder is a crystalline β-type powder, α-type powder, a mixed powder thereof or an amorphous powder. 7. A sintered body according to claim 6, wherein the silicon carbide powder has an average particle size of 10 µm or less. 8. A sintered body according to claim 7, wherein the silicon carbide powder has an average particle size of 1 µm or less. 9. A sintered body according to claim 1, wherein the boron or boron compound is metallic boron, boron carbide, boron nitride and/or boron oxide. 10. A sintered body according to claim 1, wherein the carbon or carbon compound is an organic material which provides residual carbon when sintered. 11. A sintered body according to claim 10, wherein the organic material is carbon black, acetylene black, a phenolic resin, polyvinyl alcohol, corn starch, a sugar and/or coal tar pitch. 12. A method for producing a sintered non-oxide ceramic composite high in density, high-temperature strength and toughness according to any one of claims 1 to 11, which comprises mixing silicon carbide powder with 5-95% by weight of a powder of at least one carbide, nitride or carbonitride of titanium, 0.1-5% by weight of a powder of boron or a boron compound, based on the weight of the silicon carbide and in terms of boron, and 0.1-5% by weight of powder of carbon or carbon compound, based on the weight of silicon carbide and in terms of residual carbon, compacting the resulting mixture and then sintering the compacted mixture. 13. A method according to claim 12, wherein the powder of boron or a boron compound and the powder of carbon or a carbon compound are simultaneously added to the silicon carbide powder.

Inventors: ENDO, HIDEHIRO C/O DAIICHIGIJUTSU KENKYUSHO, KUBO, HIROSHI C/O DAIICHIGIJUTSU KENKYUSHO, TANEMOTO, KEI C/O DAIICHIGIJUTSU KENKYUSHO
Applicants: NIPPON STEEL CORPORATION
Application year: 1987
Main classification: C04B35
IPC codes: C04B35