Process for producing ceramic composites

Abstract: Ceramic powders selected from the group (a) consisting of ZrB₂, HfB₂, ZrC, HfC, ZrN and HfN and ceramic powders selected from the group (b) consisting of TiC, TiN and TiO₂ are mixed, the mixture is sintered in a non-oxidizing atmosphere, a substitution reaction of chemical elements between the compound of the ceramic powders of the group (a) and the compound of the ceramic powders of the group (b) is caused in the sintering, and a ceramic composite substantially comprising compounds which do not belong to the groups (a) and (b) is produced by the substitution reaction. By this process, the affection of the particle size and the aggregation of raw powders to the sintering can be greatly reduced, the ranges of the applicable sintering conditions can be broadened, and dense ceramic composites having fine crystal grains and having excellent mechanical properties can be obtained.

Description: The present invention relates to a process for producing a ceramic composite suitable for materials in various uses requiring strength and toughness such as a part of an internal combustion engine for automobiles such as a glow plug, a turbine wheel of a turbo charger, a piston cap, a lining of a cylinder, a tappet, an exhaust valve, a hot plug in a sub-combustion chamber of a diesel engine, a part of an aircraft or a spacecraft such as a fan, an air compressor of a jet engine, a housing of a combustion chamber, a heat insulator of a nose cone, a slidable part of a pump, a die, a tool, cutlery, a thread guide, heat resisting pipe, a pipe for a high-temperature gas, a crucible for dissolution, a roll for a rolling mill, etc. Various processes which are not accompanied with reaction during sintering are known as processes for producing ceramic composites. JP-A-SHO 58-120571 discloses a process for producing TiB₂-ZrO₂, TiC-ZrO₂, TiN-ZrO₂ and TiCN-ZrO₂ composites containing TiB₂, TiC, TiN and TiCN as respective main components. JP-A-SHO 47-35011 (corresponding to U.S. Patents 3,808,012 and 3,859,399) discloses a process for producing a ceramic composite containing TiB₂, B₄C, SiC and Si. JP-A-SHO 49-85115 discloses a process for producing a ceramic composite containing TiB₂ and WC-system cemented carbide. JP-A-SHO 58-217463 and JP-A-SHO 62-292678 disclose processes for producing a ceramic composite containing TiB₂ and Al₂O₃. JP-A-SHO 59-118828 (corresponding to U.S. Patent 4,514,355) discloses a process for producing a ceramic composite containing TiB₂, BN and Al₂O₃. JP-A-SHO 60-226459 discloses a process for producing a ceramic composite containing TiB₂, B₄C and TiN. JP-A-SHO 61-270265 discloses a process for producing a ceramic composite containing TiB₂, TiC and SiC. As processes for producing ceramic composites containing ZrC, the process disclosed in "Journal of Material Science" (vol. 22, page 1135-1140, 1987) wherein ZrC is combined with Al₂O₃ and the process disclosed in "Poroshkovaya Metallurgiya" (vol. 149, page 61-64, 1975) wherein ZrC is combined with ZrB₂ are known. It is common to the conventional processes described above that raw powders (hereinafter also called "starting materials") comprising mixed powders consisting of the same compositions as those of finally obtained ceramic composite are prepared, a desired article is formed with the raw powders and the obtained green article is sintered. In such conventional processes, however, the following problems are present. First, there is a problem that it is difficult to obtain a sintered material dense and having fine crystal grains. Because the particle size of sintered materials generally becomes greater than that of raw powders on account of the growth of crystal grains, the particle size of the raw powders is needed as small as possible in order to obtain a sintered material dense and having fine crystal grains. In practice, however, it is difficult to get raw materials of fine ceramic powders with a commercial base, and particularly non-oxide ceramic powders comprising fine powders having particle size of less than submicrometer are not available in the present circumstances. Accordingly, the particle size of the raw materials which can be industrially applied is limited to an extent, and densification and fining of obtained sintered materials are also limited. If an obtained sintered material is not dense and the crystal grains thereof are not fine, high mechanical properties thereof cannot be attained. Moreover, although raw materials of ceramic powders are desirably as fine as possible as described above, the raw powders are liable to aggregate and become difficult to disperse as they become finer. Since more or less the aggregation of raw powders occurs in the conventional processes, the aggregation causes the segregation of mixed raw powders, the structure of obtained sintered material becomes nonuniform and the mechanical properties thereof decrease. Further, in the conventional processes, fairly high temperature, high pressure and long time are required as sintering conditions to make pores as small as possible and obtain a denser sintered material. Accordingly, applicable sintering conditions are limited by the kind, particle size, purity etc. of raw powders, and the sintering conditions are inevitably fairly high temperature, high pressure and long time. Satisfactory sintering cannot be achieved under the conditions of relatively low temperature, low pressure and short time in the conventional processes. Furthermore, particularly in the conventional process wherein metal powders are used as a starting material, there is the following problem. If a part of the metal powders remain in the sintered material without the reaction and without converting to a ceramic (i.e. as they are), not only the sintered material becomes porous because the metal powders remain with segregation, but also the mechanical properties of the sintered material greatly decrease because the segregated portions become origins for the fracture of the material. Meanwhile, as processes accompanied with reaction during sintering, reaction-sintering process and self-combustion process are known. The reaction-sintering process is a process wherein, after forming a desired article with raw metal powders by using one of various forming methods, a ceramic such as Si₃N₄ and SiC is produced in the article by a special heat treatment. For example, after forming an article with Si powders, the article is nitrified in ammonia or N₂ atmosphere, and Si₃N₄ is produced by the reaction of 3Si + 2N₂ → Si₃N₄ which occurs in the nitriding. The self-combustion process is a process wherein mixed powders including metal powders which should become a component of a target inorganic compound having a high melting point are ignited, and self propagation of the reaction is performed by utilizing the heat of the reaction, thereby achieving a high composition rate. For example, the mixture of Ti powders and B powders is ignited and the reaction of Ti + 2B → TiB₂ is performed. This process can be applied not only to the composition of a ceramic consisting of a single compound such as TiB₂ or TiC but also to the composition of a ceramic composite such as TiB₂-TiC system. In these reaction-sintering process and self-combustion process, Si₃N₄, SiC, TiB₂, TiC etc. are produced by a specified solid solution process, so-called "interstitial solid solution", wherein the atoms of N, B or C are intruded into crystalline lattices of Si or Ti which is a metal. Because a large thermal energy is generated by this intrusion, the condition of the sintering, in general, is limited to a specified condition of a fairly high temperature. Accordingly, it is difficult to conduct a desired sintering and obtain a dense sintered material under the condition of a relatively low temperature. It would be desirable to provide a process for producing ceramic composites which has less dependency on the particle size of a raw material and can obtain ceramic composites dense, having fine crystal grains and having excellent mechanical properties by sintering accompanied with particular reaction different from the conventional reactions. Further, it would be desirable to provide a process for producing ceramic composites which can obtain an extremely uniform structure of a sintered material even if the aggregation of its raw powders occurs a little. Still further, it would be desirable to provide a process for producing ceramic composites which can enlarge the ranges of sintering conditions and achieve a desired sintering even at relatively low temperature and low pressure and in a relatively short period of time. Still further, it would be desirable to provide a process for producing ceramic composites which can prevent the mechanical properties of an obtained sintered material from greatly decreasing even if a part of starting materials remains in the sintered material without reacting. A process for producing a ceramic composite, according to the present invention, is provided which comprises the steps of: mixing at least one kind of ceramic powders selected from ceramic powders each consisting of a compound selected from the group (a) and at least one kind of ceramic powders selected from ceramic powders each consisting of a compound selected from the group (b), (a) ZrB₂, HfB₂, ZrC, HfC, ZrN, HfN, (b) TiC, TiN, TiO₂; sintering a mixture the main component of which consists of the mixed ceramic powders in a non-oxidizing atmosphere; causing a substitution reaction of chemical elements between the compound selected from the group (a) and the compound selected from the group (b) by the sintering; and making a ceramic composite substantially comprising compounds which do not belong to the group (a) and the group (b) by the substitution reation. The process according to the present invention will be explained in detail. Mixing process: In the process according to the present invention, first, at least one kind of ceramic powders selected from ceramic powders each consisting of a compound selected from the following group (a) and at least one kind of ceramic powders selected from ceramic powders each consisting of a compound selected from the following group (b) are mixed. (a) ZrB₂, HfB₂, ZrC, HfC, ZrN, HfN (b) TiC, TiN, TiO₂ Where, in the combination of a compound of group (a) and a compound of group (b), the combination, in which the substitution reaction described later in detail would not occur between the compounds because the compounds are stable to each other, i.e. the combination including a combination of ZrC and TiC, HfC and TiC, HfN and TiN or ZrN and TiN, is not applied in the present invention. All of other combinations are available. The particle size of the ceramic powders of group (a) and group (b) is preferably not greater than 5.0 µm such that the sintering accompanied with substitution reaction for the mixture of the ceramic powders may be conducted more promptly and more uniformly. This range of not greater than 5.0 µm is much broader than that in the conventional processes aforementioned. The mixing ratio of the ceramic powders consisting of a compound selected from group (a) and the ceramic powders consisting of a compound selected from group (b) is selected from the range of 1:0.2-1:2.0 in mole ratio, although this ratio is varied in accordance with the average particle sizes of the ceramic powders, the presence and addition amount of a stabilizer described later, the presence and addition amount of other components, and the conditions of the sintering. Namely, in the present invention, an equimolar substitution reaction such as reactions shown by the following reaction formulas is presumed to occur. For example, TiC + ZrB₂ → TiB₂ + ZrC, TiO₂ + ZrB₂ → TiB₂ + ZrO₂, TiO₂ + ZrC → TiC + ZrO₂, TiO₂ + ZrN → TiN + ZrO₂, or TiO₂ + x ZrC + (1-x) ZrN → Ti(C x N (1-x) ) + ZrO₂. Accordingly, the mole ratio of 1:1 is desirable from the theoretical point of view. Even though unreacted ceramic powders of group (a) and/or group (b) remains by a small amount in the sintered material, however, it is understood that the properties of the sintered material obtained is not influenced so much by the residual unreacted ceramic powders because the unreacted ceramic are originally ceramics in the present invention and the unreacted ceramic can bond with adjacent ceramic by normal sintering bond. Therefore, the above range of the mixing ratio of ceramic powders of group (a) and group (b) is reasonable. The mixing method for the ceramic powders of group (a) and group (b) may be any of a dry mixing method and an wet mixing method. Wet mixing is more desirable from the viewpoint of the uniformity of the dispersion of raw powders. For example, an organic solvent such as isopropanol, ethyl alcohol, ethylene glycol or dimethylsulfoxide is added to the mixture of the ceramic powders, and the mixture is sufficiently blended and milled by an attrition mill. By this, secondary aggregation of the mixture can be decomposed and primary particles can be quite uniformly dispersed in the mixture. After the mixture is blended and milled, it is dried under reduced pressure by a rotary evaporator. That preferably a rotary evaporator is used is because segregation due to gravity difference is liable to occur in natural drying or constant-temperature drying. In the process according to the present invention, in a case where ZrO₂ or HfO₂ is contained in the ceramic composite obtained by sintering, the crystal structure of the ZrO₂ or HfO₂ sometimes presents a monoclinic or mainly monoclinic structure. To make this crystal structure tetragonal symmetry, a stabilizer may be added to the mixture of the ceramic powders. The stabilizer can be selected from the group consisting of Y₂O₃, CeO₂, MgO and CaO. The average particle size of the stabilizer is preferably not greater than 0.5 µm. The addition of the stabilizer is controlled to 2-5 mol.% for Y₂O₃, 8-16 mol.% for CeO₂, 6-12 mol.% for MgO and 3-15 mol.% for CaO as compared with the amount of the ZrO₂ or HfO₂ produced by substitution reaction. Of course, two or more kinds of the stabilizers may be used at a time. For example, 0.5-3 mol.% Y₂O₃ and 2.0-14.0 mol.% CeO₂, 2.0-10 mol.% MgO and the same mol.% CaO, or 2.0-14.0 mol% CeO₂ and 2.0-10.0 mol.% MgO can be used in combination, respectively. The mixture of the above ceramic powders may contain a so-called third component which is substantially irrelevant to the substitution reaction caused in sintering. Namely, oxide powders such as SiO₂, Al₂O₃ or mullite (Al₆Si₂O₁₃) or non-oxide powders such as WC, SiC, TiC, ZrC, TiN, ZrN, AlN, Si₃N₄ or WB may be added to the mixture according to the property to be desired of the obtained ceramic composite. If such a third component is added too much, the substitution reaction to be performed is likely to be obstructed because the gaps between the ceramic powders such as TiC or TiO₂ and the ceramic powders such as ZrB₂, ZrC or ZrN are enlarged by the presence of the third component. Therefore, the addition of this third component is preferably suppressed to not greater than 40 vol.%. The average particle size of the oxide powders such as SiO₂, Al₂O₃ and mullite is preferably not greater than 0.5 µm and that of the non-oxide powders such as WC, SiC, TiC, ZrC, TiN, ZrN, AlN, Si₃N₄ and WB is preferably not greater than 6.0 µm. Further, metal powders having an average particle size of not greater than 7.0 µm such as Si, Mg, Ca, Al, Ti, Zr, Hf, Ta, W, Ni, Co, Mo or Fe, metal-oxide powders having an average particle size of not greater than 0.5 µm such as MgO, Al₂O₃, SiO₂, CaO, ZrO₂ or HfO₂ or metal-carbide or metal-nitride powders having an average particle size of not greater than 3.0 µm such as B₄C, SiC, HfC, TaC, WC, NbC, Cubic BN, Si₃N₄, AlN, TiN, ZrN, HfN or TaN can be added as a third component by not greater than 40 vol.%, preferably not greater than 20 vol.%. In the addition of the metal powders, however, it is necessary to increase the dispersibility of the metal powders to prevent the uneven distribution of the metal powders. Moreover, since metal is generally liable to plastically deform and it is difficult to make fine powders, preferably the metal is added as a formation of hydride such as TiH₂ or ZrH₂ or a formation of metal oxalate, metal chloride, metal nitrate or metal alkoxide and such a metal compound is thermally decomposed in the sintering process. In this case, it is also necessary to sufficiently blend the metal-compound powders to prevent their uneven distribution. In the addition of the third component described above, the addition of ZrN, ZrC, TiN, TiC, HfN or HfC as a third component is restricted to a case wherein these compounds are not used as ceramic powders of group (a) or (b). Furthermore, whiskers may be added to the mixture of ceramic powders as a third component by 5-40 vol.%. If whiskers are added, when a crack propagates in the matrix of the ceramic composite obtained, the whiskers deflect the crack, the crack branches at the positions of the whiskers and a fracture energy can be absorbed. Moreover, since the fracture energy can be absorbed by the pull-out effect of the whiskers, the fracture strength and fracture toughness of the ceramic composite increase extremely. Whiskers having such an operation are preferably in the ranges of 0.2-2.5 µm in diameter, 5-50 µm in length and not less than 10 in aspect ratio. The whiskers are preferably at least one kind of whiskers selected from the group consisting of SiC whiskers, B₄C whiskers and Si₃N₄ whiskers, and particularly SiC whiskers are preferable because SiC whiskers are easy to be obtained commercially, the Young's modulus and rigidity thereof are higher and the oxidation-resistant property thereof under high temperature is excellent. The addition of the whiskers is preferably in the range of 5-40 vol.%. If the addition is less than 5 vol.%, the increase of the fracture strength and the fracture toughness cannot be expected so much, and if the addition is more than 40 vol.%, the density of the ceramic composite obtained decreases. When whiskers are added, desirably they are beforehand sufficiently dispersed in an organic solvent such as ethyl alcohol, toluene or isopropyl alcohol by applying ultrasonic waves etc., using polyethylene imine or trichlorooctadecylsilane etc. as a dispersant. The third component described above is added in accordance with the increase of a property etc. to be desired of the ceramic composite obtained. The third components increasing the density of the ceramic composite are preferably Si, Mg, Ca, Al, Ti, Zr, Hf, Mo, Co, Ni, Fe, W, Ta, SiO₂, HfO₂, ZrO₂, Al₂O₃, CaO and MgO. The third components increasing the strength of the ceramic composite are preferably ZrO₂, Al₂O₃, SiO₂, SiC whisker, Si₃N₄ whisker and B₄C whisker. The third components increasing the toughness of the ceramic composite are preferably ZrO₂, Si₃N₄, SiC whisker, Si₃N₄ whisker and B₄C whisker. The third components increasing the hardness of the ceramic composite are preferably B₄C, cubic BN, SiC, HfC, WC, TaC, NbC, SiC whisker, Si₃N₄ whisker and B₄C whisker. The third components increasing the thermal conductivity of the ceramic composite are preferably B₄C, cubic BN, SiC, HfC, WC, TaC, NbC, ZrN, TiN, HfN, AlN, TaN, SiC whisker, Si₃N₄ whisker and B₄C whisker. The third components increasing the electric conductivity of the ceramic composite are preferably SiC, HfC, WC, TaC, NbC, SiC whisker, Si₃N₄ whisker and B₄C whisker. Sintering process: In the process according to the present invention, the mixture prepared as described above is sintered. There are two methods on this sintering. One is a method wherein the mixture is formed as a required shape by cold isostatic pressing, die pressing, slip casting or injection molding and then the shaped mixture is pressure or pressureless sintered. The other is a method wherein the mixture is pressure sintered, i.e. sintered as well as being pressed, by hot pressing, hot isostatic pressing (HIP) etc. without forming the mixture in advance. In any case, the sintering is carried out in a non-oxidizing atmosphere preferably at a temperature of 1000-1900°C. The non-oxidizing atmosphere in the present invention means an atmosphere which substantially cannot produce an oxide in the atmosphere at the sintering temperature. The preferable atmosphere is an inert atmosphere such as nitrogen gas or argon gas, or reducing atmosphere such as carbon monoxide gas or hydrogen gas. Argon gas is more preferable in these gases because the content of oxygen can be reduced to a smaller extent. Even though an atmosphere contains an oxidizing gas, a non-oxidizing atmosphere applicable to the process according to the present invention can be formed by reducing the pressure of the atmosphere to a degree of vacuum of not greater than 13.33 Pa (0.1 Torr). In the sintering associated with pressing, the mixture is sintered preferably by heating the mixture to a sintering temperature at a heating rate of 5-10°C/min. and then by maintaining the mixture at this temperature for 0.5-2 hours, in order to prevent the temperature distribution from being nonuniform. In this sintering, the pressing may be carried out since a time before the heating, may be carried out gradually as the mixture is heated, or may be carried out after the mixture is heated to a sintering temperature without carrying out the pressing during raising the temperature. Also in the sintering without pressing the mixture, the heating rate of the mixture to a sintering temperature is preferably in the same range as that in the sintering with pressing. In the process according to the present invention, ceramic powders consisting of compounds selected from groups (a) and (b) different from compounds of the main components of the ceramic composite to be obtained are used as starting materials, and a substitution reaction of chemical elements is caused between the compound selected from the group (a) and the compound selected from the group (b) by sintering. By this substitution reaction, a ceramic composite substantially comprising compounds which do not belong to the groups (a) and (b) can be obtained. For example, in the substitution reactions represented by the following reaction formulas; TiC + ZrB₂ → TiB₂ + ZrC, TiO₂ + ZrC → TiC + ZrO₂, or TiO₂ + ZrN → TiN + ZrO₂, Ti and Zr are substituted for each other. In the substitution reaction represented by the following reaction formula; TiC + HfB₂ → TiB₂ + HfC, Ti and Hf are substituted for each other. Accordingly, in the reaction sintering according to the present invention, a ceramic composite is produced not by "interstitial solid solution" in the conventional sintering processes but by so-called "substitutional solid solution" wherein chemical elements are substituted between Ti and Zr or Ti and Hf in the compounds. Namely, in the present invention the ceramic composites are produced by a process quite different from the conventional processes. Since the mass transfer occurs between the substituted chemical elements in this sintering associated with the substitution reaction, the ceramic composite obtained can be sufficiently densified. According to the observation of the ceramic composite obtained by the process according to the present invention, the growth of the grains does not occur so much. This is presumably understood because, in the process according to the present invention, the chemical elements can be substituted for each other by a relatively small external energy and the particle size of each original grain can be maintained substantially as it is, as compared with that, in the conventional processes, atoms are incorporated into crystalline lattices by a large external energy such as that required in the "interstitial solid solution" type sintering, the grain boundaries are vanished and the growth of the crystal grains occurs. Moreover, in the present invention, there occurs even a case where, depending on raw materials, the crystal grains are decomposed in the substitution reaction at a temperature during sintering and the particle size of the crystal grains becomes smaller than the original size. Further, with respect to the substitution reaction, it can be considered that a kind of decomposition of a compound occurs when or before the substitution is performed. Because the chemical elements decomposed and to be substituted for other chemical elements disperse uniformly in the material being sintered, a very uniform structure of the sintered material can be obtained after the required substitution reaction finishes. Therefore, the dispersibility of the raw powders is not a great matter, and the structure of the ceramic composite obtained can be extremely uniform even if aggregable raw powders having a bad dispersibility and a relatively small particle size are used. Accordingly, the process of the present invention has less dependency on the particle size of the raw powders as compared with that in the conventional processes, even raw powders got with a commercial base and having a relatively large particle size can be used, and the range for selection of raw powders can be broadened as well as the restriction in the mixing process of the raw powders can be greatly relaxed. Further, in the process according to the present invention, the ranges of the sintering conditions can be greatly broadened, and the sintering at a relatively low temperature, under a relatively low pressure and in a relatively short period of time becomes possible. Presumably this is due to the fact that the external energy for the sintering is not required so much as in the conventional processes because the substitution reaction itself caused during sintering generates a large driving force for the substitution of chemical elements. In fact, according to the observation of the material being sintered on its thermal-expansion and contraction behavior, initially the substitution reaction begins at about 1000°C, relatively low temperature, and the densification is promoted accompanying with substitution reaction performed. This fact indicates that it is possible to promote the densification of the material to be sintered by maintaining the material at a temperature near the temperature at which the substitution reaction begins for a relatively long period of time as well as it is possible to complete the sintering in a short period of time by raising the sintering temperature. Therefore, the sintering conditions can be selected from the broad ranges of temperature and time. Similarly, the range of sintering pressure to be applied can be broadened. In other words, the density of the material sintered by the process according to the present invention can be increased much more than that sintered by the conventional processes under the same sintering condition. To achieve a density of the sintered material in the conventional processes similar to the density according to the present invention, the pressure must be further increased, the temperature must be further raised and the time must be further lengthened. Furthermore, in the process according to the present invention, because ceramic powders are used as raw powders, the powders or the grains adjacent to each other can bond with the normal sintering bond of a ceramic composite even if a part of the raw powders remain without reaction. Therefore, the mechanical properties of the ceramic composite obtained by the process according to the present invention can be increased as compared with those of the ceramic or the ceramic composite obtained by the conventional processes such as reaction-sintering process or self-combustion process wherein metal powders are used and a part of the metal powders remain with segregation and without reaction. Thus, a ceramic composite is produced via "substitutional solid solution" due to the substitution reaction in the process according to the present invention. The ceramic composite thus obtained has the following some interesting characteristics which cannot be observed in the ceramics or the ceramic composites produced by the conventional processes. First, the ceramic composite obtained by the process according to the present invention has a fairly large compressive strain of the crystalline lattice. This strain of the crystalline lattice is recognized by the fact that a peak shift is present in a diffraction figure when the ceramic composite is determined by X-ray diffraction method, and the peak shift is much greater than that caused by the pressing force applied in machining. Therefore, this large strain of the crystalline lattice can be considered to be caused by substitution reaction. Namely, it can be considered that the stress capable of greatly straining the crystalline lattice is generated accompanied with the substitution reaction. The large strain of the crystalline lattice presumably contributes to the increase of the mechanical properties of the ceramic composite, because of arresting crack propagation. This strain of the crystalline lattice is recognized the followings: For example, when TiB₂-ZrC ceramic composite whose raw powders are ZrB₂ powders and TiC powders is determined by X-ray diffraction method, extremely large peak shift, it means that large strain and large compression stress exist in sintered body, is observed compared to that by conventional process. It is considered that this compression stress is much larger than the compression stress by machining or hot pressing and this large compression stress is generated by distorted crystalline lattice due to substitution reaction sintering. As this large compression stress is maintained in this sintered ceramic composite, high mechanical properties are expected. Further, peculiar grain boundary layers exist in the ceramic composite obtained by the process according to the present invention. For example, when TiB₂-ZrC ceramic composite whose raw powders are ZrB₂ powders and TiC powders is observed by a transmission electron microscopy, grain boundary layers almost cannot be observed at the positions between TiB₂ grains and between ZrC grains and coherent grain boundaries at which crystal grains directly adhere to each other are formed. On the other hand, there exists a grain boundary layer between TiB₂ grain and ZrC grain, which is presumed to be a reaction product due to the substitution reaction. Although the operation of this grain boundary layer is not clear, it can be presumed that the layer increases the bonding strength between TiB₂ grain and ZrC grain as well as, when a difference of thermal expansion or contraction occurs between the grains, the layer prevents a crack from propagating by absorbing the difference, from the viewpoint of the fact that the mechanical properties, particularly strength and toughness, of the ceramic composite obtained by the process according to the present invention are higher than those of the composite obtained by the conventional processes. Furthermore, although the process according to the present invention accompanies a substitution reaction, the substitution reaction is rarely completed, and more or less, intermediate products exist in the ceramic composite obtained. For example, when TiB₂-ZrC ceramic composite whose raw powders are ZrB₂ powders and TiC powders is observed, the crystal grains of (Ti,Zr)B₂ and (Ti,Zr)C exist, and the former is TiB₂-rich and the latter is ZrC-rich. This is recognized by defining the field of view for the determination by a transmission electron microscopy (TEM), confirming the existence of B and C by an electron energy loss spectroscopy (EELS) and determining the existence ratio of Ti and Zr from the peak of X-ray spectral figure using an energy dispersion X-ray spectroscopy (EDS). Such a fact described above evidently shows the trace of mass transfer, and it is peculiar with respect to the crystal structure and bonding state of crystal grains for a sintered material. The ceramic composite obtained by the process according to the present invention has excellent mechanical properties, particularly excellent strength and toughness, and therefore, the composite can be applied to various uses such as a part of an internal combustion engine for automobiles such as a glow plug, a turbine wheel of a turbo charger, a piston cap, a lining of a cylinder, a tappet, an exhaust valve, a hot plug in a sub-combustion chamber of a diesel engine, a part of an aircraft or a spacecraft such as a fan, an air compressor of a jet engine, a housing of a combustion chamber, a heat insulator of a nose cone, a slidable part of a pump, a die, a tool, cutlery, a thread guide, heat resisting pipe, a pipe for a high-temperature gas, a crucible for dissolution, a roll for a rolling mill, etc. The objects, features and advantages of the present invention will become apparent and more readily appreciated from the following detailed description of the preferred exemplary embodiments of the invention, taken in conjunction with the accompanying drawings, in which: FIG. 1 is a microphotograph of the microstructure of a TiB₂-ZrC ceramic composite according to an embodiment (Example 1) of the present invention; FIG. 2 is a microphotograph of the microstructure of a TiB₂-ZrC ceramic composite produced by a conventional process (Comparative Example 1); FIG. 3 is a microphotograph of the microstructure of a TiB₂-ZrO₂ ceramic composite according to another embodiment (Example 17) of the present invention; FIG. 4 is a microphotograph of the microstructure of a TiB₂-ZrO₂-Y₂O₃ ceramic composite according to a further embodiment (Example 18) of the present invention; FIG. 5 is a microphotograph of the microstructure of a TiB₂-3 mol.% Y₂O₃-ZrO₂ ceramic composite produced by a conventional process (Comparative Example 2); FIB. 6 is a microphotograph of the microstructure of a TiC-ZrO₂ ceramic composite according to a further embodiment (Example 23) of the present invention; FIG. 7 is a microphotograph of the microstructure of a TiC-ZrO₂-Y₂O₃ ceramic composite according to a further embodiment (Example 24) of the present invention; and FIG. 8 is a microphotograph of the microstructure of a TiC-3 mol.% Y₂O₃-ZrO₂ ceramic composite produced by a conventional process (Comparative Example 3). Some preferred embodiments of the present invention will be described hereunder referring to the attached drawings (microphotographs). First, methods for determining the compositions and properties of the sintered materials in the Examples and Comparative Examples described later will be explained. (1) Bend strength: Three-point bend loading based on JIS-1601 was used to measure the bend strength of the ceramic composite. The shape of a sample was defined to 3 x 4 x 48 mm and the mausoleums of the sample extending in its longitudinal direction were cut by chamfering at 0.2C. The parallelism between the upper and lower surfaces of the sample was maintained within 0.2%, and the respective surface roughnesses were controlled to 0.8S by polishing them with a diamond grindstone having a grade of #400. The loading speed was 0.5 mm/min. The bend strength was defined as an average strength calculated from the measured data of ten samples. (2) Vickers hardness: It is measured by a micro-Vickers hardness meter under the conditions of a load of 500 g and a loading time of 15 seconds. The measurement was carried out at five points for each sample, and the Vickers hardness was defined as the average hardness. As a pretreatment for this measurement, the sample after the measurement of bend strength described above was polished to a mirror finish by an automatic polishing machine. The automatic polishing machine was precision polishing system MA-200 produced by Musashino Denshi (a Japanese company), and the finishing polish was conducted by cloth polishing using an abrasive with diamond powders having a particle-size distribution from 1 µm to 4 µm. (3) Fracture toughness: The measurement was carried out by a micro-indentation method and the fracture toughness was calculated using Niihara's equation which was disclosed in Journal of the American Ceramic Society, 65(7), C-116, July 1982. The same pretreatment as in the measurement of Vickers hardness was conducted. The measurement was carried out using a Vickers hardness meter under the conditions of a load of 10 kg and a loading time of 15 seconds. After an indenter was press fitted into the sample, the indent and crack caused by the press fitting were observed. When the length of the crack was not less than 2.5 times the length of the half-diagonal of the indent, the crack was defined as a median type crack, and when the length of the crack was less than 2.5 times the length of the half-diagonal of the indent, the crack was defined as a Palmqvist type crack, and then, the fracture toughness was calculated. (4) Composition of sintered material: The composition of a sintered material was identified by X-ray diffraction method. In the measurement, a Cu light source was used, the voltage thereof was set to 30 kV and the current thereof was controlled to 20A. The ratio of an emitter slit, a receiver slit and a scattering slit was set to 1:0.5:1. The scanning speed was 4°/min. and the time constant was 1/4. The schematic composition of the sample was determined from the height of main peaks in the diffraction figure obtained in the measurement range of 20°-80°. (5) Density: The measurement was carried out by Archimedian method using the sample used for the measurement of bend strength. This density was indicated as not an absolute value but a relative value (unit: %) by dividing the measured data by a theoretical value. However, since a material having a relative density of not greater than 95% is a porous material and such a material absorbs moisture, a precise relative density cannot be determined by this method. Accordingly, in the determination of such a material, the outer shape of the sample was measured and the density of the sample was determined from the weight and volume thereof. Examples 1-6, Comparative Example 1 In Example 1, ZrB₂ powders having an average particle size of 0.5 µm and TiC powders having an average particle size of 2.0 µm were mixed at 1:1 in mole ratio. The mixture was blended and milled in ethanol for six hours by an attrition mill using media of SiC balls having a diameter of 3 mm, and further the mixture was dried under reduced pressure by a rotary evaporator. Then, the mixture was hot pressed under vacuum for one hour at a temperature of 1800°C to obtain a ceramic composite. The pressure was set to 20 MPa. Thereafter, the obtained ceramic composite was cut by using a diamond to make a sample having a thickness of 3 mm, a width of 4 mm and a length of 38 mm. The bend strength of this sample was determined by the three-point bend loading method based on JIS-1601 at a room temperature and 1000°C. As a result, the room-temperature bend strength was 55 kg/mm² and the high-temperature bend strength at 1000°C was 93 kg/mm². The high-temperature bend strength at 1000°C was surprisingly 1.7 times the room-temperature bend strength. In Examples 2-6, the kind of starting materials was changed, and the mixing and sintering were carried out in a manner similar to that in Example 1. The conditions and the resulted data in these Examples 1-6 are shown in Table 1. Further, the microphotograph of the microstructure of TiB₂-ZrC ceramic composite obtained in Example 1 is shown in FIG. 1. The structure of the sintered material comprises two phases of TiB₂ and ZrC, the gray portions in the photograph are TiB₂ and the white portions are ZrC. The respective grain sizes are about 2 µm and 3 µm, and the grain size of ZrC is greater than that of TiB₂. In Comparative Example 1, ceramic powders of TiB₂ and ZrC were used as starting materials. After the ceramic powders were mixed, the mixture was sintered by hot pressing. The resulted data are shown in Table 1. The microphotograph of the microstructure of TiB₂-ZrC composite obtained is shown in FIG. 2. The structure of the sintered material also comprises two phases of TiB₂ and ZrC similarly to in FIG. 1, but the observed grain size of TiB₂ is greater than that in FIG. 1, because the aggregation of TiB₂ grains was remarkable. In Table 1, mixing ratio and composition ratio are indicated in mole ratio, "HT" in the column of bend strength means a high-temperature bend strength at 1000°C and other data in the same column show room-temperature bend strengths. Examples 7-13 The ceramic powders selected from group (a) were prepared as two kinds of raw powders of (a1) and (a2), and the two kinds of ceramic powders were mixed with the ceramic powders selected from group (b) at a mixing ratio of (a1):(a2):(b) = 1:1:2. The mixture was sintered in a manner similar to that in Example 1. The conditions and the resulted data in these Examples are shown in Table 2. Examples 14-16 In Example 14, ZrB₂ powders having an average particle size of 1.0 µm and TiC powders having an average particle size of 2.0 µm were mixed at 1:1 in mole ratio. The mixture was blended and milled in ethanol for six hours by an attrition mill using media of SiC balls having a diameter of 3 mm. 10 vol.% SiC whiskers which had been unformly dispersed using ultrasonic waves in ethanol solution containing 1% polyethylene imine were added as a third component to the mixture, and then, the mixture was dispersed and blended for twelve hours by a ball mill in which 5 mm-diameter and 10 mm-diameter SiC balls were mixed at a mixing ratio in weight of 1:1. After the blending, the mixture was dried under reduced pressure by a rotary evaporator. Then, the mixture was hot pressed under vacuum for one hour at a temperature of 1900°C to obtain a ceramic composite. The pressure was set at 25 MPa. Thereafter, the obtained ceramic composite was cut by using a diamond to make a sample having a thickness of 3 mm, a width of 4 mm and a length of 38 mm. The bend strength of this sample was determined by the aforementioned three-point bend loading method. The relative density of the sintered material reached 99%, the room-temperature bend strength was 110 kg/mm², the Vickers hardness was 2,180 kg/mm² and the fracture toughness was 5.3 MPa·m 1/2 . In Examples 15 and 16, the addition of the SiC whiskers was changed. The conditions and the resulted data are shown in Table 3. Examples 17-30, Comparative Examples 2-4 In Examples 17-30, the effect due to the addition of Y₂O₃ as a stabilizer to the mixture of the ceramic powders selected from group (a) and the ceramic powders selected from group (b) was estimated. In any case of adding Y₂O₃ or not adding Y₂O₃ in these Examples, the mixture was blended and sintered in a manner similar to that in Example 1. The conditions and the resulted data are shown in Table 4. The microphotograph of the microstructure of TiB₂-ZrO₂ ceramic composite obtained in Example 17 wherein Y₂O₃ was not added is shown in FIG. 3. The composition of the sintered material comprises TiB₂ and monoclinic ZrO₂. Because the monoclinic ZrO₂ was produced, the structure was porous. The microphotograph of the microstructure of TiB₂-ZrO₂ ceramic composite obtained in Example 18 wherein Y₂O₃ was added is shown in FIG. 4. The composition of the sintered material comprises TiB₂ and tetragonal ZrO₂. So-called sea-island structure (sea is ZrO₂ and island is TiB₂) is observed as a very dense and fine structure by adding the Y₂O₃. The microphotograph of the microstructure of TiC-ZrO₂ ceramic composite obtained in Example 23 wherein Y₂O₃ was not added is shown in FIG. 6. The composition of the sintered material comprises TiC and monoclinic ZrO₂. The two-phase structure wherein ZrO₂ densely surrounded TiC whose grain grew to a several-microns size can be observed. The microphotograph of the microstructure of TiC-ZrO₂ ceramic composite obtained in Example 24 wherein Y₂O₃ was added is shown in FIG. 7. The composition of the sintered material comprises TiC and tetragonal ZrO₂. Although the structure is similar to that in FIG. 6, pores almost cannot be observed and the structure becomes denser. In Comparative Examples 2-4, the starting materials were ceramic powders having the same compositions as those of components in the ceramic composite finally obtained. The conditions and the resulted data are shown in Table 4. The ceramic composites obtained in these Comparative Examples were porous and brittle. The microphotograph of the microstructure of the ceramic composite obtained in Comparative Example 2 is shown in FIG. 5. The structure of the sintered material was very porous and the relative density thereof was 83% and very low. The microphotograph of the microstructure of the ceramic composite obtained in Comparative Example 3 is shown in FIG. 8. In the photograph, the structure is observed as a two-phase structure having several-microns TiC grains and ZrC surrounding the TiC grains. There are many residual pores in the structure. In Table 4 and Tables shown hereinafter, (m) in the column of composition of sintered material means "monoclinic" and (t) means "tetragonal". Examples 31-39 The ceramic powders selected from group (b) were prepared as two kinds of raw powders of (b1) and (b2), the raw powders of (b2) were fixed as TiO₂ powders, the two kinds of ceramic powders were mixed with the ceramic powders selected from group (a), and further Y₂O₃ was added to the mixture as a stabilizer. Then the mixture was sintered in a manner similar to that in Example 1. The conditions and the resulted data are shown in Table 5. Examples 40-53 The ceramic powders selected from group (a) were prepared as two kinds of raw powders of (a1) and (a2), and the ceramic powders selected from group (b) were fixed as TiO₂ powders. These powders were mixed, and Y₂O₃ was added to the mixture as a stabilizer. (In only Example 50, Y₂O₃ was not added.) Then the mixture was sintered in a manner similar to that in Example 1. The conditions and the resulted data are shown in Table 6. Examples 54-72 The ceramic powders selected from group (b) were fixed as TiO₂ powders, and the kind of the ceramic powders selected from group (a) was changed. Y₂O₃ was added, and various third components were added. The mixture was sintered in a manner similar to that in Example 1. The conditions and the resulted data are shown in Table 7. Examples 73-78 The ceramic powders selected from group (a) were prepared as two kinds of raw powders of (a1) and (a2), and the ceramic powders selected from group (b) were prepared as one kind of raw powders of (b2) or two kinds of raw powders of (b1) and (b2). Y₂O₃ was added, and various third components were added. The mixture was sintered in a manner similar to that in Example 1. The conditions and the resulted data are shown in Table 8. Example 79 The mixing ratio of the ceramic powders selected from group (a) and the ceramic powders selected from group (b) was controlled in an inbalance ratio other than 1:1. The mixture was sintered in a manner similar to that in Example 1. The conditions and the resulted data are shown in Table 9.

Claims: A process for producing a ceramic composite comprising the step of: mixing at least one kind of ceramic powder consisting of a compound selected from the group (a) and at least one kind of ceramic powder consisting of a compound selected from the group (b), (a) ZrB₂, HfB₂, ZrC, HfC, ZrN, HfN, (b) TiC, TiN, TiO₂; excluding the combination of compounds (a) and (b) in which these compounds are stable to each other (i.e. ZrC and TiC, HfC and TiC, HfN and TiN or ZrN and TiN), sintering said mixed ceramic powders in a non-oxidizing atmosphere and causing by said sintering a substitution reaction of chemical elements between said compound selected from the group (a) and compound selected from the group (b) and obtaining a ceramic composite substantially comprising compounds which do not belong to the group (a) and the group (b) by said substitution reaction. The process according to claim 1, wherein said mixture is sintered after being formed. The process according to claim 1, wherein said mixture is sintered as well as being pressed. The process according to claim 3, wherein said mixture is sintered by hot pressing. The process according to any preceding claim, wherein said non-oxidizing atmosphere is an inert atmosphere. The process according to claim 5, wherein said inert atmosphere is formed by argon gas. The process according to any of claims 1 to 4, wherein said non-oxidizing atmosphere is formed by reducing the pressure of an atmosphere to a degree of vacuum of not greater than 0.1 Torr. The process according to any preceding claim, wherein the mixing ratio of said ceramic powders consisting of a compound selected from the group (a) and said ceramic powders consisting of a compound selected from the group (b) is in the range of from 1:0.2 to 1:2.0 in mole ratio. The process according to any preceding claim, wherein said ceramic powders are mixed by wet blending. The process according to claim 9, wherein said ceramic powders are mixed by wet blending using an organic solvent. The process according to any preceding claim, wherein a stabilizer is added to said mixture. The process according to claim 11, wherein said stabilizer consists of at least one compound selected from the group consisting of Y₂O₃, CeO₂, MgO and CaO. The process according to any preceding claim, wherein the particle size of said ceramic powders consisting of compounds selected from the groups (a) and (b) is not greater than 5.0 µm. The process according to any preceding claim, wherein said mixture is sintered at a temperature of 1000-1900°C. The process according to any preceding claim, wherein said mixture is heated to a sintering temperature at a heating rate of 5-10°C/min. The process according to claim 3, wherein said mixture is sintered by heating the mixture to a sintering temperature and then by maintaining the mixture at this temperature for 0.5-2 hours. The process according to any preceding claim, wherein a third component which is substantially irrelevant to said substitution reaction is added to said mixture. The process according to claim 17, wherein said third component is at least one kind of component selected from the group consisting of SiO₂, MgO, Al₂O₃, Al₆Si₂O₁₃, CaO, ZrO₂, HfO₂, WC, SiC, TiC, ZrC, HfC, TaC, NbC, B₄C, TiN, ZrN, AlN, Si₃N₄, HfN, TaN, cubic BN, WB, TiB₂, HfB₂, Si, Mg, Ca, Al, Ti, Zr, Hf, Ta, W, Ni, Co, Mo and Fe. The process according to claim 18, wherein said third component is added by not greater than 40% in volume. The process according to claim 17, wherein said third component is at least one kind of whiskers selected from the group consisting of SiC whiskers, B₄C whiskers and Si₃N₄ whiskers. The process according to claim 20, wherein said whiskers are added by 5-40% in volume.

Inventors: KUWAJIMA HIROSHI, MORI TAKAKO, OGATA TOMOHIKO, KUWAJIMA, HIROSHI, MORI, TAKAKO, OGATA, TOMOHIKO
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1990
Main classification: C04B35
IPC codes: C04B35