Abstract:
Disclosed is a ceramic composite which comprises particles of at least one inorganic compound selected from the group consisting of a carbide, a nitride, an oxide, a boride, a silicide and an oxy-nitride and particles and whiskers of Si 3  N 4 , Si 2  N 2  O or SiO 2 . This ceramic composite is small in volume change on sintering, and provides heat resistance. In addition, the composite can absorb thermal shock by its voids. It can be used in association with ceramic heaters, current collectors, commutators for starter motors and alternators, and in other areas.

Description:
This is a continuation of co-pending application Ser. No. 07/603,887 filed on Oct. 26, 1990, now abandoned, which is a divisional of co-pending application Ser. No. 07/500,102, now U.S. Pat. No. 5,130,055 filed on Mar. 26, 1990, which is a continuation of Ser. No. 179,984, filed Apr. 11, 1988, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. FIELD OF The INVENTION 
     This invention relates to a ceramic composite comprising particles of an inorganic compound and particles and whiskers of silicon nitride, silicon oxynitride and silicon oxide, and to a process for producing the ceramic composite. Further, this invention relates to an electric conductor, heater and sliding material comprising the ceramic composite. 
     2. Description of Related Art 
     Ceramics to be used as a conducting material, structural material or other material have recently been required to have properties of high strength at high temperatures, high toughness, high thermal shock resistance and high accuracy of dimension. Such ceramics are known to include inorganic compounds such as carbides, nitrides, oxides, borides, silicides and oxynitrides. For example, silicon nitride, silicon carbide, alumina anci zirconia are known inorganic compounds used in making ceramics. Each of these ceramic materials is rarely used by itself in order to achieve the excellent properties mentioned above. Therefore, many proposals have been made to achieve such properties by selecting combinations of the ceramic materials, and changing the design of the structure. 
     Hitherto, heaters have been comprised of metal or ceramic. The metal neater includes nichrome or tantalum. This heater has poor heat resistance and degrades at a temperature of 1000° C. or higher. 
     Ceramic heaters are generally comprised of materials including silicon carbide, stabilized zirconia, lanthanchromite, molybdenum silicide and the like. These ceramic materials have a relatively high electric resistance, and thermorunaway easily occurs. It is difficult to control the temperature in such heaters since the ceramic materials from which they are made have a negative resistance-temperature coefficient. Furthermore, these ceramic materials are defective in that they are poor in mechanical strength and thermal shock resistance. Therefore, attempts have been made to substitute new electrically conductive ceramics for the above-mentioned ceramic materials. 
     For example, Japanese Patent KOKAI (Laid-open) No. 57-41796 describes that SiC or Si 3  N 4  is mixed with an electrically conductive compound and subjected to hot press sintering to eliminate the problems or defects mentioned above. The hot press sintering method, however, makes it possible to densify sintered bodies. The problem with such a method is that, since a great amount of energy is required for production, production costs are high. 
     As described In Japanese Patent KOKAI (Laid Open) No. 60-44990, an electrically conductive ceramic enveloped with an electrically insulating ceramic is sintered by hot pressing. A great amount of energy is required. Further, the hot pressing makes it impossible to mold and sinter products having a complicated shape. Generally, to integrate and sinter two or more ceramics having electric resistances which differ from one another, their coefficients of thermal expansion must be adjusted. Hot pressing, however, is carried out at such high temperatures that a small difference in the coefficient of thermal expansion makes it easy for cracks to form. 
     Furthermore, Japanese Patent KOKAI (Laid-open) No. 60-60983 discloses a process in which an electrically conductive material is mixed with Si 3  N 4  particles. The mixture is sintered by pressureless sintering. In this process, a sintering aid is used and hence softening or deformation takes place inconveniently at high temperatures. Further, a volume shrinkage of about 40 to 60% is brought about on sintering to induce deformation of sintered bodies. On the other hand, the sintering of the conductive material and Si 3  N 3  particles in the absence of any sintering aid do not improve the density of the sintered bodies and, therefore, the specific resistance of the bodies is inconveniently high. Thus, the electrically conductive product according to this Patent KOKAI has unsatisfactory performance. 
     The Japanese Patent KOKAI references above do not take any account of shrinkage of molded bodies on sintering. Therefore, shrinkage is brought about in these references. 
     Generally, SiC, Si 3  N 4  or the like are known as engineering ceramics suitable for a structural material for engines or turbines. These ceramics are excellent in heat resistance, and they are sintered by pressureless sintering, pressure sintering or reaction sintering. With the pressureless sintering and pressure sintering, a rate of dimensional change before and after sintering is 15 to 20%. Thus, these sintering techniques provide deformation and poor accuracy of dimension. On the other hand, it is known that the reaction sintering technique provides a lower rate of dimensional change than that of the other sintering techniques. Nevertheless, the rate is as high as about 1 to 1.5%, as shown Japanese Patent KOKAI (Laid-Open) No. 58-140375. 
     As a material having a low rate of dimensional change, a bonded Si 3  N 4  product has been used as a refractory material. Such material has a mechanical strength of only about 50 MN/m2, as shown in Japanese Patent KOKAI (Laid-open) go. 58-88169, and is not suitable as a structural material. 
     Furthermore, Japanese Patent KOKAI (Laid-open) No. 61-201662, which has been filed by the present applicant, discloses that a fully satisfactory rate of dimensional change on sintering can be obtained, but a body having a complicated shape is not obtained due to poor flowability of starting mixture. Further, the resultant sintered body has an insufficient mechanical strength. 
     Some of the reasons for the lack of popularity of engineering ceramics are that the rate of dimensional change on sintering is high and sintered bodies are hardly workable due to their high hardness and brittleness. There has never been a practical process for producing a ceramic material having a high strength, low rate of dimensional change and high accuracy of dimension. 
     On the other hand, ceramic material has been used as a sliding material for sealing because it has an excellent resistance to thermal shock. 
     Generally, the working of a machine is always accompanied with sliding. Reduction of friction and wear will induce energy savings and will increase the life of a sliding material. For example, a floating seal comprising O-ring 4 and sealing elements 5 and 6 as shown in FIG. 17, has been used as a sealing meals for cars or construction machines. In this means, one of these sealing elements is fixed, and the other is rotated in contact with the fixed element at the sliding surface 7 thereof and set through a lubricant 8 in a floating state with respect to an axis 9, thereby keeping the sealing property of the means. Therefore, this means is most quitable for sealing or protecting from earth and sand. However, since it is made of a cast iron material, this floating seal performs poorly with respect to both heat resistance and wear resistance. Another sealing means exists wherein one of the sealing elements 10 is made of Cr cast iron and the other 11 is made by injection molding. This sealing means also has the same defect as mentioned above. 
     On the other hand, a ceramic-made sliding means has recently been introduced. Typical examples of the ceramics used for the sliding means include SiC, Si 3  N 4 , Al 2  O 3  or ZrO 2 . Practically, it is required that the sliding material have the following properties (see &#34;Kogyo Zairyo&#34;, Vol. 31, No. 12, pp. 139-146, particularly 140): 
     (1) a high hardness and an excellent resistance to wearness and seizing; 
     (2) a good affinity to a sliding partner (a low coefficient of friction and a small amount of the partner worn); 
     (3) a good corrosion resistance; 
     (4) a high strength, toughness and resistance to thermal shock; 
     (5) a small amount of deformation by stress and/or heat at sliding surfaces to keep a good lubrication state on the sliding surfaces (i.e., a high Young&#39;s modulus and a small coefficient of thermal expansion); 
     (6) a high thermal conductivity (The increase of the coefficient of friction and thermal deformation on the sliding surfaces are prevented by increasing the resistance to thermal shock and inhibiting the accumulation of frictional heat.); and 
     (7) a low specific gravity (centrifugal stress is reduced). 
     These required properties are not all satisfied by known materials. For example, a metallic material is questionable with respect to resistance to seizing. Al 2  O 3  itself tins a low resistance to thermal shock, and since it has a high coefficient of thermal expansion, it is easily deformed by heat on the gliding surfaces thereof. ZrO 2  itself also has a low resistance to thermal shock and a high coefficient of thermal expansion. Si 3  N 4  itself has a low thermal conductivity. SiC alone has a high hardness and high thermal conductivity but has a poor resistance to thermal shock. A sliding material having both excellent wear and corrosion resistance, as well as conspicuously improved resistance to thermal shock, has been demanded by the industry. 
     Among these sliding materials, SiC is widely used in a sintered form. For example, Japanese Patent KOKAI (Laid-Open) No. 61-163174 discloses &#34;a sliding material comprising an SiC sintered porous body having an excellent sliding property under wet conditions&#34;. Also, Japanese Patent KOKAI (Laid-Open) No. 55-100421 discloses &#34;a sliding means comprising a fixed part and rotating part, any one of the parts being made of Si 3  N 4  and the other made of SiC&#34;. 
     However, none of these Japanese Patent KOKAI references direct any attention to thermal shock resistance and one-side contact. 
     Thus, the prior art materials have no combination of good wear resistance and thermal shock resistance. Further, prior art materials have drawbacks such as cracks that are generated due to a rapid change of temperatures. Finally, the prior art materials can be worn. 
     SUMMARY OF THE INVENTION 
     The objects of the present invention are to eliminate the defects or drawbacks as described above. 
     The present invention incorporates the following three aspects: 
     First Aspect 
     One object of the present invention is to provide a ceramic composite containing two or more layers having different electrical resistances formed by near net shaping. 
     Another object of the present invention is to provide a ceramic composite further having a very small rate of dimensional change on sintering molded bodies. 
     A further object of the present invention is to provide a current collector ring for revolving electric machines, made from the ceramic composite. 
     Second Aspect 
     An object of the present invention is to provide an engineering ceramic composition suitable for near net shaping with a high accuracy of dimension. Another object of the present invention is to provide a ceramic sintered body which does not require any machining and has a small rate of dimensional change on sintering, high high-temperature strength and good resistance to thermal shock. Further, the composition has a good flowability in molding. 
     Third Aspect 
     An object of the present invention is to provide a sliding and sealing means having a good wear resistance, corrosion resistance and thermal shock resistance. The amount of wear an the sliding partner will also be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(A) and 1(B) are schematical cross-sectional views of one embodiment according to the FIRST ASPECT of the present invention. 
     FIG. 2 is a micrograph of crystal grains of the sintered body comprising a conductive portion and an insulating portion of a ceramic composite according to the FIRST ASPECT of the present invention. 
     FIGS. 3(A) and 3(B) are schematical cross-sectional views of another embodiment according to the FIRST ASPECT of the present invention. 
     FIG. 4 is a graph which glows the relation between contents of TiN and Al 2  O 3  and dimensional change on sintering in the FIRST ASPECT of the present invention. 
     FIG. 5 is a graph which shows the relation between bending strength and temperature in the FIRST ASPECT of the present invention. 
     FIG. 6 is a graph which shows the relation between particle size of metallic Si and bending strength in the FIRST ASPECT of the present invention. 
     FIGS. 7(A) and 7(B) are schematical cross-sectional views of a further embodiment according to the FIRST ASPECT of the present invention. 
     FIG. 8 is a graph which allows the relation between rate of dimensional change on sintering and mixing ratios of raw materials in the SECOND ASPECT of the present invention. 
     FIG. 9 is a graph which shows the relation among mixing ratios of raw materials [Si/(Si+inorganic compound)], rate of dimensional change on sintering and bending strength in the SECOND ASPECT of the present invention. 
     FIG. 10 is a graph which shows the relation between amount of binder resin and bending strength in the SECOND ASPECT of the present invention. 
     FIG 11 is a graph which shows the relation between volumetric particle packing rate of molded bodies and bending strength in the SECOND ASPECT of the present invention. 
     FIG. 12 is a graph which also shows the relation between volumetric particle packing rate of molded bodies and bending strength in the SECOND ASPECT of the present invention. 
     FIG. 13 is a graph which also shows the relation between specific surface area of raw material powder and amount of molding binder in the SECOND ASPECT of the present invention. 
     FIG. 14 is a graph which shows the relation between apparent viscosity of raw material composition measured by flow tester and volumetric particle packing rate in the SECOND ASPECT of the present invention. 
     FIG. 15 is a micrograph of ceramic composition of a sliding surface in an example of the THIRD ASPECT of the present invention. 
     FIG. 16 is a graph which shows the relation between surface pressure and coefficient of friction in the THIRD ASPECT of the present invention. 
     FIG. 17 is a cross-sectional view of an example of a conventional floating seal. 
     FIG. 18 is also a cross-sectional view of another example of a floating seal. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Aspect 
     The ceramic composite of the present invention is one prepared by integral molding and integral sintering ceramics which differ in electric resistively in order to bond with particles or whiskers of at least one of Si 3  N 4 , Si 2  N 2  O and SiO 2  produced from metallic Si or ferro-Si. 
     In the ceramic composite of the present invention, ceramics that are 1) contiguous to each other, 2) different in electrical resistively, and 3) electrically conductive compounds for changing the resistively are non-oxide type conductive materials which are nitrides, carbides, borides and silicides of substances of Groups IIIa, IVa, Va, VIa and VIII of the periodic table and preferred are TIN, Tic, TIB 2 , TiSi 2 , ZrN, ZrC, ZrB 2 , ZrSi 2 , HfN, HfC, TaN, TaC, TaB 2 , TaSi 2 , Mo 2  N, Mo 2  C, MoB, Cr 2  N, Cr 3  C 2 , CrB, CrSi 2 , NbN, NbC, NbSi 2 , VN, VC, WC and WSi 2 . TIN, TiC, ZrN, ZrC, Cr 2  N and Cr 3  C 2  are especially suitable and perform well with respect to oxidation resistance. 
     The content of electrically conductive particles in sintered bodies which are used for decreasing electrical resistivity in the present invention is preferably 80 vol % or less. If the content is more than 80 vol %, characteristics of ceramics such as mechanical strength, thermal shock resistance and oxidation resistance decrease. 
     In the present invention, electrical resistivity of sintered bodies can be optionally changed by changing content of conductive particles in the sintered bodies in the range of 5-80 vol %. Furthermore, the resistivity in the range of 10 14  Ω cm-10 -5  Ω cm can be optionally obtained by including electrically insulating particles in the sintered body. 
     In the ceramic composite of the present invention, conductive particles or insulating particles are bonded with at least one of Si 3  N 4 , Si 2  N 2  O and SiO 2  produced from metallic Si or ferro-Si. The volume change on sintering is small and no deformation occurs. Further, two layers different in electrical resistivity are bonded with Si 3  N 4 , Si 2  N 2  O or SiO 2  produced from metallic Si or ferro-Si. The bonded interface, like the body, is superior in heat resistance and thermal shock resistance. 
     The sintered bodies of the present invention preferably have a void content of 5-40%. When void content exceeds 40%, mechanical strength decreases and resistivity is difficult to reduce. When it is less than 5%, permeation resistance of nitriding gas or oxidizing gas, with which metallic Si or ferro-Si reacts, is increased. This makes it difficult to obtain good sintered bodies. The reason for this result rests in the fact that in order for a conductive compound or insulating compound, metallic Si or ferro-Si to react with nitriding gas or oxidizing gas to form Si 3  N 4 , SiO 2  or Si 2  N 2  O phase to bond the insulating compound or conductive compound, voids are needed for permeation of said gas through molded body. 
     Presence of 5-40% voids in a sintered body causes relaxation of strain produced due to differences in the thermal expansion coefficient of the layers that have different electrical resistivity. Thus, formation of cracks can be prevented. 
     It is preferred that metallic Si or ferro-Si has an average particle size of 5 μm or less. When the size exceeds 5 μm, nitriding requites large amounts of time. Also, Si remains. 
     In the present invention, thermoplastic resins such as polyvinyl butyryl and polyethylene and orginosilicone high molecular compounds such as silicone imide compounds and polysilane compounds may be used as molding binders. These binders are preferably added in amounts of 2-20 parts by weight in order to adjust the relative density of the molded body to 60% or higher. 
     In the present invention, a molded body is heated to 1350° C. or higher in a nitriding gas, oxidizing gas or an oxidizing and nitriding gas atmosphere such as nitrogen, ammonia and oxygen (if necessary, gases such as hydrogen, argon, helium and carbon monoxide may be included). 
     Commercially available metallic Si, ferro-Si, insulating compounds and conductive compounds may be used. Preferably, the materials should be pulverized into round particles by a mill. 
     In the case where whiskers are previously mixed with and dispersed in raw materials, all whiskers are not bonded to particles and remain between particles of sintered body. In the present invention though, particles and whiskers are bonded by needle-like whiskers produced from particles in the molded body. These needle-like whiskers cross nearly straight in the spaces between particles. This contributes greatly to the high thermal shock resistance and strength. 
     According to the present invention, spaces between particles and/or whiskers of the insulating compound and conductive compound are bonded three-dimensionally with whiskers produced from Si particles in the molded body. There are few whiskers in an unbonded state. Thus, sintered bodies of high toughness and high-temperature strength can be obtained. 
     The average particle size of the insulating compound and the conductive compound is preferably 100 μm or less. When it is more than 100 μm, the strength of the sintered body is reduced. When previously produced whiskers of the insulating compound or the conductive compound are used, an average aspect ratio of 2-50 and length of 0.2-100 μm is desirable. When the aspect ratio in less than 2 and length is less than 0.2 μm, they do not function as whiskers. When the aspect ratio is more than 50 and the length is more than 100 μm, mixing of raw materials becomes difficult and dispersibility in inferior. 
     In the present invention, the ceramic composite contains whiskers in an amount of 1-70 vol % (preferably 10-30 vol %) per particles and whiskers produced in sintered body. Outside this range, effects of the present invention cannot be obtained. 
     The molding method is selected from injection molding, casting, rubber press molding, extrusion molding, metal mold molding and the like depending on shape and required characteristics. 
     After removal of molding aid and the like, the molded body is subjected to heat treatment for formation of whiskers. 
     Particles or whiskers produced from metallic Si or ferro-Si are most preferably of Si 3  N 4 . 
     Among the conductive compounds used in the present invention, silicides and borides react with nitrogen in nitriding gas. This results in cracks in the sintered body if sintering time is not proper. Therefore, nitrides and carbides are preferred. 
     Whiskers of Si 3  N 4 , SiC or the like may be blended as raw materials in addition to the whiskers produced from Si particles. However, use of the former whiskers in a large amount causes nonuniformity and is not preferred. Further, whiskers may be used as an insulating compound and a conductive compound. 
     In order to obtain a void content of less than 5%, it is possible to re-sinter the sintered body. The resintering can be effected by hot pressing, hot isotactic pressing or, under normal pressure, utilization of a sintering aid. Thus, whiskers are present three-dimensionally in the sintered body. A ceramic composite of high heat resistance can be obtained. However, the difference in the thermal expansion coefficient must be made as small as possible, so cracks do not occur. 
     Further, since the sintered body of the present invention has voids, lubricants can be contained in the voids. 
     In the ceramic composite of the present invention, since the electrically conductive compound and the insulating compound are firmly bonded with particles or whiskers of Si 3  N 4 , Si 2  N 2  O or SiO 2  produced from metallic Si or ferro-Si, volume change on sintering is small. Also, the composite performs well with respect to heat resistance. Further, it has voids and can absorb thermal shock. 
     According to the present invention, ceramic composites that are small in volume change on sintering and having optional resistivity within the range of 10 14  -10 -5  Ωcm by adjusting amount of conductive compound and insulating compound. 
     Thus, the composite can be used for various heaters, current collector for generators, brushes for motors, commutators for starter motors, commutators for alternators, etc. 
     According to the present invention, the sintered body obtained by molding and sintering by near net shaping has small dimensional changes. Also, ceramic composites comprising a conductive portion and an insulating portion of optional resistivities can be easily obtained. 
     By using these ceramic composites, ceramic heaters, current collector rings for revolving electrical machinery, brushes for motors and commutators for starters and alternators can be provided. 
     Second Aspect 
     The present invention relates to a method for making ceramics of high strength and high dimensional accuracy which comprises 1) adding a thermoplastic resin as a binder to metallic Si powders or inorganic compound powders containing at least 45 vol % of metallic Si powders, 2) heating and kneading the mixture, 3) subjecting the kneader product to warm pressure molding to form a molded body of at least 70 vol % in volumetric packing of powders (ratio of volume occupied by powders in the molded body), 4) removing the binder in the molded body by heating, and 5) heating and sintering the molded body in a nitriding gas atmosphere to bond particles of Si 3  N 4  and the inorganic compound with Si 3  N 4  particles and whiskers produced from said Si. 
     Furthermore, the present invention relates to a composition for ceramics of high dimensional accuracy which contain metallic Si powders or inorganic compound powders containing at least 45 vol % of metallic Si powders, and a binder of thermoplastic resin, the compound having an apparent viscosity at 150° C. of (3-90)×104 N·s/m 2 . 
     When the amount of Si powders is less than 45, the dimensional change an sintering cannot be made small, nor can a ceramic sintered body of adequate strength be obtained. In the present invention, the dimensional change on sintering can be made less than 0.2% by adjusting the content of Si powders to at least 45 vol %. Of course, Si content may be 100%. 
     When volumetric packing of particles in the molded body is less than 70 vol %, a ceramic sintered body of sufficient strength cannot be obtained. By increasing it to at least 70 vol %, a sintered body leaving a bending strength of about 300 MN/m 2  or more can be obtained. This is the most effective means for improving strength. 
     Said inorganic compound is selected from oxides, carbides, nitrides, oxy-nitrides, silicides and borides. The inorganic compound desirably has a melting point or sublimation temperature of 1400° C. or higher. 
     As for the inorganic compound, mention may be made of, for example, TiN, Si 3  N 4 , Si 2  N 2  O, TiC, TiO 2 , AlN, Al 2  O 3 , SiO 2 , ZrN, ZrO 2 , Zrc, TiB 2 , ZrB 2 , Cr 3  C 2 , Cr 7  C 2 , CrB, Cr 2  N, WC, W 2  C, WSi 2 , TiSi 2 , ZrSi 2 , Fe 3  C, VC, VN, CeC 2 , MgO, Mo 2  N, Mo 2  C, MoB, ThC, HfN, Hfc, TaN, TaC, TaB 2 , NbN, BeO, WN, CaO, TaSi 2  and CrSi 2 . 
     Among these inorganic compounds, silicides and borides react with nitrogen during sintering in a nitriding gas. Hence, cracks might occur in the sintered body if sintering conditions are not proper. Therefore, carbides, nitrides and oxy-nitrides are preferred. 
     As the metallic Si powders, ferro-si containing Fe may be used. However, there is the possibility of Fe remaining in the sintered body to cause reduction of high-temperature strength. 
     Particle size of said Si powders should be 10 μm or less, preferably 1 μm or less. The particle size of the inorganic compound should be 100 μm or less, preferably 20 μm or less. Commercially available powders may be used. Preferably, though, the powders should be pulverized by a mill to round particles. 
     A part of said inorganic compound powders may be replaced with whiskers. In this case, the amount of whiskers desirable is 55 vol % or less in the sintered body. When the amount is mare than 55 vol %, a uniform blend of raw materials cannot be obtained. The whiskers added preferably have an average aspect ratio of 200 or less and an average length of 200 μm or less. 
     Thermoplastic resins used for preparation of premolded bodies for obtaining sintered ceramics may be used as binder resins. Examples thereof are polyvinyl butyryl, polyethylene, polysilicone, and synthetic waxes. 
     In the present invention, the binder has an important role. The additional amount thereof is needed to attain a packing of particles of at least 70 vol % in a molded body. 
     The inventors have made various studies on this point and have found that there is a very close correlation between specific surface area of raw material powders and the additional amount of the binder per 100 parts by weight of said raw material powders. The present invention is based on this finding. 
     First, the specific surface area S (cm 2  /g) of raw material powders can be obtained by the following formula: ##EQU1## wherein is a density and d is an average particle size (μm). 
     The relation between said specific surface area and amount B of the binder (part by weight per 100 parts by weight of raw material powder composition) necessary for obtaining a particle packing of molded body of at least 70 vol % is shown by the following formula: 
     
         B=[(7S/20,000)+3]±2.5 
    
     By adding the binder in an amount within the range given by the above formula, particle packing of the molded body can be made at least 70 vol % and a sintered body having a bending strength of about 300 MN/m 2  or more can be provided. 
     Raw material composition containing the binder in an amount of the above range has an apparent viscosity of (3-90)×104  N·s/m2 as explained hereinafter. By selection of this viscosity range, fluidity at molding can be estimated. Also, a molded body of at least 70 vol % in packing can be obtained. Thus, a composition suitable for near net shaping can be provided. 
     From the standpoint of fluidity of raw material composition, Si powders of 1 μm or less in particle size are preferred. Also, using blend comprising 15-60% by weight of polyethylene, 30-70% by weight of wax and 5-25% by weight of stearic acid as a binder is preferred. 
     The raw material composition to which the binder is added is well kneaded and then molded. A molding method is selected from injection molding, press molding, rubber press molding, extrusion molding, powder molding and the like depending on shape and required characteristics. Warm molding is carried out at a higher point than the softening point of binder resin. For example, when a mechanical press is used, molding pressure may be about 1000 kgf/cm2. 
     The resulting molded body is degreased (removal of binder) before sintering. Degreasing is carried out by gradually heating the molded body from room temperature to about 500° C. at about 2° C./h. 
     This molded body is preferably heated to lower than the melting point of metallic Si (lower than 1410° C.), possibly 1100°-1350° C. in a nitriding gas atmosphere comprising nitrogen and/or ammonia, and if necessary, further comprising hydrogen, argon, hellum or the like. A heating rate until sintering temperature is obtained can be 4° C./h. The molded body can be easily sintered by such heating. If necessary, hot pressing may be applied. 
     Preferably, the sintered body has a void content of 30% or less. When the void content exceeds 30% strength decreases. The void content of 30% or less can be obtained by adjusting the volumetric packing of particles in the molded body to at least 70 vol %. 
     Si3N4 whiskers ire produced in the sintered body during sintering and such whiskers are preferably contained in an amount of 1-70 vol %, possibly 10-30 vol % for the reaction-produced phase. 
     The reasons ceramic have small dimensional change (less than about 0.15%) an sintering and have a bending strength of 300 MN/m2 or higher are considered as follows. 
     First, Si 3  N 4  whiskers, produced by sintering in a nitriding atmosphere, take a large part in dimensional change an sintering. Thin is clear from accompanying FIGS. 8 and 9. 
     FIGS. 8 and 9 show the relation between blending ratio of raw materials (Si/(Si+inorganic compound)] and dimensional change on sintering or bending strength when sintered bodies were produced by adding 9 parts by weight of a thermoplastic resin to a mixture of Si and inorganic compound, heating and kneading the mixture, subjecting it to warm pressure molding, then removing the binder and sintering the molded body in nitrogen gas. With increase in the amount of Si, amount of whiskers of produced Si 3  N 4  increases and strength increases. Dimensional change on sintering increases, but this increase can be practically ignored. 
     The reason for the above is that the whiskers produced by sintering closely connect the particles of sintered body. That a sintered body having a bending strength of 300 MN/m 2  is obtained especially when Si is more than 45 vol % is considered to be a result of the increase of such connecting chains. 
     Next, relation between amount of binder resin and bending strength with amount of Si being 60 vol % is shown in FIGS. 10 and 11. FIG. 10 and 11 show that the amount of binder resin has a great effect on bending strength. This is influenced by volumetric packing of molded body. 
     The raw material powder is comprised of fragile solid fine powder particles and is difficult to pack by pressing. Therefore, it is necessary to add binder thereto to accelerate fluidity of powders and to increase strength of molded body. The strength of the sintered body changes depending on the additional amount of binder. As mentioned hereinbefore, this has a relation with volumetric packing of powders of molded body (density). With an increase in the amount of binder, the fluidity of the mixture increases tinder heating, and pressure molding becomes easy. As a result, volumetric packing of particles of the molded body is improved. However, if a binder is added in an amount that is greater than the vacancy when raw material powders are in ideal dense packing, the raw material powders are isolated in the binder, and fluidity is increased. The solid ratio in the molded body is decreased, though, resulting in decrease of volumetric packing of particles of the molded body. Thus, the void content of the sintered body increases. This causes a reduction in the strength of the sintered body. 
     As mentioned before, when the molded body is sintered in a nitriding atmosphere, whiskers produced from Si powders connect particles, fill the vacancy between the particles, and grow three-dimensionally in the sintered body. Thus, ceramics of high toughness can be obtained. 
     In the ceramics of the present invention, particles in the molded body of at least 70 vol % in volumetric packing of particles are connected by nitride produced from metallic Si powders. Hence, dimensional change on sintering is small, and no deformation occurs. 
     In the present invention, particles and whiskers produced from metallic Si are preferably nitrides. Oxy-nitride and oxide tend to decrease high-temperature strength. 
     According to the present invention, ceramics with less than a 0.2% dimensional change on sintering and with a bending strength of about 300 MN/m 2  or more can be obtained by near net shaping. Such ceramics require negligible mechanical working after sintering and can be applied to various fields of structural parts such as engines and turbines. 
     Third Aspect 
     The present invention relates to a sliding member for sealing wherein one surface of sliding surfaces comprises ceramics composed of SiC and Si 3  N 4  as a composite material. This composite material, Si 3  N 4 , contains at least one of the following: particles, or whiskers. Said ceramics have void content and the other surface has a hardness less than that of the composite material of said surface. 
     In the present invention, Si 3  N 4  particles or whiskers for bonding SiC produced by heat treatment of metallic Si are preferred. They bond particles and reduce vacancy between the particles. Thus, sintered bodies having open voids of 2-30% in void content and 100 μm or less in diameter can be obtained. 
     In the present invention, it is preferred that the average particle size of SiC particles is 100 μm or less, the average aspect ratio of SiC whiskers is 200 μm or less, and the average length of SiC whiskers is 200 μm or less. Outside these ranges, mixing of raw materials is difficult and dispersion is not uniform. This results in a reduction of mechanical strength. 
     The reason why one surface of the seal sliding member is made of a composite material of SIC and Si 3  N 4  in the present invention is that a sliding material leaving the characteristics of both the SIC and Si 3  N 4  can be obtained. Unexpectedly good thermal shock resistance can be obtained by bonding SiC, good in wear resistance, hardness and thermal conductivity, with Si 3  N 4 , low in thermal expansion coefficient. The reason why another surface is made of a composition having a hardness lower than that of SiC particles and Si 3  N 4  particles of the other sliding surface is that if it is harder than the sliding surface comprising SiC and Si 3  N 4  and voids, SiC and Si 3  N 4  are worn by the harder particles of another surface and fall off between the sliding surfaces to cause wear due to scratch. When materials with compositions having particles of the same hardness are combined and slid, marks are apt to form oil the sliding surface. This is because, even if the sliding surface is mirror-polished, the contact is microscopically point or line contact, and the surfaces are in the state of one-side contacting. This naturally forms sliding marks. On the other hand, according to the present invention, another surface is made of a material having a hardness lower than that of SiC and Si 3  N 4  particles, and, as a result, substantially no wear is caused on one surface. Thus, a sliding material superior in wear resistance can be obtained. By employing a material for one sliding surface which is softer than another sliding surface, even if one-side contact occurs, the surface of lower hardness is mirror-polished by the harder surface from sliding. Accordingly, the period of the state of one-side contact is short, and a sliding material superior in sealing property can be obtained. 
     The reason why one surface of seal sliding surfaces is made of a ceramic sintered body having open voids of 2-30% in void content is that said open voids relax thermal shock strain to provide a sliding material of unexpectedly good thermal shock resistance. When diameter of voids is more than 100 μm and void content is more than 30%, mechanical strength decreases and bonding strength between the SiC and the produced Si 3  N 4  phase decreases. This results in considerable falling-off of SiC aid Si 3  N 4  particles during sliding. Further, breakage of lubricant oil film does not occur due to these open voids. Thus, cohesion and scorch are not produced. This is because lubricant oil penetrates into the open voids and oozes out on the sliding surface to give the above effects. Therefore, as far as one surface is made of the ceramics of the present invention, another surface may be of metals, ceramics, carbon or synthetic resins lower in hardness than the particles of said one surface. It has been confirmed by experiments that, when oxide ceramics are used as the partner material (another material) sliding characteristics are improved. This is because SiC and Si 3  N 4  are high in covalent bonding. When the amount of adsorbates on the surface decreases, cohesion is apt to occur. However, if the partner surface is made of oxide, an oxygen ion is produced on the sliding surface to produce surface adsorbates which prevent the cohesion. 
     In the present invention, in case SiC contained in ceramics composed of SiC, Si 3  N 4  and voids is greater than the particles or whiskers of Si 3  N 4  or the like produced from metallic Si powders, the material is better in wear resistance and thermal shock resistance. Especially, because of composite ceramics, thermal shock resistance is superior to that of other ceramics having the same void content. 
     The characteristics ate further improved when a solid lubricant or resin, such as furan resin, is contained in the void of a sintered body. 
     According to the present invention, one surface of the sliding surfaces of the sliding member for sealing is made of ceramics composed of at least one of either particles or whiskers of SiC and Si 3  N 4 , and voids. The outer surface (partner surface) is made of a material having a hardness lower than that of the particles or whiskers of Si 3  N 4  of said one surface, whereby excellent wear resistance, corrosion resistance and thermal shock resistance can be obtained. The wear of a partner material can be reduced. Hence, performance of various seals can be markedly improved. 
     As explained hereinabove, according to the present invention, the sliding members have excellent sliding properties such as wear resistance and thermal shock resistance. So, endurance and reliability of the seal can be remarkably improved. This is effective for various sliding mechanisms such as mechanical seals, floating seals, plain bearings and the like. 
     EXAMPLE 1 
     To 100 parts by weight of a raw material comprising 22.7 wt. % of metallic Si powder 0.9 μm in average particle size, and 77.3 wt. % of TiN powder of 1.2 μm in average particle size, was added 9 parts by weight of binder comprising polyethylene based thermoplastic resin and stearic acid. The mixture was kneaded at 160° C. for 12 hours by a pressure kneader. The kneaded product was pulverized to 10 meshes or less to obtain a raw material for electrical conductor ceramics A. 
     To 100 parts by weight of raw material comprising 38 wt. % of metallic St powder of 0.9 μm in average particle size and 62 wt. % of Al 2  O 3  of 2 μm in average particle size was added 9 parts by weight of a binder comprising a low-density polyethylene, a synthetic wax and stearic acid. The mixture was kneaded at 160° C. for 12 hours by a pressure kneader. The kneaded product was pulverized to 10 meshes or less to obtain a raw material for insulant ceramics D. 
     Then, the raw materials for ceramics A and B were filled in succession in a mold at 160° C. and tinder 1000 kg/cm 2  to obtain a layered ring as shown in FIG. 1(A) and FIG. 1 (B) FIG. 1(B) is a cross-sectional view of FIG. 1 (A) at Z--Z. This molded ring was 1) heated to 500° C. at 3° C./h to remove the binder therein 2) heated to 1000° C. at 5° C./min in a nitrogen atmosphere, and, 3) heated from 1000° C. to 1350° C. at 4° C./h to obtain a sintered body having whisker/particle of Si 3  N 4  of 1/9. 
     The ratio of whisker/particle can be adjusted by changing the rate of heating of from 1100° C.-1400° C. and retaining stepwise the temperature during the heating. The ratio of whisker/particle can be obtained by observation of the sintered body under a scanning electron microscope and a transmission electron microscope. The characteristics of the obtained sintered body are shown in Table 1. Rate of dimensional change when the molded product was sintered was small, namely,±0.2. No cracks occurred. Resistivity of the portion of ceramics A was 9×10 -4  Ω cm and that of the portion of ceramics B was 7×10 13  Ω cm. FIG. 2 shows an electron micrograph of the boundary between the conductive portion and the insulating portion. It can be seen that the conductive portion and the insulating portion were firmly bonded. This bonding in due to Si 3  N 4  produced from metallic Si. 
     
                       TABLE I______________________________________       Composition ofEx-  Ce-    sintered body               Bendingam-  ram-   (vol %)        Void Resistivity                                   strengthple  ics    Si.sub.3 N.sub.4               TiN  Al.sub.2 O.sub.3                          (%)  (Ω cm)                                       (MPa)______________________________________1    A      40      60   --    22   9 × 10.sup.-4                                       280B      50      --   50    18   7 × 10.sup.13                                       270______________________________________ 
    
     The coefficient of thermal expansion of the conductive portion wan 5.2×10-6° C -1  and that of the insulating portion was 5.1×10-6° C -1 . This also indicates superior thermal shock resistance of the sintered body. 
     EXAMPLES 2-36 
     Sintered bodies were prepared in the same manner as in Example 1 except that the conductive particles were used in place of TIN particle. The results are shown in Table 2. The composition of the sintered bodies is not shown in Table 2 because the composition of insulating portion B of the sintered body was Si 3  N 4  : Al 2  O 3  =50: 50 (vol %) as in Example 1. 
     Rate of dimensional change of from molded product to sintered body was small, namely, less than+0.2%. No cracks occurred. Conductive portion A and insulating portion B were firmly banded at their boundary as in Example 1. 
     Furthermore, integral molding and sintering with optional combination of resistivity of from 10 14  Ω cm to 10 -5  Ω cm are made possible by allowing a conductive compound and an insulating compound to be present together. 
     
                                           TABLE 2__________________________________________________________________________   Raw material   ratio (wt. %)     Electro-conductive               Void content                      Resistivity                            Bending strengthNo.     Si     particles (%)    (Ω cm)                            (MPa)    Appearance__________________________________________________________________________Example2  20     TiC80     28     5 × 10.sup.-3                            161      No cracks3  20     TiB.sub.2 80               29     2 × 10.sup.-3                            145      &#34;4  20     ZrN80     27     1 × 10.sup.-3                            155      &#34;5   5     ZrC95     30     2 × 10.sup.-5                            101      &#34;6  20     ZrB.sub.2 80               28     5 × 10.sup.-3                            138      &#34;7  20     Cr.sub.2 N80               20     7 × 10.sup.-3                            167      &#34;8  20     Cr.sub.3 C.sub.2 80               21     5 × 10.sup.-3                            161      &#34;9  20     CrB80     21     6 × 10.sup.-3                            147      &#34;10 20     HfN80     23     8 × 10.sup.-1                            161      &#34;11 90     HfC10     12     3 × 10.sup.2                            355      &#34;12 20     TaN80     26     6 × 10.sup.-3                            151      &#34;13 20     TaC80     23     4 × 10.sup.-5                            162      &#34;Examples14 20     TaB.sub.2 80               27     4 × 10.sup.-3                            155      &#34;15 20     Mo.sub.2 N80               22     8 × 10.sup.-3                            171      &#34;16 20     Mo.sub.2 C80               22     7 × 10.sup.-4                            162      &#34;17 20     MoB80     27     6 × 10.sup.-3                            143      &#34;18 20     NbN80     23     6 × 10.sup.-3                            153      &#34;19 25     NbC75     27     6 × 10.sup.-3                            195      &#34;20 25     NbB.sub.2 75               26     5 × 10.sup.-3                            191      &#34;21  5     WC95      30     3 × 10.sup.-5                            108      &#34;22 22     W.sub.2 C78               28     3 × 10.sup.-3                            178      &#34;23 25     TiB75     24     4 × 10.sup.-3                            185      &#34;24 20     VN80      22     6 × 10.sup.-3                            182      &#34;25 30     WSi.sub.2 70               22     8 × 10.sup. -4                            225      &#34;Examples26 90     TiSi.sub.2 10               13     3 × 10.sup.3                            345      &#34;27 90     ZrSi.sub.2 10               13     8 × 10.sup.3                            365      &#34;28 90     NbSi.sub.2 10               13     6 × 10.sup.3                            350      &#34;29 90     TaSi.sub.2 10               13     6 × 10.sup.3                            351      &#34;30 90     CrSi.sub.2 10               12     1 × 10.sup.4                            365      &#34;31 90     MoSi.sub.2 10               14     5 × 10.sup.3                            362      &#34;32 80     VC20      14     2 × 10.sup.1                            360      &#34;33 75     Fe.sub.3 C15               13     8 × 10.sup.1                            358      &#34;34 90     ThC10     13     3 × 10.sup.2                            358      &#34;35 90     CeC.sub.2 10               13     5 × 10.sup.2                            362      &#34;36 95     Cr.sub.7 C.sub.2 5               13     4 × 10.sup.3                            360      &#34;__________________________________________________________________________ 
    
     COMPARATIVE EXAMPLES 1-2 
     For comparison, a molded product was produced in the same manner as in Example 1 except that Si 3  N 4  powder of 0.8 μm in average particle size was used in place of metallic Si powder. This was sintered by a hot press under 150 kg/cm 2  at 1800° C. for 4 hours under vacuum to obtain a sintered body. Separately, a molded product was produced in the same manner as in Example 1 with the addition of 3 vol % of Y 2  O 3  and 3 vol % of AlN as sintering aids. This was sintered at 1000° C. for 4 hours under low-pressure in nitrogen atmosphere. Characteristics of the resulting sintered bodies are shown in Table 3. 
     
                                           TABLE 3__________________________________________________________________________       Composition of       sintered body                  State of                         Void      BendingComparative (vol %)    sintered                         content                             Resistivity                                   strengthExamples  Ceramics       Si.sub.3 N.sub.4           TiN              Al.sub.2 O.sub.3                  body   (%) (Ω cm)                                   (MPa)__________________________________________________________________________1 Hot  A    40  60 --  Cracks 4   7 × 10.sup.-4                                   Unmeasurable  pressing  B    50  -- 50  Good   4   8 × 10.sup.10                                   3302 Pressure-  A    37.6           56.4              --  Cracks and                         2   8 × 10.sup.-4                                   Unmeasurable  less                 deformation  sintering  B    48  -- 48  Deformation                         2   8 × 10.sup.10                                   380__________________________________________________________________________ 
    
     Both the hot press sintered body and pressureless sintered body cracked, so good ceramic composites were not obtainable. It is proposed that cracks or deformations do not occur in the products of the present invention because sintering temperature in low and there are open voids. Thus, stress caused by the difference in coefficient of thermal expansion is relaxed. 
     EXAMPLES 37-43 
     To 100 parts by weight of raw materials comprising a mixture of metallic Si powder of 0.9 μm in average particle size and TIN powder of 1.2 μm in average particle size it the ratio as shown in Table 4 was added 9 parts by weight of a binder comprising a polyethylene based thermoplastic resin and stearic acid. The mixture was kneaded at 160° C. for 12 hours by a pressure kneader. The kneaded product was pulverized to 10 meshes or less. This was used as a raw material for conductor ceramics A. Separately, to 100 parts by weight of a raw material comprising a mixture of metallic Si powder of 0.9 μm in average particle size and Al 2  O 3  powder of 2 μm in average particle size at the ratio as shown in Table 4 was added 9 parts by weight of a binder comprising a polyethylene based thermoplastic resin and stearic acid. The mixture was kneaded by a pressure kneader at 160 ° C. for 12 hours. The kneaded product was pulverized to 10 meshes or less. This was used as a raw material for insulant ceramics B. 
     Then, the raw materials for A and B were filled in succession in a mold to produce a layered ring as shown in FIG. 3(A) and FIG. 3(B). FIG. 3(B) Is cross-sectional view of FIG. 3(A) at Z--Z. After removal of the binder, the molded product was heated to 1100° C. at 5° C./min in nitrogen atmosphere and then heated from 1100° C. to 1350° C. at a rate of 3° C./h over a long period of time to obtain a sintered body of whisker/particle of Si 3  N 4  =2/9. characteristics of the thus obtained sintered body are shown in Table 4. 
     
                                           TABLE 4__________________________________________________________________________        Composition of        sintered body        (vol %)    Void content                          Resistivity                                Bending strengthNo.     Ceramics        Si.sub.3 N.sub.4           TiN Al.sub.2 O.sub.3                   (%)    (Ω cm)                                (MPa)__________________________________________________________________________Examples37 A    95  5  --   8     4 × 10.sup.5                                420   B    90 --  10  13     8 × 10.sup.13                                40038 A    90 10  --  13     2 × 10.sup.3                                403   B    90 --  10  12     8 × 10.sup.13                                40039 A    80 20  --  14     9.7 × 10.sup.0                                395   B    80 --  20  14     6 × 10.sup.12                                39040 A    60 40  --  16     3.1 × 10.sup.-3                                380   B    60 --  40  16     8 × 10.sup.13                                38041 A    40 60  --  20     9 × 10.sup.-4                                280   B    40 --  60  18     7 ×  10.sup.12                                25042 A    20 80  --  23     2.1 × 10.sup.-4                                180   B    30 --  80  21     8 × 10.sup.13                                20643 A     5 95  --  30     8 × 10.sup.-5                                103   B    20 --  80  26     7 × 10.sup.13                                158__________________________________________________________________________ 
    
     Thus, according to the present invention, integrally molded and sintered ceramics comprising layers different in resistivity can be obtained by changing the amount of the electrically conductive compound. 
     FIG. 4 is a graph which shows the relation between content of TiN and the volume change on sintering and the relation between content of Al 2  O 3  and the volume change on sintering. 
     As a comparative example, molding and sintering were effected in the same manner as in comparative Example 2 except that the composition of sintered body was similar to that of Examples 37-43. The volume change on sintering is also shown in FIG. 4. 
     According to FIG. 4, the products of the present invention are superior sintered bodies which are extremely smaller in dimensional change than the pressureless sintered products. The products of the present invention are also free from cracks. 
     FIG. 5 shows relation between the bending strength and the test temperature. 
     According to FIG. 5, the products of the present invention show no reduction in strength, even at high temperatures. On the other hand, since the comparative products contain sintering aids, there exists a glass phase, which is softened at high temperature. This results in a reduction in strength at high temperatures. 
     EXAMPLE 44 
     To 100 parts by weight of a raw material comprising a mixture of 50 vol % (in terms of Si 3  N 4 ) of ferro-Si powder (average particle size 2 μm) and 50 vol % of ZrN powder (average particle size 2 μm) was added 10 parts by weight of a polysilane compound as a binder. From this mixed powder, ceramic A wag prepared in the same manner as in Example 1. Ceramic B was prepared in the same manner as in Example 1 except that 10 parts by weight of a polysilane compound was used as a binder. These powders were molded in the same manner as in Example 1. The molded product was heated to a maximum of 1450° C. with stepwise retention of time in nitrogen atmosphere to obtain a sintered body. 
     Characteristics of the resulting ceramics composite were relative density: 93%; resistivity: 4×10 -3  Ω cm; bending strength: 420 MPa and dimensional change on sintering:+3.8%. 
     EXAMPLE 45 
     Sintered bodies were produced in the same manner as in Example 44 except that the particle size of the raw material ferro-Si powder was changed. The relation between the particle size and bending strength was also examined. 
     The results are shown in FIG. 6. A silicon particle size of 5 μm or less is preferred. 
     It is considered that when the particle size exceeds 5 μm, unnitrided Si remains in sintered body. This is evaporated by heating, resulting in reduction of strength. 
     EXAMPLES 46-50 AND COMPARATIVE EXAMPLES 3 AND 4 
     Molded products were prepared in the same manner as in Example 1. They were sintered with changing heating rate and changing stepwise heating time from 1100° C. to 1400° C., thereby obtaining sintered bodies adjusted in the amount of Si 3  N 4  whisker produced. Characteristics of the resulting sintered body are shown in Table 5. 
     
                                           TABLE 5__________________________________________________________________________         Composition of sintered                         Void         Thermal         body (vol %)    con-                            Resis-                                 Bending                                      shock     Ceram-  Si.sub.3 N.sub.4                         tent                            tivity                                 strength                                      resistance  No.     ics Si.sub.3 N.sub.4             Whisker                  TiN                     Al.sub.2 O.sub.3                         (%)                            (Ω cm)                                 (MPa)                                      (times)__________________________________________________________________________Examples  46 A   39.6             0.4  60 --  21 9 × 10.sup.-4                                 280  45     B   49.5             0.5  -- 50  20 7 × 10.sup.13                                 272  45  47 A   32  8    60 --  21 9 × 10.sup.-4                                 281  47     B   40  10   -- 50  19 7 × 10.sup.13                                 273  47  48 A   24  16   60 --  21 9 × 10.sup.-4                                 280  50     B   30  20   -- 50  19 7 × 10.sup.13                                 270  50  49 A   16  24   60 --  22 9 × 10.sup.-4                                 270  52     B   20  30   -- 50  20 7 × 10.sup.13                                 270  52  50 A   12  28   60 --  22 9 × 10.sup.-4                                 265  48     B   15  35   -- 50  22 7 × 10.sup.13                                 245  48Comparative  3  A   39.76             0.24 60 --  22 9 × 10.sup.-4                                 280  38Examples  B   49.7             0.3  -- 50  23 7 × 10.sup.13                                 275  38  4  A   8   32   60 --  23 9 × 10.sup.-4                                 221  33     B   10  40   -- 50  23 7 × 10.sup.13                                 220  33__________________________________________________________________________ 
    
     From the above, it can be seen that the products of the present invention where Si 3  N 4  whiskers were present in an amount of 1-70 vol % of Si 3  N 4  phase produced from Si were superior in thermal shock resistance. 
     The thermal shock resistance was evaluated as follows: A sintered body was kept at 1200° C. for 30 minutes and then introduced into water to quench. This was repeated until cracks occurred. The number of the repetitions was taken as thermal shock resistance value. 
     EXAMPLES 51-57 
     Molded products were prepared in the same manner as in Example 1. Sintered bodies were obtained therefrom by changing oxygen partial pressure in nitriding gas. Characteristics of the sintered bodies are shown in Table 6. 
     
                                           TABLE 6__________________________________________________________________________       Composition of sintered body                         Resis-                              Bending                                   Thermal shock   Ceram-       (vol %)           tivity                              strength                                   resistanceNo.     ics Si.sub.3 N.sub.4           Si.sub.2 N.sub.2 O               SiO.sub.2                  TiN                     Al.sub.2 O.sub.3                         (Ω cm)                              (MPa)                                   (times)__________________________________________________________________________Examples51 A   40   0  0  60 --  9 × 10.sup.-4                              280  45   B   50   0  0  -- 50  7 × 10.sup.13                              270  4552 A   36   4  0  60 --  9 × 10.sup.-4                              280  45   B   45   5  0  -- 50  7 × 10.sup.13                              270  4553 A   24  16  0  60 --  9 × 10.sup.-4                              280  47   B   30  20  0  -- 50  7 × 10.sup.13                              272  4754 A   20  16  4  60 --  3 × 10.sup.-3                              260  40   B   25  20  5  -- 50  5 × 10.sup.14                              260  4055 A    0  32  8  60 --  8 × 10.sup.-3                              251  28   B    0  40  10 -- 50  8 × 10.sup.14                              253  28Examples56 A    0   4  36 60 --  2 × 10.sup.-2                              205  20   B    0   5  45 -- 50  8 × 10.sup.14                              210  2057 A    0   0  40 60 --  4 × 10.sup.-2                              191  15   B    0   0  50 -- 50  9 × 10.sup.14                              198  15__________________________________________________________________________ 
    
     From the above, it is seen that when amounts of Si 2  N 2  O and SiO 2  are larger than that of Si 3  N 4  in the phase produced from Si, resistivity and thermal shock resistance tend to decrease. 
     EXAMPLES 58-61 
     Sintered bodies were prepared in the same manner as in Example 1 except that the raw materials an shown in Table 7 were used in place of TiN and Al 2  O 3  particles. The results are shown in Table 7. 
     According to the present invention, integral molding and sintering with optional combination of resistivities within the range of 10 14  Ω cm to 10 -5  Ω cm are possible by compositely combining a conductive compound and an insulating compound. 
     
                       TABLE 7______________________________________  Ce-                     Resis- Bending  ram-   Raw material ratio                          tivity strengthNo.    ics    (wt. %)          (Ω cm)                                 (MPa)______________________________________Ex-  58    A      Si(25), TiC(10), ZrN(65)                            5 × 10.sup.-3                                   182am-        B      Bi(30), TiN(30), SiC(40)                            1 × 10.sup.-1                                   205ples 59    A      Si(50), ZrO.sub.2 (10), Cr.sub.2 N(40)                            2 × 10.sup.-2                                   275      B      Si(30), Al.sub.2 O.sub.3 (50),                            4 × 10.sup.13                                   232             Si.sub.3 N.sub.4 (20)60    A      Si(30), B.sub.4 C(10), TiC(60)                            8 × 10.sup.-3                                   205      B      Si(30), Si.sub.2 N.sub.2 O(10),                            3 × 10.sup.13                                   213             ZrO.sub.2 (60)61    A      Si(30), TiSi.sub.2 (5), WC(65)                            4 × 10.sup.-4                                   189      B      Si(30), CrB(5), Al.sub.2 O.sub.3 (65)                            2 × 10.sup.2                                   193______________________________________ 
    
     EXAMPLE 62 
     Si 3  N 4  /TIN conductive ceramics and Si 3  N 4  /Al 2  O 3  insulating ceramics obtained in Example 1 were integrally molded and sintered as shown in FIG. 3(A) and FIG. 3(B) as a current collector for an automobile alternator. Current collecting characteristics thereof were examined. The results are shown in Table 8. From the results, it can be seen that the product of the present invention is superior to the conventional copper/heat resisting resin in heat resistance and wear resistance. 
     
                       TABLE 8______________________________________                  Comparative        Example 62                  Example 5______________________________________Current Conductive Si.sub.3 N.sub.4 /TiN                          CopperCollect-   portioning ring   Insulating Si.sub.3 N.sub.4 /Al.sub.2 O.sub.3                          Heat resist-   portion                ing resinCurrent collector          Carbon      CarbonState of   Current    Gloss       Blackishsliding collecting             brownsurface ring   Current    Gloss       Scratch   collectorDamage  Current    1-3 μm   10-20 μm   Collecting   ring   Current    4.0         0.82   collectorSpark          No          NoState after test          No change   Scorching mark in                      the insulating                      portion                      Blackening of                      copper portion______________________________________ Condition: 30,000 rpm Current density of current collector 70 A/cm.sup.2 
    
     EXAMPLE 63 
     Si 3  N 4  /TiN conductive ceramics and Si 3  N 4  /Al 2  O 3  ceramics obtained in Example 1 were integrally molded and sintered as shown in FIG. 7(A) and FIG. 7(B) as a commutator for an automobile starter motor. Current collecting characteristics were examined. As in Example 63, it has been confirmed that the product of the present invention is superior to the conventional copper/heat resisting resin in heat resistance and wear resistance. Also, the production of a non-combustible motor is possible. 
     EXAMPLE 64 
     To 100 parts by weight of raw material comprising a mixture of 22.7 wt. % of metallic Si powder of 0.5 μm in average particle size and 77.3 wt. % of TiN whisker of 50 in aspect ratio and 50 μm in length was added 9 parts by weight of binder comprising a low-density polyethylene, a synthetic wax and stearic acid. The mixture was kneaded by a pressure kneader at 160° C. for 12 hours. The kneaded product was pulverized to 10 meshes or less. This was used as a ceramic conductor A. 
     Separately, to 100 parts by weight of raw material comprising a mixture of 38 wt. % of metallic Si powder of 0.9 μm in average particle size and 62 wt. % of Al 2  O 3  of 35 μm in average particle size was added 9 parts by weight of the above binder. The mixture was kneaded and pulverized in the same manner as above. This was used as a raw material for insulant ceramic B. In the same manner as in Example 1, a composite sintered body was prepared using these ceramics A and B. 
     Dimensional change of this sintered body was 40.2. The portion A had a resistivity of 7×10 -4  Ω cm, a void of 18%, a bending strength of 291 MPa and a coefficient of thermal expansion of 5.2×10 -6 . The portion B had a resistivity of 5×10 13  Ω cm, a void of 20%, a bending strength of 250 MPa and a coefficient of thermal expansion of 5.1×10 -6 . 
     EXAMPLES 65-70 
     To 500 g of a mixed powder comprising SiC particle of 16 μm in average particle size and metallic Si of 0.9 μm in average particle size at the ratio as shown in Table 8 was added 8-12 parts by weight of a molding binder comprising 21% of polyethylene (melt index 1.5), 43% of polyethylene wax (PE-520 manufactured by Hoecht, Japan) 21% of blend wax (D-556 manufactured by Chukyo Yushi Co.) and 15% of stearic acid. The mixture was kneaded in a pressure kneader at 160° C. for 5 hours. The kneaded product was crushed to obtain a raw material under test. This raw material was molded into a molded body of 50 mm in diameter and 20 mm thick by a mechanical press at 160° C. under a molding pressure of 1000 kgf/cm 2 . Volumetric packing of particle of the resulting molded body was at least 70 vol %. This molded body from which the molding binder had been removed was heated from 1100° C. to 1350° C. at a rate of 4° C./h in a nitrogen atmosphere. A sintered body with whisker/particle of Si 3  N 4  of 1/9 was obtained. The blending ratio of whisker/ particle can be adjusted by changing heating rate and retention time. The ratio of whisker/particle can be obtained by observation of the sintered body under scanning electron microscope and transmission electron microscope. 
     Characteristics of the obtained sintered body are shown in Table 8. The breaking toughness was measured by a notched beam method using a test piece of 3×4×40 mm having a notch of 0.5 mm made by a diamond wheel. For the thermal shock, the sintered body was kept at 1250° C. for 30 minutes and then introduced into water to quench it. This was repeated until cracks occurred. The thermal shock was evaluated by the number of the repetition. 
     Next, mixed powder of the compositions as shown in Comparative Examples 65-70 of Table 9 was prepared. This mixed powder contained Y 2  O 3  as a sintering aid. This was molded in the same manner as above and sintered at 1700° C. for 5 hours to obtain a sintered body. Characteristics of the obtained sintered body are shown in Table 9. 
     FIG. 8 shows the relation between the content of SiC and the rate of dimensional change on sintering. The product (A) of the present invention is a markedly superior sintered body with very small rate of dimensional change as compared with the product (B) comprising Si 3  N 4  powder, SiC powder to which sintering aid Y 2  O 3  was added. 
     
                                           TABLE 8__________________________________________________________________________      Volumetric packing                Composition                       Dimensional                              Bending           ThermalRaw-       of particles                of sintered                       change strength          shockmaterial   in molded body   on sin-                              (MN/m.sup.2)                                          Breaking                                                resis-(vol %)    product   (vol %)                       tering Room        toughness                                                tanceExampleSi SiC      (%)       Si.sub.3 N.sub.4                    SiC                       (%)    temperature                                     1200° C.                                          (MN/m.sup.3/2)                                                (Times)__________________________________________________________________________65   100    0 72        100  0 0.13   584    582  5     4366   90 10 75        90  10 0.12   512    510  6     4567   80 20 74        80  20 0.12   490    489  7     5068   70 30 73        70  30 0.12   430    430  7     5069   60 40 75        60  40 0.12   390    390  7     5070   45 55 74        45  55 0.12   310    310  6     49__________________________________________________________________________ 
    
     
                                           TABLE 9__________________________________________________________________________  Composition of            Bending strength  Thermal  sintered body            (MN/m.sup.2)                        Breaking                              shockComparative  (vol %)   Room        toughness                              resistanceExamples  Si.sub.3 N.sub.4      Y.sub.2 O.sub.3         SiC            temperature                   1200° C.                        (MN/m.sup.3/2)                              (times)__________________________________________________________________________5      93  7   0 753    352  5     216      84  6  10 518    321  3     217      75  5  20 462    255  3     188      66  4  30 391    211  3     179      57  3  40 358    208  3     1510     42  3  55 211    127  2     15__________________________________________________________________________ 
    
     EXAMPLES 71-113 
     Sintered bodies were prepared in the same manner as in Example 69 except that the inorganic compounds shown in Table 10 were used in place of SiC powder. Characteristics of the resulting sintered bodies are shown in Table 10. 
     It can be seen from Table 10 that the products of the present invention are superior in high-temperature strength, breaking toughness and thermal shock resistance. The rate of dimensional change on sintering was nearly the same as shown by curve (A) in FIG. 8. 
     
                                           TABLE 10(1)__________________________________________________________________________Raw materialcompound    Bending strength      Thermalin sintered (MN/m.sup.2)                   Void                       Breaking                             shockbody   Room        content                       toughness                             resistanceExamples(40 vol %)       temperature              1200° C.                   (%) (MN/m.sup.3/2)                             (times)__________________________________________________________________________71   TiN    358    351  12  7     4872   Si.sub.3 N.sub.4       362    354  12  5     4573   Si.sub.2 N.sub.2 O       341    330  13  6     4874   TiC    367    358  12  7     4875   TiO.sub.2       357    352  12  7     4776   AlN    365    360  11  7     5077   Al.sub.2 O.sub.3       351    343  12  7     4878   SiO.sub.2       342    336  13  6     4579   ZrN    361    356  11  7     5180   ZrO.sub.2       356    345  12  7     4981   ZrC    356    354  12  7     5282   TiB.sub.2       332    330  13  6     4883   ZrB.sub.2       341    335  13  6     4884   Cr.sub.3 C.sub.2       356    352  12  7     5185   CrB    332    329  13  6     4786   Cr.sub.2 N       365    359  11  7     5387   Cr.sub.7 C.sub. 2       355    353  12  7     5188   WC     345    341  12  7     4789   W.sub.2 C       346    343  12  7     4790   WSi.sub.2       331    330  13  6     4591   TiSi.sub.2       333    328  13  6     4592   ZrSi.sub.2       338    335  13  6     4593   Fe.sub.3 C       362    359  11  8     5294   VC     356    346  12  7     4995   VN     367    364  11  7     5196   CeC.sub.2       367    366  11  7     5097   MgO    336    328  13  6     4598   Mo.sub.2 N       349    346  12  7     4999   Mo.sub.2 C       365    346  12  7     50__________________________________________________________________________ 
    
     
                                           TABLE 10(2)__________________________________________________________________________Raw materialcompound    Bending strength      Thermalin sintered (MN/m.sup.2)                   Void                       Breaking                             shockbody   Room        content                       toughness                             resistanceExamples(40 vol %)       temperature              1200° C.                   (%) (MN/m.sup.3/2)                             (times)__________________________________________________________________________100  MoB    336    324  13  6     45101  ThC    368    358  12  7     50102  HfN    359    355  12  7     52104  HfC    357    354  12  7     52105  TaN    349    347  12  7     49106  TaC    347    346  12  7     51107  TaB.sub.2       341    332  13  7     46108  NbN    338    331  13  7     47109  BeO    359    358  12  7     50110  WN     356    352  12  7     52111  CaO    352    347  12  7     50112  TaSi.sub.2       335    331  13  7     48113  CrSi.sub.2       332    328  13  7     47__________________________________________________________________________ 
    
     EXAMPLES 114-117 
     Sintered bodies were prepared in the same manner as in Example 69 except that the SiC whiskers as shown in Table 11 were used in place of SiC powder. Characteristics of the resulting sintered bodies are shown in Table 11. The rate of dimensional change on sintering was less than 0.13% for all of them. From the results, it can be seen that the preferable average aspect ratio of a raw material whisker is 200 or less and the preferable average length is 200 μm or less. 
     
                       TABLE 11______________________________________Raw material                Break-whisker in                  ingsintered body  Bending      tough-  ThermalEx-  Average  Average  strength   ness  resist-am-  aspect   length   room         (MN/  anceples ratio    (μm)  temp. 1200° C.                               m.sup.3/2)                                     (times)______________________________________114  180       50      364   356    8     55115   50      150      374   375    8     55116  230      100      310   305    6     50117   50      220      312   310    6     50______________________________________ 
    
     EXAMPLES 118-124 
     Molded products were prepared in the same manner as in Examples 65-70. They were then sintered by changing the heating time stepwise to 1400° C. in a nitrogen atmosphere thereby to adjust the amount of Si 3  N 4  whisker produced. Test results of the resulting sintered bodies are shown in Table 12. From the results, it can be seen that the products of the present invention, where 1-70 vol % of Si 3  N 4  whisker was present for 100 vol % of the produced Si 3  N 4  phase, were especially tough. 
     
                                           TABLE 12__________________________________________________________________________Composition of     Bending strengthsintered body (vol %)              (MN/m.sup.2)                       Breaking                             Thermal shock     Si.sub.3 N.sub.4              Room     troughness                             resistanceExamplesSi.sub.3 N.sub.4     whisker          SiC temp.                  1200° C.                       (MN/m.sup.3/2)                             (times)__________________________________________________________________________118  49.5 0.5  50  330 334  6     50119  40   10   50  332 332  7     50120  30   20   50  334 331  7     50121  20   30   50  332 331  7     50122  15   35   50  335 332  6     50123  49.7 0.3  50  331 332  4     50124  10   40   50  334 332  4     50__________________________________________________________________________ 
    
     EXAMPLES 125-129 
     Table 13 shows the characteristics of sintered bodies made in accordance with Example 65 using 55 of Si, 45 of SiC and molding binder in the amounts changed within the range of 5-13 parts by weight whereby the volumetric particle packing rate of molding products was changed. Relation between the volumetric particle packing rate of molded products and bending strength is shown in FIG. 12. From these results, it can be seen that molded products having a volumetric particle packing rate of at least 70 vol % are preferred. 
     
                       TABLE 13______________________________________Volumetricpacking of                         Thermalparticles   Bending strength       shockin molded   (MN/m.sup.2) Breaking  resist-Exam- product   Room           toughness                                  anceples  (%)       temp.   1200° C.                          (MN/m.sup.3/2)                                  (times)______________________________________125   65        154     154    3       42126   69        257     254    3       48127   70        310     310    5       48128   73        359     358    6       50129   75        390     390    7       50______________________________________ 
    
     EXAMPLES 130-144 
     Table 14 shows the volumetric particle packing rate of molded bodies made in the same manner as in Example 65 except that the blending ratio of raw materials and the amount of molding binder are changed. 
     FIG. 13 shows relation between the specific surface area of the raw materials and the suitable amount of binder. When the relation is within the hatched area, the volumetric particle packing rate of molded bodies can be at least 70 vol %. 
     
                       TABLE 14______________________________________         Specific            Volumetric         surface   Amount of packing ofRaw material  area of   molding   particles(parts by     raw       binder    in moldedweight)       material  (part by  productExamples  Si     SiC     (cm.sup.2 /g)                         weight) (vol %)______________________________________130    60     40      17100   5.0     65131    60     40      17100   6.5     70132    60     40      17100   9.0     76133    60     40      17100   11.5    70134    60     40      17100   12.5    64135    90     10      25100   6.5     64136    90     10      25100   9.3     70137    90     10      25100   11.8    75138    90     10      25100   14.3    70139    90     10      25100   16.3    63140    50     50      14500   4.1     63141    50     50      14500   5.6     70142    50     50      14500   8.1     76143    50     50      14500   10.5    70144    50     50      14500   12.4    62______________________________________ 
    
     EXAMPLE 145 
     The apparent viscosity of the kneaded products of the raw materials and the molding binder in Examples 130-144 was measured by a flow tester. 
     The related between the apparent viscosity and the volumetric particle packing rate of the molded bodies is shown in FIG. 14. The smaller value of the apparent viscosity indicates better flowability, namely, superior moldability. 
     The apparent viscosity was measured by a flow tester having a nozzle of 6 mm diameter and 6.8 mm length at 150° C. and under a pressure of 39 MN/m 2 . 
     FIG. 14 shows that the molded products can have a volumetric particle packing rate or at least 70 vol % by using a composition having an apparent viscosity within the range of (3-90)×104 N·s/m 2 . The apparent viscosity of this range is relatively low. Thus molded products of complicated shape can be easily produced. 
     EXAMPLE 146 
     60 Parts by weight of metallic Si powder of 0.9 μm in average particle size and 40 parts by weight of α-SiC powder of 16 μm in average particle size were mixed together with methanol in a pot mill and dried. Then, thereto was added 9 parts by weight of a polyethylene wax and the mixture was kneaded by a pressure kneader at 150° C. for 5 hours. Then, the mixture was pulverized and molded at 150° C. and under 1000 kg/cm 2  to a ring of 60 mm in outer diameter and 40 mm in inner diameter. After removal of the wax, the molded body was subjected to stepwise heat treatment to 1380° C. over a long period of time in nitrogen atmosphere to obtain a sintered body. Dimensional change between the molded body and the sintered body was small, namely, 0.15%. The sintered body was superior in dimensional accuracy. Void content of the sintered body was 15% and diameter of void was 20 pm or less. 
     Sliding surface was produced by polishing. The sliding surface was subjected to a sliding test using A1203 (relative density: 98%) as a partner material. The results are shown in Table 15. As a comparative example, results of test between FC-20 cast-iron products are also shown in Table 15. 
     FIG. 15 shows a micrograph of the sliding surface of the sintered body obtained in this Example. In FIG. 15, 1 indicates a matrix phase, 2 indicates Si 3  N 4  particles and 3  indicates voids. 
     From Table 1, it can be seen that the product of the present invention is small in friction coefficient and superior in wear resistance. 
     The relation between the surface pressure and the friction coefficient when the surface pressure was changed while keeping the sliding speed at a constant 3 m/sec is shown by curve A in FIG. 16. The result on Comparative Example 11 is also shown by curve B in FIG. 16. It can be recognized that the product of the present invention is superior in sliding characteristics. 
     
                       TABLE 15______________________________________                      Total                      abrasion   Sliding   Friction wear   material  coeffi-  (mg/cm/  State of   (I)   (II)    cient    100 h) sliding______________________________________Example  146    SiC/    Al.sub.2 O.sub.3                       0.008  0.06   Good         Si.sub.3 N.sub.4Compa- 11     FC-20   FC-20 0.12   72     BraggingrativeExample______________________________________ Sliding conditons: In machine oil at surface pressure of 10 kgf/cm.sup.2 and sliding speed of 3m/sec. 
    
     EXAMPLES 147-204 
     The sliding test was conducted in the same manner as in Example 146 except that Al 2  O 3  was replaced by other materials as the partner material. The results are shown in Table 16. Results of comparative examples are shown in Table 17. Sliding conditions were surface pressure: 2 kgf/cm 2  and sliding speed: 2 m/sec. 
     From the results, it can be seen that the products of the present invention are small in friction coefficient and superior in thermal shock resistance. This is because the products of the present invention have open voids and comprise composite material. When the test was conducted using SiC, Si 3  N 4  having the same particle hardness as the partner material, particles of the products of the present invention were falling off, leaving great sliding marks. The thermal shack resistance was evaluated as follows: A sintered body was kept at 1,200° C. for 30 minutes and then quenched in water. This was repeated until cracks occurred. The number of the repetitions was taken for evaluation. 
     
                                           TABLE 16__________________________________________________________________________                 SiC/Si.sub.3 N.sub.4                       Abrasion                 Abrasion                       wear of partner                               Theraml shock            Friction                 wear  material                               resistance of   Partner material            coeffi-                 (mg/cm.sup.2 /                       (mg/cm.sup.2 /                               SiC/Si.sub.3 N.sub.4No.     for sliding test            cient                 100 h)                       100 h)  (times)__________________________________________________________________________Examples147   AlN      0.014                 0.03  0.08    51148   SiO.sub.2 glass            0.008                 0.02  0.04    51149   SiO.sub.2            0.007                 0.03  0.03    51150   TiN      0.013                 0.04  0.08    51151   TiO.sub.2            0.008                 0.03  0.04    51152   ZrN      0.012                 0.03  0.09    51153   ZrO.sub.2            0.007                 0.03  0.04    51154   carbon   0.010                 0.01  0.20    51155   MgO      0.010                 0.02  0.03    51156   apatite  0.012                 0.02  0.04    51157   ZnO      0.009                 0.01  0.04    51Examples158   ferite   0.008                 0.03  0.05    51159   UO.sub.2 0.010                 0.03  0.05    51160   BaTiO.sub.2            0.009                 0.02  0.07    51161   PZT      0.009                 0.01  0.04    51162   B.sub.4 C            0.007                 0.03  0.08    51163   ZrB.sub.2            0.012                 0.03  0.12    51164   TiB.sub. 2            0.012                 0.03  0.11    51165   TiO      0.009                 0.03  0.04    51166   BeO      0.009                 0.03  0.05    51167   (ZrO)P.sub.2 O.sub.7            0.010                 0.04  0.06    51168   beryl    0.010                 0.03  0.05    51Examples169   TiO.sub.2 --Al.sub.2 O.sub.3            0.008                 0.04  0.03    51170   cordierite            0.009                 0.03  0.02    51171   β-spodumene            0.011                 0.03  0.04    51172   ZrC      0.010                 0.03  0.11    51173   MgAl.sub.2 O.sub.4            0.012                 0.02  0.04    51174   BeAl.sub.2 O.sub.4            0.010                 0.04  0.06    51175   Be.sub.2 SiO.sub.4            0.011                 0.03  0.05    51176   ZrSiO.sub.4            0.011                 0.04  0.05    51177   celsian  0.012                 0.04  0.04    51178   willemite            0.012                 0.04  0.05    51179   3Al.sub.2 O.sub.3.2SiO.sub.2            0.009                 0.04  0.04    51Examples180   2BaO.3SiO.sub.2            0.009                 0.02  0.04    51181   MgO--SiO.sub.2            0.009                 0.03  0.04    51182   MgO      0.009                 0.04  0.03    51183   B.sub.2 O.sub.3            0.009                 0.03  0.04    51184   MoC      0.010                 0.05  0.11    51185   Al.sub.2 O.sub.3 --ZrO.sub.2            0.008                 0.03  0.04    51186   Al.sub.2 O.sub.3 --TiN            0.008                 0.04  0.08    51187   SiO.sub.2 --Al.sub.2 O.sub.3            0.008                 0.03  0.07    51188   TiC      0.009                 0.07  0.15    51189   Al.sub.2 O.sub.3 --TiC            0.008                 0.05  0.09    51Examples190   CaO      0.010                 0.02  0.05    51191   TaC      0.011                 0.04  0.13    51192   SUS304   0.009                 0.01  0.03    51193   Cr cast iron            0.009                 0.01  0.02    51194   Super hard alloy            0.009                 0.01  0.02    51195   FC20     0.009                 0.01  0.03    51196   Ni-Resist            0.009                 0.01  0.02    51197   Steatite 0.009                 0.01  0.02    51198   Ni-cast iron            0.009                 0.01  0.02    51199   Al bronze            0.010                 0.01  0.04    51200   P bronze 0.010                 0.01  0.04    51201   MoS.sub.2 --C            0.007                 0.01  0.02    51Examples202   Teflon   0.007                 0.01  0.02    51203   Fluorine conta-            0.007                 0.01  0.02    51   ining resin204   Glass-nylon            0.008                 0.01  0.02    51__________________________________________________________________________ Sliding conditions: In machine oil at surface pressure of 2 kg/cm.sup.2 and sliding speed of 2 m/sec. 
    
     
                                           TABLE 17__________________________________________________________________________                       (I)   (II)  Thermal                       Abrasion                             Abrasion                                   shock     Condition of Friction                       wear  wear  resistance     sliding materials                  coeffi-                       (mg/cm.sup.2 /                             (mg/cm.sup.2 /                                   (times)  No.     (I)   (II)   cient                       100 h)                             100 h)                                   (I)                                     (II)__________________________________________________________________________Comparative  12 SiC   SiC    0.12 0.24  0.24  31                                     31Examples  13 SiC   Si.sub.3 N.sub.4                  0.15 0.22  0.31  31                                     37  14 SiC   Al.sub.2 O.sub.3                  0.18 0.18  0.41  31                                     12  15 SiC   TiC    0.12 0.19  0.22  31                                     30  16 SiC   SiO.sub.2                  0.17 0.12  0.75  31                                     19  17 SiC   TiO.sub.2                  0.15 0.19  0.42  31                                     24  18 SiC   Carbon 0.12 0.09  0.96  31                                     --  19 SiC   ZrO.sub.2                  0.14 0.18  0.22  31                                     16  20 SiC   ZrN    0.16 0.17  0.23  31                                     30  21 SiC   AlN    0.11 0.14  0.31  31                                     28  22 SiC   MgO    0.22 0.16  0.56  31                                     10Comparative  23 SiC   B.sub.4 C                  0.14 0.21  0.26  31                                     20Examples  24 SiC   UO.sub.2                  0.15 0.19  0.18  31                                     19  25 Si.sub. 3 N.sub.4           Si.sub.3 N.sub.4                  0.14 0.24  0.24  37                                     37  26 Si.sub.3 N.sub.4           Al.sub.2 O.sub.3                  0.23 0.18  0.42  37                                     12  27 Si.sub.3 N.sub.4           TiC    0.18 0.34  0.22  37                                     30  28 Si.sub.3 N.sub.4           SiO.sub.2                  0.19 0.19  0.56  37                                     19  29 Si.sub.3 N.sub.4           TiO.sub.2                  0.18 0.20  0.22  37                                     24  30 Si.sub.3 N.sub.4           Carbon 0.12 0.09  0.89  37                                     --  31 Si.sub.3 N.sub.4           ZrO.sub.2                  0.17 0.18  0.23  37                                     16Comparative  32 Si.sub.3 N.sub.4           ZrN    0.20 0.22  0.23  37                                     30Examples  33 Si.sub.3 N.sub.4           AlN    0.24 0.21  0.31  37                                     28  34 Si.sub.3 N.sub.4           MgO    0.22 0.16  0.48  37                                     10  35 Si.sub.3 N.sub.4           B.sub.4 C                  0.18 0.31  0.28  37                                     20  36 Si.sub.3 N.sub.4           UO.sub.2                  0.17 0.21  0.18  37                                     19  37 Al.sub.2 O.sub.3           Al.sub.2 O.sub.3                  0.38 0.24  0.24  12                                     12  38 Al.sub.2 O.sub.3           TiC    0.24 0.24  0.25  12                                     30  39 Al.sub.2 O.sub.3           SiO.sub.2                  0.23 0.19  0.48  12                                     19  40 Al.sub.2 O.sub.3           TiO.sub.2                  0.24 0.21  0.21  12                                     24  41 Al.sub.2 O.sub.3           Carbon 0.10 0.09  1.17  12                                     --  42 Al.sub.2 O.sub.3           ZrO.sub.2                  0.28 0.17  0.18  12                                     16  43 Al.sub.2 O.sub.3           ZrN    0.26 0.21  0.16  12                                     30  44 Al.sub.2 O.sub.3           AlN    0.21 0.31  0.34  12                                     28  45 Al.sub.2 O.sub.3           FC-20  0.28 0.24  1.21  12                                     --  46 Al.sub.2 O.sub.3           Carbon iron                  0.28 0.23  1.02  12                                     --  47 Al.sub.2 O.sub.3           PTFE   0.17 0.14  0.56  12                                     --  48 ZrO.sub.3           Carbon iron                  0.27 0.21  0.98  16                                     --  49 Product of           Product of                  0.27 1.71  1.71  51                                     51     the present           the present     invention           inventionComparative  50 Product of           SiC    0.24 2.95  0.45  51                                     31Examples  the present     invention  51 Product of           Si.sub.3 N.sub.4                  0.28 2.24  0.53  51                                     37     the present     invention__________________________________________________________________________ Sliding conditions: In machine oil at surface pressure of 2 kg/cm.sup.2 and sliding speed of 2 m/sec. 
    
     EXAMPLES 205-210 
     The same test as in Example 146 was conducted on sintered bodies different in void content and diameter. The amount of binder was changed. The results are shown in Table 18. 
     Sliding conditions were surface pressure: 10 kgf/cm 2  and sliding speed: 1 m/sec. 
     From the results, it can be seen that when void content exceeds 30%, abrasion wear abruptly increases. When diameter of void is more than 100 μm, abrasion wear also increases. The products of the present invention are superior in sliding properties. 
     
                                           TABLE 18__________________________________________________________________________                      SiC/Si.sub.3 N.sub.4                                 Total                SiC/Si.sub.3 N.sub.4                      Maximum    abrasion                Void  void  Friction                                 wear      Sliding materials                content                      diameter                            coeffi-                                 (mg/cm.sup.2 /   No.      (I)   (II)                (%)   (μm)                            cient                                 100 h)__________________________________________________________________________Examples   205      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                 5     5    0.007                                 0.07   206      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                10     8    0.007                                 0.08   207      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                15    15    0.007                                 0.08   208      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                20    15    0.007                                 0.08   209      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                25    30    0.007                                 0.11   210      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                30    100   0.007                                 0.89Examples    52      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                35    40    0.007                                 1.57(Comparative)    53      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                40    50    0.007                                 2.96    54      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                50    80    0.007                                 3.24    55      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                30    110   0.007                                 1.54Examples    56      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                30    150   0.007                                 3.21(Comparative)    57      SiC/Si.sub.3 N.sub.4            Al.sub.2 O.sub.3                30    210   0.007                                 Dragging                                 for 70 h__________________________________________________________________________ Sliding conditions: In machine oil at surface pressure of 10 kgf/cm.sup.2 and sliding speed of 2 m/sec. 
    
     EXAMPLES 211-217 
     Sintered bodies were prepared in the same manner as in Example 146 except that the average particle size of SiC powder was changed. The sliding test was conducted using Al 2  O 3  as the partner material. The results are shown in Table 19. When SiC particle is great, dragging occurs as a result of the SiC particles which have fallen off. The products of the present invention are superior in wear resistance. Excellent sliding properties were exhibited when two or more SiC particles different in average particle size were mixed. 
     
                                           TABLE 19__________________________________________________________________________      Average particle                    SiC/Si.sub.3 N.sub.4 Abrasion                              Al.sub.2 O.sub.3 Abrasion                                      SiC/Si.sub.3 N.sub.4      size of SiC in               Friction                    wear      wear    Thermal shock      SiC/Si.sub.3 N.sub.4               coeffi-                    (mg/cm.sup.2 /                              (mg/cm.sup.2 /                                      resistance   No.      (μm)  cient                    100 h)    100 h)  (times)__________________________________________________________________________Examples   211      0.5      0.08 0.02      0.03    50   212      2        0.08 0.03      0.03    51   213      10       0.07 0.03      0.04    51   214      50       0.07 0.04      0.07    51   215      72       0.07 0.07      0.12    52   216      85       0.07 0.09      0.17    52   217      100      0.07 0.10      0.22    51Examples    58      120      0.07 0.22      1.08    51(Comparative)    59      240      0.07 0.44      1.54    51    60      400      0.07 0.81      2.12    51    61      800      0.07 1.23      5.23    51__________________________________________________________________________ Sliding conditons: Surface pressure 10 kgf/cm.sup.2 and sliding speed 5 m/sec. in SUNISO 4GS oil (made by Nippon Sun Sekyu K.K.) 
    
     EXAMPLES 218-220 
     60 parts by weight of metallic Si powder of 1 μm in average particle size and 40 parts by weight of β-Sic powder of 8 μm in average particle size were mixed together with an organosilicane high polymer and xylene as a binder in a pot mill and dried. The mixture was molded to a ring of 150 mm outer diameter, 100 mm inner diameter and 20 mm thick, at 100° C. and under 2000 kgf/cm 2 . The molded body was heated stepwise to 1400° C. over a long period of time in nitrogen and ammonia mixed atmosphere. Void content of the sintered bodies was 10% and void diameter was 10 μm or less. Sliding test was conducted in the same manner as in Example 146 and the results are shown in Table 20. From the results, it can be seen that the products of the present invention are superior in sliding properties. 
     
                                           TABLE 20__________________________________________________________________________                       SiC/Si.sub.3 N.sub.4                               Abrasion wear of   Organo Si high      Abrasion wear                               partner material   polymer compound            Partner                 Friction                       (mg/cm.sup.2 /                               (mg/cm.sup.2 /No.     (part by weight)            material                 coefficient                       100 h)  100 h)__________________________________________________________________________Examples218   Siliconimide            Al.sub.2 O.sub.3                 0.007 0.02    0.03   MW: 1500   (15)219   Polysilane            Al.sub.2 O.sub.3                 0.007 0.02    0.03   MW: 500   (10)220   Polysil  Al.sub.2 O.sub.3                 0.007 0.02    0.03   MW: 800   (12)__________________________________________________________________________ Sliding conditons: Surface pressure 10 kgf/cm.sup.2 and sliding speed 5 m/sec. in SUNISO 4GS oil. 
    
     EXAMPLES 221-229 
     Sintered bodies were prepared in the same manner as in Example 146 except that the raw material ratio of metallic Si and SiC was changed in order to change the ratio of Si 3  N 4  phase and SiC phase in the sintered bodies. Results of sliding test on these sintered bodies are shown in Table 21. The sliding conditions were same as in Example 210. 
     From the results, it can be seen that with an increase in the amount of SiC particle, the mechanical strength decreases. This results in an increase in abrasion wear. Therefore, it in preferred that the amount of SiC particle is 70 vol % or less. Considering thermal shock resistance, SiC particle is contained in an amount of at least 20 vol %. 
     Therefore, it is seen that reaction-bonded material of the present invention where SiC particles are bonded with Si 3  N 4  is superior to the material comprising only Si 3  N 4  or SiC in thermal shock resistance. 
     As comparative examples, the same tests were conducted on porous reaction-sintered Si 3  N 4  body and SiC body. The results are also shown in Table 21. 
     
                                           TABLE 21__________________________________________________________________________                       Si.sub.3 N.sub.4 /SiC                               Abrasion wear of                                        Si.sub.3 N.sub.4 /SiC     Composition of                  Friction                       Abrasion wear                               partner material                                        Thermal shock     sintered body             Partner                  coeffi-                       (mg/cm.sup.2 /                               (mg/cm.sup.2 /                                        resistance  No.     Si.sub.3 N.sub.4         SiC material                  cient                       100 h)  100 h)   (times)__________________________________________________________________________Examples  221     95   5  Al.sub.2 O.sub.3                  0.022                       0.38    0.24     37  222     90  10  Al.sub.2 O.sub.3                  0.019                       0.18    0.09     42  223     80  20  Al.sub.2 O.sub.3                  0.008                       0.04    0.04     50  224     70  30  Al.sub.2 O.sub.3                  0.007                       0.04    0.03     51  225     50  50  Al.sub.2 O.sub.3                  0.007                       0.04    0.03     52  226     40  60  Al.sub.2 O.sub.3                  0.008                       0.05    0.05     52  227     30  70  Al.sub.2 O.sub.3                  0.009                       0.09    0.15     51  228     20  80  Al.sub.2 O.sub.3                  0.012                       0.15    0.48     43  229     10  90  Al.sub.2 O.sub.3                  0.057                       0.34    0.89     40Comparative   62     100 --  Al.sub.2 O.sub.3                  0.101                       Reaction                               1.24     20Examples                    sintering                       2.10   63     --  100 Al.sub.2 O.sub.3                  0.120                       Reaction                               3.24     34                       sintering                       1.54__________________________________________________________________________ Sliding conditons: Surface pressure of 12 kgf/cm.sup.2 and sliding speed of 5 m/sec. in SUNISO 4 GS oil. 
    
     EXAMPLE 230 
     13 Parts by weight of a mixture of polyethylene, stearic acid and a synthetic wax were added to 60 parts by weight of metallic Si powder of 0.5 μm in average particle size and 40 parts by weight of SiC powder of 10 μm in average particle size. The mixture was kneaded. This raw material was injection molded into a floating seal as shown by reference number 7 in FIG. 18. After removal of the wax in the molded body, this was heated from 1100° C. 1400° C. at 6° C./h in nitrogen atmosphere. The dimensional change of the floating seal on sintering was small, namely, 0.13%. This was able to be used only by finishing a sliding surface. Therefore, it is considered that variability in dimension of bodies can be inhibited and working cost can be reduced to half. Thus, the bodies are suitable for mass production. Results of sliding tests are shown in Table 22. 
     
                       TABLE 22______________________________________                    Total  Combination            Fric-   abrasion  Abrasion  of sliding            tion    wear      wear  material  coeffi- (mg/cm.sup.2 /                              (mg/cm.sup.2 /No.      (I)     (II)    cient 100 h)  100 h)______________________________________Ex-   230    Si.sub.3 N.sub.4 /                Cr cast                      0.007 0.02    0.22ample        SiC     iron______________________________________ Sliding conditions: Surface pressure 8 kgf/cm.sup.2 and sliding speed 3 m/sec. in SUNISO 4GS oil. 
    
     EXAMPLE 231 
     The sintered body obtained in Example 146 was subjected to HIP treatment at 2000° C. for 4 hours under 80 atm in nitrogen atmosphere. Test results on the resulting sintered body are shown in Table 23. 
     
                                           TABLE 23__________________________________________________________________________   Sintered body after   HIP treatment               (I)     (II)   (I)               Partner                          Friction                               Abrasion wear                                       Abrasion wear   Void content            Therml shock                     material                          coeffi-                               (mg/cm.sup.2 /                                       (mg/cm.sup.2 /No.     (%)      resistance (times)                     (II) cient                               100 h)  100 h)__________________________________________________________________________Example231   2        53       Al.sub.2 O.sub.3                          0.007                               0.01    0.02__________________________________________________________________________ Sliding conditons: Surface pressure 10 kgf/cm.sup.2 and sliding speed 7 m/sec. in SUNISO 4GS oil. 
    
     EXAMPLE 232 
     A furan resin was impregnated into the voids of the sintered body obtained in Example 146. This was accomplished by autoclave in order to reduce open void content. Test results on the resulting sintered body are shown in Table 24. 
     
                                           TABLE 24__________________________________________________________________________   Furan resin      (I)     (II)   impregnated          Partner               Friction                    Abrasion wear                            Abrasion wear   product (I)          material               coeffi-                    (mg/cm.sup.2 /                            (mg/cm.sup.2No.     Void content          (II) cient                    100 h)  100 h)__________________________________________________________________________Example232   7      Al.sub.2 O.sub.3               0.005                    0.001   0.002__________________________________________________________________________ Sliding conditions: Surface pressure 10 kgf/cm.sup.2 and sliding speed 7 m/sec. in SUNISO 4GS oil. 
    
     EXAMPLES 233-240 
     Sintered bodies were made in the same manner as in Example 146 except that SiC whisker as shown in Table 25 was used in place of α-SiC particle. Sliding test results on the resulting sintered bodies are shown in Table 25, wherein the average aspect ratio of whisker is indicated by A and average length by L. 
     
                                           TABLE 25__________________________________________________________________________                    (I)     (II)   SiC    Partner               Friction                    Abrasion wear                            Abrasion wear   Whisker          material               coeffi-                    (mg/cm.sup.2 /                            (mg/cm.sup.2 /No.     (I)    (II) cient                    100 h)  100 h)__________________________________________________________________________Examples233   A = 50 Al.sub.2 O.sub.3               0.007                    0.01    0.02   L = 10 μm234   A = 100          Al.sub.2 O.sub.3               0.007                    0.01    0.02   L = 50 μm235   A = 200          Al.sub.2 O.sub.3               0.007                    0.03    0.05   L = 35 μm236   A = 100          Al.sub.2 O.sub.3               0.007                    0.03    0.04   L = 100 μm237   A = 60 Al.sub.2 O.sub.3               0.007                    0.09    0.11   L = 200 μm238   A = 50 Al.sub.2 O.sub.3               0.007                    0.10    0.23   L = 250 μm239   A = 210          Al.sub.2 O.sub.3               0.007                    0.15    0.26   L = 100 μm240   A =  220          Al.sub.2 O.sub.3               0.007                    0.25    0.56   L = 350 μm__________________________________________________________________________ Sliding conditions: Surface pressure 10 kgf/cm.sup.2 and sliding speed 7 m/sec. in SUNISO 4GS oil. 
    
     EXAMPLE 241 
     The raw material obtained in Example 146 was injection molded into a floating seal as shown in FIG. 18. This molded body was impregnated with a furan resin using an autoclave in the same manner as in Example 232. As a partner material, a floating seal as shown in FIG. 18 was made by injection molding of Al 2  O 3  (relative density 98%). They were incorporated into rolling wheel of a caterpillar rotating axis of a construction machine and actual working test was conducted. After 500 hours test, overhaul thereof was carried out to find no hydraulic leakage or sliding marks.