High strength composite ceramic structure and process for producing the same

Abstract: The present invention provides a composite ceramic structure of sintered ceramics having a porosity of 5 to 40 % by volume and, if necessary, a maximum pore size of 10 µ m or less comprising particles selected from the group consisting of grains and whiskers of at least one inorganic compound AB selected from the group consisting of carbides, nitrides, and oxides which are composed of an element A and an element B, where a difference in electronegativity between the element A and the element B is not higher than 1.7; or particles selected from the group consisting of grains and whiskers of an inorganic compound, between the components of which a difference in electronegativity is higher than 1.7, and having layers of the inorganic compound AB formed on the surfaces of said particles; where said particles are bonded one another with silicon nitride particles having an average particle size not more than 0.2 µ m. The composite ceramic structure is produced by sintering a shape composed of particles selected from the group consisting of grains and whiskers of at least one of the inorganic compound AB; or a shape composed of particles selected from the group consisting of grains and whiskers of an inorganic compound, between the components of which a difference in electronegativity is higher than 1.7, and having layers of the inorganic compound AB formed on the surfaces of said particles; in a gaseous atmosphere containing silicon and nitrogen to grow silicon nitride particles having an average particle size not higher than 0.2 µ m on the surfaces of said inorganic compound particles, thereby bonding said inorganic compound particles one another. A ceramic sintered shape highly accurate in dimension having a surface configuration which is a replica of the surface contour of a mold as a ceramic mechanical part of precision is also proveded.

Description: The present invention relates to a composite ceramic structure and a process for producing the same, and more particularly to a high strength composite ceramic structure suitable for structural members for use in engines and turbines and a process for producing the same. Moreover, the present invention relates to a sintered ceramic product having a surface configuration accurate in dimension as being an accurate replica of the surface contours of a mold. There have been generally known SiC and Si₃N₄ which have an excellent heat resistance as engineering ceramics suitable for structural members in engines, turbines and the like. Since the SiC and Si₃N₄ are compounds having strong covalent bonds, however, these nitrides alone are difficult to sinter and hence require sintering aids to produce sintered shapes. For example, it has been recognized that when Si₃N₄ is sintered under ambient pressure, an addition of Y₂O₃ or Al₂O₃ permits the production of highly dense sintered shapes. However, because the glass phase produced with the sintering aids is softened at high temperatures, the mechanical properties of the sintered shapes are lowered at high temperatures. In order to prevent the reduction in strength at high temperatures, an attempt has been made to use as small amount of the sintering aids as possible, or to crystallize the glass phases at the grain boundaries attributable to the sintering aids. However, no complete solution has been achieved yet. On the other hand, though an enhancement in the strength of the composites at high temperatures by incorporating whiskers of SiC, Si₃N₄, and the like, so-called a reinforcing technique with fibers has been proposed [see, J. Ceram. Soc. of Japan, 91 , [11], (1983) pp.491-497], this also employs the sintering aids causing a significant reduction in the mechanical properties of the sintered shapes. Another technique for sintering the ceramic composites comprising two kinds of inorganic compounds different in characteristics from each other without using any sintering aids, for example, a bonding reaction process has been known (see, Japanese Patent KOKAI (Laid-open) No. Sho 61-101465). Since this prior process employs metallic silicon particles of an average size of 0.5 µm or more, however, the resulting materials studied were macroporous sintered shapes having a maximum pore size of not less than 30 µm in which relatively large silicon nitrides bond spaces of SiC particles. For this reason, attainment of high strength of the sintered shapes has been limited. There has been studied no method for supplying silicon nitrides from a gaseous source. With respect to ceramic composites comprising two or more kinds of inorganic compounds, requirements for achieving high bonding strength have not been clarified and no high strength ceramic has been obtained so far. SiC and Si₃N₄ which have an excellent heat resistance are known as engineering ceramics generally suitable for structural members for use in engines, turbines and the like. There are techniques for sintering SiC and Si₃N₄ such as an ordinary pressure method, a high pressure method, and a reaction-sintering method. Among them the ordinary pressure method and the high pressure method suffer from deformation and less accuracy in dimensions as causing a rate of variation in size of 15 to 20 % so that a high level of technique is required. On the other hand, the reaction-sintering method is known to cause a lower rate of variation in size at the sintering process than those by other sintering methods, but still in the range of 1 to 1.5 % [see, Japanese Patent KOKAI (Laid-open) No. Sho 58-140375]. Materials expected to have a lower rate of variation in size at the time of sintering include Si₃N₄ binder which has been used heretofore in refractories, but which has only a mechanical strength as low as 50 MN/m² [see, Japanese Patent KOKAI (Laid-open) No. Sho 58-88169], unsuitable for use in structural members. The high rate of variation in size at the time of sintering imposes time consumption and high cost on processing after sintering, which is one of the reasons why the engineering ceramics have not been widely used. Therefore, there has been heretofore no technique, nor practical method or producing ceramics excellent in the accuracy in dimension. The publication "ADVANCED CERAMIC MATERIALS, vol. 3, No. 4, July 1988, Westerville, US, pp. 415-417, J.E. RITTER et al.; "High-strengh reaction-bonded silicon nitride" discloses a reaction-bonded silicon nitride (RBSN). According to this document, the fabrication RBSN is made by nitriding fine, high-purity Si-powder synthesized from laser-heated SiH₄. The diameter of powder is approximately 0.3 µm. The resultant nitride phase is a continuous network with characteristic solid-phased dimensions between 0.2 and 0.7 µm, which is consistent with the starting Si particle size. Document EP-A- 0 286 127 discloses a ceramic composite and a process for producing the same. Disclosed is a ceramic composite comprising particles of at least one inorganic compound selected from the group consisting of carbides nitrides, oxides, borides, silicides and oxy-nitrides and particles and whiskers of Si₃N₄, Si₂ N₂O or SiO₂. This ceramic composite is small in volume change on sintering. It is the object of the invention to provide a ceramic structure and a method of producing the same, according to which the sintering properties of a material to be sintered can be improved prior to sintering it and which leads to a sintered body being a highly accurate replica of the surface contour of a mould. This object is solved in accordance with the features of the independent claims. Dependent claims are directed on preferred embodiments of the invention. FIGURE 1 is a graph showing a relationship between the average particle size of silicon nitrides and the flexure strength for Si₃N₄/SiC sintered shape constantly with a porosity being of 10 % by volume obtained in Example 38, FIGURE 2 is a graph showing a relationship between the maximum porosity and the flexure strength for Si₃N₄/SiC sintered shape constantly with a porosity being of 20 % by volume obtained in Example 39, FIGURE 3 and FIGURE 4 are photomicrographs of the structures of the particles of Si₃N₄ whiskers and grains in the process of epitaxially growing them on the surfaces of SiC particles, photographed using a scanning electron microscope (SEM), FIGURE 5 is a photomicrograph of an example of the structures of Si₃N₄ particles epitaxially grown densely on the surfaces of SiC particles, photographed using a scanning electron microscope, FIGURE 6 is a photomicrograph of an example of the state of epitaxial growth of Si₃N₄ particles densely grown on the surfaces of SiC particles, photographed using a scanning electron microscope, FIGURE 7 is a photomicrograph of the state of epitaxial growth of Si₃N₄ particles on the surfaces of SiC particles observed at the grain boundaries, photographed using a transmitting electron microscope (TEM), FIGURE 8 is a photomicrograph of a comparative example of Si₃N₄/SiC composite materials, photographed using a scanning and transmitting electron microscope (STEM), FIGURE 9 is a X-ray photograph of the Si₃N₄/SiC at the grain boundaries as shown in FIGURE 8, FIGURE 10 is a schematic view of the lattice intersecting angle between the inorganic compound and the crystalline silicon nitrides, FIGURE 11 and FIGURE 12 are graphs showing the accuracy as expressed by a magnitude of fluctuation in dimensions of the sintered shape obtained in Example 46, FIGURE 13 is a graph showing a tolereance in outside diameters of the miniature toothed wheel manufactured in Example 47, and FIGURE 14 is a graph showing an accuracy as expressed by a degree of fluctuation in dimensions of the sintered shape manufactured in Example 49. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In an aspect of the present invention, there is provided a composite ceramic structure of sintered ceramics having a maximum pore size of 10 µm and a porosity of 5 to 40 % by volume comprising particles selected from the group consisting of grains and whiskers of an inorganic compound, between the components of which a difference in electronegativity is higher than 1.7, said particles having layers of an inorganic compound AB composed of at least one selected from the group consisting of carbides, nitrides, and oxides which comprise an element A and an element B, where a difference in electronegativity between the element A and the element B is not higher than 1.7, formed on the surfaces of said particles and being bonded to one another with silicon nitride particles or granules having an average particle size of not more than 0.2 µm. In the present invention, inorganic compound particles or whiskers are bonded with silicon nitride particles. The silicon nitrides may be derived from metallic silicon as starting material or from gaseous sources. The starting metallic silicon should be of an average particle size of 0.3 µm or smaller. The present invention will now be disclosed in detail. The reasons why the silicon nitrides for bonding the inorganic compounds should have an average particle size not more than 0.2 µm are based on the findings that if it is made so, one can achieve a distribution of pores of uniform sizes not more than several microns existing between the particles and that the mechanical strengths of the resultant composite ceramic structures can be greatly enhanced. FIGURE 1 is a graph showing a relationship between the average particle size and the flexure strength of the silicon nitrides in the Si₃N₄/SiC sintered shapes having a constant porosity of 10 % by volume obtained in Comparative Example 38. It can be seen from the graph that the strength is rapidly increased nearly by a factor of 2 as the average particle size is 0.2 µm. When the average particle size is more than 0.2 µm, the pore size becomes larger and fractures may start from the pores of the maximum size causing an reduction in the strength. FIGURE 2 is a graph showing a relationship between the maximum pore size and the flexure strength of the Si₃N₄/TiN sintered shapes having a constant porosity of 20 % by volume obtained in Example 39. It can be seen from this graph that the strength is rapidly increased nearly by a factor of 2 as the maximum pore size is 10 µm or less. This may be because fractures start from the pores of the maximum size. Therefore, it is extremely important to render the maximum pore size not more than 10 µm, preferably not more than 5 µm which implies the necessity of the average particle size being not more than 0.2 µm. In order to achieve the average particle size of 0.2 µm and the maximum pore size of not more than 10 µm of the silicon nitride particles for bonding the inorganic compounds, the metallic silicon particles to be mixed with the inorganic compounds should have an average particle size of not more than 0.3 µm, and the sintering temperature should be 1250 °C. The metallic silicon particles to be used may be those which have hydroxide coatings formed on the surfaces thereof. The present composite ceramic structures may be obtained by heating a shape composed of inorganic compounds or a combination of inorganic compounds and metallic silicon particles having an average particle size of 0.3 µm or less in an atmosphere containing silicon and nitrogen, e.g., in an atmosphere of gaseous ammonia and silane. Alternatively, they may be obtained by epitaxially growing amorphous silicon nitrides and then heat-treating the nitrides to crystallize. Still alternatively, they may be obtained by heating a shape composed of inorganic compounds and silicon nitrides having an average particle size of 0.2 µm or less as a binder in an atmosphere containing silicon and nitrogen. The silicon nitrides for bonding the inorganic compounds in the present invention should be preferably silicon nitrides produced through by epitaxial growth, because this type of silicon nitrides has in most cases a particle size of not more than 0.1 µm with smaller particles in the order of several nanometers allowing pore sizes small and uniform. Epitaxially growing particles, whiskers are predominantly of silicon nitride phase, and should preferably comprise 70 % or more silicon nitrides, because a higher amount of oxynitrides produced reduces the bonding strength between inorganic compounds. The present invention requires that a difference in electronegativity between the inorganic component A and the inorganic component B be not higher than 1.7, where the difference in electronegativity is as determined by L. Pauling, for the reasons found from our experiments that such inorganic compounds as the difference in electronegativity therebetween being higher than 1.7, in other words as having a higher ionic bonding strength therebetween have a less capability of epitaxially growing silicon nitride particles or whiskers on their surfaces so that they cause reduced bonding strengths. The electronegativity defines an ability of an atom to attract electrons at the time of forming a chemical bond, as the magnitudes of the electronegativities were determined by L. Pauling making a calculation using the following equation: Δ AB = D A-B - (1/2) (D A-A + D B-B ) where D is a bonding energy. Inorganic compounds AB having a difference in electronegativity between the element A and the element B of not higher than 1.7 include nitrides, carbides and oxides of Si, Ti, Zr, Cr, Al, Be, Ta, Th, Hf, Nb, W, V, or Mo, as the element A which have preferably a higher bonding strength. The magnitudes of the electronegativities determined by L. Pauling are tabulated in Table 1. Processes of bonding the inorganic compounds with silicon nitrides in the presence of sintering aids cause deformations due to large contractions from molds to sintered shapes as well as formations of remaining solution phases (alloy phases) softenable at high temperatures in the sintered shapes which result in a reduction in strength. In contrast, in the present invention, the bonding at least one of carbides, nitrides and oxides as the inorganic compounds having the difference in electronegativity of not higher than 1.7 one another with crystalline silicon nitride particles or epitaxially grown silicon nitride particles or whiskers enables the sintering of molds causing little contraction in dimension at the time of the sintering without any reduction in strength at high temperatures because of no formation of such solution phases softenable at high temperatures. The present invention requires carbides, nitrides and oxides of the inorganic compounds having a difference in electronegativity between one element A and the other element B composing the compounds for the reason that borides and silicides are difficult to be made of an average particle size of not more than 0.2 µm because they are reactive with silicon nitrides. In case the inorganic compounds have a difference in electronegativity of higher than 1.7, particles of the compounds can be bonded one another by oxidizing or nitriding the inorganic compounds to produce films having a difference in electronegativity of not higher than 1.7 on the surfaces of the compounds, and thereafter, bonding the particles with crystalline silicon nitride particles or epitaxially grown particles or whiskers via the intervening films. As can be seen from FIGURE 10, a lattice crossing angle  between the inorganic compound and the epitaxially grown silicon nitride should preferably be 70° or lower. If the angle is higher than 70°, shearing strength and tensile strength between particles are reduced, resulting in a reduction in mechanical strength. The reasons why a porosity is 5 to 40 % in the present invention are that the porosity over 40 % lowers rapidly the strength. The porosity lower than 5 % is inadequate for the epitaxial growth by reaction in vapor phase because pores allowing gases to pass through are required. It may be satisfactorily effected, however, to densify the shapes by HP or HIP after completing the treatment for the epitaxial growth. In this way densely sintered shapes having no glass layer in grain boundaries can be produced. Preferably particles having an average particle size of 20 µm or less and/or whiskers having an aspect ratio not higher than 100 and a length not longer than 100 µm should be used for the inorganic compounds, because the average particle size over 20 µm, the aspect ratio over 100 and the length over 100 µm make it difficult to mix the particles producing a less uniform dispersion state which in turn makes it difficult for the maximum pore size to be 10 µm or less, preferably 5 µm or less. For the inorganic compounds, commercially available ones may be used as they are. Alternatively, roundish particles produced by grinding with a mill may be used. Particularly, the more round the particles, the higher the strength due to a higher uniformity in pore size within the sintered shapes. Preferably a content of inorganic compound materials in the sintered shapes should be in the range of 5 to 80 %, since if it is higher than 80 % by volume, an amount of the materials to be bonded with the epitaxially grown silicon nitrides is reduced, which lowers the mechanical strength of the sintered ceramics. If it is lower than 5 % by volume, little effect of the inorganic compounds on preventing the propagation of cracking is achieved resulting in an reduction in the mechanical strength. In the process for epitaxially growing crystalline silicon nitrides on the surfaces of the inorganic compounds, the silicon nitrides are grown and piled on each other on the surfaces of the inorganic compounds to bond the particles, under the conditions that the shapes of the inorganic compounds are present in an atmosphere containing Si and N and that the potential energy of the inorganic compounds is lower compared with the reaction of Si and N. The crystalline silicon nitrides may be obtained by heat-treating the amorphous silicon nitrides epitaxially grown to crystallize the same. The use of molecular beams and the like are also effective. It is possible to impregnate the pores of the sintered composite ceramics with resins, particles, oils, solid lubricants and the like. The impregnation of the uniform small pores with materials as described above enables the production of slidable materials having a lower coefficient of friction. To form shapes, various forming processes may be employed such as injection molding, press molding, casting, rubber press, extrusion, powder molding with metal dies, and the like depending upon shapes and properties to be required. Alternatively, it may be possible to remove, form and sinter fine Si materials in a glow box. The characteristics of the composite ceramics to be produced may be varied depending upon the type of the inorganic compounds as well as the formulation ratio of the inorganic compounds and silicon nitrides. For example, the use of electroconductive inorganic compounds produces electroconductive composite ceramics. Ceramic composites characterized by having a electric resistivity in the order of 10⁻⁷ Ωm to 10¹⁵ Ωm can be obtained by controlling the types and the formulation ratios of the inorganic compounds. High strength composite ceramic structures can be produced by rigidly bonding at least one of carbides, nitrides, and oxides of the inorganic compounds, which have a difference in electronegativity of not higher than 1.7, one another with silicon nitride particles of an average particle size not more than 0.2 µm or silicon nitride whiskers, and sintering the bonded materials into sintered shapes having a maximum pore size of not more than 10 µm, preferably not more than 5 µm. The composite ceramic structures produced by the present invention will find broadening areas of application with regard to ceramics useful as structural members such as engines, turbines, slide parts as well as those useful for developments in airospace, iron and steel, and marine technologies. The present invention provides a precise mechanical part characterized by having a surface which is a precise replica of the surface configuration of a mold and by being a sintered ceramic shape having highly accurate dimensions. The sintered ceramic shape of highly precise dimensions having a surface which is a replica of the surface configuration of a mold can be produced by adding a molding binder to a material mix consisting of powdery metal or a combination of powdery metal and inorganic compounds, forming the mixture with a mold having a configuration near to a desired end product to produce a shape, removing the binder in the shape, thereafter heating the shape in an nitriding gaseous atmosphere to bond the particles with the nitride particles or whiskers grown from the metal grains. It may be possible particularly as disclosed in Japanese Patent KOKAI (Laid-open) No. sho 63-277576 that a sintered shape with a rate of variation in dimension at the time of sintering on the order of about +0.1 % may be produced by bonding inorganic compounds with nitrides grown from Si grains. However, there is no discussion about a mold taking account of an amount of the dimensional spring back of a pressed shape and/or a rate of variation in dimension from the shape to the sintered one. In order to achieve the sintered ceramic shape of a high accuracy in dimension having a surface which is a replica of the surface configuration of a mold in the present invention, a fluctuation in dimension of sintered shapes should be minimized by using a mold taking account of an amount of the dimensional spring back of a shape pressed with a mold and/or a rate of variation in dimension at the time of sintering, thereby enabling the achievement. The molding binders to be used in the present invention include organic high molecular weight compounds such as polyvinyl butyrals and polyethylenes and organic Si high molecular weight compounds such as silicon imido compounds and polysilane compounds. The binders should be added in an appropriate amount, preferably 8 to 12 parts by weight to allow the shape to have a particle packing ratio of the shape of 70 % by volume. The forming method may be selected from various forming methods such as injection molding, press forming with a metal die, casting, extrusion, and the like depending upon required configuration and properties. The shapes should preferably be heated in an atmosphere of nitrogen and/or ammonia with an addition of hydrogen, argon, helium, or the like, if necessary, at a temperature of below the melting point of metallic Si. In the present invention, preferably inorganic compounds in the forms of particles having an average particle size not more than 100 µm and/or whiskers having an average aspect ratio of 200 and an average length not longer than 200 µm should be used, because an average particle size over than 100 µm, an average aspect ratio over 100 and an average length over 200 µm make mixing difficult to produce an unsatisfactory dispersion state. As the powdery metals and the inorganic compounds, commercially available ones may be used as they are. Alternatively, roundish particles produced by grinding with a mill or the like may be used. Any surface configurations complicated two- or three-dimensionally of molds such as stair-type, embossed pattern, or the like can be duplicated onto the present sintered shapes. In the present invention, the sintered ceramics may be impregnated with resins, particles, oils, metals and the like into the pores thereof. The present invention can produce sintered ceramic shapes highly accurate in dimension having a suface which is a precise replica of the suface configuration of a mold, which ceramics can be used as precision mechanical parts of ceramics without any additional processing, or with little processing, if any. They are useful for various mechanical seals, miniature toothed wheels, various heaters, various far infra-red heaters, electrodeposition, electrodeposition brushes, various bearings, commutators, vanes, wire guides, rings, Oldham's rings, screw rotors, turbochargers, side combustion room, guide rails for magnetic disks, and the like. The present invention will be further described in detail with reference to the following Examples which are not intended to limit the present invention. Comparative Example 1 To SiC particles having an average particle size of 5 µm (a difference in electronegativity : 0.7), 2 parts by weight of polyvinyl alcohol (PVA) were added as a molding binder and the whole was mixed to produce a sample material. The material was pressed with a mechanical press under a molding pressure of 100 kgf/cm² into disks of 50 mm in diameter × 20 mm in thickness. From these shapes, the molding binder was removed. The binder removed shapes were placed centrally in a tube furnace having an inside diameter of 50 mm as filling the inside of the furnace with the disks, and heated at a temperature of 1300 °C for a long time with passing a flow of a gaseous mixture of SiH₄ and NH₃ through the furnace. In this way there were produced sintered shapes where Si₃N₄ whiskers/particles were epitaxially grown on the SiC particles. The resultant sintered shapes had a compositional ratio of SiC : Si₃N₄ = 45 : 55 in terms of % by volume. Figures showing the process of the epitaxial growth of Si₃N₄ whiskers and particles on the SiC particles observed with a scanning electron microscope (SEM) are given in FIGUREs 3 and 4. It is evident from the FIGUREs how the Si₃N₄ whiskers and particles were epitaxially grown on the SiC particles. FIGURE 5 illustrates the epitaxial growth of dense Si₃N₄ particles on the surfaces of SiC particles. It can be seen from the FIGURE that the Si₃N₄ particles are composed of fine particles of 0.2 µm or less. FIGURE 6 shows one of SEM photomicrographs of the finally obtained sintered shapes. It can be apparant from the FIGURE that the SiC particles are bonded peripherally with the epitaxially grown Si₃N₄ whiskers/particles having a maximum pore size of not more than 10 µm. One of photomicrographs of grain boundaries and the vicinity thereof taken by a trasmission electron microscope (TEM) is given in FIGURE 7. A lattice crossing angle between the SiC particle and the Si₃N₄ was 25 degrees. These grain boundaries were examined by an energy dispersion type X-ray analysis (EDX) and as a result, it was confirmed that no intervening material such as oxygen and the like exists. It has been found, therefore, that the products according to the present invention comprise SiC particles bonded with Si₃N₄ whiskers/particles epitaxially grown thereon. The resultant sintered shapes have a flexure strength of 480 MPa at a temperature of 1200 °C and a Weibull modulus of 18.3 indicating production of satisfactory and reliable sintered shapes. For comparison, a powdery mixture having a composition of 62 % Si₃N₄, 30 % SiC, 3 % Al₂O₃, 5 % Y₂O₃ by volume was prepared. This was molded in the same manner as above, hot pressed and sintered at a temperature of 1700 °C for 3 hours under a pressure of 29 MPa to obtain sintered shapes. One of STEM photomicrographs of the resultant sintered shapes is shown in FIGURE 8. The results of the energy dispersion X-ray analysis are given in FIGURE 9. It can be seen from these FIGUREs that the Si₃N₄ and SiC particles are bonded with glass phases composed of Al₂O₃ and Y₂O₃ at grain boundaries, but not with Si₃N₄ particles indicating that they are different from the sintered ceramics of the present invention. The resultant sintered shapes had a flexure strength of 180 MPa at a temperature of 1200 °C and a Weibull modulus of 7.3 indicating production of only less reliable sintered shapes. Comparative Examples 2 to 29 Mixtures were molded and sintered in the same manner as in Example 1, except that the SiC particles were replaced by inorganic compounds having an average particle size of 2 µm as in Table 2 to be added. As as result, in any cases, sintered shapes bonded with Si₃N₄ particles of an average particle size of 0.2 µm or less having a maximum pore size of not more than 5 µm were obtained. The sintered shapes were also examined by SEM and TEM similarly to Example 1 to confirm that Si₃N₄ particles or Si₃N₄ whiskers and particles had been epitaxially grown on the inorganic compounds. The results are shown in Table 2 as Examples 2 to 25. For comparison, the results with inorganic compounds having a difference in electronegativity of above 1.7 are also shown as Examples 26 to 29. Those inorganic compounds having a difference in electronegativity of above 1.7 confirmed to have no Si₃N₄ whiskers/particles epitaxially grown. Example No. Inorg. Compound (30 vol.%) Difference in Electronegativity Presence of Epitaxial Bond 2 Si₃N₄ 1.2 Yes 3 SiO₂ 1.7 Yes 4 TiC 1.0 Yes 5 TiN 1.5 Yes 6 AlN 1.5 Yes 7 ZrN 1.6 Yes 8 ZrC 1.1 Yes 9 Cr₃C₂ 0.9 Yes 10 Cr₂N 1.4 Yes 11 BeO 1.5 Yes 12 WC 0.8 Yes 13 WN 1.3 Yes 14 VN 1.4 Yes 15 VC 0.9 Yes 16 Fe₃C 0.7 Yes 17 Mo₂N 1.2 Yes 18 Mo₂C 0.7 Yes 19 ThC 1.2 Yes 20 HfN 1.7 Yes 21 HfC 1.2 Yes 22 TaN 1.5 Yes 23 TaC 1.0 Yes 24 NbN 1.4 Yes 25 WN 1.3 Yes 26 Al₂O₃ 2.0 No 27 TiO₂ 2.0 No 28 ZrO₂ 2.1 No 29 MgO 2.3 No Comparative Examples 30 to 37 To a mixture of TiN particles having an average particle size of 2 µm (a difference in electronegativity: 1.5) and powdery Si metal having an average particle size of 0.05 µm in a narrow size distribution, 10 parts by weight of polyethylene based binder were added as molding binder and the whole was mixed to provide a sample material. The material was pressed with a mechanical press under a molding pressure of 1000 kgf/cm² into disks of 50 mm in diameter × 20 mm in thickness. The molding binder was removed from these shapes. The binder removed shapes were heated at a temperature of 1350 °C for a long time in an atmosphere of NH₃. In this way there were produced sintered shapes where Si₃N₄ particles were epitaxially grown on the TiN particles. FIGURE 3 shows a relationship between the amount of TiN particles and the properties. As can be seen from the FIGURE, if an amount of TiN is lower than 5 % by volume, there is no effect of preventing crack propagation to reduce strength with an reduction in Weibull modulus. Therefore, one can not obtain any good and reliable sintered shapes. If an amount of TiN is more than 80 % by volume, insufficient bondings with epitaxially grown Si₃N₄ particles are achieved so as to produce only sintered shapes having a lower strength with a diminisheded Weibull coefficient. From the foregoing, it is clearly evident that an amount of TiN in the range of 5 to 80 % by volume permits the increase in Weibull modulus and the enhancement of reliability. Example No. Amount of TiN (vol.%) Flexure strength (MPa) Weibull Modulus 30 95 150 7.5 31 90 270 12.8 32 78 510 21.2 33 50 580 23.4 34 30 690 21.8 35 10 470 19.5 36 6 320 17.5 37 2 280 9.7 Comparative Example 38 To a mixture of SiC particles having an average particle size of 2 µm (a difference in electronegativity: 0.7) and various powdery Si metal having an average particle size in the range of 0.02 µm to 5 µm, 9 parts by weight of a polyethylene based binder were added as molding binder and the whole was mixed to provide a sample material. The material was pressed with a mechanical press under a molding pressure of 1000 kgf/cm² into disks of 50 mm in diameter × 20 mm in thickness. The molding binder was removed from these shapes. The binder removed shapes were heated at a temperature of 1350 °C for a long time in an atmosphere of N₂. In this way there were produced sintered shapes comprising Si₃N₄ particles having different average particle sizes where Si₃N₄ particles were epitaxially grown on the SiC particles. FIGURE 1 shows a relationship between the average particle size and the flexure strength. It is apparent from the FIGURE that the strength is rapidly increased up to near twice at the average particle size of 0.2 µm. This may be attributed to an enlargement in pore size caused when the average particle size of the Si powder is higher than 2 µm. From the pores having a maximum pore size, fractures will start to cause an reduction in strength. Comparative Example 39 To a mixture of TiN particles having an average particle size of 3 µm (a difference in electronegativity: 1.5) and various powdery Si metal having an average particle size in the range of 0.02 µm to 3 µm, 9 parts by weight of a polyethylene based binder were added as molding binder and the whole was mixed to provide a sample material. The material was pressed with a mechanical press under a molding pressure of 1000 kgf/cm² into disks of 50 mm in diameter × 20 mm in thickness. The molding binder was removed from these shapes. The binder removed shapes were heated at a temperature of 1350 °C for a long time in an atmosphere of N₂ + H₂. In this way there were produced sintered shapes having different maximum pore sizes where Si₃N₄ particles were epitaxially grown on the TiN particles. FIGURE 2 shows a relationship between the maximum pore size and the flexure strength. As can be seen from the FIGURE, it has been found that the strength is rapidly enhanced at a maximum pore size of 5 µm or less to be nearly twice. This is due to occurrence of fractures starting from the pores having a maximum pore size. This means that it is very important to controll the maximum pore size to be 5 µm or less. Example 40 TiO₂ particles having an average particle size of 2 µm (a difference in electronegativity : 2.0) were treated at a temperature of 1700 °C for one hour in a nitrogen atmosphere, to form TiN films (a difference in electronegativity : 1.5) of about 100 Å in thickness. The resistant particles were mixed with powdery Si metal having an average particle size of 0.05 µm into a compositional ratio of TiO₂ : Si = 30 : 70 % by weight with an addition of 8 parts by weight of a polystyrene based binder to produce a sample material. The material was pressed with a mechanical press under a molding pressure of 1000 kgf/cm² into disks of 50 mm in diameter × 20 mm in thickness. The molding binder was removed from these shapes. The binder removed shapes were heated at a temperature of 1400 °C for a long time in an atmosphere of N H₃. In this way there were produced sintered shapes where Si₃N₄ particles were epitaxially grown on the TiN films which had been formed on the TiO₂ particles. The produced sintered shapes had a flexure strength of 400 MPa and Weibull modulus of 18.8 indicating that they are good and reliable. Comparative Example 41 to 45 Mixtures were prepared, molded and sintered in the same manner as in Example 1, except that the SiC particles were replaced by inorganic compounds having an average particle size of 2 µm added as indicated in Table 4. As a result, in any cases, sintered shapes bonded with Si₃N₄ particles of an average particle size of 0.2 µm or less having a maximum pore size of not more than 10 µm were obtained. It was confirmed that the sintered shapes could have a wide variety of electric resistivity from insulating properties to electroconductive properties. Example No. Inorganic Compound Material Compound in Sintered Shape (vol.%) Electric Resistivity (Ωm) 41 SiO₃ 70 4 × 10¹⁵ 42 AlN 50 7 × 10¹⁵ 43 TiN 80 7 × 10⁻⁷ 44 TiN 30 8 × 10⁻³ 45 ZrN 70 4 × 10⁻⁷ Comparative Example 46 500 g of a mixture of 40 parts by weight SiC particles having an average particle size of 16 µm and 60 parts by weight powdery Si metal having an average particle size of 0.9 µm were mixed with a total 9 parts by weight of polyethylene based wax, synthetic wax, and stearic acid, as binders, and kneaded with a press kneader at a temperature of 160 °C for 5 hours. The kneaded mixture was crashed to produce a sample material. The material was pressed with a mechanical press with a metal mold of 20 mm in outside diameter and 5.1 mm in inside diameter under a molding pressure of 1000 kgf/cm² at a temperature of 160 °C. The resulting shapes had a particle packing ratio of 78 % by volume. These shapes, after the molding binders were removed, were heated at a rate of elevating temperature of 4 °C/hour from 1100 to 1350 °C for a long time in a nitrogen atmosphere to produce sintered shapes. Magnitudes of the deviation in dimension of the resistant sintered shapes are shown in FIGURE 11. It can be understood from the FIGURE that one can produce those being well within an acceptable tolereance in dimension. Moreover, taking account of the tolerance in dimension, the use of a metallic mold having dimensions of an outside diameter of 19.9 mm and an inside diameter of 5.1 mm could produce the sintered shapes, the magnitudes of the deviation in dimension of which are shown in FIGURE 12. It should be noted from the FIGURE that one can produce those having a smaller magnitude of deviation than could in FIGURE 11. Comparative Example 47 Small toothed wheels of the specification as shown in Table 5 were made in the same way as in Example 46. Magnitudes of the deviation in dimension of the metallic mold, the tooth configuration of the resistant molded shapes and the sintered ones are described in Table 6. In addition, the tolerance in dimension for the toothed wheels is shown in FIGURE 13. From Table 6, it can be seen that a difference between the metallic mold and the curve of Involute theory is 100 µm, a difference between the molded shapes and the curve of Involute theory is 112 µm at maximum, and that a difference between the sintered shapes and the curve of Involute theory is 128 µm at maximum. Therefore, a difference in dimensional tolerance between the metallic mold and the sintered shapes is very small, i.e., 28 µm. Also, the deviation in dimension is small as can be seen from FIGURE 13. These toothed wheels have been confirmed to be capable of being used without any additional processing. Specification of toothed wheel Module 1.55 Number of teeth 18 Pressure angle 20 ° Tooth tip diameter 30.9 Tooth width 6 Object Difference from Curve of Involute Theory Metallic mold 100 µm Molded shape 112 µm Sintered shape 128 µm Comparative Example 48 Mixtures were prepared, molded and sintered in the same manner as in Example 46, except that TiN (2 µm), ZrN (2 µm), and Al₂O₃ (0.8 µm) were substituted for the SiC particles to produce sintered shapes. As a result, in all cases, it has been found that those within the acceptable tolerance in dimension could be manufactured as in FIGURE 11. Surface roughnesses were determined to be 2 µm or less in all cases. Essentially, TiN and ZrN are effective to enhancement in surface accuracy. Comparative Example 49 A mixture of 50 parts by weight powdery Si metal having an average particle size of 0.2 µm and 50 parts by weight TiN particles having an average particle size of 2 µm were added with a total amount of 9 parts by weight of polyethylene based wax, synthetic wax, and stearic acid as binders, and kneaded with a press kneader at a temperature of 160 °C for 5 hours. The kneaded mixture was crashed to produce a sample material. The material was pressed with a mechanical press with a metallic mold of 110 mm in outside diameter and 90 mm in inside diameter under a molding pressure of 1000 kgf/cm² at a temperature of 140 °C. The resulting shapes were heated at a rate of elevating temperature of 3 °C/hour up to 500 °C in an argon atmosphere, and after the molding binders were removed, were heated stepwise from 500 to 1300 °C for a long time in a nitriding atmosphere to produce sintered shapes. Magnitudes of the deviation in dimension of the resistant sintered shapes are shown in FIGURE 14.

Claims: A composite ceramic structure of sintered ceramics having a porosity of 5 to 40 % by volume and a maximum pore size of 10 µm and comprising a shape composed of particles selected from the group consisting of grains and whiskers of an inorganic compound, between the components of which a difference in electronegativity is higher than 1.7, said particles having layers of an inorganic compound AB composed of at least one selected from the group consisting of carbides, nitrides, and oxides which comprise an element A and an element B, where a difference in electronegativity between the element A and the element B is not higher than 1.7 formed on the surfaces of said inorganic compound particles and being bonded one another with silicon nitride particles having an average particle size not more than 0.2 µm. The composite ceramic structure according to claim 1, in which said layered particles are bonded one another with crystalline silicon nitride particles epitaxially grown on the surfaces of said layered particles. The composite ceramic structure according to claim 1 or 2, in which lattice crossing angles between said inorganic compounds and said silicon nitrides are not higher than 70°. The composite ceramic structure according to claim 1 or 2, in which a compositional ratio of said inorganic compounds including said layers to said silicon nitrides is in the range of 80 : 20 to 5 : 95 by volume. The composite ceramic structure according to claim 1 or 2, in which said structure has an electric resistivity in the order of 10⁻⁷ Ωm to 10¹⁵ Ωm. A process for producing a composite ceramic structure of sintered ceramics having a porosity of 5 to 40 % by volume and a maximum pore size of 10 µm comprising forming layers of at least one inorganic compound AB selected from the group consisting of carbides, nitrides, and oxides which are composed of an element A and an element B, where a difference in electronegativity between the element A and the element B is not higher than 1.7 on the surfaces of particles selected from the group consisting of grains and whiskers of an inorganic compound, between the components of which a difference in electronegativity is higher than 1.7, shaping said layered particles to produce a shape, sintering said shape in a gaseous atmosphere containing silicon and nitrogen to grow silicon nitride particles having an average particle size not more than 0.2 µm on the surfaces of said layered particles, thereby bonding said layered particles to one another. A process for producing a composite ceramic structure of sintered ceramics having a porosity of 5 to 40 % by volume and a maximum pore size of 10 µm, comprising forming layers of at least one inorganic compound AB selected from the group consisting of carbides, nitrides, and oxides which are composed of an element A and an element B, where a difference in electronegativity between the element A and the element B is not higher than 1.7 on the surfaces of particles selected from the group consisting of grains and whiskers of an inorganic compound, between the components of which a difference in electronegativity is higher than 1.7, mixing the resultant layered particles with powdery silicon metal having an average particle size of not more than 0.3 µm, shaping said layered particles to produce a shape, sintering said shape in a gaseous atmosphere containing silicon and nitrogen to grow silicon nitride particles having an average particle size not more than 0.2 µm on the surfaces of said layered particles, thereby bonding said layered particles to one another.

Inventors: CHIBA AKIO, MIYOSHI TADAHIKO, SAKAI JUNJI, SOBUE MASAHISA, WATAHIKI SEIJI, YASUTOMI YOSHIYUKI, CHIBA, AKIO, MIYOSHI, TADAHIKO, SAKAI, JUNJI, SOBUE, MASAHISA, WATAHIKI, SEIJI, YASUTOMI, YOSHIYUKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: C04B35
IPC codes: C04B35, C04B38, C23C16