Ceramic composite and process for production thereof

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 Si₃N₄, Si₂N₂O or SiO₂. This ceramic composite is small in volume change on sintering and superior in heat resistance and besides can absorb thermal shock by its voids. It can be used as ceramic heaters, current collectors, commutators for starter motor and alternator and the like.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a sliding material for a sealing, comprising a ceramic composite material. DESCRIPTION OF RELATED ART Ceramics has recently been required to have a high high-tenperature strength, high toughness, high thermal shock resistance and high accuracy of dimension. As such ceramics are known inorganic compounds such as carbides, nitrides, oxides, borides, silicides and oxynitrides, for example, silicon nitride, silicon carbide, alumina and zirconia. Each of these ceramic materials is hardly used alone to achieve so excellent properties as mentioned above. Therefore, many proposals have been made to achieve such properties by selecting the combinations of the ceramic materials and changing the design of structure. Generally, SiC, Si 3 N 4 or the like is known as an engineering ceramic 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, but nevertheless, the rate is as high as about 1 to 1.5%, as shown in Japanese Patent KOKAI (Laid-Open) No. 58-140375. As a material having a low rate of dimensional change, there is known a bonded Si 3 N 4 product which has been used as a refractory material. Such material has a mechanical strength of only about 50 MN/m 2 , as shown in Japanese Patent KOKAI (Laid-Open) No. 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 is obtained but a body having a complicated shape is hardly obtained due to poor flowability of a starting mixture, and further the resultant sintered body has an insufficient mechanical strength. It is not too much to say that there have never been any 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, a ceramic material has been used as a sliding material for sealing having an excellent resistance to thermal shock. Generally, working of a machine is always accompanied with sliding. Reduction of friction and wear will induce saving of energy and long life of a sliding material. For example, a floating seal comprising O-ring 4 and sealing elements 5 and 6 as shown in Fig. 7, has been used as a sealing means 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 suitable for sealing or protecting from earth and sand and has widely been used. However, this floating seal has such a defect that it is poor in heat resistance and wear resistance since it is made of a cast iron material. Another sealing means is known in which one of sealing elements 10 is made of Cr cast iron and the other 11 is made by injection molding. This sealing means has also the same defect as mentioned above. On the other hand, a ceramic-made sliding means has recently been provided. A typical example of the ceramics for the sliding means includes SiC, Si 3 N 4 , Al 2 O 3 or ZrO 2 , but it is practically required to have the following properties as the sliding material (see "Kogyo Zairyo", 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'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 all are not satisfied by known materials. For example, a metallic material is questionable in the resistance to seizing. Al 2 O 3 itself has a low resistance to thermal shock, and since it has a high coefficient of thermal expansion, it is easily deformed by heat on the sliding 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 conductivitity but has a poor resistance to thermal shock. A sliding material having both excellent wear resistance and corrosion resistance and conspicuously improved resistance to thermal shock has been demanded. Among these sliding materials SiC is widely used in a sintered form. For example, Japanese Patent KOKAI (Laid-Open) No. 61-163174 discloses "a sliding material comprising an SiC sintered porous body having an excellent sliding property under wet conditions" and Japanese Patent KOKAI (Laid-Open) No. 55-100421 discloses "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". However, these Japanese Patent KOKAI references all do not draw any attention to resistance to thermal shock and one-side contact. Thus, the prior art materials have no combination of good wear resistance and thermal shock resistance and further have such drawbacks that cracks are generated due to a rapid change of temperatures and they allow their partners to be worn. SUMMARY OF THE INVENTION An object of the present invention is to provide a sliding member for a sealing having a good wear resistance, corrosion resistance and thermal shock resistance and a reduced worn amount of a sliding partner. This object is solved by the sliding member of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph which shows the relation between rate of dimensional change on sintering and mixing ratios of raw materials. Fig. 2 is a graph which shows the relation between volumetric particle packing rate of molded bodies and bending strength. Fig. 3 is a graph which shows the relation between specific surface area of raw material powder and amount of molding binder. Fig. 4 is a graph which shows the relation between apparent viscosity of raw material composition measured by flow tester and volumetric particle packing rate. Fig. 5 is a micrograph of ceramic composition of a sliding surface in an example of the present invention. Fig. 6 is a graph which shows the relation between surface pressure and coefficient of friction in the present invention. Fig. 7 is a cross-sectional view of an example of a conventional floating seal. Fig. 8 is also a cross-sectional view of another example of a floating seal. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 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 and this composite material, Si 3 N 4 contains at least one of particles and whiskers and said ceramics have void and besides, another surface of the sliding surfaces has a hardness smaller than that of the composite material of said surface. In the present invention, Si 3 N 4 particles or whiskers for bonding SiC are preferably those produced by heat treatment of metallic Si. They bond particles and reduce vacancy between the particles. Thus, sintered body 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 average particle size of SiC particles is 100 µm or less, average aspect ratio of SiC whiskers is 200 or less and average length of SiC whiskers is 200 µm or less. Outside these ranges, mixing of raw materials is difficult and dispersion is not uniform, resulting in reduction of mechanical strength. The reason why one surface of 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 having the characteristics of both the SiC and Si 3 N 4 can be obtained and unexpectedly excellent thermal shock resistance can be obtained by bonding SiC superior 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 of compositions having particles of same hardness are combined and slidden, marks are apt to be formed on 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 and 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 thus, substantially no wear is caused on one surface and 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, one surface of lower hardness is mirror-polished with the harder surface by sliding and so 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 unexpenctedly superior thermal shock resistance. When diameter of voids is more than 100 µm and void content is more than 30%, mechanical strength decreases and besides bonding strength between SiC and produced Si 3 N 4 phase decreases, resulting in considerable falling-off of SiC and Si 3 N 4 particles during sliding. Further, breakage of lubricant oil film does not occur due to these open voids and so cohesion and scorch are not caused. 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 especially, 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 and when amount of adsorbates on the surface decreases, cohesion is apt to occur. However, if the partner surface is made of oxide, 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 are further improved when a solid lubricant or resin such as furan resin is contained in the void of sintered body. According to the present invention, one surface of sliding surfaces of sliding member for sealing is made of ceramics composed of at least one of particles and whiskers of SiC and Si 3 N 4 and voids and another surface (partner surface) is made of a material having a hardness lower than that of 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 and wear of a partner material can be reduced and hence, performance of various seals can be markedly improved. As explained hereinabove, according to the present invention, the sliding members have extremely excellent sliding properties such as wear resistance and thermal shock resistance and so, endurance and reliability of seal can be remarkably improved. This is effective for various sliding mechanisms such as mechanical seal, floating seal, plain bearing and the like. 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 1 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 Hoechst, Japan) 21% of blend wax (D-556 manufactured by Chukyo Yushi Co.) and 15% of stearic acid and 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, whereby 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. Charactersitics of the thus obtained sintered body are shown in Table 1. The breaking toughness was measured by notched beam method using a test piece of 3 x 4 x 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 it into water to quench it. This was repeated until cracks occurred and 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 2 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 2. Fig. 1 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. Example Raw-material (vol %) Volumetric packing of particles in molded product (%) Composition of sintered body (vol %) Dimensional change on sintering (%) Bending strength (MN/m 2 ) Breaking toughness (MN/m 3/2 ) Thermal shock resistance (Times) Si SiC Si 3 N 4 SiC Room temperature 1200°C 65 100 0 72 100 0 0.13 584 582 5 43 66 90 10 75 90 10 0.12 512 510 6 45 67 80 20 74 80 20 0.12 490 489 7 50 68 70 30 73 70 30 0.12 430 430 7 50 69 60 40 75 60 40 0.12 390 390 7 50 70 45 55 74 45 55 0.12 310 310 6 49 Comparative Examples Composition of sintered body (vol %) Bending strength (MN/m 2 ) Breaking toughness (MN/m 3/2 ) Thermal shock resistance (times) Si 3 N 4 Y 2 O 3 SiC Room temperature 1200°C 5 93 7 0 753 352 5 21 6 84 6 10 518 321 3 21 7 75 5 20 462 255 3 18 8 66 4 30 391 211 3 17 9 57 3 40 358 208 3 15 10 42 3 55 211 127 2 15 Examples 114 - 117 Sintered bodies were prepared in the same manner as in Example 69 except that the SiC whiskers as shown in Table 3 were used in place of SiC powder. Characteristics of the resulting sintered bodies are shown in Table 3. The rate of dimensional change on sintering was less than 0.13% for all of them. From the results, it will be seen that preferable average aspect ratio of raw material whisker is 200 or less and preferable average length is 200 µm or less. Examples Raw material whisker in sintered body Bending strength Breaking toughness (MN/m 3/2 ) Thermal resistance (times) Average aspect ratio Average length (µm) room temp. 1200°C 114 180 50 364 356 8 55 115 50 150 374 375 8 55 116 230 100 310 305 6 50 117 50 220 312 310 6 50 Example 118 - 124 Molded products were prepared in the same manner as in Examples 65 - 70 and then they were sintered with changing stepwise the heating time until 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 4. 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 excellent in toughness. Examples Composition of sintered body (vol %) Bending strength (MN/m 2 ) Breaking toughness (MN/m 3/2 ) Thermal shock resistance (times) Si 3 N 4 Si 3 N 4 whisker SiC Room temp. 1200°C 118 49.5 0.5 50 330 334 6 50 119 40 10 50 332 332 7 50 120 30 20 50 334 331 7 50 121 20 30 50 332 331 7 50 122 15 35 50 335 332 6 50 123 49.7 0.3 50 331 332 4 50 124 10 40 50 334 332 4 50 Examples 125 - 129 Table 5 shows characteristics of sintered bodies made in accordance with Example 65 using 55 vol % of Si and 45 vol % of SiC and molding binder in the amounts changed within the range of 5 - 13 parts by weight whereby 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. 2. From these results, it can be seen that it is preferred that the molded products have a volumetric particle packing rate of at least 70 vol %. Examples Volumetric packing of particles in molded product (%) Bending strength (MN/m 2 ) Breaking toughness (MN/m 3/2 ) Thermal shock resistance (times) Room temp. 1200°C 125 65 154 154 3 42 126 69 257 254 3 48 127 70 310 310 5 48 128 73 359 358 6 50 129 75 390 390 7 50 Examples 130 - 144 Table 6 shows volumetric particle packing rate of molded bodies made in the same manner as in Example 65 with changing blending ratio of raw materials and amount of molding binder. Fig. 3 shows relation between specific surface area of the raw materials and suitable amount of binder. When the relation is within the hatched area, volumetric particle packing rate of molded bodies can be at least 70 vol %. Examples Raw material (parts by weight) Specific surface area of raw material (cm 2 /g) Amount of molding binder (part by weight) Volumetric packing of particles in molded product (vol %) Si SiC 130 60 40 17100 5.0 65 131 60 40 17100 6.5 70 132 60 40 17100 9.0 76 133 60 40 17100 11.5 70 134 60 40 17100 12.5 64 135 90 10 25100 6.5 64 136 90 10 25100 9.3 70 137 90 10 25100 11.8 75 138 90 10 25100 14.3 70 139 90 10 25100 16.3 63 140 50 50 14500 4.1 63 141 50 50 14500 5.6 70 142 50 50 14500 8.1 76 143 50 50 14500 10.5 70 144 50 50 14500 12.4 62 Example 145 Apparent viscosity of the kneaded products of the raw materials and the molding binder in Examples 130 - 144 was measured by flow tester. Relation between the apparent viscosity and volumetric particle packing rate of the molded bodies is shown in Fig. 4. 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. 4 shows that the molded products can have a volumetric particle packing rate of at least 70 vol % by using a composition having an apparent viscosity within the range of (3 - 90) x 10 4 N·s/m 2 . The apparent viscosity of this range is relatively low and 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 (100 MPa) 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% and the sintered body was superior in dimensional accuracy. Void content of the sintered body was 15% and diameter of void was 20 µm or less. Sliding surface was produced by polishing and subjected to sliding test using Al 2 O 3 (relative density: 98%) as a partner material. The results are shown in Table 7. As a comparative example, results of test between FC-20 cast-iron products are also shown in Table 7. Fig. 5 shows a micrograph of the sliding surface of the sintered body obtained in this Example. In Fig. 5, 1 indicates a matrix phase, 2 indicates Si 3 N 4 particles and 3 indicates voids. From Table 7, it can be seen that the product of the present invention is small in friction coefficient and superior in wear resistance. Relation between surface pressure and friction coefficient when surface pressure was changed with keeping sliding speed at constant 3 m/sec is shown by curve A in Fig. 6. The result on Comparative Example 11 is also shown by curve B in Fig. 6. It can be recognized that the product of the present invention is superior in sliding characteristics. Sliding material Friction coefficient Total abrasion wear (mg/cm / 100 h) State of sliding (I) (II) Example 146 SiC/Si 3 N 4 Al 2 O 3 0.008 0.06 Good Comparative Example 11 FC-20 FC-20 0.12 72 Bragging Sliding conditions: In machine oil at surface pressure of 10 kgf/cm 2 (1 MPa) and sliding speed of of 3 m/sec. Examples 147 - 204 The sliding test was conducted in the same manner as in Example 146 except that Al 2 O 3 as the partner material was replaced by other materials. The results are shown in Table 8. Results of comparative examples are shown in Table 9. Sliding conditions were surface pressure: 2 kgf/cm 2 (0,2 MPa) 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 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 fallen off, leaving great sliding marks. The thermal shock resistance was evaluated as follows: A sintered body was kept at 1200°C for 30 minutes and then quenched in water. This was repeated until cracks occurred and the number of the repetition was taken for evaluation. Examples 205 - 210 The same test as in Example 146 was conducted on sintered bodies different in void content and diameter obtained in the same manner as in Example 146 with changing amount of binder. The results are shown in Table 10. Sliding conditions were surface pressure: 10 kgf/cm 2 (1 MPa) and sliding speed: 1 m/sec. From the results, it can be seen that when void content exceeds 30%, abrasion were abruptly increases and when diameter of void is more than 100 µm, abrasion wear also increases and the products of the present invention are superior in sliding properties. Examples 211 - 217 Sintered bodies were prepared in the same manner as in Example 146 with changing average particle size of SiC powder and the sliding test was conducted using Al 2 O 3 as the partner material. The results are shown in Table 11. When SiC particle is great, dragging occurs due to the SiC particles fallen off. The products of the present invention are superior in wear resistance. Excellent sliding properties were exhibited also when two or more SiC particles different in average particle size were mixed. No. Average particle size of Sic in SiC/Si 3 N 4 (µm) Friction coefficient SiC/Si 3 N 4 Abrasion wear (mg/cm 2 /100 h) Al 2 O 3 Abrasion wear (mg/cm 2 /100 h) SiC/Si 3 N 4 Thermal shock resistance (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 51 Examples (Comparative) 58 120 0.07 0.22 1.08 51 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 conditions: Surface pressure 10 kgf/cm 2 (1 MPa) 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 organosilicone high polymer and xylene as a binder in a pot mill and dried. The mixture was solded 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 (200 MPa). 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 12. From the results, it can be seen that the products of the present invention are superior in sliding properties. No. Organo Si high polymer compound (part by weight) Partner material Friction coefficient SiC/Si 3 N 4 Abrasion wear (mg/cm 2 /100 h) Abrasion wear of partner material (mg/cm 2 /100 h) Examples 218 Siliconimide MW: 1500 (15) Al 2 O 3 0.007 0.02 0.03 219 Polysilane MW: 500 (10) Al 2 O 3 0.007 0.02 0.03 220 Polysil MW: 800 (12) Al 2 O 3 0.007 0.02 0.03 Sliding conditions: Surface pressure 10 kgf/cm 2 (1 MPa) 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 with changing the raw material ratio of metallic Si and SiC 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 13. The sliding conditions were same as in Example 210. From the results, it can be seen that with increase in the amount of SiC particle the mechanical strength decreases, resulting in increase in abrasion wear. Therefore, it is preferred that amount of SiC particle is 70 vol % or less. Considering thermal shock resistance, SiC particle is preferably 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 and the results are also shown in Table 13. Example 230 13 Parts by weight of a mixture of polyethylene, stearic acid and a synthetic wax was 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 and the mixture was kneaded. This raw material was injection molded into a floating seal as shown by reference number 7 in Fig. 8. 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% and 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 14. No. Combination of sliding material Friction coefficient Total abrasion wear (mg/cm 2 /100 h) Abrasion wear (mg/cm 2 /100 h) (I) (II) Example 230 Si 3 N 4 /SiC Cr cast iron 0.007 0.02 0.22 Sliding conditions: Surface pressure 8 kgf/cm 2 (0,8 MPa) 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 (8 MPa) in nitrogen atmosphere. Test results on the resulting sintered body are shown in Table 15. No. Sintered body after HIP treatment (I) Partner material (II) Friction coefficient (I) Abrasion wear (mg/cm 2 /100 h) (II) Abrasion wear (mg/cm 2 /100 h) Void content (%) Therml shock resistance (times) Example 231 2 53 Al 2 O 3 0.007 0.01 0.02 Sliding conditions: Surface pressure 10 kgf/cm 2 (1 MPa) 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 by autoclave to reduce open void content. Test results on the resulting sintered body are shown in Table 16. No. Furan resin impregnated product (I) Partner material (II) Friction coefficient (I) Abrasion wear (mg/cm 2 /100 h) (II) Abrasion wear (mg/cm 2 100 h) Void content Example 232 7 Al 2 O 3 0.005 0.001 0.002 Sliding conditions: Surface pressure 10 kgf/cm 2 (1 MPa) 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 17 was used in place of α-SiC particle. Sliding test results on the resulting sintered bodies are shown in Table 17, wherein average aspect ratio of whisker is indicated by A and average length by L. Example 241 The raw material obtained in Example 146 was injection molded into a floating seal as shown in Fig. 8 and 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. 8 was made by injection molding of Al 2 O 3 (relative density 98%). They were incorporated into rolling wheel of caterpiller 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 and sliding marks.

Claims: A sliding member for a sealing, wherein one of the sliding surfaces comprises ceramics composed of SiC and Si 3 N 4 as a composite material containing Si 3 N 4 particles and/or Si 3 N 4 whiskers, and having voids in an amount of 2 to 30 vol.% and with a void size of not more than 100 µm, and another one of the sliding surfaces has a lower hardness than the composite material of said one surface. A sliding member according to claim 1, wherein said other surface comprises an oxide ceramics or a metal.

Inventors: KITA HIDEKI, NAKAMURA KOUSUKE, SOBUE MASAHISA, YASUTOMI YOSHIYUKI, KITA, HIDEKI, NAKAMURA, KOUSUKE, SOBUE, MASAHISA, YASUTOMI, YOSHIYUKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: C04B35
IPC codes: C04B35, F16C33