METHOD OF MAKING A SILICON NITRIDE SINTERED MEMBER

Abstract: A silicon nitride sintered member comprises a silicon nitride sintered body and a zirconia layer coated onto the surface of the silicon nitride sintered body. The zirconia layer is preferably coated onto the surface of the silicon nitride sintered body through plasma spraying. The zirconia-coated silicon nitride sintered member has both high strength at high tempera­ tures and excellent heat insulating property.

Description: The present invention relates to a silicon nitride sintered member. Since silicon nitride sintered members are better than other inorganic materials or metals in terms of mechanical strength, thermal resistance, thermal shock resistance, corrosion resistance, etc. at high temperatures, their use as high temperature structural members has been examined, and is actively being developed. However, silicon nitride sintered members have a shortcoming that they have high thermal conductivity and therefore provide poor heat insulation. For instance, when a silicon nitride sintered member is used at a wall face of an engine combustion chamber to enhance thermal resistance, heat insulation to the exterior can be improved by an air gap etc., but proper intake gas density cannot be obtained because thermal conduct­ ivity is high and therefore the intake gas is warmed. In that case, the high thermal conductivity near the surface of the silicon nitride sintered member becomes a problem. It is disclosed in Japanese patent applications laid-open Nos. 55-28,351, 57-140,876 and 58-87,273 that the surface of a metal substrate may be coated with zirconia by plasma spraying, with the aim of improvement of thermal resistance, corrosion resistance and wear resistance. Further, in Japanese patent applications laid-open Nos. 57-135,771 and 58-125,679 it is described that the surface of a substrate of carbon is coated with zirconia. In such a technique, a cermet, platinum or Ni base alloy is used as an inter­ mediate layer for strengthening the bonding force of the zirconia layer. However, it is not suggested that a silicon nitride sintered body should be used as a substrate, nor that thermal insulation near the surface thereof is intended to be improved. As compared with a silicon nitride sintered body, zirconia has a lower themal conductivity and a better thermal insulation property, but it has a defect that it is lower in mechanical strength at high temperatures. The present invention is aimed at solving the above-mentioned drawbacks. According to the present invention, the surface of the silicon nitride sintered body which has high strength at high temperatures but high thermal conduct­ ivity and poor heat insulating property is coated with zirconia having a low thermal conductivity, thereby improving the heat insulating property near the surface of the silicon nitride sintered member. The zirconia-coated silicon nitride sintered member according to the present invention has both high strength at high temperatures and excellent heat insulating property. The silicon nitride sintered member used in this invention may be a structural member, such as an engine part, made of a silicon nitride pressure­ less sintered body, a hot press sintered body, an HIP sintered body or a reaction sintered body. Although the silicon nitride sintered member may contain various additives such as MgO, Al₂0₃ or Y₂0₃, the additives do not have essential importance in improving the thermal insulating property of the silicon nitride sintered body. The present invention is characterized in that a surface of the silicon nitride sintered member is coated with zirconia. As the zirconia, use may be made of zirconia containing one or more additives, preferably not more than 30 weight % thereof. The zirconia may for example be stabilized or partially stabilized with Y₂0₃, Mg0 or Ca0. The thickness of the zirconia layer is preferably not more than 100 µm. This is because when the thickness of the zirconia layer is too large, peeling or penetration is likely to occur through repetition of heating and cooling due to difference in coefficient of thermal expansion between the silicon nitride sintered body and zirconia and also because even the 100 µm or less thickness is sufficient to effectively reduce thermal conductivity near the surface of the silicon nitride sintered member. Next, some methods of producing the silicon nitride sintered member having a surface coated with zirconia according to the present invention will be described. The whole or part of the surface of the silicon nitride sintered member is coated with zirconia by applying or spraying a zirconia slurry onto the surface to form a zirconia layer, followed by firing, or more preferably by coating zirconia onto the surface of the silicon nitride sintered body through plasma spraying. The surface of the silicon nitride sintered body is generally a fired surface or a machined surface. Thus, in order to increase the bonding force between the zirconia slurry-applied layer or plasma sprayed layer and the surface of the silicon nitride sintered body, it is desirable to perform a surface treatment for roughening or activation. For instance, such surface treatments are polishing with abrasive grits, oxidizing under heating in air, etching with fluoric acid, etc. The optimum method of surface treatment for roughening and activation depends upon the quality of the silicon nitride and sintered body and can be ascertained in each case. Then, zirconia is coated on or plasma sprayed onto the surface of the silicon nitride sintered body which has undergone the surface treatment. As the zirconia coating raw material or a zirconia plasma spraying raw material, use may be made of stabilized or partially stabilized zirconia. Alternatively, stabilized or partially stabilized zirconia may be formed by reacting zirconia and a stabilizer during firing or plasma spraying. An intermediate layer having an intermediate coefficient of thermal expansion may be preliminarily formed on the surface of the silicon nitride sintered body so that the risk of peeling and penetration due to difference in coefficient of thermal expansion between the silicon nitride sintered body and the stabilized zirconia is reduced or avoided. In the following, examples of the present invention are given, by way of illustration, and should not be interpreted to limit the scope of the invention. In the Tables, "24 weight % Mg0-Zr0₂" means zirconia containing 24 weight % Mg0, etc. Example 1 A disc of 10 mm in diameter and 3 mm thick machined from a silicon nitride pressureless sintered body containing Sr0, Mg0, Ce0₂ and Zr0₂ as additives was oxidized at 1,200°C in air for 24 hours to form an oxidized layer on the surface thereof, thereby obtaining a substrate to be plasma sprayed. A flame sprayed agent as set out in Table 1 was plasma sprayed onto one surface of the substrate to a thickness of 50 to 70 µm, thereby obtaining Samples 1, 2 and 3 of silicon nitride sintered members according to the present invention which were to be subjected to the measurement of the thermal conductivity. The thermal conductivities of Samples 1, 2 and 3 and Comparative Example 4 in which the substrate was not coated with zirconia were measured at room temperature and at 800°C according to a laser flash method, and the results are shown in Table 1. The measurement of the thermal conductivities of Samples 1, 2 and 3 according to the laser flash method was performed by irradiating the coated surface with the laser beams. Samples 1, 2 and 3 according to the present invention suffered no peeling or penetration even after repetition of heating and cooling between room temperature and 1,200°C at 20 times. Example 2 A disc of 10 mm in diameter and 3 mm thick machined from a silicon nitride pressureless sintered body containing Y₂0₃, Mg0 and Ce0₂ as additives was immersed in conc. hydrofluoric acid at 50°C for 1 hour to perform etching of the surface thereof, thereby obtaining a substrate to be plasma sprayed. A flame spraying agent in Table 2 was plasma sprayed onto one surface of the substrate to a thickness of 50 to 70 µm, thereby obtaining Samples 5 and 6 of silicon nitride sintered members according to the present invention to be used for the thermal conductivity measurement. The thermal conductivities of Samples 5 and 6 and Comparative Example 7 in which a substrate was not coated with zirconia were measured by the same method asin Example 1, and measurement results are shown in Table 2. Samples 5 and 6 according to the present invention suffered no peeling or penetration even after repetition of heating and cooling between room temperature and 1,200°C at 20 times, either. Example 3 A disc of 10 mm in diameter and 3 mm thick machined from a silicon nitride hot press sintered body containing Mg0 as an additive was polished with #400 SiC abrasive grits to render the surface coarse, thereby obtaining a substrate to be slurry-applied. A slurry in which the slurry-applied agent given in Table 3 and polyvinyl alcohol as a binder were suspended in water was applied onto one surface of the substrate, which was dried and heated at 1,500°C in a nitrogen atmosphere for 1 hour to form a fired layer of 90-100 µm thick, thereby obtaining Samples 8 and 9 of silicon nitride sintered members according to the present invention for the thermal conductivity measurement. The thermal conductivities of Samples 8 and 9 and Comparative Example 10 in which a substrate was not coated with zirconia were measured by the same method as in Example 1, and measurement results are shown in Table 3. Samples 8 and 9 according to the present invention suffered no peeling or penetration even after repetition of heating and cooling between room temp­ erature and 1,200°C at 20 times. As shown in Tables 1, 2 and 3, the silicon nitride sintered members according to the present invention in which the surface of substrate is coated with zirconia through plasma spraying or slurry-applying have low thermal conductivities and excellent heat insulating property. As explained in the foregoing, the silicon nitride sintered member according to the present invention in which the surface of the substrate is coated with zirconia possesses both high temperature strength of the silicon nitride sintered body and the heat insulating property of zirconia in combination. Thus, the thermal resistance and the heat insulating property can be simultaneously improved by application of the silicon nitride sintered member as high temp­ erature structural members, e.g. as engine parts.

Claims: 1. A silicon nitride sintered member comprising a silicon nitride sintered body characterized by a zirconia layer coated onto the surface of the silicon nitride sintered body. 2. A silicon nitride sintered member according to claim 1, wherein said zirconia layer is formed on the surface of the silicon nitride sintered body by plasma spraying. 3. A silicon nitride sintered member according to claim 1 or claim 2 wherein the zirconia is stabilized or partially stabilized zirconia. 4. A silicon nitride sintered member according to any one of claims 1 to 3 wherein said zirconia layer is not more than 100 µm thick. 5. An engine part which is a silicon nitride sintered member according to any one of claims 1 to 4.

Inventors: MATSUI, MINORU, TAKAHASHI, TOMONORI
Applicants: NGK INSULATORS, LTD.
Application year: 1986
Main classification: C04B41
IPC codes: C04B41, F02F7, C23C4