SILICON NITRIDE CERAMICS

Abstract: Sintered silicon nitride ceramics excellent in sliding proper­ ties have the α-phase silicon nitride and the β-phase silicon ni­ tride admixed therein, and the proportion in which the α-phase silicon nitride is present in the sintered silicon nitride ceramics is controlled to be in the range of 5 to 15%. The sintered silicon nitride ceramics of this invention are suitable for heat-resistant parts such as heat-resistant bearings which are required to have high abrasion resistance.

Description: SILICON NITRIDE CERAMICS Field of the Invention and Related Art Statement This invention relates to silicon nitride ceramics of a controlled structural phase. Ceramics using silicon nitride as a principal component thereof exhibit ample resistance to heat and high strength and low thermal expantion coefficients at elevated temperatures up to 1,9000 C and , therefore, acquire qualities in resistance to thermal shock, resistance to abrasion, and resistance to corrosion. Thus, they are finding growing applications to various parts. The silicon nitride ceramics assume varying structural phases, depending on method and conditions of their fabrication. Heretofore, it has been held that in order for the sintered ceramics to acquire ample increase of density and sufficient enhancement of strength, it is essential that the phase silicon nitride as a starting material should be completely converted into the phase silicon nitride during the course of sintering. For sliding members such as bearing parts, however, sintered ceramics which have been thoroughly converted into the phase silicon nitride do not always prove to be optimum materials. When a method is developed which permits production of sintered silicon nitride ceramics retaining part of the phase silicon nitride unaltered therein and exhibiting excellent properties, it will be also convenient from the standpoint of fabrication. Object and Summary of the Invention This invention has been produced in the light of the true state of affairs of the prior art described above. An object of this invention is to provide silicon nitride ceramics which are excellent in sliding properties and are easy to produce. To be specific, this invention is directed to sintered silicon nitride ceramics which are characterized by having the i-phase silicon nitride and the phase silicon nitride intermingled therein and excelling in sliding properties. For this invention, the proportion in which the phase silicon nitride is present in the sintered silicon nitride ceramics is desired to fall approximately in the range of 5 to 15 %. The powdered raw materials for the silicon nitride ceramics do not require very exact control of grain size or purity. The sintered silicon nitride ceramics of the present invention can be obtained by admixing powdered silicon nitride as a starting material with an amount in the range of 0.1 to 15 % by weight at least one rare earth-originating compound (oxide), for example such as yttrium oxide used as a sintering aid and subjecting the resulting mixture to hot press sintering under the conditions of 1,700 to 1,8000C and 300 to 500 kg/cm2. Additionally, aluminum oxide or aluminum nitride can be added singly or multiply with an amount in the range of 0.1 to 15 % by weight. In this case, an atmospheric sintering can be carried out, and also it is effective by HIP (Hot Isostatic Press). Detailed Description of Preferred Embodiment Now, the present invention will be described below with reference to a working example. Example: Platen sintered silicon nitride ceramic samples were obtained by hot press sintering a powdered mixture, which consisted of 93% by weight of powdered phase silicon nitride (0(phase silicon nitride 99% in purity), 5% by weight of powdered yttrium oxide, and 2% by weight of powdered aluminum oxide at a fixed temperature of about 1,7800C under fixed pressure of 400 kg/cm2 for varying lengths of time ranging from 20 minutes to 4 hours. The proportions of the O(-phase silicon nitride in the sintered samples and the magnitudes of resistance of the samples to abrasion are shown in Table 1. The proportions of the phase silicon nitride were determined by the X-ray diffraction and the magnitudes of resistance to abrasion were relatively rated with the magnitude obtained of a phase structure consisting solely (100%) of phase silicon nitride as 100. Table 1 EMI4.1 <tb> Proportion <SEP> of <SEP> Time <SEP> for <SEP> Resistance <SEP> Excessive <SEP> Despersion <tb> &alpha; <SEP> phase <SEP> silicon <SEP> sintering <SEP> to <SEP> abrasion <SEP> grain <SEP> of <SEP> deflective <tb> nitride <SEP> at <SEP> 1,780*C <SEP> <SEP> growth <SEP> force(kg/mm2) <SEP> <tb> <SEP> 0 <SEP> 4 <SEP> hrs <SEP> 100 <SEP> yes <SEP> 60 <SEP> ¯ <SEP> 110 <SEP> <tb> <SEP> 2 <SEP> 3.5 <SEP> " <SEP> <SEP> 100 <SEP> yes <SEP> 80 <SEP> ¯ <SEP> 110 <tb> <SEP> 5 <SEP> 3 <SEP> " <SEP> 100 <SEP> <SEP> no <SEP> 90 <SEP> ¯ <SEP> 110 <SEP> <tb> <SEP> 10 <SEP> 2 <SEP> " <SEP> <SEP> 95 <SEP> no <SEP> 90 <SEP> ¯ <SEP> 110 <tb> <SEP> 15 <SEP> 1.5 <SEP> " <SEP> 95 <SEP> no <SEP> 90 <SEP> ¯ <SEP> 110 <tb> <SEP> 20 <SEP> 1 <SEP> " <SEP> 90 <SEP> no <SEP> 80 <SEP> ¯ <SEP> 100 <tb> <SEP> 30 <SEP> 20 <SEP> mins <SEP> x <SEP> no <SEP> 70 <SEP> ¯ <SEP> 90 <tb> The sintering conditions were about 1,7800C x 4 hours in the case of the sample whose proportion of the phase silicon nitride was 0% (ss 0% - 100%) and 1,780 C x 20 minutes in the case of the sample whose proportion of the &alpha;-phase silicon nitride was 30%. It is noted from Table 1 that the samples having the proportions of the phase silicon nitride below the level of 15% exhibited as high resistance to abrasion as the sample thoroughly converted to the phase and, what is more, enjoyed the following advantage. To obtain through conversion of the &alpha;-phase to the ss -phase, the sintering is required to be carried out at high temperatures for a long period of time. During the course of the sintering, therefore, the grains of ceramics are liable to attain excessive growth. The excessive grain growth brings about degradation of property such as dispersion of strength. To obtain silicon nitride throughly converted into the phase, therefore, the conditions of the powders as the raw materials (such as grain size) and the conditions of sintering are required to be controlled exactly. In the plant-level operation, this requirement is quite difficult to fulfil. The sample containing absolutely no phase shown in Table 1 was observed to entail excessive grain growth such its grain diameter is not less than 5 m. In contrast, the samples according with this invention required no rigid control of raw materials and were sufficiently sintered in short lengths of time to produce sintered ceramic showing no excessive grain growth. The sintered silicon nitride ceramic having the phase silicon nitride and the phase silicon nitride admixed therein and produced by the present invention are excellent in sliding properties and easy to produce as described above. Thus, they are most suitable for use in bearing parts.

Claims: What is claimed is: 1. Silicon nitride ceramics excellent in sliding properties, which are characterized by having the phase silicon nitride and the #-phase silicon nitride admixed therein. 2. Silicon nitride ceramics according to claim 1, wherein the proportion of the phase silicon nitride fall in the range of 5 to 15 %. 3. Silicon nitride ceramics according to claim 1 or 2, which are used in bearing parts.

Inventors: ABE, SHIGEO PATENT DIVISION, KOMATSU, MICHIYASU PATENT DIVISION, MIYANO, TADASHI PATENT DIVISION, OHTA, HIROYASU PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: C04B35
IPC codes: F16C33, F16J15, C04B35