Composite ceramics sintered body and method of manufacturing the same

Abstract: In order to improve strength of a composite sintered body containing yttrium oxide, a composite ceramics sintered body contains yttrium oxide and silicon carbide. A composite oxide containing yttrium and silicon is formed on the surface of the sintered body. Mixed powder containing yttrium oxide powder and silicon carbide powder is compression-molded in an inert gas atmosphere of at least 1550°C in temperature to obtain a sintered body. This sintered body is heat treated in an atmosphere containing oxygen gas.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a composite ceramics sintered body and a method of manufacturing the same, and more particularly, it relates to a composite ceramics sintered body containing yttrium oxide (Y 2 O 3 ) having heat resistance and high strength to be employed as structural ceramics, and a method of manufacturing the same. Description of the Background Art Yttrium oxide, which is a compound having a high melting point, has excellent stability under a high temperature. Such yttrium oxide is expected as a high temperature structural material, or translucent ceramics. However, yttrium oxide has not yet been put into practice as a structural material, due to inferior mechanical properties as compared with currently available oxide ceramics such as aluminum oxide (Aℓ 2 O 3 ) or zirconium oxide (ZrO 2 ). To this end, an attempt has been made to add an additive to a sintered body which is mainly composed of yttrium oxide for improving its strength. According to study made by the inventors, for example, it has been recognized possible to improve strength of a sintered body having a matrix of yttrium oxide by dispersing silicon carbide (SiC) therein. The result of this experiment was reported by the inventors in Fourth Autumn Symposium, Ceramics Society of Japan, 1991. When silicon carbide is simply dispersed in a sintered body having a matrix of yttrium oxide, however, fracture strength of the as-obtained sintered body is merely at the level of 300 MPa at the most. Thus, the sintered body which is mainly composed of yttrium oxide is still insufficient in strength as practical structural ceramics. Further JP-A-51056815 discloses a method of manufacturing a SiC-Y 2 O 3 compact sintered body by hot pressing a mixed powder including SiC and Y 2 O 3 in an air atmosphere of 1850 - 1900°C. From GB-A-2233322 it is known a refractory oxide ceramics of silicon nitride and silicon carbide which can be mechanically strengthened by incorporating up to 50 mole percent of a strengthening material selected from zirconia, silicon carbide, alumina, magnesia, tantalum carbide, zirconium carbide aluminum nitride or silicon nitride. However, the sintered compact of the two above references was shown to lack the desired mechanical strength for highest performance, especially when exposed to elevated temperatures. SUMMARY OF THE INVENTION An object of the present invention is to improve mechanical strength of a ceramics sintered body containing yttrium oxide. This object is solved according the features of claim 1 and 3. The composite ceramics sintered body according to the present invention is a composite sintered body containing at least yttrium oxide (Y 2 O 3 ) and silicon carbide (SiC). In particular, the inventive ceramics sintered body has a structure obtained by dispersing silicon carbide in yttrium oxide particles. The sintered body according to the present invention is heat treated in the atmospheric air or an atmosphere containing oxygen gas, to be improved in fracture strength. While the mechanism of such improvement in fracture strength attained by the heat treatment has not yet been sufficiently clarified, the silicon carbide contained in the sintered body may be oxidized and volume-expanded to cause compressive stress on the surface of the sintered body, or a composite oxide containing yttrium and silicon, such as Y 2 SiO 5 , for example, is deposited on the surface of the sintered body to cause residual stress. It is conceivable that the fracture stress of the sintered body is improved by the heat treatment in the atmosphere containing oxygen due to an action brought by the combination of the yttrium oxide diffusing oxygen and the oxidized silicon carbide. Further, the inventive ceramics sintered body has such high strength at elevated temperatures that fracture strength in the atmospheric air is increased by about at least 20 % at a temperature of 1000°C as compared with the room temperature. In this case, the fracture strength may be improved during a fracture test under a high temperature by an action equivalent to that of the heat treatment in the atmospheric air. According to the present invention, an effect similar to the above can be expected also when the composite ceramics sintered body contains ceramics other than yttrium oxide and silicon carbide or other compound ceramics in which yttrium oxide and silicon carbide are dispersed, so far as the sintered body contains at least yttrium oxide and silicon carbide. In other words, an effect similar to the above can be expected also in a ceramics sintered body containing a composite composition of yttrium oxide and silicon carbide. According to the present invention, as hereinabove described, it is possible to provide a composite ceramics sintered body having higher strength than a conventional ceramics sintered body containing yttrium oxide and silicon carbide, which can be put into practice as a structural material. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing relations between silicon carbide contents and fracture strength levels of composite ceramics sintered bodies according to the present invention; and Fig. 2 is a graph showing relations between fracture test temperatures and fracture strength levels of composite ceramics sintered bodies according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Yttrium oxide powder of not more than 0.4 µm in mean particle diameter and silicon carbide powder of 0.3 µm in mean particle diameter were wet-blended at a prescribed ratio in butanol for 50 hours with a ball mill. Then the mixed powder was dried and subjected to hot press sintering in an argon jet of 1550 to 1700°C in temperature for one hour. The as-obtained sintered body of about 40 by 40 by 5 mm was cut to prepare fracture test pieces of 3 by 4 by 35 mm by grinding. The as-obtained test pieces were subjected to fracture tests at the room temperature. The fracture tests were made in the worked states of the test pieces and after the pieces were heat treated in the atmospheric air of 900°C in temperature for 30 minutes. The results of measurement in the fracture tests are shown in Fig. 1, which is a graph showing relations between contents (vol. %) of silicon carbide (SiC) in the respective test pieces and fracture strength levels. Referring to Fig. 1, black circles show fracture strength levels of the test pieces measured in the worked states. On the other hand, black triangles show fracture strength levels measured after the test pieces were heat treated in the atmosphere of 900°C in temperature for 30 minutes. It is understood from Fig. 1 that the fracture strength is improved by about 30 % by the heat treatment. Example 2 Sintered bodies obtained according to Example 1 were worked to prepare fracture test pieces. The as-obtained test pieces were subjected to measurement of fracture strength in the atmospheric air at the room temperature and at temperatures of 1000°C, 1200°C, 1400°C and 1600°C respectively. The results of measurement are shown in Fig. 2, which is a graph showing relations between the fracture test temperatures and the fracture strength levels. It is understood from Fig. 2 that the fracture strength measured in the atmosphere at a high temperature is higher by at least about 20 % than that measured at the room temperature. Example 3 Among sintered bodies obtained according to Example 1, that having a content of silicon oxide (SiC) of 10 volume percent was employed to prepare fracture test pieces. The as-obtained test pieces were subjected to heat treatment of holding the pieces in the atmospheric air at temperatures of 700°C, 800°C, 900°C, 1000°C, 1100°C, 1200°C and 1300°C respectively for 30 minutes. When fracture strength levels of the test pieces were measured, those of the test pieces heat treated at the temperatures of 900°C, 1000°C and 1100°C were improved from 400 MPa (under the room temperature) to exceed 500 MPa. However, fracture strength levels of the test pieces heat treated at the temperatures of 700°C, 800°C, 1200°C and 1300°C were reduced below those at the room temperature on the contrary. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims: A composite ceramics sintered body containing yttrium oxide and silicon carbide, comprising a composite oxide containing yttrium and silicon being formed on the surface of said sintered body, characterized in that the thus-formed sintered body has a fracture strength of more than 400 Mpa and a silicon carbide content of 5 - 30 vol.%. A composite ceramics sintered body in accordance with claim 1, wherein said composite oxide includes Y 2 SiO 5 . A method of manufacturing a composite ceramics sintered body, comprising; a step of preparing mixed powder containing yttrium oxide powder and silicon carbide powder; a step of compression-molding said mixed powder in an inert gas atmosphere of at least 1550°C in temperature thereby obtaining a sintered body; and a step of heat treating said sintered body in an atmosphere containing oxygen gas or an atmospheric air gas characterized in that the oxygen gas or the atmospheric air gas is at least 900°C and less than 1200°C in temperatur, wherein said composite ceramics sintered body containing at least 5 volume percent and not more than 30 volume percent of said silicon carbide.

Inventors: NIIHARA KOICHI, YOSHIMURA MASASHI, NIIHARA, KOICHI, YOSHIMURA, MASASHI, YOSHIMURA, MASASHI, C/O ITAMI WORKS OF SUMITOMO
Applicants: NIIHARA KOICHI, SUMITOMO ELECTRIC INDUSTRIES, NIIHARA, KOICHI, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1993
Main classification: C04B35
IPC codes: C04B35, B28B3