Effects of sedimentation treatment on the mechanical properties of SiC whisker-reinforced Si3N4 composites

Abstract

Silicon nitride (Si3N4) has been recognized as a potential candidate for structural applications in advanced heat engines and many other devices because of its excellent high-temperature strength and resistance to oxidation and corrosion. It is also well known that reinforcement of ceramics by high-strength whiskers can improve the fracture toughness (Kit)' and other mechanical properties compared with unreinforced ceramics [1-5]. Indeed, it has been reported that the toughness value of Si3N 4 can be increased by 50% when reinforced with 40 vol % SiC whiskers [3]. However, there have been some inconsistencies concerning SiC whisker-reinforced Si3N 4 composites. Some researchers reported a decrease in strength and toughness of Si3N 4 with additions of SiC whiskers [2, 6], whereas others reported a small increase in strength and much increase in toughness with the same whisker additions [3-5]. These inconsistencies have been thought mainly to result from the differences in raw materials and processes such as mixing and/or milling. In the present work the effects of sedimentation treatment on the mechanical properties of SiC whisker-reinforced Si3N4 composites were investigated. The sedimentation treatment was designed to make a difference to the raw materials, especially to remove the large agglomerates andjmpurities. A commercial SiC whisker (Tokamax; Tokai Carbon, Tokyo, Japan) and Si3N 4 powder (LC-12; H.C. Starck, Berlin, FRG) were used for the experiment. Chemical analysis showed that both SiC whisker and Si3N 4 powder contained metal impurities such as Fe, AI and Ca. The typical diameter and length of the whisker was about 0.2/zm and 10-50/xm, respectively. The Si3N4 powder was mainly composed of oL-phase, the mean diameter was 0.3/zm and it had a specific surface area of 23 m 2 g-1. The purities of Y203 and A1203 used as sintering additives (10 wt % Y20~ + 5 wt % A1203) were >99.9%. The sedimentation treatment was done as follows. The whiskers or powders were blended with distilled water and then settled in a mass-cylinder. Within 10 min the sediments and after 12 h the floatings were removed. All SiC whiskers used for experiments were sedimentation-treated, but some batches of Si3N 4 powders were not sedimentation-treated and were used in the as-received state. After the conventional wet mixing and drying process, batches were prepared to contain 0, 10, 20