High temperature properties of sintered alpha silicon carbide

Abstract

Sintered alpha silicon carbides are being developed to meet the requirements of conventional and advanced heat engine systems. Through pressureless sintering, low cost, near-net complex shapes requiring little finish grinding can be fabricated from this form of silicon carbide. The use of ceramics such as silicon carbide as engine components requires detailed data collection and understanding of material's performance especially at high temperatures. Much data have been generated on sintered alpha SiC (1-5) over the past two years. Specifically, it has been shown that the fast fracture strength of a sintered alpha silicon carbide tested in air is constant from room temperature to above 1400° (2) and that an increase in strength has been observed when tested in argon (1,2). Some physical properties of sintered alpha SiC are listed in Table 1. These data are typical of the high purity, low porosity nature of a densified silicon carbide. Figure 1 shows the four point bend strength of sintered alpha SiC as a function of temperature. It has been shown (3,4) that the strength of fine grain, less than 10 µm average grain size, sintered alpha SiC is presently limited by the presence of processing defects. It has also been reported (4) that evidence of slow crack growth has been observed at 1500 °C in air for sintered alpha SiC having an extremely coarse grain structure. This coarse grain microstructure was caused by post-sintering heat treatment of a fine grain material promoting the occurance of exaggerated grain growth. An evaluation of various experimental techniques for determining the fracture toughness (K_(Ic)) of sintered alpha SiC has also been reported (5). This paper presents further work on the high temperature properties of a sintered alpha SiC. Specifically, data on the oxidation behavior and stress-rupture as a function of temperature and atmosphere will be presented.