Relationship between Grain Boundary Microstructure, Chemistry and Mechanical Properties in Silicon Carbides

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Silicon carbide has been expected as a structural material for gas turbine and nuclear fusion reactor in severe environments for example because it possesses excellent heat-resistance and corrosion-resistance. However, for practical applications, it is necessary to improve some mechanical properties related to grain boundary such as intergranular fracture, high-temperature oxidation-induce embrittlement and wear resistance. The objective of this project therefore was to obtain the knowledge concerning grain boundary microstructure and chemistry for achieving enhanced mechanical properties. The chief results obtained are summarized as follows.(1) Effects of materials processing and dopant on grain boundary microstructure: Grain boundary microstructure in β-silicon carbides could be controlled by selecting doping element (Mg, Al, P) and modifying sintering processing (hot-pressing, spark plasma sintering). It was found that the larger ionic radius mismatch between Si and dopant, the more … More grain growth was enhanced. Moreover, the spark plasma sintering yielded a high frequency of low energy special boundaries.(2) Effect of Grain Boundary Microstructure on Mechanical Properties: Fracture strength and microhardness increased with decreasing grain size with the manner of "Hall-Petch relation". Of particular importance was the observation that the grain size sensitivity in the relation depended on the grain boundary character distribution. Fracture strength and microhardness increased with increasing the frequency of special boundaries.(3) Effects of Dopant and Grain Boundary Microstructure on High-Temperature Oxidation and Oxidation-Induced embrittlement: Passive oxidation was observed irrespective of doping elements. In particular, oxidation in Mg-doped SiC was remarkably enhanced and the mass gain was approximately one order higher than undoped SiC. A remarkable strength degradation was observed, particularly in Mg-doped SiC even though passive oxidation took place. We have found that the strength degradation can be improved by decreasing the frequency of general boundaries (increasing the frequency of special boundaries) and grain size.(4) Effects of Doping Element and Environment on Friction and Wear Properties: It was found that the friction and wear properties of SiC in unlubricated condition depended on doping elements and relative humidity (RH). At lower humidity (30%RH), the effect of doping elements (Mg, Al, P) was more pronounced. In undoped condition, the friction coefficient was approximately 0.8. The friction coefficient reduced to 0.4 with Mg-doping. At higher humidity (60%RH), the friction coefficient was 〜0.25 and was independent of doping elements. Less