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

碳化硅具有优异的耐热性和耐腐蚀性，因此被期待作为例如恶劣环境下的燃气涡轮机和核聚变反应堆的结构材料。然而，在实际应用中，有必要提高一些与晶界有关的机械性能，如沿晶断裂，高温氧化诱发脆化和耐磨性。因此，本项目的目标是获得有关晶界微观结构和化学的知识，以实现增强的机械性能。所得主要结果概括如下。(1)材料工艺和掺杂对晶界结构的影响：通过选择掺杂元素（Mg、Al、P）和改变烧结工艺（热压、放电等离子烧结）可以控制β-SiC的晶界结构。发现Si和掺杂剂之间的离子半径失配越大， ...更多信息 增强了晶粒生长。此外，放电等离子烧结产生了高频率的低能特殊边界。(2)晶界组织对力学性能的影响：断裂强度和显微硬度随晶粒尺寸的减小而增大，并符合“Hall-Petch关系”。特别重要的是观察到的关系中的晶粒尺寸的敏感性取决于晶界特征分布。断裂强度和显微硬度随着特殊晶界出现频率的增加而增加。(3)掺杂剂和晶界微结构对高温氧化和氧化诱发脆化的影响：观察到与掺杂元素无关的被动氧化。特别是，在Mg掺杂的SiC的氧化显着增强，质量增益是约一个订单高于未掺杂的SiC。观察到一个显着的强度退化，特别是在镁掺杂的SiC，即使发生被动氧化。我们已经发现，强度退化可以通过减少一般边界的频率（增加特殊边界的频率）和晶粒尺寸来改善。(4)掺杂元素和环境对摩擦磨损性能的影响：发现SiC在无润滑条件下的摩擦磨损性能取决于掺杂元素和相对湿度（RH）。在较低湿度（30%RH）下，掺杂元素（Mg、Al、P）的效果更明显。在未掺杂条件下，摩擦系数约为0.8。Mg掺杂使摩擦系数降低到0.4。在较高湿度（60%RH）下，摩擦系数为0.25，与掺杂元素无关。少

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