Effect of compositional variations on the microstructural development of calcium-alpha/beta sialon materials.

摘要

The effect of compositional variations on the microstructural development of Ca-alpha/beta sialon composites was examined. Six calcium sialon compositions with varying levels of CaO, AlN, and Al₂O₃ additions were fabricated via hot pressing at 1700°C. The Ca-alpha/beta sialon compatibility region at 1700°C for compositions that were not adjusted for oxide impurities was estimated using x-ray diffraction. Hardness and fracture toughness were found to be dependent on many microstructural factors, but the Ca-alpha sialon content was the most important. As the Ca-alpha sialon content increased, hardness increased. Fracture toughness was dependent on both the Ca-alpha sialon content and grain morphology. The room temperature hardness and fracture toughness for the various Ca-alpha/beta sialon materials were comparable to published data for Y-alpha/beta sialon materials. Compared to yttrium or the rare earths, calcium was shown to impart greater stability to the alpha sialon.; Compensation for oxide impurities was required to attempt fabrication of pure Ca-alpha/beta sialon materials with a completely transient liquid phase. Carbothermal reduction and nitridation of oxide impurities was shown to be a feasible method for adjusting the oxygen/nitrogen ratio. However, the exact level of compensation provided by this technique was difficult to determine. An AlN for Al₂O₃ substitution technique was also successfully used to adjust the oxygen/nitrogen ratio. The two techniques had similar effects on densification, final phase assemblage, and resultant grain morphology. Complete compensation resulted in compositions less likely to achieve full densification.; The oxide additives and impurities determine the composition and amount of the liquid phase during sintering. The CaO-SiO2-Al₂O₃ ternary phase diagram was found to be a useful toot in predicting the probability of densification. Complete densification required an initial oxide composition that had an eutectic liquid temperature below the maximum sintering temperature and a sufficient quantity of liquid, dependent on the overall composition, that is not taken into the alpha or beta sialon crystalline phases too quickly. The level of compensation was varied to determine the minimum oxide content that allowed for complete densification. For one composition, the maximum compensation level was found to be 75 percent, but this threshold level is probably a function of composition, temperature, and processing. Although complete densification was achieved, partial compensation did alter the liquid composition and amount which had a dramatic impact on final grain morphology. Therefore, the optimum composition and/or the level of compensation for oxide impurities would depend on the desired mechanical properties.