Microstructure tailoring in hot-pressed silicon carbide
Lee, Yan Xi.
Date of Issue2013
School of Materials Science and Engineering
Defence Science Organisation, National Laboratories
SiC powder was hot pressed with 2 additive systems, BC system and ABC system, with the aim to determine the type of additive and composition that will enable to achieve a combination of high hardness, sufficiently high indentation hardness and high volume fraction of 6H polytype SiC ceramics. In the BC system, temperature was varied from 1950 oC to 2050 oC. It was found that full densification was achieved only at 2050 oC. However, BC system results in poor indentation toughness inspite of high hardness. Hence, focus was shifted to ABC system, where Al content was varied from 0 wt% to 6 wt%. It was found that the ABC system was able to cater for high hardness and high toughness in sintered SiC ceramics. The hardness of the samples with the ABC system also showed the Hall-Petch relationship when the hardness was plotted against the inverse root of the average length of grains. Improvement in indention toughness was described to be credited to the elongated grains that led to crack bridging and crack deflection mechanism. Quantitative analysis of the XRD spectrum was peak fitted and carried by the polymorphic method. The relationship between the phase composition of 3C, 4H and 6H suggested that 3C4H and 4H6H were the two dominating transformation paths. The contribution of elongation grains may very likely be a result of the 3C4H transformation as the maximum elongation seems to coincide with the maximum transformation. At the same time 6H remain as 6H and grow in equixed, at the expense of 4H.
Final Year Project (FYP)
Nanyang Technological University