Effect of Y ₂O ₃-Al ₂O ₃ ratio on inter-granular phases and films in tape-casting α-SiC with high toughness

Silicon carbide (SiC) ceramics prepared from liquid phase sintering after aqueous-tape-casting can yield high toughness when appropriate amount of Y ₂O ₃-Al ₂O ₃ are added, even though no elongated grains are present. Grain boundaries (GB), second-phases and hetero-phase boundaries (HB) in 2 samples with additive mole ratios of 3:5 and 3:7 are investigated using high-resolution and analytical electron microscopy (HREM and AEM). The meta-stable YAlO ₃ (YAP) was nucleated from SiC surfaces in the sample with Y/Al = 3:5 as revealed by crystallographic relations across the HB, whilst relatively thick amorphous films were found at GB. In contrary, the higher level of Al ₂O ₃ additives decreases the GB film thickness in the sample with Y/Al = 3:7, and the homogeneous nucleation of Y ₃Al ₅O ₁₂ (YAG) occurs at triple pockets accompanying with thick HB films. The strong variation of GB widths is a result of GB wetting in the sample with Y/Al = 3:5 and HB wetting in the sample of Y/Al = 3:7, both by liquid Al ₂O ₃. The energy of GB in the former sample is higher than the energy of HB as exhibited by the preferential nucleation of meta-stable YAP on SiC surfaces, which results in wetting of GB by the liquid; the situation is opposite in the latter sample as the wetting of HB occurs, leading to de-wetting of GB. The thermal mismatch between SiC and YAP or YAG as well as the presence of amorphous films facilitate the creation of micro-crack to promote inter-granular fracture and result in high toughness in both SiC ceramics.