The effect of heat-treatment on the performance of submicron SiC_p-reinforced α-β sialon composites: III. Mechanical properties

Agglomerate-free SiC_p-reinforced Ln-sialon (Ln=Nd and Yb) composites were fabricated by hot-pressing and by pressureless sintering to evaluate the improvement in microstructure and mechanical properties that could be achieved by incorporating sub-micron SiC particles into a sialon matrix and then inducing α → β sialon transformation by heat-treatment. Both Nd₂O₃ and Yb₂O₃ were successful in producing dense samples by hot-pressing, but Yb₂O₃ produced a more stable α-sialon phase than Nd₂O₃. As a result, Yb-sialon/SiC_p composites showed higher hardness (H_v), attributable to the higher percentage of α-sialon grains (formed in acicular morphology) in these materials. Under comparable heat-treatment conditions, the α-sialon phase present in Nd-sialon/SiC_p composites was very unstable and almost completely transformed to β-sialon, with the aluminium-containing melilite phase (M') forming at pockets in grain junctions as the dominant grain-boundary phase. Even though the Nd-densified samples contained a large proportion of β-sialon, the fracture toughness(K_Ic) was not significantly improved by SiC_p addition because of the decrease in aspect ratio and the coalescence of grains caused by the longer heat-treatment which resulted in degradation of mechanical properties, especially K_Ic.