THE EFFECT OF HEAT-TREATMENT ON THE PERFORMANCE OF SUBMICRON SICP-REINFORCED ALPHA-BETA SIALON COMPOSITES .3. MECHANICAL-PROPERTIES

Agglomerate-free SiCp-reinforced Ln-sialon (Ln=Nd and Yb) composites w ere fabricated by hot-pressing and by pressureless sintering to evalua te the improvement in microstructure and mechanical properties that co uld be achieved by incorporating sub-micron SiC particles into a sialo n matrix and then inducing alpha --> beta sialon transformation by hea t-treatment. Both Nd2O3 and Yb2O3 were successful in producing dense s amples by hot-pressing, but Yb2O3 produced a more stable or-sialon pha se than Nd2O3. As a result, Yb-sialon/SiCp composites showed higher ha rdness (H-v), attributable to the higher percentage of alpha-sialon gr ains (formed in acicular morphology) in these materials. Under compara ble heat-treatment conditions, the alpha-sialon phase present in Nd-si alon/SiCp composites was very unstable and almost completely transform ed to beta-sialon, with the aluminium-containing melilite phase (M') f orming at pockets in grain junctions as the dominant grain-boundary ph ase. Even though the Nd-densified samples contained a large proportion of beta-sialon, the fracture toughness(K-Ic) was not significantly im proved by SiCp addition because of the decrease in aspect ratio and th e coalescence of grains caused by the longer heat-treatment which resu lted in degradation of mechanical properties, especially K-Ic. (C) 199 7 Elsevier Science Limited. All rights reserved.