MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF PRESSURELESS SINTERED ALUMINA-SILVER COMPOSITES

The silver toughened alumina ceramics, which were fabricated via a con ventional ceramic processing route using commercially available alumin a and silver oxide powders as the starting materials, exhibit a higher fracture toughness (6 to 9 MPam(0.5)) than the monolithic alumina cer amic (2.5 to 3.0 MPam(0.5)). The weak interfacial bonding between the silver inclusions and the alumina matrix is characterised by the occur rence of voids or pores at the interface, the formation of which is du e to the high vapour pressure of molten silver at the sintering temper ature. The thermal expansion mismatch between the metallic phase and t he ceramic matrix generates residual strains in the composite structur e. A microstructural study using in-situ TEM observation showed that t he thermal strains were accommodated largely by the silver grains, whi ch are much more deformable than the rigid alumina grains. Both the we ll established dislocation rings and deformation twins were observed t o occur in the silver grains entrapped at the grain boundaries and gra in junctions of the alumina matrix.