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

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