Microstructure and mechanical properties of chromium and chromium/nickel particulate reinforced alumina ceramics

Abstract

A range of Al₂O₃-Cr and Al₂O₃-Cr/Ni composites have been made using either pressureless sintering in the presence of a graphite bed or hot pressing. Examination of the microstructures shows that they are fully dense (typically 98–99% of the theoretical density) and that the micrometre-scale metallic particles remain discrete and homogeneously dispersed in all composites. All of the hot pressed specimens had higher flexural strengths than the sintered materials. Within each processing route, the composites had slightly lower strength values than the equivalent monolithic alumina specimens. This was attributed to weak interfacial bonding. Fracture toughness behaviour was investigated using indentation and double cantilever beam methods. All of the composites were found to be tougher than the parent alumina and to show resistance-curve behaviour. For the composites, maximum fracture toughness values were 5–6 MPa m^1/2 (about double the value for alumina) for process zone sizes of a few millimetres, although steady state was not reached in the limited number of specimens tested. Examination of fracture surfaces and indentation cracks showed that the toughening potential of the metal particles was not exploited to any significant extent. This was mainly due to weak metal-Al₂O₃ interfaces, but also because of carbon embrittlement of the metallic particles in which chromium was the major constituent.