Mechanical properties of pressureless sintered alumina containing alumina platelets

Experiments were conducted to investigate the possibility of producing alumina platelet reinforced alumina composites by pressureless sintering of tape cast compacts. Tape casting was chosen as a means of producing aligned alumina platelets in the alumina composites in the hope of achieving high sintered densities. Two platelet sizes were used (3-7 mu m, 0.6 mu m thickness and 10-15 mu m, 1 mu m thickness), and platelet contents up to 20 vol.-% were produced for each platelet size. The mechanical integrity of the sintered material was investigated with regard to the effects of platelet size and volume fraction on 4 point bend strength and fracture toughness. The double cantilever beam (DCB) fracture test was used to measure fracture toughness. When the porosity content was >6% it was found to cause a slight reduction in strength. Generally the platelets had little effect on strength. Fracture toughness was found to generally decrease with increase in platelet content for both platelet sizes, due to the accompanied increase in porosity. The platelets did not promote crack bridging or significant crack deflection, and consequently no R curve behaviour was observed for these specimens. The need for hot pressing tape cast compacts was found to be unavoidable in reducing porosity arm possibly promoting platelet induced toughening.