Strengthening mechanism of alumina ceramics prepared by precoarsening treatments

Effects of precoarsening treatments on strength and on subcritical crack growth are examined for Al2O3 ceramics. Al2O3 polycrystals sintered by conventional hot-pressing procedure yield a 4-point bending strength of 400–500 MPa, while the precoarsening treatments increase strengths up to 750 MPa. Fracture toughness remains at the value of 3.5 MPa√m regardless of the treatments and the kind of source powder. From observations of fracture origin, it is found that the precoarsening treatments suppress the subcritical growth of flaws during testing to give a relatively small flaw size. The grain boundary toughness is evaluated qualitatively by comparing the lengths of subcritically fractured regions, and quantitatively by measuring the percentage of transgranular fracture. It is concluded that the fracture toughness of the grain boundary is increased by the precoarsening, and the toughened grain boundary plays a major role in reducing the subcritical growth rate of sintering flaws, resulting in strengthening.