STRENGTHENING MECHANISM OF ALUMINA CERAMICS PREPARED BY PRECOARSENINGTREATMENTS

Effects of precoarsening treatments on strength and on subcritical cra ck growth are examined for Al2O3 ceramics. Al2O3 polycrystals sintered by conventional hot-pressing procedure yield a 4-point bending streng th of 400-500 MPa, while the precoarsening treatments increase strengt hs up to 750 MPa. Fracture toughness remains at the value of 3.5 MPa r oot m regardless of the treatments and the kind of source powder. From observations of fracture origin, it is found that the precoarsening t reatments suppress the subcritical growth of flaws during testing to g ive a relatively small flaw size. The grain boundary toughness is eval uated qualitatively by comparing the lengths of a subcritically fractu red regions, and quantitatively by measuring the percentage of transgr anular fracture. It is concluded that the fracture toughness of the gr ain boundary is increased by the precoarsening, and the toughened grai n boundary plays a major role in reducing the subcritical growth rate of sintering flaws, resulting in strengthening.