Fractographic Investigation of Micromechanisms of Fracture in Alumina Ceramics

摘要

The variations in the fracture features along radii from fracture origins in alumina ceramics were characterized and compared at equal values of stress intensity factor. The flaws at fracture origins, flaw linking, fraction of intergranular and transgranular fracture; linear features such as cleavage steps, surface roughness profiles, and crack branching were investigated for specimens fractured at various stresses and in selected environments using multiple techniques such as SEM, TEM, selected area electron channeling, and computerized surface roughness analysis. In alumina ceramics subcritical crack growth occurs mainly by wavy transgranular fracture which gives rise to the so-called reflecting spots that surround the fracture origin. The subcritical crack growth boundaries are characterized by minima in the percent intergranular fracture and scattered pullouts and projecting grains. A particularly striking example of these pullouts and projecting grains was observed in coarse grained 99+% alumina. At greater radii the percentages of intergranularly fractured grains and grains with linear features increased and the surfaces had a more disturbed appearance. Effects of environment on the fracture mode were investigated in specimens from strength measurements and work of fracture tests. Present evidence indicates that the presence of water in the environment enhances transgranular fracture.