GRAIN-SIZE EFFECTS IN THE INDENTATION FRACTURE OF MGO

The variation of radial crack length with indentation load in a series of polycrystalline MgO materials is examined as a function of grain size. Comparison with the single-crystal response indicates that the fracture resistance of the polycrystals is significantly enhanced. Fracture surface observations suggest that the source of the enhancement is frictional interlocking of jogs in the crack plane caused by the tendency of the local crack path to maintain preferred crystalline cleavage planes. A model is developed for the fracture resistance increase and used to describe the radial crack lengths. The parameters thus derived suggest that a minimum grain size exists, below which this mechanism of fracture resistance is not operative.