Fracture toughness and hardness of zinc sulphide as a function of grain size

Abstract

The erosion properties of brittle materials depend upon plastic deformation and crack generation at an impact or indented site. Vickers indentations have been used to investigate the plastic processes and crack systems in chemical vapour deposited zinc sulphide of different grain sizes. The hardness,H, and the “local” fracture toughnessKc, are dependent upon the grain size of the material. For small grain size material (<50 Μm) the Vickers hardness was found to increase with decreasing grain size in accord with the Petch mechanism, i.e.H=H0 +kd−1/2 wherek andH0 are constants andd is the grain diameter. A maximum hardness of ca. 4 GPa has been observed for material with an average 0.5 Μm grain diameter. In large grain size material, hardness anisotropy within the grains causes significant experimental scatter in the hardness measurements because the plastic impression formed by the indenter (load 10 N and 100 N) is smaller than the grain diameter. The values ofKc obtained using an indentation technique show that for grain sizes less than 8 ΜmKc decreases with decreasing grain size. For materials with a grain size in the range 500 Μm to 8 Μm, well developed median cracks were not observed, however, the radius of the fracture zone was measured in order to estimate an “effective”Kc. The “effective”Kc was found to increase approximately linearly with the reciprocal root of the grain size. Consideration of the models for elastic/plastic impact and micromechanics of crack nucleation in conjuction with the variation ofKc andH, indicate that zinc sulphide with a mean grain size of 8 Μm will give the optimum solid particle and rain erosion resistance.