Grain size dependent alumina fracture mechanics stress intensity

Abstract

Various grain size dependent hardness test results obtained on alumina materials are brought together on a Hall–Petch (H–P) inverse square root of grain diameter basis to illustrate the normal expectation of greater strengthening for finer grained material, not unrelated to the observation that structural ceramic materials are employed often in a very much finer grain size condition than counterpart structural metals and alloys. Here, the H–P dependence is shown to carry over to describing recent, mutually confirming, hardness-based measurements of the fracture mechanics stress intensity of alumina materials at small grain sizes. The somewhat surprising dependence relates to the frequent observation that an increase in plastic flow strength of a structural material is associated with a decrease in its fracture toughness. For the case of strengthening by grain size refinement, however, the fracture stress of a material is raised more than the plastic flow stress and so greater local plastic work is required for fracturing.