Extrapolating hardness-structure property maps in WC/Co hardmetals

The concept of microstructural design has been extensively used for many years by the hardmetal industry to develop hard and tough products with consistent properties. The primary microstructural features are the size of the hard phase crystals and the volume fractions of the constituent phases. The range of commercial materials now comprises products with grain sizes in the range 0.2–5 μm, but there is interest in manufacturing materials with both finer and coarser structures. The Hall–Petch expression, that predicts high hardness for materials with ever finer grain sizes, underpins this drive. The availability of very fine powders, even down to nanosizes, has stimulated great interest in developing processing routes that can take advantage of these very fine powders. Powders in the nanometer size range are available. The technological challenge is to maintain this small crystal size in the sintered product. The current research has paid careful attention to accurate measurements of hardness and WC grain size, over the whole range of grain sizes, but especially at the finer end, with the objective of reviewing the Hall–Petch predictions for a large population of WC/Co hardmetals with either 6 or 10 wt% Co as binder-phase. Alternative property maps are discussed that may provide a better fit for the experimental data. The use of alternative maps has implications with regard to trends in hardness with grain size and these predictions are discussed. However, they are empirical and currently have no physical basis to underpin their use.