Hardness of WC-Co hard metals: Preparation, quantitative microstructure analysis, structure-property relationship and modelling

Different commercial WC-Co hard metals with carbide grain sizes ranging from ultrafine to coarse and Co contents between 4.2 and 25 wt.% have been investigated with regard to their microstructural and mechanical properties. Therefore, novel preparation strategies - including specific etching reactions - and microscopic methods for the microstructural characterization were developed. Two sets of microscopic images were generated suitable for further semi-automatic determination of either the Co volume fraction phi(Co) or the mean maximum Feret diameter d(Fer) of the WC grains considering the irregular shape of the grains. Subsequently, the determined microstructural parameters and measured Vickers hardness values were used to develop a novel model calculating the hardness of WC-Co hard metals. The total hardness is mainly determined by the hard carbide whereby its influence is reduced by the soft Co binder phase. The current model is in good agreement with the measured values within almost the whole relevant hardness range of WC-Co hard metals (700 - 2300 HV 10) and do not require any statement of the existence of a carbide skeleton within the material and thus the Co binder mean free path which is essential for hardness models established so far but also hard to determined experimentally. So the current model is a significant simplification and improvement of the prediction of the hardness of WC-Co hard metals by means of microstructural parameters.