Hardness to toughness relationship of fine-grained WC-Co hardmetals

The hardness to toughness relationship of fine-grained WC-Co hardmetals was studied based on Palmqvist indentation toughness measurements. Sixty-five commercial and lab-sintered hardmetals of different composition, microstructure and manufacturing history were investigated to build up a representative hardness/toughness measurement band. This band is then used to discuss the influence of the various alloy- and process-related parameters on the hardness to toughness relationship of WC-Co composites. Beyond that, optimal hardness/toughness combinations can be assessed for the hardness range of 1400-2200 HV30. In general, the higher the hardness of the alloys, the longer were the indentation cracks, indicating a decrease in fracture toughness with increasing hardness. However, at a certain hardness, the toughness of individual alloys varied significantly. For example, at HV30:1670, the sum of crack lengths varied between 287 mu m (high toughness) and 449 mu m (low toughness), which corresponds to fracture toughness values of 11.5 and 9.2 MNm(-3/2), respectively. Very fine-grained hardmetals (ultrafine grades) were shown to be not necessarily tougher than coarser grained alloys (submicron grades), in particular in the hardness range of 1450-2000 HV30, although they exhibit significantly more binder at a given hardness. Only in the high hardness range of > 2000 HV30 might they be of advantage. Samples, exclusively doped with Cr3C2 as growth inhibitor exhibit more favorable hardness/toughness combinations than comparable VC-doped alloys. However, other parameters, such as sintering temperature, sintering time, or the gross carbon content of the respective alloys must be taken into consideration for obtaining optimal hardness/toughness combinations. (C) 1998 Elsevier Science Ltd. All rights reserved.