Wear-Microstructure-Property Relationships in Hardmetals

摘要

The National Physical Laboratory has recently commissioned a liquid jet erosion rig capable of infinitely variable impingement velocities and erodent volume fractions. Initial work using the new tribometer is being focussed on hardmetals based on tungsten carbide, either with a cobalt binder or cobaltickel binder, using principally silica sand entrained in a water jet as the erodent stream, though a number of tests were conducted with brown alumina of the same grain size. Investigations have been conducted into transients in the wear process in a number of hardmetals and longer-term results correlated with conventional parameters used for assessing hardmetals (hardness, binder linear intercept, WC grain size). Results have been obtained using two principal impingement angles; normal incidence (90°) and 45°. Results were correlated with bulk hardness but exhibited different relationships than found in the abrasive wear of WC based hardmetals; there are indications that local microstructure in regard to the area of impact on the microscopic scale plays a more fundamental role in the respect of material response to erodent impact. Electron microscopy has been used to correlate wear modes with behaviour and its relationship to properties and microstructure. Significant differences were seen between the response of different hardmetal grades to jet impingement; with a strong dependence on grain size in respect of the role of brittle fracture as a volume removal mechanism, or a precursor thereof. The type of erodent used had a significant effect on material response; microscopic plastic grooving occurred readily with an alumina slurry jet, but was not evident in the case of silica sand. Wear was also characterised by electron microscopy of polished cross sections through wear scars to give further insights into the degradation and material removal processes at work in selected grades of WC hardmetal. 3-D spatial analysis of wear scars has also been- undertaken to characterise their shape and also a non-contact scanning probe technique to further the understanding of the nature of degradation caused by impingement.