A study of abrasive wear mechanisms in cobalt-base alloys

Abstract

Single-point scratch tests were used to investigate material removal mechanisms in two cobalt-base powder metallurgy alloys 6 and 19. Each alloy was produced with fine, medium and coarse chromium-rich M 7C 3 carbides in a cobalt-rich f.c.c. matrix phase. In a separate study, the low stress abrasion resistance was found to increase with carbide size. The present scratch test study was designed to simulate low stress abrasion conditions by using single quartz abrasive particles as scratch tools, and the results are compared with those from scratches made using regularly shaped diamond tools. Single- and multiple-pass scratches were made with several different loads using Vickers diamond pyramids and single quartz abrasive particles on metallographically prepared surfaces. Single-pass scratches were also made on preworn surfaces by using a Vickers diamond. Single-pass diamond scratches exhibited ploughing and extensive coarse slip bands in the matrix phase of metallographic specimens. Evidence of carbide deformation and of slip band cracking of the matrix material was observed also. Coarse slip bands were not observed on preworn surfaces. On both polished and preworn surfaces, thin layers of the matrix phase were often smeared over the carbides, and coarse carbides were extensively cracked. Multiple-pass diamond scratches on metallographic specimens exhibited thin detached layers of matrix material in the scratch groove and tongue-like extruded lips of material stretching away from the ridge. Particles of similar size and shape were found on the diamond pyramid. Scratch tests with quartz particles exhibited slip band cracking, smeared matrix material over the carbides and cracking and pulling out of carbides similar to observations with diamond tools. However, extensive ploughing and shear lip formation were not observed in quartz scratches, and wear debris particles in the scratch and on the tool were rounded and very much smaller than those produced by multiple-pass diamond scratches. Wear debris in multiple-pass quartz scratches were observed to pile up at the leading edges of the carbides, which protruded from the surface because of preferential wear of the matrix phase, as observed in low stress quartz abrasion. With both quartz and diamond tools, the material with the finest carbides (alloy 19A) exhibited large pits where cracks had traveled along the carbide-matrix interface and between carbides. Very little evidence of pulling out of carbides in the fine carbide materials was found.