On adhesion in tribological contacts-Causes and consequences

This paper is focused to the metallurgical consequences of severe adhesive wear of metallicmaterials. Early examples from the late 70:ies from sliding wear tests of different steels areshown together with some high-resolution TEM micrographs of a cemented carbide cutting tooledge, prepared by using a Focused Ion Beam.Irrespective of sliding conditions, severe metallic wear of the adhesive type results in a surfacelayer, the structure of which is totally different from that of the original bulk material. Theoutermost surface layer displays a nano-crystalline structure followed by a textured layer inwhich the original grains are heavily deformed. For carbon steels, the nano-crystalline layeroften represents untempered martensite.During the wear process, oxide fragments and wear particles from the counter-material may alsobe mixed into the surface layer.The consequence for all metallic materials is that severe wear generates a hard superficial layer.For carbon steels, the hardness of the outermost layer may well exceed 1000 HV. The hardeningmechanisms are well known to a metallurgist and consist of grain refinement, deformationhardening through dislocation generation and tangling, solute hardening (martensite in carbonsteels) and second phase or particle strengthening through intermixing.Consequently, the wear process generates a surface layer on metallic materials that has a muchhigher wear resistance than the original material. This was also demonstrated in one of theexperiments.