This paper considers mild-oxidational wear of metals by studying their behavior under friction with different loads. Low carbon, steel and copper are chosen as the model materials. We show that tribo-oxidation and the structure of surface layers of materials, both formed in the process of plastic deformation during friction, provide the boundary conditions of mild and severe wear. Oxidational wear is predominant when structural changes are minimal. As the load increases, oxidational wear is at first accompanied by metallic wear and afterwards the oxidational wear accompanies the metallic wear. The structure of the metal surface layers changes gradually during these processes, so that the strengthening of the metal is high enough to withstand friction forces. When the magnitude of frictional forces becomes higher than the maximal strength of the plastically deformed metal, the transition to severe wear occurs. The composition of different types of oxides and the fineness of wear particles varies with the friction conditions. Under light load friction conditions, fine wear particles are formed. These particles contain oxides of high oxygen content. As the friction conditions become tougher, in particular when the load increases, large-sized wear particles are formed. These particles contain oxides of a higher metal content. Phase composition and fineness of wear particles are used for gradation of mild wear. Analyses of phase composition of oxides and estimation of the fineness of wear particles are suggested as a method of wears character diagnostics. The electron diffraction method of the study of wear particles is used for this analysis in order to evaluate and choose appropriate friction and wear conditions.
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