Resistance to wear is determined by the ability of the metal structure to change in such a way that it can withstand friction stresses. The structure of surface layers which have undergone wear can be identified as a 'critical' structure. This is where destruction takes place. To study the surface structure specimens of low-carbon steel, aluminum and copper were subjected to wear tests and then investigated by X-ray. Under certain test conditions it was found that the changes in the surface layer structure, evidenced by the structural broadening of diffraction lines, reach a maximum level. The tests showed that as one progresses from mild wear to harsher friction conditions and correspondingly higher wear intensity, the structural broadening of the diffraction lines is first increased but than reduced. The results show that under low and moderate wear conditions, the structure of the surface layers is changed by the friction process, the surface layers being hardened by fragmentation. The level of metal hardening corresponds to the friction stresses that occur in the surface layers, and reaches a maximum when the fragment dimensions are minimal. As the friction conditions become more severe, the critical structure of the metal approaches the initial conditions, and therefore its strength is less than that of the hardened structure formed under moderate wear conditions. Such results can be explained by the difference in the rates of the plastic deformation and of plastic relaxation processes. Therefore, the critical structure under the process of wear depends on the friction conditions. To put the mechanical properties of the material to best use, one should choose optimal friction and wear conditions so that the metal will be hardened as much as possible.