On Wear Resistance of Steel-Containing Composites under Extreme Friction Conditions

The interrelation between the mechanisms of surface layer deterioration of powder composites and the elemental compositions of their primary structures under extreme friction conditions was studied. Extreme conditions were set by sliding under high pressure (>100 MPa) with boundary lubrication or by dry sliding under high-density electric current (>100 A/cm²). This resulted in plastic deformation of the surface layers and their deterioration due to low-cycle fatigue. High wear resistance of materials in such conditions should be achieved due to satisfactory stress relaxation in the surface layers. It was suggested that stresses should be relaxed due to local plastic deformation in the vicinity of the emerging stress concentrators. The ease of plastic deformation (and ease of relaxation) should be ensured by reduced doping of the structural components of composites, i.e., due to the lack of solid solutions. It was shown that the composites having Cu–steel (alloy)–TiC compositions obtained by self-propagating high-temperature synthesis with simultaneous pressing of the burning charge demonstrated strong adhesion at the sliding contact and showed low wear resistance under high boundary friction pressures. The absence of solid solutions in the primary structure of the Cu–Fe–TiC composite corresponded to high wear resistance due to the absence of adhesion at the contact and easy stress relaxation. Composites of Cu–steel-graphite compounds made by sintering in vacuum showed strong adhesion at a dry sliding electrical contact and low wear resistance due to the high content of alloying elements. It was noted that the absence of solutions in the Cu–Fe–graphite composite prevented adhesion at the contact and resulted in high wear resistance. In addition, stresses in the surface layer were also relaxed by the formation of FeO in the contact space during sliding with the current collector. Composites containing solid solutions were incapable of forming FeO on the sliding surface. This was an additional reason for the low wear resistance. It was noted that solid solutions caused a decrease in thermal conductivity of the surface layer, leading to increase in temperature gradients on the sliding surface and corresponding acceleration of friction zone deterioration.