Оn wear resistance of steel-containing composites under extreme friction conditions

Abstract

The interrelation between the mechanisms of surface layer deterioration of powder composites and the elemental compositions of their primary structures under extreme conditions of friction was studied. Extreme conditions were set by sliding under high pressure (higher 100 MPa) in boundary lubrication or by dry sliding under high density electric current (higher 100 A/cm2). It caused plastic deformation of the surface layers and their deterioration due to lowcycle 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 vicinity of the emerging stress concentrators. The ease of plastic deformation (and ease of relaxation) should be ensured by reducing the doping of the composites structural components, i.e. due to the lack of solid solutions. It was shown that the composites having the Cu – steel (alloy) – TiC compositions obtained by the method of self-propagating high-temperature synthesis with simultaneous pressing of the burning charge had strong adhesion in the sliding contact and showed low wear resistance under high pressures boundary friction. 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 in the contact and easy stress relaxation. Composites of Cu – steel-graphite compounds, made by sintering in vacuum, showed strong adhesion in 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 composite composition of Cu – Fe – graphite caused the absence of adhesion in contact and the corresponding high wear resistance. In addition, stresses in the surface layer were also relaxed by the formation of FeO oxide in the contact space during sliding with the current collector. Composites containing solid solutions were not capable of forming FeO oxide on the sliding surface. It was an additional reason for the low wear resistance realization. It was noted that solid solutions caused a decrease in the thermal conductivity of the surface layer. Therefore, it led to an increase in temperature gradients on the sliding surface and to a сorresponding acceleration of the friction zone deterioration.