On the competition between plastic deformation and material detachment in ferritic/pearlitic steel under boundary lubrication

Abstract

Microstructure evolution in polycrystalline metals, which undergo sliding contact, takes place by dislocation activity, followed by dislocation pile-up, formation of low-angle grain boundaries and consolidation of those low-angle grain boundaries as nanocrystalline grains, thus resulting in grain refinement. In dry contacts or boundary lubrication, this plasticity dominated process occurs in competition with material detachment as a consequence of wear. In order to understand the interrelation between plastic deformation and material detachment, a two-phase steel with a ferritic/pearlitic microstructure was studied under unidirectional sliding in fully-flooded conditions using an additive-free mineral oil. The test parameters were selected to assure that the contact takes place in boundary lubricated conditions. The load and the oil viscosity were varied in order to achieve different lambda ratios. The microstructural evolution was measured up to 100,000 cycles using electron backscatter diffraction. This data was used to calculate the grain sizes, densities of low-angle grain boundaries and geometrically necessary dislocations. The results show a competition between plastic deformation and material detachment. Lower lambda ratios lead to material detachment since the strain concentrates at the immediate surface due to higher shear stresses, which results in a wear-dominated system. In contrast to that, higher lambda ratios lead to wear tracks characterized by the formation of side ridges due to ploughing by plastic deformation. In this case, the strain distribution beneath the surface follows a Hertzian stress distribution with negligible strain at the immediate sub-surface zone and minor material detachment.