Shear mode M3 in the first sites of ductile metallic alloys: Some considerations on the physical mechanisms leading to internal particle flows

Abstract

This paper examines plastic mechanisms that lead to damage to the sub-surfaces of ductile metallic contact areas and, by extension, to the creation of wear particles. The word "damage" is used here as a generic term to designate all topological, morphological and/or microstructural modifications to the surfaces that are the consequence of interfacial shear stress under friction. During friction, the accommodation mechanisms of plastic deformation by the M3 mode in the contact area can be different, depending on the microstructures of the alloy and the first sites (bulk materials). For example, for tempered martensitic steels under frictional stresses, plasticity strain occurs in Tribologically Transformed Surfaces (TTS) under dislocation gliding. The creation of new dislocations and the rearrangement of all these dislocations into a "bamboo"-type structure leads to a decrease in hardness, or softening of the steel. Otherwise, depending on the stacking fault energy in alloy microstructures, plasticity may occur through mechanisms involving either perfect dislocation gliding and/or partial dislocation gliding. In addition to hardening, the mobility and morphology of dislocations are also related to stacking fault energy, which can promote phase transformation leading to shear strain in TTS. These shear strain mechanisms have a great influence on the internal flow of wear particles and on the formation of the "Third-body layers". These findings are given in a review of tribological results of measurements carried out on a wrought X38CrMoV5 grade steel and on cobalt-base thick coatings, using tribometers, under various loadings, test temperatures and sliding speeds. The cobalt-base thick coatings are deposited on steel by several processes which modify the nominal chemical composition of the cobalt alloy and make it possible to study the influence of the iron content on shear strain under friction stresses.

Coupling of surface and volume behaviors to understand the generation of TTS in pure magnesium proceeded by HPT

“…This phenomenon is related to the formation of a nanocrystalline structure near the bulk surface [1,2,7]. Commonly, the term TTS is used to describe all processes that occurred in the skin of bodies in contact and altered their essential properties such as geometrical properties, mechanical properties and thermal properties, but not chemical ones [8]. Other terms are also used to define these transformations: Tribological Transformations of the Surface [9,10] and Superficial Tribological Transformation (STT) [11,12] which refer to the location where this layer is formed in the bulk; the White Layer (WL) or White Etching Layer (WEL) [13][14][15], which reflect the appearance of the metal after etching with Nital.…”