The effect of frequency on both the debris and the development of the tribologically transformed structure during fretting wear of a high strength steel

It is well established that damage to the first bodies in fretting wear gives rise to a hard and brittle tribologically transformed structure (TTS) which can influence both wear and fatigue. While the existence and significance of TTS has been established through extensive study, the underlying physical mechanisms of its formation and their dependence on fretting conditions are still not clear. The aim of this work is to investigate the role of frequency in the evolution of tribologically transformed structures in a high strength steel subjected to fretting. In this work, fretting tests in the gross slip regime were conducted using self-mated high strength steel pairs employing a cylinder-on-flat geometry to produce a line contact, across a range of fretting frequencies. The nature of the debris (both particle size and composition) expelled from the fretting contact was not affected by the fretting frequency. Examination of worn specimens using SEM and EBSD was conducted to characterise the formation of ITS and to allow its role in the development of wear to be understood. It was found that at low frequencies, the fretting scar consisted of an oxide debris bed on top of a largely undeformed martensite substructure. However, at higher frequencies, the appearance of the sub-surface worn damage layer was very different, consisting of a debris oxide, a severely damaged/fragmented metallic layer and a plastically deformed martensitic sub-structure. A phenomenological model is presented (based upon the basic physical processes of oxide formation at the fretting surface and subsurface deformation below the contact) which accounts for the complex observations made.