Modeling Dark and White Layer Formation on Elastohydrodynamically Lubricated Steel Surfaces by Thermomechanical Indentation or Abrasion by Metallic Particles

Abstract

In a series of publications, the author has shown that the passage of ductile microparticles through elastohydrodynamic (EHD) contacts results in frictional heating that can greatly affect surface damage. The thermoviscoplastic numerical model built for those studies is extended in the present article. A more rigorous analysis of dynamic (strain-rate) effects is performed and a new element of heating is introduced, namely, that owed to plastic work of the surfaces being indented. The model is then quantitatively validated against experimental data on soft and hard particles extruded in rolling and rolling–sliding contacts. It is also compared to past numerical predictions of the author. Following its validation, the model is further expanded to predict the formation of dark and white tribochemical layers of overtempered and untempered martensite, respectively, on steel surfaces, caused by the particle-induced frictional heating. Such layers are well-known in machining processes of hardened steels as being the result of phase transformations and play a critical role in contact fatigue. The debris model in this article is used to predict the layer thickness and relative hardness for a variety of operating conditions. Layers of micrometric thickness are typically found and graphic examples are presented, linking their location to that of debris dents. A parametric study examines the role of particle size and hardness, Coulomb friction coefficient, and contact rolling velocity on dark and white layer thickness and relative hardness. The layers are zones of great inhomogeneity and thermomechanical anisotropy, increasing the risk of spalling by delamination as they are potential sources of crack initiation, particularly in sliding contacts. However, white layers in particular may actually be beneficial to contact fatigue in rolling contacts because of their substantially increased hardness. The conclusion of the study is that debris-driven surface indentation and abrasion should no longer be viewed from a purely mechanistic or geometrical perspective but has to consider the tribochemical or microstructural-modification factor for the correct evaluation of the remaining useful life of a dented or abraded contact.