Transfer layer evolution during friction in HIPIMS W–C coatings

Abstract The investigation of the evolution of transfer layer in the contact area in W–C coating/steel ball system during interrupted friction tests in humid air revealed that fast oxidation occurs simultaneously with water vapor dissociation and carbon hydrogenation. These mechanochemical reactions produced a mixture of hydrogenated carbon, ferritungstate with minor amounts of single tungsten (sub-)oxides and iron oxides forming transfer layer preferentially adhered to steel ball. The steady coefficient of friction (COF) was attributed to the development of a thin continuous tribo-film with higher hardness and stiffness in the central zone of the wear track facing the thickest zone of transfer layer on the ball. Despite direct experimental confirmation is necessary, tribo-film was proposed to consist of ordered rehydrogenated carbon film. Friction would be controlled by shear interactions between transfer layer and thin tribo-film on top surface of W–C coating. The effect of humidity on COF can be explained by a control of oxidation, dissociation and hydrogenation reactions and subsequent influence on the composition of transfer layer and tribo-film in the wear track. The existence of phases identified in transfer layers during friction were confirmed via modelling of parallel and mutually interacting mechanochemical reactions based on the minimization of free Gibbs energy. The ability to apply equilibrium thermodynamical calculations to dynamical processes in friction contact was related to very fast reactions among microscopic asperities eliminating kinetical factors necessary for thermodynamical equilibrium at macroscale. Based on the experimental observations and modelling, three stage model of transfer layer evolution was proposed to explain run-in and steady periods on friction curves.