Fretting-Induced Degradation of Quenched and Tempered Steel under Flat-on-Flat Contact : An Experimental and Microstructural Study of Friction, Wear, and Cracking

Abstract

Fretting refers to tangentially cyclic micro-movement between the preloaded contact surfaces, usually found in mechanical joints and machine components, such as those in heavy-duty internal combustion engines. It can cause two distinct types of damage: fretting fatigue, characterized by surface crack nucleation, subsequent growth, and potentially complete fracture if the cracks propagate sufficiently under cyclic loading, and fretting wear, resulting from the combined effects of various wear mechanisms. Because fretting often occurs at hidden or tightly fitted interfaces, such as shaft–hub connections or bolted assemblies, it can progress unnoticed during operation, finally leading to premature component failure.

Despite its prevalent impact, predicting and mitigating fretting damage remains a key challenge because of the complex interplay of mechanical and chemical processes at contact surfaces, posing major obstacles to the reliable design of components under high loading conditions. Motivated by the need for deeper understanding, this study investigates fretting-induced degradation of quenched and tempered steel under self-mated flat-on-flat contact, with emphasis on friction, wear, and cracking.

A series of fretting experiments were carried out using a uniquely designed in-house testing device with a large annular flat-on-flat contact, compared with small lab-scale Hertzian contacts. The effect of surface conditions was first analyzed using a unique rough-textured surface and varying contact orientations, which have not been studied in existing research. The results revealed a delay and reduction in the initial adhesive friction peaks under the gross sliding regime, along with modified wear and cracking behavior relative to the ground surface. Building on the limited existing experiments involving flat-on-flat configurations under oil lubrication, the role of lubrication was studied in the second phase. Oil-lubricated contacts exhibited a delayed and reduced friction peak, as well as lower steady-state friction relative to dry contacts under gross sliding. These effects were accompanied by distinct changes in surface damage, such as the formation of carbonaceous tribolayers from oil decomposition. Moreover, the effect of imposed displacement amplitude on fretting behavior was systematically characterized in both test series to identify the threshold values of tangential loading that ensure so-called stable friction behavior, which is defined by the absence of an initial adhesive friction peak and notable instability in the friction curves. Increasing the imposed displacement amplitude led to substantial crack growth, with transitions in wear and damage mechanisms discerned between the partial slip and gross sliding regimes.

The results elucidate how surface texture, lubrication, and applied tangential loading govern the progression of fretting damage. The findings have practical implications for the design of fretting-resistant components, recommending strategies for surface engineering, lubricant selection, and operational parameter optimization. The work also provides a foundation for future efforts to develop predictive models of fretting behavior under service-relevant conditions.