Some considerations on the mitigation of fretting damage by the application of surface-modification technologies

Fretting is a surface-degradation process due to mechanical and chemical attack by small-amplitude oscillatory movement between two contacting surfaces and it is intimately related to wear, corrosion and fatigue. The introduction of surface treatments or coatings is expected to be an effective strategy against fretting damage. This paper discusses the application of several types of advanced surface-modification methods for the mitigation of fretting damage, such as physical and chemical vapour deposition (PVD and CVD), ion implantation, laser treatment and plasma nitriding, etc. Some coatings are effective in the mitigation of the fretting wear, whereas others are more effective under fretting fatigue conditions. The effects of surface-modification methods on fretting resistance are explained using fretting maps. There are at least five different mechanisms in using surface-modification methods to increase fretting resistance: (1) inducing a residual compressive stress; (2) decreasing the coefficient of friction; (3) increasing the surface hardness; (4) altering the surface chemistry; (5) increasing the surface roughness. Apart from this, the intrinsic properties of the coatings, such as their density and mechanical and chemical properties as well as the adhesion condition with the substrate, also significantly affect the performance of the coatings under fretting conditions. Based on this rationale, a coating-selection method was proposed to select the most appropriate surface treatments or coatings to minimise the probability of fretting damage. Selection of a process is guided primarily by identification of the fretting failure modes, and the ability to adjust and obtain the required surface properties, with a balance between the precise control of the surface properties and the process cost.