Role of Surface Texture on Friction and Wear under Boundary Lubricated Conditions

Ensuring effective lubrication between sliding surfaces is one of the challenges in the field of tribology. In addition to the conventional parameters like speed, load, contact geometry and material parameters, the surface texture also influences the coefficient of friction. In the present investigation, the effect of surface texture on coefficient of friction under boundary lubricated condition was studied by sliding Al-4Mg alloy pins against EN8 steel discs of various surface textures using pin-on-disc machine. Both isotropic and directional textures were generated on the surfaces of the discs. Scanning Electron Microscopy and optical profilometer studies were carried out on the contact surfaces of both the pins and discs before and after the experiments. The result showed that the coefficient of friction varied considerably with surface textures. In addition, maximum value coefficient of friction was observed when pins slid perpendicular to the uni-directional texture and minimum when pins slid on random texture. Backscattered scanning electron micrographs revealed the transfer of iron from the disc to the pins and it was highest when pins slid perpendicular to the uni-directional texture and least for the random texture. However, no transfer layer of Al-Mg alloy was observed on the steel disc. INTRODUCTION Friction is the resistance to relative motion of two bodies which are in contact. Interacting surfaces in relative motion are encountered in almost all the natural and artificial phenomenon’s in this universe. Lubrication involves introducing thin low shear strength layers of solid, liquid or gaseous between two surfaces to reduce friction. Regimes of lubrication are normally associated with dominant lubrication mechanism involved in the mechanical system. The three main regimes of lubrication can be referred to as full film or hydrodynamic lubrication, mixed lubrication and boundary lubrication. Hydrodynamic lubrication involves two non-parallel surfaces in relative motion with a layer of fluid pulled in between the surfaces to develop adequate pressure to support the load. In boundary lubrication regime thin monolayer of fluid film is formed resulting in frequent asperity contact that leads to high values of coefficient of friction and wear compared to hydrodynamic lubrication. Mixed film lubrication is the combination of full film lubrication and boundary lubrication. Boundary lubrication regime can be defined as the regime in which average film thickness is less than the composite roughness. In the boundary lubrication regime, the load is mainly supported by asperity contacts, chemical interaction between the surfaces takes place, and the reaction products play an important role in the effectiveness of lubrication process. Boundary lubrication is a complex process and is controlled by additives in the oil with a monolayer being formed by physical adsorption, chemical adsorption and chemical reaction. The bulk properties of the lubricant are of minor importance since the separation distance is in the order of molecular dimensions and strong adsorption ensures that the entire surface is covered by a film of lubricant. Stribeck curve [1], the plot of coefficient of friction against a non dimensional number, namely Hersey’s number, was instrumental in demarcation of various lubrication regimes. Hersey’s number is given by (ηv/p), where ‘η’ is the coefficient of viscosity, ‘v’ is sliding velocity and ‘p’ is load per unit width. Values of coefficient of friction that are depicted in the original Stribeck curve remain constant in the boundary lubrication regime. Experimental work done by Fischer et al. [2] led to the modification of Stribeck curve as shown in figure 1. The authors [2] corrected that the coefficient of friction does not remain constant in boundary lubricated regime, but reduces as the value of non dimensional number (ηv/p) increases. The slope of this curve is determined by the extent of the boundary lubrication. At low values of Heresy’s number the thickness of lubricant film developed is very low, resulting in significant asperity contact and therefore high friction. With increase in sliding speed or decrease in load at constant viscosity of the lubricant, the film thickness increases resulting in low frictional values. Exact analysis of elastohydrodynamic lubrication carried out by Hamrock and Dowson [3, 4] provided the formulae for calculation of minimum film thickness in lubricated contacts. The fluid film thickness parameter (λ) decides the lubrication regime with boundary lubrication characterized by a value of λ less than 1. Specific fluid thickness (λ) is the ratio of fluid film thickness (h) to combined surface roughness (σ) as shown below, λ = h /σ ; where σ = √ (σ1 2 + σ2 2 ) ------------------(1) where σ1 and σ2 are root mean square roughness of the two mating surfaces.