The Role of Surface Texture on Friction and Transfer Layer Formation – a Study of Aluminium and Steel Pair Using Pin-on-plate Sldiing Tester

Surface texture of a tool plays an important role as it primarily controls the frictional behavior at the tribo interface. In the present investigation, pin-on-plate sliding tester was used to identify the effect of directionality of surface grinding marks on coefficient of friction and transfer layer formation. 080 M40 steel plates were ground to achieve different surface roughness with unidirectional grinding marks. Super purity aluminium pins were slid at a sliding velocity of 2 mm/s against the prepared steel plates. Grinding angle (i.e., the angle between direction of sliding and grinding marks) was varied between 0 o and 90 o in the tests. Normal load was varied from 0 to 120 N during the tests. Experiments were conducted under both dry and lubricated conditions in ambient environment. Scanning electron micrographs of the contact surfaces of pins and plates were used to study the surface features that included the morphology of the transfer layer. Surface roughness parameters of the steel plates were measured in the direction of the sliding using an optical profilometer. It was observed that the coefficient of friction and transfer layer formation depends primarily on the directionality of the grinding marks of the harder mating surface. Under lubricated conditions, stick-slip phenomena was observed, the amplitude of which depends on the plowing component of friction. The presence of stick-slip motion under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities. The grinding angle effect on coefficient of friction was attributed to the variation in plowing component of friction, which in turn depends on the mean slope of the profile of the harder mating surface. INTRODUCTION Friction is the resistance to relative motion of two bodies that are in contact. It is commonly represented by the coefficient of friction (μ), defined as the ratio of tangential force (T) to the normal force (N). The important factors that control friction are affected by (1) kinematics of the surfaces in contact (i.e., the direction and the magnitude of the relative motion between the surfaces in contact), (2) externally applied load and/or displacements, (3) environmental conditions such as temperature and lubricants, (4) surface texture and (5) material properties [1]. Considerable work has been done to study the effect of these parameters on coefficient of friction using different experimental methods [2-13]. Among the five key factors mentioned above, the role of surface texture on friction is considered in the present study. In the literature a few authors reported the effect of surface texture of soft materials on friction [14-16]. However, the surface texture of deformable material cannot explain true friction values during sliding, and thus it is important to have knowledge about the surface texture of harder material. Considerable amount of work has also been done to study the effect of surface texture of harder material on coefficient of friction during sliding [17-23]. Menezes et al. studied the effect of surface texture on friction and transfer layer formation under both dry and lubricated conditions for Al-Mg alloy [24], pure copper [25] and super purity aluminium [26] using inclined scratch test. Various kinds of surface textures – namely unidirectional grinding marks, 8-ground, and random were prepared using simple metallographic techniques. Roughness, represented by Ra, of surfaces was varied over a range and these were prepared using different grit emery papers and abrasive powders. It was found that surface texture that promotes plane strain conditions near the interface causes higher plowing component and thus the higher coefficient of friction. On the other hand, surface texture that promotes plane stress conditions at the interface results in lower value for plowing component of friction. It was found that the sliding perpendicular unidirectional grinding marks gives maximum friction force contributed by higher plowing component, and at the other extreme, random texture resulted in lower friction values. It was observed that the roughness as given by Ra within the test range does not significantly affect the friction values. Further, the surface roughness parameter like Ra is used in general to describe a surface. However, such a single roughness parameter, which is the universally recognized and most used international parameter of surface roughness, is not sufficient to describe a functional characteristic like friction [27] and it is possible that two surface textures can have the same Ra, but their frictional characteristics could be different [23-26]. Considerable amount of work has also been done to study the effect of various roughness parameters on friction [28-32]. It was observed from the literature that many individual or hybrid surface roughness parameters [33] are in vogue in the literature. These include amplitude, spatial and hybrid parameters. However, it was noticed that like friction, the correlation coefficient between coefficient of friction and roughness parameters, was system dependent. In the present study, experiments were conducted on steel counter surfaces by sliding pure aluminium pins, owing to the importance of its alloys in aerospace and automobile industries. This study mainly addresses the issue of the effect of surface texture on the coefficient of friction to enable a better understanding of the underlying principles. EXPERIMENTAL DETAILS Surface textures with varying roughness were produced on 080 M40 steel plates by dry grinding the steel plates against dry emery papers of 220, 400, 600, 800 or 1000 grit sizes. Care was taken so that the grinding marks were unidirectional in nature. Figure 1 (a) and (b) respectively shows the threeand twodimensional surface profiles of as-ground steel plate with uni-directional grinding marks. Figure 1: (a) 3D and (b) 2D profiles of unidirectionally ground steel plate. Experiments were conducted using an inclined pin-on-plate sliding tester, the photograph i.e., details and a schematic of which are shown in figure 2 (a) and (b). The stiffness of the pin-on-plate sliding tester was found to be 0.16 μm/N. The usefulness of this test is that from a single experiment, the effect of load on the coefficient of friction can be studied. The pin-on-plate sliding tester has a vertical slide and a horizontal slide, which are driven by stepper motors with step size of 2.5 μm and 10 μm respectively. A 2-D load cell is mounted on the vertical slide to measure both the normaland traction-forces. A Linear Variable Displacement Transducer (LVDT) with a resolution of 1 μm mounted on the vertical slide was used to measure the height difference over the plate holder, to allow determination of the angle of inclination of the steel plate. The pins were made of 99.997 wt % purity aluminium. The pins were 10 mm long, 3 mm in diameter with a tip radius of 1.5 mm. The dimensions of the 080 M40 steel plates were 28 mm x 20 mm x 10 mm (thickness). The pins were first machined, and then electro-polished to remove any work-hardened layer that might have formed during the machining. Hardness measurements of aluminium pin and steel plate were made at room temperature using a Vickers micro hardness tester with 100 gm load and 10-second dwell time. Average hardness numbers, obtained from 5 indentations, were found to be 31 and 208 for the pin and plate, respectively. Before each experiment, the pins and steel plates were thoroughly cleaned first in an aqueous soap solution and then with acetone in an ultrasonic cleaner. The steel plate and an aluminium pin were then mounted on the horizontal slide and vertical slide respectively of the tester. The pin was moved towards the steel plate surface slowly using the vertical slide. Once the pin touched the steel plate (indicated by load cell registering an increase in normal load), it was stopped and the height was recorded. The pin was then retracted and the steel plate was moved in the horizontal direction by a distance of 5 mm, and the pin was again made to touch the steel plate surface. Knowing the difference in the heights at which the pin touched the steel plate and the horizontal distance, the angle of the steel plate could be accurately calculated. This angle was kept within 1 o 0.05 o . The schematic diagram of the pin-on-plate with inclined steel plate is shown in figure 2(b). The advantage of 1 o inclination of the steel plate was that from a single experiment, the effect of normal load (up to the test limit of 100 N) on the coefficient of friction and the formation of transfer layer could be studied. The waviness of the steel plate was not considered, which in any case was low. The steel plate was moved to a position where a minimum wear track length of 10 mm could be obtained. The tester was programmed such that pin would be lowered until it touched the steel plate, then the horizontal slide was moved at a speed of 2 mm/s. The horizontal and vertical forces were recorded at ambient conditions once the pin touched the surface, and the forces increased as the pin was sliding uphill on the steel plate. The normal and tangential forces were continuously acquired using a computer with data acquisition system. The coefficient of friction was calculated using the formula given by equation (1). = N T = cos sin sin cos