ROLE OF SURFACE TEXTURE ON FRICTION AND TRANSFER LAYER FORMATION WHEN Mg-8Al ALLOY SLID AGAINST STEEL COUNTERFACE

Surface texture influences friction and transfer layer formation during sliding. In the present investigation, various kinds of surface texture with varying roughness were produced on steel plates. Pins made of Mg-8Al alloy were then slid against the prepared steel plates using inclined pin-on-plate sliding tester to understand the role of surface texture of the harder surface and load on coefficient of friction and transfer layer formation under both dry and lubricated conditions. Normal loads were varied from 1 to 120 N during the tests. Tests were conducted at a sliding velocity of 2 mm/s in ambient conditions. Scanning electron micrographs of the surfaces in contact for both the pins and plates were obtained to understand the morphology of the transfer layer. Surface roughness parameters of the steel plate were measured in the direction of the sliding on the bare surface away from the wear tracks using an optical profilometer. It was observed that the coefficient of friction and transfer layer formation are strongly dependent on surface texture and independent of surface roughness (Ra) of steel plate. Among the surface roughness parameters, the mean slope of the profile was found to explain the variations best. The plowing component of friction was highest for the surface that promotes plain strain conditions while it was lowest for the surface that promotes plane stress conditions near the surface. INTRODUCTION Friction is the resistance to relative motion of two bodies that are in contact. It is described in terms of a coefficient i.e., coefficient of friction (μ), defined as the ratio of tangential force (T) to the normal force (N). The important factors that control friction are surface texture, normal load, sliding speed, environmental conditions such as temperature and lubricants and material properties [1]. Considerable work has been done to study the effect of these parameters on coefficient of friction using different experimental methods [2-16]. Among the many important factors, which affect friction, as pointed out earlier, the role of surface texture on friction is considered in the present study. A few attempts have earlier been made to study the influence of surface texture on friction especially of soft metals [17-19]. However, as the surface texture of deformable material cannot explain true friction values, it is hence important to have knowledge about the surface texture of harder material. Thus, the present study focuses on the effect of surface texture of harder materials on coefficient of friction. In the literature, considerable amount of work has also been done to study the effect of surface texture of harder material on coefficient of friction during sliding [20-25]. Lakshmipathy and Sagar [20] studied the influence of die grinding marks orientation on friction in open die forging under lubricated conditions. They used commercial pure aluminium as the work piece material and H11 steel as the die material. Two sets of dies, one with unidirectional grinding and the other with criss-cross grinding marks were employed. It was found that, for the same percentage of deformation, the dies with the criss-cross ground pattern required lesser forging loads when compared with the situation prevailing with dies having uni-directionally ground pattern. The friction factor was also lower during the forging process when the die with the criss-cross surface pattern was used. The authors [20] concluded that compared to the criss-cross ground die, the lubrication breakdown tendency is more when pressing is done with unidirectionally ground die. Malayappan and Narayanasamy [21] studied the bulging effect of aluminium solid cylinders by varying the frictional conditions at the flat die surfaces. Different machining processes like grinding, milling, electro-spark machining, and lathe turning with emery finish were produced on the flat dies to vary the frictional conditions. The authors [21] concluded that the barreling depends on friction and thus on surface finish. Määttä et al. [22] studied the friction and adhesion of stainless steel strip against different tool steels. It was concluded that the composition of the tool steel does not have a marked effect on the friction between the tool and the work piece. However, the surface topography of the tool has a marked effect; for example, polishing of the tool surface to reduce the surface roughness reduces the friction between the tool and the work piece. Staph et al. [23] studied the effect of surface texture and surface roughness on scuffing using caterpillar disc tester. The authors [23] used steel discs of varying roughness and texture (honed, circumferentially ground with low and high roughness, and crossground) and concluded that both surface texture and surface roughness affect frictional behavior. The relation between friction and surface topography using various lubricants was studied by Hu and Dean [24] who reported that a random smoother surface could retain more lubricant and hence, reduce friction. Menezes et al. [2] studied the influence of directionality of surface grinding marks on coefficient of friction and transfer layer formation when soft pure Mg pins slid against hard steel flats under both dry and lubricated conditions using inclined scratch test. Grinding angle (i.e., the angle between direction of scratch and grinding marks) was varied between 0 o and 90 o during the tests. Roughness, represented by Ra, of surfaces was varied over a range as they were prepared using emery papers of different grit sizes. It was observed that the average coefficient of friction and transfer layer formation depend primarily on the directionality of the grinding marks but was independent of surface roughness on the harder mating surface. The authors [2] concluded that the grinding angle effect on the coefficient of friction, which consists of adhesion and plowing components, was attributed to the variation of plowing component of friction. Magnesium alloys offer lightweight alternatives to conventional metallic alloys, and consequently are finding structural applications in the automotive and light truck industry. Magnesium alloys would normally not be candidates for bearings, sliding seals or gears. But there could be situations in which their surfaces could come into contact with other materials so as to make the study of their friction and wear behavior of interest. Despite the growing interest in magnesium alloys, very little data exist on their friction and wear behavior. Earlier, a few attempts were made to study the tribological properties of Mg alloys [26-30]. However, no attempts were made on Mg alloys to study the tribological performance in terms of surface texture and hence it is used in this study. Thus, in the present study, various kinds of surface textures with varying roughness were produced on the steel plate using various grits of emery papers or polishing powders. Inclined pin-onplate sliding tester was used to study the effect of surface texture of the prepared steel plates on coefficient of friction by sliding Mg-8Al alloy pins, owing to the importance in aerospace and automobile industries. In the following sections, present the experimental results along with the discussion on the nature and contribution of various components of friction are considered. EXPERIMENTAL DETAILS Materials used: Casting made of binary Mg-8 (wt. %) Al alloy was prepared in the laboratory using diecasting technique. Heat treatment of the casting was done using standard procedures to get the optimum mechanical properties. To confirm the chemical composition, the casting was analyzed using EDAX (Energy Dispersive X-Ray Analysis). The product was then machined to make pins of 10 mm long, 3 mm in diameter with a tip radius of 1.5 mm size. The pins were electro-polished to remove any workhardened layer that might have formed during the machining. The counterpart, plate, was made of 080 M40 steel that had 28 mm x 20 mm x 10 mm (thickness) dimensions. Hardness measurements of the pins 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, was found to be 68 and 208 for the pin and plate respectively. Surface texture preparation: Four types of surface textures were produced on 080 M40 steel plates. Type I, namely, uni-directional surface texture, were produced on the steel plates with varying roughness by dry grinding the steel plates against dry emery papers of 220, 400, 600, 800 or 1000 grit sizes. For the directional surface texture, care was taken so that the grinding marks were unidirectional in nature. Type II, namely, non-directional surface texture, was generated on steel plates with varying roughness by moving the steel plate on dry emery papers of 220, 400, 600, 800 or 1000 grit size along a path with the shape of an “8” for about 500 times. Type III, namely, directional surface textures, similar to Type I was produced. But, here the grinding marks direction was perpendicular to that of Type I. Type IV, namely, random texture, with varying roughness was generated under wet grinding conditions using a polishing wheel with any one of the three abrasive media such as SiC powder (220, 600 and 1000 grit), Al2O3 powder (0.017 μm), and diamond paste (1-3 μm). Figures 1 (a), (b), (c) and (d) show the 2D and 3D profiles of steel plate surfaces along with its 3D roughness parameter, Ra, generated by Types I, II, III, and IV respectively. In figure 1, the surface textures, namely, Type I, II and III were generated using 1000 grit emery papers while the Type IV was produced using 1000 grit SiC powder. It was observed that the range of surface roughness values measured for different textured surfaces comparable to one another even though they were prepared using different techniques. Description of the pin-on-plate sliding tester: Experiments were conducted using an inclined pinon-plate sliding tester, the photograph of which is shown in figure 2(a). 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 and transfer layer formation can be studied. The pin-on-plate sliding tester has vertical and horizontal slides, which are driven by stepper motors with step size of 2.5 μm and 10 μm respectively. A 2D 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. 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 Mg-8Al alloy pin were then mounted on the horizontal slide and vertical slide of the tester respectively. Figure 1: 2D and 3D profiles of (a) Type I (b) Type II (c) Type III and (d) Type IV surfaces textures that are generated on steel plates. Arrow indicated the sliding direction of the pin relative to the plate in the experiment. The procedure employed to measure the angle of inclination is as follows: 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 waviness of the steel plate was not considered, which in any case was low. 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 120 N) on the coefficient of friction and the formation of transfer layer could be studied. 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 slides 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