The Effect of Sliding Speed and Normal Load on Friction and Wear Property of Aluminum

Transcription

1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: The Effect of Sliding Speed and Normal Load on Friction and Wear Property of Aluminum M. A. Chowdhury, M. K. Khalil, D. M. Nuruzzaman, M. L. Rahaman Department of Mechanical Engineering Dhaka University of Engineering & Technology, Gazipur, Gazipur , Bangladesh asadzmn@yahoo.com, Phone: Abstract The present paper investigates experimentally the effect of sliding speed and normal load on friction and wear property of an aluminum disc sliding against stainless steel pin. To do so, a pin-on-disc apparatus was designed and fabricated. Experiments were carried out under normal load N, speed rpm and relative humidity 70%. Results show that the friction coefficient decreases with the increase of sliding speed and normal load for aluminum. It is also found that the wear rates increase with the increase of sliding speed and normal load. Index Term Friction Coefficient, Normal Load, Sliding Speed, Wear Rate. I. INTRODUCTION Study of mechanics of friction and the relationship between friction and wear dates back to the sixteenth century, almost immediately after the invention of Newton s law of motion. It was observed by several authors [1-13] that the variation of friction and wear rate depends on interfacial conditions such as normal load, geometry, relative surface motion, sliding speed, surface roughness of the rubbing surfaces, type of material, system rigidity, temperature, stick slip, relative humidity, lubrication and vibration. Among these factors sliding speed and normal load are the two major factors whose play significant role for the variation of friction and wear rate. The third law of friction, which states that friction is independent of velocity, is not generally valid. The coefficient of kinetic friction as a function of sliding velocity generally has a negative slope. Changes in the sliding velocity result in a change in the shear rate which can influence the mechanical properties of the mating materials. The strength of many metals and nonmetals is greater at higher shear strain rates as stated by Bhushan and Jahsman [14, 15] which results in a lower real area of contact and a lower coefficient of friction in a dry contact. On the other hand, Bhushan reported that high normal pressures and high sliding speeds can result in high interface (flash) temperatures that can significantly reduce the strength of most materials [16]. Yet in some cases, localized surface melting reduces shear strength and friction drops to a low value determined by viscous forces in the liquid layer. Fridmen and Levesque [17] suggest that part of the observed friction reduction is due to negative slope of the dependence of the friction force upon velocity. The friction force is a function of velocity and time of contact. For most materials when the velocity increases, friction decreases and when duration of contact increases, friction increases. The dependence of friction on velocity may be explained in the following way. When velocity increases, momentum transfer in the normal direction increases producing an upward force on the upper surface. This results in an increased separation between the two surfaces which will decrease the real area of contact. Contributing to the increased separation is the fact that at higher speeds, the time during which opposite asperities compress each other is reduced increasing the level on which the top surfaces moves. In the case of materials with surface films which are either deliberately applied or produced by reaction with environment, the coefficient of friction may not remain constant as a function of load. In many metal pairs in the high-load regime, the coefficient of friction decreases with load. Bhushan [18] and Blau [19] reported that increased surface roughening and a large quantity of wear debris are believed to be responsible for decrease in friction. It was observed that the coefficient of friction may be very low for very smooth surfaces and/or at loads down to micro-to nanonewton range [20, 21]. In spite of these investigations the effects of sliding speed and normal load are yet to be clearly understood. Therefore in this study an attempt is made to investigate the effect of sliding speed and normal load on friction and wear behavior of aluminum sliding against stainless steel. It is expected that the applications of these results will contribute to the different concerned mechanical processes. II. EXPERIMENTAL DETAILS Fig. 1 shows a pin-on-disc machine which contains a pin that can slide on a rotating horizontal surface (disc). A circular aluminum test sample (disc) is to be fixed on a rotating plate (table) having a long vertical shaft welded from the bottom surface of the rotating plate. The shaft passes through three close-fit bush-bearings which are rigidly fixed with threesquare plates such that the shaft can move only axially and any

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: Load arm holder 2. Load arm 3. Normal load (dead weight) 4. Horizontal load 5. Pin sample 6. Test disc with rotating table 7. Computer 8. Belt and pulley 9. Main shaft 10. Motor 11. Speed control unit 12. Base plate 13. Rubber block 14. Rotating plate 15. Vertical square bar Fig. 1. Block diagram of the experimental set -up radial movement of the rotating shaft is restrained by the bush. To provide the rigidity to the main structure of this set-up all these three supporting square plates along with a base plate are rigidly fixed with four vertical square bars. The base plate was bolted with the foundation. A 50 mm thick neoprene rubber bearing pad was used between the base plate and the foundation. Foundation bolts were passed through this bearing pad to fix base plate with this concrete foundation. Sliding velocity can be varied by two ways (i) by changing the rotation of the shaft and (ii) by changing the radius of the point of contact of the sliding pin. A half-horsepower motor is mounted vertically to rotate the shaft with the table on a separate base having rubber damper. This separate base was used to reduce the effect of vibration of the motor, which may transmit to the main structure. The speed of the motor is varied as required by using an electronic speed control unit. Contacting foot of a 6mm diameter cylindrical pin is flat made of SS-304, fitted on a holder is subsequently fitted with an arm. The arm is pivoted with a separate base in such a way that the arm with the pin holder can rotate vertically and horizontally about the pivot point with very low friction. Pin holder is designed including the facility of putting dead weight on it so that required normal force will act on the test sample through the pin. To avoid the loss of surface material of the pin the contacting surface will remain almost constant and for this the shapes of pin were maintained cylindrical. A load cell (TML, Tokyo Sokki Kenkyujo Co. Ltd, CLS-100NA, Serial no. MR2947) was used to measure the vertical force acting on the pin. A data acquisition system was used to measure the force continuously when the system is on and these data are sent directly to the computer. The load cell along with its digital indicator (TML, Tokyo Sokki Kenkyujo Co. Ltd, Model no. TD-93A), calibrated against a standard proving ring was used for measuring loads. Losses of frictional forces at pivot points of the pin holder were determined and incorporated in the results. The total set-up was placed inside a chamber whose relative humidity can be adjusted by supplying requisite amount of moisture. A hygrometer (Wet and Dry Bulb Hygrometer, ZEAL, England) was used to measure the relative humidity of the chamber. A tachometer was used to measure the rpm of the rotating shaft. The surface roughnesses of the test sample were also measured by surface roughness tester (Taylor Hobson Precision Roughness Checker). The average roughnesses of the aluminum disc before test were found to be 0.20 m (RMS). All experiments were conducted at about 70% relative humidity. Wear rates were calculated from the measured weight loss of the disc after rubbing for definite time. Initial and final weights of the disc before and after rubbing were measured on a high-resolution weighing scale. During tests each experiment was repeated several times with new sample of pin and disc. III. RESULT AND DISCUSSION Fig. 2 shows the variation of friction coefficient with the duration of rubbing at different sliding speed for aluminum. The curve in Fig. 2 drawn for speed 500 rpm shows the variation of friction coefficient of aluminum with duration of rubbing. During the starting, value of friction coefficient is 3 which remains constant for few seconds then increases almost linearly up to 8 over a duration of 15 seconds of rubbing and after that it remains constant for the rest of the experimental time. Other curves of this figure show the values of friction coefficient at 1000, 1500, 2000 and 2500 rpm

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: rpm 1000 rpm 1500 rpm 2000 rpm 2500 rpm Duration of rubbing (sec) Sliding Speed (rpm) Fig. 2. Variation of friction coefficient with the variation of duration of rubbing at different sliding speed (Relative humidity =70%, Normal load =10N). Fig. 3. Variation of friction coefficient with the variation of sliding speed (Relative humidity =70%, Normal load =10N). sliding speed. All these curves show similar trend as before. Other parameters such as normal load (10 N), surface roughness (0 m) and relative humidity (70%) are identical for these five curves. These findings are in agreement with the findings of Chowdhury and Helali [22, 23] for mild steel and composite materials. The friction at the time of starting is low and remains at its initial value for some time and the factors responsible for this low friction are due to the presence of a layer of foreign material. This surface in general comprises of (i) moisture, (ii) oxide of metals, (iii) deposited lubricating material, etc.. Aluminum readily oxidizes in air, so that, at initial duration of rubbing, the oxide film easily separates the two material surfaces and there is little or no true metallic contact and also the oxide film has a low shear strength. During initial rubbing, the film (deposited layer) breaks up and clean surfaces come in contact which increase the bonding force between the contacting surfaces. At the same time due to the inclusion of trapped wear particles and roughening the substrate, the friction force increases due to the increase of ploughing effect. Increase of surface temperature, viscous damping of the friction surface, increased adhesion due to microwelding or deformation or hardening of the material might have some role on this increment of friction coefficient as well. After a certain duration of rubbing, the increase of roughness and other parameters may reach to a certain steady state value and hence the values of friction co-efficient remain constant for the rest of the time. In the curves of Fig. 2, it is also seen that the values of friction co-efficient decreases with the increase of sliding speed. These results are presented in Fig. 3. The decrease of friction coefficient of aluminum with the increase of sliding speed may be due to the change in the shear rate which can influence the mechanical properties of the mating materials. The strength of these materials is greater at higher shear strain rates [14, 15] which results in a lower real area of contact and a lower coefficient of friction in dry contact condition. These findings are in agreement with the findings of Chowdhury and Helali [24] for mild steel, ebonite and GFRP sliding against mild steel. Similar trends of results N 15 N 20 N Duration of rubbing (sec) Fig. 4. Variation of friction coefficient with the variation of duration of rubbing at different normal load (Relative humidity =70%, Sliding Speed =1500 rpm) Normal load (N) Fig. 5. Variation of friction coefficient with the variation of normal load (Relative humidity =70%, Sliding Speed =1500 rpm).

4 Wear rate (mg/sec) Wear rate (mg/sec) International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: are obtained for friction coefficient with the variation of normal load and duration of rubbing and these results are presented in Fig. 4 and 5. Fig. 5 indicates coefficient of friction decreases with the increase of normal load within the observed range. Increased surface roughing and a large quantity of wear debris are believed to be responsible for the decrease of friction [18, 19] with the increase of normal load Fig. 6. Variation of wear rate with the variation of sliding speed (Relative humidity =70%, Normal load =10N) Sliding Speed (rpm) Normal load (N) Fig. 7. Variation of wear rate with the variation of normal load (Relative humidity =70%, Sliding Speed =1500 rpm). Several experiments are carried out to observe the effect of normal load and sliding speed on wear rate of aluminum. Curve of Fig. 6 shows the variation of wear rate with the variation of speed. From this figure, it is observed that wear rate increases with the increase of sliding speed. This is due to the fact that duration of rubbing is the same for all sliding speed, while the length of rubbing is more in case of higher speed [25]. The materials mild steel-mild steel couples [25] also show similar behavior i.e. wear rate increases with the increase of sliding speed. Figure 7 indicates the variation of wear rate with normal load. The curve of the figure shows that wear rate increases with the increase of normal load. This is due to the fact as the normal load increases frictional heat generates at the contact surface and hence strength of materials decreases. IV. CONCLUSION The presence of sliding speed and normal load indeed affects the friction force and wear rate considerably. The values of friction coefficient decrease with the increase of sliding speed and normal load. The wear rates, on the other hand, increase with the increase of sliding speed and normal load. As the (i) the friction coefficient decreases and (ii) wear rate increases with the increase of normal load and sliding speed, therefore maintaining appropriate level of sliding speed and normal load friction and wear may be kept to some lower value to improve mechanical processes. REFERENCES [1] J. F. Archard, Wear Theory and Mechanisms, Wear Control Handbook, M. B. Peterson and W.O. Winer, eds., ASME, New York, NY, pp , (1980). [2] D. Tabor, Friction and Wear Developments Over the Last 50 Years, Keynote Address, Proc. International Conf. Tribology Friction, Lubrication and Wear, 50 Years On, London, Inst. Mech. Eng., pp , (1987). [3] S. T. Oktay, N. P. Suh, Wear Debris Formation and Agglomeration, ASME Journal of Tribology, Vol. 114, pp , (1992). [4] N. Saka, M. J. Liou, N. P. Suh, The role of Tribology in Electrical Cotact Phenomena, Wear, Vol. 100, pp , (1984). [5] N. P. Suh, H. C. 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