International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 82 Tribological Behaviour of Basalt Fiber Reinforced Aluminium Alloy Journal Bearings S. Ezhil Vannan * 1 , Paul Vizhian S 2 . Research scholar 1 , Professor 2 , Department of Mechanical Engineering, University Visvesvaraya College of Engineering, K.R. Circle, Bangalore University, Bangalore-560 001, Karnataka, India *Author for Correspondence:
[email protected] Abstract: In this study, frictional behavior of journal bearings produced from Al/basalt MMCs was investigated using a dry journal bearing test rig. The Al/basalt MMCs contains basalt short fiber from 2.5 to 10 % in steps of 2.5 wt. % and fabricated using compo-casting technique. The friction behavior of the bearings was determined by testing the bearings in a journal bearing test machine under different operating conditions. In the present investigation, friction coefficient of aluminium alloy radial bearings has been determined and effects of basalt short fiber content and load on bearings have been examined in dry, semi dry and lubricated conditions. The results showed that coefficient of friction decreased with increasing bearing pressure especially in the mixed and hydrodynamic lubrication zones. It was found that high surface roughness led to high friction coefficient. Surfaces before and after wear tests were characterized using scanning electron microscopy (SEM). Key Words: Al MMCs; Journal bearing; basalt fiber. 1. INTRODUCTION Composite materials are in limelight nowadays which needs fewer introductions. Their applications range from automobile to aerospace industries. Metal matrix composites (MMCs) have received much research interests over several years due to their excellent mechanical and thermal properties compared with the conventional materials [1] and have recently become candidates for critical structural applications because of a combination of superior mechanical properties such as better elastic modulus, tensile strength, high temperature stability and wear resistance in comparison with the parent matrix alloys [2]. MMCs possess excellent mechanical and tribological properties and are considered as potential engineering materials for various tribological applications [3]. The MMCs are commonly reinforced with high strength, high modulus, and brittle ceramic phases, which may be in the form of fiber, whiskers, or particulates. The addition of ceramic reinforcement to a metal matrix improves strength and stiffness, but at the expense of ductility [4]. Compared to the continuous fiber-reinforced composites, short fiber-reinforced MMCs offer several advantages such as improved anisotropy, ease of fabrication, and lower cost. In the past few years, wood, iron and skin have been used as journal bearing materials. Later, brass, bronze and white metal have also found some applications. Many studies have been going on to replace widely used conventional journal bearing materials such as white metal (babbit), cast iron and bronze with aluminum alloy based new journal bearing materials having superior properties [5]. Currently in addition to these materials, aluminium and zinc based materials are used as journal bearing materials. The increase in demand for lightweight, stiff and strong materials has led to development of MMCs reinforced with ceramics dispersoids. One among them is, aluminium bearing are less expensive to manufacturer and it also gets rid of lead which is an environmental concern for manufacturer [6]. Friction and wear occur to the machinery components which run together. The researchers investigate friction and wear behavior of materials because of the undesirable effect observed in the performance and life of machinery components [7]. These MMCs possess excellent mechanical and tribological properties and are considered as potential engineering materials for various wear related applications. Aluminium alloys is an important material for tribological applications due to its low density, good capability to be strengthened by precipitation, good corrosion resistance and high thermal and electrical conductivity improved are usually reinforced by Al 2 O 3 , Sic, C, SiO 2 , B, BN, B 4 C. International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 83 Therefore, the investigation of tribological behaviour of aluminum based materials is becoming increasingly important. Several studies [8, 9] suggest that MMCs under lubricated sliding distance against a smooth counterface exhibit superior wear resistance over unreinforced alloys. However, under unlubricated conditions, complex and often opposite results have been reported [10]. Sharama et al [11] have studied liquid metallurgical technique. A pin on disc wear testing machine was used to carry out the tribological tests on both composites and matrix alloy over a load range of 10–50N and sliding velocities of 1.25–3.05 m/s for various sliding distances of 0.5–3 km. The wear resistances of Al6061 matrix, garnet particulate reinforced composites are superior to that of unreinforced matrix alloy. Technological innovation, improved energy planning, enhanced property requirements and sky-rocketing costs have mandated the need for development of newer materials. The aerospace industry and many other industrial sectors are under constant pressure for materials development in order to achieve improved performance. In view of the above discussion, an attempt has been made to develop a Al/Basalt fiber composites and study the influence of basalt short fiber on bearing characteristics of a conventional as cast aluminum alloy composites. In this study, coefficient of friction of Al/Basalt short fiber radial bearings has been determined and effects of basalt fiber content and load on bearings have been examined in dry, semi dry and lubricated conditions. 2. Experimental 2.1 Materials and experimental procedure In the present study, Al 7075 alloy having the chemical composition as per the ASTM ingot specification given in Table 1 was used as the base matrix alloy. Basalt short fibers were used as reinforcement. The weight percentage of basalt short fiber was varied from 2.5–10 % steps of 2.5 wt. %. The compocasting technique was used to prepare the composite specimens, which is similar to the one used by Sharma et al. [11]. Table.1. Chemical composition of Al 7075 Alloy and basalt fiber Element Si Fe Cu Mn Mg Cr Zn Ti Al % 0.4 0.5 1.6 0.3 2.5 0.15 5.5 0.2 Bal Element SiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO Na 2 O K 2 O TiO 2 MnO % 69.51 14.18 3.92 2.41 5.62 2.74 1.01 0.55 0.04 In this process, the cu coated basalt short fiber was first pre-heated to temperature of 500C and maintained at that temperature till it was introduced into the Al alloying elements melt. The preheating of the reinforcement is necessary in order to reduce the temperature gradient and to improve wetting between the molten metal and the basalt short fiber. A known quantity of these metals ingots were pickled in 10% NaOH solution at room temperature for ten minutes. Pickling was done to remove the surface impurities. The smut formed was removed by immersing the ingots for one minute in a mixture of 1 part nitric acid and 1 part water followed by washing in methanol. These cleaned ingots after drying in air were loaded into different alumina crucibles. These crucibles kept in different furnace, which were setting metals respected melting temperature. The melts were super heated and maintained at that temperature. The temperatures were recorded using a chromel - alumel thermocouple. The molten metals were then degassed using purified nitrogen gas. Purification process with commercially pure nitrogen was carried out by passing the gas through an assembly of chemicals arranged in a row (concentrated sulphuric acid and anhydrous calcium chloride, etc.) at the rate of 1000 cc/ minute for about 8 minutes. A stainless steel impeller or stirrer coated with basalt short fiber was used to stir the molten metal and create a vortex. The impeller used for stirring was of centrifugal type with three blades welded at 45 inclination and 120 apart. The stirrer was rotated at a speed of 500 rpm and a vortex was created in the melt. The depth of immersion of the impeller was approximately one third the height of the molten metal above the bottom of the crucible. The reinforcing basalt short fiber, which were preheated in the muffle furnace, were introduced into the vortex at the rate of 120 gm/min. Stirring was continued until interface interactions between the basalt short fiber and the matrix promoted wetting. Then the melt was degassed using pure nitrogen for International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 84 about 3-4 minutes and after reheating to super heat temperature (540C), it was poured into the pre heated lower half die of the hydraulic press. The top die was brought down to solidify the composite by applying a pressure of 100 kg/sq.cm. Both the lower die and the upper dies were preheated to 280C, before the melt was poured into it. The pressure applied enables uniform distribution of the basalt short fiber in the developed composite. 2.2. Bearing Test The bearing specimens were tested under dry (unlubricated) conditions in accordance with ASTM G99 standards using a dry journal bearing apparatus as shown in fig 1.The journal bearing test apparatus were designed and modified for tribological characteristic of metal matrix composites journal bearings. The test apparatus were controlled by electronic motor driver which allows obtaining speeds varying in the range of 0-2200 rpm. The parts of journal bearing test apparatus were given in fig.1. The montages of specimens to test apparatus were given in fig 1. The temperatures of friction surfaces were measured by 3 thermometers in hollows in the bearing house (Figure 2). The bearing test indicates the frictional state of the journal bearings of the basalt short fiber reinforced composites and as cast Al7075 alloy in dry conditions due to the change in applied load. The bearing specimens for the test were prepared by machining the ingots in the as cast condition. The bearings were 60mm long, 60mm outside diameter and 40mm inner diameter. The shafts were fabricated using EN 24 steel, the chemical composition as per [12]. The shafts were heat treated to obtain a surface hardness of HRc 58-60. The shafts and the bearings were ground to a surface roughness of R 2 = 0.4-0.6m. Speed of 50 rpm was used with the loads 100-500 N in steps of 100 N. Speed as low as 50 rpm was employed since the aluminium alloys are accepted in journal bearing applications under low speeds. The higher speed may invite sharp increase in temperature, particularly when lubrication becomes marginal. The maximum allowable temperature for Al alloy is around 100 C. Moreover, usage of such low speed is not uncommon, and has been used in similar tests by Delneuville [13] and also by Deonath et al [14]. Delneuville [13] has used a speed as low as 0.2 rpm in the studies on tribological behavior of Al composites as bearing materials. The test specimens press against the shaft under a radial vertical loading. When the shaft rotates, it exerts a force to overcome the frictional resistance at the bearing / shaft interface. This force gives rise to a moment due to which an L- shaped lever fixed just above the test bearing tilts about the shaft axis in the direction of the shaft rotation and comes in contact with the load cell. The force, the load cell intern exerts to bring back the lever to its original position, is a measure of the frictional resistance force (F), which is displayed on the monitor. The coefficient of friction is given by = F/W, where W is the load applied. The bearings were run in two successive stages for friction measurement. In the first stage, the bearing was run at varying loads upto a maximum of 100 N, well below the seizure load for duration of 8 hours. The second stage consisted of running the bearing at shaft speed of 50 rpm employing the loads given above for duration of 2 hours. 3. Results and discussion 3.1 Bearing behaviour of as cast Al7075 alloy/basalt dispersed metal matrix composites The plot of the coefficient of friction () with variation in bearing parameter ZN/P under lubricated test conditions of as cast Al7075 alloy and Al7075/ basalt short fiber reinforced composites has been presented in fig 2. The measured mean values of coefficient of friction () were plotted as a function of bearing parameter ZN/P for different weight percentages of basalt short fiber. From the study it can be observed that within the scope of this investigation as the basalt short fiber was increased, there has been a decrease in the coefficient of friction. The basalt short fiber alloy bearings showed a decrease in with the increase in basalt short fiber content, but an increase with the increase in load applied. Figure 2 shows the Stribeck curves. International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 85 Figure 1: The dry Journal bearing test apparatus 1. Electric motor, 2. Clutch, 3. Sledge, 4. Aluminum bearing house, 5. Ball bearing, 6. Steel cage, 7. Weight hanger, 8. Screw rod to hold and loadcell measuring the polymer journal bearing, 9. loadcell, 10. Thermocouples. It was found that the as cast Al7075 alloy without basalt short fiber exhibited visually observable stick slip at the high loads. This stick slip became violent and led to seizure at all loads beyond 400 N. In contrast, the basalt short fiber reinforced composite did not show any signs of visually observable stick slip under these conditions even at loads of 500 N. In the present case, the reduction in of the basalt short fiber bearings is due to the smearing of the basalt short fiber onto the bearing interface and formation of a thin film such that the relative movement of the journal and the bearing promotes easy shear between the lamellar planes of the basalt short fiber. This ensures a low coefficient of friction and prevents bearing seizure in the absence of a liquid lubricant. The buildup of the film is a significant feature of basalt short fiber tribology. Basalt short fiber has a hexagon layered structure and the bonds between the parallel layers are relatively weak (Vander Walls type). The key to basalt short fiber’s value as a self lubricating solid lies in its layered lattice structure and its ability to form strong chemical bonds with gases such as water vapour. The adsorption of water vapour and other gases from the environment onto the crystalline edges weakens the interlayer bonding forces, resulting in easy shear and transfer of the crystalline platelets on to the mating surfaces. Basalt short fiber also performs well under boundary lubrication conditions because of its affinity for hydrocarbon lubricants. Powered basalt short fiber and stable suspension of basalt short fiber in lubricating oils as an additive to grease are commonly used for a variety of industrial applications. Under the lubrication test conditions, the basalt short fiber reinforced composite exhibits lower than the as cast Al7075 alloy in boundary and the hydrodynamic regions, while in the mixed region, the alloy exhibits reduced. Similar observations have been reported by other workers too [15-18]. The reason for the decrease in in the hydrodynamic region can be explained by considering the condition of hydrodynamic lubrication, which is governed by the bulk properties of the lubricants, mainly, the viscosity and the relative speed of the moving surfaces. The hydrodynamic lubrication is based on the formation of a thick lubricant film of characteristically geometric profile that develops automatically between solid surfaces having relative motion with each other [19]. The load carrying phenomenon in this case arises from the fact that a viscous fluid cannot be instantaneously squeezed out from between the two surfaces that are approaching each other. It takes time for these surfaces to meet, and during this interval due to the fluids resistance to extrusion, pressure is built up, due to which the basalt short fiber get sheared due to the load applied and gets mixed uniformly with the lubricating oil to form an effective layer of lubricant at the bearing shaft interface, due to which the basalt short fiber composites exhibit reduced than the alloy. In the boundary International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 86 region, the basalt short fiber composites showed lower levels of friction. This is attributed to the free basalt short fiber release from the bearings during the operation to form a lubricant film at the interface. In the boundary lubrication, there is considerable asperity interaction since the contacting solid surfaces move very close to each other. The solid-solid interactions get modified through the action of the boundary lubricant, so that friction and wear behavior of a boundary contact is determined by solid lubricant- solid interface. Hence, the basalt short fiber reinforced composite bearings, which liberate basalt short fiber from the subsurface exhibit reduced friction than the as cast Al7075 alloy. In the mixed region, the contact behavior is basically a combination of hydrodynamic and boundary lubrication. In this case, the asperity contact occurs although a lubricant film is present at the interface of the mating surfaces. The total friction is partly due to the asperity contact and partly due to the viscous friction of lubrication. The presence of basalt short fiber in the lubricating oil which gets released from the subsurface of the bearings is expected to add to the viscous friction of lubrication, thereby increasing friction at the interface, due to which the basalt short fiber composites exhibit slightly higher values of than the as cast Al7075 alloy in the mixed region. The visual examination of the oil used confirmed the presence of the basalt short fiber, which are also responsible for the increase in viscosity of the oil. The fresh oil, which was reddish and thinner to start with, turned into black thick slurry after the tests. The incorporation of basalt short fiber reduces friction and improves the bearing performance of the as cast Al7075 alloy. This is in good agreement with the observations made by other researchers on short fiber composites [20, 21]. The improvement in bearing performance is attributed to the changes in wear mechanism brought about by the presence of basalt short fiber. Figure 2 Stribeck curves of the as cast Al7075 alloy /basalt short fiber reinforced metal matrix composite 3.2. Coefficient of friction All the tests were conducted at atmospheric conditions. Figure 3 shows the variation of friction coefficient (μ) with sliding distance for both matrix alloy and composites at applied loads of 10 N, 20 N, 30 N, 40 N and 50 N with sliding speed of 1.25 m/s. The amplitude of the friction fluctuations was seen at all the stages. International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 87 Figure 3 Effect of coefficient of friction with load on Al/Basalt fiber composites Due to sliding surface irregularities, the speed and applied load causes amtypical stick-slip oscillation as observed in the frictional profiles. In all these cases the average coefficient of friction of the composite decreases with increasing reinforcement content. It was observed that the coefficient of friction of matrix alloy and garnet reinforced composite material increases with increasing the load. It is observed from Figure 3 that the stick-slip type frictional behaviour of both unreinforced alloy and reinforced metal matrix composites decreases and is a function of sliding distance. The sliding surface is covered with alumina layer. This layer formed is very brittle and acts as an insulator [19]. The SEM microphotographs of matrix alloy and 5 percent and 10 percent basalt short fiber reinforced composites at speed of 1.25 m/s, load of 40 N, and sliding speed of 2 km has been presented in Figure 4. From the microphotographs, it is observed that the morphology of worn surfaces of composite is different from that of as cast Al7075 alloy. It can be seen from the microphotographs that a number of parallel, continuous and deep ploughing grooves exist on the wear surface of the composite and an abrasion phenomenon is observed at low loads. The worn out surfaces in some places reveal patches from where the material was removed from the surface of the material during the course of wear. The parallel grooves suggest an abrasive wear as characterized by the penetration of the hard basalt fiber into a softer surface, which is an important contributor to the wear behavior of Al7075/basalt short fiber reinforced composites. Visual examination shows that patches were formed progressively during the wear process. The amount of dark patches formed increased with increase in the reinforcement. The area of the dark layer increased with increase in sliding distance. Wear debris produced from both the as cast Al7075 and basalt short fiber reinforced composite were predominantly metallic and dark in colour. In all the SEM micrographs, an arrow is shown to indicate the sliding direction. Cracks parallel to sliding direction at the bottom of the grooves were present. Larger groove width was observed at the test load of matrix specimen when compared to the groove that was composites specimens. 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 100 200 300 400 500 Load N C o f f i c i e n t o f f r i c t i o n 0 2.5 5 7.5 10 International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 88 3.3. Examination of the worn surfaces Fig. 4 Wornout surface of A) ascast Al7075, B) 5% basalt short fiber composites and C) 10% basalt short fiber composites at speed of 1.25 m/s rpm, load of 40 N, and sliding distance of 2 km A ) B ) c ) International Journal of Advanced Engineering Applications, Vol.7, Iss.2, pp.82-90 (2014) 89 4. Conclusion 1. The tests done on the full size journal bearing made of Al7075 alloy and the Al/basalt short fiber reinforced composite reveal that the basalt short fiber composites bearings in the lubrication test are able to run up to the regimes of boundary lubrication, but under very high friction. 2. The level of the average values of in lubrication test is lower as compared with semidry and dry tests, and it is the highest in dry test. This is due to higher temperature of mating surface. 3. Thick lubricant film was formed in hydrodynamic region that is reason why μ decreases. 4. The better performance of basalt short fiber composites bearing is mainly attributed to the formation of tribo – induced basalt short fiber rich film on the mating surface. 5. There is an improvement in conformability of the basalt short fiber composites bearings, and they can perform better under severe running conditions, small bearing journal clearance, and low rates of lubrication. 6. It is shown that, performance of basalt contain bearings under all conditions is better than bearing without basalt, this is due to fact that basalt imparts the strength and hardness to the bearing materials. REFERENCES 1. Koczak MJ, Khatri SC, Allison JE, Bader MG. 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