Tribological Properties Of Epoxy Composites Filled By Oil And Reinforced By Polyamide And Polyester Fibres

Transcription

1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Tribological Properties Of Epoxy Composites Filled By Oil And Reinforced By Polyamide And Polyester Fibres Abdel-Jaber G. T. Faculty of Engineering, Qena, South Valley University, Qena, EGYPT Abstract-- The present work discusses the possibility of developing the polymeric composited to be used as bearing materials. Epoxy matrix was filled by paraffin oil and reinforced by polyamide and polyester fibres. The friction coefficient and wear were investigated. It was found that friction coefficient showed consistent trend with increasing sliding velocity and decreasing load. As the oil content increased friction coefficient decreased. Although that the sliding was dry the friction values were close to the hydrodynamic lubrication. This observation confirmed the possibility to get hydrodynamic properties by using the proposed epoxy composites. Friction decrease was attributed to the oil that occupied infinite number of micro pores in the epoxy matrix, where it flew out and provided thin film of oil on the sliding surface. Wear of the tested composites reinforced by PA fibres drastically decreased with increasing PA content. The high wear resistance of PA fibres was responsible for wear decrease. Epoxy filled by 20 wt. % oil displayed the lowest wear followed by that filled by 10 wt. % oil. This performance can be explained on the basis that PA fibres and oil film formed on the sliding surfaces could control the epoxy transfer into the steel counterface and consequently wear decreased. Friction coefficient displayed by the tested composites reinforced by PET showed slight increase at the mixed lubrication regime compared to that reinforced by PA fibres. The lowest friction value (0.084) was recorded confirming that the tested composites can be used as self lubricated bearings at dry sliding in different application of appropriate sliding velocity and load. Wear of the tested composites reinforced by PET fibres decreased with increasing PET content. Wear values were higher than that observed for composites reinforced by PA fibres. Index Term-- Friction coefficient, wear, epoxy composites, oil, polyamide, polyester fibres. INTRODUCTION There is an increasing demand to use porous polymers in self lubricated bearing surfaces. Porous polymers have been in existence for many decades, [1]. A typical porous polymer is prepared by copolymerizing styrene and divinylbenzene (DVB), [2], resulting in the formation of very small pores (micropores) as a result of DVB tying together linear chains of styrene at various points. Then, porous polymer structures were developed, [3]. These materials, known as high-internalphase emulsions (HIPE), had been of interest to chemists and materials scientists for many years. The HIPE polymers were produced in spherical particles, [4]. These spheres are composed of large, micrometer-size cavities that are interconnected by smaller pores. Spheres can be made rigid, fragile, rubbery, sponge-like, or soft. They can be prepared to swell a great deal, as indicated in the discussion above for super absorbent applications, or they can be made not to swell at all in applications in which expansions in volume are inappropriate. Spherical cavities within polymers, being of micrometer dimensions, can be utilized as chambers for containing liquids, lubricants, or even solid particles. Self-lubricated porous bearings in which pores are impregnated with oil are widely used in domestic electrical appliances, automobile electric components, extractor fans and in some industrial devices. A considerable number of theoretical and experimental studies has been made on the performance characteristics of the porous bearings by several research workers. This includes selfacting oil bearings and externally pressurized bearings with porous matrix [5-10]. The theoretical studies have been carried out either by analytical or numerical techniques depending on the solution of modified Reynolds and Darcy equations. Hybrid porous bearings have become the subject of increasing attention because of their potential applications as support elements in cryogenic high-speed turbomachinery [11]. The operation of these bearings depends on both the self-acting principle and the external pressurization. Also they should be replaced at regular intervals for better performance to avoid the decrease of permeability with length of service life and choking of pores due to impurities in the lubricant. Recent investigation aimed to reduce the friction and wear by using a secondary particulate phase in lubricants, in combination with porous bearing materials [12]. The particles act as one way valves, obstructing flow in the pores of the porous bearing surfaces during a load decrease, and allowing for lubricant stored inside the pores to be released, following a load increase in the contact. The above studies have been limited to the metallic porous bearings and the elastic deformation of these bearings has not been attempted. In addition, porous bearings have more factors influencing lubrication characteristics than the solid bearings, such as porosity, permeability and the purity of oil within the porous matrix. This led the investigators to find out an economic porous bearing material with lower friction losses and wear. Recently, polymers permeability have received an attention as bearing materials due to their good tribological properties, [13-16]. The mechanical properties of polymers and wear resistance are improved by adding filling materials.

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: The effect of elastic deformation on the performance of polymeric porous journal bearings was investigated, [17]. The finite difference method has been adopted to solve the Reynolds and Darcy equations. The eccentricity ratio, elastic deformation coefficient, porosity of porous matrix, permeability, and journal speed have been considered for epoxy, polyamide (PA6), polypropylene (PP) and polytetrafluoroethylene (PTFE). The results showed that the porous bearings exhibit lower load carrying capacity and higher friction coefficient compared with non-porous bearings. The effect of elastic deformation demonstrates that lower friction coefficient with higher load carrying capacity have been obtained for epoxy followed by PA6, PP and PTFE respectively. Higher porosity of porous matrix has been recommended for materials of lower elastic deformation coefficient to meet the requirements of a specific load at the same eccentricity ratio. Furthermore, at high journal speed applications higher porosity for porous matrix should be selected to satisfy the requirements of higher amount of lubricating oil.it is known that, during friction on metals or dielectric couples, part of the energy consumed turns into electrical energy, [18]. Because of triboelectrification, the charged surfaces can interact with each other due to the direct electrostatic forces, [19]. Since these forces are strong and effective, they contribute a major part of the adhesion force. The electrostatic forces are proportional to the value of the voltage generated during triboelectrification. Friction and wear of epoxy resins composites reinforced by different types of fibre materials were investigated, [20, 21]. It was observed for graphite fibre, kevlar fibre and glass fibre composites that the lowest wear and friction were obtained for fibre oriented normal to the sliding surface. The tribological performance of slip resistant material made of epoxy resin filled by abrasive grain like silicon oxide, aluminium oxide and silicon carbide of different particle size and concentration was tested [22]. The friction and wear of the tested materials sliding against steel counterface was investigated. Generally, wear resistance of epoxy filled by silicon oxide displayed the best wear resistance. Abrasive wear caused by sandy soil of steel specimens coated by epoxy filled by metallic particles such as aluminium, copper, iron and tin of μm particle size. Also, epoxy coatings were reinforced by copper, steel and tinned steel wires of different wire diameters, [23]. It was found that addition of metallic particles into epoxy coatings showed relatively higher wear than that displayed by unfilled epoxy coatings. Among metallic particles, iron and aluminium filled epoxy coatings showed the lowest wear values followed by copper and tin. Wear of composites reinforced by copper wires slightly decreased down to minimum then significantly increased with increasing wire diameter. Perpendicular orientation represented the lowest wear followed by 45 cross plied, cross plied and parallel wire orientations. When the steel was coated by tin and used as reinforcement inside epoxy coatings, significant decrease in wear was observed. Tin coatings provided steel wires by an increased elastic deformation which can absorb the impact and withstand the abrasive action of sandy particles. It was observed that coating steel surface by epoxy reinforced by tinned steel wires displayed lower wear than that observed for uncoated steel. EXPERIMENTAL Experiments were carried out using wear tester, Fig. 1. Test specimens were of epoxy in form of cubes of mm. The counterface, in form of stainless steel disc of 40 mm diameter and 11 mm width, was fastened to the rotating shaft of the tester. Load was applied by weights. The sliding velocity was varied in the range of m/s and the load was varied between 1.0 and 240 N. The experiments were performed under laboratory conditions (25 C temperature and 30% humidity) using a block on ring rig assembled in Amsler testing machine. The effects of sliding velocity and load on the friction coefficient of the tested material pairs were studied. Friction coefficient was determined by measuring the friction torque using a pendulum device, which is a part of Amsler machine. Experiments were carried out at dry sliding. The tested oil (S. A. E. 30) was prefiltered by 0.45 µm filter. It was added to epoxy during moulding at 0, 10, 20 and 30 wt. %. Besides, polyamide (PA6) and polyester (PET) fibres of 0.3 mm diameter were reinforcing the epoxy matrix at 0, 5, 10, 15 and 20 wt. % content. The rings, of 40 mm diameter and 10 mm width, made from stainless steel {403 S17 (12 % Cr, 0.5 Ni %, 1.0%, Mn, 0.8% Si)} slide against a block, in form of epoxy cube, ( mm). The surface roughness of the test specimens that were finished by grinding was 3.4 μm R a, while the roughness of the stainless steel ring was 0.17 μm R a. The tested block was held in the stationary shaft, while stainless steel discs were assembled to the rotating shaft. Based on the analysis of the friction coefficient under different loads and sliding velocities, the Stribeck curves were presented for all tested conditions.

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: RESULTS AND DISCUSSION The tribological properties of the proposed bearing materials depend on the load and sliding velocity as well as the viscosity of the lubricant. It was found that the functional relationship between the coefficient of friction and the product of sliding speed and viscosity divided by the normal load well known as the Stribeck curve has been experimentally explored much earlier by Adolf Martens in 1888 long before Richard Stribeck did his pioneering measurements in 1902, [24]. Although the proposed materials were dry tested, it was expected that the contained oil would form a film on the contact area and the dry sliding would be mixed or hydrodynamic lubricated one. The experiments discusses the frictional performance of the lubricated surfaces by Stribeck curve which expresses the Fig. 1. Arrangement of the test rig. relationship between the friction coefficient, viscosity of the lubricating fluid [η], load per unit length [F], and velocity [U]. The curve illustrates the characteristics of various lubrication regions, including boundary lubrication (BL), mixed lubrication (ML) and elastohydrodynamic lubrication (EHL). These three parameters are included in Stribeck curve which contains three lubrication regimes, Figs. 2 and 3. The first regime is the boundary lubrication regime (BL) where the film thickness equals zero. While in the mixed lubrication regime (ML), where the oil film is lower than the surface roughness, the slope is negative. The third regime is the hydrodynamic lubrication (EHL), where the oil film is higher than the surface roughness, the slope is positive. The minimum value of friction coefficient is observed in the mixed lubrication regime (ML). As the sliding velocity increased the oil film increased and consequently friction coefficient increased.

4 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: BOUNDARY LUBRICATION MIXED LUBRICATION HYDRODYNAMIC LUBRICATION Fig. 2. The relationship between friction coefficient and viscosity of the lubricating fluid, load per unit length, and velocity. Boundary Lubrication Mixed Lubrication Friction coefficient displayed by the tested composites reinforced by PA fibres of different contents and filled by 10, 20 and 30 wt. % oil is shown in Figs At 5.0 wt. % PA fibres, friction coefficient showed consistent trend with increasing sliding velocity and decreasing load, Fig. 4. The highest friction value was 0.148, while the lowest value was 0.09 displayed by composites filled by 30 wt. % oil. It is shown that as the oil content stored in epoxy matrix increased, friction coefficient decreased. In hydrodynamic lubrication zone epoxy containing 20 wt. % oil gave the highest friction coefficient indicating that the oil film thickness was relatively high. In mixed lubrication, epoxy containing 30 wt. % oil displayed the lower friction due to the relative inctraese of oil Fig. 3. Characteristics of the lubrication regions. Hydrodynamic Lubrication amount formed on the sliding surface. Although that the sliding was dry the friction values were close to the hydrodynamic lubrication. This observation confirmed the possibility to get hydrodynamic properties by using the proposed epoxy composites. Friction decrease was attributed to the oil that occupied infinite number of micro pores in the epoxy matrix, where it flew out and provided thin film of oil on the sliding surface.

5 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Fig. 4. Friction coefficient displayed by the tested composites reinforced by 5 wt. % PA fibres. When PA fibres content increased up to 10 wt. %, friction coefficient showed relatively higher values at lower velocity and higher load, Fig. 5. This behavior may be attributed to the mixed lubrication regime where the oil film was not enough to cover the contact area. As the sliding velocity increased, friction recorded slight decrease compared to the composites reinforced by 5 wt. %. It seems that PA fibres were responsible for that decrease due to their relatively low friction coefficient compared to epoxy. The lowest friction values were displayed by composites containing 30 wt. % oil. At low sliding velocity and high load, friction decreased as the oil content increased. This observation confirmed the possibility of using the proposed composited in such applications. The same trend was observed for epoxy composites reinforced by 15 and 20 wt. % PA fibres content, Figs. 6 and 7 respectively. Composites, containing 20 wt. % oil, displayed the lowest friction in the hydrodynamic regime. This behavior was attributed to the fact that the epoxy matrix could store oil up to 20 wt. % content inside the micro pores, while oil excess flew out of the matrix. It was expected that the effect of reinforcing epoxy composites by PA fibres can be shown in the wear resistance more than in friction. The dependency of friction coefficient on the oil content decreased as the PA6 fibres content increased.

6 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Fig. 5. Friction coefficient displayed by the tested composites reinforced by 10 wt. % PA fibres. Fig. 6. Friction coefficient displayed by the tested composites reinforced by 15 wt. % PA fibres.

7 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Fig. 7. Friction coefficient displayed by the tested composites reinforced by 20 wt. % PA fibres. Fig. 8. Wear of the tested composites reinforced by PA fibres.

8 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Fig. 9. Friction coefficient displayed by the tested composites reinforced by 5 wt. % PET fibres. Fig. 10. Friction coefficient displayed by the tested composites reinforced by 10 wt. % PET fibres. Wear of the tested composites reinforced by PA fibres drastically decreased with increasing PA content, Fig. 8. The high wear resistance of PA fibres was responsible for wear decrease. Epoxy filled by 20 wt. % oil displayed the lowest wear followed by that filled by 10 wt. % oil. This performance can be explained on the basis that PA fibres and oil film formed on the sliding surfaces could control the epoxy transfer into the steel counterface and consequently wear decreased. Friction coefficient displayed by the tested composites reinforced by PET fibres is shown in Figs Composites filled by 20 and 30 wt. % oil displayed the lowest friction

9 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: coefficient, where the friction values were 0.09 and Generally, friction coefficient showed slight increase at the mixed lubrication regime compared to that reinforced by PA fibres. Besides, the friction behavior of the tested composites showed the possibility of using them in applications of marginal lubrication. Besides, the oil effect was more clear for those composites than that observed for composites reinforced by PA6 fibres. The lowest friction value (0.084) was recorded confirming that the tested composites can be used as self lubricated bearings at dry sliding in different application of appropriate sliding velocity and load. The lowest friction values were shown in the hydrodynamic regime. In mixed lubrication, the effect of oil content was more pronounced than in hydrodynamic lubrication, where composites filled by 30 wt. % oil displayed the lowest friction. The same trend was observed for composites containing 10, 15 and 20 wt. % PET fibres, Figs. 10, 11 and 12 respectively. Fig. 11. Friction coefficient displayed by the tested composites reinforced by 15 wt. % PET fibres.

10 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: Fig. 12. Friction coefficient displayed by the tested composites reinforced by 20 wt. % PET fibres. Wear of the tested composites reinforced by PET fibres, Fig. 13, decreased with increasing PET content. Wear values were higher than that observed for composites reinforced by PA fibres. Composites filled by 20 wt. % oil showed the lowest wear. Wear decrease was attributed to the ability of PET fibres to decrease the epoxy transfer into the steel counterface as well as the high wear resistance of PET fibres. Fig. 13. Wear of the tested composites reinforced by PET fibres.

11 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No: The action mechanism of the polymeric fibres as well as oil filling epoxy is illustrated in Fig. 14. It is shown that oil occupied infinite number of very small pores in epoxy matrix. When sliding began oil flew on the sliding surface and provided thin film of lubricant. This proposed mechanism of lubrication changed the dry sliding into lubricated one. The amount of oil depended on the filling content, where it was observed that the optimum oil content was 20 wt. % which gave hydrodynamic lubrication. minimum wear. Increasing oil content caused wear increase due to the reduced adhesion between sand polymeric fibres and epoxy matrix. The role of polymeric fibres was to reinforce the matrix and enhance its wear resistance. Besides, fibres could decrease friction coefficient by removing the transferred epoxy layer formed on the steel surface causing also wear decrease. This behavior can be explained on the fact that friction coefficient displayed by epoxy sliding against steel is relatively higher than that observed for PA6 and PET sliding against steel. CONCLUSIONS 1. Friction coefficient showed consistent trend with increasing sliding velocity and decreasing load. 2. As the oil content increased friction coefficient decreased. 3. When PA fibres content increased, friction coefficient showed relatively higher values at lower velocity and higher load. 4. Composites filled by 20 wt. % oil, displayed the lowest friction in the hydrodynamic regime. 5. Wear of the tested composites reinforced by PA fibres drastically decreased with increasing PA content. Epoxy filled by 20 wt. % oil displayed the lowest wear followed by that filled by 10 wt. % oil. 6. Friction coefficient displayed by the tested composites reinforced by PET fibres showed slight increase at the mixed lubrication regime compared to that reinforced by PA fibres. Fig. 14. Action mechanism of the proposed porous epoxy composites. 7. Composites filled by 20 and 30 wt. % oil displayed the lowest friction coefficient. 8. Wear of the tested composites reinforced by PET fibres decreased with increasing PET content. Wear values were higher than that observed for composites reinforced by PA fibres. Composites filled by 20 wt. % oil showed the lowest wear. REFERENCES [1] Benson, J., Highly Porous Polymers, American Laboratory May 2003, (2003). [2] Barby, D. and Haq, Z., Low Density Porous Cross Linked Polymeric Materials and Their Preparation and Use as Carriers for Included Liquids, U.S. patent no. 4,522,953, (1985). [3] Hong, L. and Benson, J., Polymeric Microbeads and Method of Preparation, U.S. patent no. 5,583,162, (1996). [4] Landgraf W., Hong, L. and Benson J., Polymer Microcarrier Exhibiting Zero Order Release, Drug Deliv Technol, 3(1), (2003). [5] Kumar, V., Porous metal Bearings - A Critical Review, Wear, Vol. 62, pp , (1980). [6] El-Mahdey, M. R., Investigation into the Lubrication of Porous Journal Bearing M. Sc. Thesis, Faculty of Eng., Cairo Univ., Cairo, (1980). [7] Malik, M., Sinhasan, R. and Chandra, M., Finite Element Analysis of a porous Slider, Wear, Vol. 76, pp. 1-13, (1983). [8] Singh, K. C., Rao, N. S. and Majumdar, B. C., Effects of Velocity Slip, Anisotropy and tilt on the Steady State Performance of Aerostatic Porous Annular Thrust Bearing, Wear, Vol. 97, pp , (1984). [9] Chattopadhyay, A. K. and Majumdar, B. C., On the Stability of Rigid Rotor in Finite Porous Journal Bearings with Slip, ASME J. Tribology, Vol. 108, pp , (1986). [10] Kumar, A. and Rao, N. S., Turbulent Hybrid Bearings with Porous Bush: A Steady State Performance, Wear, Vol. 154, pp , (1992). [11] Kumar, A. and Rao, N. S., Stability of a Rigid Rotor in Turbulent Hybrid Porous Journal Bearings, Tribology Int., Vol. 27, pp , (1994). [12] Nikas, G. K. and Sayles, R. S., A Study of Lubrication Mechanisms Using Two-Phase Fluids with Porous Bearing Materials, Imperial College, EPSRC, Grant GR/89658, (1998).

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