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Wear and friction behaviour of soft particles filled random direction short GFRP composites
Materials Science and Engineering A 458 (2007) 25–33
Wear and friction behaviour of soft particles filled random direction short GFRP composites V.K. Srivastava ∗ , S. Wahne Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India Received 12 September 2006; received in revised form 5 December 2006; accepted 25 January 2007
Abstract The random direction short E-glass fibre reinforced epoxy resin composites filled with the particles of mica and tricalcium phosphate (TCP) were prepared by hand lay-up method. The wear and friction behaviour of random direction short E-glass fibre reinforced epoxy resin (GFRP) composites sliding against AISI-1045 steel in a pin-on-disc configuration were evaluated on a TR-20LE wear and friction tester. The microhardness, density, tensile strength and compressive strength of the filled and unfilled mica as well as TCP particles were determined. The morphology of the worn surfaces of the unfilled and filled random E-glass fibre composites and the transfer films were analyzed with the scanning electron microscope. It was found that the particles as the fillers contributed significantly to improve the mechanical properties and wear resistance of the E-glass fibre. This was because the particulates as the fillers contributed to enhance the bonding strength between the fibre and the epoxy resin. Moreover, the wear and friction properties of the random E-glass fibre composites were reduced by increasing filler weight of particles. © 2007 Elsevier B.V. All rights reserved. Keywords: Random E-glass composites; Microhardness; Fillers; Wear and friction
1. Introduction Polymer–matrix composites have many applications in industry as a class of tribo-engineering materials [1–4]. However, the tribological application of polymers and polymer– matrix composites is limited because of their relatively poor thermal stabilities and wear resistance [5,6]. Therefore, efforts have been made to increase the wear resistance of polymers. The reinforcing with carbon fibre has been found to be effective in improving the tribological and mechanical properties of various polymer-based composites [7,8]. The wide use of carbon fibre as the reinforcing agent of polymers is not only attributed to its high strength and modulus but also to its excellent thermal stability. It is anticipated that the plain carbon fabric made of carbon fibre would have better mechanical and tribological properties than the carbon fibre, because of the orderly aligned structure, good integrity, and good load-carrying capacity of the carbon fabric as compared with the carbon fibre counterpart. This could account for the increased focus on the friction and wear properties of polymer-based composites such as carbon/carbon composites
∗
Corresponding author. Tel.: +91 542 2315619; fax: +91 542 2368174. E-mail address: vk [email protected] (V.K. Srivastava).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.096
reinforced with fabric [9–12]. However, little has been currently reported on the friction and wear behaviour of the fabric composites, though some of the fabric composites have been done by applying in the areas of aerospace and aviation industries [13–16]. Of these applications, it has been found that the fabric bearings made of a fabric surface and metal matrix in the presence of a polymeric adhesive have good self-lubricity, anti-wear ability, and load-carrying capacity, apart from the low density, which is essential to the application in many high-tech areas such as aerospace, aviation, automobile, etc. However, the brittleness and modest anti-wear ability of carbon fabric still hinders its tribological application. The tribological application of glass fibre reinforced plastic (GFRP) composite is not very effective because high amount of temperature is induced within the counter parts of the sliding surfaces. Therefore, it is imperative to seek for the effective ways to enhance the mechanical and tribological properties of carbon fabric composites, so as to increase its applicability in the bearing industry where the integration and multi-functionalization of the bearings made of various composites are of particular interest. Many inorganic particles have been used as the fillers to modify the polymer–matrix composites for this purpose [17–19]. However, little has been so far on the modification of random direction E-glass fabric by various particulates. In view of this, we selected mica and tricalcium
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2. Experiments
TCP powder obtained in this way is -TCP (Ca3 (PO4 )2 ). The chemical interaction between tricalcium phosphate and polymer enhances the mechanical strength of composite. Hardness also increases with inclusion of small amount of tricalcium phosphate. In the present work, extra pure tricalcium phosphate with a particle size of 25 m is used which was obtained from Himedia Laboratories Ltd., Mumbai, India.
2.1. Epoxy resin
2.5. Preparation of specimens
Polymeric materials gain importance owing to their advantageous mechanical properties. One such polymer is epoxy resin, which has good resistance to alkalis and good adhesive properties owing to the cross-linking chain between the resin polymer and the hardener. The epoxy resin is a viscous liquid, and the viscosity is a function of degree of polymerization. Each epoxy molecule is end-capped with the epoxy group. A curing agent (hardener) is mixed in the liquid epoxy to polymerize the polymer and form a solid network cross-linked polymer. The type of epoxy resin used in present investigation is CY-205 while the hardener HY-951 was obtained from Ciba-Geigy Ltd., India.
Randomly oriented short fibre (length 6 cm) composites are made from epoxy resin and E-glass fibre. Epoxy resin is mixed with hardness in the ratio of 10:1 by weight. To avoid the formation of bubbles by liberated CO2 in resin, it was degassed in vacuum at 110 ◦ C before use. The required amount of filler material mica/tricalcium phosphate was added to epoxy and then they were mixed to get a macroscopically homogeneous mixture. Then a small amount of this mixture was poured in mould so that a layer of it was formed. A layer of randomly oriented fibre was put in the mould. This procedure is repeated with several layers of fibres to get the required thickness. The castings were cured overnight at room temperature and post-cured at 90 ◦ C for 6 h in an oven. Also, composite rods are moulded with a glass tube of 8 mm diameter and 30 mm length. Different specimens were prepared for wear and friction test. Using these composites, the specimens for hardness, density, tensile, compressive, friction and wear test were prepared according to the prescribed standards.
phosphate (TCP) particles to modify random orientation E-glass fibre so as to endow it with good self-lubricating and increased mechanical strengths and wear resistance. The present work is expected to broaden the application of E-glass fibre composites in dry-sliding bearings.
2.2. Glass fibre Glass fibres are most common reinforcing fibre. The principal advantages of glass fibres are the low cost and high strength. Glass fibre increases mechanical strength, impact resistance, stiffness and dimensional stability of a resin. Two forms of fibreglass can be produced by continuous fibre and staple (discontinuous) fibre. Both forms are made by the same production method up to the fibre drawing stage. Glass fibres are amorphous solids. Chemically, glass is composed primarily of a silica (SiO2 ) back bone in the form of (–SiO4 –)n tetrahedra. Modified ions are added for their contribution to glass properties and manufacturing capability. In the present investigation, random orientation short (length 6 cm) E-glass fibre is used which was obtained from Fibre Glass Pilkington Ltd., India. 2.3. Mica Mineral-filled epoxy resins have been used in a variety of applications for controlling the viscosity, differential thermal expansion and isothermal shrinkage of the cured material. Mica is a solid lubricant, which separates moving surface under boundary lubrication conditions. It is well known that freshly cleaved faces of mica adhere very strongly to one another. The adhesiveness decreases with time of exposure of the mica to the atmosphere. Mica is a plate like crystalline aluminosilicate and has been used widely as reinforcing filler in polymeric matrices due to its excellent mechanical, electrical and thermal properties. In this work, mica powder with a particle size of 60 m is used which was obtained from India Rare Earth Ltd., India. 2.4. Tricalcium phosphate Tricalcium phosphate powder is generally synthesized by a wet method using Ca(OH)2 and H3 Po4 as starting material. The
2.6. Evaluation of microhardness and mechanical properties of random composites Microhardness test is conducted on SHIMADZU MICROHARDNESS TESTER (HMV-2T). Indenter is made of diamond in the form of a squared-based pyramid with an included angle of 136◦ between opposite faces. The hardness tester is semiautomatic in which the specimen surface is brought close to indenter. The preset load is applied for some definite time and load is removed automatically. The loads are slowly applied to avoid error due to inertia effects. The time of load application and load duration can be controlled. In the present study, the load ranging from 1 to 10 N is applied for a duration of 5 s. The deformations of surfaces at different loads are shown in Fig. 1. Vicker’s hardness number is calculated using the following equation Hv = 0.1891 and, d=
P d2
H +V 2
(1)
(2)
where P is applied load (N), d the diagonal of square impression (mm), and H and V are the horizontal length (mm) and vertical length (mm), as shown in Fig. 2. The tensile and compressive strength of particles filled and unfilled random orientation short E-glass fibre composites spec-
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Fig. 1. Indentation of Vicker’s hardness indenter.
imen were prepared according to the ASTM D3039-76 standard. The tensile and compressive tests were performed on the universal instron testing machine at a constant speed of 10 mm/min. The load–displacement results were analyzed to calculate the tensile and compressive strength of composite samples. 2.7. Measurement of wear and friction test The aim of the present work is to improve the friction and wear properties of randomly oriented short E-glass fibre rein-
Fig. 2. Vicker’s microhardness indentation.
forced epoxy resin by adding mica particles and Tri Calcium Phosphate particles. The “friction and wear monitor TR-ZOLE”; a pin on disc type machine (Fig. 3) is used to conduct the experiments. The wear and friction monitor TR-20LE is a pin-on-disk type with facilities to monitor wear and friction under dry condition. This is a sturdy versatile machine, which facilitates the study of friction, and wears characteristics in sliding contacts under desired conditions. Sliding occurs between the stationary pin and a rotating disc. Normal load, rotational speed and wear track diameter can be varied to suit the test conditions. Tangential frictional force and wear are monitored with electronic sensors and recorded. These parameters are available as a
Fig. 3. Photograph of friction and wear monitor machine.
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function of load, speed or lubrication condition for continuous monitoring. Finally, the morphology of the worn composite surfaces was analyzed on a JSM-5600 LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Mechanical behaviours of the composites Fig. 4 shows that the density of random orientation short Eglass fibre reinforced epoxy resin composite reduces with the increase of mica and TCP particles weight. In addition, mica filled GFRP sample gives the lower density values than the TCP filled GFRP sample. The variation in the observed behaviour is attributed to increase of Vicker’s microhardness. This indicates that the formation of air bubbles and voids is practically reduced, thereby increasing the adhesive bonding strength of epoxy resin. Figs. 5 and 6 indicate that the Vicker’s microhardness of GFRP increases with the increase of filler weight and load. However, mica filled GFRP composite gives higher values than the GFRP and TCP filled GFRP composites. This clearly shows that the tensile strength and compressive strength are influenced by the homogeneity of microstructure, which reduces the tensile strength of GFRP and increases the compressive strength of GFRP with addition of mica and TCP filler
Fig. 6. Variation of Vicker’s microhardness with the load.
as shown in Figs. 7 and 8. Such behaviours are observed to increase as the amount of mica and TCP particles increases to 4% by weight. After this value, these properties deteriorate with increase in proportion of filler particles. Although the load is very efficiently transferred to the dispersed phase, and hence the epoxy resin and the fibre have to carry only a part of the load, the material is completely brittle and no yielding of the matrix is observed. Even through the dispersed phase, it does not take any shear stress [19]. It is supposed that the micro particles of mica and TCP of high surface energy help to strengthen
Fig. 4. Variation of density with filler weight.
Fig. 7. Variation of tensile strength with filler weight.
Fig. 5. Variation of Vicker’s microhardness with filler weight.
Fig. 8. Variation of compressive strength with filler weight.
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Fig. 9. Variation of weight loss with sliding velocity of 5 g TCP filled GFRP composite with different load.
the interface bonding between the glass fibre and the adhesive resin, which contribut to increase the compressive strengths and microhardness of GFRP composites and hence to increase the wear resistance. 3.2. Wear and friction behaviours of the composites It is observed that the weight loss of mica or TCP filled GFRP composites is saturated after the 4 g of particles and becomes constant up to 5 g particles. Therefore, 5 g mica or TCP filled GFRP composites are considered for the measurements of the wear and friction results. The dependence of weight loss of the specimen containing 5 g concentration of TCP and mica particles in epoxy resin reinforced random orientation short Eglass fibre, on different load and sliding velocity are shown in Figs. 9 and 10. The results show that the weight loss increases with load and sliding velocity. It is seen that all the filled random E-glass fibre composites show better weight loss-reducing ability than the unfilled one, and mica as the filler is most effective in decreasing the weight loss. At the same time, mica and TCP filled random GFRP composites have also better wear resistance than the unfilled GFRP composite as well, and the wear rate is reduced by 30–68%, respectively, by the addition of mica and TCP micro particles, as can be seen from Figs. 11 and 12. This shows that the wear rate is reduced for filled GFRP composites,
Fig. 10. Variation of weight loss with sliding velocity of 5 g mica filled GFRP composite at different loads.
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Fig. 11. Variation of wears volume with sliding distance for different filler weight of mica at sliding speed 2 m/s and load 70 N.
sliding against the steel pin at a fixed velocity 2 m/s and load 70 N and at room temperature. All the composites filled with mica and TCP micro particles show better load-carrying capacity than the unfilled GFRP composite, which confirms the better wear resistance of the filled GFRP composites than the unfilled one. This is because, when there is contact between metal and epoxy, the surface is sheared and ploughed. This shearing and ploughing take place more or less at the same rate. This gives rise to loss of metallic particles from the counterface. These particles are then assumed to oxidize, ultimately resulting in an iron oxide surface, and thse represent an increased wear rate. However, the presence of fibre in epoxy resin increases the flow stress of the matrix, thus tending to increase the wear resistance [18]. When GFRP composites are filled with mica or TCP particles, the wear is at slower rate in the beginning which almost stabilizes at a later time. This is because, when particles filled GFRP composites are in contact with the disc, the surface is sheared giving rise to loose mica or TCP particles, and starts forming a film at the metal surface. The loose particles at the contact interface now start abrading the surface together with the shearing. These abrading particles give rise to more loose particles and in this way a higher wear rate of the surface starts in a cumulative fashion. Fig. 13 shows the effect of sliding time on the friction coefficient of the 5 g filled mica and TCP particles filled GFRP composites at different sliding speed and load. It is seen that
Fig. 12. Variation of wears volume with sliding distance of different filler weight of TCP at sliding speed 2 m/s and 70 N.
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Fig. 16. SEM picture of Glass Epoxy + 5 g TCP filled GFRP sample at load 70 N load and sliding velocity 2 m/s. Fig. 13. Variation of coefficient of friction with sliding time at different load 70 N and sliding speed.
Fig. 14. Variation of coefficient of friction with load at different sliding speed at different sliding speed.
the inclusion of micro size mica particulate leads to lower values of friction coefficient than the TCP filled composites. The result clearly indicates the coefficient of friction increases with increase of sliding time up to 2 h, then reduces with increase of sliding time and stabilizes after the 4 h. The same behaviours are observed in Figs. 14 and 15, when friction coefficient of 5 g mica and TCP filled GFRP composites is measured against the different load and sliding speed. These results show that the friction
Fig. 15. Variation of coefficient of friction with sliding speed at different load.
coefficient of 5 g TCP filled GFRP composite decreases with increase of load and increases with increase of sliding speed, whereas mica filled GFRP composite gives lower value than the TCP filled GFRP composite and increases with increase of load and sliding speed. This is because there is contact between metal and epoxy resin containing mica and TCP particles. Filler particles are removed and form a transfer lubricating film on the disc. As a result of formation of the film, there is mow contact between transfer film and epoxy resin containing particles. This transfer film on the disc reduces the mechanical interlocking at the counter interface, thus reducing the friction coefficient. However, in the case of mica filled GFRP composites, the coefficient of friction is relatively lower than the TCP filled GFRP composites, because mica particles transfer thick lubricating films on the counterface, which reduces the friction coefficient. 3.3. Morphology of worn surfaces The SEM morphology of the worn surfaces of the counterpart 5 g TCP filled GFRP pins sliding against the steel disc at load 70 N and sliding speed 2 m/s is shown in Fig. 16. It is seen that the debris formation and broken fibres take place. Keeping the load constant, but increasing the sliding velocity leads to appearance of smaller sized fibres with matrix debris well spread out (Fig. 17). Keeping the sliding velocity constant, when we decrease the load to 30 N, it yields a sample with good spread of the matrix, cracks in the matrix and fewer debris (Fig. 18). A rise in the velocity (5 m/s) at this load results in greater wear of the matrix and exposure of the fibres (Fig. 19). On the other hand 2 g mica filled GFRP sample in Fig. 20 shows lesser spread of the matrix debris and longer fibres compared to 5 g TCP filled GFRP as can be seen from Fig. 16. Fig. 21 displays large fibres similar to fibres seen in the 2 g mica filled GFRP sample subjected to a lower sliding velocity of 2 m/s (Fig. 20). The length of these fibres appears longer compared to the fibres observed for 5 g TCP filled GFRP sample under corresponding test conditions (Fig. 17). Lowering of load and velocity in 5 g mica filled GFRP sample yields surfaces, which show a less smearing wear of the matrix region (Fig. 22) compared to 5 g TCP filled GFRP samples (Fig. 18). Also, the
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Fig. 17. SEM picture of 5 g TCP filled GFRP sample at higher sliding velocity 5 m/s and load 70 N.
Fig. 18. SEM photograph of 5 g TCP filled GFRP sample at lower load 30 N and sliding velocity 2 m/s velocity.
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Fig. 20. SEM picture of 2 g mica filled GFRP sample at high load 70 N and low sliding velocity 2 m/s.
Fig. 21. SEM picture of 2 g mica filled GFRP sample at higher sliding velocity 5 m/s and load 70 N.
matrix has a tendency to show dark-colored matrix zones at the higher velocities (Fig. 23) compared to the lower velocities (Fig. 22). The coloration change in 2 g mica filled GFRP samples, not noticed in type 5 g TCP filled GFRP samples, point towards the difference in the material tribological behaviour, i.e., sliding wear and coefficient of friction characteristics.
However, 5 g mica filled GFRP samples also show spreading of the matrix with exposed long fibres (Fig. 24). Even with a rise in velocity, long fibres are retained despite some tendencies to fragment (Fig. 25). Darkening of the resin, a feature seen for 2 g mica filled GFRP samples, is noticed in some areas (for example, in the lower right region of Fig. 24). Also comparing 2 g mica filled GFRP and 5 g mica filled GFRP samples, the
Fig. 19. SEM picture of 5 g TCP filled GFRP sample at sliding velocity 5 m/s and load 30 N.
Fig. 22. SEM picture of 2 g mica filled GFRP sample at load 30 N and lower sliding velocity 2 m/s.
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Fig. 23. SEM picture of 2 g mica filled GFRP sample at 30 N load and higher sliding velocity 5 m/s.
Fig. 24. SEM picture of 5 g mica filled GFRP sample exhibiting spread of the matrix on lengthy fibres despite the use of higher load 70 N and lower sliding velocity 2 m/s.
extent of wear is less in the latter, which is supported by the features on the surface of the sample, where lesser de-laminated worn conditions are noticed (Fig. 24). As for the high load 70 N and high sliding speed 5 m/s, it is seen that the 5 g TCP filled GFRP sample shows broken fibres and greater debris (Fig. 17) than the 2 g mica filled GFRP sample (Fig. 21), while the debris and the length of the fragmented fibres
Fig. 25. SEM picture of 5 g mica filled GFRP sample showing features of load 70 N subjected to higher sliding velocity 5 m/s displaying least matrix debris formation and some fibre fragmentation tendencies.
are the least for the 5 g mica filled GFRP sample (Fig. 25). Also noticed are hardly any breakages of fibre with very less matrix wear in addition to network of very small cracks at 30 N load and 2 m/s sliding velocity for 5 g mica filled GFRP samples. These observations are consistent with the experimental weight loss data presented earlier [19]. A decreasing wears loss with increasing amount of mica in GFRP system is observed. To explain the reduced wear in 2 g mica filled GFRP compared to 5 g TCP filled GFRP samples, the abilities of mica to form a film (lubricant) layer by reducing frictional drag forces can be invoked. Indeed, mica with the basal plane shear is known to be good solid lubricants. Mica forms a film, which reduces the wear as seen in 2 g mica filled GFRP composite. This is further supported with the data obtained for 5 g mica filled GFRP samples. The 5 g mica filled samples containing higher amount of mica of two layers have a lower coefficient of friction as well as lower wear loss values. The greater ease of film formation assists in lubricating the contact surface thereby causing less wear of the material. This is supported by lower friction coefficient obtained for 5 g filled GFRP sample compared to 2 g mica filled GFRP composites. The change in the pattern of friction with either sliding velocity or load, being different compared to 5 g TCP filled GFRP can be ascribed to the frictional heat [17] and attendant property changes in mica and to the optimum level of mica film thickness above which the film layer formed may have differing response to the operating conditions. In the present study, for the velocity range 2–5 m/s, the friction coefficient values increased steadily with load for 2 g mica filled GFRP sample. Incidentally, it was noticed that removal of debris formed during experiments contributed to increased values of friction coefficient, thus supporting the contention that thickness of already formed mica layer during the build up and its retention on the surface during continuous contact with the steel counter face has an important effect on the friction and wear performance of the GFRP composites. 4. Conclusions 1. Vicker’s microhardness, and compressive strength of random orientation short E-glass fibre reinforced epoxy resin composite increases with addition of filler material. This is because mica and TCP (tricalcium phosphate) particles resist the initiation of the cracks along the fibre direction, whereas density and tensile strength of GFRP composites decreases with increase of filler weights. 2. GFRP composites when loaded with mica or TCP particles exhibits lower wear loss. This value further drops down as the content of fillers in the composite is raised because fillers like mica and TCP act as self-lubricating materials. The wear rate was found to be more efficient in case of TCP than mica because loose fragments generated in case of mica acts as very soft and solid lubricants thereby improving the wear properties. On the other hand, fragments generated in case of TCP are not effective. 3. In the case of mica, the effect of load on coefficient of friction is not significant. However, at higher loads there is an increase
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in coefficient of friction with load whereas in case of TCP coefficient of friction decreases with increase in load. The value of coefficient of friction increases with sliding time and reaches an optimum value beyond which it decreases. This occurs due to the effect of the temperature between the mating surfaces and mechanical interlocking at the interface. The value of coefficient of friction is not much influenced by sliding speed when mica is used as filler material. However, it increases with increase in sliding speed in case of TCP. References [1] L. Zsidai, P.D. Baets, P. Samyn, G. Kalacska, A.P.V. Peteghem, F.V. Parys, Wear 253 (2002) 673–688. [2] J.A. Jia, H.D. Zhou, S.Q. Gao, J.M. Chen, Mater. Sci. Eng. A 356 (2003) 48–53. [3] Z.Z. Zhang, Q.J. Xue, W.M. Liu, Shen WC, J. Appl. Polym. Sci. 76 (2000) 1240–2124.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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J. Khedkar, I. Negulescu, E.I. Meletis, Wear 252 (2002) 361–369. Q.H. Wang, Q.J. Xue, W.M. Liu, J.M. Chen, Wear 243 (2000) 140–146. Y.J. Yamamoto, M.S. Hashimoto, Wear 257 (2004) 181–189. ¨ J. FlOck, K. Friedrich, Q. Yuan, Wear 225–229 (1999) 304–311. Y.J. Wang, D.M. Zhao, Composites 26 (1995) 115–124. H. Pihtili, N.H. Tosum, Wear 252 (2002) 979–984. N.R.L. Pearce, J. Summerscales, F.J. Guild, Composites Part A 31 (2000) 1433–1441. J. Bijwe, J. Indumathi, A.K. Ghosh, Wear 256 (2004) 27–37. H.K. Shin, H.B. Lee, K.S. Kim, Carbon 39 (2001) 959–970. R.Y. Luo, X.L. Huai, J.W. Qu, H.Y. Ding, S.H. Xu, Carbon 41 (2003) 2693–2702. F.H. Su, Z.Z. Zhang, W.M. Liu, Mater. Sci. Eng. A 392 (2005) 359–365. T. Iizuka, H. Kita, T. Hirai, K. Osumi, Wear 257 (2004) 953–961. G. Shi, M.Q. Zhang, M.Z. Rong, B. Wetzel, K. Friedrich, Wear 256 (2004) 1072–1081. G. Shi, M.Q. Zhang, M.Z. Rong, B. Wetzel, K. Friedrich, Wear 254 (2003) 784–796. F.H. Su, Z.Z. Zhang, W.M. Liu, Wear 260 (2006) 861–868. V.K. Srivastava, J.P. Pathak, K. Tahzibi, Wear 152 (1992) 343–350.
Wear and friction behaviour of soft particles filled random direction short GFRP composites V.K. Srivastava ∗ , S. Wahne Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India Received 12 September 2006; received in revised form 5 December 2006; accepted 25 January 2007
Abstract The random direction short E-glass fibre reinforced epoxy resin composites filled with the particles of mica and tricalcium phosphate (TCP) were prepared by hand lay-up method. The wear and friction behaviour of random direction short E-glass fibre reinforced epoxy resin (GFRP) composites sliding against AISI-1045 steel in a pin-on-disc configuration were evaluated on a TR-20LE wear and friction tester. The microhardness, density, tensile strength and compressive strength of the filled and unfilled mica as well as TCP particles were determined. The morphology of the worn surfaces of the unfilled and filled random E-glass fibre composites and the transfer films were analyzed with the scanning electron microscope. It was found that the particles as the fillers contributed significantly to improve the mechanical properties and wear resistance of the E-glass fibre. This was because the particulates as the fillers contributed to enhance the bonding strength between the fibre and the epoxy resin. Moreover, the wear and friction properties of the random E-glass fibre composites were reduced by increasing filler weight of particles. © 2007 Elsevier B.V. All rights reserved. Keywords: Random E-glass composites; Microhardness; Fillers; Wear and friction
1. Introduction Polymer–matrix composites have many applications in industry as a class of tribo-engineering materials [1–4]. However, the tribological application of polymers and polymer– matrix composites is limited because of their relatively poor thermal stabilities and wear resistance [5,6]. Therefore, efforts have been made to increase the wear resistance of polymers. The reinforcing with carbon fibre has been found to be effective in improving the tribological and mechanical properties of various polymer-based composites [7,8]. The wide use of carbon fibre as the reinforcing agent of polymers is not only attributed to its high strength and modulus but also to its excellent thermal stability. It is anticipated that the plain carbon fabric made of carbon fibre would have better mechanical and tribological properties than the carbon fibre, because of the orderly aligned structure, good integrity, and good load-carrying capacity of the carbon fabric as compared with the carbon fibre counterpart. This could account for the increased focus on the friction and wear properties of polymer-based composites such as carbon/carbon composites
∗
Corresponding author. Tel.: +91 542 2315619; fax: +91 542 2368174. E-mail address: vk [email protected] (V.K. Srivastava).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.096
reinforced with fabric [9–12]. However, little has been currently reported on the friction and wear behaviour of the fabric composites, though some of the fabric composites have been done by applying in the areas of aerospace and aviation industries [13–16]. Of these applications, it has been found that the fabric bearings made of a fabric surface and metal matrix in the presence of a polymeric adhesive have good self-lubricity, anti-wear ability, and load-carrying capacity, apart from the low density, which is essential to the application in many high-tech areas such as aerospace, aviation, automobile, etc. However, the brittleness and modest anti-wear ability of carbon fabric still hinders its tribological application. The tribological application of glass fibre reinforced plastic (GFRP) composite is not very effective because high amount of temperature is induced within the counter parts of the sliding surfaces. Therefore, it is imperative to seek for the effective ways to enhance the mechanical and tribological properties of carbon fabric composites, so as to increase its applicability in the bearing industry where the integration and multi-functionalization of the bearings made of various composites are of particular interest. Many inorganic particles have been used as the fillers to modify the polymer–matrix composites for this purpose [17–19]. However, little has been so far on the modification of random direction E-glass fabric by various particulates. In view of this, we selected mica and tricalcium
26
V.K. Srivastava, S. Wahne / Materials Science and Engineering A 458 (2007) 25–33
2. Experiments
TCP powder obtained in this way is -TCP (Ca3 (PO4 )2 ). The chemical interaction between tricalcium phosphate and polymer enhances the mechanical strength of composite. Hardness also increases with inclusion of small amount of tricalcium phosphate. In the present work, extra pure tricalcium phosphate with a particle size of 25 m is used which was obtained from Himedia Laboratories Ltd., Mumbai, India.
2.1. Epoxy resin
2.5. Preparation of specimens
Polymeric materials gain importance owing to their advantageous mechanical properties. One such polymer is epoxy resin, which has good resistance to alkalis and good adhesive properties owing to the cross-linking chain between the resin polymer and the hardener. The epoxy resin is a viscous liquid, and the viscosity is a function of degree of polymerization. Each epoxy molecule is end-capped with the epoxy group. A curing agent (hardener) is mixed in the liquid epoxy to polymerize the polymer and form a solid network cross-linked polymer. The type of epoxy resin used in present investigation is CY-205 while the hardener HY-951 was obtained from Ciba-Geigy Ltd., India.
Randomly oriented short fibre (length 6 cm) composites are made from epoxy resin and E-glass fibre. Epoxy resin is mixed with hardness in the ratio of 10:1 by weight. To avoid the formation of bubbles by liberated CO2 in resin, it was degassed in vacuum at 110 ◦ C before use. The required amount of filler material mica/tricalcium phosphate was added to epoxy and then they were mixed to get a macroscopically homogeneous mixture. Then a small amount of this mixture was poured in mould so that a layer of it was formed. A layer of randomly oriented fibre was put in the mould. This procedure is repeated with several layers of fibres to get the required thickness. The castings were cured overnight at room temperature and post-cured at 90 ◦ C for 6 h in an oven. Also, composite rods are moulded with a glass tube of 8 mm diameter and 30 mm length. Different specimens were prepared for wear and friction test. Using these composites, the specimens for hardness, density, tensile, compressive, friction and wear test were prepared according to the prescribed standards.
phosphate (TCP) particles to modify random orientation E-glass fibre so as to endow it with good self-lubricating and increased mechanical strengths and wear resistance. The present work is expected to broaden the application of E-glass fibre composites in dry-sliding bearings.
2.2. Glass fibre Glass fibres are most common reinforcing fibre. The principal advantages of glass fibres are the low cost and high strength. Glass fibre increases mechanical strength, impact resistance, stiffness and dimensional stability of a resin. Two forms of fibreglass can be produced by continuous fibre and staple (discontinuous) fibre. Both forms are made by the same production method up to the fibre drawing stage. Glass fibres are amorphous solids. Chemically, glass is composed primarily of a silica (SiO2 ) back bone in the form of (–SiO4 –)n tetrahedra. Modified ions are added for their contribution to glass properties and manufacturing capability. In the present investigation, random orientation short (length 6 cm) E-glass fibre is used which was obtained from Fibre Glass Pilkington Ltd., India. 2.3. Mica Mineral-filled epoxy resins have been used in a variety of applications for controlling the viscosity, differential thermal expansion and isothermal shrinkage of the cured material. Mica is a solid lubricant, which separates moving surface under boundary lubrication conditions. It is well known that freshly cleaved faces of mica adhere very strongly to one another. The adhesiveness decreases with time of exposure of the mica to the atmosphere. Mica is a plate like crystalline aluminosilicate and has been used widely as reinforcing filler in polymeric matrices due to its excellent mechanical, electrical and thermal properties. In this work, mica powder with a particle size of 60 m is used which was obtained from India Rare Earth Ltd., India. 2.4. Tricalcium phosphate Tricalcium phosphate powder is generally synthesized by a wet method using Ca(OH)2 and H3 Po4 as starting material. The
2.6. Evaluation of microhardness and mechanical properties of random composites Microhardness test is conducted on SHIMADZU MICROHARDNESS TESTER (HMV-2T). Indenter is made of diamond in the form of a squared-based pyramid with an included angle of 136◦ between opposite faces. The hardness tester is semiautomatic in which the specimen surface is brought close to indenter. The preset load is applied for some definite time and load is removed automatically. The loads are slowly applied to avoid error due to inertia effects. The time of load application and load duration can be controlled. In the present study, the load ranging from 1 to 10 N is applied for a duration of 5 s. The deformations of surfaces at different loads are shown in Fig. 1. Vicker’s hardness number is calculated using the following equation Hv = 0.1891 and, d=
P d2
H +V 2
(1)
(2)
where P is applied load (N), d the diagonal of square impression (mm), and H and V are the horizontal length (mm) and vertical length (mm), as shown in Fig. 2. The tensile and compressive strength of particles filled and unfilled random orientation short E-glass fibre composites spec-
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Fig. 1. Indentation of Vicker’s hardness indenter.
imen were prepared according to the ASTM D3039-76 standard. The tensile and compressive tests were performed on the universal instron testing machine at a constant speed of 10 mm/min. The load–displacement results were analyzed to calculate the tensile and compressive strength of composite samples. 2.7. Measurement of wear and friction test The aim of the present work is to improve the friction and wear properties of randomly oriented short E-glass fibre rein-
Fig. 2. Vicker’s microhardness indentation.
forced epoxy resin by adding mica particles and Tri Calcium Phosphate particles. The “friction and wear monitor TR-ZOLE”; a pin on disc type machine (Fig. 3) is used to conduct the experiments. The wear and friction monitor TR-20LE is a pin-on-disk type with facilities to monitor wear and friction under dry condition. This is a sturdy versatile machine, which facilitates the study of friction, and wears characteristics in sliding contacts under desired conditions. Sliding occurs between the stationary pin and a rotating disc. Normal load, rotational speed and wear track diameter can be varied to suit the test conditions. Tangential frictional force and wear are monitored with electronic sensors and recorded. These parameters are available as a
Fig. 3. Photograph of friction and wear monitor machine.
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function of load, speed or lubrication condition for continuous monitoring. Finally, the morphology of the worn composite surfaces was analyzed on a JSM-5600 LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Mechanical behaviours of the composites Fig. 4 shows that the density of random orientation short Eglass fibre reinforced epoxy resin composite reduces with the increase of mica and TCP particles weight. In addition, mica filled GFRP sample gives the lower density values than the TCP filled GFRP sample. The variation in the observed behaviour is attributed to increase of Vicker’s microhardness. This indicates that the formation of air bubbles and voids is practically reduced, thereby increasing the adhesive bonding strength of epoxy resin. Figs. 5 and 6 indicate that the Vicker’s microhardness of GFRP increases with the increase of filler weight and load. However, mica filled GFRP composite gives higher values than the GFRP and TCP filled GFRP composites. This clearly shows that the tensile strength and compressive strength are influenced by the homogeneity of microstructure, which reduces the tensile strength of GFRP and increases the compressive strength of GFRP with addition of mica and TCP filler
Fig. 6. Variation of Vicker’s microhardness with the load.
as shown in Figs. 7 and 8. Such behaviours are observed to increase as the amount of mica and TCP particles increases to 4% by weight. After this value, these properties deteriorate with increase in proportion of filler particles. Although the load is very efficiently transferred to the dispersed phase, and hence the epoxy resin and the fibre have to carry only a part of the load, the material is completely brittle and no yielding of the matrix is observed. Even through the dispersed phase, it does not take any shear stress [19]. It is supposed that the micro particles of mica and TCP of high surface energy help to strengthen
Fig. 4. Variation of density with filler weight.
Fig. 7. Variation of tensile strength with filler weight.
Fig. 5. Variation of Vicker’s microhardness with filler weight.
Fig. 8. Variation of compressive strength with filler weight.
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Fig. 9. Variation of weight loss with sliding velocity of 5 g TCP filled GFRP composite with different load.
the interface bonding between the glass fibre and the adhesive resin, which contribut to increase the compressive strengths and microhardness of GFRP composites and hence to increase the wear resistance. 3.2. Wear and friction behaviours of the composites It is observed that the weight loss of mica or TCP filled GFRP composites is saturated after the 4 g of particles and becomes constant up to 5 g particles. Therefore, 5 g mica or TCP filled GFRP composites are considered for the measurements of the wear and friction results. The dependence of weight loss of the specimen containing 5 g concentration of TCP and mica particles in epoxy resin reinforced random orientation short Eglass fibre, on different load and sliding velocity are shown in Figs. 9 and 10. The results show that the weight loss increases with load and sliding velocity. It is seen that all the filled random E-glass fibre composites show better weight loss-reducing ability than the unfilled one, and mica as the filler is most effective in decreasing the weight loss. At the same time, mica and TCP filled random GFRP composites have also better wear resistance than the unfilled GFRP composite as well, and the wear rate is reduced by 30–68%, respectively, by the addition of mica and TCP micro particles, as can be seen from Figs. 11 and 12. This shows that the wear rate is reduced for filled GFRP composites,
Fig. 10. Variation of weight loss with sliding velocity of 5 g mica filled GFRP composite at different loads.
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Fig. 11. Variation of wears volume with sliding distance for different filler weight of mica at sliding speed 2 m/s and load 70 N.
sliding against the steel pin at a fixed velocity 2 m/s and load 70 N and at room temperature. All the composites filled with mica and TCP micro particles show better load-carrying capacity than the unfilled GFRP composite, which confirms the better wear resistance of the filled GFRP composites than the unfilled one. This is because, when there is contact between metal and epoxy, the surface is sheared and ploughed. This shearing and ploughing take place more or less at the same rate. This gives rise to loss of metallic particles from the counterface. These particles are then assumed to oxidize, ultimately resulting in an iron oxide surface, and thse represent an increased wear rate. However, the presence of fibre in epoxy resin increases the flow stress of the matrix, thus tending to increase the wear resistance [18]. When GFRP composites are filled with mica or TCP particles, the wear is at slower rate in the beginning which almost stabilizes at a later time. This is because, when particles filled GFRP composites are in contact with the disc, the surface is sheared giving rise to loose mica or TCP particles, and starts forming a film at the metal surface. The loose particles at the contact interface now start abrading the surface together with the shearing. These abrading particles give rise to more loose particles and in this way a higher wear rate of the surface starts in a cumulative fashion. Fig. 13 shows the effect of sliding time on the friction coefficient of the 5 g filled mica and TCP particles filled GFRP composites at different sliding speed and load. It is seen that
Fig. 12. Variation of wears volume with sliding distance of different filler weight of TCP at sliding speed 2 m/s and 70 N.
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Fig. 16. SEM picture of Glass Epoxy + 5 g TCP filled GFRP sample at load 70 N load and sliding velocity 2 m/s. Fig. 13. Variation of coefficient of friction with sliding time at different load 70 N and sliding speed.
Fig. 14. Variation of coefficient of friction with load at different sliding speed at different sliding speed.
the inclusion of micro size mica particulate leads to lower values of friction coefficient than the TCP filled composites. The result clearly indicates the coefficient of friction increases with increase of sliding time up to 2 h, then reduces with increase of sliding time and stabilizes after the 4 h. The same behaviours are observed in Figs. 14 and 15, when friction coefficient of 5 g mica and TCP filled GFRP composites is measured against the different load and sliding speed. These results show that the friction
Fig. 15. Variation of coefficient of friction with sliding speed at different load.
coefficient of 5 g TCP filled GFRP composite decreases with increase of load and increases with increase of sliding speed, whereas mica filled GFRP composite gives lower value than the TCP filled GFRP composite and increases with increase of load and sliding speed. This is because there is contact between metal and epoxy resin containing mica and TCP particles. Filler particles are removed and form a transfer lubricating film on the disc. As a result of formation of the film, there is mow contact between transfer film and epoxy resin containing particles. This transfer film on the disc reduces the mechanical interlocking at the counter interface, thus reducing the friction coefficient. However, in the case of mica filled GFRP composites, the coefficient of friction is relatively lower than the TCP filled GFRP composites, because mica particles transfer thick lubricating films on the counterface, which reduces the friction coefficient. 3.3. Morphology of worn surfaces The SEM morphology of the worn surfaces of the counterpart 5 g TCP filled GFRP pins sliding against the steel disc at load 70 N and sliding speed 2 m/s is shown in Fig. 16. It is seen that the debris formation and broken fibres take place. Keeping the load constant, but increasing the sliding velocity leads to appearance of smaller sized fibres with matrix debris well spread out (Fig. 17). Keeping the sliding velocity constant, when we decrease the load to 30 N, it yields a sample with good spread of the matrix, cracks in the matrix and fewer debris (Fig. 18). A rise in the velocity (5 m/s) at this load results in greater wear of the matrix and exposure of the fibres (Fig. 19). On the other hand 2 g mica filled GFRP sample in Fig. 20 shows lesser spread of the matrix debris and longer fibres compared to 5 g TCP filled GFRP as can be seen from Fig. 16. Fig. 21 displays large fibres similar to fibres seen in the 2 g mica filled GFRP sample subjected to a lower sliding velocity of 2 m/s (Fig. 20). The length of these fibres appears longer compared to the fibres observed for 5 g TCP filled GFRP sample under corresponding test conditions (Fig. 17). Lowering of load and velocity in 5 g mica filled GFRP sample yields surfaces, which show a less smearing wear of the matrix region (Fig. 22) compared to 5 g TCP filled GFRP samples (Fig. 18). Also, the
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Fig. 17. SEM picture of 5 g TCP filled GFRP sample at higher sliding velocity 5 m/s and load 70 N.
Fig. 18. SEM photograph of 5 g TCP filled GFRP sample at lower load 30 N and sliding velocity 2 m/s velocity.
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Fig. 20. SEM picture of 2 g mica filled GFRP sample at high load 70 N and low sliding velocity 2 m/s.
Fig. 21. SEM picture of 2 g mica filled GFRP sample at higher sliding velocity 5 m/s and load 70 N.
matrix has a tendency to show dark-colored matrix zones at the higher velocities (Fig. 23) compared to the lower velocities (Fig. 22). The coloration change in 2 g mica filled GFRP samples, not noticed in type 5 g TCP filled GFRP samples, point towards the difference in the material tribological behaviour, i.e., sliding wear and coefficient of friction characteristics.
However, 5 g mica filled GFRP samples also show spreading of the matrix with exposed long fibres (Fig. 24). Even with a rise in velocity, long fibres are retained despite some tendencies to fragment (Fig. 25). Darkening of the resin, a feature seen for 2 g mica filled GFRP samples, is noticed in some areas (for example, in the lower right region of Fig. 24). Also comparing 2 g mica filled GFRP and 5 g mica filled GFRP samples, the
Fig. 19. SEM picture of 5 g TCP filled GFRP sample at sliding velocity 5 m/s and load 30 N.
Fig. 22. SEM picture of 2 g mica filled GFRP sample at load 30 N and lower sliding velocity 2 m/s.
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Fig. 23. SEM picture of 2 g mica filled GFRP sample at 30 N load and higher sliding velocity 5 m/s.
Fig. 24. SEM picture of 5 g mica filled GFRP sample exhibiting spread of the matrix on lengthy fibres despite the use of higher load 70 N and lower sliding velocity 2 m/s.
extent of wear is less in the latter, which is supported by the features on the surface of the sample, where lesser de-laminated worn conditions are noticed (Fig. 24). As for the high load 70 N and high sliding speed 5 m/s, it is seen that the 5 g TCP filled GFRP sample shows broken fibres and greater debris (Fig. 17) than the 2 g mica filled GFRP sample (Fig. 21), while the debris and the length of the fragmented fibres
Fig. 25. SEM picture of 5 g mica filled GFRP sample showing features of load 70 N subjected to higher sliding velocity 5 m/s displaying least matrix debris formation and some fibre fragmentation tendencies.
are the least for the 5 g mica filled GFRP sample (Fig. 25). Also noticed are hardly any breakages of fibre with very less matrix wear in addition to network of very small cracks at 30 N load and 2 m/s sliding velocity for 5 g mica filled GFRP samples. These observations are consistent with the experimental weight loss data presented earlier [19]. A decreasing wears loss with increasing amount of mica in GFRP system is observed. To explain the reduced wear in 2 g mica filled GFRP compared to 5 g TCP filled GFRP samples, the abilities of mica to form a film (lubricant) layer by reducing frictional drag forces can be invoked. Indeed, mica with the basal plane shear is known to be good solid lubricants. Mica forms a film, which reduces the wear as seen in 2 g mica filled GFRP composite. This is further supported with the data obtained for 5 g mica filled GFRP samples. The 5 g mica filled samples containing higher amount of mica of two layers have a lower coefficient of friction as well as lower wear loss values. The greater ease of film formation assists in lubricating the contact surface thereby causing less wear of the material. This is supported by lower friction coefficient obtained for 5 g filled GFRP sample compared to 2 g mica filled GFRP composites. The change in the pattern of friction with either sliding velocity or load, being different compared to 5 g TCP filled GFRP can be ascribed to the frictional heat [17] and attendant property changes in mica and to the optimum level of mica film thickness above which the film layer formed may have differing response to the operating conditions. In the present study, for the velocity range 2–5 m/s, the friction coefficient values increased steadily with load for 2 g mica filled GFRP sample. Incidentally, it was noticed that removal of debris formed during experiments contributed to increased values of friction coefficient, thus supporting the contention that thickness of already formed mica layer during the build up and its retention on the surface during continuous contact with the steel counter face has an important effect on the friction and wear performance of the GFRP composites. 4. Conclusions 1. Vicker’s microhardness, and compressive strength of random orientation short E-glass fibre reinforced epoxy resin composite increases with addition of filler material. This is because mica and TCP (tricalcium phosphate) particles resist the initiation of the cracks along the fibre direction, whereas density and tensile strength of GFRP composites decreases with increase of filler weights. 2. GFRP composites when loaded with mica or TCP particles exhibits lower wear loss. This value further drops down as the content of fillers in the composite is raised because fillers like mica and TCP act as self-lubricating materials. The wear rate was found to be more efficient in case of TCP than mica because loose fragments generated in case of mica acts as very soft and solid lubricants thereby improving the wear properties. On the other hand, fragments generated in case of TCP are not effective. 3. In the case of mica, the effect of load on coefficient of friction is not significant. However, at higher loads there is an increase
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in coefficient of friction with load whereas in case of TCP coefficient of friction decreases with increase in load. The value of coefficient of friction increases with sliding time and reaches an optimum value beyond which it decreases. This occurs due to the effect of the temperature between the mating surfaces and mechanical interlocking at the interface. The value of coefficient of friction is not much influenced by sliding speed when mica is used as filler material. However, it increases with increase in sliding speed in case of TCP. References [1] L. Zsidai, P.D. Baets, P. Samyn, G. Kalacska, A.P.V. Peteghem, F.V. Parys, Wear 253 (2002) 673–688. [2] J.A. Jia, H.D. Zhou, S.Q. Gao, J.M. Chen, Mater. Sci. Eng. A 356 (2003) 48–53. [3] Z.Z. Zhang, Q.J. Xue, W.M. Liu, Shen WC, J. Appl. Polym. Sci. 76 (2000) 1240–2124.
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