friction and slide wear characteristics of glass-epoxy...

Indian Journal of Engineering & Materials Sciences Vol. 13, December 2006, pp. 535-541

Friction and slide wear characteristics of glass-epoxy and glass-epoxy filled with SiCp composites

B Sureshaa*, G Chandramohana, P R Sadananda Raob, P Sampathkumaranc, S Seetharamuc & Vartha Venkateswarluc

aDepartment of Mechanical Engineering, PSG College of Technology, Coimbatore 641 004, India bDepartment of Mechanical Engineering, National Institute of Engineering, Mysore 570 008, India

cMaterials Technology Division, Central Power Research Institute, Bangalore 560 080, India

Received 5 October 2005; accepted 18 August 2006

This paper highlights the friction and wear behaviour of glass-epoxy (G-E) composites with and without silicon carbide particles (SiCp) filler content as a function of sliding distance, keeping the sliding velocity and applied load constant. It is seen that the wear rate increases with increasing sliding distance, but the gradient is not maintained the same all through. An attempt has also been made to correlate the wear loss of the worn surfaces using scanning electron microscopic (SEM) observations. The coefficient of friction is found to be almost same over a wide range of sliding distance employed. Further, the study indicates that the SiCp-G-E composites show lower coefficient of friction and lower slide wear loss compared to G-E composites irrespective of the sliding distance employed. It is found that during the running in wear, the wear of the resin mix as well as very few broken fibers are noticed. The breakage of fibers, the matrix debris formation and interface separation take place at a much later stage (i.e., in the severe wear region). In the steady state region some of the broken fibers are getting disoriented in the matrix and also agglomerations of the debris are seen. Other interesting SEM features have been noticed and discussed taking into account the addition of SiCp filler content (2.5 and 5.0 wt. %).

IPC Code: C03C 14/00, G01N 19/02

It is reported in the literature1 that the energy intensive processes involved during metal forming have encouraged many researchers to look for new and alternate type of materials. Among these, composites and fiber reinforced polymeric materials are the most attractive ones. They are used in many mechanical components such as gears, cams, wheels, impellers, brakes, seals, bushes and bearings2,3. One of the areas where their use is finding advantageous in situations involving contact wear. The other advantages seen are the diverse range of mechanical and tribological properties achievable using different reinforcement types, their orientation and varied volume fractions. The addition of fillers along with reinforcements give rise to improved properties such as higher load bearing capability, reduced coefficients of friction, improved wear resistance and thermal properties and higher mechanical strength. An internally lubricated composite with thermal stability and increased resistance to wear is an area wherein research is being attempted4,5.

The wear of polymeric composites with different reinforcements is the topic of investigation in recent

times6-11. One of the well-known composites that widely used is glass reinforced polymer material. The rigidity and strength of polymeric materials are less by 25 times when compared with that of glass while its thermal expansion is greater than 25 times that of glass. Generally, the diameter of these glass fibers is in the range 5-12 µm. The commonly used resins are the epoxy resins; they possess better mechanical and thermal properties. Moreover, a wide spectrum of friction and wear properties can be achieved with this polymer by the addition of a variety of fillers and fiber reinforcements.

The use of fillers in polymeric composites helps to improve tensile and compressive strengths, tribological characteristics, toughness (including abrasion), dimensional stability, thermal stability, and other properties. In addition to the higher mechanical strength obtained due to the addition of fillers in polymeric composites, there is cost reduction in terms of consumption of resin material. The critical and final selection of filler is primarily depends upon the requirements of the end product, the interface compatibility and the dimension/shape of particles. Various researchers12-18 have reported that the wear resistance of polymers is improved by the addition of

_______________ *For correspondence (E-mail: [email protected])

INDIAN J ENG. MATER. SCI., DECEMBER 2006

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fillers. Some of the fillers that are effective in reducing wear are graphite, MoS2, CuO, CuS and Al2O3. Kishore et al.16 have analyzed the influence of sliding speed and load on the friction and wear behaviour of G-E composite, filled either with rubber or oxide particles, wherein they have been reported that the wear loss increases with increase in load/speed. The use of fillers and fibers has been reported16-18 to be very effective in reducing wear under adhesive wear conditions. Bahadur et al.17 studied the decomposition and physicochemical reactions of the fillers with the counter face contributed to the formation of a thin, stable and adherent transfer film resulting in lower coefficient of friction and in turn higher wear resistance. The studies on the friction and wear of polymer composites containing PTFE have reported that the PTFE reduces the coefficient of friction and wear rate of some composites, but the converse is true in other types18. The use of SiCp filler in polymeric composites system has not been much reported in the literature, as SiCp is known to improve the mechanical properties and in turn the tribological properties also. Hence, in the present work SiCp addition as filler material in G-E system has been taken up for investigation from the point of characterizing them for friction and slide wear behaviour. Experimental Procedure

Materials and method A hand lay up technique has been adopted for

making G-E and SiCp-G-E composites. In the present study, a 7-mil E-glass plain-weave fabric reinforcement material is used. The epoxy resin is mixed with the hardener in the ratio 12:1 by weight19. The weight fraction of the glass fiber in the composite

is 47±3%. The matrix system consists of a medium viscosity epoxy resin (LAPOX L-12) and a room temperature curing polyamine hardener (K-6) supplied by ATUL India Ltd, Gujarat, India. Some details including density of the constituents of the matrix system and mechanical properties of epoxy and E-glass fabric are given in Tables 1 and 2 respectively. The filler material used is silicon carbide of particle size 10-30 μm. For obtaining the SiCp-G-E composites, in one set of samples about 2.5 wt.% of silicon carbide particles in the resin was introduced, while in the second set, roughly 5 wt% was included. The details of samples regarding the matrix, reinforcement material, filler and wt% are given in Table 3 and Fig. 1 shows the geometry of the 7-mil plain weave woven roving glass fabric. Wear test samples of geometry 5 mm × 5 mm × 2.8 mm were cut from the laminates using a diamond tipped cutter and then tested for characterizing the friction and wear properties. Wear test set-up and parameters

The test set-up used in this investigation is the widely used pin-on-disc type set-up (Fig. 2). The test samples were glued to pins of size 6 mm Φ × 22 mm length using suitable adhesive and their initial weights recorded using a high precision electronic balance (0.1 mg accuracy) after thorough cleaning. The counter face disc used is EN-32 steel hardened to 62

Table 1—Physical properties of epoxy resin and hardener

Material Trade name and Chemical name

Epoxide equivalent Density, kg/m3 Supplier Parts by weight

Resin LAPOX, L-12 Diegycidyl Ether of Bisphenol A (DGEBA)

182-192 1162 Atul Industries LTD, Gujarat, India 100

Hardener K-6 Triethylene Tetro amine (TETA)

---- 954 Atul Industries LTD, Gujarat, India 10-12

Table 3—Details of samples regarding the matrix, reinforcement material, filler and wt.%

Sample Matrix Reinforcement material

Weight % Filler Weight %

G-E Epoxy E-glass fabric 47±3 Silicon carbide ⎯ SiCp-G-E Epoxy E-glass fabric 47±3 Silicon carbide 2.5 SiCp-G-E Epoxy E-glass fabric 47±3 Silicon carbide 5

Table 2—Mechanical properties of Epoxy resin and Glass fibers

Property Epoxy Glass

Tensile strength, GPa 0.11 3.4 Tensile modulus, GPa 4.1 72.3 Strain to Failure % 4.6 4.8 Poisson’s ratio 0.3 0.2

SURESHA et al.: FRICTION AND WEAR BEHAVIOUR OF GLASS-EPOXY COMPOSITES

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HRC, 120 mm in diameter and 8 mm thick with a surface roughness of 1.8 μm Ra. The sample is dead weight loaded through a string to which a pan assembly is attached. A 20 kg load cell is fixed tangential to the lever arm through which the frictional force is measured. The test is conducted by selecting the test duration, load and velocity and performed in a track of 115 mm diameter20. The surface (5 mm × 5 mm) of the composite specimen shown in Fig. 3 makes contact with the counter surface. Prior to testing, the samples are rubbed over a 600 grade SiC paper to ensure proper contact with the counter surface. The surfaces of both the sample and the disc are cleaned with a soft paper soaked in acetone before the test. After fixing both the disc and the sample pin in their respective positions, the normal load to the pin was applied through a pivoted

loading lever with a string as shown in Fig. 2. After the preset time is reached, the test was stopped using the timer mechanism provided in the machine. The weight of the sample after the completion of the test was measured.

The samples were subjected to a normal load of 40 N; the disc was rotated continuously till select distance in the range of 500-6000 m was reached and the weight loss changes were recorded. In a select few samples, after the required wearing distances were reached, SEM examinations were conducted. All the experiments were carried out at a sliding speed of 4 m/s. A minimum of three tests were carried out for each set of test conditions and weight changes were recorded. The average of these samples is determined and used in the data representation. SEM examinations

The worn surface features have been analyzed on sputter coated samples run for selected sliding distances using SEM pictures. Results and Discussion Wear measurement

Fig. 4 shows the details of the weight loss with respect to sliding distance for G-E samples, the sliding velocity being 4 m/s and applied normal load being 40 N. For the same test conditions, the weight loss values recorded for the SiCp-G-E composites are shown in Fig. 5. In both the cases (Figs 4 and 5), the trend shows that wear loss increases with sliding distance. To show the relative wear loss for varying sliding distances, the results are now displayed in the form bar diagram in Fig. 6. From the bar chart, it is seen that of the three samples evaluated, the SiCp-G-E (5 wt. %) composite exhibits the least wear loss where as the G-E composite shows the highest. These data get further support from

Fig. 4—Wear loss versus sliding distance at load of 40 N and a sliding velocity of 4 m/s in G-E samples.

Fig. 1—Fabric geometry of plain weave woven roving glass.

Fig. 2—Pin-on-disc experimental set-up.

Fig. 3—Test sample details.


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the SEM surface features concentrated at the center region. It is obvious from the result that the wear is initially high and at a distance of 2000 m run, the wear rate comes down considerably. This may be attributed to the change in the wear process. The SEM photograph of the sample in the as received condition is shown in Fig. 8 for comparison purpose. The wear behaviour of unfilled and SiCp filled G-E composites may be substantiated as follows.

The wear loss is low for SiCp filled G-E composite compared to unfilled G-E composite. In G-E sample, at the start of sliding, all the asperities of the two surfaces are in contact with each other. As shear forces are applied, the asperities deform. The epoxy matrix wears out more in the initial stage up to a sliding distance of 2000 m since the top and bottom layers of the G-E sample is having resin rich surface. This can be termed as the running in region. As the sliding distance increases from 2000 m to 4000 m, the exposed glass fibers of the G-E sample is in contact with the counter surface. At this instance, wear debris formed consists of shear deformed polymer matrix containing broken pulverized glass particles and wear powder of the metallic counter surface. The particles can either be

lost from the contact zone or remains there for a fixed time as a transfer layer. In such cases, polymer component in the debris can cushion the counter surface asperities and reduce the effective roughness. Further, the pulverized glass component in the debris can act as a third body abrasive leading to enhanced roughening of the counter surface and hence increases the wear rate, it being lesser than that observed up to a sliding distance of 2000 m. Further, increase in sliding distance beyond 4000 m, increases the wear loss leading to higher wear rate. This may be attributed to increase in interface temperature resulting in destruction of the thin film formed on the counter surface.

However, in SiCp filled G-E composites, in the initial stage epoxy matrix wear out is higher (between sliding distance 500 m to 2000 m) and as sliding distance progresses the SiCp particles protrude out from the sample surface. At this instance, the SiCp particles along with the exposed glass fibers wear the counter surface leading to decrease in wear rate (up to 4000 m). Further, increase in sliding distance beyond 4000 m, increases the wear rate due to the SiCp particles getting crushed at the interface resulting in powdery SiCp particles and broken fibers are being present in the wear debris. The wear debris which consists of powdery SiCp and pulverized glass fibers contributed to higher wear rate. Coefficient of friction

Different trends of coefficient of friction (μ) measurements are observed and plotted in Fig. 7. For G-E sample with an increase in sliding distance, μ increases initially and then reaches steady state.

In the case of G-E composites, wear debris consists of shear deformed polymer matrix containing broken pulverized glass particles and wear powder of the

Fig. 7—Coefficient of friction versus sliding distance of G-E and SiCp-G-E samples at 4 m/s sliding velocity and a load 40 N.

Fig. 5—Wear loss versus sliding distance at load of 40 N and a sliding velocity of 4 m/s in SiCp-G-E samples.

Fig. 6—Bar chart of G-E and SiCp-G-E samples at 4 m/s sliding velocity, load 40 N for different sliding distances.


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metallic counter surface. The particles can either be lost from the contact zone or remains there for a fixed time as a transfer layer. In such cases, the polymer component can cushion the counter surface asperities and reduce the effective roughness, but the pulverized glass component in this debris can act as a third body abrasive leading to enhanced roughening of the counter surface. Thus the friction coefficient of the G-E composite depended on the constituents of the wear debris. Due to this coefficient of friction was high. Further, with increase in sliding distance the coefficient of friction reaches steady state which may be attributed to pulverized glass fiber and soft matrix present in the wear debris.

In SiCp filled G-E composite, at the start of sliding, the sample surface consists of epoxy matrix and SiCp particles which are in contact with the steel counter surface. As shear forces are applied, the asperities deform. The SiCp particles protrude out from the sample surface and wear the counter surface. Due to

this the frictional force increases. It may be worth noting that, by increasing the sliding distance, friction trend changed from smooth to rough stick-slip type behaviour. Further increase in sliding distance for SiCp filled G-E composite, caused vibration and noise from the pin-on-disc assembly. This phenomenon was attributed to fiber cracking at the sample surface leading to increased coefficient of friction. The behaviour is probably due to an increase in temperature at the interface at higher sliding distance. Further increase in sliding distance beyond 2000 m results in decrease in coefficient of friction which may be attributed to powdery SiCp particles in the wear debris forming a thin layer on the counter surface. Scanning electron microscopy

Figs. 8-12 show the SEM features of G-E composite worn out samples taken at different sliding distances. Thus, Fig. 9 illustrates the features at 500 m run. It is observed from Fig. 10 that the resin is still

Fig. 8—SEM picture of as received G-E sample.

Fig. 9—SEM picture of G-E sample at low sliding distance (500 m) showing wear out of the matrix region.

Fig. 10—SEM picture of G-E sample at sliding distance 1000 m showing long fibers.

Fig. 11—SEM picture of G-E sample at sliding distance 4000 m showing wear debris and few broken fibers.


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adhering to the glass fiber like in the as received sample (Fig. 8). Further, breakages of fibers are hardly noticed, less matrix wear in addition to network of very small cracks. For the sliding distance of 1000 m run sample, it is seen that the G-E sample shows the appearance of glass fibers getting exposed

and less debris formation are seen (Fig. 10). Evidence of few fiber breakages and occurrence of voids due the debonding of fibers are observed in Fig. 10. However, for 4000 m sliding distance run condition an increasing trend in the wear behaviour is seen (Fig. 4). These data are well corroborated by the SEM pictures shown in Fig. 11. Further, the appearance of small debris formed on the matrix with broken glass fibers in good numbers is noticed.

When the test is continued for high sliding distance (6000 m) run (Fig. 12), extensive debris formation with large number of broken fibers are seen. Also, it is interesting to note that the resinous material is well spread out. The process of wear of the matrix resin might have helped in dislodging such glass fibers. The glass fibers are not only getting fragmented, but also show an inclined type of fracture (Fig. 12). When Fig.11 is compared with Fig. 9, it may be inferred that the increase in the sliding distance contributes to the higher wear loss and well supported by the slide wear data (Fig. 4).

Fig. 12—SEM picture of G-E sample at higher sliding distance (6000 m) exhibiting spread of the matrix and breakage of fibers.

Fig. 13—SEM image of SiCp-G-E sample at low sliding distance (1000 m) showing SiC particles and wear out of the matrix.

Fig. 14—SEM image of SiCp-G-E sample at sliding distance 4000 m showing wear out of the matrix and masking of fibers.

Fig. 15—Higher magnification SEM image of SiCp-G-E sample at sliding distance 4000 m showing long broken fiber.

Fig. 16⎯SEM image of SiCp-G-E sample at higher sliding distance 6000 m featuring more broken fibers, some of which are fragmented.


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The worn surface SEM features of SiCp-G-E composite run for different sliding distances are shown in Figs. 13-16 respectively. Fig. 13 shows the SEM features at 1000 m run. From this figure it is seen that there is a distinct evidence of fiber breakages and as well as damage to the epoxy resin matrix with SiC particles protruding (SiC particle size in the range 10 to 30 μm) are noticed. A few voids are also noticed which may be due to the dislodging of SiC particles. The reason for this may be attributed to the fact that the highly angular SiC particles formed may act as abrading media causing breakage of fibers and also higher matrix wear. For 4000 m run SiCp-G-E sample increased debris formation and cleavage type of fiber breakage are observed as seen in Fig. 14. Further, masking of fibers due to the smearing of the resinous material (marked ‘A’ on the top mid region) are noticed. The debris in large quantities (cluster of debris formed marked ‘B’ in Fig. 14) is also seen. It is also seen that glass fiber (diameter 7.14 μm) have come out of the matrix and standing out as shown in Fig. 15. Further, increase of sliding distance to 6000 m in respect of SiCp-G-E composite (Fig. 16) shows removal of debris along the length of the fibers and also heavy fiber breakages as supported by the slide wear data (Fig. 5). Conclusions

Based on the above studies, the following key points emerge.

(i) SiCp-G-E composites show better resistance to wear at sliding distance in the range 2000 m to 4000 m compared to G-E composites. The better behaviour of SiCp-G-E composite is attributed to the SiCp particles, which act as good wear resistance medium.

(ii) At higher sliding distance (6000 m run), addition of SiCp filler lowers the wear loss of the G-E composite by about 14-20 %.

(iii) The coefficients of friction obtained in SiCp-G-E composite are higher than that of G-E composite for all sliding distances employed. On the other hand, SiCp filler influenced the coefficient of friction.

(iv) SiCp-G-E composites are the right choice material since they have shown better wear behaviour

and high friction for applications in automotive industries such as brake liners and clutches. Acknowledgements

The authors would like to acknowledge the assistance of Mr S Vynatheya for the help rendered in taking SEM photographs. The authors thank the Central Power Research Institute for providing the experimental facility. The authors also thank Mr. Sathish C.H and Mr. Shankar. J for their help in conducting the experiments. References 1 Pascoe M W, Tribology, 6 (1973) 184. 2 Lancaster K, Tribology, 5 (1972) 249. 3 Briscoe B J & Tabor D, in Fundamentals of tribology, edited

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friction and slide wear characteristics of glass-epoxy...

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