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Dry sliding wear characteristics of glass–epoxy composite filled with silicon carbide and graphite particles
Wear 296 (2012) 491–496
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Dry sliding wear characteristics of glass–epoxy composite filled with silicon carbide and graphite particles S. Basavarajappa n, S. Ellangovan Department of Studies in Mechanical Engineering, University B.D.T. College of Engineering, Davangere 577004, India
a r t i c l e i n f o
abstract
Article history: Received 19 November 2011 Received in revised form 30 July 2012 Accepted 2 August 2012 Available online 10 August 2012
The dry sliding wear characteristics of a glass–epoxy (G–E) composite, filled with both silicon carbide (SiCp) and graphite (Gr), were studied using a pin-on-disc test apparatus. The specific wear rate was determined as a function of sliding velocity, applied load and sliding distance. The laminates were fabricated by the hand lay-up technique. The volume percentage of filler materials in the composite was varied, silicon carbide was varied from 5 to 10% whereas graphite was kept constant at 5%. The excellent wear resistance was obtained with glass–epoxy containing fillers. The transfer film formed on the counter surface was confirmed to be effective in improving the wear characteristics of filled G–E composites. The influence of applied load is more on specific wear rate compared to the other two wear parameters. The worn surfaces of composites were examined with scanning electron microscopy (SEM) to investigate the probable wear mechanisms. It was found that in the early stage of wear, the fillers contribution is significant. The process of transfer film, debris formation and fiber breakage accounts for the wear at much later stages. & 2012 Elsevier B.V. All rights reserved.
Keywords: Sliding wear Polymer matrix composites Wear testing Surface topography
1. Introduction Over the past decades, polymer matrix composites are made and most widely used for structural applications in the aerospace, automotive, and chemical industries, and in providing alternatives to traditional metallic materials [1]. The features that make composites so promising as industrial and engineering materials are their high specific strength, high specific stiffness and opportunities to tailor material properties through the control of fiber and matrix compositions. Composites are developed for superior mechanical strength and this objective often conflicts with the simultaneous achievement of superior wear resistance [2]. As a result of this, these materials are found to be used in mechanical components such as gears, cams, wheels, impellers, brakes, clutches, conveyors, transmission belts, bushes and bearings. In most of these services the components are subjected to tribological loading conditions, where the likelihood of wear failure becomes greater. Of the large number of matrices available commercially, only a small portion is in significant use for these kinds of applications. The use of fillers in the matrix, gives rise to many combinations that provide increasing load withstanding capability, reduced coefficient of friction, improved wear resistance and
n
Corresponding author. Tel./fax: þ 91 8192 2224567. E-mail address: [email protected] (S. Basavarajappa).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.08.001
improved thermal properties. In addition to this, fillers in polymeric composite reduce the cost due to the less consumption of matrix material. Fibers are the principal constituents in a fiber reinforced composite materials. They occupy the largest volume fraction and share the major portion of the load acting on a composite [3]. In case of dry sliding it is effective in reducing the wear rate, this reduction in wear is due to the load carrying capacity of the fibers, their higher creep resistance and thermal conductivity. But the higher load makes it more sensitive to fiber breaking, pulverizing of the fibers and transfer [4]. Generally, the wear behavior of polymer matrix composites is different from that of conventional metallic materials. The material removal from the polymer matrix composites in contact with a counter surface is characterized by several mechanisms. The primary one is adhesive wear, wherein fine particles of polymer gets removed from the surface, and also fiber–matrix debonding and fiber breaking. On the other hand, the presence of either the fused polymer or the grooves at the interface is interpreted to indicate that the materials are wearing out by abrasion instead of adhesion [5]. The question of why fibers and fillers usually improve the wear resistance of a polymer matrix has been the subject of intense study in recent years [6–9]. Zhang et al. [10] studied dry sliding friction and wear behavior of PEEK and PEEK/SiCcomposite coatings and concluded that the influences of SiC fillers in the composite effectively reduce the plough and the adhesion between the two relative sliding parts. Chauhan et al. [11]
492
S. Basavarajappa, S. Ellangovan / Wear 296 (2012) 491–496
reported a study on the effect of SiCp filled glass fiber–vinyl ester composites on dry sliding wear and water lubricated conditions. Pure vinyl ester material has higher specific wear rate because of the lower mechanical properties and reinforcement of glass fiber and SiC filler improves the wear resistance both under dry and water lubricated conditions. Basavarajappa et al. [12] studied the effect of graphite fillers in glass–epoxy composites under dry sliding conditions. They reported that addition of graphite in glass–epoxy composite leads to lower wear volume loss. This is due to a thin coherent and uniform film that was transferred on the disc and the interphase also contained lubricant particles, thereby reducing the severity of the wear. Hyung Cho et al. [13] investigated tribological properties of solid lubricants (graphite, Sb2S3, MoS2) for automotive brake friction materials wherein it is reported that the friction stability, fade resistance, anti-fade, and wear of gray iron disks and friction materials were affected by the relative amounts of solid lubricants in the friction materials. Shyam [14] found that wear depends upon the cohesion of the transfer film, adhesion of the transfer film to the counterface and the protection of rubbing polymer surface from metal asperities by transfer film. Hashmi et al. [15] demonstrated graphite modified cotton fiber reinforced polyester composites under sliding wear conditions. They stated that significant reduction in the contact-surfacetemperature was observed on addition of graphite in cotton– polyester composites. Bahadur and Polineni [16] investigated the glass fabric-reinforced polyamide composites filled with CuO and PTFE, and reported that 11.3 vol% glass fabric–25 vol% CuO– 10 vol% PTFE composite showed the lowest steady state wear rate. It was 60–75% lower than the wear rate that could be obtained by using glass fabric or CuO reinforcement alone or in combination. Suresha et al. [17] described the role of SiC and Gr on friction and slide wear characteristics in glass–epoxy composites by adding them separately. They stated that the influence of these inorganic fillers has a significant role in reducing friction and exhibited better wear resistance properties under dry sliding conditions. In view of above an attempt is made in this present investigation to combine the benefits of two inorganic fillers SiCp and Gr into the G–E composite, to enhance the wear resistance. The dry sliding wear behavior of G–E composites was characterized by observing the scanning electron microscopy (SEM) images. This approach was adopted to elucidate the mechanism of wear in the composites.
Table 1 Details of composites prepared. Specimen code
A B C
Matrix volume (%)
50 40 35
Reinforcement volume (%)
50 50 50
Fillers volume (%) SiCp
Gr
– 5 10
– 5 5
to yield wear test specimen of size 6 mm 6 mm 3 mm. Dry sliding wear behavior tests were performed on 6 mm 6 mm face. 2.2. Test details A pin-on-disc wear test apparatus was used for the dry sliding wear experiments (as per ASTM G-99 standard). The disc used was an alloy steel with 165 mm diameter and 8 mm thick, hardness of 62 HRc and with a surface roughness of 1.2 mm. The test was conducted on a track of 130 mm diameter for a specified test duration, applied load and sliding velocity. The surface of the specimen was perpendicular to the contact surface. Prior to testing, the specimen pin was rubbed over a 600-grade SiC paper to ensure proper contact between the specimen surface and the disc counter surface during sliding. The surfaces of both the specimen and the disc were cleaned with a soft paper soaked in acetone before the test. The initial and final weights of the specimen were measured by using an electronic digital balance with an accuracy of 0.0001 g. The difference between the initial and final weights is the measure of weight loss. The weight loss was then converted into wear volume using the measured density data. The specific wear rate (Ws) parameter provides a more comprehensive measure of the wear loss characteristics of the materials. The specific wear rate was calculated from W s ¼ DV=Ld ðmm3 =NmÞ
ð1Þ 3
where DV is the volume loss in mm , L is the applied load in Newton and d is the sliding distance in meters. SEM observations were carried out; the features of interest regions were recorded. The specimen being non-conducting, it was sputter coated with a layer of gold before SEM examination.
3. Results and discussion 2. Experimental details 2.1. Specimen details The matrix material used was a medium viscosity epoxy resin (LAPOX L-12) and a room temperature curing polyamine hardener (K-6). This matrix was chosen since it provides good adhesive properties owing to the cross-linking chain between the resin polymer and the hardener. Hence, the shrinkage after curing is usually lower. The reinforcement material employed was bidirectional perpendicular yarns of 7-mil E-glass fiber. SiCp (15 mm) and Gr (15 mm) powders were selected as the filler materials on the basis of their demonstrated ability to withstand high temperatures, and to form transfer film during sliding and low thermal expansion. The composites were prepared in the form of blocks (250 mm 250 mm 3 mm) by the hand lay-up technique. The fillers SiCp and Gr are mixed with known amount of epoxy resin. The detail composition of the composite is given in Table 1. The laminate was cured at room temperature for a period of about 24 h. The cured laminates are cut using a diamond tipped cutter
When SiCp and graphite fillers are embedded in the G–E composite, the wear trend is as shown in Figs. 1–3. The specific wear rate of G–E composite has been found to be affected by the sliding speed. This is true in both filled and unfilled G–E composites. In both the cases a general trend has been found for the effect of sliding speed as show in Fig. 1. The effect of sliding speed on wear of polymer matrix composites has been investigated quite extensively. The specific wear rate increased as the sliding speed increased. Plowing by the wear debris and the asperities on the counter surface is the major activity on the surface. At high speed the interface temperature increases because of the poor conductivity of the polymer composite. The high temperature can give rise to a molten layer at the interface and it can affect the fiber–matrix bonding on the subsurface. It can also promote degradation wear and crack propagation on the subsurface. So, the specific wear rate increases exponentially at higher speeds [18]. The higher thermal conductivity of the fillers is one of the reasons why filled G–E composites have superior wear resistance to that of unfilled G–E composite.
S. Basavarajappa, S. Ellangovan / Wear 296 (2012) 491–496
14
Ws X10-6 mm3/Nm
12 10 8 6
A
4
B
2
C
0 2.72
4.08
5.44 6.8 Sliding Velocity in m/s
8.16
9.52
Fig. 1. Variation of specific wear rate against sliding velocity at constant applied load of 60 N and at a sliding distance of 3000 m.
25 A Ws X10-6 mm3/Nm
20
B C
15 10 5 0 20
40
60 80 Applied Load in N
100
120
Fig. 2. Variation of specific wear rate against applied load at constant sliding velocity of 5.44 m/s and at a sliding distance of 3000 m.
12
Ws X10-6 mm3/Nm
10 8 6 A
4
B 2 0 1000
C
2000
4000 3000 Sliding Distance in m
5000
6000
Fig. 3. Variation of specific wear rate against sliding distance at constant sliding velocity of 5.44 m/s and at an applied load of 60 N.
Variations of specific wear rate with sliding velocity 5.44 m/s and sliding distance 3000 m under various loads are shown in Fig. 2. With reference to the figure, it is noticed that the specific wear rate of the composites is seen to be high at the applied load of 20 N. On increasing the applied load to 40 N the specific wear rate drops down considerably, indicating a change in wear process. The specific wear rate appears to be minimum at 60 N and then rises again with increasing applied load. This is because,
493
during the initial run in period when epoxy comes in contact with the counter surface, severe adhesive wear occurs and the specific wear rate increases. Further the specific wear rate is controlled by glass fiber reinforcement. The subsequent escape of exposed glass fiber debris expelled within the contact zone is likely to exacerbate wear still further by three-body abrasion. This type of wear behavior has been explained by Rajesh et al. [19]. Basavarajappa et al. [20] studying dry sliding wear behavior of aluminum metal matrix composites reinforced with SiC and Gr particles using Taguchi techniques, found that sliding distance is the wear factor that has the highest physical as well as statistical influence on the wear of the composites. In contrast, in the dry sliding wear behavior of G–E composite filled with SiCp and graphite particles, sliding distance has less effect on the wear as shown in Fig. 3. From the figure it can be observed that the specific wear rate exhibits an initial steep drop and remains practically insensitive to sliding distance thereafter. High specific wear rate was noticed for unfilled G–E composite compared to filled G–E composites. The formation of air bubbles and voids is practically unavoidable in unfilled G–E composite, and these voids affect the matrix–fiber interfacial zone. In this zone a viscous matrix flow appears to occur during sliding, probably because of matrix softening. The synergistic effect of SiCp filler and solid lubricant Gr in G–E composite not only reduces the voids but also increases the wear resistance. During the sliding, SiCp particles embedded in the matrix are capable of enhancing the adhesion by forming physical interaction with the counter surface, resulting in significant reduction in matrix wearing. At higher loads, however additional frictional heat released in a contact tends to soften the resin; thus the contact surface is sheared giving rise to loose SiCp particles. Due to this some SiCp particles are easily removed from the surface layer together with graphite film and transferred on to the counter surface to form a high lubricity in a cumulative fashion. The transfer film effectively reduces the extent of frictional heating, leading to less damage to the matrix, fibers and their adhesion. The nature of transferred film on the counter surface plays a key role in controlling the wear performance of a composite. In addition, fillers serve to cushion asperities from shock, subsequent fracture and the resultant enhanced wear. At the same time fillers provide more protection from glass fibers on the counter surface and have strong influence on the wear resistance. In the presence of more volume percentage of SiCp filler, the lack of transfer film causes unsatisfactory levels of specific wear rate at higher loads. But from the published data [21] it appears that the wear rate increases when SiCp portion exceeds the optimum value. The wear characteristics of the G–E composite appear to depend on the hard SiCp and graphite fraction depositing a thin transfer film on the counter surface.
4. Worn surface morphology In order to investigate the wear mechanism for filled and unfilled G–E composites, the worn surfaces of composite specimens were examined using SEM. Fig. 4 depicts the SEM features of worn out surface taken out at 60 N applied load for unfilled G–E composite. From the figure the poor fiber–matrix bond strength (marked ‘A’) can be observed, which caused an accelerative breakage of the matrix especially in the interfacial region. As a result, the surface damage was remarkably increased with serious fibers breakage, and also imprints left by the separating fibers (marked ‘B’) are evident from the micrograph. Moreover, the fibers were removed with larger patches and undertook little wear process [18]. The asperities of the harder surface of the steel disc exerted
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C
D
Fig. 4. Worn surface of unfilled G–E composite at 60 N applied load.
Fig. 6. G–E composite filled with 5% SiCp–5% Gr at 60 N applied load.
E
Fig. 5. G–E composite filled with 5% SiCp–5% Gr at 20 N applied load.
a ploughing action on the surface of the composite. Thus the wear debris was produced during the sliding process and further decreased the wear resistance of the composite owing to an abrasive wear effect. Contrary to the above, the worn surface of filled G–E composites has nearly invisible peeling-off at the same sliding conditions. This indicates that the fillers incorporated in the G–E composite effectively act to enhance the bonding strength among the fibers and the matrix. Figs. 5–10 are micrographs of the worn surfaces of the filled G–E composites in the order of increasing load, showing that the fillers are protruded from the matrix. The protrusion of fillers indicates that the fillers take up some portion of the load during sliding and prevent severe adhesion between the matrix and the counter surface (Fig. 5). Here the fibers are less distinctly seen due to smearing by resinous material in which glass fibers are arranged. In the absence of severe adhesion the surface fracture is significantly reduced. The matrix surface is covered with small shallow and irregular patches of the thin dark film (marked ‘C’ in Fig. 6) which are different from the markings on the unfilled G–E composite. Here a few abrasion grooves (marked ‘D’ in Fig. 6) and a number of ripple markings are exhibited. From Fig. 8, there are
Fig. 7. G–E composite filled with 5% SiCp–5% Gr at 100 N applied load.
Fig. 8. G–E composite filled with 10% SiCp–5% Gr at 20 N applied load.
S. Basavarajappa, S. Ellangovan / Wear 296 (2012) 491–496
F
Fig. 9. G–E composite filled with 10% SiCp–5% Gr at 60 N applied load.
G
495
By comparing Fig. 10 with Fig. 7, it is possible to highlight the effect of more SiCp particles in G–E composite. Here the epoxy matrix and glass fibers are damaged more severely by crushing and cutting action of abrasive particles (marked ‘G’). The worn surface shows evidence of poor adhesion of matrix to the fibers as several clean fibers appear on the worn surface. In fact, this topography looks more likely as an abrasive wear case rather than the adhesive wear. As a result the wear rate of the composites increases slightly with increasing applied load but never reaches very high levels. The synergistic effect of fillers hinders the wear of G–E composites surface layer. Thus, a smoother worn surface and hence lower wear rate was observed under dry lubrication as compared with unfilled G–E composite. The better wear resistance exhibited by the filled G–E composites depends on factors such as increasing bonding strength, less voids and formation of transfer film by filler materials.
5. Conclusions The study of the wear behavior of filled and unfilled G–E composites at various sliding velocity, applied load and sliding distance reveals the following. An increase in sliding velocity increased the specific wear rate. Applied load has much more predominant effect, whereas sliding distance has less effect. Inclusion of fillers in G–E composites leads to better wear resistance; however higher the percentage of the SiCp filler along with graphite higher the wear due to deteriorated abrasive wear performance of the parent material and it is also seen to depend on the amount and nature of the transfer film formed on the steel counter surface. The wear mechanisms involved are well indicated with SEM micrographs, which reveal multiple microcracking, debris formation, fiber thinning, fiber breakage, fiber pull outs, peeling of the matrix and fiber–matrix debonding.
References
Fig. 10. G–E composite filled with 10% SiCp–5% Gr at 100 N applied load.
some extent cracks, which are parallel and perpendicular to the sliding direction (marked arrow). Further, these cracks become more susceptible to propagation on to the surface, giving rise to loosening of the fillers. Furthermore, the worn surface gives rise to laminate type of debris (marked ‘F’ in Fig. 9) when applied load is increased to 60 N. Hence it can be concluded that an increase in the volume percentage of filler materials leads to less bonding between the filler and matrix material. To produce a good bonding and better wear resistance, an optimum level of filler is required. In the case of higher loads most of the matrix material has already been removed and loosening of the fillers results in exposure of the fibrous region to the sliding contact. In the regime under adhesive forces, often transmitted through a film, the fiber ends undergo severe thinning along their length (marked ‘E’ in Fig. 7); a similar feature was observed by Hui Zhang et al. [22]. The thinning process fractures the skin of the fibers and separates it from the surface, whereas the rest remain embedded, still contributing to the wear resistance of the composite for a certain time.
[1] Li Chang, Zhong Zhang, Lin Ye, Klaus Friedrich, Tribological properties of epoxy nanocomposites III characteristics of transfer films, Wear 262 (2007) 699–706. [2] W. Giwdon, Stachowiak Andrew, W. Batchelor, Engineering Tribology, third ed., Elsevier, 2005, p. 676. [3] Hasim pihtili, Nihat Tosun, Effect of load and speed on the wear behavior of woven glass fabrics and aramid fiber-reinforced composites, Wear 252 (2002) 979–984. [4] J. Quintelier, P. De Baets, P. Samyn, D. Van Hemelrijck, SEM features of glass– polyester composite system subjected to dry sliding wear 261 (2006) 703–714Wear 261 (2006) 703–714. [5] P. Sampath kumaran, S. Seetharam, A. Murali, P.K. Kumar, Kishore, Sliding wear studies in glass–epoxy system through scanning microscopic observations, Bulletin of Materials Science 21 (4) (1998) 335–339. [6] V.K. Srivastava, S. Wahne, Wear and friction behavior of soft particles filled random direction short GFRP composites, Materials Science and Engineering A 458 (2007) 25–33. [7] B. Suresha, G. Chandramohan, J.N. Prakash, V. Balusamy, K. Sankaranarayanasamy, The role of fillers on friction and slide wear characteristics in glass–epoxy composite systems, Journal of Minerals and Materials Characterization and Engineering 5 (1) (2006) 87–101. [8] Xiubing Li, Yimin Gao, Jiandong Xing, Yu Wang, Liang Fang, Wear reduction mechanism of graphite and MoS2 in epoxy composites, Wear 257 (2004) 279–283. [9] Kishore, P. Sampathkumaran, S. Seetharamu, P. Thomasb, M. Janardhana, A study on the effect of the type and content of filler in epoxy–glass composite system on the friction and slide wear characteristics, Wear 259 (2005) 634–641. [10] G. Zhang, H. Liao, H. Lia, C. Mateus, J.M. Bordes, C. Coddet, On dry sliding friction and wear behavior of PEEK and PEEK/SiC-composite coatings, Wear 260 (2006) 594–600. [11] S.R. Chauhan, Anoop Kumar, I. Singh, Sliding friction and wear behavior of vinylester and its composites under dry and water lubricated sliding conditions, Materials and Design 31 (2010) 2745–2751.
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[12] S. Basavarajappa, S. Ellangovan, K.V. Arun, Studies on dry sliding wear behavior of graphite filled glass–epoxy composites, Materials and Design 30 (2009) 2670–2675. [13] Min Hyung Cho, Jeong Ju, Seong Jin Kima, Ho Jang, Tribological properties of solid lubricants (graphite, Sb2S3, MoS2) for automotive brake friction materials, Wear 260 (2006) 855–860. [14] Shyam Bahadur, The development of transfer layers and their role in polymer tribology, Wear 245 (2000) 92–99. [15] S.A.R. Hashmi, U.K. Dwivedi, Navin Chand, Graphite modified cotton fibre reinforced polyester composites under sliding wear conditions, Wear 262 (2007) 1426–1432. [16] Bahadur, V.K. Polineni, Tribological studies of glass fabric-reinforced polymide composites filled with CuO and PTFE, Wear 200 (1996) 95–104. [17] B. Suresha, G. Chandramohan, Siddaramaiah, P. Samapthkumaran, S. Seetharamu, Three-body abrasive wear behaviour of carbon and glass fiber reinforced epoxy composites, Materials Science and Engineering A 443 (2007) 285–291.
[18] Kishore, P. Sampathkumaran, S. Seetharamu, S. Vynatheya, A. Murali, R.K. Kumar, SEM observations of the effects of velocity and load on the sliding wear characteristics of glass fabric–epoxy composites with different fillers, Wear 237 (2000) 20–27. [19] J.J. Rajesh, J. Bijwe, U.S. Tewari, Influnce of fillers on abrasive wear of short glass fibre reinforced polyamide composites, Journal of Materials Science 36 (2001) 351–356. [20] S. Basavarajappa, G. Chandramohan, J. Paulo Davim, Application of Taguchi techniques to study dry sliding wear behavior of metal matrix composites, Materials and Design 28 (2007) 1393–1398. [21] Qun-Ji Xue, Qi-Hua Wang, Wear mechanisms of polyetheretherketone composites filled with various kinds of SiC, Wear 213 (1997) 54–58. [22] Zhong Hui Zhang, Zhang, Klaus Friedrich, Effect of fiber length on the wear resistance of short carbon fiber reinforced epoxy composites, Composites Science and Technology 67 (2007) 222–230.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Dry sliding wear characteristics of glass–epoxy composite filled with silicon carbide and graphite particles S. Basavarajappa n, S. Ellangovan Department of Studies in Mechanical Engineering, University B.D.T. College of Engineering, Davangere 577004, India
a r t i c l e i n f o
abstract
Article history: Received 19 November 2011 Received in revised form 30 July 2012 Accepted 2 August 2012 Available online 10 August 2012
The dry sliding wear characteristics of a glass–epoxy (G–E) composite, filled with both silicon carbide (SiCp) and graphite (Gr), were studied using a pin-on-disc test apparatus. The specific wear rate was determined as a function of sliding velocity, applied load and sliding distance. The laminates were fabricated by the hand lay-up technique. The volume percentage of filler materials in the composite was varied, silicon carbide was varied from 5 to 10% whereas graphite was kept constant at 5%. The excellent wear resistance was obtained with glass–epoxy containing fillers. The transfer film formed on the counter surface was confirmed to be effective in improving the wear characteristics of filled G–E composites. The influence of applied load is more on specific wear rate compared to the other two wear parameters. The worn surfaces of composites were examined with scanning electron microscopy (SEM) to investigate the probable wear mechanisms. It was found that in the early stage of wear, the fillers contribution is significant. The process of transfer film, debris formation and fiber breakage accounts for the wear at much later stages. & 2012 Elsevier B.V. All rights reserved.
Keywords: Sliding wear Polymer matrix composites Wear testing Surface topography
1. Introduction Over the past decades, polymer matrix composites are made and most widely used for structural applications in the aerospace, automotive, and chemical industries, and in providing alternatives to traditional metallic materials [1]. The features that make composites so promising as industrial and engineering materials are their high specific strength, high specific stiffness and opportunities to tailor material properties through the control of fiber and matrix compositions. Composites are developed for superior mechanical strength and this objective often conflicts with the simultaneous achievement of superior wear resistance [2]. As a result of this, these materials are found to be used in mechanical components such as gears, cams, wheels, impellers, brakes, clutches, conveyors, transmission belts, bushes and bearings. In most of these services the components are subjected to tribological loading conditions, where the likelihood of wear failure becomes greater. Of the large number of matrices available commercially, only a small portion is in significant use for these kinds of applications. The use of fillers in the matrix, gives rise to many combinations that provide increasing load withstanding capability, reduced coefficient of friction, improved wear resistance and
n
Corresponding author. Tel./fax: þ 91 8192 2224567. E-mail address: [email protected] (S. Basavarajappa).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.08.001
improved thermal properties. In addition to this, fillers in polymeric composite reduce the cost due to the less consumption of matrix material. Fibers are the principal constituents in a fiber reinforced composite materials. They occupy the largest volume fraction and share the major portion of the load acting on a composite [3]. In case of dry sliding it is effective in reducing the wear rate, this reduction in wear is due to the load carrying capacity of the fibers, their higher creep resistance and thermal conductivity. But the higher load makes it more sensitive to fiber breaking, pulverizing of the fibers and transfer [4]. Generally, the wear behavior of polymer matrix composites is different from that of conventional metallic materials. The material removal from the polymer matrix composites in contact with a counter surface is characterized by several mechanisms. The primary one is adhesive wear, wherein fine particles of polymer gets removed from the surface, and also fiber–matrix debonding and fiber breaking. On the other hand, the presence of either the fused polymer or the grooves at the interface is interpreted to indicate that the materials are wearing out by abrasion instead of adhesion [5]. The question of why fibers and fillers usually improve the wear resistance of a polymer matrix has been the subject of intense study in recent years [6–9]. Zhang et al. [10] studied dry sliding friction and wear behavior of PEEK and PEEK/SiCcomposite coatings and concluded that the influences of SiC fillers in the composite effectively reduce the plough and the adhesion between the two relative sliding parts. Chauhan et al. [11]
492
S. Basavarajappa, S. Ellangovan / Wear 296 (2012) 491–496
reported a study on the effect of SiCp filled glass fiber–vinyl ester composites on dry sliding wear and water lubricated conditions. Pure vinyl ester material has higher specific wear rate because of the lower mechanical properties and reinforcement of glass fiber and SiC filler improves the wear resistance both under dry and water lubricated conditions. Basavarajappa et al. [12] studied the effect of graphite fillers in glass–epoxy composites under dry sliding conditions. They reported that addition of graphite in glass–epoxy composite leads to lower wear volume loss. This is due to a thin coherent and uniform film that was transferred on the disc and the interphase also contained lubricant particles, thereby reducing the severity of the wear. Hyung Cho et al. [13] investigated tribological properties of solid lubricants (graphite, Sb2S3, MoS2) for automotive brake friction materials wherein it is reported that the friction stability, fade resistance, anti-fade, and wear of gray iron disks and friction materials were affected by the relative amounts of solid lubricants in the friction materials. Shyam [14] found that wear depends upon the cohesion of the transfer film, adhesion of the transfer film to the counterface and the protection of rubbing polymer surface from metal asperities by transfer film. Hashmi et al. [15] demonstrated graphite modified cotton fiber reinforced polyester composites under sliding wear conditions. They stated that significant reduction in the contact-surfacetemperature was observed on addition of graphite in cotton– polyester composites. Bahadur and Polineni [16] investigated the glass fabric-reinforced polyamide composites filled with CuO and PTFE, and reported that 11.3 vol% glass fabric–25 vol% CuO– 10 vol% PTFE composite showed the lowest steady state wear rate. It was 60–75% lower than the wear rate that could be obtained by using glass fabric or CuO reinforcement alone or in combination. Suresha et al. [17] described the role of SiC and Gr on friction and slide wear characteristics in glass–epoxy composites by adding them separately. They stated that the influence of these inorganic fillers has a significant role in reducing friction and exhibited better wear resistance properties under dry sliding conditions. In view of above an attempt is made in this present investigation to combine the benefits of two inorganic fillers SiCp and Gr into the G–E composite, to enhance the wear resistance. The dry sliding wear behavior of G–E composites was characterized by observing the scanning electron microscopy (SEM) images. This approach was adopted to elucidate the mechanism of wear in the composites.
Table 1 Details of composites prepared. Specimen code
A B C
Matrix volume (%)
50 40 35
Reinforcement volume (%)
50 50 50
Fillers volume (%) SiCp
Gr
– 5 10
– 5 5
to yield wear test specimen of size 6 mm 6 mm 3 mm. Dry sliding wear behavior tests were performed on 6 mm 6 mm face. 2.2. Test details A pin-on-disc wear test apparatus was used for the dry sliding wear experiments (as per ASTM G-99 standard). The disc used was an alloy steel with 165 mm diameter and 8 mm thick, hardness of 62 HRc and with a surface roughness of 1.2 mm. The test was conducted on a track of 130 mm diameter for a specified test duration, applied load and sliding velocity. The surface of the specimen was perpendicular to the contact surface. Prior to testing, the specimen pin was rubbed over a 600-grade SiC paper to ensure proper contact between the specimen surface and the disc counter surface during sliding. The surfaces of both the specimen and the disc were cleaned with a soft paper soaked in acetone before the test. The initial and final weights of the specimen were measured by using an electronic digital balance with an accuracy of 0.0001 g. The difference between the initial and final weights is the measure of weight loss. The weight loss was then converted into wear volume using the measured density data. The specific wear rate (Ws) parameter provides a more comprehensive measure of the wear loss characteristics of the materials. The specific wear rate was calculated from W s ¼ DV=Ld ðmm3 =NmÞ
ð1Þ 3
where DV is the volume loss in mm , L is the applied load in Newton and d is the sliding distance in meters. SEM observations were carried out; the features of interest regions were recorded. The specimen being non-conducting, it was sputter coated with a layer of gold before SEM examination.
3. Results and discussion 2. Experimental details 2.1. Specimen details The matrix material used was a medium viscosity epoxy resin (LAPOX L-12) and a room temperature curing polyamine hardener (K-6). This matrix was chosen since it provides good adhesive properties owing to the cross-linking chain between the resin polymer and the hardener. Hence, the shrinkage after curing is usually lower. The reinforcement material employed was bidirectional perpendicular yarns of 7-mil E-glass fiber. SiCp (15 mm) and Gr (15 mm) powders were selected as the filler materials on the basis of their demonstrated ability to withstand high temperatures, and to form transfer film during sliding and low thermal expansion. The composites were prepared in the form of blocks (250 mm 250 mm 3 mm) by the hand lay-up technique. The fillers SiCp and Gr are mixed with known amount of epoxy resin. The detail composition of the composite is given in Table 1. The laminate was cured at room temperature for a period of about 24 h. The cured laminates are cut using a diamond tipped cutter
When SiCp and graphite fillers are embedded in the G–E composite, the wear trend is as shown in Figs. 1–3. The specific wear rate of G–E composite has been found to be affected by the sliding speed. This is true in both filled and unfilled G–E composites. In both the cases a general trend has been found for the effect of sliding speed as show in Fig. 1. The effect of sliding speed on wear of polymer matrix composites has been investigated quite extensively. The specific wear rate increased as the sliding speed increased. Plowing by the wear debris and the asperities on the counter surface is the major activity on the surface. At high speed the interface temperature increases because of the poor conductivity of the polymer composite. The high temperature can give rise to a molten layer at the interface and it can affect the fiber–matrix bonding on the subsurface. It can also promote degradation wear and crack propagation on the subsurface. So, the specific wear rate increases exponentially at higher speeds [18]. The higher thermal conductivity of the fillers is one of the reasons why filled G–E composites have superior wear resistance to that of unfilled G–E composite.
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14
Ws X10-6 mm3/Nm
12 10 8 6
A
4
B
2
C
0 2.72
4.08
5.44 6.8 Sliding Velocity in m/s
8.16
9.52
Fig. 1. Variation of specific wear rate against sliding velocity at constant applied load of 60 N and at a sliding distance of 3000 m.
25 A Ws X10-6 mm3/Nm
20
B C
15 10 5 0 20
40
60 80 Applied Load in N
100
120
Fig. 2. Variation of specific wear rate against applied load at constant sliding velocity of 5.44 m/s and at a sliding distance of 3000 m.
12
Ws X10-6 mm3/Nm
10 8 6 A
4
B 2 0 1000
C
2000
4000 3000 Sliding Distance in m
5000
6000
Fig. 3. Variation of specific wear rate against sliding distance at constant sliding velocity of 5.44 m/s and at an applied load of 60 N.
Variations of specific wear rate with sliding velocity 5.44 m/s and sliding distance 3000 m under various loads are shown in Fig. 2. With reference to the figure, it is noticed that the specific wear rate of the composites is seen to be high at the applied load of 20 N. On increasing the applied load to 40 N the specific wear rate drops down considerably, indicating a change in wear process. The specific wear rate appears to be minimum at 60 N and then rises again with increasing applied load. This is because,
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during the initial run in period when epoxy comes in contact with the counter surface, severe adhesive wear occurs and the specific wear rate increases. Further the specific wear rate is controlled by glass fiber reinforcement. The subsequent escape of exposed glass fiber debris expelled within the contact zone is likely to exacerbate wear still further by three-body abrasion. This type of wear behavior has been explained by Rajesh et al. [19]. Basavarajappa et al. [20] studying dry sliding wear behavior of aluminum metal matrix composites reinforced with SiC and Gr particles using Taguchi techniques, found that sliding distance is the wear factor that has the highest physical as well as statistical influence on the wear of the composites. In contrast, in the dry sliding wear behavior of G–E composite filled with SiCp and graphite particles, sliding distance has less effect on the wear as shown in Fig. 3. From the figure it can be observed that the specific wear rate exhibits an initial steep drop and remains practically insensitive to sliding distance thereafter. High specific wear rate was noticed for unfilled G–E composite compared to filled G–E composites. The formation of air bubbles and voids is practically unavoidable in unfilled G–E composite, and these voids affect the matrix–fiber interfacial zone. In this zone a viscous matrix flow appears to occur during sliding, probably because of matrix softening. The synergistic effect of SiCp filler and solid lubricant Gr in G–E composite not only reduces the voids but also increases the wear resistance. During the sliding, SiCp particles embedded in the matrix are capable of enhancing the adhesion by forming physical interaction with the counter surface, resulting in significant reduction in matrix wearing. At higher loads, however additional frictional heat released in a contact tends to soften the resin; thus the contact surface is sheared giving rise to loose SiCp particles. Due to this some SiCp particles are easily removed from the surface layer together with graphite film and transferred on to the counter surface to form a high lubricity in a cumulative fashion. The transfer film effectively reduces the extent of frictional heating, leading to less damage to the matrix, fibers and their adhesion. The nature of transferred film on the counter surface plays a key role in controlling the wear performance of a composite. In addition, fillers serve to cushion asperities from shock, subsequent fracture and the resultant enhanced wear. At the same time fillers provide more protection from glass fibers on the counter surface and have strong influence on the wear resistance. In the presence of more volume percentage of SiCp filler, the lack of transfer film causes unsatisfactory levels of specific wear rate at higher loads. But from the published data [21] it appears that the wear rate increases when SiCp portion exceeds the optimum value. The wear characteristics of the G–E composite appear to depend on the hard SiCp and graphite fraction depositing a thin transfer film on the counter surface.
4. Worn surface morphology In order to investigate the wear mechanism for filled and unfilled G–E composites, the worn surfaces of composite specimens were examined using SEM. Fig. 4 depicts the SEM features of worn out surface taken out at 60 N applied load for unfilled G–E composite. From the figure the poor fiber–matrix bond strength (marked ‘A’) can be observed, which caused an accelerative breakage of the matrix especially in the interfacial region. As a result, the surface damage was remarkably increased with serious fibers breakage, and also imprints left by the separating fibers (marked ‘B’) are evident from the micrograph. Moreover, the fibers were removed with larger patches and undertook little wear process [18]. The asperities of the harder surface of the steel disc exerted
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C
D
Fig. 4. Worn surface of unfilled G–E composite at 60 N applied load.
Fig. 6. G–E composite filled with 5% SiCp–5% Gr at 60 N applied load.
E
Fig. 5. G–E composite filled with 5% SiCp–5% Gr at 20 N applied load.
a ploughing action on the surface of the composite. Thus the wear debris was produced during the sliding process and further decreased the wear resistance of the composite owing to an abrasive wear effect. Contrary to the above, the worn surface of filled G–E composites has nearly invisible peeling-off at the same sliding conditions. This indicates that the fillers incorporated in the G–E composite effectively act to enhance the bonding strength among the fibers and the matrix. Figs. 5–10 are micrographs of the worn surfaces of the filled G–E composites in the order of increasing load, showing that the fillers are protruded from the matrix. The protrusion of fillers indicates that the fillers take up some portion of the load during sliding and prevent severe adhesion between the matrix and the counter surface (Fig. 5). Here the fibers are less distinctly seen due to smearing by resinous material in which glass fibers are arranged. In the absence of severe adhesion the surface fracture is significantly reduced. The matrix surface is covered with small shallow and irregular patches of the thin dark film (marked ‘C’ in Fig. 6) which are different from the markings on the unfilled G–E composite. Here a few abrasion grooves (marked ‘D’ in Fig. 6) and a number of ripple markings are exhibited. From Fig. 8, there are
Fig. 7. G–E composite filled with 5% SiCp–5% Gr at 100 N applied load.
Fig. 8. G–E composite filled with 10% SiCp–5% Gr at 20 N applied load.
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F
Fig. 9. G–E composite filled with 10% SiCp–5% Gr at 60 N applied load.
G
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By comparing Fig. 10 with Fig. 7, it is possible to highlight the effect of more SiCp particles in G–E composite. Here the epoxy matrix and glass fibers are damaged more severely by crushing and cutting action of abrasive particles (marked ‘G’). The worn surface shows evidence of poor adhesion of matrix to the fibers as several clean fibers appear on the worn surface. In fact, this topography looks more likely as an abrasive wear case rather than the adhesive wear. As a result the wear rate of the composites increases slightly with increasing applied load but never reaches very high levels. The synergistic effect of fillers hinders the wear of G–E composites surface layer. Thus, a smoother worn surface and hence lower wear rate was observed under dry lubrication as compared with unfilled G–E composite. The better wear resistance exhibited by the filled G–E composites depends on factors such as increasing bonding strength, less voids and formation of transfer film by filler materials.
5. Conclusions The study of the wear behavior of filled and unfilled G–E composites at various sliding velocity, applied load and sliding distance reveals the following. An increase in sliding velocity increased the specific wear rate. Applied load has much more predominant effect, whereas sliding distance has less effect. Inclusion of fillers in G–E composites leads to better wear resistance; however higher the percentage of the SiCp filler along with graphite higher the wear due to deteriorated abrasive wear performance of the parent material and it is also seen to depend on the amount and nature of the transfer film formed on the steel counter surface. The wear mechanisms involved are well indicated with SEM micrographs, which reveal multiple microcracking, debris formation, fiber thinning, fiber breakage, fiber pull outs, peeling of the matrix and fiber–matrix debonding.
References
Fig. 10. G–E composite filled with 10% SiCp–5% Gr at 100 N applied load.
some extent cracks, which are parallel and perpendicular to the sliding direction (marked arrow). Further, these cracks become more susceptible to propagation on to the surface, giving rise to loosening of the fillers. Furthermore, the worn surface gives rise to laminate type of debris (marked ‘F’ in Fig. 9) when applied load is increased to 60 N. Hence it can be concluded that an increase in the volume percentage of filler materials leads to less bonding between the filler and matrix material. To produce a good bonding and better wear resistance, an optimum level of filler is required. In the case of higher loads most of the matrix material has already been removed and loosening of the fillers results in exposure of the fibrous region to the sliding contact. In the regime under adhesive forces, often transmitted through a film, the fiber ends undergo severe thinning along their length (marked ‘E’ in Fig. 7); a similar feature was observed by Hui Zhang et al. [22]. The thinning process fractures the skin of the fibers and separates it from the surface, whereas the rest remain embedded, still contributing to the wear resistance of the composite for a certain time.
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