Tribology Engineers, Part J: Journal of Engineering

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Effect of organically modified montmorillonite clay on wear behavior of naturally woven coconut sheath/polyester composite N Rajini, JT Winowlin Jappes, B Suresha, S Rajakarunakaran, I Siva and N Azhagesan Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology published online 3 January 2014 DOI: 10.1177/1350650113515199 The online version of this article can be found at: http://pij.sagepub.com/content/early/2014/01/03/1350650113515199

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Original Article

Effect of organically modified montmorillonite clay on wear behavior of naturally woven coconut sheath/polyester composite

Proc IMechE Part J: J Engineering Tribology 0(0) 1–15 ! IMechE 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1350650113515199 pij.sagepub.com

N Rajini1, JT Winowlin Jappes2, B Suresha3, S Rajakarunakaran1, I Siva1 and N Azhagesan2

Abstract Coconut sheath fiber was surface treated with silane-coupling agent and alkali modification, respectively. The coconut sheath fiber reinforced unsaturated polyester composites with and without organically modified montmorillonite (1, 2, 3, and 5 wt %) were produced using hand lay-up technique followed by compression molding. The dry sliding wear behavior of unfilled and organically modified montmorillonite filled coconut sheath fiber reinforced unsaturated polyester composites, slid against a hardened steel disc (Rc 32), were investigated on a pin-on-disc wear test apparatus. The chemically modified fibers, examined by scanning electron microscope, revealed the changes of structure on the morphology of fiber surface. Experimental results revealed that silane treatment largely reduced the specific wear rate of the organically modified montmorillonite filled coconut sheath fiber reinforced unsaturated polyester composites. Scanning electron microscope images of the worn surfaces of the organically modified montmorillonite filled coconut sheath fiber reinforced unsaturated polyester composites showed that silane modified composites had the strongest interfacial adhesion and the smoothest worn surface under given load and sliding velocity. The maximum reduction in specific wear rate was observed for silane treated coconut sheath with 2 wt % organically modified montmorillonite into polyester for different velocities. Scanning electron microscope studies were made to investigate the wear mechanisms. Keywords Organically modified montmorillonite filled coconut sheath fiber reinforced unsaturated polyester composites, chemical treatment, specific wear rate, scanning electron microscopy Date received: 13 June 2013; accepted: 11 November 2013

Introduction Natural fibers are emerging as low cost, light weight and apparently environmentally superior alternatives to glass fibers in polymer-based composites.1 Natural fiber reinforced polymer composites (NFRPCs) such as hemp fiber–epoxy, flax fiber–epoxy/polypropylene, and sisal fiber–epoxy are particularly attractive in automotive applications because of lower cost and lower density.2–4 While natural fibers have been used to reinforce thermoset polymers, especially unsaturated polyester (USP) composites have attracted greater attention due to their added advantages such as low cost and curing time.5 NFRPCs are also claimed to offer environmental advantages such as reduced dependence on nonrenewable energy/material resources, lower pollutant emissions, enhanced energy

recovery, biodegradable, and possess good physical and mechanical properties. Most of the natural fibers, namely, coconut, oil palm, bamboo, and sugarcane fibers are discarded as waste. However, much attention has been drawn 1 Centre for Composite Materials, Department of Mechanical Engineering, Kalasalingam University, Krishnankoil, Tamil Nadu, India 2 Department of Mechanical Engineering, Cape Institute of Technology, Tirunelveli, Tamil Nadu, India 3 Department of Mechanical Engineering, The National Institute of Engineering, Mysore, Karnataka, India

Corresponding author: N Rajini, Centre for Composite Materials, Department of Mechanical Engineering, Kalasalingam University, Krishnankoil 626126, Tamil Nadu, India. Email: [email protected]



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Proc IMechE Part J: J Engineering Tribology 0(0)

to take advantage of their attractive characteristics. Natural fibers are not a problem-free alternative and they possess certain deficits in properties. Majority of natural fibers allow moisture absorption from the environment which leads to poor bonding with the matrices.6 Furthermore, the chemical structures of the fibers and matrix are different and couplings between these two phases are challenging. This leads to ineffective load transfer from the matrix to the fibers. Therefore, certain chemical treatments on the surface of natural fibers are essential. These treatments are usually based on the use of reagent functional groups that are capable of reacting with the fiber structures and changing their composition. As a result, the tendency of moisture absorption of the fibers is reduced and this facilitates greater compatibility with matrix material.7 Therefore, the usage of surface-treated natural fibers to reinforce polymer has increased drastically. For example, polyester was strengthened with oil palm fibers and showed an improvement in wear resistance.8 In general, most of the properties including wear behavior, the performance of natural fibers is influenced by the fiber weight percentage, orientation, and chemical modifications.9–12 Many of the researchers have studied the wear behavior of natural fibers such as sisal, oil palm fiber, bamboo, and jute fiber reinforced polymer composites.13–16 Several methods to modify the surface of natural fibers have been proposed. These are the graft copolymerization of monomers onto the fiber surface, the use of maleic anhydride, alkyl succinic anhydride, stearic acid, etc. It has also been reported that the use of coupling agents such as silanes, titanates, zirconates, triamine, etc. also improve fiber–matrix adhesion.17 The tribological behavior has been studied for potential use of sugarcane fiber/polyester composite by ElTayeb et al.18 The study reflects that sugarcane fiber/ polyester is a capable composite which can be a competitive to glass fiber/polyester composite. The dry sliding wear behavior of bamboo with the function of load, velocity, and relative orientation has been studied by Tong et al.19 It was observed that the normal orientated bamboo specimen shows better wear resistance. Nirmal et al.20 studied the use of alkali treated betel nut fibers as reinforcement in polyester composites in different conditions. It was reported that the wear and frictional performance of the composite were enhanced under wet contact conditions by about 54 and 95%, respectively. Eleiche and Amin21 reported the effect of sliding speed, fiber volume fraction, and fiber orientation of the unidirectional cotton fiber reinforcement on the friction and sliding wear characteristics of polyester matrix. It was also reported that the wear and friction performance of kenaf fibers reinforced epoxy composite in three different fiber orientations with respect to the sliding direction exhibited better wear performance in normal compared to parallel and antiparallel orientations.22

The higher abrasive wear resistance was noticed while placing the fibers normal to the sliding direction in unidirectional sisal fiber reinforced epoxy composites.23 Efforts were also made to study the influence of fiber modification using certain treatments on friction and wear. Bisanda and Ansell24 studied the effect of silane treatment on the mechanical and physical properties of sisal/epoxy composites and found improved mechanical properties on treatment. Chand and Dwivedi25 studied the sliding wear and friction characteristics of sisal fiber reinforced polyester composites with and without silane modification. They found that the silane treatment plays a significant role for reducing the wear and increasing the friction coefficient. Yousif et al.26 studied the effect of the chemical treatment on tribo-performance of coir fiber reinforced polyester composite. Surface modification (alkaline treated) on the coir fibers exhibited good interfacial adhesion when used as a reinforcement in the polyester composites. In recent years, the polymer nanocomposites received considerable attention owing to their high stiffness, strength, and excellent barrier properties with the addition of lower nanofiller content.27,28 The reinforcement effect of the nanofiller in the polymer nanocomposites on the wear performance strongly depends on the filler size, shape, and the homogeneity of the particle distribution.29 Recent investigations reveal that the polymer nanocomposites also exhibit good tribological properties.30,31 The formation of layered silicate nanocomposites at much lower of volume fraction reinforcements can avoid many of the costly and cumbersome processing techniques common to conventional fiber reinforced polymer composites.32 Addition of a little amount of nanoclays, typically in the range of 3–5 wt %, could provide an efficient upgrade on the mechanical and wear performances of conventional polymer-based composites.33–35 Mohan and Kanny36 studied the nanoclay filled sisal fiber reinforced composites and showed that these composites possess better wear properties than that of unfilled and microclay (>5 wt %) filled composites. Possibly, the increased hardness and strength due to nanoclay additions may have caused the improved wear properties of composites. For applications in hard working conditions, it is of great importance to develop composite materials that possess a high stiffness, toughness, and wear resistance. Less work37,38 has been reported on natural fibers and modified nanoclay reinforcement into matrix individually and proved that the nanoclay can offer better wear and mechanical performance. In the various natural plants, the fibers in the form of mat are rare; the research of this paper has used such rare mat-fibers found in coconut sheath. The mat form of coconut sheath can be able to address the solution for the fiber inconsistency and fiber orientation problems in the case of NFRPCs. The addition of modified



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montmorillonite (MMT) clay hard filler as secondary reinforcement in polymer matrix can offer better wear resistance properties and it may be expected to exhibit the solution for the moisture up taking problems in NFRPCs due to its excellent barrier characteristics. However, there is no specific work reported on wear resistance by considering the hybridization of nanoclay with coconut sheath fiber reinforced polyester composites. Therefore, it is essential to study the dry sliding wear behavior of the natural fiber/organically modified montmorillonite (oMMT) reinforced hybrid polymer nanocomposites. Hence, the current work aims to study the effect of oMMT loading on the tribological behavior of coconut sheath fiber reinforced unsaturated polyester composites (CSFUSPCs). The wear performance of the oMMT filled coconut sheath/polyester nanocomposites is focused, based on the interfacial bonding at the fiber–matrix interface after the chemical treatment.

Experimental details Materials used USP isophthalic resin (Grade 4503) is used as matrix, procured from Vasivibala Resins (P) Ltd, Chennai. The naturally woven coconut sheath is extracted from the outer bark of a coconut tree in the local areas of Virudhunagar, Tamil Nadu, India. The organically modified MMT was procured from Sigma Aldrich (P) Ltd, Bengaluru and is used as tertiary reinforcement. Proportionate wt % of the methyl ethyl ketone peroxide and the cobalt naphthenate has been used as catalyst and accelerator, respectively. The fiber architecture of the coconut sheath is shown in Figure 1.

Chemical modification Coconut sheaths were soaked in 4% NaOH solution in a water bath for 60 min and then washed with fresh water until all the NaOH deposition over the fiber

surface was removed. The mats were then left to dry at room temperature for 24 h followed by drying in an oven at 80 C for next 1 h. The untreated and NaOH treated coconut sheath based composites are designated as UTC and ATC, respectively. A 0.5% of trichlorovinyl silane coupling agent was mixed with water uniformly using hand stirrer and the mixture was allowed to stand still for 1 h. The pH of the solution was maintained at 3.5 by adding acetic acid. The coconut sheath mats were dipped in the above solution and were allowed to remain there for 1 h. The solution was drained out and the coconut sheath fibers were dried in air for 24 h at room temperature. The silane treated coconut sheath based composite is designated as STC.

Fabrication of hybrid nanocomposites In this work the fabrication of the coconut sheath fiber/oMMT clay reinforced hybrid polymer composites has been carried out in two steps. In the first step, the clay/resin mixture is stirred using mechanical shear mixer at 500 r/min for 120 min, based on the work reported in Ref. 39 to obtain a uniform dispersion of the nanoparticles within the matrix. In the second step, the CSFUSPCs were fabricated by hand layup followed by compression molding. The resin mix collected from the high shear mixer was allowed for degassing. The mats were impregnated with the resin mix having 1.25 vol. % of cobalt naphthenate and 1.25 vol. % of methyl ether ketone peroxide. During impregnation of coconut sheath fibers, the roller was used to disperse the resin mix uniformly and care was taken to remove the air bubbles. Polishing wax was applied at the sides of the cavities in the middle, top, and the bottom plates of the steel molds for the easy removal of the cured composite slab. All impregnated layers of coconut sheath mat were stacked together (six layers) and the consolidation was placed inside the mold cavity (size 300 125 10 mm3). The mold was closed completely by applying 15 MPa pressure in order to obtain the 10 mm thick composite slabs. The compressed laminate was kept for curing at room temperature for 24 h. The loading of the fiber in the matrix was about 40 2 wt %. The slab of the prepared composite was shown in Figure 4(d). The prepared composites were machined into small specimens of required size for various tests.

Microstructure

Figure 1. The fiber architecture of coconut sheath mat.

The X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to study the quality of clay dispersion in polyester. The XRD was performed using SHIMADZU, XD-DI by the step scanning rate of 2 /min with Cu-Ka radiation. The TEM images (Tecnai Sprit, FEI, and Netherlands) were taken on 50 nm thin sections from samples containing



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1, 2, 3, and 5 wt % of nanoclay. The chemically treated fiber surface of the coconut sheaths and worn surfaces of the composites were examined using scanning electron microscope (SEM) (Carl Zeiss Pvt Ltd, UK, EVO MA15 model).

Wear test details The dry sliding wear tests of CSFUSPCs were carried out using the pin-on-disc machine. The pin-on-disc friction and wear testing machine was designed and developed by Magnum Engineers, Bengaluru, and is primarily intended for determining the tribological characteristics of wide range of materials under conditions of different normal loads and temperatures. A stationary pin mounted on a pin holder is brought into contact against a rotating disc at a specified speed as the pin is sliding, and the resulting frictional force acting between the pin and disc is measured by arresting the deflecting pin holder against a load cell. Both normal load and speed can be set as desired. Some of the important technical features include: normal load range—up to 200 N, frictional force range—up to 200 N with a resolution of 1 N, wear measurement range—0–4 mm, sliding speed—0.26– 10 m/s (disc speed 100–3000 r/min), wear disc diameter—160 mm (EN31 disc 55-60 HRC). The wear test was performed on the composite specimen of size 10 mm 10 mm 9 mm according to the ASTM standard G-99, as shown in Figure 2(b). Prior to testing, the test samples were rubbed against a 600grade SiC paper. The prepared composite specimens were cleaned by a dry soft brush before and after the test. The surface of the sample comes in contact with a hardened disc of hardness 62 HRC and surface roughness (Ra) of 0.48 mm. The mounting arrangement of the CSFUSP composite sample is made in such a way that the thickness side of the laminate containing the layup consisting of coconut sheath fiber layers and

USP resin system is made to come in contact with the disc (as shown in Figure 2(b)). The tests were carried out keeping the orientation of coconut sheath layers in the composites as normal to the track radius. At the end of the test, the sample was again weighed in the same balance. The difference between the initial and final weights was a measure of the slide wear loss. For each sliding distance, a new specimen was used. The wear was calculated by the loss in weight, which was then transformed into wear volume using the measured density data. After the wear test, the sample was once more cleaned. The specific wear rate (Ks) was calculated from the following equation Ks ¼

v mm3 LXD Nm

ð1Þ

where V is the volume loss, L is the applied normal load, and D is the sliding distance. In the present study, the tests were conducted with a steel disc on composite pin samples. The experimental sequences can be explained by the two stages as described below. Initial experiments were conducted, in stage 1, on untreated CSFUSPC samples, to understand and optimize the tribological parameters for further tests. In this stage, the tests were conducted at various sliding distances of 1–6 km in steps of 1 km, at constant velocity of 3.33 m/s and for two normal loads: 40 and 60 N, respectively. In stage 2, tribological studies were conducted at a constant load of 60 N, sliding distance 3 km, and for the sliding velocities 1.66 and 3.33 m/s. The tests were conducted on UTC, ATC, and STC to investigate which composite provides the good tribological performance. The results of the stage 1 (Figure 3) were used to select optimum tribological parameters, i.e. sliding distance and load. It shows the variation of wear volume

Figure 2. Wear testing: (a) pin-on-disc machine and (b) sample with holder.



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of untreated CSFRUPCs tested under various sliding distances: 1, 2, 3, 4, 5, and 6 km for two normal loads, 40 and 60 N. As shown in Figure 3, the slopes of the specific wear rates curves are extremely steep only after 3000 m sliding distance at both applied loads. However, results show that the wear performance of the CSFRUPCs is more sensitive to the higher sliding distance than the applied normal load. Hence, the medium sliding distance of 3000 m with higher load (60 N) was selected for the further tribological tests in stage 2 by taking sliding velocities, 1.66 and 3.33 m/s.

Results and discussions Structure and morphological studies The XRD pattern of pure nanoclay powder and nanocomposites containing 1, 2, 3, and 5 wt % clay is shown in Figure 4. The XRD patterns of oMMT clay show diffraction peak at 4.5 , which corresponds to a d-spacing of 1.963 nm. The sharp peak at around 4.5 is responsible for the crystalline nanoclay. It can be seen that there is no diffraction peak of clay inclusion in 1, 2, 3, and 5 wt % clay nanocomposites. This

Figure 3. Specific wear rate as a function of sliding distance for CSFUSPCs.

Figure 4. XRD pattern for 1, 2, 3, and 5 wt % of oMMT filled CSFUSPCs.



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Figure 5. TEM images of oMMT filled polyester nanocomposites: (a) 1 wt %, (b) 2 wt %, (c) 3 wt %, and (d) 5 wt %.

indicates that either the d-spacing of the intercalated clay layers is larger than 1.963 nm (2y < 3 ) or the layers are exfoliated (i.e. clay layers are separated/dispersed more than 7.5 nm) in the polyester resin. The increased d-spacing may be attributed to the intercalation of the polyester chain into the clay layers during processing. Addition of 5 wt % of nanoclay leads to a shifting of peak moving toward lower angle which indicates less d-spacing between platelet which causes the formation of intercalation. Figure 5 shows the TEM images of different weight percentages of oMMT nanoclay with the polyester matrix. Figure 5(a) to (d) shows the separation of the clay platelet (1, 2, 3, and 5 wt %) of oMMT nanoclay in polyester by means of beach mark patterns, which attributed to the formation of intercalated/ exfoliated structure. The distance between the clay platelets gets increased by the delamination at lower clay content from the clay tactoid sheet. When the wt % of oMMT nanoclay (5 wt %) increases, the presence of clay aggregates has been noticed from the appearance of a dark black band river pattern (Figure 5(d)). This could happen due to the immobility of clay in a highly viscous clay mixture, and it was controlled by the presence of the surrounding clay platelets during mixing. Figure 6(a) to (c) shows the SEM images of UTC, ATC, and STC treated CSFUSPCs. The appearance of the waxy layer over the coconut sheath was identified from the white color cup like structure and it

leads to the incompatibility between the fiber and the matrix (Figure 6(a)). The NaOH treatment removes the waxes and any impurity present on the fiber surface and thus increases the roughness of the fiber surface and causes the fibers to become stiffer. The removal of waxy layers can expose the segregation of fibril in fiber surface. The parallel orientation of fibril to the fiber axis can make the fiber more rigid, inflexible, and high stiffness.40 Rout et al.41 have done the SEM studies of the coir fibers and show significant morphological changes after the removal of waxy layer from the surface by giving an alkali treatment. The arrangement of parallel fibrils after the alkali treatment was indicated by the arrow in the SEM image shown in Figure 6(b). The inherent changes in structural integrity on fiber surface after the chemical treatment could be the reason for providing higher stiffness. The same kind of results was also reported for the ATC sheath fiber by Obi Reddy et al.42 Fibrillation (breaking down the fibers into smaller ones as shown in (Figure 6(b)) takes place after the alkali treatment and it increases the effective surface area available for contact. The rough surface of fibers tends to increase the interfacial adhesion between the fiber and the matrix by mechanical interlocking mechanism as shown in Figure 6(b). On the other hand, a coating of thin film deposition over the surface of STC sheath can be seen in Figure 6(c). Once the surface of the coconut sheath was covered by a thin film of trichlorovinyl silane, a good physical adhesion



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Figure 6. SEM images of (a) untreated coconut sheath fiber, (b) alkali treated coconut sheath fiber, and (c) silane treated coconut sheath fiber.

between the fiber and the polyester matrix is promoted.

Specific wear rate Specific wear rate is the key parameter to study the tribological performance of the material and it was defined as a volume of material removed/unit time and it was calculated by measuring the weight loss of the specimen. The addition of filler in the form of particulate or fibers in matrix can offer better resistance to the material loss of composites. The variation in the specific wear rate of unfilled and oMMT filled CSFUSPCs is discussed in the following sections. Effect of velocity on untreated and treated CSFUSPCs. The specific wear rate changes with respect to the sliding velocity and the chemical modification of the coconut sheath at a constant load of 60 N are shown in Figure 7. The variation of the specific wear rate of the untreated and the treated composites with the sliding velocity (Figure 7) showed that wear rate of untreated CSFUSPC increased with increase in the sliding velocity, whereas for treated composites, the

wear rate increases marginally with the increase in the sliding velocity. The dissimilar wear behavior of these composites with the sliding velocity can be due to the different surface morphology of the coconut sheath mats which, in turn, affects the fiber–matrix interface. The chemical modifications largely affect the wear rate of CSFUSPCs. Several studies have been carried out43–46 on the alkali treatment of natural fibers and illustrate that the formation of irregularities on the fiber surface leads to the mechanical interlocking at the interface which can enhance the mechanical strength of natural fiber composites. Siva et al.47 investigated the dry sliding wear behavior of coconut sheath/glass fiber reinforced hybrid polyester composite. They concluded that the removal of the waxy layer in by alkali wash followed by silane treatment increased the mechanical interlocking between the coconut sheath fibers and polyester matrix. In the case of the UTC, the poor interfacial adhesion was observed as a result of the waxy layer over the fibers and it leads to a weak interfacial bonding at the interface. Further, it can be seen from Figure 7 that there is a significant improvement in the wear resistance for the unfilled silane treated



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Figure 7. Specific wear rate of unfilled CSFUSPCs with different chemical treatment.

CSFUSPCs. Normally, the NaOH treatment has been done before the silane activation to remove the waxy layer and to create a rough surface over the fiber. A thin film deposition of silane coating over the fiber surface leads to the formation of a strong interfacial bonding between the fiber and matrix. From Figure 7, it is also clear that the specific wear rate of the silane treated CSFUSPCs has shown maximum reduction of 48% as compared to the untreated CSFUSPCs at a velocity of 3.33 m/s. The improved specific wear rate at different sliding velocity for the silane treated CSFUSPC can be attributed to the fiber matrix interfacial characteristics of the composite. Although, at higher sliding velocity, little debonding of fibers is seen due to repeated axial thrust, no detachment of fiber pull out has occurred during the sliding. This indicates a good interfacial adhesion between the matrix and the fiber. Also, fiber breakage was very less which indicates higher load carrying capacity of the coconut sheath fiber.47 However, for alkali (NaOH) treated CSFUSPCs, the wear rate is less as compared to the untreated CSFUSPCs. This could be due to the strong mechanical interlocking between the fiber and the matrix. It was caused by the formation of open sites over the fiber surface (Figure 8) after the removal of waxy layers and such morphological changes on fiber surface were observed in our previous work.48 Therefore, it can be stated that both silane and alkali treatment of the coconut sheath mat helped in improving the wear resistance of USP matrix. As for the specific wear rate of the untreated and chemically treated CSFUSPCs, at a lower velocity (1.66 m/s) no significant variation was observed between alkali and untreated condition and it may be due to the absence of interfacial separation at the interface of composites by the insufficient sliding

Figure 8. SEM image of alkali treated coconut sheath fiber with circular pores.

speed. Incorporation of trichlorovinyl silane as coupling agents in the pretreated (alkali) coconut sheath fibers significantly improved the stiffness of the composites. Moreover, the silane treated CSFUSPCs were found to have a wear resistance higher by 27% more than that of alkali treated CSFUSPCs at 3.33 m/s velocity. Even at lower velocity (1.66 m/s) also, significant specific wear rate reduction (15%) was observed compared to alkali and untreated case. The presence of thin film coating over the fiber surface could create the chemical reaction between fiber and matrix and it executes a strong chemical bonding between the fibers and the matrix. This formation of covalent bonds after the silane treatment provides more resistance at the sliding surface. The patching of silane after the sliding wear could form a polished layer over counterface which could separate the debris from the contact surface.



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Figure 9. Specific wear rate of untreated, treated (alkali and silane) CSFUSPCs at 3.33 m/s without and with oMMT. oMMT: organically modified montmorillonite.

Effect of oMMT addition and chemical modifications at different velocity. Besides, that the addition of oMMT in CSFUSPCs could further improve the interfacial adhesion by acting as a crack arrester and transfer the load effectively. As shown in Figure 9, the specific wear rates are decreased with respect to the wt % of oMMT at the velocity of 3.33 m/s. The transition in specific wear occurred beyond 2 wt % of oMMT wherein decreasing trend shifted to slight increase in the wear rate for 3 and 5 wt % oMMT filled CSFUSPCs. The highest wear resistance was obtained for 2 wt % of oMMT addition for all CSFUSPCs and thus was found to depend on the microstructure associated with fine, well-dispersed oMMT platelets. The influence of the oMMT platelets on wear resistance depends on their hardness relative to the matrix hardness. Rashmi et al.49 studied the effect of varying wt % of clay content as reinforcement in polyester matrix. The experimental results showed that the inclusion of oMMT nanoclay increased the wear resistance of the epoxy nanocomposite significantly. Mechanical and abrasive wear studies of carbon fabric/oMMT/epoxy hybrid reported by Suresha et al.50 showed that the tensile strength and modulus of hybrid composites increase with the addition of oMMT clay content. Jawahar et al.51 studied the clay/thermoset polyester nanocomposites and conventional clay filled composites are produced using oMMT clay as the reinforcement and unmodified inorganic clay as filler, respectively. They found that the wear resistance increases significantly on addition of organically modified nanoclay, whereas wear resistance increases with increase in clay content. Most recently, the effect of silane-treated halloysite nanoparticles into highly crosslink USP matrix was studied by Albdiry and Yousif52 to explore the morphological structure and tribological performance of nanocomposite. They empathized that the uniform morphological dispersion of halloysite particles in the USP

Figure 10. Specific wear rate of untreated, treated (alkali and silane) CSFUSPCs at 1.66 m/s without and with oMMT.

matrix can induce a modest decrease in a specific wear rate for silane-treated halloysite particles. Similar results were obtained in the present work. The failure of matrix by thermal softening can be minimized by adding oMMT clay because the hard particles embedded in the matrix prevent the catastrophic failure of the softened polyester at the sliding surface. The same kind of specific wear rate trend obtained from 3.33 m/s was also observed for the UTC, ATC, and STC CSFUSPCs with the addition of oMMT at the velocity of 1.66 m/s as shown in Figure 10. The addition of 3 and 5 wt % oMMT content shows the increasing wear rate than the clay unfilled composites but that is not the case for 3.33 m/s velocity. Lower sliding velocity can shear out the oMMT platelets at the weaker clay/matrix interface and it could be due to the agglomeration of oMMT platelets at the higher loading. However, it is clear that the lower wt % of oMMT addition can improve the wear resistance of all the samples (both untreated and treated) significantly. It is well known that, owing to their dispersed layer structure, the oMMT platelets can be easily separated by shear force during sliding and can form a transfer film on the counterpart, which can effectively reduce both frictional force and thus improve the wear resistance. The enhancement of the mechanical properties of polyester matrix was observed in 2 wt % clay loading.39 The bonding strength between the transfer film and the counterface improved and the frictional effect decreased due to the rolling effect of nanoparticles, as in the reports presented by others.53–55 Transfer film on the counterface during sliding wear test. There are several reasons for adding fiber/fillers into polymers to make composites. One reason is to increase the polymer load carrying capacity. In general, fiber reinforced composites have better load carrying capacity. A second reason for making a



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Figure 11. Optical micrographs showing transfer films in steady state sliding wear by unfilled and nanoclay filled CSFUSP composites against steel counterfaces of (a) unworn 0.48 mm, (b) 0.43 mm, and (c) 0.41 mm Ra roughnesses (sliding directions are indicated by white arrows).

composite is to include lubricating additives to help in reducing the friction and increasing the wear resistance of composites. Usually, the formation of transfer film layer on the counterface during sliding wear process helps in reducing the coefficient of friction as well as wear rate. Optical micrographs in Figure 11(a) to (c) show CSFUSPCs samples slide against a steel counterface. From Figure 11(b) and (c), it is seen that a thin transfer film formed on the counterface during sliding wear process. Continuous transfer film can be seen (Figure 11(c)) on the counterface for the oMMT filled CSFUSP composite sample. From Figure 11(c), it is also clear that the increase of transfer film bond strength by the addition of oMMT nanoparticles is accounted for mechanical effects. The most plausible mechanical action is obviously the interlocking of the transfer film into the counterface asperities. The dependence of the transfer film bonding strength on the counterface surface roughness supports this explanation. The surface roughness of the counterface was measured using Mitutoyo (Surftest SJ-301) instrument for the unfilled and oMMT clay filled composites. The significant decrease in surface roughness value (0.41 mm) was observed on wear track of clay filled composites compared to unfilled composites (0.43 mm) and unworn (0.48 mm) surfaces of the track. It is believed that oMMT nanoparticles in the transfer film provide stronger anchoring and hence increased bond strength by virtue of their positioning in the

surface asperities of the counterface. Thus, the continuous transfer film formed on the counterface during sliding wear test of oMMT nanoparticles is responsible for the significant reduction in the wear rate and coefficient of friction of CSFUSP composites. SEM analysis of worn surfaces. The SEM of the worn surface of the UTCs, ATCs, and STCs with (2 wt %) and without clay addition is shown in Figure 12. Figure 12(a) to (c) shows the SEM photomicrographs of the worn surfaces of the composites made of unmodified and modified CSFUSPC (without oMMT) under dry sliding conditions with 60 N load, 3000 m sliding distance, and at a sliding velocity of 3.33 m/s. Concerning the effect of chemical modifications, in both cases, i.e. either untreated or treated (alkali and silane), the treated CSFUSPCs exhibited always a better wear resistance than the untreated. The effect, especially due to the interfacial debonding, seemed to be more pronounced with untreated composites, as shown in Figure 12(a). The sliding direction of the composites is marked with the white arrowhead. In fact, at a constant load of 60 N, macro-cracks and pulverized fibrous debris can be seen in Figure 12(a), while they were not observed in the case of the treated composites. This reflected that the treated fiber composites possessed a higher load-bearing capacity, which was probably due to their better stiffness and strength. From Figure 12(a), it can be seen that a pronounced



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Figure 12. SEM of worn surfaces of CSFUSPCs without and with 2 wt % of oMMT at 3.33 m/s: (a) UTC, (b) ATC, (c) STC, (d) UTC with 2 wt % oMMT, (e) ATC with 2 wt % oMMT, and (f) STC with 2 wt % oMMT.

crack formation is identified due to sheared destruction of coconut sheath fiber–matrix interface on the worn surface of untreated composite; this indicates that the coconut sheath fiber performs poor interface bonding with the polyester matrix. The debonding of the coconut sheath fiber mat from the polyester resin matrix shifted the wear mechanisms from adhesive wear to abrasive wear. As shown in Figure 12(b) and (c), no significant difference can be recognized on the worn surfaces under the same loading conditions, although the

worn surfaces of the silane treated CSFUSPCs (Figure 12(c)) look a bit smoother than those of the other ones (Figure 12(b)). However, the worn surface of silane treated composite was relatively smoother (Figure 12(c)), and the phenomenon of pronounced crack propagation at the interface between the layers stopped and it offers high resistance to wear of composite. For NaOH treated CSFUSPCs, Figure 12(b) shows destruction of surfaces due to matrix broken on the worn surface. This worn surface shows coconut sheath fibers debonding (the small gap



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around the fiber) with polyester matrix that was deformed under sliding action. In the case of the nanocomposite filled with 2 wt % clay loading, as seen from SEM, the fine and smooth surface is noticeable as shown in Figure 12(d) and (e). The dispersed nanoclay also acts as reinforcement, bears the loads, and reduces the wear rate. The improved hardness is due to the addition of clay which also has been one of the reasons for the wear improvement. Due to the presence of soft (matrix) and hard (clay) phases, the region in the matrix phase can be removed easily. Very fine micro cracking are noticed in Figure 12(d) to (f) and it could be resulting from the nanoclay pulled out of the matrix, and then further encouraged across the surface by scratching and rolling. Due to their smaller in size, the nanoparticles should be able to travel into gaps and asperities on the counterpart surface and may then be exposed to rolling rather than sliding/ scratching movement. More matrix damage was not found. Probably, due to this reason, the worn surface became relatively smooth, and consequently, the frictional force between the mating surfaces and the wear rate dropped to some extent, as shown in Figure 12(d) to (f). However, the wear resistance reduces if the clay

loading exceeds 3 wt % of oMMT and the higher clay loading cannot offer any wear reducing effect. In particular, the worn surface of 2 wt % oMMT filled silane treated CSFUSPCs was the smoothest and there are no signs of coconut sheath fiber damage to be seen (Figure 12(f)), which confirmed to the best wear resistance of the composite. From the above, it is clear that the surface treatment of the coconut sheath fiber can improve the adhesion between the coconut sheath fiber and the polyester resin matrix, which can transmit the load from the matrix to the fibers efficiently and prevent the peeling off of the coconut sheath layers and resulted in better tribological behavior. The fibers are protected from destruction by the mask of silane coating and only the matrix is subjected to micro-ploughing and micro-cutting and to some extent, attacks by steel asperities so that by all these sources, less wear of the matrix only occurs. Figure 13 shows the magnified SEM morphologies of the worn surfaces of the composites made of unmodified and modified coconut sheath fibers under 1.66 m/s velocity. As shown in Figure 13(a), the surface of the coconut sheath fibers is subjected to micro growing, micro-cutting and is with fiber–adhesive interface gap which is significantly

Figure 13. SEM of worn surfaces of unfilled CSFUSPCs at 1.66 m/s: (a) UTC, (b) ATC, and (c) STC.



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small, which indicated the adhesion between the coconut sheath fiber and the polyester matrix of unmodified CSFUSPCs. For ATC and STC CSFUSPCs, the coconut sheath fiber and the polyester resin were still bonded to some extent after friction indicating that the bonding between the coconut sheath fiber and the polyester resin matrix was improved (Figure 13(b) and (c)), which was responsible for the slight increase in wear resistance of the composite compared to the unmodified one. The decrease in velocity (worn under 60 N) decreases the severity of fiber damage process leading to lower damage to the surface and fibers. This fact is supported by the topography of surfaces. For the untreated CSFUSPCs, Figures 12(a) and 13(a) are compared; severity of increased damage in terms of few debris formation and macro-crack at interface can be seen in Figure 12(a). The difference in the fiber–matrix adhesion at the interface for UTC and STC CSFUSPCs can also be seen in Figure 13(a) and (c). Thus, worn surfaces showed fairly good correlation with wear performance and fiber–matrix interface.

Conclusions Based on the studies on oMMT filled composites developed with polyester resin matrix and coconut sheath fiber surface treated with NaoH and silane modifications, the following conclusions were drawn: 1. Both silane and NaoH chemical treatments can help improve the wear resistance of oMMT filled coconut sheath fiber reinforced polyester resin matrix composites. 2. Treated composites showed better wear resistance. The enhancement in adhesion between matrix and coconut sheath fiber led to improved wear performance properties of oMMT filled polyester composites. However, the wear resistance did not increase linearly for untreated and NaOH treated coconut sheath with oMMT filled polyester composites. 3. The worn surface morphology of the oMMT filled composites with silane modified fiber obviously revealed improved interfacial bonding between the fiber and the polyester matrix, which is responsible for the improvement in wear resistance of the composites. 4. The significant reduction in surface roughness value was observed for oMMT clay filled composites compared to the unfilled composites and it ensures the formation of transfer film at the counterface surface. 5. Studies on wear mechanisms indicated that the dominating wear mechanisms during adhesive mode of composites were micro-cracking, microcutting, and peeling off of pulverized fibrous debris from the matrix.

6. Overall it was concluded that silane treatment to coconut sheath fiber into polyester resin matrix is beneficial only when the loading of oMMT employed judiciously. Funding This work was supported by the Department of Science and Technology [grant number SR/FTP/ETA -92/2009].

Conflict of Interest None declared.

Acknowledgements The authors wish to thank the Department of Science and Technology, India for the funding through SR/FTP/ETA92/2009 project and also the Center for Composite Materials, Department of Mechanical Engineering, and Kalasalingam University for the constant encouragement throughout the work and for their kind permission to carry out the preparation and testing of the composites.

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Tribology Engineers, Part J: Journal of Engineering

Tribology Engineers, Part J: Journal of Engineering

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