In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres

Materials and Design xxx (2012) xxx–xxx

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In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres A. Shalwan ⇑, B.F. Yousif Centre of Excellence in Engineered Fiber Composites (CEEFC), Faculty of Engineering and Surveying, University Southern Queensland, Toowoomba, 4350 QLD, Australia

a r t i c l e

i n f o

Article history: Available online xxxx

a b s t r a c t In this article, a comprehensive literature review on the mechanical and tribological behaviour of polymeric composites based on natural fibres is introduced. The effects of volume fraction, orientations, treatments and physical characteristics of different types of natural fibres on the mechanical and tribological properties of several thermoset and thermoplastic polymers are addressed. The effects of the tribological operating parameters (applied load, sliding velocity and sliding distance) on the frictional and wear performance of natural fibre polymer composites are demonstrated. The collected date and analyses revealed that volume fraction, orientations, type of treatment and physical characteristics of the natural fibres significantly influence the mechanical and tribological behaviour of composites. The most influence key in designing natural fibre/polymer composite is the interfacial adhesion of the fibre with the matrix. NaOH chemical treatment found to be the most useful treatment method to enhance the interfacial adhesion of the natural fibres with the matrix, while other techniques exhibited either no effect or deterioration on the fibre strength. Frictional characteristics of the natural fibre composites are poor and solid lubricants are recommended to reduce the friction coefficient of the materials. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Natural fibres Mechanical Tribological

applications in which the synthetic fibres have been replaced with natural fibres in composites. These cases are as follows:

1. Introduction In the last few years, natural fibres have become an attractive reinforcement for polymeric composites from economically and ecologically points of view. There is increase in the environmental awareness in the world which has aroused an interest in the research and the development of biodegradable and highperformance materials. Natural fibres can be obtained from natural resources such as plants, animals or mineral. With the increase of global energy crisis and ecology risk, the unique advantages of plant fibres such as abundant, non-toxic, non- irritation of the skin, eyes, or respiratory system, non-corrosive property, plant-based fibrereinforced polymer composites have attracted much interest owing to their potential of serving as alternatives reinforcement to the synthetic [1,2]. Natural fibres have lower cost (US$ 200–1000/ton) and energy to produce (4 GJ/ton) than glass (cost: US$ 1200– 1800/ton and energy to produce: 30 GJ) and carbon (cost: US$ 12500/ton and energy to produce: 130 GJ) [3]. The lower weight (20–30 wt.%) and higher volume of natural fibres compared to synthetic fibres improve the fuel efficiency and reduced emission in auto applications [4–6]. Joshi et al. [6] had discussed and compared the life cycle environmental performance of natural fibre composites (NFCs) with glass fibre composites (GFCs) by reviewing three ⇑ Corresponding author. Fax: +61 46315331. E-mail addresses: (A. Shalwan).

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(1) replacing a side panel for Audi A3 car made from Acrylonitrile–Butadiene–Styrene (ABS) co-polymer with hemp fibre/epoxy resin composite [4], (2) replacing an insulation component for a Ford car made from ethylene propylene diene copolymer (EPDM), polypropylene (PP) and reinforced with glass fibres with hemp fibre/EPDM/ PP composite reported [7], (3) and replacing transport pallets made from glass fibre reinforced PP with china reed fibre reinforced PP [5]. In all these applications, natural fibre composites (NFCs) has emerged more environmental friendly than GFCs and appeared a realistic alternative for the following reasons: (1) natural fibre (NF) production has consumed of non-renewable energy lesser than glass fibre (GF) and thus lesser pollution emissions, (2) the higher volume fraction of NF than GF for equivalent performance has decreased the volume and weight of base synthetic polymer matrix, that decreases the energy use and emissions in production of polymer, (3) the lower weight (20–30 wt.%) and higher volume of NF compared to GF has improved the fuel efficiency and reduced emission in the use phase (auto applications),

0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.07.014

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(4) and the incinerated of NFC direct to positive carbon credit, and enhanced the net effects on air emissions and energy recovery due to the lower mass of base polymer in NFCs. In the recent years, natural fibres such as hemp, flax, jute, linen, kenaf, oil palm, and bamboo have been drawn considerable attention in numerous applications, e.g. automobiles, furniture, packing and construction, [6,8–17]. This is due to their superiors advantages over synthetic fibres in term of relatively low cost, low weight, less damage to processing equipment, improved surface finish of moulded parts composite, good relative mechanical properties, abundant and renewable resources [4–6]. This attracts the researchers to investigate the possibility of using natural fibres in various applications and under different loading conditions. Fig. 1 shows the number of articles published on synthetic and natural fibres for the past 5 years, which indicates the interest of the researcher in this topic. However, the participant authors found a necessity to address the common issues in using natural fibres and possible solutions to those issues. In the light of this, the current review effort is motivated to address these issues and limitations from mechanical and tribological point of views. 2. Mechanical properties of natural fibres/polymer composites From mechanical point of view, natural fibres may enhance mechanical properties of polymers with some considerations and improvement to the surface characteristics natural fibre. There are several factors related to the natural fibres which influence the performance of the composites such as the interfacial adhesion, the orientation, the strength, physical properties, etc. The mechanical efficiency of the fibre-reinforced polymer composites depends on the fibre–matrix interface and the ability to transfer stress from the matrix to fibre as reported by many researchers, [9,11,14,18– 26]. Moisture absorption, impurities, orientation, volume fraction and physical properties of natural fibres play constitutive role to determine the mechanical properties of fibre polymer composites. Fig. 2 shows the effect of reinforcing polymers with different types of natural fibres on the mechanical properties of polymers. In most cases, natural fibres reinforced polymer composites exhibit better mechanical properties than the pure matrix. In other words, the using natural fibres as reinforcement for polymeric composites introduces positive effect on the mechanical behaviour of polymers. Addition of jute fibres [27] to poly(lactic acid) (PLA) showed 75.8% enhancement to the tensile strength of PLA while flax fibres exhibited negative impact, i.e. decrease the tensile strength of composite by 16%. On other hand, kenaf [28], hemp [2], and cotton [29] improved the tensile strength of PP composite. Tensile strength of epoxy improved with the addition of jute fibres [30] but the jute fibres deteriorated the compressive strength. Meanwhile, jute enhanced all the mechanical properties of polyester

Fig. 1. Number of synthetic and natural fibre reinforced polymeric composite articles. Source: http://www.ScienceDirect.com, keywords used: natural fibres, reinforcement, polymers, synthetic fibres.

composites [30]. Jute/polyester composite has showed the maximum improvement in tensile strength by 121% compared to pure polyester. 2.1. Interfacial adhesion of natural fibres Mechanical properties of polymeric composites based on natural fibres strongly depend on the interface adhesion between the fibres and the polymer matrix [14,15,31–33]. This is mainly because natural fibres are rich in cellulose, hemicelluloses, pectins and lignin, which are hydroxy1 groups, i.e. natural fibres tend to be strong polar and hydrophilic materials whilst polymers exhibit significant hydrophobicity. In other words, there are significant problems of compatibility between the fibre and the matrix, which weakens interface area between natural fibres and matrices. However, many investigators reported that chemical treatments such as bleaching, acetylation and alkali treatment may improve the matrix–fibre interfacial adhesion [9,11,13,31,34–36]. These chemical treatments are process of cleaning the surface of the fibres from impurities which in turn increases the roughness of the fibre surface and disrupting the moisture absorption process through removing the coat of OH groups in fibre as illustrated in Fig. 3. Several works have been attempted to study the influence of the type and concentration of chemical solution on the fibres’ characteristics and their interfacial adhesion with various matrices. For example, Alawar et al. [37] investigated the effects of two kind of chemical treatments in different concentrations (NaOH 0.5–5% and HCL 0.3–1.6 N) on surface morphology, and mechanical properties of date palm fibre (DPF). The results of that work revealed that NaOH is able to enhance surface morphology of fibre and increases the number of pores on fibre surface with the increase of the concentration. This could be owing to increase the cruelty of reaction of NaOH on fibre whenever increasing the soda concentration. In addition, the tensile strength and young’s modulus of the fibre have been improved compared with untreated fibre at low concentrations of alkali treatment. The optimum alkali concentration was at 1% where enhance the tensile strength 300% compared with untreated fibre. At high NaOH concentrations, the solution attacked the main construction components of the fibres which weakened the fibre strength. On the other hand, HCL treatment deteriorated the tensile and huge distortions on natural fibre surface has been observed which was due to the attack of acid. Similar findings of HCL effects has been reported on bamboo fibres in [38]. With regards to the chemical treatment technique and conditions, Saha et al. [13] studied the influence of alkali treatment (NaOH) on the tensile strength of jute fibres under ambient temperatures and elevated temperatures at high pressure steaming conditions. The use of NaOH under all conditions had resulted in a rougher surface and better separation and removing of impurities, non-cellulosic materials, inorganic substance and wax. The range representing O–H stretching of hydrogen bond became less intense upon alkali treatment compared to untreated fibres. The tensile strength and elongation at break of the fibre have been improved by 65% and 38%, respectively at elevated temperature and his pressure steam conditions. Many studies have been used the single fibre fragmentation tests (SFFTs) to evaluate the interfacial shear (s) of natural fibre with matrix [32,33,39–45]. An example to this method, the interfacial shear of untreated and pre-treated sisal fibres (with stearic acid) with polyester matrix has been studied, [32]. The treatment on the fibres had improved, i.e. s has been increased by about 23% with respect to untreated fibre due to reduce the size and number of fibre clumps and agglomerates during standard processing operations which enhances in bonding between the fibre and the matrix. That has been observed on the fractographs of the

Please cite this article in press as: Shalwan A, Yousif BF. In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres. J Mater Design (2012), http://dx.doi.org/10.1016/j.matdes.2012.07.014

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160

140 Tensile Strength

140

Compressive strength

120

Modulus of Elasticity

Strength, MPa

120

Flexural Strength Flextural Modulus

100

80 80 60 60 40

Modulus , GPa

100

40

20

20

0

0

Fig. 2. Some of the mechancial properties of natural fibre/polymer composites.

lower than the neat PLA by 30%, whilst the treated bamboo/PLA composite exhibited higher tensile strength and modulus by about 10%, 4%, respectively, compared to the neat PLA. The effects of NaOH or silane treatment on the tensile strength of bamboo/PP and henequen/HDPE are similar to the bamboo/PLA composite results. On the other hand, treated bamboo/PLA and ramie/PLA with NaOH is more efficiency than silane treatment. 2.2. Effect of orientation of fibre on mechanical properties

Fig. 3. Scheme of reaction of fibre surface with NaOH treatment.

samples after tests, where the untreated fibre showed large fibre pullout compared to the treated fibre. For the composites based on natural fibres, Fig. 4 displays the influence of NaOH and silane treatment of natural fibres on the tensile strength of bamboo/PLA [46], ramie/PLA [47], bamboo/PP [46], and henequen/HDPE [48] composites. It can be seen that the treated fibre/polymer composite showed better tensile strength compared to untreated fibre/polymer composite. Moreover, the tensile strength of untreated bamboo/PLA composites is

In fibre polymeric composites, the shape of composite and its surface appearance were awarded by matrix while fibres acts as carriers of load and stress (stiffness and strength) under loading conditions. Therefore, the orientation of natural fibres will have significant effects and play an important role in controlling the mechanical properties of the composites. It has been reported that Young modulus (E), Poisson ratio (m) and tensile strength of the Alfa/Polyester composites decreased with the increase of the fibres orientation (0°, 10°, 30°, 45° and 90°) with respect to the direction of the applied load, [49]. The reduction percentages of tensile strength with respected to the change of fibres’ angle were 78% and 88% when the fibres’ oriented in 45° and 90° (transverse direction) respectively. At the transverse direction, the mechanical properties of fibre/polymer composite are controlled by the matrix rather than the fibres, and vice versa. On the same level, all

Tensile Strength MPa

80 70 60 50 40 30 20 10 0

Fig. 4. Tensile strength of polymeric composites based on natural fibres with/without treatment.

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reported works on the natural fibre/polymer composites reached the same conclusion, e.g. sisal/oil palm/natural rubber [50], and others [18,19,50,51]. However, this could not be correct in the case of other loadings, e.g. under tribological loading, orientation of the fibres exhibited different influence on the tribological performance of the composite. This will be clarified in the tribology section.

tion of this behaviour ambushes in the fibre structure and the probability of defects increase in the fibre. By nature, natural fibres are bundle of fine fibres called microfibrils and the strength keeping these microfibrils together is a lot lower than their tensile strength. With large fibres’ diameter, the number of microfibrils increase in the fibre which increases the probability of inter cells failure and lower overall mechanical properties, [20,21,43,64,65].

2.3. Effect of volume fraction In theoretical model, the increase of high strength fibre volume fraction (Vf) leads to high tensile properties of fibre polymer composite, i.e. proportional relation [25,52,53]. However, from experimental point of view, the increase of Vf over specific value always deteriorates the mechanical properties. Several studies have been conducted to find the optimum value of the natural fibre volume fraction in composites to gain the optimum mechanical properties. Some of the reported works on the optimum volume fraction of natural fibres in some polymeric composites are summarized in Table 1. It seems that there is no universal value of fibre volume fraction of natural fibres in which optimum tensile strength can be achieved, i.e. for each type of fibre there is an optimum volume fraction exhibiting good tensile strength. This can be related to the nature of natural fibres and their characteristics in term of strength, interfacial adhesion, physical property, etc. Moreover, those reported works agreed to that at high volume fraction 50%, the fibres tend to aggregate in the composite which weakens the interfacial area and deboning between the fibres and matrix [31,49,50,52–55]. 2.4. Effect of fibre physical properties The individualism increase in length or decrease in diameter of natural fibre have positive effects on the mechanical properties of polymer composite, [20–23,43,53,60–65]. Decrease of fibre length more than the critical length reduces the stress transfer efficiency between the matrix and the fibre. Moreover, maintaining optimum volume fraction of fibres with short fibres increases the number of fibre’s ends that act as crack initiators, i.e. deteriorate the mechanical properties of composites. Liu et al. [62] found that the critical fibre length for kenaf/soy composite is 6 mm. Mylsamy and Rajendran [63] found the optimum fibre length for Agave/epoxy composite is 3 mm. This difference in critical length is mainly due to the differences in the interfacial adhesion of the fibre with the matrix that in turn controls the shear between the fibre and the matrix. On the other hand, small natural fibre diameters have positive effect on the mechanical properties of fibre/polymeric composites. The explana-

Table 1 Comparisons between various existing fibre-reinforced composites. Materials

Optimum tensile strength at Vf (%)

Sisal–oil palm/natural rubber [50] Coir/PP [31] Palm/PP [31] Hemp/PP [56] Flax/HDPE [52] Rice/HDPE [55] Kenaf/PP [57] Jute/PP [57] Hemp/PLA [58] Jute/PBS [59] Alfa/polyester [49] Sisal/rubber [50] Oil palm/rubber [50] Kenaf/ corn-starch [53] Bagasse corn-starch [53] Ramie cloth/ polyester [54]

30 15 15 40–50 20 5–10 40 40 35 20 44 30 30 50 50 30

3. Tribological performance of polymeric composites based on natural fibres Most of the industrial and manufacturing parts are exposed to tribological loadings such as adhesive, abrasive, etc. in their service. Therefore, tribological performance of materials becomes an essential element to be considered in design mechanical parts. In other words, understanding the tribological behaviour of natural fibre/polymer composites has an equal role to be considered with the mechanical properties of those materials [17]. Nevertheless, less work has is found on the effects of natural fibres on the tribo-performance of polymeric composites in the literature. Some studies have been emphasized that the tribology behaviour of composite polymers based on the natural fiber is not intrinsic behaviour and it strongly depend on many processing’s parameters such as operating parameters, characteristics of polymer martial, physical and interfacial adhesion properties of fibre, additives and contact condition. Few works have been attempted to investigate the tribological behaviour of polymeric composites based on natural fibres such as Kenaf [15], Oil palm [36], Sisal [66], Cotton [67], Jute [68], Betelnut [17], Bamboo [69]. From those reported works, there are few issues can be addressed which are 1. Operating parameters influence the wear and frictional behaviour of the major of polymer composites. 2. Reinforcing the polymer with natural fibres may enhance the wear performance of the composite. However, it is not true for the entire composites. Therefore, further investigation is required. 3. Chemical treatment to the natural fibres enhances the interaction between the fibres and the matrix at the interface. However, the type and the concentration of the chemical treatments vary from type to type. 4. Addition of solid lubricants controls the shear resistance in the interface zone. This could either enhance both friction or wear behaviour of enhance the friction and worsen the wear. Optimization is recommended. 5. Mathematical modelling always far away from the real experimental results. However, introducing artificial neural networks modelling may save the time and predict some of the results. Polymers have displayed different tribology behaviours with different type of natural fibres. Table 2 lists most recent works on the tribological performance of natural fibre/polymer composite associated with glass/polyester one for comparison purposes. Furthermore, the specific wear rate and fractional trends against sliding distance are given in the table. In general, one can say that after certain sliding distance, steady state can be achieved. However, in the running period (first stage of the sliding), there is difference in the wear behaviour of the composites. In some cases such as polyester [70], chopped glass/polyester [71], cotton/polyester [66] and kenaf/epoxy [15], the composite showed low specific wear rate and an increase in the steady state stage which is due to the adaption period of the two rubbed surfaces in the running in stage. Moreover, in those works, it has been reported that the film generated on the counterface became smoother at the steady state than the running in. However, in coir/polyester [70],

Please cite this article in press as: Shalwan A, Yousif BF. In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres. J Mater Design (2012), http://dx.doi.org/10.1016/j.matdes.2012.07.014

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Table 2 Adhesive wear and coefficient friction result of neat polymers and natural-fibre composites under dry contact.

sisal/polyester [66], betelnut/polyester [17] and bamboo/epoxy [69] showed the opposite, i.e. high specific wear rate at the first stage and then reduced at the steady state due to the smoothening process occurred on both rubbed surface. From that, it can be said that the characteristics of generated film on the counterface is the main key in determining the wear behaviour of the composite. For the frictional behaviour of those composites, there are four catrogery of frictional trends. In [65,66,69], there is increase in the friction coefficient at the running in stage and then followed by the steady state. This indicates the stability of the rubbed surface characteristics. In [36,69,70], there is reduction in the friction

coefficient in the steady stage compared to the running in, and this is due the smooth film transfer generated in on the counterface and its high stability. On the other hand, [15,17,36,68] showed fluctuated and increase in the friction coefficient value which represent the instability of the rubbed surface characteristics and modifications took place during the sliding process. The date in Table 2 is extracted and represented in Fig. 5 for comparison purpose. Fig. 5 shows the specific wear rate (a) and friction coefficient (b) of the polymeric composite based on natural fibres. In designing of components subjected to tribological loading, it is desirable to have low specific wear rate and either

Please cite this article in press as: Shalwan A, Yousif BF. In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres. J Mater Design (2012), http://dx.doi.org/10.1016/j.matdes.2012.07.014

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Specific Wear Rate, mm3/N.m E-6

25

(a) 20

15

10

5

0

Friction Coefficient

1.2 1

(b)

0.8 0.6 0.4 0.2 0

Fig. 5. Specific wear rate and friction coefficient of some polymeric composites under dry contact conditions.

high or low friction coefficient. From Fig. 5, it seems that there is no correlation between the wear and frictional behaviour of the composites. For example, sisal/polyester, [66], exhibited low specific wear rate and relatively high friction coefficient. In another example [66], cotton/polyester composites showed very high friction coefficient with low specific wear rate. In bearing applications, it is desired to have a low friction coefficient with low specific wear rate. Therefore, it is suggested to reduce the friction coefficient of natural fibre/polymer composite with the addition of solid lubricants and/or use the designed components in lubricant conditions. However, those suggestions may affect other characteristics, i.e. composite strength, physical properties, degradation ability, etc., i.e. further investigation is required. 3.1. Effect of treatments The interfacial adhesion of fibre with the matrix play substantial role in controlling the tribological properties of polymeric composite based on the natural fibre [15,36,72]. It is well known that natural fibres lack of good interfacial adhesion with polymers. Chemical treatment of fibre is one of the common and useful techniques that used to enhance the interfacial adhesion between the natural fibres and the synthetic matrices and showed good results from mechanical point of view as mentioned previously. With regards to the effect of chemical treatment on natural fibres as reinforcement and the tribological behaviour of polymeric composites, few works have been reported on kenaf, oil palm, betelnut and sisal [15,36,45,66]. These studies have emphasized on that the stronger interfacial adhesion between the fibres and matrix offers

better tribological performance. Yousif and El-Tayeb have investigated alkali treatment consequence (6% NaOH) on the tribological performance of treated and untreated oil palm fibre reinforced polyester (T-OPRP and UT-OPRP) composites using block-on-Ring (BOR) technique, [36]. T-OPRP and UT-OPRP composites were experienced at different sliding distance (0.85–5 km), sliding velocities (1.7–3.9 m/s) and applied loads (30–100 N) under dry contact conditions. In general, the presence of either untreated and treated oil palm fibres in the polyester matrix promotes its wear and friction performance by about 40–80% and 40–70%, respectively. Neat polyester showed poor result due to rapid polyester debris worn away from the interface especially at the longer sliding distances, high-applied load and sliding velocity. T-OPRP showed less specific wear rate by about 11% compared to UT-OPRP due to the enhancement of the adhesion characteristic between the oil palm fibres and the polyester matrix. Moreover, the SEM observation on UT-OPRP worn surface showed debonding and bending of fibres, and fragmentation and deformation on the resinous regions. Meanwhile, T-OPRP composite showed less damages compared to UT-OPRP, where no sign of fibres debonding was observed. The friction coefficient of T-OPRP composite seems to be at a steady state compared to UT-OPRP which showed fluctuated behaviour due to the modifications occurred on the counterface surface during the rubbing process, i.e. unstable behaviour. Wear behaviour of untreated and treated betelnut fibres as reinforcement in polyester (BFRP) composites have been studied at same conditions (applied load = 30 N, sliding velocity: 2.8 m/s and under dry contact condition), [73]. The comparison revealed that serious enhancement of wear performance occurred when

Please cite this article in press as: Shalwan A, Yousif BF. In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres. J Mater Design (2012), http://dx.doi.org/10.1016/j.matdes.2012.07.014

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(a)

Empty fibre

Resin Debonding

(b)

Resin

Penetration of resin

Coarse voids

Small voids

Natural fibre Natural fibre

(c)

Rich surface with pulled out fibres

(d)

Pull out of

Small broken fibres

Resin

Macro-crack Fibres carrying the load

Hard debonding Rough surface

Sliding direction Sliding Fig. 6. Schematic drawing showing the effect of treating the natural fibres on the wear behaviour of polymeric composites.

the betelnut fibres were treated with 6% NaOH. In general, the specific wear rate was in order of 10 8 for the treated betelnut fibres and 10 5 for the untreated betelnut fibres. This is due to high interfacial adhesion of betelnut fibres with polyester matrix preventing fibre pullout during the sliding, i.e. low removal of material. Chand et al. [66] studied the effect of chemical treatment (saline) of sisal fibres/polyester composite (SP) on the tribological behaviour of the composites. The specific wear rate of untreatedSP (UT-SP) was greater than treated-SP (T-SP). This was attributed to the poor bonding between UT-S fibre and polyester that led to the easy detachment of the sisal fibres from the matrix during sliding. In contrast, the coefficient of friction of the material followed the order, pure polyester (.05) < SP (0.6–0.8) < T-SP (invalid value >1) at an applied load of 20 N. The improvement of interfacial bonding reduces the pull out of fibre which increases the resistance in the interface leading to high friction coefficient. Fig. 6 suggests two different scenarios when the natural fibre/ polymer composites slide against hard and smooth counterface such as steel. In the case of untreated fibre, Fig. 6a suggests that there is initial deboning of fibres which weakens the contact surface leading to high damage on the composite surface such as tear, breakage, and pull out of fibres during sliding conditions, Fig. 6c. This has been reported by most of the works on untreated natural fibre/polymer composites such as oil palm/polyester [36] and coir/ polyester [69]. Meanwhile, in the case of treated natural fibre/polymer composite (Fig. 6b), the possibility of pulling out the fibre is less due to the high interfacial adhesion property of the treated fibre compared to the untreated one. In other words, the materials removal from the surface of treated fibre composite surface (Fig. 6d) will be lesser than the untreated ones (Fig. 6c). This appeared when the kenaf/epoxy composite has been tested against stainless steel coutnerface, [15]. 3.2. Operating parameters El-Tayeb [74] reported that wear resistance of sugarcane fibre/ polyester composite (SCRP) increased significantly with increasing load (20–80 N). At higher load, larger frictional heat generation resulted in large extent of back transfer patches of polymer film, which were intermittently spread over the surface and shield the composite surface from further damage. Chand and Dwivedi [66] studied the effect of increasing load applied on the abrasive wear behaviour of sisal–polyester and found that the increasing applied load

decreased the specific wear rate which was caused by the greater frictional heat which softened the matrix on the composite surface. Chin and Yousif [15] have found insignificant effect of applied load (30–100 N) and sliding velocity (1.1–3.9 m/s) on the specific wear rates (Ws) of kenaf fibre reinforced epoxy (KFRE) composite. Ws of KFRE composites exhibited steady state after about 3 km sliding distance. Increase of applied load have slight increased the friction coefficient and the interface temperature. The worn surface revealed that the fibre ends are still well adhered in the matrix and no sign of debonding or pullout at lower applied load (50 N). While at higher applied load (70 N), the worn surface showed debonding of fibres and this was due to the high thermo-mechanical loading which fastened the material removal from the resinous regions and weakened the interfacial area between the fibres and the matrix but there is no sign of pull-out of fibres. At higher applied load of 100 N, micro-cracks became clear on the surface which is due to the high side force indicating the high wear resistance in the rubbing zone. Yousif [70] has investigated the effect of operation parameters i.e. applied load (10–30 N) and sliding distance (0–4.2 km) on the tribological behaviour of coir fibre-reinforced polyester (CFRP) composites. There is not much different in the friction coefficient and the interface temperature of CFRP composite at different applied load, i.e. the average friction coefficient of CFRP composite at 10, 20, and 30 N was 0.61, 0.63, and 0.68, respectively. At short range of sliding distance (0–.7 km), there is insignificant different in the friction of CFRP composites at all applied loads while there is a reduction in friction coefficient of CFRP composites at longer sliding distance. 3.3. Frictional behaviour In general, friction is the measure of energy that dissipated at the surface. Based on the three friction mechanisms, namely asperity deformation, adhesion and ploughing, quantitative treatment is proposed to determine the total friction coefficient. The behaviour of each one of these mechanisms depends on the contact surface topography, operation conditions and the type of material. Few works focused to investigate the friction behaviour of polymeric composites based on the natural fibres under dry sliding conditions [15,36,66,67,70]. Chin and Yousif [15] have found that the presence of kenaf fibres in the composite reduced the frictional coefficient of epoxy from 0.75 to 0.56. In [17,36,70], it has been reported that a

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reduction of friction coefficient can be achieved when polyester was reinforced with coir, betelnut and oil palm fibres by about 30%, 46% and 60%, respectively. This is attributed to the modification that took place on the counterface wear track and the topographical composite surfaces, i.e. the role of this modification in the roughness of film transfer generation on counter face. In other words, the presence of natural fibres on the composite surface smoothen the film transfer on the counterface, which led to decrease the interlock between the asperities in contact leading to low friction coefficient. However, the friction coefficient is still high for such materials to be used in tribological applications such as bearing, sliding and bushes. Moreover, the low friction coefficient may sacrifice the wear performance of the material. The addition of cotton and sisal fibres to polyester composites significantly increased the friction coefficient by 46% and 50% compared to the neat polyester. This was due the low sensitivity of the cotton fibre to heat of friction as compared to polyester resin which offered higher resistance to sliding movement. There are two possible solutions to overcome the high friction coefficient of the polymeric composites based on natural fibres which either introducing solid lubricants to the composites or operate the composites under wet contact conditions. This will be explained further in the coming sections. 4. Possible reduction of friction coefficient 4.1. Liquid lubricants From the literature, the contact condition (wet/dry) has significant effects on the tribo-performance of polymeric composites

Specific Wear Rate, mm3/N.m E-6

10 9

based on synthetic fibres and/or additives. This effect could be positive or negative as reported on different polymers. the tribological behaviour of some polymeric composites such as PPS and PEEK were broke down under wet contact condition compared to dry due to the lowering in the hardness of the surface layer of the composite [75]. Moreover, the absence of the film transfer on the counterface led to transfer the wear mechanism from adhesive into abrasive, i.e. the worn debris and fibres in the interface attacked both surfaces [76]. However, the tribological behaviour of some polymeric composites such as PA, UHMWPE [77], and epoxy [76] were enhanced under wet contact conditions compared to dry. This is mainly due to that the water acted as cooler and cleaner, i.e. the water absorbs the heat generated by friction and removes the wear debris from the rubbing area. In [78], the friction and wear characteristics of carbon and glass/PEEK and PPS composites have been studied under wet contact conditions. The wear performance of the glass fibre/PEEK slightly improved the friction but exhibited poor resistance to wear. On the other hand, carbon/PEEK showed good friction and wear characteristics under wet contact condition compared to the dry ones. In the light of this, there is an argument on the effect of the water as lubricant on the tribological behaviour of synthetic fibre/polymer composites. With regards to the natural fibre/polymer composites, the tribological behaviour of untreated oil palm fibre reinforced polyester (U-OPFFP) composites was studied under dry/wet adhesive mode [79]. Under dry condition, the untreated fibre/polyester exhibited poor wear characteristics leading to debonding the oil palm fibres during the sliding, especially under severe conditions. The high interface temperature during dry adhesive condition led to soften the polyester region and then pullout of fibre from the bulk to

(a)

8 7 6 5 4 3 2 1 0

0.5 0.45

(b)

Friction Coefficient

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

Fig. 7. Specific wear rate and friction coefficient of some polymeric composites under wet contact conditions.

Please cite this article in press as: Shalwan A, Yousif BF. In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres. J Mater Design (2012), http://dx.doi.org/10.1016/j.matdes.2012.07.014

A. Shalwan, B.F. Yousif / Materials and Design xxx (2012) xxx–xxx

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Friction Coefficient

1.2 1 0.8 0.6 0.4 0.2 0

Fig. 8. Influence of solid lubricants on the frictional behaviour of polymeric composites.

the surface. Presence of the water assisted to absorb the frictional heat resulting in lower material removal and low friction coefficient compared to the dry contact conditions behaviour. Recently, the adhesive wear and frictional performance of Betelnut fibres reinforced polyester (BFRP) composite under dry and wet conditions at different operating parameters has been investigated [17]. Under wet condition, coefficient frictional of BFRP composite was lower by about 94% than the dry ones. This is due to reduction in the thermo-mechanical loading during the sliding, [8,80]. Under dry contact conditions, the roughness of the generated film transfer on the counterface was significantly increased during the sliding, while under wet contact conditions, there is no significant change in the wear track roughness. It should be considered here that the presence of water may lead to increase the moisture of natural fibres and considerably decrease the mechanical properties of natural fibre, [2,81–83]. Fig. 7 summarises the wear and friction of natural and synthetic fibre/polymer composite under wet contact conditions. Compared to the dry contact conditions, Fig. 5, presence of the water significantly reduces the specific wear rate and friction coefficient of the natural fibre/polymer composite, e.g. oil palm/polyester [36,78], betelnut/polyester [17]. Moreover, one can notice that natural fibre/polymer composite exhibited much better wear and frictional performance than the synthetic fibre/polymer composite. This is due to the abrasive nature of the synthetic fibres which act as third body in the interface causing severe damage on the composite surface as reported by [84]. 4.2. Solid lubricants In polymer composites, selecting the right compositions assists to reach special requirements for tribological applications. In other words, filling polymer composite with desired fillers and/or reinforcement is frequently employed for specific objective. Many researchers studied the tribological behaviour of polymers containing solid lubricants aiming to reduce the friction coefficient of the composites and maintain good wear performance. In [85–93,67], frictional characteristics of polymers enhanced dramatically by incorporation solid lubricants such as graphite, molybdenum disulfide (MoS2) and poly-tetrafluoroethylene (PTFE) where fluid lubricants are undesirable and ineffective. Fig. 8 summarises the influence of solid lubricants on the frictional performance of polymeric composites. In general, it can be seen that the addition of the graphite to the cotton/polyester composite significantly reduced the friction coefficient of the composites, [66]. PTFE showed remarkable reduction in the friction coefficient value (<0.1) of the epoxy composite compared to the reported friction coefficient value of neat epoxy [15], kenaf/epoxy

[15], and MoS2/epoxy [87]. PTFE could be promising fillers, which helps in enhancing the frictional performance of polymeric composites. Zhang et al. [88] studied the tribological properties of two solid lubricants, PTFE and graphite filled polyphtalazinone ether sulfone ketone (PPESK) composites. That research revealed that the friction coefficient and wear rate of PPESK composites noticeably reduced by 65% compared with neat PPESK due to the lubricating transfer film formation steel counterface. However, it is known that PTFE has low support to the composite in term of wear performance of the composites [84–86,89]. Therefore, most of the reported works on the solid lubricant proposed the graphite as solid lubricant for polymeric composite aiming for low friction coefficient and good wear performance. Ben Difallah et al. [85] investigated the effects of graphite addition (0–7.5 wt.%) on the wear resistance and friction coefficient of Acrylonitrile Butadiene Styrene (ABS) matrix. In that work, ABS matrix had exhibited lower friction coefficient and high wear resistance with the increase of the weight fraction of graphite in the polymer matrix, i.e. composite with 7.5 wt.% graphite exhibited the lowest value of friction coefficient (0.18) compared to neat ABS (0.34). The reason of the low friction of polymers filled with graphite is that the transfer film contains graphite particles acting as lubricant layer which in turn reduces and hamper the direct contact between the polymer and the hard counterface. Hashmi et al. [67] investigated the friction behaviour of cotton– polyester composites (CPCs) and the effect of adding graphite as a filled to polyester composites. In that work, the friction coefficient decreased (0.95, 0.65, 0.6, 0.58 and 0.47) as the concentration of graphite increased (0, 1.96, 3.84, 5.66, 7.4 vol.%). In the case of graphite filled CPCs, the contact temperature drastically reduced due to graphite’s lamellar crystal structure. This is another reason of the low friction coefficient and good wear performance of the composites. 5. Conclusion In this article, several reported works have been reviewed and several issues have been addressed with the usage of synthetic and/or natural fibres as reinforcements for the polymeric composites. The main points can be concluded as follows: 1. Surface characteristics, volume fraction, physical properties and orientation of natural fibre have significant influence on the mechanical and the tribological performance of the composites. The nature of the fibres controls the mechanical and the tribological behaviour of the composites. In other words, modification and critical selection of the fibres are necessary to gain high composite performance.

Please cite this article in press as: Shalwan A, Yousif BF. In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres. J Mater Design (2012), http://dx.doi.org/10.1016/j.matdes.2012.07.014

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A. Shalwan, B.F. Yousif / Materials and Design xxx (2012) xxx–xxx

2. There is no remarkable correlation between the mechanical and tribological performance of major polymeric composites. However, for natural fibre/polymer composites, treatment of the fibres has influenced both mechanical and tribological behaviours of the composites. Treating the natural fibres assist to stabilise the bonding area between the fibre and the matrix which enhanced the ability of the fibre to carry the load under mechanical and tribological loadings. 3. Natural fibre polymeric composites suffer from high friction coefficient. It is suggested to use graphite as solid lubricant for such composites, which may reduce the friction coefficient of the composite and maintain high wear characteristics. On the other hand, using water as lubricant for natural fibre/polymer composite may deteriorate the composite strength. 4. The use of water as lubricants deteriorate the wear performance of synthetic fibre/polymer composites by transfer the adhesive into three-body abrasion wear mode. On the other hand, water as lubricant assisted to cool the rubbed surface, which in turn exhibited better wear and frictional performance, compared to the dry contact conditions.

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Please cite this article in press as: Shalwan A, Yousif BF. In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres. J Mater Design (2012), http://dx.doi.org/10.1016/j.matdes.2012.07.014