POLYMER BEHAVIOUR UNDER ADHESIVE WEAR LOADING CONDITIONS




Krasnov et al. 2010 studies different processes having polymers and polymer based materials running in contact. The studies related to systems and regularities of friction and wear of such materials covers under polymer tribology. However  Mitrus, Wojtowicz&Moscicki, (2009) explained that a single layer molecule developed by joining the chain of  small molecules in a  repeated manner is named as polymer. Moreover the main purpose behind the development of polymers is to have a product having unique physical and chemical properties. Chemists manipulate large, complex molecules and capitalize on the connections between their molecular structure and properties that make them light weight, hard strong and flexible with distinct thermal, electrical and optical properties, thus making them useful in major industries as packaging, transportation, fiber and communication industries. Harsha &Tewari, (2003) emphasize that the polymer composites are developed to enhance the wear resistance capability and also to minimize the coefficient of friction. It concludes that polymer composites thus developed are for tribological applications.  Furthermore they also studies the five types of wear observed in the practical scenarios. These wear are abrasive, adhesive, fretting, erosion and fatigue wear.   Chiu et al. (2012) explained that the thermoplastics as polyaryletherketone (PEEK) and polyimides when blend with solid lubricants results in reducing the coefficient of friction. Unal, Sen & Mimaroglu, (2005) defined wear as loss of material when two material are placed in contact with each other and there exist a relative motion between the contact surfaces, thus wear is a damage to a solid surface.

Myshkin, Petrokovets&Kovalev(2006) explained that a polymer surface which is under elastic, plastic and viscos-elastic deformation depends upon material characteristics. The properties of polymers like wear, friction resistance can be used with high effectiveness. This effectiveness was explained by Sinha& Briscoe (2009) with the explanation that the polymers like films, bulk and composites are being used in shoes, tires, brakes, bearings, MEMS, hair products and artificial human joints.  Wegner (2000) develop the understanding about the importance of polymer surface properties particularly in paints and thin coatings. These coatings are extensively used for corrosion protection as adhesive or anti reflection layers. The best example is the wetting behavior change of the film with respect to water from hydrophilic to hydrophobic. This behavioral change is carried out by the addition of small amount of a surface active component to a polymer material. Shalwan & Yousif(2012)  discussed the significance change in the tribological properties of polymer composites when glass fiber is added.  The common forms of glass fiber produced in the industry are woven or chopped strand mats.  However  Parallel (P-O), Anit-Parallel (AP-O), Normal (N-O) are the three principle orientations of composites with respect to sliding directions. Also there are several studies have been investigated the tribological behaviour of glass/polymer composites considering one orientation (Normal, N-O). Chiu et al(2012) concluded by simulation investigation that the highest  friction corresponding gross interfacial chain rearrangement is on the perpendicular sliding direction and this is independent of the polymer type. Baldan. A,  (2004) suggested that performance of the adhesive bond, the adhered surface (metallic or non metallic) can be improved by using some pre treatment. This pretreatment may be either by using the physical, mechanical, chemical, photochemical, and thermal, or plasma method. However Zhou & Yan (2004) revealed that although the adhesion of the transfer film to the counter face may be enhanced by the fillers which is an attribute of wear reduction. However, the friction reducing and wear resistant mechanisms of various fillers in different polymer matrices are not well understood.  According to Zhou & Yan, (2004), it is feasible to improve the dispersion of fillers by incorporating the compatibilizers in the composites. This will also increase the adhesive ability between the polymers and the fillers.  Interfacial adhesion canbe enhanced and the depressed phase size can be reduced by some copolymers or graft polymers with reactive groups and similar chemical structures. For example, polyethylene-graft-maleic anhydride (PE-g-MAH) has been used as a compatibilizing agent in many polymer–polymer alloys. Yan, Xue& Wang (2002)& Lai, Yue& Li (2007) introduced that preferential load of fillers also help to enhance the wear reducing ability of fillers. This method was proposed by Lancaster and further theoretically by Tanaka and Kawakami. Yan, Xue & Wang (2002) divided the actions of metallic fillers on transfer film in to two aspects. The hard filler exposed on the wear surface of the composite can scrape the polymer transfer film on the counterpart surface; and the transfer film composed of metallic filler can mechanically hold the polymer forming the composite transfer film, because its mechanical strength is higher than that of the polymer matrix. A steady state model of wear of polymer composite material was explained by Blanchet & SUNG (1998). This model also includes the effects of preferential load support by and surface accumulation of wear-resistant filler particles. This theory was further developed by Han and Blanchet both experimentally and theoretically. Yan, Xue& Wang  (2002)  studied the transfer process and preferential load support of hard fillers. The study was conducted to understand the characteristics of metallic fillers under a type of extreme friction condition as high load and reciprocating motion and the influence on the tribological behavior and transfer of polymer composite was recorded. Moreover, Chiu et al (2012) developed the hypothesis that the transfer film  in the polymer/polymer composites during sliding provide the protective and low shear interfaces due to which the wear resistance is increased and friction coefficient is reduced. Yousif & Ku (2012) suggested that natural fiber due to low density, high strength, high flexural modulus and high impact strength are more preferable for developing polymer composites. Furthermore the polymeric composites can be found in housing construction material, industrial and automotive parts. Suresha et al. (2010) explained that by adding the fillers or reinforced fibres,  the mechanical properties of the polymers are improved at higher rate as compared to the effect in the wear rate which do not invariably beneficent. Also, Suresha et al,  (2013) added the extensive applications of high strength and stiffness polymers and its composites like epoxy, vinyl ester, polyamide (PA), polyetheretherketone (PEEK), and polyphenylenesulfide (PPS). These polymers can be manufactured by compression moulding/injection moulding with ease and economics.

Skeist&Miron, (1990) reported adhesive cure is necessary to join together the small resin molecules (mers) so that large molecules known as polymers could be formed. For example, one of some adhesives, such as this wood adhesive, is used only with specific substrates because of the cure time needed to allow for good bonding. Although Adhesion is considered as friction impact and Johnson- Kendal-Roberts model theory used small and big sizes having the properties of polymers (Myshkin et al. 2005).  They also explained the way the special instruments can be used to measure the molecular forces acting between the solids. According to Deryagin et al. (cited in Myshkin et al. 2005, p. 911), molecular attraction forces can be measured by applying the concept of feedback balance. However these molecular forces are now being measured by using atomic force microscopy. Also the mechanical energy dissipates when deformation occurs between the two sliding surfaces. The deformation may be elastic deformation, plastic deformation and visco-elastic deformation. Thus this concludes that the adhesion and deformation is the source of fractional forces (Myshkin et al. 2005, p. 913).  Load, velocity and the temperature has the direct effect on the friction. Firstly, friction force is proportional to the applied load and coefficient of friction staying constant at load between 10-100 Newton. Secondly, sliding velocity has a relationship with friction. By increasing velocity, viscous resistance will increase and at high velocity, friction force will reduce due to short duration of contact. Thirdly, temperature resulted from friction by converting mechanical energy to heat and this heat is generated from friction and the source of this heat is deformation of materials.

Yousif (2008) explained the performance evaluation of polyester in terms of frictional and wear performance using dry sliding contact against the smooth stainless steel using block-on-disk (BOD) machine.  The polyester is reinforced by the multilayered (mainly three or four layers) of coir fiber. The results were the worn surface analyzed on scanning electronic microscope and wear rate and friction coefficient were calculated in different aspects including;

·         Different applied load between 10N to 30N.
·         Different sliding distance between, (0km) to (4.2km).

This experiment concluded that the coir fibre-reinforced polyester (CFRP) composites with three layers are better than four layers with better frictional and wear performance. Moreover, CFRP composites with four layers gave a high friction coefficient (Yousif 2008). Yousif (2008) concludes with some outcomes as follows:

·         Strong interfacial adhesion between the polyester and the coir fibres which overcome the pull out of fibres at the interface.
·         High coefficient of friction was observed.
·         Reinforced polyester with coir fibres has improved adhesive wear characteristic of (NP).

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