The components that are engaged in relative motion with their counterparts in any product demand synergy of mechanical and tribological behavior for service longevity. Traditional materials often demonstrate better performance in either of these, resulting in early failure of the part. This has provided the scope for evolution of tailor‐made materials, i.e. composites, to suit the desired applications. Among the available composites, polymer matrix composites (PMCs) offer benefits – for instance, superior strength to weight ratio, resistance to corrosion, simplicity in processing, energy consumption, relatively low processing temperature, and recyclability. This has made PMCs the preferred choice of materials engineers for the development of products. PMCs consist of an engineering polymer as the matrix material. The matrix element is a continuous and homogeneous material into which the reinforcements are entrenched to amend the functioning of the matrix material. Commonly used matrix materials in PMCs are thermoplastics and thermosets.
9.1.1 Matrix Material
Thermoplastics melt and soften upon application of heat and are suitable for liquid flow forming. They derive their strength and stiffness from the monomer units, which are of very high molecular weight. Composites with thermoplastic matrices are generally manufactured by extrusion, followed by injection molding. Polyethylene (PE), polypropylene (PP), polystyrene (PS), polycarbonates (PCs), polyamides/nylons (PAs), polymethyl methacrylate (PMMA), polyacetal (POM), polyurethane (PU), polyvinyl chloride (PVC), polyimide (PI), polyetherimide (PEI), polyaryletherketones (PAEKs), polyamideimide (PAI), etc. come under this type.
Thermoset matrices are high‐density liquid polymers that are converted into hard brittle solids by polymerization, resulting in the formation of a covalently bonded three‐dimensional network. The mechanical behavior of the composites made of thermosets depends on this networking. They do not melt on heating once curing is over. However, a loss in stiffness at the heat distortion temperature is noted. Phenol formaldehyde (PF), urea formaldehyde (UF), polyester, vinyl ester, and epoxy resins are some of the examples of thermoset polymeric materials. Of these, epoxy, vinyl ester, and unsaturated polyester resins cover a very broad class of thermosets. Compared to polyesters, epoxies have fair toughness and environmental resistance, low absorption of moisture, and considerably less shrinkage during curing .
Many researchers have reported the synergistic performances of several PMCs reinforced with natural fibers such as bamboo, sugarcane bagasse, jute, kenaf, flax, grass, sisal, hemp, coir, ramie, and abaca, and synthetic fibers such as carbon, glass, aramid, nylon or rayon embedded in a polymer matrix and their performances have been evaluated by several researchers [2–7]. However, in order to enhance their performances two or more reinforcing elements, in the form of fibers/particulate fillers, are included in the polymer matrix, leading to the evolution of hybrid polymer matrix composites (HPMCs). The polymer matrix element surrounds and tightly binds the reinforcements, which are in the form of fibers or particulate fillers.
HPMCs represent a new class of alternative materials to conventionally filled, i.e. two‐phase PMCs, which can overcome the problems associated with traditional composites [8, 9]. These HPMCs have been immensely used in thermal power sectors, sports, automotive, aircraft, military, and aerospace industries. They have exhibited significant weight savings for aircraft structures and remarkable resistance to corrosion and fatigue damage. Mercantile aircraft applications vary from minor flight control surfaces to more and more prime edifices . Several investigators have studied the tribo performances of various HPMCs [11–17].
22.214.171.124 Fiber Reinforcements
A fibrous reinforcement in HPMCs is categorized by its length being much larger than its cross section. The relation between the length and cross‐sectional feature is known as aspect ratio. The diameter of fibers used in structural composites varies from 5 to 140 μm. The key tasks of the fibers are conveying the load, thermal stability, strength, and providing stiffness and other structural behavior. To achieve these tasks, the fiber in composite must possess increased ultimate strength, modulus of elasticity, and a little distinction of strength amid the fibers, increased stability of their strength during usage, and homogeneity of diameter. Carbon, glass, and Kevlar are the most important fibrous materials used for making composites. These fibers can be designed with unique architecture (fabrics) depending on the product requirement and manufacturing process. The fabrics are made of fibers orientated along two perpendicular directions. One is called the warp and the other is called the weft directions. The fibers are interlaced together, which means the weft fibers (tow) pass over and under the warp fibers, following a constant pattern. Intertwined fabric reinforced composites are becoming well known because of their well‐adjusted behavior in the fabric plane as well as their ease of fingering during processing. Major types of weave forms of fabrics are illustrated in Figure 9.1.
Plain Interweave Fabric
Each warp fiber passes alternatively under and over each weft fiber as shown in Figure 9.1a. The fabric is symmetrical and possesses good stability. Nevertheless, it is most intricate to drape the interweaves, and the increased fiber gathering displays comparatively subdued mechanical behavior compared with the other styles of weaving.
Twill Interweave Fabric
One or more warp fibers are sequentially interweaved over and under two or more weft fibers in a steady repetitive pattern as shown in Figure 9.1b. This is portrayed by excellent wet out and drape over the plain weave with a little decline in stability. With decreased fiber gathering, the fabric has a smoother surface with faintly better mechanical behavior.
Satin Interweave Fabric
They are basically twill interweaves customized to make a few junctions of warp and weft as shown in Figure 9.1c. It is designated by a harness number, which is the sum of fibers crisscrossed and conceded under, before the fiber recaps the pattern. Satin weaves are awfully flat, and have beneficial wet out and an improved level of drape. The decreased gathering of fibers in the interweave result in the demonstration of better mechanical behavior. Satin interweave fibers can be woven closely, which eventually produces fabrics with tight weave. But this weave style has low stability and is asymmetric in nature. The irregularity results in one face of the fabric having fibers running chiefly in the warp direction while the other face has fibers running chiefly in the weft direction.
Basket Interweave Fabric
They are similar to plain interweave except that two or more warp fibers consecutively intertwine with two or more weft fibers as shown in Figure 9.1d. This grouping of two warps crisscrossing two wefts is described as 2 × 2 baskets, but the grouping of fibers will essentially not be even. Basket interweave is relatively flat, and, through less crimp, sturdier than a plain interweave, but less steady.
126.96.36.199 Particulate Reinforcements
Functional fillers of various shapes and sizes are used as reinforcing particles in HPMCs. The size ranges from a few microns to several hundred microns. Fillers in HPMCs can account for considerable weight saving. In comparison to resins and reinforcements, fillers are the least expensive of the major ingredients. A wide range of fillers are used starting from metallic powders to elastomeric fillers. Oxides such as aluminum oxide (Al2O3), zirconium dioxide (ZrO2), silicon dioxide (SiO2), copper oxide (CuO), copper sulfide (CuS), titanium dioxide (TiO2), copper fluoride (CuF2), lead sulfide (PbS), tungsten disulfide (WS2), calcium sulfide (CaS), and boron nitride (BN) are some of the commonly used metallic fillers. Lubrication with liquids is limited in both technological and economic aspects such as physical and chemical degradation. Hence, solid lubricants such as graphite, molybdenum disulfide (MoS2), lead sulfide (PbS), tungsten disulfide (WS2), calcium sulfide (CaS), and polytetrafluoroethylene (PTFE) can be used.
Particulate fillers filled PMCs acquire better mechanical behaviors and these fillers are added from the techno‐commercial aspects. The insertion of fillers in the polymer materials improves the physical, mechanical, and thermal behavior [18–22]. Even in composites reinforced with fiber, the addition of fillers has resulted in enhancement in behavior of the composites. For applications taking place in hard working conditions, such as friction bearings, chute liners used in power plants, gears, agricultural equipment, and pumps handling industrial fluids, the development of polymer based composites, which acquire high stiffness, toughness, and wear resistance, becomes crucial.
9.1.3 Friction and Wear
Wear is described as impairment to a solid surface, normally concerning continuing loss of material, due to relative motion with contacting surfaces. The major types of wear are adhesive, abrasive, fretting, erosion, and fatigue, which are generally witnessed in practical circumstances. Adhesive wear or dry sliding wear is caused by local adhesion between contacting solid surfaces, leading to material transfer between the two surfaces or loss from either surface. The friction and wear performances of PMCs are assessed by this wear test.
Abrasive wear is the most important among all forms of wear, because it contributes to about 63% of the total cost of wear . Abrasive wear is caused by hard particles sliding on a softer solid surface and displaying or detaching material , in which hard asperities on one body moving across a softer body under some load penetrate and remove material from the surface of the softer body, leaving a groove. These hard asperities may in fact be discrete particles. Abrasive wear is classified as two‐body abrasive wear (2‐BAW) and three‐body abrasive wear (3‐BAW) according to the type of contact. It is straightforward to nominate a pair of terms such as constrained abrasive and free abrasive mode.
- Two‐body abrasion is where a hard rough body ploughs into a softer body as shown in Figure 9.2a.
- Three‐body abrasion wear is where a third body (usually hard granular matter) is placed between the sliding surfaces, which rolls and slides to cut grooves on the surface of the softer body as shown in Figure 9.2b.
Two‐body wear is generally a low stress type of wear, where hard asperities plough into the soft surface. In three‐body wear, due to the high stress, the hard abrasive particles between the two sliding surfaces roll and slide to result in ploughing, cutting, and cracking at the micro level as shown in Figure 9.3.
The wear mechanisms, i.e. micro‐ploughing and micro‐cutting, are chiefly ruled by plastic deformation of the wear material; however, micro‐cracking mechanism results in fracture of the wear material. Therefore, the prevailing wear mechanisms that arise for a specific operational circumstance are persuaded to an excessive magnitude by the plastic deformation and fracture behavior of the wear material . Three‐body abrasion is commonly regarded as more realistic: however, its study has attracted very less interest relatively. Investigations on abrasive wear performance of polymer composites are reported by a few researchers [12, 26–30]. However, the researchers have concluded that the abrasion process is a complex phenomenon as it is affected by various process parameters. Hence the authors have penned this chapter to share their acquired knowledge with the technical students, young budding researchers, and the dedicated research community on the tribological performance of hybrid carbon‐epoxy PMCs with various reinforcing particulate fillers. Further, the wear mechanisms are discussed with the help of scanning electron micrograph images.
9.2 Materials and Methods
9.2.1 Matrix Material
In the present investigation, LY 556 epoxy resin (procured from Hindustan Ciba Geigy) is used as the matrix material, owing to its exceptional mechanical performance and good resistance to corrosion/chemicals. Another attribute, which has placed epoxies above others, is the easiest possibility for the addition of a curing agent, with/without the use of heat. LY 556 resin is a bifunctional epoxy resin, i.e. di‐glycidyl ether of bisphenol‐A (DGEBA). This was cured or hardened by the addition of HY 951, which is an aliphatic primary amine, viz. tri‐ethylene tetramine (TETA) along with diluents DY 021, both purchased from Hindustan Ciba Geigy. They react with epoxide groups at room temperature and hence accomplish simple curing. The detailed compositions of the carbon‐epoxy composite system are provided in Table 9.1.
Table 9.1 Constituents of matrix system.
|Constituent||Trade namea)||Chemical namea)||Epoxide equivalenta)||Density (g cm−3)a)||Supplier||Parts by weighta)|
|Resin||LY‐556||Di‐glycidyl ether of bisphenol A (DGEBA)||182–192||1.16||Hindustan Ciba Geigy Ltd.||100|
|Hardener||HY‐951||Tri‐ethylene tetra amine (TETA)||—||0.95||Hindustan Ciba Geigy Ltd.||10–12|
a)From supplier’s data sheet.
9.2.2 Reinforcement Materials
The reinforcement material used in the present investigation is bidirectional carbon fiber fabric (Thornel™ T300). This fabric had a fiber area density of 208 g m−2. The carbon fiber has high strength/tenacity with 3000 filaments per tow of the warp and weft of the fabric as shown in Figure 9.4. The yarns had a twist of 15 turns per meter (TPM) and fineness (Tex) of 200 g km−1. The diameter of the carbon fiber is 6–8 μm.
Bidirectional plain woven fabric is beneficial in load bearing in both longitudinal and lateral directions of the fabric plane. Woven reinforcement offers good stability in the warp and weft direction. The carbon atoms are bonded together in microscopic crystals and the crystal alignment makes the fiber very strong. Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric as shown in Figure 9.4.
The different properties of the carbon fabrics (T300) used in the investigation are listed in Table 9.2. The properties exhibited by carbon fibers make them a suitable candidate for both structural and non‐structural applications.
9.2.3 Particulate Fillers
The two particulate fillers used in the investigations are molybdenum disulfide (MoS2) and aluminum oxide (Al2O3), where MoS2 is regarded as lubricant while the Al2O3 is a ceramic filler.
188.8.131.52 Molybdenum Disulfide
Molybdenum disulfide (MoS2) is a black solid, and is similar to graphite in appearance. It is slippery in nature or greasy to touch. It has a hexagonal layer‐lattice structure. Molybdenum atoms are sandwiched between layers of sulfur atoms. Because of weak van der Waals interactions between the sheets of sulfide atoms, MoS2 has a low coefficient of friction and is thus known as a solid or dry lubricant. MoS2 powder of particle size 20–25 μm is used as particulate filler in the present study and is shown in Figure 9.5.
MoS2 has advantages such as (i) practically no tendency to flow, creep, or migrate; (ii) minimum tendency to contaminate products or environment; (iii) awfully less volatility allowing it to be utilized in vacuum (e.g. space applications); (iv) chemical inertness allowing it to be utilized in reactive chemical situations; (v) normally steady to radioactivity (beneficial in nuclear power plants); (vi) fair load carrying ability; and (vii) nontoxicity. In contrast to graphite, MoS2 does not depend on the existence of adsorbed fumes to act as lubricant.
184.108.40.206 Aluminum Oxide
Aluminum oxide is artificially manufactured, and is a white or almost colorless crystalline material. It is also commonly used as fillers in different polymer matrices. Aluminum oxide (Al2O3) is absolutely insoluble in water. In its most usually occurring crystalline form, called corundum or α‐aluminum oxide, its hardness makes it fit for usage as an abrasive and as a constituent in cutting tools. Alumina is the most cost‐effective and most widely used among the engineering ceramics. It has high strength and stiffness coupled with good hardness; its wear resistance to chemical attack enables it to be widely used as engineering ceramics. A combination of excellent properties and low cost has made fine grade alumina to be used in a wide range of applications. Al2O3 powder of particle size 20–25 μm is used as particulate filler in the present study and is shown in Figure 9.6.
The important properties of alumina are (i) good hardness and wear resistance, (ii) excellent dielectric properties, (iii) good thermal conductivity, (iv) high strength and stiffness, (v) resistance to strong acid and alkali attack at high temperatures, and (vi) excellent size and shape capability. These key properties of alumina found varied applications in different fields such as wear pads, seal rings, abrasion resistant tube and elbow liners, gas laser tubes, high‐temperature and high‐voltage insulators, furnace liner tubes, grinding media, and many more. All these fillers are treated with a 2% organo‐reactive silane coupling agent. Table 9.3 presents the physical properties of the fillers used in the present study.
9.2.4 Composite Fabrication
The composite materials deliberated in the current discussion comprise of bidirectional carbon fabric of around 6–8 μm diameter as reinforcement. LY 556 epoxy resin with HY 951 grade room temperature curing hardener with diluents DY 021 mixture was cast off for the matrix material. Predetermined quantity of epoxy resin and the fillers were assorted by means of high shear mixture (T‐T18 ULTRATRURRAX Basic) at an operative speed of 2000 rpm for 10 min. A temperature of about 50 °C was maintained during mixing. To obtain 3‐mm‐thick laminates eight layers of fabrics were used. The carbon fabric reinforced epoxy (C‐E) composites were prepared by hand lay‐up technique followed by the autoclave molding process (pressure 0.75 MPa and temperature 90 °C). Autoclave curing is preferred over other curing, to prevent voids and to obtain good surface finish. Figure 9.7 shows the different stages involved in the fabrication of the composite under investigation. Table 9.4 illustrates the particulars with respect to designation and weight percentage of the carbon fabric, epoxy, and micro‐fillers utilized in this study.
Table 9.4 C‐E composites utilized in the current study.
|Material description||Designation||Carbon fabric (wt%)||Matrix (wt%)||Filler (wt%)|
|5% MoS2 filled C‐E||C‐E‐5M||60||35||05|
|10% MoS2 filled C‐E||CE‐10M||60||30||10|
|5% Al2O3 filled C‐E||CE‐5A||60||35||05|
|10% Al2O3 filled C‐E||CE‐10A||60||30||10|
9.2.5 Dry Sliding Wear Test
The pin‐on‐disk apparatus was used for the characterization of sliding wear by evaluating friction and wear. The test device designed and developed by Magnum Engineers, Bangalore, India, is mainly envisioned for defining the tribological features of an extensive choice of materials under circumstances of varied normal loads and velocities, and is depicted in Figure 9.8. A static pin fixed on a pin holder is brought into interaction beside a rotating disk at an indicated speed as the pin slides, causing frictional force acting between the pin and disk. A graphic representation of the pin‐on‐disk test rig, presenting the various elements, is shown in Figure 9.9. The applied normal load, wear track diameter, and rotational speed are followed in accordance with ASTM G99‐17  test standard.
The surface of 5 mm × 5 mm × 3 mm composite sample, attached to a mild steel pin of 6 mm diameter and 25 mm length called as pin assembly, interacts with the disk made of EN32 steel of hardness 65 HRC having dimensions 160 mm diameter, 8 mm thickness, and 0.1–0.2 μm surface roughness (Ra).
Prior to experimentation, the test specimens were polished against 600 grade SiC paper to confirm apt interaction with the counterface. The surfaces of both the specimen and the disk were cleansed with a soft paper wetted in acetone and completely dried before the test. The pin assembly was primarily and after the slide wear test, it was weighed in an electronic balance (Mettler Toledo) of accuracy of 0.1 mg. Three specimens were run for a particular set of conditions and the average of the three readings was taken for further analysis. The wear loss measured was then converted to wear volume utilizing the measured density of the sample. The details of dry sliding wear test factors utilized in the current study are enumerated in Table 9.5.
Table 9.5 Particulars of the slide wear test features employed in the current study.
|Sliding velocity||m s−1||0.5, 1.0, 1.5, and 2.0|
|Normal load||N||20, 40, 60, and 80|
The following equations were used to calculate wear volume loss and specific wear rate.
where m1 is the mass of the specimen before the test, m2 is the mass of the specimen after the wear test, ρ is the density of the composite in g mm−3, ΔV is the volume loss in mm3, L is applied load in Newton, and D is the sliding distance in meter.
9.2.6 Three‐Body Abrasive Wear Test
Three‐body abrasion experiments were carried out using a dry sand/rubber wheel abrasion tester (RWAT) in accordance with ASTM G65‐17  standard except for modification to the applied normal load as shown in Figure 9.10.
The schematic diagram of the same is portrayed in Figure 9.11. The dry sand particles of AFS 60 grade were used as abrasives and they are jagged in shape with sharp edges. The standard dimensions (in millimeters) of the composite test specimen used in three‐body abrasion are depicted in Figure 9.12. The abrasive particles were forced between the test specimen and the rotating rubber wheel (rotational speed 200 rpm), made of chlorobutyl material with a hardness of 58–62 Shore A. The feed rate of the abrasive particles was in the range of 255–265 g min−1. The specimen was cleaned with acetone in an ultrasonic cleaner and then dried. Its initial weight was measured in a digital measuring balance – Mettler, Toledo – with an accuracy of 0.1 mg and the specimen was mounted in the holder. After the test, the specimen was removed, thoroughly cleaned, and again the final weight was measured. The difference in weight calculated is a measure of the abrasive wear loss. At least three tests were performed and the average value obtained was used in the analysis.
The particulars of 3‐BAW test factors utilized in the current discussion are enumerated in Table 9.6. The investigations were conducted at two different loads (23 and 34 N) at an unceasing sliding velocity of 2.15 m s−1. Further, the abrading distances were varied in steps of 270 m, from 270 to 1080 m. The abrasive wear was determined by the weight loss, and then transformed into wear volume utilizing the assessed density data. After the wear test, the sample was again cleaned. The wear volume loss (ΔV) and specific wear rate (Ks) were calculated using Equations (9.1) and (9.2) respectively.
Table 9.6 Particulars of 3‐BAW test in the current discussion.
|Abrading distance||m||270, 540, 810, 1080|
9.3 Results and Discussion
The present work attempts to produce and evaluate the characteristics of particulate filled and C‐E composites for tribological appliances. Earlier research reviews on wear of polymeric composites demonstrate anisotropic characteristics. Besides, investigations on the effect of micro‐fillers loading and applied normal load/abrading distance on wear rate revealed some discrepancies. Therefore, the friction and wear performance of unfilled C‐E and C‐E composites filled with MoS2 and Al2O3 have been reviewed in terms of the coefficient of friction (μ), wear volume loss, and specific wear rate (Ks).
9.3.1 Dry Sliding Wear Performance of Carbon‐Epoxy Composites
220.127.116.11 Wear Volume Loss
The plot of wear volume loss of unfilled and particulate filled C‐E composites as a function of applied normal load is illustrated in Figure 9.13a–d. The wear volume loss increases with increase in load and velocity for all the composites. The wear volume loss as a function of velocity for unfilled and filled C‐E composites is shown in Figure 9.14a–d.
Wear volume loss increases with increase in sliding velocity for all the composites and decreases with increase in filler loading, as depicted in Figure 9.15, which highlights the beneficial effect of inclusion of micro‐fillers into the C‐E composite. The wear volume loss of unfilled C‐E composite is the maximum among all the composites examined. It was also observed that inclusion of particulate micro‐fillers to the C‐E composite reduced the wear volume loss. The wear volume decreased with increase in filler loading, and 10 wt% MoS2 filled C‐E (C‐E‐10M) composite showed the least wear volume loss under different loads. The reduction in wear volume loss due to the addition of particulate fillers such as Al2O3 and MoS2 can be accredited to the existence of particulate fillers that act as active blockades to avoid significant scale fragmentation of the epoxy matrix. It can be realized from Figure 9.13a–d that the wear volume loss of composites with 10 wt% MoS2 (C‐E‐10M) decreased about by 68% compared to unfilled C‐E under a sliding velocity of 0.5 m s−1 and at 80 N load. In the case of C‐E with 10 wt% Al2O3 (C‐E‐10A), the wear volume loss was approximately reduced by about 36% that of unfilled C‐E. It must be observed that the decrease in wear volume loss was superior in MoS2 filled C‐E composites. The wear volume loss for C‐E‐10M was 1.34 mm3 under a sliding velocity of 0.5 m s−1 while that of C‐E‐10A was 2.66 mm3. In addition, it can be seen that the wear volume loss of unfilled and particulate filled C‐E composites increased with increase in sliding velocities from 0.5 to 2 m s−1. Unfilled C‐E and C‐E‐5A composites showed excessive wear volume loss after 1.5 and 2 m s−1 in the investigation, and the corresponding wear fragments were in the form of large wedges under a load of 80 N and at 2 m s−1 sliding velocity (Figure 9.13). These results are further substantiated with scanning electron microscopy (SEM) micrographs in Section 18.104.22.168.
It is known that in polymer composites wear resistance can be essentially increased by decreasing the coefficient of friction or by increasing their hardness. In the present work, the outcomes exhibit that the wear resistance of unfilled C‐E was increased by addition of Al2O3 and MoS2 microparticles. In the study of sliding wear behavior of epoxy matrix composites filled with silica particles by Xing and Li , the neat epoxy behaved in a brittle manner and macro‐cracks were formed, resulting in larger wear debris. However, with the addition of micro‐silica particles into the epoxy matrix, the propagation of cracks was prevented by the particles near the surface layer. Hard filler particles have the ability to partially support normal loads during sliding, which eventually reduces the wear volume of the PMC. Similar observations were also made in the dry sliding wear tests on PTFE filled MoS2/graphite composites .
Molybdenum disulfide (MoS2) is a dark blue–gray or black solid, and has a hexagonal layer‐lattice structure [35, 36]. The layer comprises flat sheets of atom or molecules, and the configuration is termed a layer‐lattice configuration. The significant outcome is that the materials can shear effortlessly parallel to the layers than crosswise. They can thus assist fairly hefty loads at right angle to the layers while still being able to slide easily parallel to the layers. This characteristic is being successfully applied for lubrication activity. Unlike graphite, MoS2 does not depend on the existence of adsorbed vapors to act as lubricant. Hence, it can be used adequately at elevated vacuum and temperatures. MoS2 starts to oxidize at 350 °C in air, although it can even be employed at small intervals up to 450 °C. The oxidation generates molybdic oxide (MoO3), which is a good lubricant at elevated temperatures but wears quickly. Apart from oxidation, it is stable with most chemicals; however, it is affected by powerful oxidizing acids and alkalis. To sum up, MoS2 is a versatile and beneficial material where oils or greases cannot be expended or do not have adequate load‐carrying capability . Li et al.  studied the wear diminution mechanism of graphite and MoS2 in epoxy composites. They found that MoS2 with lamellar structure becomes MoO3 during the wear process because of the thermal influence on friction and stresses of compression or shear, and hence MoS2 has a minute influence on the coefficient of friction. Finally, they concluded that the presence of MoS2 can confine the transportation of the matrix to the steel counterface, so that the wear resistance of the composite increases effectively.
22.214.171.124 Specific Wear Rate
The specific wear rates (Ks) of unoccupied and particulate occupied C‐E composites with respect to normal load and for various sliding velocities are illustrated in Figures 9.16 and 9.17 respectively. The ks of all the composites decreases with increase in load and increases with increase in sliding velocity. The ks of unfilled C‐E was the highest for all loads/sliding velocities. It was also noticed that the increase in filler loading led to decrease in ks for constant load and velocity. The 10 wt% MoS2 occupied C‐E (C‐E‐10M) demonstrated least ks over Al2O3 occupied and unoccupied C‐E composites. The addition of particulate fillers into C‐E composite reduces the wear volume and ks. The reduction in ks was due to the formation of a transfer film on the counterface. Kishore and Kumar  noticed that the inclusion of alumina particles (size <1 μm) into epoxy increased the wear resistance. Durand et al.  investigated the sliding wear performance of epoxy matrix composites reinforced with several kinds of micron‐sized ceramic particles (size ranging from 5 to 100 μm), such as TiC, Al2O3, TiN, SiC, ZrO2, and TiO2. Pettarin et al.  studied the sliding wear behavior of MoS2 filled high molecular weight, high‐density polyethylene matrix. They reported that the wear behavior was altered by the addition of MoS2 and it was also reported that the composites demonstrated the lowest wear rate for 10 wt% of MoS2 and the same is accredited to the development of a uniform and adherent transfer film on the counterface. The wear diminution mechanism of graphite and MoS2 fillers in epoxy composites was investigated by Li et al.  and they reported that the graphite in epoxy matrix can reduce the coefficient of friction effectively and increase the wear resistance of the composite. In MoS2 filled epoxy composites, the coefficient of friction cannot be decreased but the wear resistance can be efficiently increased by controlling the epoxy transfer from the surface of the composite to the steel counterface.
The dry sliding wear behavior of nylon 1010 reinforced with short carbon fiber and filled with MoS2 was studied by Wang et al. . They revealed that the accumulation of carbon fiber was effective in decreasing friction and wear, but the MoS2 filler increased its wear. Wear and friction diminution was more substantial while the carbon fiber was used as reinforcement along with MoS2 filler. The ability of the synergistic fillers in facilitating the development of a thin, identical, and continuous transfer film would help improve the wear resistance. The influence of fillers such as graphite and silicon carbide particles (SiC) on the dry sliding wear of glass fabric reinforced epoxy (G‐E) composites was investigated by Suresha et al. , and the study revealed that wear loss increases with increase in sliding velocity for both filled and unfilled G‐E composites.
The magnitude of wear loss was less in filled G‐E than in unoccupied G‐E composites. The SiC filled G‐E composite exhibited maximum resistance to wear. The increased exposure of the reinforcing fibers to the counterface resulted in increased fiber fracture. Graphite filled G‐E composite exhibited reduced coefficient of friction and this can be accredited to the existence of graphite – a solid lubricant. In the recent studies of dry sliding wear of C‐E filled with graphite, Suresha et al.  demonstrated that specific wear rate increased with increase in sliding velocity and the specific wear rate of graphite filled C‐E was lower than that of the unoccupied C‐E composite. The decrease in the wear rate of graphite filled C‐E composites was due to the growth and expansion of the transfer film.
In the present work, the wear volume loss data indicate that MoS2 filler could improve the transfer film of filled C‐E to steel counterface. For unoccupied and Al2O3 filled C‐E composites, the transfer film was relatively thin, non‐uniform, and uninterrupted on the counterface surface. The cause for this could be the quantity of fiber and hard Al2O3 particles in epoxy, which grazed the transfer film partly. When 10 wt% MoS2 was included to the composite with 60 wt% carbon fiber, the creation of transfer film was thin, even, and uninterrupted. From the SEM micrographs (Section 126.96.36.199), it can be determined that the synergism between carbon fiber and MoS2 filler may help to form a good and persistent transfer film on the counterface surface, thus enhancing the resistance to wear and decreasing the friction coefficient.
188.8.131.52 Coefficient of Friction
The graph of friction coefficient (μ) with respect to normal load for filled and unoccupied C‐E composites is presented in Figure 9.18. For unoccupied C‐E, the μ for lower loads remained almost constant, and at higher loads it increased marginally. The μ for Al2O3 filled C‐E composites increased and the increment was high for C‐E‐10A sample, while μ of MoS2 filled C‐E composites decreased and the maximum reduction was obtained for the C‐E‐10M sample.
The wear loss and μ of particulate filled and unfilled C‐E composites are closely linked to the formation and development of a transfer film on the counterface. During sliding of the unfilled C‐E composite, the epoxy wears and only bidirectional carbon fibers are in interaction with the counterface. As normal load increases, the interface temperature increases, leading the carbon fiber to fracture, resulting in a complex condition at the interface. The fractured and debonded fibers act as the third body  resulting in increased abrasion. This leads to marginal increment in μ in the unfilled C‐E composite. In the case of Al2O3 filled C‐E composite, when epoxy comes in contact, adhesive wear occurs and further sliding leads to the detachment of particulate filler along with carbon fibers. The interface during the later part of dry sliding consists of Al2O3 particles and carbon fibers. The presence of hard ceramic particles in the interface of the sample and counterface increases the μ. But for MoS2 filled C‐E composites, the interface is filled with solid lubricant MoS2 particles and during further sliding these particles smear the interface, forming a transfer film that reduces the μ. A similar trend in the dry sliding of nylon 1010 reinforced with MoS2 and short carbon fiber was observed by Wang et al. . They conducted tribo‐chemical studies by X‐ray photoelectron spectroscopy (XPS), which revealed that MoS2 decomposed, and MoO3, FeS, FeSO4, and Fe2 (SO4)3 were generated through sliding. It was determined that FeS, FeSO4, and Fe2 (SO4)3 compounds could enhance the adhesion between the transfer film and the counterface surface. The formation of a thin, uniform, and continuous transfer film would be a factor contributing to the decrease in μ of nylon composites.
184.108.40.206 Worn Surface Morphology
The SEM examination of worn surfaces of unoccupied and particulate filled C‐E composite specimens against steel counterface under a sliding velocity of 0.5 m s−1 and normal load of 20 N is shown in Figure 9.19a. The filled arrow marks show the sliding direction in Figures 9.19 and 9.20.
The matrix debris generation and fiber matrix debonding start at different locations. The worn surface is relatively smooth and some of the exposed fibers are worn. The fiber micro‐cracks and fiber breakage are very less. When the sliding velocity and applied normal loads are high at 2 m s−1 and 80 N respectively, the worn surface feature shows additional fiber breaking and removal of fibers from the surface along with increased matrix debris accumulation (Figure 9.19b). At higher sliding velocity and normal loads, the contact temperature increases, which results in the rupture of the matrix, particularly at the interfacial region. This results in increased surface damage with ripping of the matrix and wreckage formation. Hence, the wear volume loss of unoccupied C‐E composite is high and it enhances with increase in sliding velocity and applied normal load. The different stages in the wear of fiber reinforced polymer composites as stated by Tsukizoe and Ohmae  can be observed such as wear thinning of fibers (marked A), breakdown of fibers (marked B), and peeling off of the fibers from the matrix (marked C).
Figure 9.20a,b presents the worn surfaces of MoS2 particulate occupied C‐E composites with different loads and sliding velocities. The 10 wt% MoS2 filled C‐E composite sample subjected to low sliding velocity (0.5 m s−1) and low applied normal load (20 N) is shown in Figure 9.20a. Small distinct parallel grooves (marked J) formed due to the ploughing of asperities present on the disk along with matrix breakage (marked K) at different places are seen. The increased applied normal load and sliding velocity effect on C‐E‐10M are shown in Figure 9.20b. It shows debonding and peeling off of the fiber from the matrix material and fiber rupture. These features observed when compared with the unfilled C‐E sample at same operating conditions showed less damage. Particulate fillers used in the composite were silane treated, which improves the interfacial adhesion of matrix. Additionally, the particulate filler MoS2 acts as a solid lubricant.
9.3.2 Abrasive Wear Performance
220.127.116.11 Abrasive Wear Volume Loss
Figure 9.21a,b presents the wear volume loss with abrading distance for diverse loads. It is obvious from these figures that regardless of the kind of filler material utilized, there is a near linear trend of wear volume loss. The filled (both MoS2 and Al2O3) C‐E composites demonstrated significantly inferior wear volume loss than the unoccupied C‐E. Furthest, the wear volume loss of MoS2 filled C‐E is much less than that of Al2O3 filled and unoccupied C‐E composites.
Diverse kinds and intensity of fiber reinforcement and/or solid lubricants are used to increase several physical and/or mechanical and tribological behavior of polymers. Harsha et al.  studied 2‐BAW and 3‐BAW of PAEK composites. They reported that fillers such as PTFE and graphite were unfavorable to abrasive wear resistance related to glass fiber PAEK composites. Suresha et al. [28, 30] studied the 3‐BAW of graphite/SiC filled G‐E composites and determined that the graphite filler increased the specific wear rate and SiC reduced the specific wear rate of glass–epoxy composite. Suresha et al.  also studied the dispersed silane treated graphite particles in C‐E composites. The resulting composites showed improved mechanical properties and improved abrasion resistance. Bijwe and coworkers  demonstrated that alumina is quite effective in lowering the wear rate of phenolic resin. The effect of fibers and/or fillers on the abrasive wear resistance of pure polymers was intricate and variable and diverse trends were registered by researchers. Lancaster  investigated 13 polymers reinforced with 30 wt% short carbon fiber and stated that the wear behavior of 7 composites improved with reinforcement and that of 6 composites reduced. The prime motive for including the fillers is to enhance the mechanical behavior, but the influence on tribo‐behavior is not always beneficial. The tribo‐mechanical performance of polymer reinforced composites depends upon the type of filler, geometry, and chemical treatment. The role of inclusion of molybdenum disulfide on tribological behavior of high molecular weight, high‐density polythene was studied by Pettarin et al. . They adopted dry sliding and abrasive wear tests to determine the wear performance of HMWHDPE composites. Adhesive wear mechanism was dominant and characterized by the formation of an even and adherent transfer film in dry sliding wear test. However, under abrasive situations an encouraging rolling effect of molybdenum disulfide particles was observed. In the present work, based on earlier literature, the MoS2 and Al2O3 fillers were altered with organo‐reactive silane treatment, which is certain to improve the interfacial bonding with the polymer and fiber. The MoS2 modified by silane treatment filled in C‐E composite demonstrated significantly less wear than Al2O3 filled and unoccupied C‐E composites. This is because the carbon fiber reinforcement in epoxy reduces the abrasive wear resistance owing to debonding and ripping at the carbon/matrix interface. Further, the wear volume damage of MoS2 filled C‐E composite is much less than that of unoccupied C‐E composite. The fibers (warp fibers) in C‐E composites that were aligned along the abrading direction could have worked as a barricade in fracturing the crosswise fibers (weft fibers) as they were intertwined with the warp fibers.
Surface treatments of micro‐fillers influence the tribological properties of polymer composites. The present write up focuses on the 3‐BAW behavior of the composites consisting of epoxy, carbon fiber, and Al2O3/MoS2 microparticles with silane treatment. Aluminum oxide is renowned for its hardness and is frequently utilized as a grinding agent. As pondered by Schwartz and Bahadur , it would not be an apt filler in the micro‐scale powder structure, owing to its jaggedness, which tends to scratch the mating counterface. The material in the submicron or nanoscale powder structure possesses considerably less jaggedness and hence is not that abrasive. Kishore and Kumar  noticed that the inclusion of Al2O3 particles (size <1 μm) into epoxy escalates the sliding wear resistance of the material. Our earlier work indicated that glass fiber reinforced epoxy resin (G‐E) with unmodified Al2O3 (fine particles) exhibits higher efficiency than graphite in advancing the abrasion resistance of the G‐E composite . The abrasive wear properties of G‐E composites filled with Al2O3, SiC, and pine bark dust were examined by Patnaik et al. . They have determined that the wear rate is susceptible to the type of filler used. The results showed that the G‐E composite filled with SiC filler demonstrated the lowest wear resistance and the fibers were detached effortlessly with least resistance to the action of abrasive particles, whereas G‐E filled with pine bark dust filler showed highest resistance to the material elimination related to other composites used.
Molybdenum disulfide is a well‐known solid lubricant . Its lubrication capability, i.e. capacity for effortless cleavage with little friction, is innate to its crystal layered edifice . Each crystal layer comprises two coatings of sulfur atoms detached by a film of molybdenum atoms. The atoms resting on a similar crystal film are closely stored and intensely joined to each other, and the coatings themselves are separated comparatively far off. For example, the van der Waals bonds are weak. Certain manufacturers of MoS2 modified polymers attribute the increase in wear resistance to the nucleating effect of the filler  and not to its lubrication effect. In the present study, the wear volume was low in the MoS2 filled C‐E composite and it can be accredited to the intrinsic mechanical performance and self‐lubricating characteristic of MoS2. Further, the interaction between the surface treated MoS2 units and the epoxy leads to improved adhesion because of better polymer–filler activities. The effect of fibers and/or fillers on the abrasive wear resistance of unmixed polymer was considerably complex, and erratic and diverse developments were observed [47, 53]. Thus, in the present work microparticles of Al2O3/MoS2 into C‐E showed different wear rates. The Al2O3 filled C‐E composite demonstrated poor resistance to abrasion owing to its jaggedness, which tends to scrape the mating counterface. However, C‐E filled with MoS2 indicated improved abrasion resistance under similar test conditions. This is attributed to a positive rolling effect of MoS2 particles.
18.104.22.168 Specific Wear Rate
The specific wear rate (Ks) of unoccupied C‐E and MoS2 and Al2O3 filled C‐E composites at 23 and 34 N loads are presented in Figures 9.22a,b respectively. The Ks data reveals that the Ks tend to reduce with increasing abrading distance from 810 to 1080 m. However with increase in load from 23 to 34 N, they show increased Ks and finally reaching a saturation level in all the cases. It was observed that wear behavior of C‐E improved with the addition of silane treated MoS2 and Al2O3 fillers. However, the higher loading of fillers in the C‐E composite (10 wt% MoS2 and Al2O3) leads to lower Ks at all the loads and abrading distances. The maximum Ks of unoccupied C‐E and Al2O3 and MoS2 filled C‐E are 2.74 × 10−11, 1.23 × 10−11, and 1.01 × 10−11 m3 N−1 m−1 respectively. The reason for the improvement in wear characteristics may be attributed to the type, surface treatment, and filler loading.
22.214.171.124 Consequences of Factors on Wear Volume Loss
It can be observed that wear volume loss increases with increase in applied load and abrading distance while it decreases with increase in filler loading. At higher loads the composite material under investigation is fractured while with increase in abrading distance, it will be exposed to prolonged interaction with the abrasive particles, which contributes to the increase in wear volume . The decrease in wear volume loss with increase in filler stacking is due to the silane coupling agent, which enriches the interfacial bonding with the polymer and fiber . The type of particulate filler material has less significance than other factors on wear volume. Among the two filler materials used in the present study, MoS2 leads to less wear volume loss compared to Al2O3. This may be attributed to the encouraging rolling outcome of MoS2 particles during the abrasive wear process . The MoS2 particles seem to be pulled out by sand units along with matrix material during wear. These act as a rolling third body between the rubber wheel and the polymer composite, as shown in Figure 9.23. The MoS2 elements behave like a ball‐bearing element, meaning that the elements trundle relatively than glide among the two contacting surfaces, thereby reducing shear stress and μ. This consequence of fillers has been previously mentioned in the works as “rolling effect” [56, 57], which is a pseudo‐lubricant effect.
126.96.36.199 Worn Surface Morphology
To associate the wear information effectively, SEM photomicrograph of the silica sand abrasives, scanned wear scar of selected samples, and worn surface heights of samples by vision measure scope (VMS, Mitutoyo make) are presented in Figures 9.24–9.26 respectively. The SEM photomicrographs of the damaged surfaces of unoccupied and particulate filled C‐E samples are depicted in Figures 9.27–9.29 respectively. These specimens are exposed to an applied load of 34 N and abrading distance of 1080 m at a sliding velocity of 2.17 m s−1. In the current discussion, silica sand (density 2.6 g cm−3 and Knoop hardness 875) was cast off as the abrasive. The abrasive specks of American Foundry Society (AFS) grade silica sand were jagged in shape with pointed edges (Figure 9.24).
The distinctive wear damage of MoS2 and Al2O3 stuffed C‐E composite specimen is illustrated in Figure 9.25a,b respectively. Three different regions, i.e. entrance (denoted as 1), center (indicated as 2), and exit (denoted as 3), can be viewed under 3‐BAW test at 34 N, 1080 m abrading distance. At the entrance and exit zones, the pressure on the abrasive is least, and the impairment morphologies are stable with particle rolling. The wear information and Figure 9.27 indicate comparatively soaring early wear rates when surfaces are fresh, and these slowly decrease with increase in abrading distance.
This is owed to resin rich at the outer surface of the specimen, being rapidly worn until reinforcements resulting in the formation of stable surface. The wear volumes of unoccupied C‐E and Al2O3 and MoS2 filled C‐E composite specimen at 34 N and 1080 m abrading distance are 0.567 × 103, 0.2189 × 103, and 0.1912 × 103 mm3 respectively. To associate the wear volume damage with the depth of wear of dissimilar composites, the disparity in the depth of wear from entry to exit of the wear mark is illustrated in Figure 9.25. These illustrations indicate the ploughing marks in the abrading track and the length of the wear mark for unoccupied and particulate filled C‐E specimens. The length of wear mark is more for the unoccupied C‐E (41.4 mm) and least for MoS2 filled C‐E (35.95 mm) composite (Figure 9.25). Further, the wear scar of MoS2 filled C‐E is smoother (less coarse) than that of unoccupied and alumina filled C‐E samples. The surface and nano‐tribological behaviors were examined by XPS, atomic force microscopy (AFM), and nano‐scratching experiments by Zhang et al. . The surface geography was investigated by surface imaging. High‐ and low‐order roughness was decided by AFM categorization and was related to the diamond‐like carbon (DLC) evolution mechanism and revealed the flattening effect of silver. They discovered that bonding friction was the leading friction mechanism and concluded that the bonding force between the scraping tip and DLC surface was diminished by hydrogenation and amplified by silver doping. In the present work, the surface geography was investigated by scanning and wear scar depth by VMS. The damaged surfaces exhibited better bonding between matrix and fiber/filler and smoothing effect of molybdenum disulfide than that of alumina.
Figure 9.27 demonstrates the damaged surface topographies of the unoccupied C‐E composite specimen. The photomicrograph indicates that there is severe damage on the worn surface. The carbon fiber reinforced epoxy matrix (Figure 9.27) was impaired more harshly than the particulate filled C‐E composites (Figures 9.28 and 9.29). Figure 9.27 shows that the matrix is deformed and damaged (marked as D) by ploughing and the cutting action by jagged silica abrasive specks and uncovering of fibers. These uncovered fibers are inclined to rupture (marked as F) and get removed from the worn surface of the C‐E composite. The micrograph also specifies crevice proliferation of the matrix, worsening of the fiber–matrix bonding owing to cyclic mechanical stress, and some fiber withdrawal from the matrix. Additional features such as the paced presence of carbon fibers (marked as S), fiber–matrix debonding (marked as B), and leaning end breakage of fibers (marked as I) can be seen from the photomicrograph in Figure 9.27. The breakage of fiber is due to abrasion and crosswise bending by pointed abrasive specks, resulting in breaking of fibers torn from the matrix (Figure 9.28).
Conversely, the SEM topographies of MoS2/Al2O3 filled C‐E samples are shown in Figures 9.28 and 9.29. It is apparent from the SEM photomicrographs (comparing Figure 9.27 with Figures 9.28a,b and 9.29a,b) that the 10 wt% MoS2/Al2O3 filled C‐E exhibits lower degree of damaged surface topographies compared with unoccupied C‐E specimen. These composites with abrasion examination possess lower rough surfaces, preserving the ploughing path along the abrading route (Figure 9.25). Wear damages emerge in the shape of lumped mass, indicating that they were created by micro‐cutting. In addition to micro‐cutting, micro‐ploughing and micro‐cracking appear in the worn surface. The abrasion is highly dependent on the asperity geometry. Zhang and Komvopoulos  studied the nanoscale pseudo elastic behavior of Cu‐Al‐Ni shape memory alloy (SMA) induced by partial indentation unloading and confirmed the direct implications in micro‐system fabrication with SMAs. To have uniform surface roughness for all specimens, before abrasive wear test, the specimens were pre‐worn using 1200 grit SiC paper and cleaned with a wire brush. Subsequent to the normal test duration, the sample was detached, meticulously gutted, and again weighed for accurate weight loss measurement. These topographies are important for the abrasive wear information, wherein the level of wear volume damage is elevated for the unoccupied C‐E associated to the particulate filled C‐E samples.
The role of ceramic and lubricating micro‐fillers on friction and wear behavior of C‐E composites has been evaluated by adhesive and abrasion wear processes. Unoccupied C‐E composites were unfavorable to wear under upper normal loads and sliding velocities. Amalgamation of ceramic and lubricating fillers appreciably improved the wear performance of the C‐E composite. Wear volume damage and friction coefficient of all the composites increased with increase in the normal load/sliding velocity. Al2O3 filled C‐E composites exhibited slightly higher wear volume loss compared to MoS2 filled C‐E composites. Lower friction coefficient was observed in 10 wt% MoS2 filled C‐E composites, which demonstrated better resistance to wear. Al2O3 filled C‐E composites exhibited slightly higher friction coefficient compared to MoS2 filled and unoccupied C‐E composites. The wear mechanism of the unfilled C‐E composite during dry sliding wear was dominated by plastic deformation, fiber thinning, bending of transverse fiber, and fiber breakage, whereas in filled composites, micro‐cracking, fine debris formation, less fiber breakage, and a thin continuous transfer film on the counterface were the principal wear mechanisms. Lastly, it can be concluded that C‐E composites exhibited superior wear presentation when filled with a lubricating filler, namely MoS2.
The experimental outcomes of 3‐BAW showed increase in Ks with increase in normal load while it decreased with increase in abrading distance for unfilled as well as particulate filled C‐E composites. Also, particulate filled C‐E composites performed better in contrast with unoccupied C‐E composite. The micrographs of three‐body abrasive worn surface features exhibit matrix and fiber damages, fiber breakage, and fiber elimination, the magnitude of which changed according to the system, load, type of filler material, and abrading distance. The chief wear mechanisms in particulate filled C‐E composites are micro‐cutting, micro‐ploughing, and micro‐cracking.
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