Cotton reinforced polymer composites

5 Cotton reinforced polymer composites

Abstract: Cotton grows around the seeds of the cotton plant which belongs to the genus Gossypium. This chapter discusses the utility of cotton as reinforcement in polymer composites for automotive applications. Polymer composites based on plant fibers are extensively used for thermo-acoustic insulation, and such insulating materials are mainly based on cotton fibers. The chapter deals with the structural, thermal and mechanical characterization of cotton fibers and polymer composites based on them. Cotton as a reinforcement influences the tribological properties of cotton polymer composites in different wear modes. The material acts as a lubricant in wear related situations. These issues are discussed in detail in this chapter. Key words: cotton; thermo-acoustic insulation; lubricant; cotton–polymer composites; wear and friction properties.

5.1

Cotton fiber

Cotton (derived from the arabic word al qutun) is an important agricultural crop belonging to the genus Gossypium, sub-tribe Hibisceae family Malvaceae.1 It is the main source of clothing and means of livelihood for a large population across the world. Cotton cultivation needs a long period of growth with plentiful sunshine and water; the fibers are harvested in dry weather. These climatic conditions are native to tropical and warm subtropical latitudes in the Northern and Southern hemispheres, the USA being the largest cottongrowing nation. The American cottons have 26 chromosomes while Asian and African cottons have only 13. Four species of Gossypium account for practically all the world’s supply of cultivated cotton.2 These are: • •

Gossypium hirsutum: These varieties of Central American origin constitute 87 % of the world’s production. Their maximum height is 1.8 m. Gossypium barbadense is believed to have originated in Peru. Its height is between 1.8 and 4.6 m. This species includes the Sea Island and Pima S-2 cottons and some of the Egyptian varieties. 129

130

• •

Tribology of natural fiber polymer composites

Gossypium arboreum includes the tree cottons found in Nigeria and the native cottons of India and Pakistan. It grows as tall as 4.6–6.1 m. Gossypium herbaceum has an average height of 1.2–1.8 m and is slow yielding. It has short fibers.

5.1.1 • • •

Advantages and disadvantage of cotton fiber

Cotton fibers have high strength and durability and possess absorbency. Cotton is a biodegradable fiber. It can be easily blended with other fibers.

The only disadvantage is that cotton cultivation requires a high input of chemical fertilizers and insecticides and plenty of water.

5.1.2

Chemical composition of cotton fiber

Cotton grows around the seeds of the cotton plant (see Fig. 1.1b on p. 4). It consists of pure cellulose of high molecular weight, typically ≥ 6 × 105, comprising long chains of D-glucose units joined by β-1,4 glycosidic links. The rigidity of the cellulose structure in cotton, conferred by the anhydroglucopyranose units with their β-1,4 glycosidic (COC) links and intermolecular hydrogen bonding, makes cotton resistant to environmental extremes. The typical arrangement of cellulose gives cotton fibers a high degree of strength and absorbency. A single cotton fiber contains more than

28

Yield stress (MPa)

26 24 22 20

C0 C1 C2 C3

18 16 14 0

10

20 30 Fiber content (wt%)

40

50

5.1 Influence of fiber content as well as three compatibilizers on yield stress of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite).24

Cotton reinforced polymer composites

131

20 layers of cellulose coiled as springs. The chemical composition of cotton includes cellulose 82.7 %, hemicellulose 5.7 %, pectin 5.7 %, water soluble 1.0 %, wax 0.6 % and water 10 %. The degree of polymerization (Pn) of cotton fiber is around 7000. Details of these constituents are given in the Appendix.

5.1.3

Physical structure of cotton fiber

Cotton fibers vary considerably in their gross morphology, namely convolutions, cell wall thickness, cross-sectional shape, etc., and in their fine structure, namely fibrillar orientation, reversals, the packing density of microfibrils, etc., from variety to variety and species to species. Table 5.1 shows the dimensional and structural characteristics of different varieties of cottons.3,4 The two varieties, namely G. arboreum and G. herbaceum cottons, are short and coarse and have few convolutions and structural reversals per unit length as compared to G. hirsutum and G. barbadense cottons. G. arboreum and G. herbaceum cottons have a significantly higher percentage of fibers with a circular cross-section than G. hirsutum and G. barbadense cottons.5 These species also vary in the convolution angle, spiral angle and x-ray angle. For instance, the cell wall of G. herbaceum cotton is composed of finer micropores and smaller crystallites than the cell wall of G. Hirsutum cotton.6 In general, cotton fiber of all varieties has a ribbon-like shape with twists or convolutions at regular intervals. The molecular chains aggregate in an extended and non-folded form into elementary fibrils that combine to form a microfibril. The fibrils are 4–10 nm wide and ~mm long.7–9 There is no lattice coherence along the elementary fibril and it breaks down after every 50 nm. Consequently the fibril contains mismatched crystal blocks with the same axial orientation of the cellulose chains but different orientation of the a- and c-axes.10 The microfibrils are arranged as a helix in concentric cylindrical growth layers. These fibrils combine to form bundles (macrofibrils) of larger diameter due to physical coalescence as a result of the reduced surface free energy. They are interconnected and have widths around 100 nm.11 The helix angle is constant throughout the cross-section and along the length for all cottons. The apparent variations in different varieties are attributed to the superposition of the convolutions and the helical angles.7–9 However, the sense of the helix reverses from 30–100 times in the fiber. This reversal frequency primarily depends on the variety of cotton and the growth conditions.12 The crystalline orientation has been shown to be high at the reversal points. Cotton is essentially crystalline; only about one third of the total molecules constitute the amorphous phase. The disorder is mainly due to small crystalline units that are randomly packed. The structure of cotton is considered to be paracrystalline. Some physical characteristics of the cotton fibers, including the mean fiber length, linear density, convolution

132

Cotton species

G. G. G. G.

arboreum herbaceum hirsutum barbadense

Length (mm)

10–25 10–25 20–32 35–45

Width (µm)

17–25 17–30 16–20 14–18

Avg. cross-sectional shape (75 % mature) Round

Elliptical

Flat

40.5 34.8 15.3 18.2

48.0 51.5 67.6 65.3

11.5 13.7 17.1 16.5

Convolutions/ cm

Reversals/ cm

Convolution angle

Spiral angle

x-ray angle

30–60 30–50 50–75 30–55

2–6 2–5 10–27 12–20

5.1–11.8 6.2–8.9 8.8–12.3 3.9–8.5

29.6–39.8 34.4–37.5 35.4–38.0 28.2–35.0

22.6–35.0 26.5–31.8 28.6–34.6 22.8–30.5

Tribology of natural fiber polymer composites

Table 5.1 Dimensional and structural characteristics of different cotton species13

Cotton reinforced polymer composites

133

angle* and moisture absorption, are given in Table 5.2.13 The data on the degree of crystallinity and crystalline orientation are given in Table 5.3.13

5.1.4

Mechanical properties of cotton fiber

The stress–strain curve of the cotton fiber is very similar to that of a glassy solid. However, the elongation at break is relatively large for the former compared to that of the latter. Under tensile loading, the dominant feature is the splitting of the structure along its length due to the fibrillar nature of the fiber.14 This splitting occurs between fibrils and the break adjacent to a reversal, causing a tear that develops along the fiber and follows the helical path of the fibrils around the fiber. In cotton fibers, since the interaction between the chain molecules is strong, the elasticity of cotton is dominated by changes in internal energy. Crystallinity, crystal size and the links between crystalline units offer high correlation with Young’s modulus in the fiberaxis direction.15 The crystallites take part in the deformation even at low strains during measurement of the modulus at room temperature. A good correlation also exists between birefringence and crystalline orientation.7–9 Mature fibers exhibit higher elastic modulus than less mature fibers. The strength of cotton fiber is inversely proportional to the x-ray orientation Table 5.2 Some physical characteristics of two important species of cotton13 Cotton species

Mean fiber length (cm)

Linear density (mtex)

Convolutions/cm

Convolution angle

Moisture regain

G. hirsutum G. herbaceum

2.775 2.310

168.80 179.82

80 60

13°34′ 11°40′

6.410 6.765

Table 5.3 Degree of crystallinity and crystallite orientation of two species of cotton13 Cotton species

Degree of crystallinity

G. hirsutum G. herbaceum

0.685 0.730

Crystallite orientation X-ray angle ————————————— 40% 50% 75%

Herman’s orientation factor

38.7 43.5

0.592 0.547

34.0 37.5

22.5 24

* During drying from a swollen cellular tube to the collapsed-fiber form, in-built strains and stresses are locked-in in the fibers. In fact, the asymmetry of mechanical strains during drying is thought to be responsible for the typical convoluted structure of the fiber.

134

Tribology of natural fiber polymer composites

angle, which in turn is a measure of the convolution and the spiral angle.16 However, the strength–orientation correlation is relatively poor.17,18 These correlations decrease with increasing test length, which indicates that the strength of cotton fibers is determined partly by the orientation and partly by the presence of weak places along the fiber length. The decrease of strength with increasing number of reversals was higher for highly oriented samples. The strength of cotton is linearly related to its molecular weight. However, a paracrystalline-lattice distortion adversely affects the strength.19 The elongation at break for cotton fiber also shows a good correlation with the x-ray angle,7–9 increasing with the increasing angle. Since crystals are elastic up to only relatively low elongations, beyond which plastic deformation takes place, the elongation of cotton depends partly on the alignment of the fibrils in the direction of stretch and partly on their deformation. The mechanical properties of cotton fiber, including the stiffness, strength, elongation at break, toughness and bundle strength are listed in Table 5.4.13 These properties are discussed in the subsequent section. Stiffness The stiffness of cotton fiber depends on the molecular as well as the crystalline orientation. At very low strain rates (below 0.5 %), a linear stress–strain curve is obtained.15 The average molecular orientation fmol is related to the crystallite and amorphous orientation in terms of the following relation:20 fmol = Xfc + (1–Xfa) where X is the degree of crystallinity and fc and fa are the Herman’s orientation factors for the crystalline and amorphous regions, respectively. The results for cotton are listed in Table 5.5.13 The data show that the amorphous regions in cotton are in an oriented state and their orientation is same as that of the crystalline phase. Thus, the degree of crystallinity is not a critical factor in determining the fiber modulus. Firstly, the amorphous regions will be in the glassy state and therefore rigid. Secondly, they have high orientation, close to that of the crystalline phase.

Table 5.4 Mechanical properties of two species of cotton13 Cotton species

Average Young’s modulus (gf/tex)

Average tenacity (strength) (gf/tex)

Elongation (%)

Bundle strength (gf/tex)

Toughness index (gf/tex)

G. hirsutum G. herbaceum

383.01 488.39

22.65 22.52

6.93 5.00

48.24 48.78

0.92 0.62

Cotton reinforced polymer composites

135

Table 5.5 Details of three compatibilizers24 Compatibilizer

Temperature (°C)

Reaction time (min)

MAH (phr)

DCP (phr)

Graft content (wt%)

Intrinsic viscosity (dl/g)

C1 C2 C3

120 140 150

5 5 5

7 7 7

0.3 0.7 0.7

0.84 1.72 2.14

0.82 0.68 0.57

Strength of single fibers and bundle strength The strength of cotton fibers depends on the molecular weight, orientation, number of reversals and gauge length of the test specimen.16 The bundle strength of cotton is higher than the single fiber strength (Table 5.4). This difference is attributed to the difference in the gauge lengths; zero for the bundle strength measurement and 1 cm for single fiber strength measurements. Bundle strength shows good correlation with x-ray orientation and maturity coefficient. Thus, an increase in molecular weight and crystallite orientation leads to an increase in the bundle strength of cotton. However, single fiber strength does not show any correlation with orientation. This is attributed to the fact that as the gauge length increases the correlation with orientation decreases and the reversal frequency starts to dominate. For average fineness, the breaking strength values show more scatter. It has been observed that the breaking load of fibers increases with increasing linear density up to the average linear density of the cotton and then becomes steady. Consequently, values of breaking strength obtained are much lower than the average, indicating that the distribution of linear density influences the breaking strength. The breaking of fibers involves the rupture of molecules, i.e. the tensile failure occurs when hydrogen bonds are overcome. In cellulosic fibers, overcoming hydrogen bonds provides the measured strength values. Once hydrogen bonds are overcome, the stress can concentrate on fibrils close to the reversals and breakage occurs.21 Elongation at break and toughness The elongation at break shows a good correlation with toughness. It also shows good correlation with x-ray angle, linear density and the maturity coefficient. Similarly, the toughness shows good correlation with the x-ray angle. Thus the main factors that make elongation high are: low crystalline orientation, low linear density and low maturity coefficient. The toughness is determined by the elongation of the cotton fiber.

136

5.2

Tribology of natural fiber polymer composites

Cotton–polymer composites

It has already been mentioned in Chapter 1 of this book that plant fibers are currently used in the interior of passenger cars and truck cabins as trim parts, door panels or cabin linings. Composites based on plant fibers are also used extensively for thermo-acoustic insulation. Such insulating materials are mainly based on cotton fibers recycled from textiles and comprise cotton fibers up to 80 % by weight. Polyester is frequently used to develop cotton fiber reinforced polymer composites since polyester is inexpensive, easily available as a liquid, easily processed and cured and possesses good mechanical properties when reinforced with fibers and fillers. Polyesters are suitable for a variety of applications and are adaptable to the fabrication of structures of complex and intricate shapes.

5.2.1

Cotton–polyester composites

Polymer composites based on cotton fibers have gained significant importance both in technical applications, such as the automotive industries, and in terms of strength requirements.22,23 However, the main disadvantage of these composites is the lack of good fiber–matrix interfacial adhesion, which adversely affects the properties of composites. The poor adhesion between fiber and matrix is due to the hydrophilic nature of the former, which if decreased either by chemical modification or the use of a compatibilizer can greatly enhance the composite properties. The compatibilizers strongly affect the mechanical properties as discussed in subsequent sections. Effect of cotton content on mechanical properties The addition of cotton fibers in polymers decreases yield stress, increases Young’s modulus and decreases elongation at break and impact strength. The decrease of yield stress with increasing fiber content (Fig. 5.1) is attributed to the poor adhesion between the two phases (i.e. fiber and matrix). However, a significant increase of Young’s modulus (Fig. 5.2) is due to the significantly high Young’s modulus of cotton fiber24 that increases the stiffness of the composites as the fiber content increases. The reduction of matrix amount as fiber loading is increased contributes to the decrease of impact strength because the matrix is primarily responsible for the absorption of the impact energy. The incorporation of cotton fibers reduces the elongation at break of the composites. The significant reduction in mechanical properties at high fiber content is attributed to the presence of many fiber ends in the composites, which could cause crack initiation.25 All the mechanical properties discussed in the preceding paragraphs improve with the use of compatibilizers. These agents modify the interface by interacting

Cotton reinforced polymer composites

137

2800

C0 C1 C2 C3

Tensile stress (MPa)

2400 2000 1600 1200 800 400 0

10

20 30 40 Fiber content (wt%)

50

5.2 Influence of fiber content as well as three compatibilizers on tensile modulus of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite).24

with both the fiber and the matrix, thus forming a link between the two phases of the composite. This is attributed to the ability of the maleic anhydride (MAH) to react with the hydroxyls of the cotton fibers and the compatibility of the grafted copolymer bionolle chains with the main bionolle phase. The yield stress of the composites with 50 wt% fiber content increases to 23.9 and 26.4 MPa with the addition of lowest and highest compatibilizers (Table 5.5), respectively, compared with the value of 19.7 MPa for the noncompatibilized composite. Similarly, the addition of the compatibilizers leads to a slight improvement in Young’s modulus (Fig. 5.2) and a significant increase in impact strength (Fig. 5.3). The elongation at break is not affected significantly by the addition of the compatibilizer. A slight improvement in Young’s modulus (Fig. 5.4) is obtained when the compatibilizer content is increased. The impact strength also increases with increasing compatibilizer content (Fig. 5.5), the effect of the compatibilizer becoming more pronounced with increased fiber content. The addition of compatibilizer reduces the water uptake of composites (Fig. 5.6) due to the formation of covalent bonds between the functional groups of MAH and the hydroxyl groups at the surfaces of cotton fibers.26 With the increase in the compatibilizer content, less water is absorbed. Since there are more functional MAH groups as compatibilizer content increases, so more bonds are formed between matrix and fibers. The same conclusion is drawn from the use of different grafting content compatibilizers. Thickness swelling of cellulosic materials occurs when the cell wall is bulked by water. Composites with compatibilizer show lower thickness swelling compared with non-compatibilized composites (Fig. 5.7).

138

Tribology of natural fiber polymer composites 120 C0 C1 C2 C3

Impact strength (J/m)

100

80

60

40

20

30 40 Fiber content (wt%)

50

5.3 Influence of fiber content as well as three compatibilizers on impact strength of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite).24 3500 –

Young’s modulus (MPa)

3000

5 phr 10 phr 15 phr

2500 2000 1500 1000 500 0

10

20 30 40 Fiber content (wt%)

50

60

5.4 Influence of fiber content as well as compatibiliser content on Young’s modulus of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite).24

Figures 5.8 and 5.9 show the modulus of rupture (MOR), modulus of elasticity (MOE) and tensile strength of cotton stalks–polyester composites containing different ratios of fibers that were milled using different screen sizes.27 These properties depend on fiber size as well as fiber content. MOR increases on increasing fiber content up to 25 %. Tensile strength of the composites also decreases at the lower fiber content (15–20 %). A critical fiber content is

Cotton reinforced polymer composites

139

120

Impact strength (J/m)

– 5 phr 10 phr 15 phr

100

80

60

40

20

30

40 50 Fiber content (wt%)

60

5.5 Influence of fiber content as well as compatibilisers content on yield stress of cotton fiber-based composites (solid square legend shows data for untreated cotton fiber composite).24

10

50 wt% fiber content

Water absorption (%)

8

6 – 10 phr C1 10 phr C2 10 phr C3 5 phr C2 15 phr C2

4

2

0 0

2

4

6 8 10 Immersion time (days)

12

14

5.6 Influence of compatibilizers and their content on moisture absorption characteristics of cotton fiber-based composites containing 50 wt% fiber.24

required before the strength of the composites becomes greater than that of the polymer matrix. Maximum MOR is achieved for the composite containing 25 % cotton stalk fibers milled using a 0.2 cm screen. Tensile strength of the composites exceeds that of neat polyester only on using 20–25 % cotton stalk fibers milled using a 0.35 cm screen. Water absorption and thickness swelling decrease with decreasing fiber size and fiber content.

140

Tribology of natural fiber polymer composites 10

Thickness swelling (%)

Without compatibilizer 8

With compatibilizer

6

4

2

0 20

30

40 50 Fiber content (wt%)

60

5.7 Histogram showing thickness swelling of cotton fiber based composites with and without compatibilizer (C2; 15 phr).24

Effect of esterified cotton fibers on mechanical properties The infrared (IR) spectra of the esterified cotton fibers show a carbonyl absorption peak at 1725 cm–1 due to the presence of the bark*, which contains waxes, resins, starches and a high percentage of tannic acid (Fig. 5.10). The intensity of this peak increases as a result of esterification. No improvement in MOR, MOE and tensile strength of the composites could be achieved due to esterified fibers (Table 5.6).27 In fact, there was a slight decrease in the values of MOR and MOE. Notwithstanding the lower strength properties obtained using esterified fibers, esterification is still preferred because it leads to an increase in the fiber–matrix interaction that could compensate for the lower fiber content and, at sufficiently high ester content, could result in higher strength properties. Although water absorption increases, thickness swelling decreases because of the introduction of the ester groups into the cell wall polymers. Esterification prevents further swelling caused by water absorption. Effect of hybridization Cotton/ramie–polyester composites The tensile strength of plain weave ramie/cotton hybrid polyester resin matrix composites increases significantly for the composites with high fiber volume fractions.28 However, no linear relationship between the tensile strength and the volume fraction of fibers exists. The volume fraction of longitudinal *Bark present in the cotton contains a high percentage of UV-absorbing groups that increases the UV resistance of cotton–polyester composite.

Cotton reinforced polymer composites

141

35 0.55

0.35

0.2

0.15

30

MOR (MPa)

25

20

15

10

5

0 0

15

20 % Fiber (a)

25

30

1.4 0.55

0.35

0.2

0.15

1.2

MOE (GPa)

1

0.8

0.6

0.4

0.2

0 0

15

20 % Fiber (b)

25

30

5.8 Influence of fiber content and fiber size on (a) MOR and (b) MOE of cotton stalk–polyester composites.27

142

Tribology of natural fiber polymer composites

16 0.55

0.35

0.2

0.15

14

10

8

4 2

0

0

15

20 % Fiber

25

30

5.9 Influence of fiber content and fiber size on tensile strength of cotton stalk–polyester composites.27

Tannic acid (%)

Tensile strength (MPa)

12

a

b

3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 Wavenumber (cm–1)

5.10 Comparative FTIR spectra of (a) untreated and (b) MAH esterified cotton stalks.27

Cotton reinforced polymer composites

143

Table 5.6 Effect of esterification of cotton fibers on mechanical properties (average values) of cotton stalks–polyester composites (fiber wt% ~25) milled fiber size (0.2 cm)27 Fiber

MOR (MPa)

MOE (GPa)

Tensile strength (MPa)

Water absorption (%)

Thickness swelling (%)

Cotton stalks Esterified cotton stalks

29.73 25.30

1.25 1.16

10.42 9.44

12.52 15.21

4.34 2.25

Table 5.7 Tensile stress of the cotton–ramie hybrid composites with (90/0) configurations28 Material

Total volume fraction of fibers (%)

Fabric I

49.7 55.3 55.3 57.2 45.3 49.3 49.3 50.9 54.1 60.2 60.2 60.9 52.9 58.0

Fabric II

Fabric III

Fabric IV

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.7 1.2 1.2 3.4 3.1 3.4 3.4 3.4 1.8 1.4 1.4 1.3 0.5 0.6

Ramie Vf % (parallel ) to the test direction

Stacking sequence

σ (MPa)

12.9 ± 0.4 19.2 ± 0.6 9.6 ± 0.6 14.8 ±1.8 12.7 ± 2.3 18.5 ± 2.5 9.2 ± 2.5 14.3 ± 2.6 19.5 ± 1.3 28.9 ± 1.0 14.5 ± 1.0 21.9 ± 0.9 20.6 ± 0.4 22.5 ± 0.5

0/90 0/90/0 90/0/90 0/90/0/90 0/90 0/90/0 90/0/90 0/90/0/90 0/90 0/90/0 90/0/90 0/90/0/90 0/90 0/90/0/90

46.3 61.3 43.5 60.8 56.2 61.4 43.2 54.0 55.2 85.0 51.7 70.3 60.5 68.9

± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.7 4.1 3.6 3.0 3.4 4.1 6.2 3.3 4.0 9.3 1.9 11.7 4.3 3.5

ramie fibers controls the overall tensile properties. The transverse cotton fibers (along the tensile axis) do not contribute significantly to the tensile properties. Table 5.7 shows the effect of different (0/90) fabric stacking sequences on the tensile strength of the polyester/ramie–cotton composites. (0) direction indicates that the ramie fibers are aligned along the test direction, whereas (90) direction is referred to whenever the ramie fibers are perpendicular to the test direction. The tensile strength obtained is greater than that of the matrix, and an increase of up to 243 % is achieved. Cotton/kapok–polyester composites Polymer matrix composites (PMCs) developed using kapok/cotton fabric as reinforcement and isotactic polypropylene (iPP) and MAH grafted

144

Tribology of natural fiber polymer composites

polypropylene as matrices exhibit better mechanical properties.29 Untreated kapok/cotton fabric–iPP shows less stress resistance compared to composites with mercerized kapok/cotton fabric. Mercerization forms a ductile interface and good fiber–matrix interfacial adhesion. Weathered kapok/cotton fabric–iPP composite has low interfacial bond strength and low stress resistance. The tensile strength of iPP matrix reinforced with untreated kapok/cotton fabric is much higher than that of composites reinforced with alkali treated fabric and acetylated fabric. Both alkali treatment and acetylation reduce the crystalline cellulose, resulting in low tensile strength of the composites (Fig. 5.11). Kapok/cotton–MAH-iPP composites have the lowest tensile strength compared to other composites, and this is due to the brittle characteristics of the MAH-iPP matrix causing poor load transmission. The stiffness or modulus of the untreated iPP composites increases with an increase in fiber volume fraction and then decreases beyond about 23 % fiber volume fraction. On the other hand, the addition of the same amount of fiber causes a significant increase in the initial tensile modulus of the MAH-iPP reinforced with untreated kapok/cotton fabric. It also has the highest stiffness properties at higher fiber content (Fig. 5.12). The flexural properties of iPP and MAH-iPP–kapok/cotton fabric composites are shown in Figs 5.13 and 5.14 together with glass fiber–MAH-iPP composites. The increase in the cellulose reinforcements on the iPP- and MAH-iPPbased composites shows a trend similar to that of the increases in their flexural strength and modulus. The maleated polypropylene fiber reinforced composite gives superior flexural properties compared with the conventional 40

Tensile strength (N/mm2)

35 30 25 20 Untreated kc-iPP

15

Mercerized kc-iPP Acetylated kc-iPP

10

Weathered kc-iPP

5

Untreated kc-MAH-iPP

0 0

5

10 15 20 Fibre volume fraction

25

30

5.11 Effect of chemical treatment and weathering on tensile strength of kapok/cotton (kc)–iPP composites.29

Cotton reinforced polymer composites

145

Tensile modulus (N/mm2)

2

1.5

1 Untreated kc–iPP Untreated kc-MA–iPP Mercerized kc–iPP

0.5

Acetylated kc–iPP Weathered kc–iPP

0 0

5

10 15 20 Fibre volume fraction

25

30

5.12 Effect of chemical treatment and weathering on tensile modulus of kapok/cotton (kc)–iPP composites.29

Flexural strength (N/mm2)

70 60 50 40 30 Untreated kc–iPP

20

Untreated kc-MAH–iPP 10 0

Glass fiber MAH–iPP 0

5

10 15 20 Fibre volume fraction

25

30

5.13 Comparison of flexural strength of cotton/kapok (kc) and glass fiber reinforced iPP composites.30

polypropylene composites. MAH-iPP–kapok/cotton reinforced composite shows an increasing trend of toughness with an increase in fiber volume fraction. The overall toughness (represented by work done, J) is, however, much lower than that of the fiber reinforced iPP composite (Fig. 5.15). This is mostly due to the brittle characteristics of the MAH-iPP matrix. The slight increase in toughness of the acetylated fabric composite is attributed to the plasticization effect of the acetylation process on the fibers.

146

Tribology of natural fiber polymer composites

Flexural modulus (N/mm2)

5

4

3

2

Untreated kc–iPP 1

Untreated kc–MAH-iPP Glass fiber–MAH-iPP

0 0

5

10 15 20 Fiber volume fraction

25

30

5.14 Comparison of flexural modulus of cotton/kapok (kc) and glass fiber reinforced iPP composites.29

12

Untreated kc–iPP Untreated kc–MAH–iPP Mercerized kc–iPP Acetylated kc–iPP Weathered kc–iPP

Work done (J)

10 8 6 4 0

18

22.98 Fiber volume fraction

27.76

5.15 Comparison of toughness (represented by work done) of treated and untreated cotton/kapok (kc) fabric reinforced iPP composites.29

5.3

Tribological behavior of cotton–polyester composites

Tribological applications of unidirectional cotton fiber reinforced polyester composite, as bearings in conjunction with water lubrication and cooling, are quite old.30 However, published work on the friction and wear behavior of cotton reinforced polymer composites is scarce. One important work on the effect of orientation of cotton fibers on friction and wear behavior of polyester composites in sliding wear mode is a pioneering study which showed that the fiber diameter affected the friction coefficient µ of composites.31

Cotton reinforced polymer composites

147

For pins sliding in the direction perpendicular to sliding (normal, N) a small increase in µ accompanied the increase in fiber volume fraction Vf, whereas in the longitudinal (L) and transverse (T) directions µ slightly decreased initially and then attained a steady value, essentially constant for values of Vf > 0.15. This was obviously due to the larger area of exposed cotton fiber in the case of the L and T directions than in the N direction. When the percentage of cotton fibers increases in the area of contact, the friction coefficient tends to decrease. The decrease in µ in the L and T directions was due to the easy detachment of fibers from the bulk of the composite, in contrast with the N direction in which fibers are less exposed and their detachment is difficult. With respect to the L and T directions, the latter showed a lesser decrease in µ compared to those in the former. This is probably because, for the fibers to be pulled out of the surface, the frictional forces in the T direction must overcome an additional resisting force to deformation exerted by the polyester matrix backing up the fibers along their length, which does not exist in the L direction. The specific wear rate of all the three samples decreased initially with the increase in Vf and then became almost constant for Vf > 0.15 (Fig. 5.16). The highest wear rate occurred in the L direction of sliding for the same reason discussed above. Additionally, the improvement in the mechanical properties of the polymer also contributes to this factor (Table 5.8). The formation of a fiber-rich composite surface in the N and T directions and, to a smaller extent, in the L direction indicates that cotton particles act as lubricant in 0.9

Friction coefficient µ

0.8

0.7

0.6

0.5

0 0

0.1 0.2 Fiber volume fraction Vf (a)

0.3

5.16 Friction coefficient of cotton polyester composites as a function of fiber volume fraction for (a) normal, (b) longitudinal and (c) transverse orientation of fibers. Dashed lines are for speed 32 m/s while solid line shows data at a sliding speed of 10 m/s.31

148

Tribology of natural fiber polymer composites 0.9

Friction coefficient µ

0.8

0.7

0.6

0.5

0 0

0.1 0.2 Fiber volume fraction Vf (b)

0.3

0

0.1 0.2 Fiber volume fraction Vf (c)

0.3

Friction coefficient µ

0.8

0.7

0.6

0.5

0

5.16 (Continued)

decreasing the wear rate (Fig. 5.17). The diameter of the cotton fibers was either 0.3 or 0.45 mm, which increased by a factor of 2–3 during sliding. Some of them acquired an oval shape with the major axis being oriented in the direction of sliding. Above the PV limit, the behavior was quite different. In the N direction µ increased with increasing Vf then attained a steady value at Vf = 0.15, whereas µ remained almost constant throughout in the L and T directions. The values of µ determined were always smaller than the corresponding values below the pv limit. The specific wear rate decreased in all three cases with increasing Vf then became constant for Vf > 0.15 irrespective of the fiber diameter df. At any given Vf the N direction of sliding showed the lowest specific wear rate followed by the T and L. In the N direction of sliding, the long cotton fibers extending through the matrix prevented the

Table 5.8 Mechanical properties of cotton–polyester composites32 Property

Neat polyester

Reinforced polyester

Energy absorbed for impact per unit width (kg m/s2) Flexural strength at mid-point (µm/mm) Max. strain at mid-point (µm/mm) Modulus of elasticity at bending (N/mm2) Strain energy density at maximum stress (N/mm2) Tensile strength at the point of break (N/mm2) Tensile strength at yield (N/mm2) Modulus of elasticity (N/mm2) Strain of fracture (%) Plain strain fracture toughness (N/m3/2)

6.11 101.83 44.75 2476.09 2.375 53.88 53.88 702.06 11.86 44.22

df = 0.45 mm

Vf = 0.127

Vf = 0.275

Vf = 0.167

Vf = 0.286

22.27 103.10 50.185 2727.3 2.759 71.75 71.56 1122.13 8.65 61.54

97.1 142.02 63 4251.61 3.56 117.51 108.05 1235.21 11.29 113.489

34.33 116.1 59.48 2773.49 2.923 87.03 76.3 1109.54 10.69 78.122

105.37 143.17 65.35 3774.85 3.37 116.48 110.28 1256.79 10.65 112.76

Cotton reinforced polymer composites

df = 0.3 mm

149

150

Tribology of natural fiber polymer composites

catastrophic failure of the frictional heat softened polyester at the sliding surface. This is in contrast to the higher specific wear rate observed in the T and L directions where the fibers extending parallel to the sliding surface are easily pulled out from the softened matrix. The micrograph of the worn surface of a specimen (Vf = 0.127, df ~0.3 mm) after testing in the N direction above the pv limit showed smudging of the surface with deformed cotton fiber that was much greater than that observed below the pv limit.

200

Wear rate W (10–8 g/cm)

160

120

80

40

0 0

0.1 0.2 Fiber volume fraction Vf (a)

0.3

Wear rate W (10–8 g/cm)

200

160

120

80

40

0 0

0.1 0.2 Fiber volume fraction Vf (b)

0.3

5.17 Wear rate of cotton polyester composites for (a) normal, (b) longitudinal and (c) transverse orientation of fibers. Dashed lines are for speed 32 m/s while solid line shows data at a sliding speed of 10 m/s.32

Cotton reinforced polymer composites

151

Wear rate W (10–8 g/cm)

200

160

120

80

40

0 0

0.1 0.2 Fiber volume fraction Vf (c)

0.3

5.17 (Continued)

Graphite filled polyester composites Cotton reinforced polyester composites are used for making fabric bearings. Hence, the friction and wear performance (low friction coefficient and low wear rate) of such anti-friction composites should be very high. In view of this, efforts have been made to reduce the friction coefficient of cotton filled composites. Graphite acts as a solid lubricant due to its lamellar crystal structure. When used as a solid lubricant in cotton–polyester composites, the tribological properties are improved significantly due to the lubricating action of the layer-lattice structure of graphite. However, the friction and wear of graphite filled composites depend on the concentration of graphite powder as observed in the pin-on-disk sliding wear tests.32 The specific wear rate of pure polyester reduces with the increased load, and it fails at a smaller pv value (0.597 MPa pressure; 2.22 m/s sliding speed) (Fig. 5.18). However, at higher loads the wear rate increases due, obviously to the greater frictional heat which softens the matrix. The transfer film of polyester improves the wear performance. Simultaneously, the increase in interface temperature causes a deterioration in the mechanical properties and the load-carrying capacity beyond ~0.6 MPa pressure. When the polyester was reinforced with cotton waste, the pv limit increased to ~1.2 MPa. However, it was accompanied by a higher friction heat and interface temperature that led to higher wear rates. When these composites were filled with graphite (5 phr), the wear performance (reciprocal of specific wear rate) was much better than that of the cotton–polyester composite. The specific wear rate was almost steady in the graphite filled cotton–polyester composite which could be tested even up

152

Tribology of natural fiber polymer composites

Specific wear rate, × 10–4 (m3/mN)

8 Cotton–polyester 5 phr Gr–CP 10 phr Gr–CP

6

15 phr Gr–CP 20 phr Gr–CP Polyester

4

2

0 0

20

40 60 Applied load (N)

80

100

5.18 Specific wear rate of graphite filled cotton reinforced polyester composites.32 1.2 0 vol. % 1.96 vol. %

1

3.84 vol. %

Friction coefficient

5.66 vol. % 7.40 vol. %

0.8

Polyester

0.6

0.4

0.2

0 0

10

20

30

40 50 60 Applied load (N)

70

80

90

5.19 Coefficient of friction of graphite filled cotton reinforced polyester composites.32

to ~1.6 MPa. With increased concentration of graphite, the wear rate reduced, indicating the lubrication effect of graphite. The addition of graphite also reduced the friction of the cotton–polyester composite (Fig. 5.19). The friction coefficient also decreased with increasing graphite content. With frictional heat, thermal softening of the composite caused a larger contact between the specimen pin and sliding disc and hence a larger friction coefficient value.33,34

Cotton reinforced polymer composites

153

In contrast, the friction coefficient increased with cotton reinforcement, probably due to the reduced area of sliding contact between pin and counterface. It is proposed that resin bonded cotton fibers increase the friction coefficient while the loose cotton fibers might align in the direction of motion and hence reduce frictional force.35 Operating parameters such as load and temperature had contrasting influence on the friction behavior. For instance, the cotton polyester composite showed a higher friction coefficient and corresponding higher sliding interface temperature. Although the temperature increased with load (Fig. 5.20), the friction coefficient reduced, which is possibly due to the deterioration of fiber bonding with increased temperature, leading to the easy pull-out of fibers aligned in the sliding direction. In the case of graphite filled cotton fiber reinforced polyester composite, the temperature of the contact surface drastically reduced due to the reduced frictional heat as a consequence of the lubrication effect of graphite. Moreover the higher conductivity of graphite increased the heat dissipation and prevented accumulation of heat at the contact. Ultra high molecular weight polyethylene (UHMWPE) filled polyester composite The incorporation of a small fraction of UHMWPE particles improves the wear resistance of base polymers like polypropylene, polyester, etc.36,37 The wear characteristics of cotton polyester composites measured on a pin-on100

Temperature (°C)

90

80

70

0 vol. % 1.96 vol. %

60

3.84 vol. % 5.66 vol. %

50

7.40 vol. % Polyester

40 0

20

40 60 Applied load (N)

80

100

5.20 Rise in temperature of counterface disc with increasing load during sliding wear tests.32

154

Tribology of natural fiber polymer composites

disk machine show that among neat polyester, cotton waste–polyester composite and cotton waste–UHMWPE filled polyester composite, neat polyester possesses a low pv limit and low wear resistance (Fig. 5.21). Cotton waste reinforced polyester composites show a better structural integrity and a higher pv limit. When filled with UHMWPE particles, the pv limit of cotton polyester composite is not adversely affected. Furthermore, when UHMWPE modified polyester resin is used as matrix, not only is the pv limit increased, but the wear resistance increases as well. The increase in the concentration of UHMWPE reduces the wear rate even at higher loads, possibly due to the lubricating effect of UHMWPE. For the same reason, UHMWPE filled cotton fiber reinforced polyester composite exhibited low friction coefficient which decreased further with the increase in the UHMWPE content (Fig. 5.22). The value of µ which increased with cotton reinforcement was reduced to below 0.40, nearly half of that of a polyester resin and nearly one-third of that of a cotton–polyester composite. The increased friction coefficient on cotton reinforcement is due to the decrease in the area of contact between pin and counterface. The cotton fibers resist frictional heat more than the polyester resin and therefore offer a resistance to sliding movement that resulted in the increased friction coefficient of the cotton–polyester composite. Figure 5.23 shows the effect of applied load on the friction coefficient of polyester, cotton–polyester and UHMWPE filled cotton–polyester composite. The friction coefficient increased with load for pure polyester. The cotton– polyester composite showed higher friction coefficient as well as higher sliding surface temperature. The temperature increased with load but the friction coefficient reduced and this was attributed to the deterioration of

Specific wear rate (m3/Nm)

9.0E-14 Cotton–polyester 3.87 vol.% of UHMWPE 7.41 vol.% of UHMWPE 13.97 vol.% of UHMWPE 14.19 vol.% of UHMWPE Polyester resin

6.0E-14

3.0E-14

1.0E-16 0

20

40

60 Load (N)

80

100

5.21 Specific wear rate of UHMWPE filled cotton reinforced polyester composites.36,37

Cotton reinforced polymer composites

155

1.30 1.20 1.10

Cotton–polyester

1.00

Friction coefficient

0.90 0.80

Polyester

0.70 0.60

3.87 vol.% of UHMWPE

0.50 0.40

14.19 vol.% of UHMWPE

0.30 0.20 0.10 0.00 0

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 170018001900 Time (seconds)

5.22 Coefficient of friction of UHMWPE filled cotton reinforced polyester composites.36,37

Friction coefficient µ

1.2

Polyester Cotton–polyester 7.41 vol.% UHMWPE 10.86 vol.% UHMWPE 13.97 vol.% UHMWPE 14.19 vol.% UHMWPE

0.9

0.6

0.3 0

20

40 Normal load (N)

60

80

5.23 Coefficient of friction of UHMWPE filled cotton reinforced polyester composites as a function of load.36,37

fiber binding with increased temperature that led to the easy pull-out of fibers and their subsequent alignment in the sliding direction. The addition of UHMWPE changed the trend and, due to the lubrication effect, the additional UHMWPE content in cotton–polyester composite resulted in a reduction in the friction coefficient.

156

Tribology of natural fiber polymer composites

The increased load further reduced the value of µ due to the formation of a transfer layer of UHMWPE on steel disc. Worn surfaces when observed under scanning electron microscope (SEM) revealed the morphological changes of studied materials subjected to sliding wear. The worn surface of cotton reinforced polyester composites showed maximum damage to the surfaces (Fig. 5.24). Cotton fibers along with the polyester matrix were damaged under sliding action. Cavities were formed due to the removal of material. Bigger cavities were expected in this case because cotton fiber may not allow small debris to be removed easily; instead, the combined cotton polyester lumps were removed leaving behind larger cavities. The addition of UHMWPE in the cotton–polyester composite modified the wear, and the worn surfaces showed smooth topography and less material removal (Figs 5.25 and 5.26). SEM examination also revealed the incomplete wetting of cotton fiber with polyester resin and therefore an air entrapped region is observed at certain places (Fig. 5.27). Loose cotton and fragments were observed in a cavity, which was formed during sample preparation. The high viscosity of the unsaturated polyester may be attributed to such pockets wherein resin could not penetrate sufficiently to embed properly in the bundles of cotton fibers. The addition of UHMWPE in cotton–polyester composites provided microdots having excellent wear resistance properties and protected the composite from further wear (Fig. 5.28). The maximum load was carried by UHMWPE particles and hence an increased volume percent of UHMWPE significantly reduced wear rate as well as friction coefficient.

5.24 Micrograph of worn surfaces of cotton polyester composites.36,37

Cotton reinforced polymer composites

157

5.25 Worn surface of UHMWPE (13.97 vol.%) filled cotton–polyester composites.36,37

5.26 Magnified view of the worn surface of UHMWPE (13.97 vol.%) filled cotton–polyester composite.36,37

Lubrication behavior of cotton The lubrication mechanism of cotton fiber can be understood better using a cotton transfer film in a steel-on-steel contact on a ball-on-disc machine.38 The film is formed during the sliding wear. The friction coefficients of steel-

158

Tribology of natural fiber polymer composites

5.27 Worn surface of UHMWPE (14.19 vol.%) filled cotton fiber reinforced polyester composite.36,37

Polyester

Cotton–polyester

3.87 vol.% UHMWPE in cotton–polyester

14.19 vol.% UHMWPE in cotton–polyester

5.28 Schematic showing formation of friction dots of UHMWPE during sliding wear tests.36,37

Cotton reinforced polymer composites

159

on-steel measured in the presence and absence of a cotton transfer film are shown in Fig. 5.29 for relative humidity (RH) of 30, 50 and 75 %. The friction coefficient is smaller for the experiments with a cotton film, confirming the lubricant effect of these films. At a low RH of 30 %, the friction coefficient for the steel–steel contacts was high (0.6–0.8) and very unstable. The friction coefficient of the steel/cotton–steel contact was unstable at the beginning of the experiment, then assumed a constant value of about 0.4. In the presence of a transfer film, the friction coefficient was about 0.3 during the first third of the run, then it rose sharply to the value typical for a steel–steel contact. 1 PH = 30% Without cotton film

0.8 0.6 0.4

With cotton film 0.2 0.9

Friction coefficient

RH = 50%

Without cotton film

0.7

0.5 0.3 With cotton film 0.1 0.8 RH = 75% Without cotton film

0.6 0.4 0.2

With cotton film

0 0

50

100

150 200 Time (s)

250

300

5.29 Coefficient of friction vs time with and without transferred cotton film at different RH conditions.38

160

Tribology of natural fiber polymer composites

At a high RH of 75 %, the friction coefficient of the steel–steel contact was between 0.5 and 0.6, but in the presence of a transfer film it was as low as 0.25. This value was stable during the first 50 s, then the friction coefficient rose sharply to the steel–steel value. In addition, changes of roughness of the sliding track were hidden by the presence of the cotton film which increased the apparent roughness of the disc both inside and near the wear track. The wear rate generally increased with the RH, but the dispersion of the results also increased. The lubricant effect may be due to the cellulose itself or the wax, or due to a synergistic effect of the two. However, the formation of the transfer film is not an intrinsic property of cellulose but is the result of complex phenomena. The transfer film is composed of fragmented cotton fibers, which agglomerate and adhere to the metal. Fragmentation is essentially a mechanical phenomenon and therefore depends on the resistance of the fibers and of the yarn. In principle, weak fibers favour fragmentation and film formation compared to stronger fibers. On the other hand, adhesion is strongly affected by surface chemical effects: the presence of specific chemical components within the fibers may promote adhesion and cohesion of cotton fiber fragments, thus facilitating the formation of the transfer film as well as its tribological properties. The presence of wax in cotton is a prerequisite for the formation of a transfer film.39,40

5.4

References

1. I.V. de Gury, J.H. Carra, W.R. Goynes, The Fine Structure of Cotton, Dekker, New York (1973). 2. N. Chand, P.K. Rohtagi, Natural Fiber and Composites, Periodical Experts, New Delhi (1994). 3. S.M. Betrabet, K.P.R Pillay, R.L.N. Iyenger, Text. Res. J. 33 (1963) 720. 4. S.M. Betrabet, R.L.N. Iyenger, Text. Res. J. 34 (1964) 46. 5. B.M. Petkar, P.C. Oka, V. Sundaram, Proc. 18th Technol. Conf. (ATIRA, BTRA and SITRA) (1977) 28, 1. 6. K.L. Datar, S.M. Betrabet, V. Sundaram, Text. Res. J 43 (1973) 718. 7. R. Meredith, Text Prog. 7 (4) (1975). 8. J.W.S Hearle and R. Greer, Text Prog. 2 (4) (1970). 9. J.O. Warwicker, R. Jeffries, R.L. Colbran, R.N. Robinson, The Cotton Silk and Man Made Fibers Research Association, Shirley Institute, Manchester (1966). 10. A. Peterlin, P. Ingram, Text. Res. J. 40 (1970) 353. 11. H.H. Dolmetsch, H. Dolmetsch, Text. Res. J. 39 (1969) 568. 12. H. Wakeham, T. Radhakrishnan, G.S. Vishwanathan, Text. Res. J. 29 (1959) 450. 13. V.B. Gupta, A.V. Manohar, B.C. Panda, in P.W. Harrison (ed.), Proc. 63rd Annual Conf. of the Textile Institute, IIT Delhi, Jan. 18–23 (1979). 14. W.E. Morton, J.W.S. Hearle, Physical Properties of Textile Fibers, 2nd edn, The Textile Institute and Heinemann, Manchester and London, (1975). 15. I.M. Ward, Mechanical Properties of Solid Polymers, Wiley, London (1971). 16. S.P. Rowland, M.L. Nelson, C.M. Welch, J.J. Hebert, Text. Res. J. 46 (1976) 194. 17. L. Rebenfeld, W.P. Virgin, Text. Res. J. 27 (1957) 286.

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18. A. Rajagopalan, N.B. Patil, V. Sundaram, Proc. 15th Technol. Conf. (ATIRA, BTRA, SITRA) (1974). 19. A. Vishwanathan, Cellulose Chem. Technol. 9 (1975) 103. 20. R.J. Samuels, Structured Polymer Properties, Wiley, New York (1974). 21. A.V. Tobolsky, H.F. Mark (eds) Polymer Science and Materials, Interscience, New York (1971). 22. A.K. Bledzki, J. Gassan, Prog. Polym. Sci. 24 (1999) 221. 23. A.K. Mohanty, M.A. Khan, G. Hinrichsen, Compos. Sci. Technol. 60 (2000) 1115. 24. V. Tserki, P. Matzinos, C. Panayiotou, J. Appl. Polym. Sci. 88 (2003) 1825. 25. A.K. Mohanty, M.A. Khan, G. Hinrichsen, Composites: Part A 31 (2000) 143. 26. J. Gassan, A.K. Bledzki, Composites: Part A 28A (1997) 1001. 27. M.L. Hassan, A.M.A. Nada, J. Appl. Poly. Sci, 87 (2003) 653. 28. C.Z. Paiva Junior, L.H. de Carvalho, V.M. Fonseca, S.N. Monteiro, J.R.M. d’Almeida, Polym. Test. 23 (2004) 131. 29. L.Y. Mwaikambo, E. Martuscelli, M. Avella, Polym. Test. 19 (2000) 905. 30. J.K. Lancaster, in A.D. Jenkins (ed.) Polymer Science, Vol. 2, North Holland, Amsterdam (1972) 959. 31. A.M. Eleiche, G.M. Amin, Wear 112 (1) (1986) 67. 32. S.A.R. Hashmi, U.K. Dwivedi, N. Chand Wear 262 (11–12) (2007) 1426. 33. P.V. Vasconcelos, F.J. Lino, A.M. Baptista, R.J.L. Neto, Wear 260 (2006) 30. 34. A.M. Hagger, M. Davis, in K. Friedrich (ed.), Advances in Composite Tribology, Elsevier, Amsterdam (1993) 107. 35. S.A.R. Hashmi, U.K. Dwivedi, N. Chand, Tribol. Letts 21(2) (2006) 79. 36. S.A.R. Hashmi, S. Neogi, A. Pandey, N. Chand, Wear 247 (2001) 9. 37. S.A.R Hashmi, U.K. Dwivedi, N. Chand, Tribol. Letts 21 (2) (2006) 79. 38. V. Fervel, S. Mischler, D. Landolt, Wear 254 (2003) 492. 39. B.C. Jiang, Tribol. Trans. 34 (3) (1991) 369. 40. H.P. Stout, Wear 15 (1970) 149.