Effect of fiber volume fraction on tribological properties

Fig. 3.12A—C show the effect of increasing CNF content in the bio-epoxy matrix on coefficient of friction (COF) at different normal loads and sliding speeds. In the figures, the connecting data points refer to the average values of the three independent tests while error bars show the corresponding scatter. Fig. 3.12A shows the variation in COF with increasing volume fractions of nanocellulose in the bio-epoxy matrix at 4, 7, and 10 N for a sliding speed of 0.15 m/s. It is clear that the COF decreases with increasing volume fraction of nanocellulose in the bio-epoxy for all the three normal loads during the low sliding speed of 0.15 m/s. Also, the reduction in COF is higher at larger normal loads. It is also obvious that the reduction in friction is more significant when the volume fraction of nanocellulose is increased from 0 to 0.9% than from 0.9 to 1.4%. It can be concluded from Fig. 3.12A that the bio-epoxy sample showed better tribological performance at lower normal loads and the 1.4% CNF composite showed enhanced tribological performance at higher normal loads due to the presence of nanocellulose on contact surfaces.

Fig. 3.12B shows the variation of COF with volume fraction of nanocellulose in the bio-epoxy matrix at 4, 7, and 10 N when the tests were performed for a constant sliding speed of 0.25 m/s. Similar to the 0.15 m/s observations, the COF decreases when 0.9% volume fraction of CNF was added to the bio-epoxy for all the three normal loads. However, the COF increases for 4 and 7 N when 1.4% CNF added to the bio-epoxy matrix, although they have lower COF than the pure bio-epoxy. Also, for 10 N, the COF constantly decreases although the reduction in COF is less significant. Incidentally, the 10 N load led to the lowest COF. In addition, the pure bio-epoxy showed better tribological performance at lower normal loads, while the 1.4% CNF composite showed superior tribological performance at higher normal loads.

Effect of nanocellulose content in bio-epoxy on coefficient of friction

Figure 3.12 Effect of nanocellulose content in bio-epoxy on coefficient of friction (COF) at different normal loads for (A) 0.15 m/s, (B) 0.25 m/s, and (C) 0.35 m/s sliding speeds and effect of nanocellulose in bio-epoxy on wear-volume at different normal loads for (D) 0.15 m/s, (E) 0.25 m/s, and (F) 0.35 m/s sliding speeds.

Source: Journal of Carbohydrate Polymer, vol. 17 pp. 282—293.

Fig. 3.12C displays the variation of COF with increasing volume fraction of nanocellulose in the bio-epoxy matrix at 4, 7, and 10 N for a constant sliding speed of 0.35 m/s. At this higher sliding speed, the variation of COF with the volume fraction of CNF showed totally different trends. At 4 N normal load, the COF increases when 0.9% volume fraction of nanocellulose is added to the bio-epoxy matrix; later the COF decreases slightly when the fiber volume fraction is raised to 1.4%. The COF was found to be the lowest for the pure bio-epoxy as compared to the nanocellulose composites. For the 7 N normal load, the COF does not change much with the increasing nanocellulose volume fraction. Interestingly, at the highest normal load of 10 N, the COF decreases with the increasing volume fraction of nanocellulose for the 0.15 and 0.25 m/s sliding speeds. From the above results, it can be concluded that pure bio-epoxy has poor tribological properties and is preferable only for low load and low sliding speed applications. However, for more severe tribological conditions, a higher volume fraction of nanocellulose would be preferred in the CNF composites.

Fig. 3.12D—F show the variation in wear rate (volume loss) with increasing volume fractions of CNF in the bio-epoxy matrix for various normal loads (4, 7, and 10 N) at sliding speeds of 0.15, 0.25, and 0.35 m/s, respectively. It can be observed that for a given volume fraction, the wear rate increases with the increasing normal load for all sliding speeds. The wear rate decreases with the increasing fiber volume fraction of the cellulose in the bio-epoxy matrix. In addition, the wear rate decreases with the increasing sliding speed. The volume loss in nanocellulose/bio-epoxy composites is significantly less than that in the neat bio-epoxy. This is because the incorporation of nanocellulose in the bio-epoxy matrix effectively improves the mechanical and tribological properties of bioepoxy due to the enhancement in strength properties as shown in Fig. 3.8 and the ability of nanocellulose fibers to resist the bending force as reported in the literature [22]. Although, the hardness of the material has long been observed as a primary material property that expresses the wear resistance, there are strong evidences to suggest that the elastic modulus can also have an important influence on the wear behavior [23]. Therefore, the bio-epoxy matrix reinforced by 1.4% CNFs have better wear properties owing to higher elastic modulus as shown in Fig. 3.8. In addition, the cellulose fibers play a major role in improving the fracture toughness of polymer matrix through several energy absorbing mechanisms, such as fiber pull-out, fiber fracture, and fiber bridging [24]. These mechanical factors seem to have significant effects on the tribological properties of nanocel- lulose/bio-epoxy composites as well. Also, the addition of nanocellulose into bioepoxy results in an improvement in the COF and reduction in the volume loss of nanocomposites. In general, as demonstrated in Fig. 3.12D—F, superior improvement in wear rate occurred at the highest volume fraction of CNFs due to the increase in stress transferred to the fiber and the ability of cellulose fibers to resist the bending forces [25], to carry the higher load, and to support the bio-epoxy matrix. Table 3.1 displays quantitatively the improvements in COF and wear rate of the CNFs reinforced composites.

Fig. 3.13 presents a schematic illustration of the wear mechanisms dominant during sliding wear of the bio-epoxy composites reinforced by nanocellulose. The continuous transfer film formed during the running-in stage can effectively reduce the direct contact of the composite with the asperities of hard metallic counterface [26,27]. During the sliding process, the CNFs reinforcement may carry most of the contact load and hence may wear against the counterface. In addition, thermal—mechanical failure of the material in this contact region may take place due to high local friction and heating [28]. In such a situation, the polymeric matrix in the interfacial region around CNF solid phase suffers higher stress and temperature. As a result, the CNF solid phase will be removed more easily, which is associated with a progressive increase in the wear rate of the composites. Therefore, the detached CNF solid phase can fill the gap between the asperities and reduce the roughness of the counterpart surface, and consequently decrease the COF and volume loss.

Table 3.1 Summary of improvements in the COF and wear rate of CNF/bio-epoxy composites

Sliding speed

(m/s)

Load (N)

CNFs volume fraction

0.9%

1.4%

0.9%

1.4%

COF improvement (%)

Volume loss improvement (%)

0.15

4

6.0

19.4

73.8

82.5

7

25.4

31.0

71.7

72.6

10

50.0

55.1

67.5

71.6

0.25

4

8.2

-10.2

21.9

67.9

7

27.8

9.3

25.0

70.2

10

41.7

45.0

4.5

57.4

0.35

4

-27.5

-20.0

25.6

66.9

7

2.2

0.0

31.3

65.8

10

28.8

34.6

16.5

43.6

Schematic illustration of the failure mechanism for the sliding wear of CNF/ bio-epoxy nanocomposites

Figure 3.13 Schematic illustration of the failure mechanism for the sliding wear of CNF/ bio-epoxy nanocomposites.

Source: Journal of Carbohydrate Polymer, vol. 17 pp. 282—293.

To understand the effect nanocellulose fibers on the wear mechanism, two composite samples with different nanocellulose fiber volume fractions and a neat bio-epoxy sample were chosen for comparison. The worn surfaces were further studied using stereo-macroscopy and SEM. The texture of worn surfaces clearly depicts the composition-dependent wear behavior. All selected coupons were compared for the same normal load and sliding speed. The surface damage on the worn surface is found to decrease with an increase in the nanocellulose content interspersed with the bio-epoxy matrix. This reduced the wear-volume loss and hence the nanocomposites displayed improved wear resistance when compared with the neat bio-epoxy materials (since no CNF solid phase was there to reinforce the matrix of the latter). Hence, the load-bearing ability of cellulose in the bio-epoxy matrix is an important reason for the better wear resistance of the composites. Furthermore, by increasing the CNF content, more fibers can bear the load which tends to decrease the wear rate at higher volume fractions of CNF. The SEM image of the worn surface of neat bio-epoxy showed more debris and fine cutting chips similar to those produced during machining. It implies that the wear mechanism for the neat bio-epoxy sample is abrasive. The detached debris act as third body abrasives. These debris can further increase the depth of the grooves by ploughing action and the material is continually displaced sideways to form ridges adjacent to the developing grooves by plastic deformation. In addition, the debris can cut the surface in a way similar to micromachining and all the material displaced by the debris is removed as a chip. Consequently, the volume loss increases in neat bio-epoxy as confirmed by the experimental results presented.

In contrast, a minimal amount of debris can be seen on the surface of 0.9% CNF/bio-epoxy composite and no debris are found on the surface of 1.4% CNF/ bio-epoxy composite. Therefore, the composites showed lower wear-rates. The SEM image of the worn surfaces show that the dominant wear mechanism for the composites is adhesion due to plastic deformation and transfer layer is seen in some regions on the composite surfaces. A peculiar feature of this wear process is the back transfer of material from one surface to another where iron from steel is transferred to the polymer surface [29]. A similar observation is reported in the literature where a soft aluminum material slides against a hard steel counter material and iron from the steel counter material is transferred to the soft aluminum sample surface [30,31]. This phenomena occurs due to the localized bonding between the contacting solid surfaces [26]. Because of the presence of some debris on the worn surface of 0.9% CNF/bio-epoxy composite, the wear mechanism for this composite is a combination of abrasion and adhesion with the latter being dominant. On the other hand, the wear mechanism of 1.4% CNF/bio-epoxy composite is completely adhesive in nature.

By investigating the worn surfaces, it is clear that the surfaces of the composites are smoother than that of the neat bio-epoxy. The smooth surfaces imply that the lower wear-rates are prevalent for the composites. The 3D images of the worn surfaces (Fig. 3.14) confirm that the surface of composites are less coarse with the roughness parameter, the maximum heights of the profile (Rt), being 37.3, 22.9, and

21.6 цш for the neat bio-epoxy, 0.9% CNF/bio-epoxy, and 1.4% CNF/bio-epoxy, respectively. Generally, once the nanocellulose fibers were incorporated in the bioepoxy matrix, the worn surfaces appear much smoother even at severe wear conditions (at the highest normal load and sliding speed) in comparison with the neat bio-epoxy without nanofibers.

D surface topography of (A) neat bio-epoxy, (B) bio-epoxy reinforced by 0.9 vol.% nanocellulose, and (C) bio-epoxy reinforced by 1.4 vol.% nanocellulose at 7 N normal load and 0.15 m/s

Figure 3.14 3D surface topography of (A) neat bio-epoxy, (B) bio-epoxy reinforced by 0.9 vol.% nanocellulose, and (C) bio-epoxy reinforced by 1.4 vol.% nanocellulose at 7 N normal load and 0.15 m/s.

Source: Journal of Carbohydrate Polymer, vol. 17 pp. 282—293.

 
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