3.1 Tensile behaviour
Tensile tests were performed on three specimens of each graphite/epoxy composite as well as the neat polymer. Typical engineering stress-strain curves are shown in Figure 4. These curves illustrate evidence of a brittle behaviour of all the specimens with and without graphite filler. It is clear that the mechanical properties of a composite are dependent on the characteristics of the filler used. In the present work, all composites exhibited lower tensile strength than the neat epoxy, the higher the graphite filler content, the lower the tensile strength measured. Tensile properties derived from the curves are given in Table 3. Results show that an addition of 0.1wt% graphite reduced the tensile properties of the neat epoxy by 32.7% (from 52.11MPa to 35.05MPa). This reduction is expected as graphite is a brittle material and its incorporation into the epoxy matrix makes the epoxy more brittle. A 43.5% reduction is also observed with the addition of 0.5wt% graphite particles. A final reduction of 56.03% was concluded with the specimen having 1wt% graphite particles. A similar trend was observed by Suresha et. al. [3] in their study on the role of micro/nanoparticles on the mechanical and tribological properties of polymer composites. The strain at break for each specimen as given in Table 3 shows a drastic reduction in elongation with the addition of graphite particles. This higher brittleness can be attributed to the increase in stress concentration areas as well as the reduced bonding between the matrix and the filler. It is expected that higher graphite content introduces agglomerates which in turn help in developing micro cracks as well as air bubbles from where there is larger interfacial distance between matrix and filler. This is shown in Figure 5. Additionally, the graphite microparticles may provide obstacles to cracks formed in the composite which results in a less convenient crack path which makes the fracture brittle.
Table 3: Tensile properties of graphite/epoxy composite
Specimen
|
Tensile strength (MPa)
|
Tensile Modulus (GPa)
|
Strain at break
|
Neat
|
52.11
|
2.96
|
0.0190
|
0.1% Graphite
|
35.05
|
3.0
|
0.0119
|
0.5% Graphite
|
29.41
|
3.08
|
0.0104
|
1% Graphite
|
22.91
|
3.26
|
0.0072
|
3.2 Flexural behaviour
3.2.1 Homogenously mixed composite
Flexural properties are of great importance for any structural element. Composite materials used in structures are prone to fail by bending and therefore the development of new composites with improved flexural characteristics is essential. Figure 6 shows the flexural strength of the homogenously mixed graphite/epoxy composites compared to neat epoxy. It is observed that all graphite epoxy composites exhibited higher flexural properties than the neat epoxy. This is as a result of the mechanism of load transfer from the matrix to the high-strength graphite particles in bending. This is not the case in tension because the stresses are in the opposite direction. The best improvement in the flexural properties of graphite/epoxy composite is with the addition of 0.1wt% graphite microparticles which increased the flexural strength of neat epoxy from 46.8MPa to 80.08MPa, this is a 71.1% increase. With a graphite content of 1wt%, an increase in flexural strength compared to neat epoxy was 59.79MPa which was a reduction from the 0.1wt% and 0.5wt% graphite content but still an increase of 27%.
A comparison between tensile strength and flexural strength is shown in Figure 7. It can be seen that the advantages of graphite as a filler are more prominent in bending than they are in tension. This was confirmed in literature where different inorganic fillers are added in a polymer matrix composite [26, 27].
3.2.2 Layered composite
The development of layered composites in this study is to determine the possibility of successfully bonding together different layers of epoxy and graphite/epoxy composites to have a new material that has different internal and external flexural properties. This investigation is so stiffer materials can be combined with weaker materials to reduce the general stiffness of the bulk material and to combine cheaper and expensive materials to reduce cost among other benefits. Figure 8 presents the flexural strength of the graphite/epoxy layered composites with regards to the neat epoxy. It is concluded that the best layered-specimen in terms of flexural strength is the composite with a layer of neat epoxy and 0.1wt% graphite/epoxy composite which had a 64.8% increase compared to the neat epoxy. This is close to the improvement noticed with the homogenously-mixed graphite/epoxy composite. The layered composite with neat+1wt% graphite had the least flexural strength compared to the other layered specimen which was 49.9MPa. This is still an improvement compared to the neat epoxy by 6.6%. The specimen with a triple-layer of Neat+0.1+1wt% graphite had a flexural strength of 61.28MPa. This is an encouraging output in the layered composites which means flexural properties can be improved with the layering of a composite with another composite that has better flexural properties. Failure mechanism of the layered specimens were noticed to be of a delamination type because of the clear interlayer debonding and the change of direction of the crack path when propagating between layers as seen in Figure 9.
3.3 Hardness and impact strength
The hardness and impact strengths of all graphite/epoxy composites were improved with the addition of higher graphite content. This is expected as the graphite particles have a higher density, surface hardness and mechanical strength compared to the neat epoxy. Table 4 shows the hardness and impact strengths of the composites. It can be seen that the Shore D hardness value of neat epoxy was 50SHD, with the final addition of 1wt% graphite, hardness values increased to 64SHD, this is a 28% increase in the hardness value. For the impact strength of the composite, a 1wt% graphite content increased the impact strength by 29%. These improvements can be attributed to the load transfer mechanism in a homogenously dispersed composite where the surface of the graphite particles become load-bearing during compression (Shore D hardness testing) and hence prove to be hard. On the impact strength, the change in direction created by graphite particles as a crack propagated during failure reduces the ease of fracture and hence increases the toughness of the material. It has been reported in [28, 29, 30] that the addition of high-density and high-strength inorganic particles enhances the surface hardness and the impact strength of a composite.
Table 4: Hardness and impact results
Specimen
|
Hardness value (SHD)
|
Impact value (J)
|
Neat
|
50
|
6.75
|
0.1% G
|
57
|
7.25
|
0.5% G
|
61
|
7.65
|
1% G
|
64
|
8.75
|
3.4 Friction and Sliding wear results
3.4.1 Friction
The friction coefficients of neat and graphite/epoxy composites were obtained under different conditions of operation. A sliding speed of 200 and 600rpm, applied loads of 10,20 and 40N, and a constant time of 300sec. In this study, it was observed that as applied load increases, the coefficient of friction increases for all composites and neat epoxy, this is to be expected as higher normal loads means the pressure exerted on the specimen and counterface increases and this increases the frictional force generated in the system. The lowest coefficient of friction was observed in the specimens with 1wt% graphite content for all applied loads and sliding speeds. For specimens tested at 10N applied load and 200rpm sliding speed, a reduction in the coefficient of friction of 14% was observed from the neat epoxy to the 1wt% graphite/epoxy composite as shown in Figure 10. For the specimens tested at 20N applied load and 600rpm sliding speed, it can be seen from Figure 11 that the composites with the higher graphite content started with lower friction coefficients, a decrease of 12% was observed between the neat epoxy and the specimen with 1wt% graphite content, this is as a result of the ease with which the graphite particles come off with higher graphite content and form the lubricating film as discussed earlier [23]. This is also comparable to results recorded in experiments by [31, 32]. Similar trends are noticed in all other operational conditions as shown in Figure 12. The reduction of the coefficient of friction is most noticeable when applied load is changed, despite the effect of sliding speed on the coefficient of friction of the composites, very little change is observed as seen in the figures presented. A more prominent effect is observed when higher loads are applied. An increase of 52% was noticed when the applied load is changed from 10N to 40N. A higher graphite content would certainly mean much lower friction coefficients but that will lead to instability of the composite as it becomes weak and crumbles easily due to the weak bonding between the matrix and the filler [33].
3.4.2 Wear
The variation in wear rates of the neat epoxy and the graphite/epoxy composites under different conditions were presented in Figure 13. The experimental results show that the wear rate reduces with an increase in graphite content as the specimen with 1wt% graphite exhibited a reduced wear rate compared to the neat epoxy of more than 75%. The difference was similar for the higher sliding speed of 600rpm which was 73% in reduction. This is a result of the solid lubrication abilities of graphite particles [34]. An interesting trend that was observed in this study is the reduction of wear rate with increasing sliding speed. This can be attributed to the faster formation of a sliding film on the specimen and counterface which then reduces further disintegration of the composite, the film generated stabilises the stick-slip process which becomes a low adhesion region where the wear rates are low. But this trend was not in agreement with the conclusions in the literature [35, 36]. However, a study by Tripathy et al. [37] stated that the wear rate increases with an increase in sliding velocity but with an exception. In their research, there was a sharp decrease in wear rate at velocities ranging between 3-4 m/s but then increases exponentially up to 7m/s. It could be argued that the reduction in wear rate with increase in sliding speed in this research is still between the 3-4m/s decrease observed in the research above and hence acceptable. Figure 14 shows the specific wear rate of graphite/epoxy composite for 10N applied load at different sliding distances.
The Worn surfaces of the specimens tested during friction were studied to determine the wear mechanism and surface morphology. From Figure 15, it can be seen that the difference between the worn surface of the neat epoxy composite was much more coarse and had deep scratches due to the high frictional force and the brittle nature of the epoxy while the composite with 1wt% graphite particle had a smoother surface even though some scratches were visible. These smooth scratches were as a result of the initial friction and break-off of the surface of the composite before a smooth lubrication film was generated which in turn becomes a layer with low friction and less debris formation. This is in agreement with results gotten in previous studies on the review of tribological properties of polymer composite [38]. The worn surface of the specimens with graphite filler in higher quantity can be described as quality surface [39] because there is no sign of extreme debonding or high porosity. The relative appearance of micro-cracks shows good resistance to shear stresses displayed by graphite particles. Similar things have been reported with carbon fibre and epoxy polymer where there was relatively less damage than the glass fibre epoxy polymer [40]. In other words, it can be concluded that graphite has excellent properties in reducing both the friction and wear rates in polymer composites.