2. Formulation and Manufacturing Process
Several studies have reported that natural fiber-reinforced composite (NFRC) materials showed better tribological and mechanical properties
[43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. These studies also showed the effect of the adhesion between the ingredients and how that affects the mechanical and tribological properties.
One of the factors that influences the adhesion of fibers is the alkaline treatment applied to them. This treatment is utilized to remove the unwanted elements, such as impurities, from the fiber
[68][29]. The alkali treatment is a cleaning and modifying treatment for the surface of fibers to reduce surface tension and enhance the interfacial adhesion of natural fibers
[69,70][30][31].
3. Influence of Natural Fiber on the Mechanical and Tribological Properties
When natural fibers are used as reinforcement elements, the tribological properties can be improved by controlling the fiber content at certain levels
[43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. The natural fiber content affects the tribological properties, mainly due to the change in the binder (matrix) to the natural fiber ratio. The binder to the natural fiber ratio affects the adhesion force, which has a direct influence on the tribological properties
[71][32]. The use of natural fibers improves the mechanical properties until a certain point, then the mechanical properties start to decrease. This could be observed for banana fiber and palm kernel fibers with different ingredients and different weight percentage of the fibers. For instance, when this ratio is below an optimum value, the fiber content needed to reinforce the matrix is not enough. In addition, when it is above that optimum value, the fibers will need more binder to bind together. Hence, the deterioration of mechanical and tribological properties is expected in cases when the fiber content is above or below these optimum values.
Researchers have focused their efforts on measuring different mechanical and tribological properties of natural fiber composite materials that are prepared by using various binders and natural fiber types and contents. They also measured the porosity, density, hardness, thermal conductivity, compressibility, wear rate, and coefficient of friction (COF) of the prepared composites.
Figure 1 and
Figure 2 show the testing machines (T-Mc) and standard (St) used by some of the studies to determine the tribological properties. These properties are illustrated
in Table 3 and Figure 3,
Figure 4,
Figure 5,
Figure 6 and
Figure 7.
Figure 1. (
a) The schematic of the pin-on-disc machine and (
b) pin-on-disc used in the wear rate and measuring COF
[72][33].
Figure 2. The disc brake machine used for measuring the wear rate and COF
[73][34].
Figure 3. The density variation with variation of fiber weight percentage for different fibers
[43,44,62,67][4][5][23][28].
Figure 4. The porosity percentage variation with variation of fiber weight percentage for different fibers
[56,67][17][28].
Figure 5. (
a) The variation of hardness using the Brinell tester for banana fiber with variation in the fiber weight percentage
[43][4], (
b) illustrates variation of hardness using inertia dynamotor palm kernel fiber with variation in the fiber weight percentage
[56][17], and (
c) variation of hardness using Rockwell tester for different fibers with a variation in the fiber weight percentage
[44,57,62,67][5][18][23][28].
Figure 6. The variation of COF for different fibers with a variation in the fiber weight percentage
[43,44,56,67][4][5][17][28].
Figure 7. (
a) The variation of wear rate for different fibers for different fiber weight percentages in mg/m
[56][17], (
b) the variation of wear rate for different fibers for different fiber weight percentage in mm
3/Nm
[44,57][5][18], (
c) and the variation of wear rate for different fibers for different fiber weight percentage in g
[43,62][4][23].
In order to provide better comparative observations of the effect of these natural fiber types and their contents on the resulting properties, the results obtained by other researchers are plotted, as shown in
Figure 3,
Figure 4,
Figure 5,
Figure 6 and
Figure 7. For example, palm kernel fibers
[62][23] show values of wear rate and hardness, as the fiber content is constant while the pressure used during manufacture varies from 10 to 60 tons. In addition, the inverse relationship between the density and the porosity is quite clear when examining
Figure 3 and
Figure 4. Moreover, the density increased for banana fiber weight percentages (Wt.%) of 63% and 82% parent matrices, until reaching a certain fiber weight percentage before decreasing. However, the density of the 63 Wt.% composite increased again after the decrease happened at the 14% Wt.% fiber content. Palm kernel with a 54% Wt.% fiber content recorded a density drop when the fiber weight percentage increased. This drop could be due to the change in fillers and the friction modifier MoS4 used (
Figure 3).
There was a boosting up trend for porosity percentage in the palm kernel of 46% in the parent matrix. It could be observed that the enlargement of the porosity decreased the density value, which could be referred to as the particle size of the palm kernel fibers
[56][17]. This could be attributed to the fact that the particle size of the palm kernel fibers was higher than that of the space filler used
[67][28]. The uniform curing caused a uniform dispersion, which led to a high porosity value
[67][28] (
Figure 4). Furthermore, the measured hardness improved for all of the tested natural fiber composites. This improvement was observed to occur with the fiber weight percent rising, except for the case of palm kernel with the 46% parent matrix. The hardness value of each formulation was quite close to the other one and this could be due to applying the same pressure and temperature for the same duration during the hot compaction process
[43][4]. In addition, this rise could be attributed to the cross-linking between the fiber and the resin that takes place during the curing process
[44][5] (
Figure 5a). The hardness of the palm kernel with the 46% parent matrix decrease is suggested to occur because of the increase in porosity as the fiber weight percent increased (
Figure 5c). The palm kernel of the
[56][17] hardness decayed as the fiber weight percent increased, followed by a jump at 35 and 40% fiber weight percent (
Figure 5b). The hardness values of the palm kernel 40% fiber weight percent showed an increase as the compression force used in the fabrication process increased. Increasing the pressure had a role in reducing the voids in the NFRC-based composites. The effect of having a compact and composite material, with a high density and low porosity, is known to improve the mechanical properties significantly
[43][4].
The tribological properties recorded an obvious improvement as the fiber weight percentage increased. Most formulations recorded a trend for enhancement of the properties. The banana fibers recorded a trend of increasing the COF among all of the formulations, except that of the 63% parent matrix (
Figure 6). The COF in
Figure 6 is the average value for banana fibers
[43,44][4][5] as it was examined at different temperatures and different loads.
The wear rate in
Figure 7 is the average value for the results from several studies
[43,44,56,57,62][4][5][17][18][23]. An average value was used because of the differences in the testing conditions. The 63% parent matrix recorded an increase followed by a drop, then stability for the COF value. This drop could be due to the failure of bonding between the phenolic resin and the banana fibers. Moreover, it is an indication of the deteriorated COF for these composites at elevated temperatures
[43][4]. The palm kernel fibers recorded a different COF trend, which is obvious in the graphs. The 46% parent matrix recorded a slight increase followed by a minor drop. This could be attributed to the degradation of the palm kernel fiber at higher temperatures
[67][28]. Ikpambese et al.
[56][17] recorded a decreasing trend followed by an overshot trend, which seems to be different that the results reported in other similar studies. However, this difference could be attributed to the change in the natural fiber weight percent, the absence of a parent matrix, and variations in the ingredients. Furthermore, the results seem to be stable at different ranges, which could be due to the absence of steel fiber
[56][17], as steel fibers play a role in increasing the COF
[44,57,74,75][5][18][35][36].
In general, the wear rate showed trends for both banana and palm kernel. Some of them increased with increasing the fiber weight percent, while others showed a decreasing trend after an initial increase. In addition, some of them showed an increase followed by a decrease and then another increase (
Figure 7). For example, the banana fibers with a 63% and 82% parent matrix showed an increase in wear rate as the fiber weight percent increased. This boost could be due to the degradation of the samples at elevated temperatures
[43][4] (
Figure 7b,c). Finally, studies that showed an increase followed by a decrease and then an increase were that of palm kernel fiber weight percentages of 5–25% and phenolic resin of 35% weight percent. This could be attributed to the unique adhesion properties between the wheat and Nile rose with the phenolic resin. The formulation that did not contain the Nile rose and wheat showed better results at the same fiber weight percentage, and followed a trend of increasing followed by decreasing, as better wear resistance was obtained through the strong adhesion of natural fibers with resin
[71][32].