## 4 Discussion  
In Figure 7, it can be seen that increasing the load increased the mass loss of the materials. The ability of composite materials to resist wear relies on the composites forming adherent, uniform, and thin transfer film on the counter wheel [43,44]. The transfer film along the counter wheel stops the direct interaction between the materials (PTFE matrix composites) and metal counter wheel face, preventing abrasive action and hence leading to reduced mass loss. Therefore, the reason for the increase in mass loss due to an increase in the load is attributed to the high load breaking the tribolayer between the composites and the counterface. Moreover, the decrease in the mass loss due to an increase in speed, distance, and grit size is related to the formation of an adherent, thin, and uniform tribolayer. The insignificant lower mass loss of the CF25 composite compared to the BF40 composite was connected to the carbon fiber's inherent properties. This finding tallies with [19,43,45]. The decrease in volume loss due to an increase in the sliding distance and speed is attributed to an increase in temperature at high speed, forming a layer preventing direct contact with the counter wheel face. The CF25 composite exhibited the best and worst volume loss at run 6 and 26, respectively. The minimum volume loss is related to the lower grit size, while the maximum was as a result of a higher grit size and the highly brittle nature of carbon fiber. As noticed in Figure 9, the lowest \(\mu\) was attained at 9 N. The reason for this is the formation tribolayer at the interacting state, as well as the temperature and visco-elastic-related behaviors of the PTFE matrix composites. This layer prevented the PTFE matrix composites from coming into direct contact with the abrasive surface, thus minimizing \(\mu\). Similar observations were made by [46]. The high \(\mu\) at 6 N was perhaps due to the tearing of the transfer film at the contact region. This finding was contrary to the results obtained by [47]. High \(\mu\) at 150 mesh is related to the high roughness of the SiC paper that offered a significant amount of resistance, while the low \(\mu\) at 1000 mesh is attributed to the smoothness of the abrasive paper that offered little resistance to the samples--all due to the formation of the protective layer at the contact surface. The high friction due to an increase in the sliding distance is explained on the basis that the distance acted as a  
Figure 21: Taylor diagram for the models in the (**a**) calibration and (**b**) verification stages. lubricant to the rubbing surfaces. Thus, when the distance is increased, the lubrication is lost, thereby leading to excessive wear. The reason for the lower \(\upmu\) owing to an increase in the sliding speed is attributed to the adhesion between the matrix and the fibers as a result of the elevation in temperature. This is because when the sliding speed increases, the materials' strength is reduced, but their adhesion (tribolayer) is enhanced due to the elevated temperature. The low \(\upmu\) due to the increase in speed is related to the decrease in the contact time of the abrasives due to the high speed, thereby reducing the intensity of the tangential forces. As presented in Figure 10, \(\mathrm{T_{s}}\) decreased with an increase in the load, grit size, sliding distance, and sliding speed. The low \(\mathrm{T_{s}}\) at high load (9 N) was because of the significant increase in the apparent contact area at high load, thereby leading to an increase in the contact area, permitting a large number of particles to meet at the interface and share the stress. This, in turn, reduced the abrasive tribological rate. The decrease in the tribological rate due to the increase in the abrasive size is related to clogging of the wear track with wear debris and a reduction in the cutting efficiency of the abrasives due to transfer.