# Tribological Analysis of Fused Filament Fabrication PETG Parts Coated with IGUS
## 3 Results
It is important to note that traditionally, the roughness of a part is analyzed perpendicular to the manufacturing marks, which, in the case of FFF, would be perpendicular to the layer deposition. However, in this case, since the pin will slide over the outer face of the sample, the upper face of the samples is being analyzed, the roughness of which is conditioned by the deposition path, which is the same for all samples. Therefore, it makessense that the coating thickness does not influence the roughness significantly because, even though increasing the thickness removes certain defects, these are smaller than the inherent marks from the material deposition. Therefore, it should be clear that this behaviour will be important when studying the tribological behaviour of parts, as generally, a rougher surface tends to have a higher coefficient of friction. This is because irregularities and roughness on the surface increase the resistance to sliding between two surfaces in contact. In the context of parts manufactured by FFF, higher surface roughness can increase the coefficient of friction due to greater interaction between the peaks and valleys of the contact surfaces [41; 42; 43]. However, it should not be forgotten that in this case, there is another determining factor, which is load variation. The load also affects the coefficient of friction of surfaces. Generally, as the applied load increases, the coefficient of friction can vary due to the deformation and mechanical behaviour of the material under pressure [44; 45]. This also occurs in the case of FFF, where it is observed that the coefficient of friction increases with the normal load applied [46; 47]. Analysing the tests conducted on specimens coated with a tribomaterial layer (0.3 mm), it is observed that as the load increases, the average coefficient of friction seems to increase (Figure 4). However, there is a much greater difference between 5 and 10 N than beyond. Similarly, it is observed that at the maximum load, this coating layer is worn away, implying that the part could not continue to function properly. On the other hand, in all three cases, the behaviour is very stable, with no significant jumps in the amplitude of the curve. Increasing the coating and material load shows a similar but less-pronounced behaviour (Figure 5). The lower load in the case of 15 N seems to show the best coefficient of friction, increasing with the load until the coating breaks, which, in this case, is 0.6 mm. At 20 N, there are jumps in the width and depth of the groove that can be related to the stick and slip phenomenon. This phenomenon occurs when two surfaces in contact alternate between sticking phases (where they adhere and do not move) and sliding phases (where  
Figure 4: Evolution of tests conducted on specimens coated with 0.3 mm coating. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Coating Thickness** & **Sa** & **Sz** & **Sdc** \\ \hline F-value & 4.197 & 2.774 & 4.355 \\ \(p\)-value & 0.0797 & 0.1398 & 0.0753 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: ANOVA results of surface quality metrics vs. coating thickness. they slide over each other) and is closely related to the variation in the coefficient of friction under different loads, especially on FFF-fabricated surfaces [46]. On FFF-fabricated surfaces, variability in layer structure and surface roughness can make the stick and slip behaviour more pronounced. The applied load directly influences the transition between sticking and sliding phases, affecting the stability and smoothness of relative movement between the surfaces [48]. It can be observed that in the specimen tested at 25 N, from around 150 m, the curve behaviour changes, probably caused by slipping and coating loss. This phenomenon is even more pronounced in the case of thicker coatings, probably due to increased load (Figure 6).