# Tribological Analysis of Fused Filament Fabrication PETG Parts Coated with IGUS
## 3 Results
Increasing the coating thickness does not eliminate it at 25 N, although there are areas where the substrate of the specimen is visible, and a different behaviour appears from 125 m, which may indicate interference with the substrate. At 35 N, the coating breaks again, at a point similar to the previous specimen. In all cases, a morphology consistent with the stick-slip phenomenon is observed. Figure 5: Evolution of tests conducted on specimens coated with 0.6 mm coating. Figure 6: Evolution of tests conducted on specimens coated with 0.9 mm coating. Analysing the average behaviour of the data (Figure 7), disparate behaviours are observed. In the thinnest coating, it is observed that the coefficient of friction increases with the load, completely wearing away the coating in certain areas at higher loads. On the other hand, increasing the coating shows more-stable behaviour, even manifesting a slight trend of decreasing the coefficient of friction, which is very unusual when compared to previous observations [46]. However, comparing it with the previously analysed graphs, the difference is insignificant compared to the amplitude of the tests (Figure 4), which are also quite stable. However, comparing it with the thinner coating, it is observed that the coefficient of friction is significantly lower, which is consistent with previous observations, as depositing more material attenuates defects and levels the surfaces, positively impacting the coefficient of friction. Also, it is observed that increasing the coating thickness increases the coefficient of friction. This could be related to the previously mentioned stick-slip phenomenon and the formation of debris. Accumulating more material generates more detached particles that can hinder the pin's movement, resulting in higher outcomes. In thinner coating layers, these particles are not generated, resulting in more stable behaviour. This is noticeable in Figure 5, where the formation of burrs generating these particles is observed, except in the intermediate case of 30 N, which offers the best result. Therefore, the formation of particles caused by filament breakage and burr formation seems to have an opposite and negative behaviour. Additionally, these particles (Figure 8) have a morphology different from those observed in other FFF materials [10,11]. In this case, they form elongated and almost-curled particles, more similar to chips, which may indicate greater plastic deformation of the material. Performing the statistical analysis of the data (Table 4), it is observed that for the average coefficient of friction case, the ANOVA results against the effect of coating thickness are F = 0.811 and \(p\)-value = 0.4024, indicating that the F-value is low and the \(p\)-value is greater than 0.05, suggesting that the coating thickness does not have a statistically significant effect on the average coefficient of friction. This suggests that variability in the coefficient of friction is not significantly influenced by coating thickness under the conditions of this study. A similar result is found with the effect of load, with F = 0.459 and \(p\)-value = 0.5233, indicating that the applied load also does not have a statistically significant effect on the average coefficient of friction. The amplitude of the friction curves also does not show significance with the coating layer thickness (F = 0.584; \(p\)-value = 0.4737), although the highest significance would be with the effect of load (F = 4.256; \(p\)-value = 0.0847), where, although it exceeds the 5% level, it is the closest, indicating greater dependence. Additionally, there is no significance in the interaction of both factors. Previous studies  
Figure 7: Evolution of the average coefficient of friction. have already reported that despite the performance improvement that can be achieved with the use of coatings, conditions can vary the performance; therefore, they do not show clear significance [40; 41].