# Tribological Analysis of Fused Filament Fabrication PETG Parts Coated with IGUS
## 3 Results  
Using more than one material in the fabrication of a specimen implicitly carries a series of problems. The filament flow is not always homogeneous, so the entire surface of the specimen will not be characterised as having the same properties [33; 34]. In the case of FFF, this poses a significant problem as it hinders interlayer adhesion. Additionally, interlayer adhesion between different materials is another added problem as adhesion may not be correct. In this case, this problem may arise when transitioning from PETG to IGUS. As mentioned, PETG will act as the substrate, and IGUS will act as the coating with a certain thickness. Upon performing the morphological analysis of the specimens with different coating thicknesses (Figure 1), it is observed that as the coating thickness decreases, the coating becomes more translucent, revealing the PETG base material. On the other hand, colouration changes caused by irregular coating thicknesses related to flow and deposition trajectories affecting the quality of the parts are observed [35]. Additionally, certain defects associated with these specimens are observed, which are related to material deposition defects that evolve as more coating thickness is added. In Figure 2a, it can be seen that with the lowest thickness, gaps between the filaments caused by irregular flow appear, as well as marks consistent with the appearance of porosities. As the thickness increases (Figure 2b), these gaps are covered and are no longer visible, but in some areas, dark colouration still appears, indicating a thinner coating thickness as the substrate is partially visible. Additionally, flow defects continue to appear but in a much more discontinuous manner. These defects continue to appear in the greater thickness (Figure 2c), although the rest of the defects are no longer visible due to the thickness of the layer. Therefore, it can be determined that increasing the thickness stabilises the surface of the specimen. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Material** & **Coating Thickness (mm)** & **Load (N)** & **Test Speed** & **Distance (m)** & **Revolutions** (rpm)** & **Radius (mm)** \\ \hline \multirow{3}{*}{PETG + Iglidur I50} & 0.3 (1 layer) & 5/10/15 & & & \\  & 0.6 (2 layer) & 15/20/25 & & & \\ \cline{1-1}  & 0.9 (3 layer) & 25/30/35 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Materials and testing parameters. These defects will impact the surface quality and texture of the parts. If Sa is analysed (Figure 3a), understood as the average height deviations of the surface from a mean line within the measurement area, it is observed that as the coating thickness increases, there is a trend towards decreasing roughness. In fact, it is observed that when the coating is thinner, the deviations are very large, indicating that the average surface is very rough. However, there seems to be no significant difference between an internal and external area, which may mean that the surface is homogeneous. Analysing Sz (Figure 3b), understood as the maximum amplitude of the surface, it is observed that there are also higher values with the thinner coating thickness, although the  
Figure 3: Evolution of surface quality in terms of (**a**) Sa, (**b**) Sz, and (**c**) Sdc. Figure 2: Images of PETG specimens coated with IGUS manufactured with FFF: (**a**) with 0.3 mm coating thickness; (**b**) with 0.6 mm coating thickness; (**c**) with 0.9 mm coating thickness. difference is not as significant. However, there is a greater difference between the internal and external areas. This may indicate that the temperature accumulation in the central area due to the infill geometry generates more significant peaks, but as the coating thickness increases, this stabilises, probably due to the higher melting temperature of the coating. Sdc, being a parameter more similar to Sa, behaves similarly (Figure 3c). In this case, Sdc provides a measure of how many peaks of a surface exceed a given height threshold per unit area.