# Tribological Analysis of PETG Fire-Retardant Samples Manufactured by FFF
## 3 Results
### Coefficient of Friction Analysis  
Regarding the coefficient of friction (CoF) obtained through the BoD tests, different behaviors were observed depending on the material and the process parameters (Figures 7 and 8). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**PETG FP**} & \multicolumn{4}{c}{**PETG**} \\ \cline{2-10}  & \multicolumn{2}{c}{**Ra**} & \multicolumn{2}{c}{**Rz**} & \multicolumn{2}{c}{**Ra**} & \multicolumn{2}{c}{**Rz**} \\ \hline
**Factors** & **F Value** & \(p\) **Value** & **F Value** & \(p\) **Value** & **F Value** & \(p\) **Value** & **F Value** & \(p\) **Value** \\ \hline Temperature & 79.89 & 0.000 & 106.32 & 0.000 & 1.39 & 0.256 & 1.72 & 0.186 \\ Layer Thickness & 322.28 & 0.000 & 276.57 & 0.000 & 701.55 & 0.000 & 351.71 & 0.000 \\ Temperature \(\times\) Layer Thickness & 11.03 & 0.000 & 3.71 & 0.008 & 5.14 & 0.001 & 4.75 & 0.002 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: ANOVA results for surface roughness with respect to extrusion temperature and layer height by material. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Ra**} & \multicolumn{2}{c}{**Rz**} \\ \hline
**Factors** & **F Value** & \(p\) **Value** & **F Value** & \(p\) **Value** \\ \hline Material & 6.53 & 0.012 & 14.31 & 0.000 \\ Measure zone & 0.95 & 0.389 & 1.15 & 0.318 \\ Material \(\times\) Measure zone & 0.03 & 0.969 & 0.07 & 0.934 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: ANOVA results for surface roughness with respect to material. Figure 7: CoF evolution for PETG specimens. (**a**) Constant layer height of 0.15 mm and test radius of 10 mm. (**b**) Constant layer height of 0.35 mm and test radius of 20 mm. For PETG, the extrusion temperature affected the results at lower layer heights but had less influence at higher layer heights (Figure 8). Thus, layer height appears to be the more influential parameter. However, once the first printed layer begins to wear, the CoF becomes irregular--an effect that occurs sooner in specimens with thinner layers. This negative effect also tends to appear earlier at lower temperatures (Figure 9). Notably, the CoF values obtained were low overall, which is consistent with observations in the literature [27; 28; 29; 35]. PETG FP, on the other hand, exhibited a different behavior (Figure 9). First, it showed generally higher CoF values, indicating that the presence of flame-retardant additives negatively impacts CoF. There was also a general trend for improvement with increased layer height, which contrasts with the results observed in the roughness analysis--suggesting that the CoF may not be directly correlated with roughness. This effect may be due to the enhanced stability resulting from the extrusion of larger volumes of material, which helps fill potential voids. Additionally, extrusion temperature had a less pronounced effect on PETG FP, although a greater overall data dispersion was observed at higher temperatures (Figure 10). When analyzing the complete dataset--averaging out the initial and final segments of the tests, which are considered stabilization phases--similar trends were observed regarding layer height. Temperature had much less impact overall, particularly in the case of PETG FP (Figure 11). Figure 8: CoF evolution for PETG specimens. (**a**) Constant extrusion temperature of 220 \({}^{\circ}\)C and test radius of 10 mm. (**b**) Constant extrusion temperature of 245 \({}^{\circ}\)C and test radius of 20 mm. Figure 9: CoF evolution for PETG FP specimens. (**a**) Constant layer height of 0.15 mm and test radius of 10 mm. (**b**) Constant layer height of 0.35 mm and test radius of 15 mm. The ANOVA results reinforce many of these findings. The material type shows a strong statistical significance, which is visually evident in the performance graphs (Table 5). Furthermore, it shows (Table 6) lower with radius, which is related to the homogeneity of the specimens observed in the roughness study.