# Tribological Analysis of PETG Fire-Retardant Samples Manufactured by FFF
## 2 Experimental Procedure  
The objective of this study is to determine how the tribological properties of PETG parts manufactured via FFF are altered when flame-retardant additives are incorporated. To this end, tests were conducted to analyze the relationship between manufacturing parameters, the characteristics of the fabricated specimens, and their influence on the coefficient of friction (CoF) and wear resistance. Figure 1 illustrates the steps followed during the tribological testing process. Figure 1: Methodology followed for the dry tribology test. For this purpose, cylindrical specimens (Figure 2) were fabricated using two materials: PETG and flame-retardant PETG. In both cases, commercial-grade materials were selected from the manufacturer Smartmaterials 3D (Smartmaterials 3D, Alcala la Real, Jaen, Spain). The flame-retardant PETG is commercially known as PETG _Fire Retardant_ (FP). The microhardness of both materials has been analyzed and is very similar in both cases: 10.34 HMV for PETG and 11 HMV for flame-retardant PETG. The manufacturing parameters were selected based on the manufacturer's recommended settings (230 \({}^{\circ}\)C for PETG and 250 \({}^{\circ}\)C for PETG-FP), with adjustments made by increasing and decreasing the extrusion temperature. The remaining parameters were chosen according to values reported in the literature. Table 1 summarizes the parameters used. For the infill pattern, an _Archimedean chord_ structure was selected to facilitate extrusion, which has been identified in the literature as an optimal solution [27; 28; 29; 35]. After fabrication, all specimens were examined using Stereoscopic Optical Microscopy (SOM). A Nikon SMZ 800 microscope (Nikon, Tokyo, Japan) was employed. In addition, the surface roughness of the specimens was characterized. For this purpose, nine measurements were taken radially on each specimen using a Mahr Marsurf Ps10 roughness tester (Mahr, Gottingen, Germany), measuring both Ra and Rz parameters according to UNE-EN ISO 4288:1998 [39]. To evaluate the tribological behavior of the specimens, Ball-on-Disk (BoD) tests were conducted using Microtest series MT equipment (Microtest, Madrid, Spain). This study aimed to assess the friction and wear performance of PETG modified with flame-retardant additives, compared to its unmodified counterpart manufactured using FFF. In these tests, a sphere rotates around a fixed axis on a disk-shaped surface. The sphere of a 3 mm diameter is made of AISI 316L stainless steel, with around 200 HB and 0.17 \(\upmu\)m roughness in terms of Ra. This material is commonly used in these tests under similar conditions [27; 28; 29; 35]. The general ASTM G99-23 standard [40] was followed and customized based on the relevant literature for FFF-manufactured parts. The test parameters are summarized in Table 2.  
The results of the BoD wear tests were measured using a non-contact system capable of capturing surface microgeometry via Focus Variation Microscopy (FVM). This technique reconstructs images taken at different heights, merges them, and represents the surface topography at various levels. In this study, a Bruker Alicona G5+ microscope (Bruker Alicona, Raaba/Graz, Austria) with a vertical resolution of 10 nm was used. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **Temperature (\({}^{\circ}\)C)** & **Layer Height (mm)** & **Speed (mm/s)** & **Infill (\%)** \\ \hline PETG & 220 & 0.15 & & \\  & 230 & 0.25 & 60 & 100 \\  & 245 & 0.35 & & \\ \hline PETG FP & 235 & 0.15 & & \\  & 250 & 0.25 & 60 & 100 \\  & 260 & 0.35 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters used on the manufacture process of the PETG FP samples. Figure 2: Representation of the 3D model of the sample. The statistical significance of the data obtained was analyzed using analysis of variance ANOVA by analyzing the \(p\)-value as the measure of the probability of observing the results obtained in the test, assuming that the null hypothesis is true.