# Mechanical and Tribological Performance of Carbon Fiber-Reinforced PETG for FFF Applications
## 1 Introduction  
Additive Manufacturing (AM), also known as 3D printing, has emerged as a more sustainable alternative to traditional subtractive manufacturing in the industry. According to the F42 ASTM International Committee, additive manufacturing encompasses a set of processes that enable the layer-by-layer fabrication of physical objects from CAD models using materials of various compositions [1]. Among the techniques within Additive Manufacturing, Fused Filament Fabrication (FFF) is widely used. FFF employs a nozzle to melt filament material and progressively deposit it. Despite the extensive use of this method, ongoing efforts are dedicated to enhancing the quality of manufactured parts, reducing print times, and exploring new materials and applications. FFF primarily uses thermoplastic materials like polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polyethylene terephthalate glycol (PETG), nylon (Polyamide), or thermoplastic polyurethane (TPU) [2]. Carbon fiber as reinforcement may modify the surface finish of the developed parts. Several studies are focused on the improvement of the surface quality of the manufacturedparts, mainly due to the limitations of the printing process, and some post-processed methods are considered to improve the surface of the parts [3; 4; 5; 6; 7]. To improve their properties, especially their mechanical characteristics [8], some variations of these materials are reinforced with non-thermoplastic materials such as metals, ceramics, and synthetic or natural fibers. Among these, PETG is gaining prominence due to its minimal environmental impact. PETG, a modified PET copolymer, includes glycol to enhance flexibility, reduce moisture absorption, and provide transparency, making it suitable for FFF. However, the mechanical behavior of both materials becomes remarkably similar, differing mainly during flat deformation, where PET exhibits a more significant increase in strain, especially at 90 \({}^{\circ}\)C [9; 10]. Below the glass transition temperature, these materials exhibit a distinctive yield point, followed by strain softening and moderate hardening at moderate strain levels, with substantial hardening occurring at large strains. Strain-induced crystallization plays a limited role under most strain conditions and predominantly affects the mechanical behavior during loading at higher strain levels in deformation conditions that result in the nearly uniaxial molecular orientation of the polymer structure [9; 10]. However, compared to other materials used in FFF, such as PLA or ABS, PETG exhibits superior load-bearing capability [11]. Tensile tests indicate that PETG outperforms PLA and ABS, with a load of 2308 kN (0.077 kN/mm\({}^{2}\)) compared to 2083 kN (0.053 kN/mm\({}^{2}\)) and 1905 kN (0.049 kN/mm\({}^{2}\)) for PLA and ABS, respectively. PETG also demonstrates the highest elongation percentage at 14.27%, compared to 5.263% and 8.772% for PLA and ABS, respectively [11]. Consequently, there is a growing interest in PETG for use in FFF, although the manufacturing of this material can be challenging. Despite the absence of common warping defects, manufacturing parameters play a crucial role in the process's performance. For example, the best mechanical properties for PETG were achieved at a 265 \({}^{\circ}\)C extruder temperature, 20 mm/s printing speed, 0.4 mm layer height, and 100% infill density [12]. Printing orientation significantly affects mechanical properties, particularly the elastic modulus and tensile strength [13]. A cross-axial orientation yields the highest fatigue strength, while the longitudinal orientation results in greater stiffness [13]. The material's performance is strongly influenced by the extrusion temperature. According to [PERSON] et al. [14], as the printing temperature increases, so do the mechanical properties. This phenomenon has also been observed in other materials, such as PLA [14; 15], although there is a threshold beyond which materials can degrade [15].