## 1 Introduction  
Due to their high wear resistance, efficiency, weight and price, spur gears are the most commonly used power and motion transmission elements. Nowadays, metal gears are increasingly being supplanted by polymer gears, which have many engineering applications, such as for mechatronics, medical and laboratory equipment, household appliances, office devices, etc. The main advantages of polymer gears over metal gears are their low specific weight, low coefficient of friction, self-lubrication and low manufacturing costs. In particular, polymer gears can absorb and dampen the shocks and vibrations to which the gear is subjected, thus reducing noise during operation [1; 2; 3]. The good mechanical and tribological properties of polymers, primarily from the Polyoxmethylene (POM), Polyamide (PA) or Polyether Ether Ketone (PEEK) family, enables the meshing of polymer gears with metal gears [4; 5]. When polymer and metal gears mesh together, the quality of the tooth flank surface of the metal gears has a high impact on the wear of the polymer gears [6]. Polymer gears are mainly produced by injection molding, machine cutting or additive manufacturing. Injection molding produces high-quality products and is cost-efficient for mass quantities, while machine-cut polymer gears are used for small batches and are produced with the same tools used for metal gears manufacturing. Regardless of whether the injection molding or the machine cutting process is employed, there is no difference in the wear behavior of the gears [7; 8]. Additive manufacturing is a modern technology whose accelerated development allows for the energy-efficient production of high-quality polymer gears. Despite the many advantages of polymer gears, there are also limitations in terms of the permissible operating temperature and load. The main drawback of polymer gears is primarily manifested in their operation at elevated temperatures. Under such conditions, their structure, and thus their mechanical properties, as well as the resistance to impact loads and wear behavior change. Compared to metals, they have inferior thermal conductivity and a higher coefficient of thermal expansion. In addition, some polymers such as PA tend to absorb moisture, which leads to a change in dimensions [9; 10; 11]. Therefore, it is important to choose the appropriate polymer for a specific application depending on the operating conditions. Standard gear design guidelines and analytical models according to VDI 2736 [12] are employed to determine the load-carrying capacity and dimensions of polymer gears. These guidelines use the same calculation models as for steel gears and can serve as a good basis, but they only contain material data for a few polymers, which makes them insufficient. Polymer gears usually fail due to several failure modes, i.e., melting due to the overheating of the tooth flank, tooth root fracture due to flexural stress, tooth flank fracture or pitting due to excessive Hertzian stress, tooth wear or thermal overload, which can lead to plastic deformation [13; 14]. Recently, many studies have focused on failure modes and the influence of various factors on the polymer gear wear behavior, even at the microstructural level [15]. The wear mechanisms differ depending on the mechanical and tribological properties of the polymer and the load intensity. POM gears were investigated under different loads, and it was found that tooth flank wear occurs at lower loads. At higher loads, severe tooth flank wear leads to incorrect meshing, due to which the gear heats up quickly. Eventually, higher teeth deflections due to material softening, plastic deformation or even tooth melting appears once the tooth temperature reaches 80 \({}^{\circ}\)C [16; 17; 18]. [PERSON] et al. [19] carried out an experimental investigation of the wear behavior of gears made of biopolymer PA 610. The tooth flank wear was recorded at lower loads, while the tooth root fracture occurred at higher loads, as in the case of metal gears.