# Wear Effect on the Contact between a Metallic Pin and a Rotating Polymeric Specim
## 1 Introduction  
Ti6 Al4V alloy is commonly used for prosthetic components together with Ultra-High-Molecular-Weight-Polyethylene (UHMWPE), which acts as the bearing polymer. From a mechanical point of view, this alloy has excellent mechanical properties, even at high temperatures [1], making it useful in the aerospace and motorist industry (gas turbine blades, parts of combustion chambers, or high-temperature gas exhaust pipes) and in the biomedical field (joint elements in prostheses). However, due to the possible toxic effects, vanadium can be an issue for the examined alloy. For this reason, research is underway to replace vanadium with other elements, such as iron and niobium, as pointed out by [PERSON] et al. [2]. Nowadays, biomedical implants and prostheses are extensively produced in a much shorter time than conventional manufacturing technologies through Electron Beam Melting (EBM) of Ti alloy powders deposited layer by layer to obtain complex components that canbe adapted to specific needs [3]. In this way, the typical issue of conventional machining processes due to heat generation and friction, resulting in long production times and material waste, can be avoided [4; 5; 6]. However, EBM products are characterized by some roughness and poor surface finishing [7]. Moreover, powder melting gives rise to a lack of fusion defects and entrapped gas porosity, which are detrimental to mechanical strength and can be overcome through surface smoothing and hot isostatic pressing with the purpose, respectively, of lowering roughness and reducing any internal porosity [8]. Due to their favorable tribological properties, polymers are attractive materials to be placed between metal parts in contact in prosthetic joints. Among them, ultra-high molecular weight polyethylene (UHMWPE) is appreciated for good biocompatibility and high resistance to wear and abrasion [9]. In particular, mechanical strength can be improved by adding suitable fillers (see the review [10] and findings in [11; 12]). Unfortunately, the wearing contact against metal surfaces is the cause of debris production, limiting the use of UHMWPE [2]. Debris, in turn, gives rise to adverse cellular reaction response and, consequently, premature failure of the joints [13]. To limit these damages, the effect of lubrication was investigated in [14], while in [15], reinforced polymers with enhanced performance in wear behavior were developed and characterized. The \"pin-on-disc\" test is based on the relative sliding between a fixed pin and a rotating disc, resulting in wear on both the contact surfaces of the tribological pair. In this regard, many authors set up polymeric pins against metal plates [16; 17], and others adopted the opposite configuration, placing UHMWPE sheets on the movable disc [18; 19]. In this way, the addition of fluids keeps the polymer surface submerged, thus maintaining unchanged lubricating conditions throughout the test. The wear behavior of UHMWPE is not an absolute datum but depends on the opposed pin, according to the specific characteristics of the material and the production process, which determines the state of the surfaces, such as discontinuities and high roughness. In this regard, the EBM products differ from cast or forged ones due to their metallurgical and surface features. The originality of a recent work carried out by the authors [20] lies in the scarcity of data in the literature on the specific tribological couple consisting of a Ti6 Al4V alloy pin produced by EBM and a rotating UHMWPE specimen. In that article, the mass loss of polymeric material was measured at regular intervals by a weight scale having an accuracy equal to 0.1 mg. Then, the volume loss obtained by dividing by the density was plotted as a function of the test time. Thus, the specific wear coefficient was referred to, as recommended in [21], as the volume loss per sliding length unit during the stationary phase.