Experimental Investigation of the Tribological Contact between Ti6Al4V-EBM Pin and UHMWPE Rotating Sheet for Prosthetic Applications

1. Introduction

The selection of materials capable of withstanding over time, and above all minimizing abrasions and stresses due to continuous tribological contacts, represents a challenge for research in the field of endoprostheses. With the purpose of ensuring mobility and load capacity, and also minimizing the problems of friction and wear, the use of biocompatible metal alloys, such as those based on titanium and the latest generation of polymers, has become widely established to produce prosthetic joints. Of course, they should be able to satisfy the demand for mechanical resistance and to guarantee, as much as possible, health safety needs [ 1 ]. Concerning titanium, and in particular its most widespread alloy Ti6Al4V, recent studies have highlighted the ergonomic efficiency of the components produced with the Electron Beam Melting (EBM) additive technique [ 2 ]. As known, one of the main applications of EBM is the production of biomedical implants customized for the individual patient, starting from computed tomography scans of the affected area [ 3 ].

Among the most suitable polymeric materials for prosthetic joints, ultra-high molecular weight polyethylene (UHMWPE) is considered, thanks to its excellent physical, chemical and mechanical characteristics, such as biocompatibility and good resistance to wear and abrasion, which can be increased by appropriate reinforcers as reported in the review by Macuvele et al. [ 4 ] and in more recent articles [ 5 7 ] concerning the use of carbon nanofiller and graphite. However, wearing contact with metal parts can lead to the release of debris, limiting the use of UHMWPE. Therefore, several experimental researches have been addressed to studying the improving effects on wear behavior due to lubrication [ 8 ] or to the development of suitable polymer composites [ 9 ]. In these works, the metal–polymer contact was reproduced by means of a “pin on disc” apparatus, in which a Ti6Al4V alloy pin, produced by EBM, acts through the pointed end on a polymer sheet, which is placed on a rotating disc. In these works, the wear tests were aimed at determining the volume of material removed over a predetermined time interval. It allows a simple comparison between the different experimental conditions considered, such as the use of lubricants or the introduction of reinforcing fillers into the polymer. In particular, the best performances were obtained by carrying out the tests in simulated synovial fluid (SSF) and in bovine synovial fluid (BSF). However, this procedure does not allow documenting of the evolution of the wear process over time, nor the obtaining of the wear coefficient.

Concerning these latter aspects, it can be observed that the debris formation evolves from an initial transient phase with decreasing derivative to a stationary phase with a linear trend of the removed volume as a function of time. Thus, the wear coefficient, calculated at the stationary phase according to Archard’s law, is the most suitable parameter to evaluate the behavior of a tribological couple [ 10 ]. In this regard, data relating only to some metal–metal systems [ 11 ] and ZrO2-polymer [ 12 ] are available.

Therefore, the main objective of the present work consists in studying how wear of the tribological pair Ti6Al4V/UHMWPE progresses over time, and characterizing the involved surface to achieve an accurate assessment of the phenomenon. The wear tests were carried out with a “pin on disc” tribometer with fixed metallic pin and movable polymer specimen as in a similar apparatus utilized by Tai et al., and Guezmil et al. [ 12 13 ]. The tribological behavior has been tested under different lubrication conditions, for increasing times up to 240 min, using a Ti6Al4V alloy pin, produced by EBM. The pointed end of the pin acts on UHMWPE specimens, produced by pressure-forming of powders at 200 °C, and placed on the rotating disk. The conditions of adhesive friction, which are generated on contact, led to the formation of grooves on the polymer surface and therefore to the production of debris. The identification of the stationary phase in the experimental curve of mass release as a function of test time allowed us to determine the specific wear rate. Consequently, the results obtained in the different lubrication conditions were compared.

Finally, the originality of this research work is given by the fact that in the literature there are no recent works on the tribology of the customizable Ti6Al4V alloy produced with EBM technology coupled with biomedical UHMWPE, to the best of our knowledge.