Tribocorrosion behaviour of Ti6Al4V and AISI 316L in simulated normal and inflammatory conditions

Frictional effects are often associated with the relative motion between certain metallic components used in bioimplants. In particular, it applies to metal-on-metal (MOM) friction joints, which occur in hip joint endoprostheses and spine implants. In hip joint endoprostheses, friction occurs between the head and the acetabulum. In the case of spine implants, friction plays the most significant role in the artificial intervertebral disc total disc arthroplasty (FDA). Friction between metallic surfaces is the consequence of the patients movements. Reciprocating friction, high contact pressure and aggressive environments can cause tribocorrosive wear of the contact surfaces. Inflammation of the tissues may develop after the implantation of orthopaedic devices, and their chemical products may affect subsequent tribocorrosion behaviour. The primary goal of this study was to investigate how normal and inflammation conditions influence the tribocorrosion processes of popular metalic biomaterials. The research concerns two metal-on-metal friction couples. One contained self-mated stainless steel AISI 316 L, and the other self-mated titanium alloy Ti6Al4V. The friction pairs were submerged in either of two fluids: Ringer's solution (RS) and a solution of acidic sodium lactate (ASL), which provided a corrosive environment. Ringer's solution simulates normal conditions, and the acidic sodium lactate simulates inflammatory conditions. After the friction tests, the contact surfaces of the samples were characterized by their roughness, micro-indentation hardness, and by SEM observations and EDS analysis. The ASL proved to be a more aggressive environment than the RS. In combination with friction, ASL intensified the wear of the metal surfaces. The tribocorrosion process also affected the roughness and hardness of the contact surfaces. In the case of the RS, a tough oxide layer is formed, especially on the 316 L steel. Such oxidised layers were found to reduce abrasion damage and mass wear. SEM analysis indicated the different amount and structure of oxidised particles on the friction surfaces. It strongly depends on the material and solution. A form of oxidation process - locally oxidised regions - increased the surface roughness of the Ti6Al4V alloy. Both the smaller oxidised layer thickness and the bigger surface roughness contributed to wear intensification of the Ti6Al4V alloy. The oxidation process of the steel sample was faster and deeper. The generated oxidised layer was stable and relatively smooth, and therefore provided better protection against abrasive wear. Based on the results of these experiments, a friction and wear model is proposed.