Tribological and electrochemical performance of PVD TiN coatings on the femoral head of Ti–6Al–4V artificial hip joints

Replacing the femoral head and the acetabular socket with an artificial prosthetic component is practiced throughout the world to restore painless joint function for patients suffering from disabling hip joint disease. Moving and anchored parts of the prosthesis are exposed to different mechanical stresses and chemical actions; thus, various mechanical, chemical, tribological, and corrosion processes are experienced by the implants. TiN coatings were deposited on Ti–6Al–4V substrates using a plasma-assisted electron beam PVD technique, with the intention of enhancing the tribological and electrochemical performance of the femoral head. The thickness, microhardness, surface roughness, and interfacial adhesion of the TiN coatings were evaluated by means of ball-crater testing, micro-indentation, surface profilometry, and scratch testing, respectively. Impacting under a dynamic repetitive impact load, and multi-pass scratching with a 0.2 mm radius diamond indentator on the coated surface under a load of 50% of the critical load for debonding, were carried out to study wear and contact fatigue failure of the TiN coated systems. The corrosion performance was also evaluated by electrochemical testing including d.c. potentiodynamic and a.c. impedance spectroscopic (EIS) techniques in 0.5 N NaCl solution. It was found that adhesive failure during impact was observable after 5×104 cycles. The number of traversals Pc at which the coatings started to fail during multi-pass test was in a range of 300–400. The PVD TiN coatings significantly reduced the corrosion rate, icorr by approximately 2 orders of magnitude. The coated systems also exhibited superior pitting resistance, which greatly suppressed the localised deterioration. In addition, the polarisation resistance of the TiN coated Ti–6Al–4V systems, determined through the EIS modelling with an equivalent circuit, was 2–4 MΩ cm2 after 15 days of immersion.