Fretting Wear Characteristics of Suspension Plasma-Sprayed Hydroxyapatite Coating on Titanium Substrate for Orthopedic Applications

The hydroxyapatite coating on the metallic implant is preferred to improve biocompatibility and faster bone integration. The reliability of the coating during fixation and after surgery is important to achieve the expected bone growth. The micromotion between the implant and bone due to patient movement causes fretting and deteriorates the coating. New coating techniques are attempted to improve the strength of the HAp coating. The suspension plasma spray (SPS) process is a new process where fine grain-structured coating with controlled porosity can be developed. The fretting wear behavior of titanium (Ti) substrate and SPS-coated HAp on Ti substrate fretted against alumina are discussed in this paper. The relative displacement expected in the bone and stem interface is estimated using numerical studies, and fretting wear experiments are carried out with the maximum relative displacement estimated. Fretting wear tests are performed using the ball-on-flat configuration at different contact stresses. The fretting friction coefficient variation in the coated samples during the initial contact period is different compared to the uncoated samples. A high friction coefficient during the early stages of fretting influences the reliability of the coating, although the steady-state friction coefficient is nearly the same as that of the uncoated titanium. The friction force vs. displacement loops obtained from the tests revealed 'gross-slip' conditions. The wear scar observations using a scanning electron microscope and 3D-profilometer revealed significant damage in coated samples compared to the uncoated titanium samples. The porous nature of the SPS coating and fine wear particles facilitates a smooth layer formation in longer run.