In vitro mechanical integrity of hydroxyapatite coatings on Ti-6Al-4V implants under shear loading

A new test method for the mechanical behavior of coatings on metal substrates under shear loading has been developed. Finite element simulations show that this new test method provides an almost identical shear load on the coatings to that of the conventional test. Using the new method, the static and fatigue behavior of plasma-sprayed hydroxyapatite (HA) coatings on Ti-6Al-4V substrates were studied at room temperature as a function of processing conditions. The results of the static tests show that the nominal interface shear strength is in the range of 25 approximately 40 MPa. The fatigue resistance to cyclic shear loading was characterized by shear stress amplitude versus cycle-to-failure for the samples that failed within 1010(7)cycles, and by residual nominal interfacial shear strength for the samples that survived 1010(7)cycles. The experimental results indicate that a threshold level of shear stress amplitude exists for introducing fatigue damage to HA coatings. AES and XPS studies indicated that bonding between the coating and substrate does not occur over about 12 percent of the area of the interface while bonding in the contact area of the interface is mainly mechanical interlocking. A failure mechanism of interfacial microflaw coalescence is suggested.