The Effects of Substrates' Materials on Hardness, Creep, and Residual Stress of the Hydroxyapatite Coating, Deposited by HVOF Method

The aim of this study was to investigate the effects of the substrates' materials on the mechanical properties and behavior of hydroxyapatite (HA) coating, deposited by HVOF method. High velocity oxy-fuel (HVOF) deposition was used as the coating technique, and HA was coated on different substrates, i.e. commercial pure titanium (CPTi), Ti6Al4V, Cobalt chrome alloy (Co-Cr), and 316L stainless steel (SSL). The hardness, elastic modulus, creep, and residual stress of HA coatings, for the four afore-mentioned substrates, were then measured using nanoindentation technique. The results revealed a decreasing trend in the creep behavior in HA coating employed on: CPTi, Ti6Al4V, Co-Cr, and SSL substrates, respectively. Similar to the creep behavior, a descending trend was observed in the indentation depth, as well as in the tensile residual stress, while the hardness tests showed an increasing trend on the mentioned samples, respectively. This investigation showed that the coefficient of thermal expansion (CTE), as well as residual stress of the substrates had a direct impact on the hardness of the HA coatings. Interestingly, the elastic modulus of the substrates' materials had no direct effect on the elastic modulus, and on the hardness of HA coatings. This study suggests that the substrate's material properties can affect the behavior of the coating, and consequently on the interaction between the implant and neighboring bone.