In-vitro electrochemical and bioactivity evaluation of SS316L reinforced hydroxyapatite functionally graded materials fabricated for biomedical implants

Three sets of FGMs of stainless steel 316L (SS) reinforced with micro-, nano- and mixed (1:1 mass ratio) hydroxyapatite (HA) were fabricated by powder metallurgy route. The concentration of the HA was varied from 0 to 20 wt% with the increment of 5 wt% in all sets to strengthen the discrete layers of FGMs. The sintered densities along the discrete layers of FGMs continually decreased as a function of HA content which enhanced the bioactivity of the FGMs towards the end with high content of HA. All FGMs experienced an excellent corrosion response in the 0.9% NaCl solution with high passivity. Heavy diffusion of chromium (Cr) form SS to HA made the matrix Cr deficient. The chromium depletion regions around the interface of SS and HA caused an active corrosion behavior of FGMs in 0.1 M HCl solution. FGMs with micro-sized HA demonstrated the better corrosion properties than that of other FGMs. After being immersed in the simulated body fluid (SBF) solution for 7 days, the apatite layer formed on the surface of FGMs with micro-sized HA had a mature spherical morphology and leaf like shape for the other two FGMs. The apatite morphology and gained weight results proved the highest bioactivity for micro-sized HA reinforced FGMs.