Effect of hydroxyapatite coating on nickel release of the porous NiTi shape memory alloy fabricated by SHS method

The amount of nickel release from a porous NiTi shape memory alloy (SMA) fabricated by the self-propagating high-temperature synthesis (SHS) method is about two orders of magnitude more than that of a solid NiTi sample with the same nominal surface exposure to simulated body fluid (SBF). A hydroxyapatite coating has been employed to limit the nickel release from porous NiTi samples without blocking the pores. After 32.5% HNO3 and 1.2 M NaOH aqueous solution treatments, and subsequent sample immersion in SBF for 5 days, a uniform hydroxyapatite layer forms, not only on the surface of the porous NiTi alloy, but also on the inside of the pores. This greatly decreases the amount of nickel release from the porous NiTi shape memory alloy. The exceedingly fast nickel releases from the porous NiTi alloy and mechanism of hydroxyapatite layer formation have been discussed in this paper.