SURFACE NITRIZATION OF TITANIUM-ALLOY FOR PROSTHETIC IMPLANTS

A titanium nitride layer was formed on the surface of sintered Ti, sintered Ti-6 A1-4V, forged Ti and forged Ti-6 Al-4 V by heating at 850-degrees-C for 16 hours in a nitrogen atmosphere. The nitride layer has been investigated by X-ray diffraction, Vickers hardness measurement and SEM. The corrosion resistance of the nitrided Ti and Ti-6 Al-4 V was evaluated in physiological NaCl,pH 5 HCl, 5% H3PO4 and 5% HCl solutions. The cytotoxicity of the wear debris generated by friction of nitrided Ti-6 Al-4 V against UHMWPE was also evaluated. The thickness of the nitride layer was 50-60-mu-m. Diffusion coefficients of nitrogen in Ti and Ti-6 Al-4 V calculated from Vickers hardness distribution were 2.97 +/- 0.79 x 10(-11) and 5.61 +/- 1.83 x 10(-11) cm2/s, respectively. Released ions from the nitrided Ti and Ti-6 Al-4 V, as well as Ti and Ti-6 Al-4 V, were less than 0.1 mg/l in both physiological NaCl and pH 5 HCl solutions. However in 5% H3PO4 and 5% HCl solutions, the released ions from the nitrided Ti and Ti-6 Al-4 V were 6-225 times less than those from Ti and Ti-6 Al-4 V. The inert nature of the nitrided Ti and Ti-6 Al-4 V indicated superior biocompatibility compared with Ti and Ti-6 Al-4 V. The wear debris generated by the friction of nitrided Ti-6 Al-4 V against UHMWPE was less toxic than those generated by the friction of alumina, zirconia and hydroxyapatite against UHMWPE.