Evaluation of Plasma Nitriding Efficiency of Titanium Alloys for Medical Applications

Wide use of titanium alloys results from their unique physical and chemical properties 1 . Combination of low density and improved corrosion resistance with good plasticity and mechanical properties determines application of titanium alloys in such industries as aviation, automotive, power and shipbuilding industries or architecture as well as medicine and sports equipment. Despite numerous advantages, a fundamental drawbacks which limits wider use of titanium alloys include their tribological properties. This causes a more intensive wear as a result of creation of adhesion couplings and mechanical instability of passive layer of oxides, particularly with presence of third bodies. Analysis of application of titanium and its alloys also reveals unfavourable impact of reactive nature of titanium and its susceptibility to oxidation. Layer of oxides created in ambient temperature is thin and has low strength, which favours its removal during friction. Negative impact on wear in surfaces made of titanium alloys is produced by low friction resistance resulting from hexagonal crystal structure of -Ti 2, 3 . Electron configuration causing high titanium reactivity, crystal structure and inefficiency of used conventional lubrication limit application of titanium an its alloys for conditions which require resistance to tribological wear 4 . The first titanium alloy used for medicine was Grade 5 5 . This alloy, due to its contents of aluminium and vanadium in particular, turned out to be an alloy with low biotolerance, thus its composition was modified by means of exchanging these two components with elements better tolerated by the body, such as: Nb, Zr, Ta or Fe. Currently preferred titanium alloys with additions of niobium and zirconium are characterized with very good biotolerance 5, 6 . An alternative solution is modification of surface layers in relatively cheap alloys, such as e.g. titanium alloy Grade 5 ELI, causing change in chemical properties of the surface and in tribological properties, and, in consequence, change in biotolerance 5÷7 . One of the methods of surface treatment of titanium alloys is nitriding using glow discharge methods. Glow discharge processes enable constitution of surface layers with particular structure, phase and chemical composition, which enables considerable improvement in tribological properties of titanium alloys 8 . Factor which limits thickness of layers obtained by means of nitriding is low nitrogen diffusion index in titanium 8 . Thus, such nitriding is performed in most of cases within the range of temperatures of 973 ÷1423 K. Considerable improvement in wear and corrosion resistance in titanium alloys