Wear of diffusion hardened Ti–6Al–4V sliding against tool steel

Titanium has very poor tribological properties. Various coatings can be used, for example TiN and DLC, to improve them, but the loading is limited by the low strength of the substrate. In recent years thermochemical diffusion treatments have been developed. In order to produce a layer sufficiently thick to support a load in a reasonable time, these treatments have to be carried out at high temperatures: 950 °C and 1050 °C for oxidation and nitriding processes respectively. Such high treatment temperatures can degrade the core properties to such an extent that they must be heat treated again after the surface layer has been produced. An alternative treatment was found that gave a substantive load bearing layer at a treatment temperature of 850 °C or lower without degrading the core properties. The optimised diffusion hardened layers had a thin TiO2 and/or TiC surface layer. Three such layers with different surface chemistries were found to have formed on T1–6Al–4V by exposure to oxygen, carbon dioxide or carbon monoxide. Under dry wear conditions the layer formed during carbon monoxide exposure was found to give the best results. Under the same conditions this layer gave an order of magnitude less wear than carburised 21NiCrMo2 steel. In lubricated wear tests, the wear was very low.