Improving the surface characteristics of Ti-6A1-4V and Timetal 834 using PIRAC Nitriding treatments

Abstract

Titanium alloys are used in various industries due their advantageous properties such as their high strength-to-weight ratio, an excellent corrosion resistance and a maximum application temperature of about 600 °C. However titanium alloys do suffer from poor tribological performance, limiting their use to static structural applications. Powder Immersion Reaction Assisted Coating (PIRAC) nitriding is considered able to provide an improvement in the surface properties of titanium alloys in a cost-effective manner compared to established nitriding techniques such as plasma nitriding and Physical Vapour Deposition (PVD) processes. The principal aim of this work was to carry out PIRAC processing of two very different Ti alloys; Ti-6Al-4V and Timetal 834, and then characterise the resultant effects on their surfaces anJ bulk. material. X-ray diffraction analysis shows that the Ti-6Al-4V alloy forms ThN and TiN while Timetal 834 alloy responded differently to the nitriding process, forming a nitrogen diffused zone with only a very thin layer of TiN at the surfaces evident only at the highest nitriding temperature used in this work - 900 °C. Hardness measurements indicate an improvement in the surface hardness of the coupons for both alloys tested with the maximum hardness value approaching 17 GPa. Analysis of the cross-sectional hardness variation indicates a gradual decrease in hardness in the near-surface area suggesting the fonnation of a diffusion strengthened zone. Electron backscatter diffraction measurements show that PIRAC treatments carried out at 800 °C for 16 h and 900 °C for 2 h do in fact result in a significant level of grain growth in Ti-6Al-4V, while Timetal 834 exhibits significant grain growth at 900 °C for 2 h. The adhesive strength of the modified surfaces was found to be satisfactory with no brittle failure mechanisms observed during nanoscratch testing. Tribological tests using reciprocating dry-sliding apparatus showed a considerable improvement in the siding-wear characteristics of Ti-6Al-4V following all processing parameters used. The improvement in sliding-wear characteristics was also evident for the Timetal 834 alloy albeit to a lesser extent - with treatments carried out at 800 °C and 900 °C for 2 hours providing the best tribological performance for this alloy. Oxidative tests indicated that the PIRAC nitriding treatment does not significantly improve the high temperature oxidative behaviour for Ti-6Al-4V, but does provide some improvement to Timetal 834 treated at 900 °C.