Ion nitriding of a near-β titanium alloy: Microstructure and mechanical properties

In this study, a low-temperature ion nitriding treatment has been developed with the aim of improving the sliding behavior of Ti-10V-2Fe-3Al, a near-β titanium alloy, without adverse effects on its load bearing properties. The nitriding treatment was carried out at 600 °C, above the ω precipitation temperature (250-450 °C) and below the β-transus temperature of the alloy (800 °C), to inhibit the formation of brittle precipitates and undesirable phase transformations. The alloy was nitrided in a low-pressure (67 Pa) and dilute (3% N2) nitriding atmosphere for 8 h. The microstructural features of the nitrided samples were thoroughly characterized using analytical microscopy techniques. A 0.5 μm thick compound layer consisting of TiN Fm3-m, Ti2N (P42-mnm), V2N P3-1m, with the possible formation of Fe8N (I4-mmm) after the treatment. The microhardness measurements revealed the presence of an approximately 35 μm thick nitrogen diffusion zone beneath the compound layer. The maximum hardness value for this zone was observed near the interface and gradually decreased towards the substrate. The load bearing capacity and friction properties of the ion nitrided surfaces under sliding were investigated by microscratch tests. It was found that the coefficient of friction was reduced by more than 72% and the nitrided surfaces exhibited an excellent load bearing capacity. The compound layer maintained a good adhesion to the underlying diffusion zone up to the critical load of 17 N where partial chipping of the coating was observed at the scratch rims.