Development of Low Temperature Nitriding Process and its Effects on the 4-Points Bending Fatigue Properties of Commercially Pure Titanium

In order to improve both of the fatigue and tribological properties of commercially pure (CP) titanium, a low temperature nitriding process was developed. Cold rolling was introduced as pre-treatment of plasma nitriding to create fine grains which could accelerate the diffusion of nitrogen into the material. Surface microstructures of the nitrided specimens pre-treated with cold rolling were characterized using a micro-Vickers hardness tester, an optical microscope, a scanning electron microscope (SEM), X-ray diffraction (XRD) and electron backscatter diffraction technique (EBSD). Titanium-nitrides (TiN and Ti2N) were formed on the surface nitrided at the temperature greater than 600 °C. Moreover, thicker compound layer was formed in the nitrided CP titanium pre-treated with cold rolling in comparison to the only nitrided one, resulting in showing higher hardness. 4-points bending fatigue tests were performed for the specimens treated with low temperature nitriding (600 °C), which could suppress the grain-coarsening, under the stress ratio R = 0.1 at room temperature. In addition, fatigue fracture mechanism of nitrided CP titanium was discussed based on the observations of microstructure and fracture surface.