Synthesis and Evaluation of Properties and Detachment Mechanism of Ti(C,N) Coatings Produced on the Surface of Ti-6Al-4V Alloy by Pack Nitro-Carburizing Process

In this research, titanium carbo-nitride coatings were synthesized on the surface of Ti-6Al-4V alloy by nitro-carburizing route and the detachment mechanism of the coatings were investigated. For this purpose, rectangular blocks were embedded in a carbon containing graphite cup. Then, the cup was placed in a tubular furnace containing nitrogen gas. After coating in different temperatures between 1200 and 1400 ˚C for 20 to 60 min soaking times, structure and microstructure of the coatings were investigated by X-ray diffraction, Raman analysis and scanning electron microscope equipped with energy dispersive spectroscopy (EDS). Moreover, the effect of coatings on the corrosion resistance of the alloy in an aqueous solution containing 5.2 mol HCl, 3.2 mol HNO3 and 3.2 mol HF was investigated. Results showed that the maximum thickness of the titanium carbo-nitride coating for the sample treated at 1400 ˚C for 60 min was about 50 μm. However, the possibility of coating detachment increased with increasing the coating temperature. It was due to the formation of Al-rich layer at the interface of the coating and the substrate and the formation of Ti3Al brittle phases, as well as the difference between the thermal expansion coefficient of the coating and the substrate. Corrosion test results showed that the formation of the coatings decreased the corrosion rate of the Ti-6Al-4V alloy from 130g.m-2.min-1 for the bare alloy to 10g.m-2.min-1.