Effect of nitriding and TiN coating temperatures on the corrosion resistance of the combined surface modification layer

There are great demands that components become smaller, lighter and more efficient, and the environment of their use become severe. The combined surface modification techniques consisting of TiN coating on the nitrided steel have been studied to improve wear resistance for components used in severe environment. Our earlier studies indicate that the combination layer of nitriding and PVD TiN coating improves adhesion between coating layer and substrate of a tool steel, SKD11, but the corrosion resistance of the tool steel could not improve. The nitriding at approximately 400 °C improved the corrosion resistance of the steel compared to nitriding at 500–550 °C. The corrosion resistance of the steel coated with TiN at 450 °C after low temperature nitriding could not improve. The purpose of this study is to confirm an influence of coating temperature on corrosion resistance. The corrosion resistance of the steel heated at 480 °C in vacuum after nitriding at approximately 400 °C was less than that of the steel without heated at 480 °C in vacuum. Diffraction from ‘S-phase’ was detected in XRD measurement of the steel nitrided at approximately 400 °C. The S-phase did not appear from the nitrided layer at high temperature in the earlier study and could not be found from the steel heated at 480 °C in vacuum after nitriding at approximately 400 °C. When the S-phase appeared, the corrosion resistance of the layer improved. The relation between the S-phase and the high corrosion resistance was clarified. Adhesion of the combined layers was measured with a scratch tester. The adhesion of the steel nitrided at approximately 400 °C and with TiN at 350 °C was superior to the steel coated with TiN at the higher temperature without nitriding in a scratch examination. It clarified that the combined layer with superior adhesion and corrosion resistance was formed by the steel coated with TiN at low temperature after low temperature nitriding.