Formation of a hybrid coating by the use of plasma nitriding and hard chromium electroplating on the surface of H۱۱ hot work tool steel

Abstract: Formation of a hybrid coating by the use of plasma nitriding and hard chromium electroplating on the surface of H۱۱ hot work tool steel was investigated. Firstly, specimens were plasma nitrided at a temperature of ۵۵۰ °C for ۵ hours in an atmosphere of ۲۵ vol. % H۲: ۷۵ vol. % N۲. Secondly, electroplating was carried out in a solution containing ۲۵۰ g/L chromic acid and ۲.۵ g/L sulphuric acid for ۱ hour at ۶۰ °C temperature and ۶۰ A/dm۲ current density. Thirdly, specimens were plasma nitrided at a temperature of ۵۵۰ °C for ۵ and ۱۰ hours in an atmosphere of ۲۵ vol. % H۲: ۷۵ vol. % N۲. The obtained coatings have been compared in terms of composition and hardness. The compositions of the coatings have been studied by X-ray diffraction analysis. The surface morphology and elemental analysis was examined by using scanning electron microscopy. The improvement in hardness distribution after third step is discussed in considering the forward and backward diffusion of nitrogen in the chromium interlayer. Also, the formed phases in the hybrid coating were determined to be CrN+Cr۲N+Cr+Fe۲-۳N+Fe۴N.

Keywords:

H۱۱ hot work tool steel , plasma nitriding , hard chromium electroplating , hardness.