Selective Surface Heat Treatment of Cr and CrN Coatings on Steel

Chromium and chromium nitride coatings on steels have been prepared with pulsed dc magnetron sputter deposition at a frequency of 185 kHz and a duty cycle of 96%. The coating process was carried out at 150 W with the substrate temperature maintained at 200°C. For nitride coatings, reactive sputter deposition was carried out in Ar with 3, 5, or 7%N₂. Heat treatments were performed for coated steels with an infrared (IR) processing technique in an attempt to modify the interfaces and change coating properties. T he heat treatment was carried out at 400°C for 2 and 5 min. Characterisation of coated samples included X-ray diffraction (XRD), optical microscopy, and microhardness tests to verify the effect of the heat treatment. Results from XRD analysis indicated that IR heat treatment at 400°C for 2 or 5 min did not change the phases present in the coatings. Optical images of the coating microstructure show that cracks were developed on coatings deposited under 7%N₂ even after only 2 min heat treatment at 400°C. Severity of cracking is reduced for coatings deposited under low nitrogen contents. No cracks were observed on coatings deposited without nitrogen. Microhardness measurements showed that for all coated specimens, 2 min IR treatment at 400°C gave the highest microhardness. For samples treated for 5 min, the microhardness decreased. X-ray diffraction analysis with slow scanning showed that CrN peaks were developed in the coatings deposited under 7%N₂ and heat treated at 400°C for 2 and 5 min.