Sputter-deposited nanocrystalline Cr and CrN coatings on steels

Chromium and chromium-nitride coatings on steels have been deposited with a magnetron sputter-deposition system. The deposition power was 200 W pulsed DC with a frequency of 185 kHz and 96% deposition duty. The substrate temperature was maintained at 200°C. The sputtering gas was argon mixed with 0, 3, 5 or 7% nitrogen. Results show that the average deposition rate was 1.2 μm/h and was not affected by the nitrogen content in the sputtering gas. Using X-ray diffraction analysis, it was observed that the deposited Cr under pure argon condition, was nanocrystalline bcc chromium with a particle size in the range of 7–8 nm. With an increasing nitrogen content in the sputtering gas the amount of CrN increased. The measured microhardness of the chromium-coated steel increases with the increasing nitrogen content. With less than a 2-μm Cr coating, the steel hardness increases from 129 to 255 HV when the nitrogen content in the gas is 7%. The microhardness of Cr/CrN-coated steel prepared with sputter deposition is superior to that prepared with electroplating. The hardness of the coating layer calculated from these data is 1270 HV. Scratch tests were used to characterize coating adhesion. The critical load was determined to be the applied load under which an acoustic noise was found and cracks in the scratch track were first observed. The critical loads of deposited films with 0, 3, 5 and 7% N2 in the gas, were 1.57, 5.68, 8.33 and 20.29 N, respectively.