Abstract
Electroless nickel (EN) coatings are wildly used in many industries as functional coatings due to their excellent properties such as uniform thickness, corrosion and wear resistance. Recently, chromium nitride has been proposed to be a promising hard coating material owing to its better wear, corrosion, and oxidation resistance than TiN. In this study, the electroless Ni-P coating is introduced as an interlayer to modify the properties of CrN/mild steel assembly. The acid hypophosphite-reduced bath is applied to plate electroless Ni-P coating on mild steel (MS) substrate, and CrN film is further prepared by reactive rf magnetron sputtering on the electroless Ni-P coated mild steel. Due to the elevated substrate temperature during rf sputtering, the electroless nickel layer crystallizes with the precipitation Ni3P phase, and thus a coating assembly of CrN/Ni-Ni3P/MS is expected. The properties of the heat-treated EN/MS assembly are also evaluated to investigate the contribution provided by the electroless interlayer. The microhardness of the duplex coatings ranges from 2400 to 2700 HK at the load of 10 gf, in which corresponding penetration depths are around 1/4 of the CrN-overlayer thickness. The indentation behavior and Young's modulus of the coatings can be evaluated through the indentation and cracking technique (ICT). The scratch test is further applied to evaluate the adhesive and cohesive properties and the critical load of the coating assemblies. The critical load judged from the acoustic emission (AE) signal is increased with the decrease in phosphorus contents of the EN interlayer in the duplex coating assembly. The adhesive failure is postponed by the increase in thickness of the EN buffer layer. The wear behavior and resistance of the coating assemblies are studied using the pin-on-disc sliding contact accompanied by an AE detection system. The heat-treated EN coating with 12.4%P exhibits cohesive failure during sliding process, while the one with 6.23%P reveals only adhesive failure. The deformation zone of the CrN/MS assembly caused during sliding process is narrowed down with the modification of the EN intermedia. The load bearing ability of the EN interlayer incorporated between the CrN overcoating and the mild steel substrate is then evident.