Abstract
The objective of this study was to deposit thick ZrN coatings on AISI D2 steel substrates, and to investigate the structure and properties of the ZrN coatings with varying thickness. The controlling deposition parameters were the opening level of the gate valve between the deposition chamber and the turbomolecular pump (TP), and the deposition duration. The pumping speed of the gas inside the deposition chamber was changed with the different opening level of gate valve. Two deposition conditions were adopted, which were the full opening of gate valve (FV) and half opening of gate valve (HV) conditions. The results showed that ZrN coatings with thickness over 6μm were successfully deposited on the D2 steel substrates. The hardness of the ZrN coatings was ranged from 26.0 to 27.7 GPa without significant variation with thickness. The HV-series possessed higher adhesion strength and wear resistance. This could be due to the higher gas ionization rate in HV-condition, which promoted the stress relief during deposition and leaded to better-arranged structure. However, target poisoning occurred in both deposition conditions and the macro-particles generated from arcing were buried into the coatings. The region near the substrate was less affected by the particles and therefore denser . By contrast, the buried particles resulted in the internal voids and loose packed structure in the upper region of coatings which is away from the substrate. Due to larger portion of loose packed region in the thicker coatings, the residual stress of the ZrN coatings was decreased with increasing thickness. Furthermore, the loose packed region was less resistant to wear, leading to the increasing wear rate with thickness. In comparison of the two deposition conditions, ZrN coatings with higher quality could be produced by HV-condition with higher deposition rate, lower energy cost and lower working temperature of TP, indicating that HV-condition was more suitable for industrial application.