Abstract
The purpose of this study was to investigate the effect of Zr interlayer on the residual stress of the upper ZrN thin film and to evaluate the usability of ZrN/Zr bilayer coatings. It is commonly acknowledged that using a metal interlayer can relieve the residual stress and enhance the adhesion of transition metal-nitride hard coatings on metallic or Si substrates. The interlayer thickness usually ranges from 50 to 300 nm. However, there were few studies on the separate measurement of stresses in the top coating and interlayer. In this study, two nondestructive stress measurement techniques were applied to obtain the residual stress in the entire bilayer specimen and the individual layers. The wafer curvature method was used to measure the bulk average stress of the specimen and a newly developed average X-ray strain (AXS) technique combining with nanoindentation was adopted in accurately determining the stresses in ZrN thin film and Zr interlayer, respectively. From the bulk average stress of the entire specimen and the stresses in ZrN coating and Zr interlayer, the extent of stress relief by Zr interlayer could be quantitatively revealed. The results showed that the Zr interlayer could significantly relieve the residual stress of the upper ZrN thin film, whereas delamination was found to occur in more than 50% of samples Z12, Z22, Z23 and Z33. Therefore, adding a Zr interlayer could not improve the adhesion of the top ZrN film. The delamination was due to the stored energy exceeding the interfacial fracture toughness of the ZrN/Zr interface. The interfacial fracture toughness was much lower than the fracture toughness of the monolayer ZrN thin film, and estimated to be about 2.8 J/m2. The residual stress of the film was an indirect factor for the delamination of bilayer specimen. The major factor that determined the adhesion of the bilayer ZrN/Zr coating was the interfacial fracture toughness of the ZrN/Zr interface.