Evaluation of stress and energy relief efficiency of ZrN/Ti and ZrN/Zr

The purpose of this study was to investigate the extent of stress relief (Delta sigma%) and energy relief efficiency (xi tot) of metal interlayers in bilayer ZrN/Zr and ZrN/Ti thin films using an energy balance model based on the concept that the stored elastic energy in the ZrN coating could be partly relieved by the plastic deformation of metal interlayer. ZrN/Zr and ZrN/Ti specimens with different thicknesses of metal interlayers ranging from 100 to 300 nm were deposited on Si substrate using unbalanced magnetron sputtering. ZrN/Ti specimens were also deposited at different substrate bias voltages for investigating the energy relief efficiency by Ti interlayer under different stress levels in ZrN. The metal interlayers, Zr and Ti, were selected due to their different strain hardening behavior. Laser curvature method (LCM) and average X-ray strain combined with nanoindentation technique were employed to accurately measure the residual stresses in the entire specimen and the ZrN coating, respectively. Experimental results showed that Delta sigma% was more significant by Ti interlayer than by Zr interlayer. Delta sigma% of the entire specimen increased with interlayer thickness, while decreased with increasing stress of the top ZrN layer. The energy balance model was employed to evaluate xi tot by the interlayers. The contributions of energy relief from the ZrN coatings (eta ZrN) and bending curvature relaxation of Si substrate (eta Si) could be separately assessed by the model. The results revealed that xi tot decreased by increasing Zr or Ti interlayer thickness, where the energy relief was mainly contributed from eta Si for ZrN/Zr specimens, while eta ZrN was substantial for ZrN/Ti specimens, and there was an optimum interlayer thickness close to 100 nm, by which xi tot could reach up to 79.0 and 83.1% for Zr and Ti interlayers, respectively. For the ZrN coatings with different stress level ranging from -2.87 to -3.94 GPa, the variation of xi tot was less distinct, ranging from 39.1 to 48.8%, and the energy relief was mostly from eta ZrN, except for sigma ZrN close to -4 GPa, where the energy relief was mostly from eta Si. Although xi tot could serve as a better measure than Delta sigma% in evaluating the effectiveness of a metal interlayer, eta ZrN and eta Si are the critical indexes that could reveal the actual energy relief in the hard coating. It is found that eta ZrN and eta Si are related to the strength coefficient (k) of the interlayer. Therefore, in selecting a metal interlayer, it should be considered not only the capacity of energy absorption (Wp,max) but also the malleability (k) of the metal.