Evaluation of stress and elastic energy relief efficiency in a hard coating with a metal interlayer—Using TiN/Ti as a model system

This study measured the stress relief extent of a hard coating by a metal interlayer in a bilayer system. An energy-balance model was used to evaluate the energy relief efficiency (ξ_tot) by the interlayer. The objective of this study was to understand the relationship between plastic deformation and the energy relief efficiency of the metal interlayer in a bilayer thin film system. A TiN/Ti bilayer thin film was chosen as the model system. TiN/Ti samples were prepared with different interlayer thicknesses and under different stress levels in TiN coating using unbalanced magnetron sputtering. The overall stress of the bilayer samples was determined by the laser curvature method, and the stress in the individual layer was measured by average x-ray strain combined with nanoindentation method. For the TiN/Ti sample with Ti interlayer thickness >78 nm, a maximum ξ_tot was reached at an interlayer thickness about 110 nm; further increasing the interlayer thickness may decrease ξ_tot. This was mainly due to plastic deformation of the Ti interlayer being localized near the TiN/Ti interface. The results also showed that ξ_tot increased with increasing stress in the TiN coating. The model analyses revealed that the energy relief was mostly contributed from the TiN coating, while less than 30% was from curvature relaxation of the Si substrate. For the sample with insufficient thickness (52 nm) of an Ti interlayer, the stress of the TiN coating could not be effectively relieved and the interlayer was subjected to compressive stress. In this case, the energy-balance model was not valid, while our previous elastic model could be used to account for the stress state transition. The residual stress state of the Ti interlayer can serve as an index to assess the effectiveness in relieving film stress by the interlayer. The interlayer is functioning by sustaining tensile stress, whereas it is ineffective if the interlayer is subjected to compressive stress.