Analysis of the effect of antiwear CrN coating thickness on the evolution of thermomechanical interactions in the substrate/PVD coating system

The main goal of this study was to analyze the application possibilities of modulated temperature thermomechanical analysis (MT TMA) for the evaluation of thermomechanical interactions in substrate/PVD coating systems on the example of examination of influence of changes in thickness of a coating on internal stress evolution in these systems. As an indicator of the stress state in the coating at a given measurement temperature, the changes in a value of the equivalent thermal expansion coefficient of the system alpha(AC), which is calculated based on registered changes in temperature and elongation of the sample, were assumed. Independently, numerical simulations of the residual thermal stresses were carried out using finite element method. The objects of research were systems composed of substrates of HS6-5-2 steel and a single layer of chromium nitride CrN with thickness 2, 4, 8 and 10 mu m, deposited on the substrate by cathodic arc evaporation method. In order to determine the effect of the CrN coating thickness on evolution of thermomechanical interactions in the system, the samples were annealed according to the assumed curve comprising nine-step isothermal annealing in the range of 150-500 degrees C. Obtained results have shown that MT TMA is useful to study the thermomechanical interactions in substrate/PVD coating systems. In particularly, obtained values of alpha(AC), calculated on the basis of measured elongation changes for substrate/PVD coating systems, are lower than for the substrate without coatings, and they increase with the thickness of the deposited coating. This means that there is a decrease in compressive stresses in the coatings with increasing coating thickness. It was also found that with increasing temperature there is a local maximum value of alpha(AC), which indicates the temperature of deposition of the coating. This is a key parameter from a technological point of view of the PVD processes.