Design, development and characterization of stress-optimized Ti-TiN multilayer PVD-coatings
Residual stresses in PVD-coating deteriorate the adhesion of coatings and adversely affect the performance of coated components. Although it might have positive effects such as resistance to cracking, wear and fatigue failure. A proper control of residual stress is essential for successful coating development. Multilayer system with alternate hard (ceramic) and soft (metallic) layers in comparison with monolayer, offer much more ease: to control residual stresses, improve adhesion, increase overall thickness and produce the toughening response. However, in order to control the residual stresses, it is important to find the optimal thickness of individual layer and especially the thickness of metallic layer. An increase in metallic layer thickness significantly decrease the functionality of multilayer, hence the metallic layer thickness is a key factor for the optimum performance of multilayer coatings.
In the present work, finite element modeling (FEM) of residual stress analyses coupled with ANSYS optimization algorithm was used to design stress-optimized Ti-TiN multilayer coatings. The currently available stress optimization models did not take into account the intrinsic residual stresses, hence in the present study, we optimize individual layer thickness in the multilayer configuration while taking into account both intrinsic residual stresses as well as thermal stresses. The flow chart of the whole optimization procedure is summarized in the figure.
The framework adopted for multilayer coating design
Finally, multilayer coatings corresponding to those of FEM are experimentally produced using magnetron sputtering physical vapor deposition system. The coatings are characterized for their adhesion, in-plane residual stress, hardness and elastic-modulus. The results show significant improvement in adhesion of stress-optimized multilayer coatings. The developed optimization procedure can be used to design interfacial stress optimized multilayer coating comprising of many layers. These findings were published in international conference on surface modification technologies, XXVIII (2014) 185-193 and the recent work is accepted for publication in a peer-reviewed journal. http://dx.doi.org/10.1016/j.matdes.2015.03.007