Design, development and characterization of stress-optimized Ti-Tin multilayer PVD-Coatings

Abstract

Residual stresses in the PVD-coatings usually deteriorate the adhesion of coatings and adversely affect the performance of coated components. Nonetheless, it might have some positive effects such as resistance to crack nucleation and propagation, as well as wear and fatigue failures. A proper control of residual stress always remains essential for successful coating development. Multilayer systems with alternate hard (ceramic) and soft (metallic) layers compared with a monolayer, usually offer a much more ease to control the residual stresses, improve adhesion, increase overall thickness and produce the toughening response. However, in order to control the residual stresses, it is always important to find the optimal thickness of individual layers. An increase in metallic layer thickness significantly relieves the residual stress in ceramic layer. However, the performance of multilayer systems could be affected. Alternatively, an increase in thickness of ceramic layer close to the substrate could mostly increase the residual stress level. Hence the thickness of individual layers remains a key factor for the optimal performance of multilayer coating systems. In the present work, finite element analysis (FEA) of residual stress coupled with ANSYS optimization algorithm was used to develop stress-optimized Ti-TiN multilayer coatings. Thickness of individual layer was optimized for the coating configuration comprising of sixlayer while taking into account the thermal as well as thickness dependent intrinsic residual stresses. Multilayer coatings corresponding to those of FEM stress-optimization were experimentally produced in comparison with bilayer using magnetron sputtering physical vapour deposition system. The nanoindentation hardness and elastic modulus of multilayer coatings in comparison with bilayer was investigated for the assessment of deterioration in stiffness taken place by the incorporation of Ti (titanium) interlayers. Further, investigations of the stress-optimized multilayer configurations were performed for the influence on in-plane residual stress and practical scratch-adhesion. Analytical description of the failure mode under scratch adhesion testing was demonstrated for an accurate measurement of practical adhesion of coatings to the stainless steel substrate. Finally, the scratch adhesion, nanoindentation and experimental in-plane residual stresses results clearly demonstrated the significance of preliminary stress-optimization measures for the development of Ti-TiN multilayer wear-resistance coating systems. The dissertation consists of six chapters. Chapter 1 is about the introduction in which Gap of knowledge, significance of the investigation and objective, approach and scope of investigation is described. Chapter 2 presents a literature review on the evolution of residual stress in magnetron sputtering PVD-coating and their influence on failures in coating. The multilayer approach to control the residual stress and procedure adopted so far for the stress-optimization of multilayer coating architecture. Chapter 3 is about the basic details about the modelling activities used in the present studies to design stress-optimized multilayer coating configuration and in Chapter 4, experimental and characterization techniques are described in details. Chapter 5 summarizes the modelling and experimental results in three sections. In the first section results of finite element based design of stress-optimized Ti-TiN multilayer coating is summarized. In the second section, influence of Ti-TiN multilayer coating design on the mechanical properties and practical scratch adhesion are described in detail. The third section is about the influence of coating design on the in-plane residual stress. Finally, the thesis is summarized in Chapter 6 with the contributions and the remaining interesting tasks.