Pedro Avila, Martin Crouan, Oleg Zabeida, J.E. Klemberg-Sapieha, Ludvik Martinu, Polytechnique Montréal, Montréal, Québec, Canada
Mechanical stress developing in thin films during their deposition and their subsequent use plays a critical role in the performance and durability of the coated system. Indeed, excessive stress developed during film deposition can impact mechanical properties and lead to complete failure. Therefore, thorough understanding of the mechanisms leading to stress generation and their dependence on processing parameters and the effects of the working environment is fundamental for achieving the required characteristics and durability of new structures and nanomaterials, which may consist of single layers, multilayers, and micro- and nanocomposites.
In this talk, we discuss the different techniques used to evaluate stress evolution during film growth as well as stress dynamics when the coating system is exposed to harsh environments such as high temperatures and high humidity. We particularly focus on two case studies:
1) We closely follow the growth of TiAlN films with in-situ substrate curvature measurements. This allows one to track the stress development and its variation following changes in the deposition conditions, particularly the substrate bias. Combining this analysis with ex-situ techniques such as crystallographic Cos2α*Sin2ѱ evaluation allowed us to distinguish between three competing mechanisms contributing to the stress evolution, namely grain boundary closing, atom insertion and defect generation. The input of each of these mechanisms is shown to greatly depend on multiple parameters including temperature, ion bombardment and adatom mobility to name a few.
2) We evaluate the stress dynamics of multilayer optical filters during a heat-treatment procedure, as exposure to high temperatures can induce complex structural and compositional changes which ultimately affect the overall stress level. A detailed investigation of the stress evolution of each individual material included in the multilayer architecture allowed us to both predict the changes in the stress distribution throughout the filter at different temperatures, as well as calculate the shear stress at each interface. Such an approach can be used to determine which interface is the most prone to delamination and, taking into account this risk analysis, design durable optical filters accordingly.