Effects of Residual Stress on Nano-Mechanical Behavior of Thin Films

In this chapter, we present an overview of an optimized method for the determination of surface elastic residual stress in thin ceramic coatings by instrumented sharp indentation. The methodology is based on nanoindentation testing on focused ion beam (FIB) milled micro-pillars. Finite element modeling (FEM) of strain relief after FIB milling of annular trenches demonstrates that full relaxation of pre-existing residual stress state occurs when the depth of the trench approaches the diameter of the remaining pillar. Under this assumption, the average residual stress present in the coating can be calculated by comparing two different sets of load-depth curves: the first one obtained at the center of stress-relieved pillars, the second one on the undisturbed (residually stressed) surface. The influence of substrate’s stiffness and pillar’s edges on the indentation behavior can be taken into account by means of analytical simulations of the contact stress distributions. Finally, the effect of residual stress on fracture toughness and deformation modes of a TiN PVD coating is analyzed and discussed here.