Nano-impact test on a TiAlN PVD coating and correlation between experimental and FEM results

Nano-impact test on PVD coatings is an efficient method for investigating film failure mechanisms. During this test, the coating is subjected to repetitive impacts by a diamond indenter, inducing high local deformations and stresses into the film material, which may lead to coating failure. In the paper, coated specimens with a TiAIN PVD film were investigated by nano-impact tests. The nano-impacts were conducted at several loads and for various test durations. For explaining the attained results, the nano-impact test was simulated by a developed three dimensional finite elements method (FEM) model, considering a piecewise linear plasticity material law. The employed software was the LS-DYNA package; its feature of constrained tied nodes failure was used for simulating crack formation and propagation, as the plastic strain develops and exceeds the coating failure strain. The film elasto-plastic properties, used in the FEM-calculations, were determined by nanoindentations and analytical evaluation of the related results. During the nano-impact indenter penetration, it was assumed that the coating material at the FEM model node regions can withstand the applied load up to a maximum value, which corresponds to the coating rupture stress. Over this load limit, the related nodes are disconnected from the neighboring finite elements. If all nodes of an element are disconnected, the element is released for simulating a crack formation and it becomes an inactive separate entity. In this way, the stress fields developed in the film material and its coating fracture progress in terms of imprint depth versus the repetitive indenter penetrations are analytically described. The attained results converge sufficiently with the experimental ones. The developed nano-impact FEM-simulation predicts the film failure initiation and evolution, which depend on the impact load. (C) 2011 Elsevier B.V. All rights reserved.