A closed-form analytical approach for the simple prediction of hard-coating failure for tooling systems

Hard coatings are used extensively to protect tools in forming, forging and milling operations. While modern deposition of hard coatings using plasma methodologies has opened industry up to a new range of potential coating systems, challenges remain regarding coating optimization under specific conditions and environments. In the presented work a previously developed contact mechanical failure map, based on analytical solutions for a coating/substrate system under a contact load, was extended to include a thin-plate analytical solution for plastic deformation of a substrate, giving improved alignment with finite element solutions when compared to the Boussinesq solution utilized in literature. Analytical solutions were applied to aluminium chromium nitride based ceramic coatings, which have strong potential application in cutting tools and other contact-based applications, and were verified using finite element simulations. The failure map was investigated experimentally using scratch-testing, and it was found that cohesive coating failure initiated at forces predicted by the model for the ceramic coating, taking into account both the mechanical properties of the coating and substrate. An industry standard functional hard chrome coating was then tested, and it was found that cohesive failure initiated at loading conditions also predicted by the analytical model. The results show that such closed-form analytical modelling is a quick and versatile tool for industrially-relevant hard-coating systems, and could be utilized during the coating design process for parameter and material justification or combined with micro-mechanical testing, including high-temperature testing, to predict thin-film and coating failure in harsh industrial environments.