Modeling of Indentation Damage in Single and Multilayer Coatings

Abstract

In many coating applications damage resistance is controlled by the mechanical properties of the coating, interface and substrate. As coatings become thinner and more complex, with multilayer and graded architectures now in widespread use, it is very important to obtain the mechanical properties (such as hardness, elastic modulus, fracture toughness, etc.) of individual coating layers for use in design calculations and have failure-related design criteria which are valid for such multilayer systems. Nanoindentation testing is often the only viable approach to assess the damage mechanisms and properties of very thin coatings (< 1 µm) since it can operate at the required scale and provides fingerprint of the indentation response of the coating/substrate system. Finite element analysis of indentation load displacement curves can be used to extract materials properties for design; as coating thicknesses decrease it is observed that the yield strength required to fit the curves increases and scale-dependent materials properties are essential for design. Similarly the assessment of fracture response of very thin coatings requires modeling of the indentation stress field and how it is modified by plasticity during the indentation cycle. An FE approach using a cohesive zone model has been used to assess the locus of failure and demonstrates the complexity of adhesive failure around indentations for multilayer coatings.