Mechanical properties of a graded B-C-N sputtered coating with varying Young's modulus: deposition, theoretical modelling and nanoindentation

New design concepts for coatings are a promising way to improve the protection of surfaces against mechanical damage. For maximal load carrying capacity, the coating should not only be a single layer but an optimised layer stack or a coating with graded mechanical properties. The successful structural and mechanical characterisation of such a coating is the key for its optimisation. However, commonly used characterisation methods such as nanoindentation with the analysis of the force-displacement curve can only consider a coated substrate with one or only very few layers. Applying these methods to coatings with variable Young's modulus will result in inaccurate or even incorrect results. Therefore, a new approach is needed. We report on the deposition of a coating with graded mechanical properties by sputtering of materials of the ternary system B-C-N and the characterisation of this coating by nanoindentation with Berkovich and spherical indenters. A theoretical model was used to handle the contact problem of such a graded coating for purely elastic indentation with a spherical indenter. It will be shown that a graded coating cannot clearly be distinguished from a homogenous layer by elastic indentation with only one spherical indenter. However, a thorough investigation of a graded coating with a variety of properly chosen different spherical indenters does not only reveal the true gradient character of the coating but also allows the determination of its concrete structure.