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. (c) 2004 Elsevier B.V All rights reserved.