A simple method for the assessment of the contact modulus for coated systems

Abstract

This paper describes a simple method to determine the elastic modulus of a coating on a substrate using nanoindentation based on the load support of a truncated cone of material beneath the indenter. The cone angle is shown to be 32.48° in all cases. The effect of the coating on the measured contact modulus increases more rapidly as the contact scale is reduced. The model describes the behaviour of a range of coating substrate systems very well, but deviations are observed in cases where pile-up or tip end-shape variations affect the experimental data. It is possible to use a fit of a simplified form of the model to experimental data for coatings on stiff substrates to determine the elastic properties of the coating independent of the substrate. The model can be generalized for multilayer coatings; this is essential when compliant coating layers are sandwiched between stiffer layers. The model shows that the mechanical properties of thin stiff coatings on compliant substrates cannot be successfully determined from indentation data. The ISO14577 extrapolation method should therefore not be used in such circumstances.

Keywords:

• nanoindentation
• coating
• elastic properties
• Young’s modulus

Acknowledgements

The author would like to thank the Royal Academy of Engineering and the Leverhulme Trust for provision of a Senior Research Fellowship which allowed him to complete this work.