Substrate and elastic recovery effects in hardness measurement of CVD WC-based coatings

Abstract

Hard coatings are used in a very broad range of applications, for example in the automotive, tooling and gas industries. The apparently simplest way to compare different coatings is to measure their hardness. However, interpreting such measurements is not a trivial problem for thin coatings (10 μm or less); standard micro-indenters (with a typical minimum applied load of 10gf) usually produce a penetration depth such that the substrate influences the measurements, giving a ‘composite’ hardness value of the coating and the substrate. At the same time there might be a more or less evident recovery of the indentation, because of residual elasticity around the sub-indentation plastic zone or because of residual stresses. One possible solution is the use of a nano-indenter. An alternative method for obtaining ‘true’ Vickers hardness values for the coating is described here, using changes in shape of Knoop micro-indentations to characterize the elastic recovery effects, together with use of a composite model for thin film microhardness. Results of hardness testing on thin WC-based coatings, using nano-indentation, Vickers and Knoop indenters, are compared.

Acknowledgements

The authors would like to thank the company Hardide Ltd (Unit 11, Wedgwood Road, Bicester OX26 6UL, UK), which funded the research. We also thank Dr A. B. Mann for assistance with the nano-indenter experiments, which were carried out at the Manchester Materials Science Centre, University of Manchester and UMIST.

Notes

 Copyright by Daniel Hyams; http://curveexpert.webhop.biz/