Adaptive Protocol for Robust Estimates of Coatings Properties by Nanoindentation
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Adaptive Protocol for Robust Estimates of Coatings Properties by Nanoindentation Nigel M. Jennett1 and Andrew J.Bushby2. 1
National Physical Laboratory, Materials Centre, Teddington, Middlesex, TW11 0LW, UK. Department of Materials, Queen Mary, University of London, London E1 4NS, UK.
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ABSTRACT The determination of the elastic modulus and hardness of a wide range of coating systems has been studied recently in the EC project INDICOAT. This paper describes a protocol for determining the coating properties, which has been developed and tested in that project. The procedure contains simple to implement strategies to evaluate the response of the coating and design a suitable series of indentation experiments that enable a reasonable estimation of ‘coatingonly’ properties. The protocol directs how the experimental design adapts the experimental parameters to each sample. An adaptive protocol is essential to cope adequately with the different indentation responses. Indentation response depends, for example, on coating thickness and the relative properties of coating and substrate and creep response. The protocol also has to adapt itself so that it can reliably target the range of indentation depth with respect to the coating thickness necessary to obtain the coating properties from the composite indentation response. Results presented show that the parameters and approach for measurement of hard coatings are very different to those required for soft or ductile coatings. The systems studied are DLC on tool steel, Au on Ni, aluminium oxide on Ni and aluminium on optical glass (BK7). INTRODUCTION Nanoindentation is one of the few methods capable of measuring both the elastic and plastic properties of small volumes of material such as coatings. The technique is playing an increasing role in the characterisation of the mechanical properties of thin layers used in surface technology, microelectronics, micromechanics, optics etc. However, indentation into thin (less than about 2µm thick) coatings, or at high loads, is unable to determine coating properties such as hardness and Young’s modulus directly. The measured indentation response is not a pure coating response but contains a component from the substrate. The growing need for the measurement of coating properties has meant that a number of “rules of thumb” have evolved that suggest various maximum indentation depths within which it is possible to obtain a ‘coating-only’ response. The most widely applied of these is the so called “10% rule”, which was created for coating hardness determination and has gained such popularity as to be regularly misapplied to modulus measurement. The actual limits of ‘coating-only’ response are determined by parameters such as: the indenter geometry, coating thickness and the ratio of the mechanical properties of coating and substrate. The aim of the EC project “INDICOAT” was to find a general and robust indentation method for determination of coating properties that could use different indenter geometries and adapt to the indentation respo
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