An overview of the potential of quantitative coating adhesion measurement by scratch

The scratch test has been used to assess the adhesion of thin hard coatings for some time now and is a useful tool for coating development or quality assurance. However, the test is influenced by a number of intrinsic and extrinsic factors which are not adhesion-related and the results of the test are usually regarded as only semi-quantitative. The stress state around a moving indenter scratching a coating/substrate system is very complex and it is difficult to determine the stresses which lead to detachment. Furthermore, the interfacial defect state responsible for failure is unknown. However, by a careful analysis of the observed failure modes in the scratch test (not all of which are related to adhesion) it is possible to identify adhesive failures and in some cases these occur in regions where the stress state is relatively simple and quantification can be attempted. Ideally engineers would like a material parameter (such as work of adhesion or interfacial toughness) which can be used in an appropriate model of the coating-substrate system stress state to determine if detachment will occur under the loading conditions experienced in service. This data is not usually available and the development of such models must be seen as a long-term goal. In modern indentation and scratch systems the work of friction (or indentation) can be directly measured and the relationship between this parameter and adhesive failure can be demonstrated in some cases. This paper reviews the main adhesion-related failure modes and the stresses responsible for them and indicates where quantification is possible illustrating this with results from hard coatings on steel, thermally grown oxide scales and optical coatings on class. The use of empirical calibration studies, directly measured work of friction and quantification by finite element methods is discussed. (c) 2005 Elsevier Ltd. All rights reserved.