Can the scratch adhesion test ever be quantitative?

摘要

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 always 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 find some which 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. These data are not usually available and the development of such models must be seen as a long term goal. It must be questioned whether the scratch test can ever give quantitative information suitable for this modelling analysis. This paper highlights 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 and thermally grown oxide scales. The use of empirical calibration studies and quantification by finite element methods is discussed.