The effect of intrinsic parameters on the critical load as measured with the scratch test method

Abstract

In the scratch test method, scratches are generated on the coated sample using a diamond indenter (usually a Rockwell C profile) which is drawn across the surface under either constant or progressively increasing load. The sample is displaced at constant speed and at a certain load damage occurs along the scratch path. This value of critical load, L c, can be used to accurately characterise the adhesive strength of the coating–substrate system. With modern scratch testing instruments, the critical load can be determined by acoustic emission, optical microscopy, variation in penetration depth or variation in the tangential frictional force between tip and sample. However, it is difficult to express the adherence of a coating–substrate system in a quantitative way because the critical load depends on several parameters related to the testing conditions. Experience in the scratch testing field has shown that various different radii of diamond indenter are often required in order to adequately characterise the adhesion of modern thin films and coatings. The constant reduction in thickness of coatings, as well as the increase in development of softer (polymeric) coatings has meant that a whole range of indenter geometries are needed to cover the scope of elastic and plastic properties measured by scratch testing. This paper considers the effect of intrinsic parameters such as scratching speed, loading rate and diamond indenter radius on the measured critical load for scratch tests performed on various different coating–substrate combinations (TiN, W, DLC, Al and Au). Results are presented which suggest definite correlations between critical load and indenter radius, and which should aid users in making the right choice of indenter for a given coating–substrate system.