The basic principle of the scratch test is that an indenter of known geometry (commonly a spherical diamond) is dragged across a surface with either constant or progressive loading. In the case of coatings, a progressive load is usually used for generating an increasing stress until failure events occur at specific points along the scratch track, such critical failure points (or critical loads, Lc) being observed by subsequent optical microscopy, variations in frictional force, acoustic emission or changes in penetration depth.
Not all the observed failure events in a progressive load scratch test are related to delamination at the coating-substrate interface and only certain, therefore, can be truly used as a measure of adhesion. Other types of failure, such as cohesive damage within the coating or substrate, may be equally important in evaluating the in-service behavior of a coated component in a specific application. In many cases, the scratch test has now become accepted as a versatile tool for assessing the mechanical integrity of a surface, whether bulk or coated, and has found application in many different fields of materials research and industrial R&D. In general, it is most effective if the substrate does not deform plastically to any great extent. In such cases the coating is scraped off the surface and the load required to do this gives some measure of adhesion, although it is not a simple analysis and therefore a quantitative number is difficult to obtain. For coating-substrate systems that have different friction coefficients, it is often possible to detect a frictional force change when the indenter penetrates through the coating and arrives at the substrate.
The Alemnis Standard Assembly (ASA) can be used for scratch testing in various different configurations and has the advantage of in situ SEM observation of the scratch as it develops in real time. This is particularly useful for understanding scratch deformation phenomena, such as whether cracking is occurring in front of the advancing indenter by compression, or behind the indenter by tension. It is also possible to directly observe any viscoelastic relaxation that might occur in the deformed material immediately after load removal.
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