Adhesion testing by the scratch test method: The influence of intrinsic and extrinsic parameters on the critical load

Abstract

The critical load determined by the scratch test is widely regarded as representative of coating adhesion. Depending on the coating-substrate system to be measured, the critical load can be detected by acoustic emission, and/or by optical or scanning electron microscopy and/or by the variation of the tangential frictional force applied to the sample. It remains, however, difficult to express quantitatively the adherence because the critical load depends on several parameters related to the testing conditions and to the coating-substrate system. Both intrinsic parameters, such as scratching speed, loading rate, diamond tip radius and diamond wear, and extrinsic parameters, such as substrate hardness, coating thickness, substrate and coating roughness, friction coefficient and friction force, are considered in order to improve the interpretation of the critical load results. As well as the experimental relations between critical load and these different parameters, the results of a theoretical approach are also presented. The deformations and stress distributions near the interface caused by the static indentation of a spherical point have been simulated by the finite element analysis. Despite the fact that these calculations have been made in the static mode, they permitted a better understanding of the failures observed in and near the scratches; this represents a justification for the use of a scratch test to characterize the mechanical resistance (adhesion and cohesion) of a coating deposited on a tough substrate.