Characterization of surface coatings by the scratch adhesion test and by indentation measurements

Summary

Functional coating and surface modified layers can be characterized by engineering related criteria such as the ultramicrohardness test and the scratch adhesion test. The ultramicrohardness test is based on penetration depth measurements during a loading and unloading cycle. The depth sensing indentation provides a means of evaluating the elastic and plastic deformations of the tested material, from which, by calculation, the microhardness, HV, and the elasticity modulus, E, can be derived as illustrated on a number of examples of hard and soft materials and coating composites. The scratch test (REVETEST^R) and microscratch test (MST^R) has been applied to determine the adhesive and/or cohesive strength of coatings deposited on softer or tougher substrates. A Rockwell C diamond cone is used to scratch the surface of the coated or surface-modified specimen at a constant speed and under a continuously increasing load. The smallest load at which the coating is damaged, called the Critical Load, L_c, is determined by optical, electron optical, acoustic emission detection and/or by frictional force measurement, i.e. by the sudden increase of the driving force. This is illustrated on hard, brittle, and on soft coatings.