A review of adhesion test methods for thin hard coatings

Adhesion to the substrate is the most important property of a coating. Previously unattainable adhesion levels can be achieved by modern physical vapor deposition and chemical vapor deposition techniques. There is an urgent need to develop standard techniques of adhesion evaluation, to ensure user confidence and promote the widespread use of coating methods. An adhesion test for hard tribological coatings should be easy and quick to carry out and the results should be easy to interpret. The test should be practicable for real tools and machine parts. Special sample preparation should not be required. There are at least nine different basic techniques for adhesion assessment: pressure sensitive tape test, acceleration (body force) testing, electromagnetic stressing, shock wave testing, tensile and shear testing, laser techniques, acoustic imaging, indentation tests, and scratch testing. These methods are discussed in this paper in the light of the requirements mentioned above. In the author’s view only laser techniques, acoustic imaging, and the scratch test are practical for evaluating the adhesion of hard coatings on tools and machine parts. Of these, the scratch test has the widest commercial use at the moment, although there still exist some phenomena which are not well understood. For example, variations in the friction between the indenter and the coating can affect the test result. However, the effect of different parameters, such as substrate hardness and coating thickness, are well understood, at least empirically.