The use of scratch adhesion testing for the determination of interfacial adhesion: The importance of frictional drag

For many years, despite a basic lack of understanding of the mechanics on which the test is based, the scratch adhesion test has provided a simple, rapid means of assessing the adherence of thin, hard wear-resistant coatings such as titanium nitride. Recently, some progress has been made on the modelling of the scratch adhesion test and these previously published models are critically reviewed. The test can be considered as a combination of (i) an indentation stress field, (ii) an internal stress field and (iii) a frictional stress field. The importance of each of these in determining the levels of adhesion for a number of titanium nitride/substrate systems will be discussed. In particular, the results obtained from changes in diamond stylus-coating (surface) interfacial friction (a frictional drag term) are presented and the apparent changes in coating-substrate adhesion (critical load Lc) which result are discussed.