In the scratch test, the most commonly used method for evaluating the adhesion of thin surface coatings, the coated surface is scratched by a conical diamond indenter as load is gradually increased to the critical value Lc, at which the coating fails. As local deformation and failure occur in the material, some of the energy stored in the coating is released as acoustic emission.
This paper analyzed the acoustic emissions while scratch testing high-speed tool steel (SKH 51) that was TiN-coated by PVD method. It was found that acoustic emissions were detected as an event rate signal when cracking of the coating occured, and that the average amplitude of the signals suddenly increased when chipping or spalling failure occured. It was concluded that the analysis of the average amplitude of signals was an effective means of estimating Lc, the value of which depended on the thickness both of the TiN coating and of the Ti undercoating.