Testing the adhesion of hard coatings including the non-destructive technique of surface acoustic waves

摘要

Direct measurement of the adhesion of surface coatings is still a challenge. Surface engineers, however, desperately need practical quantities to evaluate the adhesion of their coatings, since this property essentially determines the applicability of their products in industrial practice. Several adhesion test methods came into use for this reason such as the scratch test, bending test, Rockwell test, cavitation test, and impact test. These techniques gained acceptance due to their easy application. The conflicting results often reported by such tests gave reason to perform a comparison of these techniques by utilizing them to test the adhesion of TiN hard coatings on steel (film thickness 1.2 ...2.4 μm). The films were deposited by ion plating. Adjusting the duration of an in-situ cleaning process (argon ion pre-sputtering) modified the interface conditions for the subsequent film deposition. The measured results from the various standard tests were investigated with regard to their correlation with the pre-sputtering time. Several test parameters of these methods were used to evaluate the adhesion: the friction work, acoustic emission activity and critical load of the scratch test, the critical strain and the defect density of the four-point bending test, the proportional damage area of cavitation test, the critical number of loading cycles of the impact test, and Young's modulus of the film as measured with the laser-acoustic method. For the majority of these parameters, a significant correlation with the pre-sputtering time was found, but while some of them indicated the expected improvement of the adhesion with increasing pre-sputtering, other parameters suggested an opposite effect. Summarizing these surprising results revealed that the evaluation of the film quality differed for local and global adhesion behaviors. An interesting aspect of the non-destructive adhesion testing was the utilization of laser induced surface acoustic waves. The laser-acoustic technique is gaining acceptance as a quick and robust technique for determining Young's modulus of thin films with thickness down to nanometers. The results demonstrate the promising possibility of laser acoustic method for fast, non-destructive and inexpensive evaluation of the adhesion behavior.