A comparative study of adhesion test methods for hard coatings

Abstract

The feasibility of seven techniques for testing and evaluating the adhesion of thin films was investigated: scratch test, four-point bending test, Rockwell test, cavitation test, impact test, laser-acoustics and acoustic microscopy. The studies were performed with TiN films (thickness 1.2–2.45 μm) deposited on the annealed steel 42CrMo4. The adhesion of the TiN films was varied by varying the time of pre-sputtering the steel substrate with argon ions before the film deposition. Argon pre-sputtering for 15 min is recommended to guarantee an optimal adhesion. The pre-sputtering time, tS, was reduced down to 0.5 min to reduce the adhesion. The following test parameters 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 measured with the laser-acoustic method. These test parameters were examined for any correlation with the pre-sputtering time, tS. The results are summarized in a table that shows which test methods and test parameters yield corresponding and contradictory evaluations of the film quality. The effect of defect density and residual stresses is discussed. Laser-acoustics and acoustic microscopy are non-destructive methods. Young's modulus, as measured by laser-acoustics, is sensitive to the density of micro-defects. This is expected to indicate their effect on the adhesion.