On the evaluation of adhesion of coatings by automatic scratch testing

Abstract

Automatic scratch testing is an expedient technique for comparatively evaluating the cohesive failure load and adhesion failure load of thin coatings on various substrates. In combination with SEM examination of the scratch track, this technique has been used herein to detect and evaluate various effects on coating strength and adhesion. For soft Triballoy T-800 and Stellite SF-6 cobalt-base coatings on 4340 low alloy steel, adhesion was found to be strong and failure was found to be cohesive in the coating. In the presence of a plated chromium interlayer, pre-existing cracks lowered substantially the cohesive failure load, which was also lowered by an increase in the coating deposition pressure. The spacing of transverse cracks within the coating was found in all cases to decrease with increasing applied normal load. In soft aluminum coatings on depleted uranium (DU)-0.75% Ti alloy specimens, alloying aluminum with magnesium or zinc enhanced the coating strength and adhesion. In (Al-Mg) coatings on this substrate, a smoother surface led to a lower friction coefficient and a higher adhesion failure load. In hard, thin TiN coatings on 17-4 PH steel, a lower bias voltage applied to the substrate yielded higher cohesive and adhesion failure loads. In hydrogenated amorphous SiC thin coatings on 4340 steel, loss of hydrogen by annealing converted the residual compressive stresses into tensile stresses and lowered both the cohesive and the adhesion failure loads. Finally, automatic scratch testing proved helpful in determining delamination loads in multilayer TiN/Ti/TiN coatings on DU-0.75% Ti alloy.

Keywords:

• Cohesive failure load
• adhesion failure load
• friction coefficient
• automatic scratch test
• cobalt-based coatings
• aluminum-magnesium coatings
• TiN coatings
• SiC coatings
• depleted uranium substrate
• coated 4340 steel