Scratch Adhesion Testing of Thick HVOF Thermal Sprayed Coatings

The cohesion bond strength of two different families of HVOF coatings was evaluated by cross-sectional scratch testing at multiple loads, according to the ISO working draft (ISO/WD 27307). Diamalloy 3001 coatings with and without regions of enhanced oxidation were investigated. Stellite 6 coatings with and without Cr3C2 carbide phases were also observed. The effects of oxidation and composition on the cohesion bond strength of these metallic and composite HVOF coatings were analyzed. Cross-sectional scratch testing revealed to be a useful qualitative tool for comparing the cohesion strength of different thick HVOF coatings, as well as identifying their failure mechanisms and observing their relationship to the coating microstructure and composition. Indeed, in both coating families tested here, the presence of ceramic compounds was linked to a decrease in the overall cohesion strength of the coating. Cracking and/or microcracking in these brittle ceramic phases was observed and is the likely mechanism behind the lower overall cohesion strength in the coatings. Splat boundary failure occurred preferentially at heavily oxidized splat boundaries in the Diamalloy 3001 coatings. Cracking within the Cr3C2 phase of the Stellite 6 composite coating significantly decreased the cohesive bond strength at high scratch loads.