Degradation of Coating Adhesion Measured by AFM Scratching

Organic coatings are widely used to provide corrosion protection to metal substrates.  However, these coatings can degrade as the result of environmental exposure.  One effect of the environment is degradation of coating adhesion.  If the adhesive strength of a coating decreases to the extent that there is general loss of adhesion, the corrosion protection can be seriously compromised.  Sensitive measures of coating adhesion to assess early stages of adhesion loss are not available.  In this work, AFM scratching is being developed as a sensitive tool for the assessment of coating adhesion and adhesion degradation.  The contact force and pressure between an AFM tip and the sample surface can be controlled accurately in an AFM.  In AFM scratching, the AFM tip is rastered in contact mode across the surface of a coated sample and the conditions required to remove the coating by tip scratching, i.e. the tip force and number of raster scans,  are determined.   The samples were polymer-coated low carbon steel (1018), some of which were first exposed to an environment containing high RH and ozone, and with irradiation by UV light.  The samples were also tested by infrared spectroscopy to probe changes in polymer structure. Some samples were pre-exposed in a salt spray chamber to study the degradation of polymer coating adhesion.  Environmental exposure resulted in a decrease in the number of raster scans required to remove the coating compared to unexposed samples, indicating a decrease in adhesion. Furthermore samples exposed to both ozone and UV radiation more degradation than samples exposed to UV radiation only. Oxygen atoms generated by the interaction of UV and ozone apparently enhance coating degradation.