Quantifying abrasive-blasted surface roughness profiles using scanning electron microscopy
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Quantifying abrasive-blasted surface roughness profiles using scanning electron microscopy Stuart G. Croll, Scott A. Payne
Ó American Coatings Association 2020 Abstract Rough surface profiles are specified for metal surfaces so that corrosion preventative coatings adhere well and provide long-term protection. Simple stylus scans and replica tape are widely used in industry to characterize surfaces and control the quality of surface preparation. Unfortunately, a scientific, quantitative connection between adhesion and surface profile parameters remains unclear. Stereo-pair images from scanning electron microscopy, SEM, were used to digitally reconstruct and then characterize 3D surfaces, which is a technique that seems previously unreported in coatings applications. SEM results were consistent with those from a stylus profilometer, but the data from a surface scan are much more numerous than from a line scan, and SEM can provide very high magnifications. 3D surfaces at high magnification gave larger values for the increase in area developed by the abrasive blasting than were determined at lower magnifications and by the stylus profilometer. Nevertheless, both techniques showed that the surface roughness height and ramification was far less than might be expected from illustrations in the existing literature on coatings’ adhesion. Quantified characterization, with details shown at high magnification, may provide scientific insight into how the various features of a roughened surface enhance the adhesion of protective coatings. Keywords 3D surface profile, Digital surface reconstruction, Statistical parameters, Fractal dimension, Scanning electron microscopy S. G. Croll (&) Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, USA e-mail: [email protected] S. A. Payne Electron Microscopy Center Core Facility, North Dakota State University, Fargo, USA
Introduction Metal surfaces must be cleaned and roughened prior to being painted to ensure the best adhesion of the coating which, in turn, protects the metal against corrosion. Many tools are available to achieve this roughening, ranging from simple sandpaper to abrasive blasting. The finish on a roughened surface can be assessed by its feel or appearance, but it is common to use a stylus profilometer1 or replica tape2 during manufacturing or in field repair to quantify the results. A typical stylus profilometer used for evaluating an abrasive-blasted metal has a vertical range of 300 lm or more and scans a length of 2.5 mm (0.1 inch) each time with a probe that has a tip radius, q, of 5 lm. Such profilometers can resolve surface features of a few nanometers in height under ideal conditions. The profile is recorded as a line trace, and various statistical expressions of the variation in height are available together with evaluations of the peak density. The lateral resolution of a profilometer can be calculated if the local amplitude, a, of the surface profile is known3. pffiffiffiffiffiffi k ¼ 2p aq If the (sinusoidal) profile amplitude is 25 l
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