A study on porosity measurements of thermally sprayed coatings and the influence of sample preparation

Assessing the porosity of a thermally sprayed coating is crucial to evaluate its quality and to predict its performance. Multiple destructive and non-destructive techniques have been established to measure the porosity with benefits and drawbacks to each of them. However, most commonly used is the light microscopic or scanning electron microscopic graphical analysis by grey scaling the image and applying thresholds to binarize the image and measure the ratio of the black and white areas. Subsequent algorithms may be used to additionally characterize the pores’ geometry and frequency and may also filter out areas of unreasonably large or negligible small scale. This method yields the outstanding advantage of enabling not just the porosity assessment but the vast majority of microstructural investigation (i.e., visual inspection, thickness, hardness, substrate bonding evaluation etc.). The drawback to this method of porosity assessment is, that multiple steps can strongly influence the results. Therefore, this study investigates the influence of metallographic sample preparation and variations during analysis and lastly human influence for testing of HVOF sprayed hardmetal and iron based hard alloy as well as APS sprayed chromia coating samples. The results are compared to other dominant techniques, for example mercury intrusion porosimetry.