Study on porosity of plasma-sprayed coatings by digital image analysis method

The porosities of plasma-sprayed Al2O3, ZrO2, and TiO2 coatings deposited on 304 stainless steel plates were evaluated by the digital image analysis method. As the accuracy of this method depends significantly on metallographic preparation and metallography procedure for coating specimens, the effects of cross-surface roughness, magnification, and number of fields of view on the porosity were studied. The results indicate that the porosity value from polished specimen with cross-surface roughness no more than 0.1 mu m is acceptable. The porosity value obtained at higher magnification is a little bit higher, especially when the real porosity is higher; more fields of view have to be considered in this case. Both experimental results and statistic analysis suggest that 15 fields of view at 1000x magnification can be implemented to evaluate porosity of plasma-sprayed coating considering both the domain size and the resolution at the same time.