Application of image analysis for characterization of porosity in thermal spray coatings and correlation with small angle neutron scattering

The microstructures of thermally sprayed coatings are very complex and incorporate process-dependent defects such as globular pores, interlamellar pores, cracks (in case of ceramics), etc. Porosity is a prevalent feature in the microstructure and affects a wide range of coating properties such as elastic modulus, thermal conductivity and dielectric behavior. Various methods are employed for quantitative measurement of porosity, which forms an important and integral part of microstructural characterization of thermal spray coatings. Studies have been conducted here to establish image analysis (IA) as a reliable method for characterization of porosity in thermally sprayed coatings. The versatility of IA methods for microstructural quantification has been investigated for diverse coatings deposited with distinct spray processes and material feedstock characteristics. The IA methods put forth have been successfully applied to a variety of thermally sprayed coatings; materials of interest in this study being produced from partially stabilized zirconia (PSZ, ZrO2+8% Y2O3) and alumina ceramics. The results have been complemented with the microstructural information obtained using advanced characterization techniques, such as small-angle neutron scattering (SANS) and computer micro-tomography (CMT) carried out on similar coatings. The trends shown by these different methods for variation in porosity with respect to feedstock characteristics were in agreement. This study confirms the applicability of image analysis as a straightforward, versatile, reliable and inexpensive method for the characterization of porosity.