Adhesion of plasma sprayed zirconia splats on stainless steel

Thermal spray technology is an alternative material fabrication technique to the traditional solidification and powder processing methods for producing thick coatings and bulk free-forms. Extensive research has enabled the extension of this technique to a wider range of material classes including polymers, bioceramics and functionally gradient materials. A key area of application of thermal spraying is the formation of thermal barrier coatings for turbine components used in power generation and propulsion. Continuing research intends to improve the quality of coatings produced by this technique to compete with other technologies like physical vapor deposition to make use of some of the advantages like higher throughput that thermal spraying affords.; Understanding the adhesion of plasma sprayed coatings is essential to improving the service life of coated components. Progressively research has focussed on the nature of the unique building blocks of plasma sprayed coatings called splats. The current research intends to characterize the microadhesion at the splat substrate interface using nondestructive methods based on the analysis of images obtained using the scanning electron microscope (SEM). A model system of yttria stabilized zirconia, a traditional thermal barrier material, on steel substrate is chosen for the study. Two techniques are developed based on the analysis of through thickness crack distribution and fragmentation of thin brittle films on ductile substrates and that based on the analysis of interface cracking. A novel imaging technique is used to determine the extent of interface cracking from the contrast observed in SEM images.; Based on the understanding of ceramic splat formation on metal substrates a shear lag theory of tensile residual stress generation is used to explain the fragmentation observed in splats. An earlier analysis of cracking in brittle films due to uniaxial stress is extended to the present case of equibiaxial thermal residual stress. Three geometric features are identified to analyze the observed fragment geometries and correlated with local interfacial adhesion in splats. The measurements are extracted from secondary and back-scattered electron images using image segmentation software. Measurement of cracked interfacial areas was accomplished using charging contrast in the secondary and specimen current images of splats. Based on these measurements it was found that micro adhesion decreases within splats from center to the periphery. This variation in adhesion was attributed to the temperature and pressure distribution in the splat-substrate interface during formation.