Porosity and microstructural features of plasma sprayed Yttria stabilized Zirconia thermal barrier coatings

In this paper effects of spray distance on the microstructural and porosity features of air plasma spray (APS) deposited Yttria stabilized Zirconia (YSZ) thermal barrier coatings (TBCs) have been investigated. Microstructural details including porosity and crack densities, pore and crack types and their morphologies were studied using Field Emission Scanning Electron Microscopy (FESEM). The detailed porosity characteristics were investigated through an image analyzing process. The results indicated that in the shorter spray distances i.e. 70 mm, some voids within splats are generated due to the gas entrapment because of high velocity of molten droplets/particles. By increasing the spray distance from 70 mm to 130 mm; (i) morphology and arrangement of the splats are changed from completely flat and well-adhered forms to coarse and less-adhered ones, (ii) the density of horizontally branched and segmented vertical cracks are decreased, (iii) pore to crack ratio is increased from 0.33 to 0.77, (iv) relative area fraction of pores in the 1–8 μm size range is decreased from 42% to 27%. Among the studied coatings; the YSZ coating deposited at spray distance of 90 mm exhibited desirable microstructural aspects in terms of level, size and morphology of the porosity.