Evolution of Porosity in Suspension Thermal Sprayed YSZ Thermal Barrier Coatings through Neutron Scattering and Image Analysis Techniques

Porosity is a key parameter on thermal barrier coatings, directly influencing thermal conductivity and strain tolerance. Suspension high velocity oxy-fuel (SHVOF) thermal spraying enables the use of sub-micron particles as feedstock, increasing control over porosity and introducing nano-sized pores; these fine-scale porosities being challenging to measure. Neutron scattering represents a non-destructive technique, capable of studying porosity with a pore size range of 1 nm to 10 um, thanks to the combination of small-angle (SANS) and ultra-small-angle neutron scattering (USANS) techniques. Image Analysis (IA) on digital images allows for the study of porosity with a size above ~100 nm. In this work, two yttria-stabilised zirconia (ZrO2 with 8 wt.% Y2O3) suspensions were sprayed and heat treated at 1100, 1200, 1300 and 1400 {\deg}C for 72 h. For the first time in SHVOF 8YSZ, pore size distribution, total porosity and pore morphology were studied using SANS & USANS and IA to determine the effects of heat treatment. X-ray diffraction and micro-hardness measurements were performed to study the phase transformation, and its effects on the mechanical properties of the coating. The results show an abundant presence of nano-pores in the as-sprayed coatings, which are eliminated during heat treatment at 1100 {\deg}C; a transition from inter-splat lamellar pores to globular pores and the appearance of micro-cracks along with the accumulation of micro-strains associated with the phase transformation from the initial metastable tetragonal into tetragonal and cubic phases at 1200 {\deg}C. The phase transformation was completed at 1400 {\deg}C, with no presence of monoclinic phase