Evolution of Porosity in Suspension Thermal Sprayed YSZ Thermal Barrier Coatings through Neutron Scattering and Image Analysis Techniques
Abstract
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 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 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 C. The phase transformation was completed at 1400 C, with no presence of monoclinic phase