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, increasing control over porosity and introducing nano-sized pores. Neutron scattering is capable of studying porosity with radii between 1 nm and 10 mu m, thanks to the combination of small-angle and ultra-small-angle neutron scattering. Image analysis allows for the study of porosity with radii above -100 nm. For the first time in SHVOF 8YSZ, pore size distribution, total porosity and pore morphology were studied 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. The results show an abundant presence of nanopores in the as-sprayed coatings, which are eliminated after heat treatment at 1100 degrees C; a transition from inter-splat lamellar to globular pores and the appearance of micro-cracks along with the accumulation of microstrains associated with the phase transformation at 1200 degrees C.