This paper examines the microstructure and morphology of zirconia coatings and demonstrates the calculation of elastic modulus and Martens hardness based on instrumented indentation test results. Coatings samples varying in microstructure, phase content, and chemical composition were deposited by suspension plasma spraying using different torches and different suspension formulations. Coatings produced from low-concentration suspensions with submicron-size powders had a columnar structure with long vertical pores between the columns and fine spherical pores within the columns. Coatings made from suspensions with high concentrations of solids and coarser, more irregular powders, on the other hand, were more uniform and their surfaces smoother. They are also shown to be harder and have higher elastic modulus based on indentation test results.

Plasma generated by an SG-100 torch was applied to spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) and ZrO 2 +24 wt% CeO 2 +2.5 wt% Y 2 O 3 (24CeYSZ) as solid phases. The solids have the mean size of about 4.5 µm for 8YSZ and 3.9 µm 24CeYSZ and were obtained by milling of commercial powders Metco 204 NS and Metco 205NS, respectively. The suspensions were formulated with the use of 20 wt% solid phase, 40 wt% water, and 40 wt.% ethanol. The plasma spray parameters were optimized by keeping constant: (i) the electric power at 40 kW (ii) the working gases composition 45 slpm of Ar and 5 slpm of H 2 . On the other hand, the spray distance was varied from 40 to 60 mm and torch linear speed was varied from 300 to 500 mm/s. The coatings were sprayed onto stainless steel substrates to reach the thickness ranging from 70 to 110 µm (8YSZ) and about 70 µm (24CeYSZ). The coating microstructures were analyzed with the use of a scanning electron microscope. Mechanical properties were tested with the use of indentation and scratch tests. The indentation test was carried out with various loads ranging from 100 to 10,000 mN to determine elastic modulus and Martens microhardness. Young’s modulus of the coatings was in the range 71 to 107 GPa for 8YSZ and 68 to 130 GPa for 24CeYSZ. Scratch tests were conducted to determine the scratch macrohardness.

The gas-cooled fast reactor is a 4th generation nuclear reactor currently under development. Its design concept requires protective coatings able to operate at 850°C and protect the underlying structure in case of extreme cases, where the functional temperature can increase up to 1250°C and there is depressurization from 70 bars to atmospheric pressure. The parts to be covered are made in 1-mm thick materials resistant to heat and erosion with high mechanical properties at high temperatures, such as the Haynes 230 nickel-based alloy. In this study, the potential of the suspension plasma spraying technique for forming the first layers of a ceramic coating on smooth 1-mm thick Haynes substrate was explored. In order to meet these specifications, the coating material selected was partially stabilized zirconia of standard composition (8 mol.% Y 2 O 3 -ZrO 2 ).