The cubic 18YSH coating has good mechanical properties including comparable microhardness and low Young’s moduli, compared to YSZ coating. However, the fracture toughness value is relatively lower, indicating a weak propagation resistance.
The cubic 18YSH coating has temperature-independent conductivity over a wide temperature range with minimum conductivity of 0.832 W/m⋅K at 800 ◦C. With more YO1.5 doping, stronger phonon scattering resulted from oxygen vacancies, mass fluctuation and elastic lattice strain becomes the main reason for lower thermal conductivity than YSZ coating.
The 18 YSH coating experiences early failure after thermally cycling for 28 cycles at 1100 ◦C. Thermal expansion mismatch and low fracture toughness of the coating both contribute a lot to the delamination at interface between bond coat and ceramic topcoat.
The 18YSH coating can be severely attacked by CMAS melt at 1250/1300 ◦C within 8 h duration. As temperature raises from 1250 ◦C to 1300 ◦C, severer CMAS reaction and infiltration can even lead to a large scale buckling of the TBC, causing mechanical damage. 18YSH coating chemically reacts with CMAS at the two temperatures following dissolution-precipitation scenario. Alternating Y-depleted and Y-rich HfO2 will be reprecipitated from melt to reach local equilibrium, which has drastically changed the coating and degraded its performances. It is believed that thermomechanical effect and thermochemical reaction would accelerate spallation of 18YSH ceramic layer when exposingthe coating to CMAS melt.
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