Suspension Plasma Sprayed Coatings at High Temperature- Evolution of the Microstructure and Mechanical Properties

Strong increasing trend in turbine efficiency and inlet gas temperature requires advanced solutions in TBC systems, one of such solutions is introduction to TBCs made of YSZ nano-sized microstructural features like fine splats, nanometric crystals and submicron porosity by SPS. Our previous investigations showed that heat treatment of SPS coatings could lead to degradation of their mechanical properties (e.g. embrittlement) and short lifespan in thermal cyclic test.

In this study, several SPS coatings were deposited- various preheating temperature, plasma enthalpy and spraying distances were intended to differentiate ceramic particles’ temperature, velocity and cooling rate during the coating build-up. Then, the coatings were submitted to heat treatments which imitate TBC operation condition in a turbine engine. Toughness and Young’s modulus of coatings were evaluated. It turned out that comparing with APS fine microstructure of SPS coatings improves mechanical properties particularly fracture toughness. The investigation done by FE-SEM and FIB/TEM revealed that microstructure evolution is highly sensitive to spraying conditions and coatings’ pristine microstructure. The microstructure could evolve into either dense almost poreless structure or structure with stable fine submicron porosity. The formation of the arrays of fine pores (<100 nm) could cause degradation of coatings’ mechanical properties and poor reliability.