Yb₂Si₂O₇/Si environmental barrier coatings (EBCs) were produced by air plasma spray (APS) and low-pressure plasma spray (LPPS) processes. The phase composition, microstructure, and bonding strength of APS and LPPS EBCs were investigated. Thermal cycling tests were performed in air and in steam atmosphere respectively at 1316 °C for both APS and LPPS EBCs. There is no coating failure in air atmosphere for both APS and LPPS EBCs after 900 cycles. In contrast, APS EBCs have an average life of 576 cycles in a steam cycling test in 90% H₂O + 10% air at 1316 °C while LPPS EBCs survived 1000 cycles without failure. The superior durability of the LPPS EBCs compared to APS EBCs in the same steam cycling environment is attributed to the significantly reduced thermally grown oxide (TGO) growth rate because of the denser and crack-free microstructure, higher bonding strength, and reduced coefficient of thermal expansion (CTE) mismatch (less Yb₂SiO₅ phase) in the LPPS Yb₂Si₂O₇/Si EBCs.

Higher operating temperatures for gas turbine engines require highly durable thermal barrier coatings (TBCs) with improved insulation properties. A suspension plasma spray process (SPS) had been developed for the deposition of columnar-structured TBCs. SPS columnar TBCs are normally achieved at a short standoff distance (50.0 mm–75.0 mm), which is not practical when coating complex-shaped engine hardware since the plasma torch may collide with the components being sprayed. Therefore, it is critical to develop SPS columnar TBCs at longer standoff distances. In this work, a commercially available pressure-based suspension delivery system was used to deliver the suspension to the plasma jet, and a high-enthalpy TriplexPro-210 plasma torch was used for the SPS coating deposition. Suspension injection pressure was optimized to maximize the number of droplets injected into the hot plasma core and achieving the best particle-melting states and deposition efficiency. The highest deposition efficiency of 51% was achieved at 0.34 MPa injection pressure with a suspension flow rate of 31.0 g/min. With the optimized process parameters, 1000 μm thick columnar-structured SPS 8 wt% Y₂O₃-stabilized ZrO₂ (8YSZ) TBCs were successfully developed at a standoff distance of 100.0 mm. The SPS TBCs have a columnar width between 100 μm and 300 μm with a porosity of ~22%. Furnace cycling tests at 1125 °C showed the SPS columnar TBCs had an average life of 1012 cycles, which is ~2.5 times that of reference air-plasma-sprayed dense vertically cracked TBCs with the same coating thickness. The superior durability of the SPS columnar TBCs can be attributed to the high-strain-tolerant microstructure. SEM cross-section characterization indicated the failure of the SPS TBCs occurred at the ceramic top coat and thermally grown oxide (TGO) interface.