Solution Medium Suspension Plasma Spray (SMSPS): A Microstructure and High-Temperature Properties Analysis of YSZ Thermal Barrier Coating

Abstract

Suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) are advanced thermal spraying techniques that enable creation of the coatings with desirable properties. In both techniques, it is necessary to entrain the liquid feedstock into the thermal jet so that it can be effectively transformed into oxide particles. In this study, the suspension containing 3wt.% solid load of 40 nm ZrO₂-7%Y₂O₃ (7YSZ) powder with a medium of zirconium acetate-yttrium nitrate solution (instead of an aqueous or non-aqueous solvent) was used as titled solution medium suspension plasma spray (SMSPS) to produce the YSZ thermal barrier coating (TBC). The microstructure of SMSPS coating exhibited a columnar structure, with an inter-columnar spacing of approximately 5 microns. This columnar structure was attributed to the trajectory of nano-sized particles being affected by the plasma jet and their deposition at shallow angles on surface asperities, resulting in shadowing effect. The presence of vertical cracks within some of the columns in the microstructure of SMSPS coating was similar to the structure observed in SPPS coatings, highlighting the unique nature of this TBC structure. The oxidation testing at 1000°C for 12, 50, 120, and 250 hours revealed the formation of a thermally grown oxide (TGO) layer consisting of aluminum oxide and mixed oxides. The TGO layer growth rate was initially high, but then significantly decreased during the diffusion-controlled and steady-state stages of the test. It was found that the inter-columnar spacing in the coating facilitated oxygen diffusion, resulting in an accelerated oxidation of the bond coat during the initial stages. In addition, the SMSPS coating exhibited an average life of 753 cycles in 1-hours, 1000 ˚C thermal cycling test. The failure mechanism observed involved insular collapsing of adjacent clusters due to crack propagation which was attributed to the presence of vertical cracks and the columnar structure.