Column Formation in Suspension Plasma-Sprayed Coatings and Resultant Thermal Properties

Abstract

The suspension plasma spray (SPS) process was used to produce coatings from yttria-stabilized zirconia (YSZ) powders with median diameters of 15 lm and 80 nm. The powder-ethanol suspensions made with 15-lm diameter YSZ particles formed coatings with microstructures typical of the air plasma spray (APS) process, while suspensions made with 80-nm diameter YSZ powder yielded a coarse columnar microstructure not observed in APS coatings. To explain the formation mechanisms of these different microstructures, a hypothesis is presented which relates the dependence of YSZ droplet flight paths on droplet diameter to variations in deposition behavior. The thermal conductivity (k th ) of columnar SPS coatings was measured as a function of temperature in the as-sprayed condition and after a 50 h, 1200°C heat treatment. Coatings produced from suspensions containing 80 nm YSZ particles at powder concentrations of 2, 8, and 11 wt.% exhibited significantly different k th values. These differences are connected to microstructural variations between the SPS coatings produced by the three suspension formulations. Heat treatment increased the k th of the coatings generated from suspensions containing 2 and 11 wt.% of 80 nm YSZ powder, but this k th increase was less than has been observed in APS coatings.

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

“…Overall the process changes have the effect of increasing the overall size of the atomized suspension droplets due to decreased plasma velocity [39]. According to the understood theory of SPS coating deposition, larger droplets will lead to more continuous or dense coatings rather than columnar coatings [36,39].…”

“…In particular, the recent developments of columnar-type coatings that mimic Electron BeamPhysical Vapor Deposition coatings have shown great promise as new TBC coatings [36][37][38][39].…”

Thermal Conductivity Analysis and Lifetime Testing of Suspension Plasma-Sprayed Thermal Barrier Coatings

“…As the particle size reduces, then the trajectory before deposition becomes increasingly parallel to the substrate. VanEvery et al proposed differing deposition regimes depending on the particle size and, therefore, impact trajectory [16]. When particles are sufficiently small, the shallow angle at which the particle will come into contact with the substrate results in the generation of columnar structures.…”

“…Suspension plasma spraying has become a promising contender for the production of high performance thermal barrier coating systems, as vertically cracked [14,15], columnar [16] or highly porous coatings [17] are possible to manufacture. Suspension plasma spraying (SPS) coatings can be produced with high levels of porosity that are not normally achievable with APS or EB-PVD coatings [18].…”

Influence of Isothermal Heat Treatment on Porosity and Crystallite Size in Axial Suspension Plasma Sprayed Thermal Barrier Coatings for Gas Turbine Applications

“…EB-PVD, on the other hand, is relatively expensive but allows coatings with an inherently strain tolerant columnar structure with relatively denser coatings than APS [8]. EB-PVD coatings, hence, typically have higher lifetime but also higher thermal conductivity than APS coatings for a given ceramic layer thickness [9][10][11][12].…”