Kent Van Every, M. Krane and R. Trice
Parametric Study of Suspension Plasma Spraying

Suspension plasma spraying is an emerging injection technology that is being employed to produce ceramic coatings. In this process, submicron to nanometer sized powders are dispersed in a solvent to create a suspension; the suspension is then sprayed directly into a plume (plasma, HVOF, etc) where the solvent is evaporated and the powders are melted. Because the process is relatively new, the processing parameters for suspension plasma spraying are not yet well-defined. In this presentation we will discuss our initial findings on spraying suspensions of ethanol and 80 nm diameter 4.5 mol% Y2O3-ZrO2 powders using this process. The spray conditions investigated include standoff distance, torch power, powder loading, and suspension injection velocity. Based upon electron microscopy, coatings sprayed with the following conditions: 50 mm standoff distance, 38 kW torch power, 20 m/s suspension injection velocity, and 5 wt% solid loading in the suspension produced fully-melted lamellae. Additionally, X-ray diffraction of the coatings showed a nominal average grain size of 50 nm; however, this value was a function of spray parameters.

Chris Petorak,⁽¹⁾ J. Ilavsky ⁽²⁾ and R. Trice⁽¹⁾
Microstructural Characterization of Stress Relaxed 7 wt% YSZ Plasma-Sprayed Coatings Using the SANS Technique

Currently, plasma sprayed 7wt% yttria stabilized zirconia, or YSZ, coatings are being used in gas turbine engines as thermal barrier coatings in order to increase the operating efficiency of the engines. Comprehension of the time-temperature deformation behavior of this material is critical to understanding the failure mechanisms of coatings currently in service, as well as further extending lifetimes of new coatings. Our current research focuses on the stress relaxation behavior of the coatings associated with engine startup when large thermal fluxes are present. We have been successful in observing some microstructural changes on the surface of the coating via SEM due to stress relaxation; particularly load orientation dependent intralamellar crack closure. But, any changes occurring in the interior of the coating, along with any contributions of inter and intralamellar porosity to stress relaxation, are unknown. Traditional polishing of ceramic cross sections leaves room for error intrinsic to the particular polishing method, while only providing data for small volume percent of material. Therefore, our current research has applied small angle neutron scattering (SANS) to provide important insight to the void microstructure of a plasma-sprayed YSZ coating. The SANS technique allows a larger volume of material to be characterized, as well as detecting any anisotropic changes that take place during stress relaxation.

(1) MSE, Purdue University, West Lafayette, IN, USA.
(2) Argonne National Laboratory, Argonne, IL, USA.