Title

Diagnostics and modeling of yttria-stabilized zirconia formation in solution-precursor plasma-spray process

Date of Completion

January 2005

Keywords

Engineering, Mechanical|Engineering, Materials Science

Degree

Ph.D.

Abstract

Thermal barrier coatings produced by solution-precursor plasma-spray (SPPS) process have been shown to offer superior thermal properties and durability. The microstructure of these coatings combines favorable properties of conventional air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD) coatings by providing evenly spaced, through-thickness vertical cracks and uniformly distributed porosity resulting in good strain tolerance and low thermal conductivity. This experimental and computational study aims at clarifying some of the key aspects of this process through diagnostics of the actual process, modeling of vaporization and precipitation within droplets and through model experiments which utilize a combustion flame instead of a plasma jet. The work also includes characterization of the combustion flame and study of the possibility of coating formation utilizing combustion processes instead of plasmas. Plasma-extracted sample indicate presence of spherical sintered polycrystalline particles of 100 nm to 1 micron. Characterization of the precursor spray show that the mean droplet size is about 40 micron suggesting droplet disintegration in the process. Modeling of the heat and mass transfer around the droplets and solute precipitation predict formation of shell type structures and ceno-spheres supported by the experimental evidence of shell type structures in the single pass plasma experiments. Combustion flame experiments were found to produce sintered polycrystalline tetragonal yttria-stabilized zirconia particles similar to the plasma-extracted samples. The microstructural and compositional evolution of the ceramic particles was characterized as a function of downstream distance in the flame jet. ^