Phase Formation and Transformation in Alumina/YSZ Nanocomposite Coating Deposited by Suspension Plasma Spray Process
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. Tarasi, M. Medraj, A. Dolatabadi, J. Oberste-Berghaus, and C. Moreau (Submitted July 22, 2009; in revised form August 17, 2009) Suspension Plasma Spray process was used for deposition of pseudo-eutectic composition of aluminayttria-stabilized zirconia as a potential thermal barrier coating using Mettech axial III torch. Process variables including feed and plasma parameters were altered to find their effects on the formation of phases in the composite coating. The in-flight particle velocity was found to be the crucial parameter on phase formation in the resulting coatings. Low particle velocities below 650 m/s result in the formation of stable phases i.e., a-alumina and tetragonal zirconia. In contrast, high particle velocities more than 750 m/s favor the metastable c-alumina and cubic zirconia phases as dominant structures in as-deposited coatings. Accordingly, the plasma auxiliary gas and plasma power as influential parameters on the particle velocity were found to be reliable tools in controlling the resulting coating structure thus, the consequent properties. The noncrystalline portion of the coatings was also studied. It was revealed that upon heating, the amorphous phase prefers to crystallize into pre-existing crystalline phases in the as-deposited coating. Thus, the ultimate crystalline structure can be designed using the parameters that control the particle velocity during plasma spray coating.
Keywords
alumina/yttria-stabilized zirconia, amorphous phase, ceramic composite coating, crystalline structure, suspension plasma spray, thermal changes
1. Introduction Alumina-zirconia coatings are being considered as potential alternative to the present thermal barrier coatings (TBCs). These TBCs include cubic or tetragonal zirconia stabilized by substituting some of the Zr atoms with one or more of the base elements from alkali metal oxides such as CaO, MgO, transient metal oxides as in Y2O3, Sc2O3, Er2O3 (Ref 1), rare-earths such as CeO, Yb2O3, or the whole Lanthanides group (Ref 2). Stabilization is to prevent the adverse effects of the phase transformation of zirconia coatings into monoclinic structure. Alternatively, stabilization can be attained in a composite with alumina, which can further enhance its thermodynamic stability at various temperatures (Ref 3), and improve its mechanical properties (Ref 4) as well as thermal resistivity (Ref 5). F. Tarasi, M. Medraj, and A. Dolatabadi, Department of Mechanical and Industrial Engineering, S-EV 4411, Concordia University, Sir George Williams Campus, 1515 St. Catherine W., Montreal, QC, H3G 2W1, Canada; and J. Oberste-Berghaus and C. Moreau, NRC-IMI, Boucherville, QC, Canada. Contact e-mails: [email protected] and [email protected].
Journal of Thermal Spray Technology
Both zirconia and alumina, during deposition by plasma spraying, have their own phase formation preferences. According to Golozar et al. (Ref 6), as-deposited yttria-stabilized zirconia (YSZ) coating applied by atmospheric plasma spray (APS) process shows mainly tetragonal along with
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