La 2 Zr 2 O 7 (LZ) Coatings by Liquid Feedstock Plasma Spraying: The Role of Precursors
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William Duarte, Sylvie Rossignol, and Michel Vardelle (Submitted April 24, 2014; in revised form July 3, 2014) Solution precursor plasma spraying (SPPS) is an innovative process for obtaining finely structured coatings from metallic salt solutions. Lanthanum and zirconium precursors were studied to understand their influence on lanthanum zirconate (La2Zr2O7) synthesis by SPPS. Thermal analysis revealed that the nature of the precursor and the solvent affected mixture decomposition by changing the decomposition temperature. The surface tensions of precursor solutions in various media were investigated and revealed the influence of the nature of the counter-cation. Different solutions of precursor mixtures were used to obtain La2Zr2O7 splats on metallic substrates. A decrease in solution surface tension led to an increase in splat size. Coating mechanisms by SPPS are governed by the nature of the precursors and solvents.
Keywords
aerospace, ceramics, oxides, thermal gravimetry measurement, thermophysical properties
1. Introduction Rocket engines used in launchers are subject to thermal and mechanical stress. Ceramic materials are used in rocket engines (Ref 1, 2) for their thermal and mechanical properties, but the greatest limitation of these materials is their brittle nature, which results in low elongation and low tenacity. Currently, yttria-stabilized zirconia (ZrO2Y2O3 or YSZ) is the standard material for thermal barrier coatings (Ref 3–5). However, this material exhibits phase instability at high temperature (>1200 C), leading to volume expansion and increased stresses within the coating. The combination of phase transformation and sintering of the material is one of the failure mechanisms of the thermal barrier coating system (Ref 6–9). Engine manufacturers want to increase yield by increasing the inservice temperature to over 1200 C, leading to studies on YSZ substitution. Among the different materials studied, lanthanum zirconate crystallized in the pyrochlore structure, La2Zr2O7, can be used as a substitute at high temperature (Ref 5, 10–12). Lanthanum zirconate exhibits high microstructural stability (>2000 C) and high sintering resistance. Furthermore, the thermal conductivity of lanthanum zirconate is 20% lower than that of YSZ. However, the thermal expansion coefficient of lanthanum William Duarte, Groupe dE´tudes des Mate´riaux He´te´roge`nes (GEMH), Limoges, France; Science des Proce´de´s Ce´ramiques et de Traitements de Surface (SPCTS), Limoges, France and Centre National dEtudes Spatiales (CNES), Paris, France; Sylvie Rossignol, Groupe dE´tudes des Mate´riaux He´te´roge`nes (GEMH), Limoges, France; and Michel Vardelle, Science des Proce´de´s Ce´ramiques et de Traitements de Surface (SPCTS), Limoges, France. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
zirconate is slightly lower than that of YSZ, but this deficit can be compensated for by a low Youngs modulus (Ref 5, 10–12). Moreover, improvement can be achieved with innovative thermal processes, such as
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