Phase Analysis and Microstructural Investigations of Ce 2 Zr 2 O 7 for High-Temperature Coatings on Ni-Base Superalloy Substrates

• Published in: High temperature materials and processes 2019-02, Vol.38 (2019), p.773-782
• Main Authors: Venkatesh, G., Subramanian, R., John Berchmans, L.
• Format: Article
• Language: English
• Subjects: Air plasma; Aluminum oxide; Cerium; Combustion synthesis; Crystal structure; Crystallinity; Cubic lattice; Diffusion coating; Evaluation; Fluorite; High temperature; Lattice parameters; Microstructure; Morphology; Nickel base alloys; Oxidation tests; Plasma spraying; Scanning electron microscopy; Shock resistance; Spalling; Substrates; Thermal cycling; Thermal cycling tests; Thermal resistance; Thermal shock; Thickness
• ISSN: 0334-6455; 2191-0324
• DOI: 10.1515/htmp-2019-0033

Crystalline cerium-zirconate (CZ) powders were synthesized via solution-assisted combustion synthesis route and calcined at 850°C for 8 h to obtain coarse crystalline powders. SEM (scanning electron microscopy) characterization studies were done to evaluate the morphology of the powders. XRD analysis of the resulting powder confirmed the presence of crystalline α-Ce 2 Zr 2 O 7 along with a Ce 2 Zr 2 O 8 phase having a disordered fluorite cubic lattice. Phase composition, lattice parameters, and the atomic positions were also investigated. Refinement of XRD data was done to quantify the amount of α-Ce 2 Zr 2 O 7 and Ce 2 Zr 2 O 8 phases. Ni-base superalloy Inconel 625 was chosen as the coating substrate, and the powders were coated using an air plasma spraying (APS). A thermally grown oxide (TGO)/Al 2 O 3 layer was observed owing to the high temperature of the substrate as well as the diffusion of bond coat material into the substrate. Coated samples were characterized by SEM to study the surface morphology, coating thickness, and interface microstructures. The thickness of the coated sample was found to be 400 μm. Thermal cycling test of the coated sample was carried out at 750°C for 50 h to evaluate the thermal shock resistance of the coating as well as the spalling behavior of the coating. Preliminary oxidation tests were carried out for 50 h at 750°C to evaluate the oxide growth by measuring weight gain of the oxide layers formed. Oxide growth signifies the gradual increment of layers over a period with a parabolic rate constant of about K p = 1.18 × 10 –3 mg 2 cm –4 h –1 .