High-Temperature Oxidation and Oxide Scale Formation in Plasma-Sprayed CoNiCrAlYRe Coatings
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INTRODUCTION
NI-BASED superalloy components operating in power plants and aircraft engines are exposed to severe attack by oxygen and molten salts, which induce fast degradation. The application of an overlay metal coating allows to protect their surface and to increase their durability, thus involving lower maintenance costs as well as higher efficiency and reduced environmental impact. MCrAlY (M = Ni and/or Co) coatings can be properly tailored to enhance the oxidation and hot corrosion resistance of these components.[1,2] During service, a thermally grown oxide (TGO) typically grows on their top surface owing to Al depletion. A dense and continuous Al2O3-rich TGO with slow growth rate is able to retard the oxidation at the substrate because of its low oxygen ionic diffusivity. Otherwise, the fast and extended formation of NiO and spinels produces localized tensile stress concentration, thus assisting cracking and delamination.[3–6] This is recognized as one of the main failure mechanisms of a ceramic TBC when applied on the surface of MCrAlY coating, because the tensile stresses generated at the interface and the volume expansion within the TGO can promote
GIOVANNI DI GIROLAMO, LUCIANO PILLONI, and EMANUELE SERRA, Researchers, are with the Technical Unit of Materials, Casaccia Research Centre, ENEA, Rome, Italy. Contact e-mail: [email protected] ALIDA BRENTARI, Researcher, is with the Technical Unit of Materials, Faenza Research Centre, ENEA, Rome, Italy. CATERINA BLASI, Technical Staff, is with the Technical Unit of Materials, Brindisi Research Centre, ENEA, Brindisi, Italy. Manuscript submitted February 12, 2014. Article published online August 14, 2014 5362—VOLUME 45A, NOVEMBER 2014
microcraking and TBC spallation during service, as well discussed in previous works.[7–9] It has been reported that the addition of Re to the MCrAlY alloy is able to improve the mechanical and anti-oxidant properties of low-pressure plasma-sprayed (LPPS) coatings exposed at 1403 K (1130 C) for 4 hours. This enhancement can be ascribed to lower Al depletion and better Al diffusion, which involves the rehealing of alumina scale after TGO spallation.[10–12] Coatings with enhanced crack and fatigue resistance can be obtained.[13] Otherwise, other investigators reported that by adding Re to the alloy, coatings with lower thermal expansion coefficient and higher hardness can be manufactured, but their oxidation resistance decreased in comparison with pure MCrAlY coatings.[14] From this point of view, the study of microstructural and mechanical properties of MCrAlYRe coatings after early-stage high-temperature oxidation is meaningful to understand their stability and performance during longterm service. The formation and growth of mixed oxides on their surface should be properly minimized. Many thermal spraying techniques are suitable for depositing thick MCrAlY coatings on components with complex geometry. Among them, vacuum plasma spray (VPS) and high-velocity oxy fuel (HVOF) are usually employed, whereas air plasma-sprayed co
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