Size effects on high temperature oxidation of MCrAlY coatings processed via APS and HVOF depositions

• Published in: Surface & coatings technology Vol. 440; p. 128483
• Main Authors: Kalush, Ali, Texier, Damien, Ecochard, Maxime, Sirvin, Quentin, Choquet, Karim, Gheno, Thomas, Vanderesse, Nicolas, Jomaa, Walid, Bocher, Philippe
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 25.06.2022; Elsevier BV; Elsevier
• Subjects: Air plasma; Aluminum oxide; Chemical Sciences; Chromium oxides; Coatings; Consumption; High temperature; Intrinsic chemical failure; Material chemistry; Mechanically induced chemical failure; Oxidation; Oxidation MCrAlY coating; Oxidation rate; Reservoir effect; Scale (corrosion); Size effects; Spallation; Thickness; Ultrathin samples; Reservoir effect; Intrinsic chemical failure; Mechanically induced chemical failure; Oxidation MCrAlY coating; Ultrathin samples
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2022.128483

The paper demonstrates how defects inherited from the deposition processes can severely impair the lifetime of MCrAlY coatings in service. The oxidation behavior of two NiCoCrAlY coatings was investigated at 1150 °C up to 500 h. The coatings had the same nominal composition but were processed by two different projection techniques: air plasma spray (APS) and high velocity oxy fuel (HVOF). Freestanding coating specimens were extracted from the coated system and thinned down to different thicknesses ranging from 520 to 15 μm in order to investigate size effects inherent to the oxidation response. The oxidation rate of the APS coating was found to be insensitive to the specimen thickness, while that of the HVOF coating increased with the specimen thickness, due to greater intersplat oxidation. APS specimens thinner than 60 μm experienced intrinsic chemical failure (InCF) due to Al consumption to form the Al2O3 scale. In comparison, HVOF specimens with a thickness of 367 μm were subject to InCF after 250–350 h oxidation. This first stage of InCF resulted in the formation of a Cr2O3 layer at the Al2O3/metal interface once Al activity in the MCrAlY coating was low enough to thermodynamically allow Cr2O3 to form. In addition, thick HVOF specimens developed mechanically induced chemical failure (MICF) resulting in the formation of (Ni,Co)(Cr,Al)2O4 spinels on top of the Al2O3 scale and within oxide intrusions. The occurrence of MICF was associated with the concomitant effects of Al consumption due to intrusive oxidation and the spallation of the external Al2O3 scale. •High temperature oxidation of thin freestanding HVOF and APS coatings was studied.•Oxide intrusion developed on process-inherited defects such as unmelted particles and intersplat regions.•Difference in process-inherited defects for APS and HVOF specimens severely affects time to intrinsic chemical failure.•100 to 350 μm-thick HVOF specimens developed InCF for thermal exposure at 1150 °C shorter than 350 h.•HVOF specimens were more prone to oxide spallation due to intrusive oxidation.