Comparison of the oxidation behavior and microstructural evolution of NiCoCrAlY coatings processed via HVOF and APS

Abstract

This comprehensive study investigates the high-temperature oxidation behavior of NiCoCrAlY coatings processed via High Velocity Oxy Fuel (HVOF) and Air Plasma Spray (APS) techniques. The research demonstrates how defects from these deposition processes can significantly impair the service life of MCrAlY coatings. The oxidation behavior of two types of NiCoCrAlY coatings, processed with APS and HVOF, was examined at high temperatures for up to 500h. NiCoCrAlY coatings were deposited by HVOF and APS processes on 304 stainless steel plates. Both coatings were deposited by PRAXAIR Surface Technologies for this specific investigation, using NiCoCrAlY powder (NI-191-4) also provided by PRAXAIR©. Both HVOF and APS, as coated and heat-treated coatings were oxidized for periods of up to 500h at 1050 °C and 1150 °C using different interrupted isothermal oxidation method. The study found that the oxidation rate of APS coatings was thickness-insensitive, while that of HVOF coatings increased with thickness due to greater intersplat oxidation. APS specimens thinner than 60 μm experienced intrinsic chemical failure (InCF) due to Al consumption to form Al2O3. After 240h of high-temperature oxidation, the mass gain of the thick sample was nine times that of the cast material. The oxidation rate of both HVOF and APS samples followed a power law with an exponent close to 3. InCF and MICF failures were observed for HVOF samples thicker than 55 μm, while thinner samples were only sensitive to InCF due to the lack of oxide spallation. Therefore, both oxide intrusion and oxide spallation are central to the Al2O3 failure and the service life of NiCoCrAlY products. The HVOF coating experienced poor oxidation resistance compared to the APS coating. The experimental analysis revealed critical findings on phase transformations, mass gain, oxidation kinetics, and mechanisms, as well as the formation and behavior of the TGO layer. The study underscores the importance of optimizing deposition techniques and heat treatment processes to enhance the coatings’ microstructural integrity and performance in high-temperature environments.

Date	8 May 2025
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Philippe Bocher (Supervisor) & Damien Texier (Co-supervisor)