Abstract
The continued development of the aero-engine industry puts forward the hot end components such as the blades with a higher operation temperature. In order to improve the operation temperature of the blades and prolong its service life, the NiCrAlY coatings were deposited on the superalloy K465 by arc ion plating. According to the standard of GB/T13303-91 and HB5258-2000,and the actual operation conditions and temperature of the blades, the isothermal oxidation testing of the NiCrAlY coatings and alloy samples at 1100℃ in air for 200 hours was carried out. The oxidation kinetics law, oxidation thermodynamics,oxidation resistance,the surface and cross-section morphology of the oxides, the phase composition,the rules of elemental diffusion and the degradation mechanism of the coating were discussed by the analysis and detection methods of SEM, EDS, XRD and so on.
The NiCrAlY coating after vacuum diffusion were deposited continuously and densely on the surface of alloy K465. There was a diffusion layer between the coating and the substrate alloy. The diffusion layer connected the coating and the substrate alloy by the metallurgical bonding. The coating and diffusion layer consisted of ρ-NiAl andγ'-Ni3Al.
The results showed that:The oxidation kinetic of alloy K465 with coating followed parabollic rate law under 1100℃ for 200h. The alloy K465 with NiCrAlY coating retained excellent oxidation resistant performance. The uninterrupted α-Al2O3 and θ-Al2O3 membrane was formed in the early and θ-Al2O3 into α-Al2O3 at later stage. The oxide film was Complete and dense because of better performance of θ-Al2O3. Because Al was consumed, there was ρ-NiAl→γ'-Ni3Al→γ-Ni in the coating and diffusion layer. Although the coating was a certain degree of degradation, but the coating still has a very effective antioxidant.
In the initial stage of high temperature oxidation, the K465 sample’s surface was covered by protective oxide film of Al2O3 and Cr2O3 to protect he samples. TiO2 and NiCr2O4 increased in the medium-term of oxidation and undermine the integrity of the oxide film due to their different physical properties such as thermal expansion coefficient of Al2O3 and Cr2O3.
It reduced the adhesion of the oxide film and dense and lead to a large number of oxide film initiation of cracks and spalling of the oxide film. The surface of samples was covered again by protective oxide film. The newly formed oxide film does not have good adhesion, and spill again. The loss of beneficial elements in the alloy specimens formed many defects in the alloy surface for the O2 –to spread. The oxide film of sample with varying degrees of internal oxidation was loose even empty. So the alloy K465 do not have the ability to wok long time at high temperatures.
Key words: K465 Nickel-based superally；NiCrAlY coating；high temperature oxidation
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