Static Oxidation Behavior Of Superalloy GH99 And Mcraly(Ta) (M=Ni,Co) Coatings At High Temperatures

Air plasma spraying (APS) and laser cladding techniques were utilized to prepare two kinds of MCrAlY(Ta) (M=Ni,Co) coatings on a GH99 Ni–base superalloy. Based on microstructural observations, the oxidation kinetics of GH99 alloy and these MCrAlY(Ta) coatings were further investigated at temperatures of 900oC, 1000oC and 1100oC in laboratory atmosphere, respectively. The surface and cross–sectional morphologies of the as–prepared and oxidized samples were observed by scanning electron microscope (SEM), and the phase identifications were determined by X-ray diffraction (XRD), thereafter, the oxidation mechanisms were further proposed.APS MCrAlY(Ta) coatings have a mechanical bonding interface and a lamellar morphology together with some defects such as pores, oxides, etc.β-NiAl phase, which is rich in Al, is observed in APS CoNiCrAlY coatings. Instead ofβphase, a small amount of Ta-rich phase is found in APS NiCoCrAlYTa coatings. Laser clad MCrAlY(Ta) coatings exhibit a metallurgical bonding interface and have almost no defects. The grains grow up perpendicular to the interface as dendrites. Onlyγphase is identified in laser clad MCrAlY(Ta) coatings.The oxidation kinetics curves of GH99 alloy accord more with the parabolic law at temperatures of 900oC and 1000oC than at 1100oC.The result obtained at 1100oC follows a two-stage parabolic law. APS and laser clad MCrAlY(Ta) coatings basically follow the parabolic law at temperatures of 900¹100oC. Under the conditions of identical preparation technique and oxidation temperature level, CoNiCrAlY coatings show a better oxidation resistance than NiCoCrAlYTa coatings. The MCrAlY(Ta) coatings prepared by laser cladding show a better oxidation resistance than those by the APS, except CoNiCrAlY coating at 1100oC.The GH99 alloy has an oxidized layer, which is composed of Cr₂O₃, NiCr₂O4 and TiO₂. The outer oxidized layer consists mainly of TiO₂. However, an inner oxidized layer containing Al₂O₃ is observed after exposing to 1100oC for 200h in air.APS CoNiCrAlY coatings have an oxidized layer of Cr₂O₃, Al₂O₃ and (Cox,Ni1–x)Cr₂O4. A compact Al₂O₃ inner layer forms after an extended oxidation experiment. A lot ofβ–NiAl remains after exposing to 900oC for 200h in air. At the initial stage of oxidation at 1100oC in air, an inner Al₂O₃ oxidized layer forms due to the abundance of aluminum. At higher temperature, theβ–NiAl phase is consumed more quickly. APS NiCoCrAlYTa coatings have an oxidized layer of Cr₂O₃, Al₂O₃, NiO, NiCr₂O4 and NiTa2O6. The oxidized layer is not so compact as the APS CoNiCrAlY coating. Neitherβ–NiAl nor an obvious inner oxidized layer of Al₂O₃ is observed. Under the conditions of elevated temperatures and prolonged oxidation time, more spinel NiCr₂O4 phase is generated as well as NiTa2O6.Laser clad CoNiCrAlY coatings have an oxidized layer of Cr₂O₃ and spinel (CoxNi1–x)Cr₂O4. In the initial stage, a large amount of internal oxidized products are formed. At the later stage of oxidation, a dense, coherent and highly–adherent inner Al₂O₃ oxidized layer is formed. Laser clad NiCoCrAlYTa coatings have an oxidized layer consisting of NiO, Cr₂O₃, NiCr₂O4 and NiTa2O6. Cr₂O₃ grows up quickly at the early stage of oxidation, and produces a porous outer layer. The oxidized layer wrinkles and spalls off later, and reveals the inner matrix of coating, which exhibits a wrinkled morphology. There exist some Ta–rich spheres in the inner coating at 1100oC. The oxidized layer spalls off seriously at the later stage of oxidation. Although more internal Al₂O₃ oxidized product is observed, no inner Al₂O₃ layer is formed.