Effects Of Oxidation On Micromechanical Properties And Interfacial Microstructures Of Thermal Barrier Coatings

Three kinds of MCrAlY coatings such as NiCrAlY, NiCoCrAlY and NiCrAlYSiB, and three types of thermal barrier coatings (TBCs) such as 8 wt.% yttria partially stabilized zirconia (8YSZ/8YPSZ)+NiCrAlY,8YSZ+NiCoCrAlY and 20 wt.% Magnesia Stabilized zirconia (20MSZ)+NiCrAlY were prepared via low pressure/air plasma spraying (LPPS/APS), electron beam physical vapor deposition (EB-PVD) and arc ion plating (AIP) methods onto the nickel-based single crystal superalloy substrates. With nanoindentation testing technique, the microscopic mechanical properties of the coatings and their evolution during high-temperature oxidation were studied. Interfacial microstructure evolution and failure mechanisms of coatings during 1100℃ isothermal oxidation were analyzed by means of transmission electron microscopy (TEM), combined with X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS).Co element may reduce the elastic modulus and increase the hardness of the as-deposited MCrAlY coatings. During the initial oxidation period, Co improves the stability and numerical values of elastic modulus and hardness of the coatings. Si and B elements may reduce the hardness and elastic modulus of the MCrA1Y coatings. However, Si and B can stabilize the elastic modulus and improve the hardness of the MCrAlY coatings in the oxidation process. Because of the different phase compositions, densities and thermal stabilities, the average elastic modulus and hardness of the as-deposited 20MSZ TBCs were smaller than those values of 8YSZs. Due to preferred orientation texture and porous sedimentary state, the elastic modulus and the hardness of as-deposited EB-PVD 8YSZ coatings are smaller than those values of the APS 8YSZ coatings. However, in the initial oxidation periods, the elastic modulus and the hardness of the EB-PVD 8YSZ coatings were larger than those values of the APS 8YSZ coating because of sintering.Since the interfacial element interdiffusion and selective oxidation, thermally grown oxide (TGO) layers formed at interfaces between the NiCrAlY coatings and the substrates after 1100℃ isothermal oxidation. The TGO layers mainly composed of Cr2O3 and α-Al2O3 phases, with distribution of dense Cr2O3 layers close to the coating while dense α-Al2O3 layers near the substrate. As oxidation time extends, because of the decomposition of γ’-Ni3Al, γ-Al precipitated in the substrate near the interface. With further internal nitridation and internal oxidation, hcp-AIN and fcc-TiN formed within the substrate near the interface. Due to the competition between the nitridation and oxidation, hcp-AIN changed into α-Al2O3 through replacement reaction. With the isothermal oxidation continued, TGOs constantly thickened and their ingredients increased with new phases such as Ni(Al,Cr)2O4, NiO and Y3Al5O12, the thermal stress may be concentrated and thus causes the delamination and failures of the coatings along the interfaces between the TGO layers and coatings.During the early stage of 1100℃ isothermal oxidation, two layers of TGO appear successively at the interfaces between the top coats and the bond coats, and between the bond coats and the substrates of the APS 8YSZ+LPPS NiCrAlY TBCs. As for the interfaces between the top coats and the bond coats, a thin TGO layer near the top coats consists of a mixture of α-Al2O3, CrO3, NiO and Ni(Al,Cr)204, and a thin TGO layer composed of dense α-Al2O3 and/or Cr2O3 located next to the bond coats. In regard to the interfaces between the bond coats and the substrates, a thin TGO layer near the bond coats consists of a mixture of α-Al2O3, Cr2O3 and Ni(Al,Cr)2O4, and a thin TGO layer composed of dense α-Al2O3 and/or Cr2O3 located close to the substrates. Because of the decomposition of the strengthen phase γ’in the substrate, and the oxygen partial pressure is too low to oxidize the decomposed products [AI] and [Ti], y-Al and β-Ti precipitated in the substrate near the interface region. With prolonged oxidation time, Al-containing phases such as P-NiAl and AI2Y in the bond coats gradually disappeared, therefore, the degradation of the TBCs gradually took place. In addition, many factors such as TGO layers constantly thickening, the porosity of the TGO layers increasing, complex composition and phase transformation of the top coats, led to the generation of thermal stresses concentration and microcracks at the interfaces. With the microcracks expansion and propagation, large cracks or gaps were formed in the TGO layers between the bond coats and the substrates. As a result, the whole TBCs finally delaminated after 1100℃ isothermal oxidation for 3000 hours.The oxidation kinetics curves of MCrAlY coating and TBCs at 1100℃ basically follow the parabolic oxidation law. During the initial oxidation period, average parabolic oxidation constant Kp of MCrAlY coatings is about 2.3×10-11 g2cm-4s-1, and that of TBCs is about 1.8×10-11g2cm-4s-1; In the stable oxidation period, average parabolic oxidation constant Kp of MCrAlY coatings is about 6.5×10-12g2cm-4s-1, and that of TBCs is about 3.5×10-12g2cm-4s-1. Oxidation process of MCrAlY coatings and TBCs is controlled by adsorption, diffusion, mass-transfer and continuous supply of oxygen from environment to the coating interior for internal oxidation. Degradation of the coating system is a gradual depletion process of the oxygen-affinitive metal elements Al and Cr. Growth and propagation of cracks in the interfacial TGO layers is finally ascribed for the failures of the isothermal oxidation coating systems, i.e. failure of coatings is drove by TGO thermal stresses.