Hongbo Guo

Development of gradient thermal barrier coatings and their hot-fatigue behavior

Abstract In this paper, a new type of gradient thermal barrier coating was produced by the co-deposition of a tablet of Al–Al 2 O 3 –YSZ (Yttria Stabilized Zirconia, ZrO 2 +8 wt.% Y 2 O 3 ), or Al 2 O 3 –YSZ and YSZ onto a NiCoCrAlY bond coat by EB-PVD. The analysis of SEM with EDS showed that the composition and microstructure changed continuously across the thickness of the gradient coating. Due to interdiffusion of elements between the bond coat and the gradient zone, a thin layer of a γ′ phase (Ni 3 Al) formed at the surface of the bond coat. The coatings were subjected to a series of tests and characterization studies, including thermal cycling, isothermal oxidation, hot-corrosion and microanalyses. Results showed that the substrate temperature during deposition had great effect on the properties of the coatings. The higher the temperature of the substrate, the lower the microhardness of the gradient coating, which can be considered as the formation of microporosity in the two-phase ZrO 2 +Al 2 O 3 region in the gradient coating, and its epitaxy into the YSZ topcoat. The thermal cyclic tests of specimens were performed by exposure to air at 1323 K for 0.5 h, and then cooled to room temperature within 5 min by forced air cooling, with a lifetime of more than 500 h. In addition, compared with a conventional two-layered coating, this gradient coating exhibited a better resistance to hot-corrosion than a conventional two-layered coating.

Investigation on hot-fatigue behaviors of gradient thermal barrier coatings by EB-PVD

Gradient thermal barrier coatings (GTBCs) have been produced by co-deposition of a tablet of mixtures of Al-Al2O3-ZrO2-8wt.%Y2O3 and an ZrO2-8wt.%Y2O3 ingot onto NiCoCrAlY bond coat by means of EB-PVD. Thermal cycling tests at 1323 K were conducted to evaluate the hot-fatigue behaviors of the coating. The microstructure and composition distribution of the coating- was studied by SEM with EDS, and constituent phases identified by XRD. It was found that the gradient structure significantly increased the lifetime of the coating. Due to the selective oxidation of Al in the NiCoCrAlY bond coat and a Ni3Al film on the surface of the bond coat, an Al2O3 layer was developed between the bond coat and the transition layer. Failure of the gradient coating eventually occurred by cracking near the interface between the Al2O3 and transition layer. In addition, Compared with that before the test, the gradient coating after thermal cycling tests becoming softer than that before the test

Effect of thermal exposure on the microstructure and properties of EB-PVD gradient thermal barrier coatings

Abstract The effects of thermal exposure on the microstructure, properties and failure of electron beam physical vapor deposited Al 2 O 3 –yttria stabilized zirconia (YSZ) gradient thermal barrier coatings (GTBCs) were studied. The GTBCs, with lifetimes of more than 500 h for 1-h cycles and 13 h for 0.25-h cycles at 1100 °C, exhibited a better thermal shock resistance than two-layered-TBCs consisting of a NiCoCrAlY bond coat and a YSZ topcoat. The GTBCs also showed a relatively low oxidation rate under cyclic exposure, due to the formation of a pre-deposited Al 2 O 3 film on the bond coat. The oxidation of the bond coat is dominated by the selective oxidation of aluminum. The values of hardness and modulus for the Al 2 O 3 –YSZ graded layer are in the range of 2.0–3 and 170–230 GPa, respectively, whereas after 500 h exposure a considerable reduction in the mechanical properties occurred in a rich Al 2 O 3 area of the graded layer. Micro-cracks initiated and propagated in the rich Al 2 O 3 area, and finally resulted in the spallation failure of the coating. The thermal conductivity of a GTBC was found to be 1.7 W/mK, which was marginally lower than that of the two-layered-TBC at 2.2 W/mK. However, the value of thermal conductivity increased up to 2.0 W/mK after 500 h exposure.

Preparation of Al2O3–YSZ composite coating by EB-PVD

Abstract An Al2O3–YSZ thermal barrier coating was prepared by co-deposition of Al2O3 and YSZ (8 wt.% Yttria Stabilized Zirconia) onto NiCoCrAlY bond coat by means of electron beam physical vapor deposition (EB-PVD). Analysis of SEM/EDXS showed that the structure and composition distribution across the thickness of the coating changed continuously. Simultaneously, a gradient micro-porous microstructure formed in the coating, which resulted in micro-hardness of the coating increasing gradually towards its surface. Additionally, the Al2O3–YSZ coating exhibited a lower thermal conductivity than traditional YSZ coating, which was caused by the micro-porous microstructure in the coating and the vacancies appearing in the ZrO2 lattice.

A study on gradient thermal barrier coatings by EB-PVD in a cyclic high-temperature hot-corrosion environment

Gradient thermal barrier coatings (GTBCs) have been produced by electron beam physical vapor deposition (EB-PVD). Their performance was evaluated by isothermal oxidation and cyclic high-temperature hot-corrosion tests. It is found that the GTBCs exhibited better resistance to high-temperature oxidation and cyclic high-temperature hot-corrosion (HTHC) than traditional two-layered TBCs. A dense Al2O3 layer on the bond coat of GTBCs can effectively prohibit inward diffusion of oxidants such as O and S and outward diffusion of Al and Cr. On the other hand, an “inlaid” interface, the formation of which resulted from the oxidation of Al diffusion into the gaps between the columns of bond coat during the fabrication of the GTBCs, contributes to reinforce the adherence of the Al2O3 layer to the bond coat. During fluxing of the Al2O3 layer, S and O diffused into the bond coat. Cracks developed in the surface layer of bond coat by the combined effect of sulfidation of the bond coat and thermal cycling, and finally led to failure of the GTBC.

Measurements of the thermal gradient over EB-PVD thermal barrier coatings

Conventional two-layered structure thermal barrier coatings (TBCs), graded thermal barrier coatings (GTBCs) and graded thermal barrier coatings with micropores were prepared onto superalloy DZ22 tube by electron beam physical vapor deposition (EB-PVD). Thermal gradient of the TBCs was evaluated by embedding two thermal couples in the surfaces of the tube and the top coat at different surrounding temperatures with and without cooling gas flowing through the tube. The results showed that higher thermal gradient could be achieved for the GTBCs with micropores compared to the two-layered structure TBCs and GTBCs. However, after the samples were heated at 1050°C, the thermal gradient for the GTBCs with or without micropores decreased with the increase of heating time. On the other hand, the thermal gradient for the TBCs increased with the increase of heating time. Cross-section observations by scanning electron microscopy showed that the change in microstructure was the main reason for the change of the thermal gradient.