Papers by Author: Sheng Kai Gong

Abstract: Numerical method was used to simulate the stress state of thermal barrier coatings (TBCs) under thermal-mechanical coupled environment. The finite element (FE) model was built as hollow tube and boundary conditions including heating rate, cooling air convection and mechanical loadings were considered. The maximum stress locates where maximum temperature gradients is formed. This failure mode is consistent with the experimental results.

Abstract: The Hf doped NiAl coatings were co-evaporated and co-deposited onto the superalloy substrate by electron beam physical vapor deposition (EB-PVD). During heat-treatment, HfO2 was formed on the NiAl coatings. And, Hf enriched at the interface between the coating and the interdiffusion zone, which could prevent outward diffusion of elements in the substrate. The NiAl coating doped with 0.5% Hf effectively improved the high temperature oxidation resistance compared to the Hf free NiAl coating and the high Hf content coating. Also, the addition of Hf to the coating contributed to enhancing the adherence of TGO layer to coating.

Abstract: The hot corrosion behavior of titanium alloy and AlCuFeCr quasicrystalline coating on titanium alloy in the presence of a solid mixture of NaCl and Na2SO4 deposit at 700°C was studied. The result shows that weight-gain kinetics for titanium alloy exhibited a linear rate law, while the kinetics of AlCuFeCr quasicrystalline coating displayed parabolic growth rate. The corrosion resistance of the titanium alloy was improved by applying the AlCuFeCr quasicrystalline coating. The corrosive oxide morphology formed on titanium alloy was porous. For AlCuFeCr quasicrystalline coating with the mixture of NaCl and Na2SO4 deposit, the scale formed on the coating surface was compact and uniform. Oxide formed on the surfaces of Al-Cu-Fe-Cr quasicrystalline coatings after hot corrosion consisted of Al2O3.

Abstract: 10mol% Nd2O3 and Yb2O3 co-doped YSZ thermal barrier coatings were produced by electron beam physical vapor deposition (EB-PVD). Compared to the traditional YSZ coating, the deposited coating has shown tree-like microstructure in each column. Due to this, the co-doped coating is more porous than the YSZ coating. The microstructure evolution of the coating during high temperature exposure at 1373 K was studied. The tree-like microstructure disappeared due to joining of sub-grains during sintering. Thermal growth oxide (TGO) grew quickly at the first few hours and then the growth of TGO became slow in the subsequent high temperature exposure. Cracks generated and propagated in the ceramic top coat and along the interface of the top coat and TGO layer. Finally, the coalescence of such cracks resulted in failure of the TBCs.

Abstract: The effect of Mo on the oxidation behavior of TiNiAl at 1073K has been investigated. It is found that 1at.% Mo addition can increase the diffusion of Al in the alloys and promote the formation of dense and continuous Al-rich oxide layer. Therefore the oxygen diffusion can be effectively impeded and the oxidation behavior of TiNiAl is improved. The observation of the cross-sectional oxidation layer showed that beneath the top oxide scale a Mo-rich oxide layer formed. Because the oxide of Mo is volatile at high temperature, voids formed in the oxide scales during the oxidation process. 3at.% Mo addition could cause cracks between the oxide scale and the substrate, resulting in poor adhesion of the oxide scale to the substrate.

Abstract: The effect of Ru on the microstructure of a high content refractory elements nickel base Superalloy has been studied in the present investigation. The results showed that, the adding of 3%Ru did not prevent the precipitation of TCP phase during high temperature exposure, and even accelerated the formation of TCP phase,however the addition of Ru decreased the growth rate of μ phase significantly. In addition, the adding of Ru decreased the solving temperature of γ/γ′ eutectic by 5°C, and increased the tendency of γ′ rafting during high temperature exposure.

Abstract: Thermal barrier coatings with one-layered/ two-layered NiAl bond coat were produced by electron beam physical vapor deposition (EB-PVD). Compared to the TBC with one-layered bond coat, the TBC with two-layered bond coat improved the thermal cycling resistance significantly. The failure mechanism of the two-layer NiAl bond coat TBC was investigated in this paper.

Abstract: The influence of YSZ addition on isothermal oxidation behavior of EB-PVD NiAl bond coatings on Ni based superalloy was investigated. The oxidation rate of the YSZ doped bond coat is almost the same as the bond coat without the doping of YSZ. Four different areas exist on the surface of the coating after 100 h oxidation, including the spalled area, the high Al-contained area, the high Cr&Ni-contained area and the high Ti-contained area.

Abstract: Lanthanum-cerium oxide (La2Ce2O7, LC) is considered as a new candidate material for thermal barrier coatings (TBCs) because of its low thermal conductivity and high phase stability between room temperature and 1673K. The LC coatings with different La2O3 contents were prepared by air plasma spraying (APS) and their lifetime was evaluated by thermal cyclic testing from room temperature to 1373 K. The structures of the coatings were characterized by XRD and SEM and the deviation of the composition from the powder was determined by EDS analysis. Long time annealing for the freestanding coating at 1673K reveals that the near stoichiometric LC coating is stable up to 240h, and the stability decreases with increasing the deviation from stoichiometric LC composition. During thermal cyclic testing, spallation was observed within the top coat near the bond coat. It is considered that the effect of intrinsic stress caused by the coefficient of thermal expansion (CTE) mismatch between top coat and bond coat is larger than that of thermally grown oxide (TGO) and the bond adherence of top coat with TGO.

Abstract: The cyclic oxidation of thermal barrier coating (TBC) specimens consisting of nickel-base superalloy, low pressure plasma sprayed Ni-24Cr-6Al-0.7Y (wt.%) bond coatings and air plasma sprayed 7.5 wt.% yttria stabilized zirconia top coatings was studied at 1050°C in air, (air + 5%H2O), O2 and (O2 + 5%H2O) respectively. The oxidation kinetics of the TBC in each test environment accords with parabolic law at the initial stage and obeys almost liner law at the final stage. The cyclic oxidation life of the TBC is 500h (1h/cyc) in O2 and (O2 + 5%H2O) and 900 h in air and (air + 5%H2O). The SEM observations indicated the oxide formed along the bond coat and top coat interface after failure at 1050°C in different environments are all consisted of Al2O3, Ni(Al,Cr)2O4, NiO and Cr2O3.