ICMCTF 2023 Session A2-1-TuA: Thermal and Environmental Barrier Coatings I

The efficiency of gas turbines depends to a large extent on the gas inlet temperature. Thus, increasing the temperature capability of thermal barrier coatings (TBCs) represents a major scientific challenge, especially since segregation of yttria occurs in the t’ phase of yttria partially-stabilized zirconia (8YSZ) at temperatures above 1250 °C in conventional use. For a more precise understanding of resulting critical phase transformations from tetragonal into the monocline phase, gradient tests were carried out on burner rigs in the present paper. The influence of different microstructures of YSZ TBCs on phase transformations at constant cooling rates in the range of 1-10 K/s from aging temperatures as high as 1600 °C were examined for two layer systems.

Nickel-based superalloys are high performance materials used for turbines blades. Due to the long duration of use of these structures and the complex thermomechanical load during a cycle of operations, a coating with an adequate resistance to corrosion and oxidation is essential. The material studied is a polycrystal nickel-based superalloy coated with an aluminure (NiAl) designs as a local aluminum reservoir to form a protective alumina film. NiAl coatings exhibit a brittle behavior up to 650-750°C. This brittle-to-ductile transition makes it sensitive to cracking during service, especially at low temperatures. Such cracking in the coating can lead to premature failure of the coated blade, then propagating under cyclic thermomechanical loading.

The objective is to link the microstructure specificities, with respect to the material’s thermal history, to the mechanical response of the coating in temperature. Thermal treatment up to 1100°C, leads to progressively transform β-NiAl phase in γ’-Ni₃Al with the decrease of the Al content in the coating coupled with the diffusion of Ni from the substrate. It has been reported in NiPtAl coatings that thermal aging delayed the apparition of the first crack. This improvement in the ductility, observed at room temperature, is attributed to the γ’ phase formation in aged specimens. From these results, our aim is to better understand how the brittle-to-ductile transition occurred in NiAl coatings and how thermal aging will impact it. The experimental approach is twofold. First, to study the crack onset, tensile tests have been carried out on dedicated specimens up to 900°C. The same protocol has been applied on thermal-aged specimens. Several thermal treatments conditions have been explored. As for NiPtAl coatings, results show a gain in ductility at room temperature. To assess the aging impact in the vicinity of the brittle-to-ductile transition temperature, attention has been paid on short time treatments. Mechanical results coupled with a detailed microstructure characterization from post-mortem materials will be presented. Then, micromechanical tests have been carried out using ultrathin bond-coating specimens, after thermal treatment or not. Indeed, due to the thickness of the coating, the direct thermomechanical measurement properties of the coating is not trivial. Assessing local properties within the gradient of microstructure may be useful for the understanding the effect of thermal treatment.

The presentation will compare the experimental results from these two campaigns to discuss the effect of thermal aging on the thermomechanical behavior of NiAl coating.

When implemented in an aero-engine, SiC/SiC CMCs require environmental barrier coating (EBC) systems in order to withstand the atmospheric conditions. Physical vapour deposition processes enabled advanced design concepts of the EBC systems since its advantages lie within the precise control over microstructure and chemistry. A favourable EBC system consists of an Si-based bond coat followed by a rare earth, Y or Yb, disilicate as intermediate layer and finalised by a rare earth, Y or Yb, monosilicate top layer which protects the component against water vapour attack.

The EBC systems presented in this talk were manufactured by using magnetron sputtering for the bond coat and reactive magnetron sputtering for the intermediate and top layer. The X-ray amorphous coating system underwent a subsequent crystallization treatment. The macroscopic homogenous surface as well as local defaults like buckling or spallation of the silicate layers was defined during the crystallization process. Afterwards, the three layered EBC system was tested under cyclic oxidation at 1200 °C up to 1000 cycles where no major macroscopic changes appeared to the EBC system. For comparison, an uncoated SiC substrate and a SiC substrate with the bond coat as single layer were tested as well. The microscopic development in terms of phase stability, layer density and interface reactions were examined by SEM, XRD and TEM at different cycle numbers. While the bond coat and the disilicate remained hermetic dense, the monosilicate forms horizontal pores which agglomerated during long term testing. The disilicate and monosilicate remained phase stable while the bond coat started to form a SiO₂-based TGO layer on top. The oxidation kinetics of the bond coat within the EBC system was compared to the single layer bond coat during the thermocyclic testing. The behaviour of the EBC system in water vapour was investigated first as short-term experiment under aggressive conditions in 100 % water vapour steam with a gas velocity of about 10^-1 m/s and secondly for a longer duration in about 30 % water vapour steam with a gas velocity of about 10^-2 m/s at 1200 °C. The degradation behaviour was analysed and correlated to chemistry and morphology of the PVD coatings.

The mechanical stability at high temperatures of Ceramic Matrix Composites (CMC) material based on SiC compounds in combination with low density is the key concept to replace the Ni-based superalloys utilized in today’s aircraft engines. However, the surfaces of these materials react with high pressure water vapour at high temperatures and volcanic ash (CMAS) and show instability due to volatilization, oxidation and diffusion. We present the capabilities of using a combined PVD-CVD technology in a continuous vacuum process to produce Environmental Barrier Coatings (EBC) to stabilize the surface of the CMC against chemical erosion processes and structural degradation. The approach for the coating design is based on a combination of adhesion layer and a chemical barrier dedicated for the CMC for the conditions of application in the turbine. Results for Si bond coat and Yb2Si2O7 barrier coating will be presented, with tests in water vapor up to 510h at 1316°C. SEM, TEM and XRD investigations were done to fully characterize the TGO growth and the diffusion along the interfaces. In addition to Si bond coat, other thin film interfaces have been tested in annealing experiments up to 1400°C in vacuum and in water vapour at 1316°C.

Several studies point out that noise contributes to hearing impairment and poor mental health. With the advancement of technology, the continuously increasing noise is unbearable, and it needs to be solved. To solve the noise problem, this research uses the technique of thermal spray to prepare the polyethylene coatings with different concentrations of ceramic microspheres, which we expect to increase the noise immunity.

The results show that the noise immunity of the coatings and substrate varies at different frequencies. Ceramic microspheres in the polyethylene coatings makes structural of coating changes, and the structure, the thickness and the mass density will affect the noise immunity. This research shows that the polyethylene coating with ceramic microspheres can make the higher sound transmission loss effect than the polyethylene coating without ceramic microspheres, because ceramic microspheres change the structure of coatings.