Oxidation Performance of Ytterbium Disilicate/Silicon Environmental Barrier Coating via Optimized Air Plasma Spraying

Environmental barrier coatings (EBCs) play a critical role in mitigating the degradation of SiC f /SiC ceramic matrix composites (CMCs) in complex combustion environment, and improve the service life of thermal engine components. In this paper, by adjusting the parameters of atmospheric plasma spraying (APS), the spraying process of ytterbium disilicate (Yb 2 Si 2 O 7 ) under a lower power has been optimized. A two-layer EBC system consisting of ytterbium disilicate and silicon is prepared on the SiC f /SiC composite sub-strate by using optimized technological parameters. The thermal resistance and water oxygen corrosion resistance of such two-layer EBC system are in-vestigated. The results indicate that the current ytterbium disilicate/silicon EBC system exhibits good phase stability, excellent water vapor and oxygen corrosion resistance. However, the exposed silicon bonding layer tends to generate an excessive thermal growth oxide (TGO) layer known as SiO 2 , leading to an early spallation of the coating.


Introduction
The improvement of thrust-to-weight ratio of advanced aero-engines is highly dependent on the development and application of advanced materials. To improve the thrust-to-weight ratio, gas turbine's inlet temperature has to be increased, which severely challenges hot-section components. Thanks to thermal How to cite this paper: Jian, Y.J., Wang, Y.F., Liu, R.J. and Wan, F. (2021) [9] is leading the research on EBCs, and the research in China has made great progress in these years. It can be divided into four stages for EBC systems. Currently, researchers mainly focus on rare earth silicate systems [10] and move towards material systems of multi-layered thermal barrier coupled with environmental barrier [11]. The main preparation methods of EBCs include air plasma spraying [12] (APS), electron beam physical vapor deposition [13] (EB-PVD), plasma spraying physical vapor deposition [14] (PS-PVD), ultra-low pressure plasma spraying [15] (VLPPS), etc. APS technology has been widely used as the preparation of EBCs because of its simple operation, high efficiency and low cost. But in the process of spraying, there are inevitably unmelt powder existed in rare earth silicate coatings. Hence, it is difficult to form a coating with very high density [16]. Meanwhile, high spraying power leads to the volatilization of Si causing the deviation of coating composition. Further, the slight oxidation occurs on silicon bond coat during the spraying process [17].
In this paper, an Yb 2 Si 2 O 7 /Si EBC prepared by APS is investigated. After optimization of APS parameters, the microstructure, phase composition, thermal stability and water vapor corrosion resistance of this EBCs system were characterized.

Sample Preparation
The SiC f /SiC substrates were prepared by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. The substrates were polished, cleaned and dried before APS, without sandblasting treatment. The raw materials of Yb 2 Si 2 O 7 powders were commercially available (Beijing Sandspray New Material Co., Ltd.). To ensure the fluidity of the powder, the powders were subjected to a spray pelletization process. The silicon powders with particle size between 50 -70 μm were purchased from

Characterization
Thermal treatment in air as well as in water vapor and oxygen corrosion of the bi-layer EBCs are investigated. The thermal treatments were under 1300˚C, 1400˚C, and 1500˚C respectively for 10 hours; the water vapor and oxygen corrosion test was carried out at 1300˚C in an alumina tube furnace, where the volume ratio of oxygen to water vapor was kept as 1:1. The test was carried out in a cycle of 20 h, and samples were taken out every 2 cycles for characterization.
Scanning electron microscope (SEM, MIRA3, Tescan, China) was used to observe the surface and cross-section morphology of the coatings. Energy dispersive spectrometer (EDS, x-max20, Oxford Instrument, UK) was used to measure element distribution. The image analysis software (ImagJ) was used to perform the threshold segmentation of the images. The specific method was to partition the samples and select 5 images evenly, where the resolution of the images and the width of the field of view were consistent. XRD (D8-advance, Bruker, 0.02˚/step, Cu-Kα, 10˚ -90˚) analysis was carried out to investigate the phase composition of the coating under various treatments.  on the whole with a morphology of multi-layer stacking, and the defects in the topcoat are mainly spherical pores and interlayer cracks. The average porosity measured by image method are 11.5%, 8.1% and 10.0%, respectively. The change of grayscale of the photo is mainly caused by the loss of Si. The more Si loss is, the higher the brightness of the area will be. It is difficult to characterize the exact Si loss and the corresponding phase composition of the image. But it can be observed in Figure 1 that the brightness region of the coating increases with the increase of power.

Optimization of Spraying Parameters
In the Parameter 4, interlayer cracks and mud cracks obviously increased, and the reason can be summed up in the increasing of Yb 2 SiO 5 or even Yb 2 O 3 . As the high melt point of the two phases (Yb 2 SiO 5 for 1950˚C, Yb 2 O 3 for 2415˚C), the molten particles in the spraying process are not able to make the previous layer melt, leading to an obvious interlayer interface. When the particles are rapidly cooling in the substrate, lamella contraction occurs with the release of the thermal stress, which results in the growth of micro cracks. Comparing to the cross-section morphologies of parameter 2 and parameter 3, the areas of pores, cracks and unmelt particle areas are greatly reduced, although there are still some remained. Figure 2(a) shows the XRD patterns of the topcoats prepared by different three parameters at 1300˚C. And the XRD patterns of the raw powder, the as-sprayed topcoats and the topcoats after annealing are shown in Figure 2(b). After thermal treatment at 1300˚C, the coatings in three parameters transform into a highly crystalline state, and the main phase composition is Yb 2 Si 2 O 7 and Yb 2 SiO 5 . The phase content does not change significantly with the change of parameters. The other XRD pattern indicates that the as-sprayed coatings are amorphous, and the phase composition of raw powder and the annealed topcoat are nearly the same. These two patterns all suggest that the phase change in the spraying process is slight due to the selection of low power parameters.
Based on the calculation results of porosity, cross-section morphologies and phase evolutions, it can be concluded that parameter 3 is an optimal spraying parameter for APS and the low power is necessary to keep the original phase Figure 2. XRD patterns of (a) different parameters prepared at 1300˚C; (b) the raw powder, the as-sprayed topcoats and the topcoats after annealing. Journal of Materials Science and Chemical Engineering composition. Then thermal treatment and water oxygen corrosions will be performed using optimized parameter 3.

Thermal Stability
XRD patterns of Yb 2 Si 2 O 7 topcoats with parameter 3 after thermal treatment at 1300˚C -1500˚C are shown in Figure 3(a), which is corresponding to the surface morphologies in Figures 3(b)-(d) separately. The XRD patterns show that with the increasing of annealing temperature, the phase composition is stable, consisting of Yb 2 Si 2 O 7 and little Yb 2 SiO 5 , and the intensity of Yb 2 SiO 5 phase is reduced. There is no evidence that any reaction was taken in the isolate thermal treatment in air [18]. According to the microstructure, it can be inferred that the rapid grain growing may bring some influence on the XRD detecting. The surface morphologies show that the surface layers are not in dense stacks, and there are pores remained and microcracks occurred due to the thermal stress.

Water Vapor and Oxidation Resistance
The overall structure is shown in Figure 4(a) and Figure 4   layer called thermal growth oxide (TGO) after corrosion for 160 h. The TGO is a critical cause for the decline of EBC system, due to its special phase transition and volume change at 220˚C. When the Si or SiC is exposed to the corrosion environment, the excessive formation of TGO will face great stress and rapid volatilization resulting in the failure of the EBC system [19].
The product has even better resistance compared to other rare earth silicates [20].
Though, it is difficult to avoid the spraying defects in the EBC, which weakens the mechanical properties of the coatings. The effects of EBC systems for protecting SiC f /SiC composites from oxidation and corrosion are proved to be indeed excellent.

Conclusions
The APS parameters are optimized in the low power level. The bi-layer EBC prepared by the optimized process underwent a thermal treatment as well as a water vapor and oxygen corrosion at 1300˚C. The results can be summarized as following: Y. J. Jian et al. Journal of Materials Science and Chemical Engineering 1) Low power in APS is needed for less phase change taking place in the process. And parameter 3 is convinced to be the better one due to its fewer pores and better morphology.
2) Oxidation is mainly taken place between the Yb 2 Si 2 O 7 topcoats and Si bond coat. Corrosion occurs on the surface of Yb 2 Si 2 O 7 topcoats, and Yb 3 Al 5 O 12 makes a contribution to the oxidation resistance. The bi-layer Yb 2 Si 2 O 7 /Si EBC system is eventually convinced to be an excellent choice for future gas engine components.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.