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 barrier coatings and sophisticated cooling technology, nickel-based superalloys can tackle current temperature challenges, but are not capable of satisfying future development of continuous increase of service temperature. Instead, silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiCf/SiC-CMCs) are proposed as an ideal material for engine components such as regulating piece, seal, heat insulation screen and turbine blades etc. due to their high melting point, low density, good thermo-mechanical properties and anti-oxidation properties   . However, while considering the high temperature combustion environments with high humidity, the dense film of SiO2 on the surface of SiCf/SiC composites tends to react with water vapor to form volatile products, leading to the rapid degradation of SiCf/SiC composites  . Therefore, it is necessary to deposit environmental barrier coatings (EBCs) on the surface of SiCf/SiC composite components to physically isolate components from the complex engine environment and ultimately extend the service life of the CMC components  .
NASA   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  and move towards material systems of multi-layered thermal barrier coupled with environmental barrier . The main preparation methods of EBCs include air plasma spraying  (APS), electron beam physical vapor deposition  (EB-PVD), plasma spraying physical vapor deposition  (PS-PVD), ultra-low pressure plasma spraying  (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 . 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 .
In this paper, an Yb2Si2O7/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.
2.1. Sample Preparation
The SiCf/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 Yb2Si2O7 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 Metco with a brand of 4810. A low-power APS with different current level was used to prepare Yb2Si2O7 topcoat while Si bond coat was employed by another set of parameters. All parameters are listed in Table 1.
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.
3. Result and Discussion
3.1. Optimization of Spraying Parameters
Figure 1 shows the SEM images of cross-section morphology of Yb2Si2O7 coatings prepared by three different parameters of low power. The topcoat is dense
Figure 1. Cross-sectional morphologies of Yb2Si2O7 topcoats. (a) parameter 2; (b) parameter 3; (c) parameter 4.
Table 1. Air plasma spraying parameters for Si and Yb2Si2O7 coatings.
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.
In the Parameter 4, interlayer cracks and mud cracks obviously increased, and the reason can be summed up in the increasing of Yb2SiO5 or even Yb2O3. As the high melt point of the two phases (Yb2SiO5 for 1950˚C, Yb2O3 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 Yb2Si2O7 and Yb2SiO5. 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.
composition. Then thermal treatment and water oxygen corrosions will be performed using optimized parameter 3.
3.2. Thermal Stability
XRD patterns of Yb2Si2O7 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 Yb2Si2O7 and little Yb2SiO5, and the intensity of Yb2SiO5 phase is reduced. There is no evidence that any reaction was taken in the isolate thermal treatment in air . 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.
3.3. Water Vapor and Oxidation Resistance
The overall structure is shown in Figure 4(a) and Figure 4(b). The system is clearly divided into three layers which are well bonded. There are no obvious cracks or declination. For as-sprayed Yb2Si2O7 topcoat, the morphology of cross-section is quite different from the corroded sample for 120 h, which changes from layered rough state to dense smooth state after corrosion. Meanwhile, the phase distribution changed from unobvious to obvious dark and bright two-phase distribution. The topcoat along with the bond coat provides oxidation resistance for SiCf/SiC composites, and there is slight oxidation due to the diffusion of oxygen. As the Figure 4(c) and Figure 4(d) shown, the interface between Yb2Si2O7 topcoats and Si bond coat combines well as sprayed, and forms a SiO2
Figure 3. (a) XRD patterns of Yb2Si2O7 topcoats deposited using plasma spray parameter 3 which were separately annealed at 1300˚C, 1400˚C, 1500˚C for 10 h; Surface morphologies at (b) 1500˚C; (c) 1400˚C; (d) 1300˚C.
Figure 4. Cross-sectional SEM photos of EBC (a) as-sprayed; (b) corrosion for 160 h; Interfaces between Yb2Si2O7 topcoat and Si bond coat (c) as-sprayed; (d) corrosion for 160 h at 1300˚C.
Figure 5. Surface of Yb2Si2O7 topcoats (a) annealed in 1300˚C; (b), (c) different areas corroded for 160 h.
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 .
Figure 5 shows the microstructure on the surface of Yb2Si2O7 topcoats. After corrosion for 160 h, the grain boundary is corroded, but in other areas, the topcoat was protected by a new, dense material. According to the EDS analyzation, the new material is rich of Al, which is mentioned to have the reactions as following:
The product has even better resistance compared to other rare earth silicates .
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 SiCf/SiC composites from oxidation and corrosion are proved to be indeed excellent.
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:
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 Yb2Si2O7 topcoats and Si bond coat. Corrosion occurs on the surface of Yb2Si2O7 topcoats, and Yb3Al5O12 makes a contribution to the oxidation resistance. The bi-layer Yb2Si2O7/Si EBC system is eventually convinced to be an excellent choice for future gas engine components.
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