Microstructure and high-temperature oxidation behavior of plasma-sprayed Si/Yb2SiO5 environmental barrier coatings

Jie XIAO, Qiomu LIU, Jinghen LI, Hongbo GUO,*, Huibin XU

a School of Materials Science and Engineering, Beihang University, Beijing 100083, China

b Key Laboratory of High-Temperature Structure Materials and Protective Coatings (Ministry of Industry and Information Technology), Beihang University, Beijing 100083, China

c AEEC Sichuan Gas Turbine Establishment, Chengdu 610500, China

KEYWORDS

Abstract An environmental barrier coating (EBC) consisting of a silicon bond coat and an Yb2-SiO5 top-coat was sprayed on a carbon fibers reinforced SiC ceramic matrix composite (CMC) by atmospheric plasma spray (APS). The microstructure of the coating annealed at 1300°C and its high-temperature oxidation behavior at 1350°C were investigated.The significant mass loss of silica during the plasma spray process led to the formation of Yb2SiO5 and Yb2O3 binary phases in the top-coat. Eutectics of Yb2SiO5 and Yb2O3 were precipitated in the top-coat, and channel cracks were formed in the top-coat after 20 h annealing because of the mismatch between the coefficients of thermal expansion(CTEs) of Yb2SiO5 and the SiC substrate.The EBC effectively improved the oxidation resistance of the CMC substrate. The channel cracks in the Yb2SiO5 top-coat provided inward diffusion channels for oxygen and led to the formation of oxidation delamination cracks in the bond coat, finally resulting in spallation failure of the coating after 80 h oxidation.

1. Introduction

SiC ceramic matrix composites(CMCs)with excellent temperature capability, low coefficient of thermal expansion (CTE),high fracture toughness, and superior resistance to sintering have been proposed to be the most promising propulsion materials to achieve high efficiency and low emission performance in gas turbines.1-3Although CMCs exhibit superior resistance to oxidation in dry air environment by forming a dense and stable silica scale,the protective scale can react with water vapor and produce gaseous species in the combustion environment of engines with high-velocity steam, resulting in high-rate recession of CMCs.4-8Environmental barrier coatings (EBCs) have shown promising potential in improving the environmental durability and service life of Si-based CMC hot-section components.

Based on the second EBCs architecture, rare earth silicates have replaced barium strontium aluminosilicate(BSAS)whose limit temperature is near 1300°C due to the formation of a glass phase of the top-coat.9-14Because of their superior chemical compatibility with substrates, moderate CTE matching to SiC, excellent phase stability, and lower volatility in turbine engine combustion environment,15-19ytterbium monosilicates(Yb2SiO5)have been considered to be one of the most promising candidates for EBCs materials.20-22

The processing and resistance to moisture vapor corrosion of Yb2SiO5coatings have been investigated. Richards and Wadley produced silicon/mullite/Yb2SiO5coatings onto SiC substrates using APS, and the microstructures of the coatings were concerned.19The resistance to moisture vapor corrosion at 1300°C of an APS-deposited silicon/mullite/Yb2SiO5coating has been reported by He et al.23Begley et al. have also studied the water vapor corrosion behavior and the fracture mechanism of the silicon/mullite/Yb2SiO5coating between 1316°C and 1410°C in a 90%H2O+10%O2environment.24

The reported CTE of Yb2SiO5is (3.5-4.5)×10-6°C-1,which is rationally matched to that of a SiC substrate((4.5-5.5)×10-6°C-1).19Hence, in this paper, a bi-layer EBC Yb2SiO5/silicon coating system was deposited on a Cf/SiC CMC substrate by the APS process.The microstructures of the as-sprayed coating and the coating annealed at 1300°C were investigated. The oxidation behaviors and associated failure mechanisms of the coatings were studied at 1350°C.

2. Experimental

Fused and crashed (FC) silicon powders are provided by Macklin Co., Ltd, while Yb2SiO5powders are supplied by Guangzhou Research Institute of Non-ferrous Metals and produced by drying spray. The morphologies of FC silicon powders and Yb2SiO5powders are shown in Fig. 1(a) and Fig. 1(b), respectively. The average particle size of the FC silicon powders is about 50 μm, while that of the Yb2SiO5powders is in a range of 3-50 μm. Rectangular specimens of Cf/SiC CMC substrates were machined to 10 mm×8 mm×4 mm.A substrate was preheated to around 800°C before spraying of EBCs.A silicon bond coat was firstly sprayed onto the substrate using a DH-80 air plasma spray system and followed by deposition of the Yb2SiO5top-coat. Before deposition, the substrate was heated by torch to above 800°C.The processing parameters for spraying Si and Yb2SiO5coatings are shown in Table 1, in which the difference of spray distances between Yb2SiO5and Si layers is that the evaporation temperature of Si is lower than that of Yb2SiO5.

As-sprayed specimens were annealed in vacuum at 1300°C for 20 h. High-temperature oxidation tests of the annealed specimens were carried out at 1350°C. Besides, the weights of both non-sprayed CMC specimens and coated specimens were measured every several hours to obtain oxidation gain curves and study the high-temperature oxidation behaviors of coatings.

The phases and components of coatings were characterized by X-ray diffraction (XRD, Rigaku D/max 2500), and the microstructures were observed by scanning electron microscopy (SEM, Quanta 200F) with energy dispersive spectra(EDS).

3. Results and discussion

3.1. Microstructure of the Yb2SiO5/silicon EBC

Fig. 2(a) shows a cross-section SEM micrograph of the sprayed EBC.The Yb2SiO5top-coat of about 120 μm in thickness was intimately bonded to the silicon bond coat with a thickness of about 60 μm. In terms of image analysis, the porosity values of the Yb2SiO5top-coat and the silicon bond coat are approximately 3.5% and 1.8%, respectively. Besides,some vertical cracks can be seen on the Yb2SiO5topcoat,which was caused by thermal stresses due to the CTE mismatch between the top-coat and bond coat during the cooling stage of spraying. After 20 h annealing treatment at 1300°C,the number of vertical cracks increased, as shown in Fig. 2(b).In addition,the porosity of the EBC decreased apparently,which could be ascribed to the sintering of the Yb2SiO5topcoat and silicon bond coat. In addition, a thin layer of SiO2was formed at the Yb2SiO5-silicon interface as a result of the oxidation of the silicon bond coat.

Fig.3 shows an SEM micrograph of the cross-section of the sprayed Yb2SiO5top-coat. The top-coat reveals a typical lamellar structure and is basically comprised of a light-grey zone and a dark-grey zone. A contrast difference in BSE images is caused by the difference in the electron backscattering coefficient η which can be calculated as follows:

Fig. 1 SEM micrographs of the powder feedstock for plasma spray.

Table 1 Processing parameters for plasma-sprayed EBCs.

Fig. 2 BSE micrographs of the cross-sections of Yb2SiO5/silicon EBC.

Fig. 3 Micrograph of the cross-section of the as-deposited Yb2SiO5 top-coat.

Annealing of the as-sprayed specimens was conducted in vacuum at 1300°C to complete the crystallization of the coating and enhance the interface bonding between the coating and the substrate.It can be seen that after 20 h annealing,the contrast difference between the dark-grey zones (lower atomic number)and light-grey zones(higher atomic number)becomes less apparent when compared to that in the as-sprayed coating,as shown in Fig. 4. Besides, a large number of fine light-grey particles are present in the dark-grey zones.The phase compositions of the as-sprayed and annealed Yb2SiO5coatings were determined by XRD. As seen in Fig. 5(a), there are apparent hump peaks, suggesting the existence of amorphous phases in the as-deposited coating due to rapid cooling during spray deposition. After 20 h annealing, the Yb2SiO5coating is mostly comprised of Yb2SiO5phase. Besides, small amount of Yb2O3phase is also present in the coating, as shown in Fig. 5(b). This indicates that crystallization of the amorphous coating is completed during annealing.

where Ciis the weight fraction for the ith element in the compound, and Ziis the atomic number of the ith element. The factors of the related compounds calculated using the formulas are: η (Yb2O3)=0.4220 and η (Yb2SiO5)=0.3828. The factor η is negatively correlated with the contrast of the composition in the BSE images, which indicates that the chemical compositions of dark- and light-grey zones are close to Yb2-SiO5and Yb2O3, respectively.

The significant BSE contrast difference between splats is attributed to the compositional variations from the loss of silicon during spraying.19,25,26The actual chemical composition of the sprayed coating is deviation from that of the Yb2SiO5powders, possibly due to the volatilization of SiO2in droplets during spraying.

Fig. 4 Higher magnification of the BSE image of the Yb2SiO5 top-coat annealed at 1300°C for 20 h.

Fig. 5 XRD patterns for the Yb2SiO5 coating.

According to the SiO2-Yb2O3pseudo-binary phase diagram in Fig.6,the phase constituents are shifted to a two-phase zone of Yb2O3and Yb2SiO5with decreasing the content of Si.Thus,it can be understood that eutectic transformation occurred in the annealed coating. As a result, extremely fine eutectics of Yb2O3and Yb2SiO5, surrounded by proeutectics of Yb2O3and Yb2SiO5, were precipitated during annealing treatment,as shown in Fig. 4, accompanied with the depletion of silicon in the droplets during spraying.

3.2. High-temperature oxidation of the EBCs

Oxidation behaviors of the EBC samples were investigated at 1350°C. Photographs of the EBC samples after 20, 50, and 80 h oxidation are shown in Fig. 7(a)-(c), respectively. Some cracks were generated at the edge of the sample after 20-h oxidation.The cracks became larger after 50 h,and finally further development of the cracks led to the partial spallation of the coating after 80 h.

The edge delamination might be attributed to two processes. The first is the CTE mismatch between the top coat and the substrate which would create a tensile stress in the Yb2SiO5layer on cooling causing the formation of channel cracks especially at the edge of the sample.Then,the oxidation of silicon exposed to form SiO2was subjected to more than a 100% volume increase. This enabled thermally grown oxide(TGO) to be formed rapidly at the edge and resulted in the spallation of the coatings eventually.24

Fig. 6 SiO2-Yb2O3 pseudo-binary phase portrait.19

The mass gains between the EBC and naked samples during isothermal oxidation at 1350°C are shown in Fig. 8. At the beginning of 10-h oxidation, both the coated and naked samples revealed significant weight loss, which can be considered to be caused by the oxidation of carbon fibers in the matrix.The vertical cracks in the coating shown in Fig. 2 provided inward diffusion channels for oxygen,resulting in rapid oxidation of carbon fibers in the composite.

A backscattered SEM micrograph of the coating after 50 h oxidation is shown in Fig. 9. Large channel cracks were formed in the top-coat because of the CTE mismatch between Yb2SiO5and the SiC substrate. An apparent TGO region can be observed at the interface between the Yb2SiO5top-coat and the silicon bond coat close to the channel cracks in the topcoat. This confirms that oxygen diffused into the substrate along the channel cracks as shown in Fig. 9.

After 80 h oxidation,more vertical cracks were generated in the coating as a result of the mismatch between the top coat and the substrate (Fig. 10), and these cracks were extended from the top-coat to the silicon bond coat. The oxidation of carbon fibers in the matrix led to the formation of a large number of pores in the matrix.

In addition,delamination cracks were also formed between the interface of the top-coat and the bond coat, as shown in Fig. 10. Obviously, the delamination cracks finally led to the spallation of the coating (Fig. 7(c)). Two adjacent channel cracks propagated into the silicon bond coat because of the inward diffusion of oxygen,then joined with each other during oxidation,and finally formed oxidation delamination cracks in TGO. The mechanism for the formation of delamination cracks is consistent with the study of Richards et al.18,27,28

Fig. 7 Photographs of EBC samples after oxidation at 1350°C.

Fig.8 Isothermal oxidation kinetics of EBC and naked samples at 1350°C.

Fig. 9 BSE micrograph of the cross-section of the EBC after 50 h oxidation at 1350°C.

4. Conclusions

(1) A silicon/Yb2SiO5EBC was fabricated by APS,and the oxidation behavior of the coating was investigated at 1350°C. A few vertical cracks were formed in the sprayed Yb2SiO5top-coat. After 20 h annealing at 1300°C, the number of vertical cracks increased, and some of the vertical cracks propagated to the silicon bond coat.

(2) The sprayed Yb2SiO5top-coat contained a number of amorphous phase and Yb2O3phase due to the volatilization of SiO2during spraying. After 20 h annealing, the coating was mainly comprised of extremely fine eutectics of Yb2O3and Yb2SiO5surrounded by proeutectics of Yb2O3and Yb2SiO5.

Fig.10 BSE micrograph of the cross-section of the EBC after 80 h oxidation at 1350°C.

(3) The EBC effectively improved the oxidation resistance of the CMC substrate at 1350°C. The channel cracks in the Yb2SiO5top-coat provided inward diffusion channels for oxygen, which led to the formation of TGO on the silicon bond coat and delamination cracks in the bond coat, and finally resulted in the spallation failure of the coating after 80-h oxidation.

Acknowledgement

This research was sponsored by the National Natural Science Foundation of China (NSFC) under grant Nos. 51590894,51425102, and 51231001.