Water vapor and CMAS corrosion tests of Si/Y2SiO5 Thermal and Environmental Barrier Coating

China Abstract Thermal and environmental barrier coating (TEBC), the up-to-date concept, is introduced to protect silicon-based ceramics matrix co mposites (CMC) from not only high temperature water vapor but also the alkali salt from volcanic ash and dust suspending in atmosphere. Because both of high temperature steam and CMAS will make Si-based CMC deteriorate rapidly. By executing the corrosion test against high temperature water vapor, we find that Si/Y 2 SiO 5 double-layer TEBC can effectively protect SiC f /SiC CMC from water vapor at 1300 ℃ for over 205 hours. Almost all Y 2 SiO 5 transform into Y 4.67 (SiO 4 ) 3 O after corrosion test. It is also found that in CMAS corrosion test, the reaction zone formed between CMAS and Y 2 SiO 5 layer prevents the mutual diffusion of elements in CMAS and Y 2 SiO 5 layer. The apparent activation energy of reaction between CMAS and Y 2 SiO 5 in 1200~1300℃ temperature ranges is calculated to be 713.749kJ/mol. These findings provide a reference to select appropriate materials for


Introduction
In order to further develop the high thrust-weight ratio aero engine, decreasing the weight of ho section structural components and improving the inlet temperature of combustion room simultaneously is the most efficient approach [1]. Nickle-based high temperature alloy is used as the hot section structural materials in aero engine.
However, the operating temperature in combustion room has already out of the usage temperature limit of the Ni-based alloy which restricts the further improvement of thrust-weight ratio. Si-based ceramics matrix composites (CMC) such as carbon fiberreinforced silicon carbide CMC (Cf/SiC CMC) and silicon carbide fiber-reinforced silicon carbide CMC (SiCf/SiC CMC) are promising candidates for hot section structural components due to low density and exceptional thermo-mechanical stability and good oxidation resistance [2][3][4]. In dry air, Si-based CMC will react with oxygen to form the silica which can isolate the oxygen and prevent Si-based CMC not to be oxidize further. However, the operating environment of Si-based CMC is full of hot gas containing high temperature water vapor. The passive silica will react with water vapor to form volatile Si(OH)4 [5]. The volatile Si(OH)4 can be easily brushed away by high pressure hot gas, making Si-based CMC continuously exposure to hot gas.
Continuous corrosion from water vapor makes Si-based CMC deteriorate rapidly.
The environmental barrier coating (EBC) fabricated on the surface of Si-based CMC aims at protecting Si-based CMC from hot gas at first. The EBC is always made up by two or three layers of coatings containing top layer and bond layer. The materials used as top layer of EBC should satisfy some constraints. For example, excellent water vapor corrosion resistance is the most fundamental condition. The coefficient of thermal expansion (CTE) of materials should be as close to the CTE of Si-based CMC as possible to avoid cracking and delamination of coating. Besides, materials must be stable and compatible with bond layer so that deleterious reaction won't happen. The bond coating is deposited to alleviate the thermal mismatch between top layer and Si-based CMC resulting from the widely varying CTE between them. When oxygen permeating into coating along the cracks in top layer, the bond layer will consume oxygen and stop them continuously diffuse inward [6]. Over the past thirty years, researches about EBC have made great breakthrough. It has been proved that EBC effectively extends the service time of Si-based CMC [7]. The up-todate EBC system containing rare earth silicate top layer and silicon bond layer is proved to be the most excellent candidate.
Researches have shown that resistance against high temperature water vapor of monosilicates is much more excellent than that of rare earth disilicates [6]. Besides, there are at most seven types of crystal structures of one disilicate over the wide temperature range [8,9]. The CTE of different crystal structures varies greatly which has an adverse influence on the integrality of EBC. There are only two types of crystal structures of every monosilicates [8,9]. So, the monosilicate is much more suitable to be used as top layer of EBC. Y2SiO5 is the most outstanding candidate out of many rare earth monosilicates due to its superior thermal and mechanical properties in high temperature condition [10][11][12][13][14][15][16][17]. The water vapor corrosion resistance of Y2SiO5 is also proved to be more excellent than other monosilicates from both experiments and simulated calculations [13][14][15].
The damage to Si-based CMC caused by high temperature water vapor can be alleviated through EBC as illustrated by several researches [14,18,19]. But Si-based CMC still faces the risk of corroded by foreign alkali salt. Foreign alkali salts will react with each other at high temperature to form the CMAS glass. CMAS can slowly permeate into the SiO2 thin layer on the surface of Si-based CMC, destroying the protection of SiO2 for Si-based CMC. The Si-based CMC itself can be operated in extremely high temperature, so in addition to high temperature water vapor, the EBC only need protect Si-based CMC against CMAS, justly like the role of TBC to Nibased alloy. And such thermal and environmental barrier coating (TEBC) is in urgent need [20]. However, most of researches only focus on the water vapor corrosion resistance of rare earth silicate. There is hardly any investigation about the resistance against the hot gas and CMAS simultaneously.  wt.%), and deionized water. After spray granulation, the Y2SiO5 powders were calcined at 1100 ℃ in furnace to get rid of PVA and achieve the densification. The densified Y2SiO5 powders were sieved to obtain the spraying powders meeting the flowability required in APS process.
The Y2SiO5 coatings was deposited at room temperature. The thickness Y2SiO5 coatings was 120 μm, respectively. The substrate coated with silicon coating was ultrasonically cleaned in ethanol to keep the surface clean before coating deposition.
The substrates were fixed on the steel wire gauze on the surface of steel shelf. The spraying parameters were shown in Table 1. The moving rate during coating deposition is 0.5 m/s. After deposition, the coatings were annealing in Ar atmosphere at 1400℃ to release the stress generated in preparing process. The Si/Y2SiO5 coated substrates were then ultrasonic cleaned in ethanol for further test. atmosphere on a 10-hour rotation. The gas generator (LVD-F1, Hefei Kejing Material Technology Co., Ltd.) was used to generate the vapor and the flow rate of steam was 0.17 cm/s. In order to accurately obtain the weight change tendency of samples during water vapor corrosion process, the samples on the alumina boat were taken out from the tube furnace and measured the weight of the sample every 10 h.

CMAS corrosion test
The 38CaO-5MgO-8AlO1.5-49SiO2 (CMAS) was used as corrosive medium to simulate practical situation of aero-energy in service. The CMAS powders were

Analysis and characterization
The morphologies of coatings before and after corrosion and the elements distribution were observed by scanning electron microscope (SEM, Philips S-4800, Hitachi Ltd., Yokohama, Japan) with attached energy dispersive spectrometer (EDS).
The SEM can also be used to measure the thickness of emerging layer between Si/Y2SiO5 is also apparently excellent than ZrSiO4 coating [21]. The works mentioned above demonstrate Si/Y2SiO5 TEBC system has great potential against high temperature water vapor.
The surface microstructure of Si/Y2SiO5 TEBC before and after water vapor corrosion in 90%H2O/10%O2 atmosphere at 1300 ℃ are shown in Fig. 1. It can be seen clearly from Fig. 1(a) Fig. 1(b) and Fig. 1(c).    respectively. It can be seen from Fig. 7 Table 2 lists the thickness of reaction zone when Si/Y2SiO5 TEBC were corroded at 1200 ℃, 1250℃ and 1300 ℃ for different times. It can be found from Table 2 that at the same reaction temperature, as reaction time is prolonged, the reaction zone gets thicker. The thickness of reaction zone also increases with reaction temperature when corrosion time is same.  Growth of the reaction zone seems to obey the parabolic law:

Reaction kinetics between CMAS and Si/Y2SiO5 coatings
Where y is the thickness of reaction zone, t is the oxidation time, k is the reaction rate constant, b is constant. Fitting process is conducted on thickness data listed in Table 2. According to the fitting result, the reaction rate constant at 1200℃, 1250℃ and 1300 ℃ are 0.1466μmh -1/2 , 0.8425μmh -1/2 and 1.9963μmh -1/2 , respectively. The reaction rate constant represents the growth rate of reaction zone at different temperature. When CMAS corrosion test is conducted at 1200 ℃, the growth rate of reaction zone is very slow. As reaction temperature raising to 1250 ℃, the growth rate of reaction zone increases more than four-fold compare with that at 1200 ℃. When corrosion temperature continues to raise to 1300 ℃, thickness of reaction zone will dramatically increase more than twelve-fold compare with that at 1200 ℃. The obtained reaction rate constants at elevated temperature are plotted in Fig. 9.
The apparent activation energy for the reaction between CMAS and Y2SiO5 layer can be calculated using Arrhenius formula as follow: Where E is apparent activation energy for CMAS corrosion test, R is the ideal gas constant, T is temperature of CMAS corrosion test.
The natural logarithm of equation 5 is shown as follow: ln k − ln k 0 = −E/2RT (6) According to the fitting result shown in Fig. 9, the relationship between the natural logarithm of reaction rate constant and reciprocal temperature obeys linear relation.
The slope of fitting line represents the apparent activation energy for CMAS corrosion, which is calculated to be 713.749k J/mol.