2022 年 63 巻 12 号 p. 1670-1676
SiC ceramic matrix composites (CMC) are potential materials for hot section gas turbine components due to their high heat resistance and low density. However, SiC CMC degrades over time in steam oxidation environments. Therefore, environmental barrier coatings (EBC) are necessary to protect SiC substrates. Yb2Si2O7 is one of the most potent materials for barrier coatings. In this study, self-healing property is further added to Yb2Si2O7 EBC by incorporating SiC in the form of Yb2Si2O7/SiC granulated powders. We investigated the effect of different SiC contents and particle sizes on the long-term oxidation behavior of Yb2Si2O7/SiC coatings. The self-healing property was observed in the microstructural changes of artificial cracks induced by Vickers indentation. The samples were oxidized at 1300°C, and the structural change before and after the oxidation was evaluated by SEM, XRD, and EDX. The cross-sectional SEM images show that SiC was entirely oxidized in the first 50 h. Meanwhile, the surface SEM results show that small particle size and high SiC content exhibited a significant volume expansion during the oxidation of SiC. In addition, the self-healing property was significantly active in the initial stage of oxidation, but drastically deteriorated as the oxidation progressed.
This Paper was Originally Published in Japanese in J. Japan Thermal Spray Soc. 59 (2022) 27–32.
In recent years, global warming has become an urgent issue worldwide, requiring the development of aircraft engines with low fuel consumption and low environmental impact. Increasing the combustion temperature improves thermal efficiency and reducing the weight of the engine components allows for flights with lower energy consumption.1) Many studies are ongoing on how to increase the combustion temperature and reduce weight, e.g. by converting components made of Ni-based superalloys to SiC-based ceramic matrix composites (CMC).2,3) However, SiC reacts with H2O in a high-temperature steam environment, as shown in the chemical reactions (1) and (2), resulting in thinning the SiC substrate by volatilization of Si(OH)4.1) Introducing an environmental barrier coating (EBC) to protect SiC substrates from steam oxidation is necessary.
| \begin{equation} \text{SiC} + \text{2O$_{2}$}\to \text{SiO$_{2}$} + \text{CO$_{2}$} \end{equation} | (1) |
| \begin{equation} \text{SiO$_{2}$} + \text{2H$_{2}$O}\to \text{Si(OH)$_{4}$(g)} \end{equation} | (2) |
The physical and chemical properties required for EBC include not only a high heat resistance and water corrosion resistance but also chemical stability against CMAS (CaO, MgO, Al2O3, and SiO2), which is the main components of volcanic ash, and a high fracture toughness against flying object debris (FOD), such as dust, ice flakes, and birds, which causes damage to aircraft engines.4) In addition, EBC should have a coefficient of thermal expansion close to that of the substrate material SiC to prevent coating delamination. Some materials have been considered for EBC top coats,5,6) and rare earth silicate is one of the leading candidate materials.7) Among the rare earth silicates, Yb2Si2O7 has a similar coefficient of thermal expansion to SiC8) and shows excellent water corrosion resistance and heat resistance.9) The previous studies10) have shown that the addition of SiC to Yb2Si2O7 EBC exhibits self-healing properties that can fill cracks in the coating, as well as microstructural stability of coating chemical composition. The chemical reactions of this mechanism are shown in Reactions (3) and (4) and Table 1. The transformation of SiC to SiO2 is accompanied by a volume expansion of approximately 200%, and the entire reaction of formation of Yb2Si2O7 from SiC and Yb2SiO5 results in a volume expansion of approximately 11.7%. When a crack occurs in the EBC, this volume expansion leads to self-healing, thereby sealing the crack.11) This self-healing property contributes to an extension in the life of the EBC because damage to the EBC may otherwise expose the SiC substrate to high-temperature steam.12) In addition, Yb2Si2O7 is produced from Yb2O3 and SiO2, making it difficult to produce a single substance phase only, and generally, a mixed phase of Yb2Si2O7 and Yb2SiO5 is produced.13) In a high-temperature steam environment, the composition of EBC may gradually change to Yb2SiO5 or Yb2O3 because of the preferential volatilization of SiO2. However, the addition of SiC can compensate for this SiO2 volatilization and stabilize the microstructure.
| \begin{equation} \text{SiC} + \text{2O$_{2}$}\to \text{SiO$_{2}$} + \text{CO$_{2}$} \end{equation} | (3) |
| \begin{equation} \text{Yb$_{2}$SiO$_{5}$} + \text{SiO$_{2}$}\to \text{Yb$_{2}$Si$_{2}$O$_{7}$} \end{equation} | (4) |
Plasma spraying, vapor deposition,14) and liquid-phase deposition15) are the conventional coating processes for the EBC deposition. In this study, we focused on the plasma spraying process, which has a high deposition rate and allows the microstructure to be controlled by the form of material powder.16) In plasma spraying process, there is a concern that the chemical composition of the particles may change due to the thermal effects of the plasma. Spraying a mixed powder of Yb2Si2O7 and SiC, the problem is the volatilization of the SiO2 component, which may be produced by the oxidation of SiC due to thermal effects or contained in Yb2Si2O7.17,18) The previous study19) showed that using Yb2Si2O7/SiC granulated powder reduces the thermal effect on SiC, suppressing volatilization of the SiO2 component. The granulated powder is a mixture of several materials in a single particle. This powder is made by spray-drying a mixed slurry composed of a binder and several raw powders which are sintered and ground. Granulated powder was also used in this study because the composition of the particles can be controlled by changing the ratio of the chemical species in the granulated powder.
Previous studies10,19) on microstructure control by SiC addition confirmed that new cracks appeared after high-temperature oxidation when the SiC content was 10 wt%. This phenomenon is thought to be due to the excessive volume expansion caused by SiC addition. Therefore, further studies are required to determine the optimal SiC addition conditions. In particular, the SiC content, which affects the self-healing properties, and the SiC particle size, which affects the reactivity of the SiC particles, are considered important parameters. In addition, considering that the on-wing life, the life span in which a jet engine can operate without being dismounted from the aircraft, is longer than 10,000 h,20) it is necessary to observe the long-term self-healing behavior. Two aspects of the oxidation behavior of a Yb2Si2O7/SiC coating need to be considered: the self-healing property and the microstructural stability due to the oxidation of SiC in high-temperature oxidation environments, and SiO2 volatilization causing the degradation of the coating in high-temperature steam environments. In previous studies, the effect of the SiC particle size on the self-healing properties was evaluated for oxidation times of 100 h or less. However, few studies have examined the combined effects of two parameters, SiC content and particle size, and evaluated the oxidation behavior over long oxidation times of 100 h or more. Therefore, in this study, we evaluated the effects of the SiC content and particle size on the self-healing properties on a 500-h scale. This study aims to provide a basic direction for the future design of self-healing EBC by evaluating the effects of the SiC addition conditions on the coating properties.
The coatings were deposited onto a ZrO2 substrate of approximately 2 mm thickness using atmospheric plasma spraying (APS). The spraying conditions are presented in Table 2. We used Yb2Si2O7/SiC granulated powders with four different contents and two different sizes of SiC particles (green silicon carbide, Fujimi Inc.), as shown in Table 3. Figure 1 shows an overview of the granulated powder mixed with 10 wt% SiC of 2.6 µm particle size. The coated surface of the specimens was polished using an automatic polishing machine (EcoMet 250, BUEHLER). First, wet polishing was conducted using #600 polishing paper, and then buffing was conducted using single-crystal diamond suspensions in the sizes 9 µm, 3 µm, and 1 µm.
Overview of the granulated powder.
The self-healing property was evaluated based on changes in the cracks before and after high-temperature oxidation. The initial cracks were introduced into the coating surface using Vickers hardness tester. The indentation load of the Vickers hardness tester (HM2000, Helmut Fischer GmbH) was 1600 mN for 5 s, and the loading and unloading times were 15 s each. Cracks before and after high-temperature oxidation were observed using scanning electron microscopy (SEM) (SU-70, Hitachi High-Tech Corporation).
To verify crack healing in the depth direction, we used a focused ion beam (FIB) (FB2200, Hitachi High-Tech Corporation) to drill the cracks, and cross-sectional SEM images of the healed cracks were obtained. Cross-sectional SEM images were also obtained to evaluate the microstructural and compositional changes, and the composition was analyzed by X-ray diffraction (XRD) (Maxima XRD-7000, Shimadzu Corporation) and energy dispersive X-ray spectroscopy (EDX).
2.3 High-temperature oxidation and steam exposure testsTo evaluate the self-healing property and time variation in the microstructure in high-temperature environments, the specimens were oxidized at 1300°C in atmospheric and steam environments. A tabletop high-temperature muffle furnace (SSFS-130-S, Tokyo Garasu Kikai Co., Ltd.) and high-temperature electric furnace (NHT-2035D, Motoyama Co., Ltd.) were used for high-temperature oxidation tests in atmospheric and steam environments, respectively. The temperature was increased from room temperature to 1300°C over 5 h, and the retention time at 1300°C was defined as the oxidation time. The oxidation times in air and steam were 500 h and 100 h, respectively. For the high-temperature oxidation test in air, the test was interrupted at 50, 100, 200, and 300 h of oxidation to evaluate time variations in the coating morphology.
As mentioned in the introduction, the chemical reactions that mainly occur in Yb2Si2O7/SiC coatings are the oxidation reaction of SiC shown in Reaction (3) and the reaction of Yb2SiO5 with SiO2 to form Yb2Si2O7 as shown in Reaction (4). As shown in Table 1, the rate of volume change for each reaction is approximately 200% in volume expansion for Reaction (3) and approximately 4.3% in volume reduction for Reaction (4).11) Hence, the oxidation reaction of SiC with a volume expansion of approximately 200% is especially significant with regards to the self-healing properties. Therefore, it is possible to qualitatively estimate the potential self-healing properties by checking whether SiC particles remain in the coating. First, we focused on the microstructure and composition of the coating cross-section. Figure 2 shows cross-sectional SEM images of the as-sprayed specimens and the specimens oxidized at 1300°C for 50 h in the air. Figures 2(a)–(c) show 10% SiC-2.6 specimens with 10 wt% SiC of 2.6 µm particle size, and Figs. 2(d)–(e) show 10% SiC-0.3 specimens with 10 wt% SiC of 0.3 µm particle size. Figure 2(a) shows that the black particles are uniformly distributed in the 10% SiC-2.6 as-sprayed coating. EDX analysis confirmed that the particles were SiC. Furthermore, Figs. 2(b) and (c) show that black particles can also be observed after oxidation. By EDX analysis, these black particles were confirmed to be SiO2 produced by the oxidation of SiC. Although it is difficult to discern from the images owing to the small SiC particle size, the same phenomenon was observed for the 10% SiC-0.3 shown in Figs. 2(d) and (e). In the coating cross-section at 50 h of oxidation, the SiC particles changed to SiO2 particles, and the density of the SiO2 particles decreased, indicating that the self-healing property is strongly expressed in the initial stage of oxidation and may slow after that. Figure 3 shows the changes in the composition ratio of Yb2SiO5 and Yb2Si2O7 obtained by XRD analysis. The composition ratio R-value was defined from the intensity ratio of Yb2SiO5 to Yb2Si2O7 calculated from each strongest intensity peaks of both materials in XRD. The diffraction planes of the strongest peaks used in the calculation were ($\bar{4}02$) for Yb2SiO5 and (021) for Yb2Si2O7.11) The vertical axis of the graph shows the result of dividing Rt, the R-value at each oxidation time, Rpowder, the R-value of the feedstock powder. A value greater than unity indicates a coating composition with a higher proportion of Yb2SiO5 than the feedstock powder, and a value lower than unity indicates a coating composition with a higher proportion of Yb2Si2O7 than the feedstock powder. As mentioned in the introduction, in the chemical reaction that occurs in the coating, SiO2 reacts with Yb2SiO5 to produce Yb2Si2O7 via the reaction shown in Reaction (4). From the composition ratio changes in Fig. 3, the R values of both 10% SiC-0.3 and 10% SiC-2.6 show a clear decreasing trend up to approximately 100 h of oxidation, indicating that Reaction (4) is not yet complete at 50 h of oxidation time. The R-value of 10% SiC-0.3, which has a small SiC particle size, drops remarkably fast, suggesting that the appearance and slowing of the self-healing property may also occur earlier for smaller particle sizes.
Cross-sectional SEM images, (a) As-sprayed 10%SiC-2.6 coating, (b) 50 h exposed 10%SiC-2.6 coating, (c) Magnified image of sample in (b), (d) As-sprayed 10%SiC-0.3 coating, and (e) 50 h exposed 10%SiC-0.3 coating.
Relationship between exposure time and Yb2SiO5/Yb2Si2O7 composition rate normalized to feedstock powder.
Next, the effects of the SiC content and particle size on the self-healing properties are described. Due to limited paper space, the following specimens are described here: 7.5% SiC-0.3 specimen with 7.5 wt% SiC of 0.3 µm particle size, 7.5% SiC-2.6 specimen with 7.5 wt% SiC of 2.6 µm particle size, 2.5% SiC-0.3 specimen with 2.5 wt% SiC of 0.3 µm particle size. Surface SEM images of the as-sprayed specimen, the specimen oxidized at 1300°C for 50 h in the air, and the specimen oxidized at 1300°C for 500 h in the air are shown in Fig. 4. The rhombic dashed line indicates the location of the Vickers indentation. Figures 4(a), (d), and (g) show some cracks around the Vickers indentations. After oxidation, healing of these cracks and changes in the surface microstructure were observed. The behavior showed different tendencies depending on the SiC content and particle size. Comparing Figs. 4(a)–(c) and Figs. 4(d)–(f) with a focus on the SiC particle size, new cracks were observed after oxidation, as shown in the dashed ellipse, in addition to crack healing. This phenomenon indicates that a reaction accompanied by volume expansion occurs. In particular, 7.5% SiC-0.3 led to a considerable crack of the coating, and 7.5% SiC-0.3 showed a larger volume expansion than observed for 7.5% SiC-2.6, even considering the difference in surface conditions before oxidation. Regarding crack healing and new crack initiation, smaller SiC particles may have caused more rapid volume expansion owing to their larger specific areas. In a previous study,21) it was reported that a Yb2Si2O7/SiC sintered body using two types of SiC shapes, SiC particles with a specific surface area of 17.16 m2/g and SiC whiskers with a specific surface area of 8.47 m2/g, showed better self-healing properties when SiC particles with a larger specific surface area were used. The current results of our study are considered valid, as similar results have been also reported19) for the thermal spray coatings at an oxidation time of 100 h. Next, we focused on SiC content. Comparing Figs. 4(a)–(c) and Figs. 4(g)–(i), the cracks shown in the elliptical dashed lines remained after oxidation in 2.5% SiC-0.3, although crack healing progressed in some parts, and 7.5% SiC-0.3 resulted in a larger volume expansion than for 2.5% SiC-0.3. Fundamentally, volume expansion is caused by the oxidation of SiC. Although it is reasonable to assume that the self-healing property is proportional to the SiC content, the self-healing area tends to be localized and be reduced for low SiC contents. This is because self-healing occurs only in the periphery of the area in which SiC particles are located. In addition, the surface microstructure changed rapidly during the first 50 h of oxidation, after which the change was moderate. This is because SiC was wholly oxidized to SiO2 after 50 h of oxidation, and no new volume expansion could occur. Surface observations confirmed the inference from the cross-sectional observations. In other words, the self-healing property is strongly expressed in the early oxidation stage but may decrease significantly after 100 h of oxidation. These results suggest that the optimum SiC content tends to decrease with decreasing SiC particle size to suppress the generation of new cracks owing to volume expansion. This study also showed that it might be challenging to maintain the self-healing property over a long time by changing only the SiC particle size and SiC content. The direction of improvement should be oriented toward improving the coating properties by utilizing the self-healing property in the early oxidation stage or a continuous expression of the self-healing property. In the former case, a multilayer structure with a composition that has a higher SiC content on the substrate side can be adopted because vacancies were observed inside the coating in cross-sectional observation. The use of a multilayer structure is expected to have the effect of densifying the coating inside and suppressing cracking on the coating surface due to volume expansion during the initial oxidation stage. The latter includes applying some type of oxidation resistance treatment to the SiC particles so that the self-healing property is continuously expressed. Although there have been no specific reports on the oxidation resistant treatment of SiC particles, it has been reported22) that graphite powder can improve their oxidation resistance by coating Al2O3, POCl3, LaP5O14, and others. If a similar treatment can be applied to the SiC particles, it may be possible that the oxidation reaction of SiC in a high-temperature environment can be delayed by changing the thickness of the oxidation-resistant coating on the SiC particles, thereby providing the EBC coating with a continuous self-healing property.
Surface SEM images of the indentation mark and induced cracks, (a) As-indented 7.5%SiC-0.3 coating, (b) 50 h heat-treated 7.5%SiC-0.3 coating, (c) 500 h heat-treated 7.5%SiC-0.3 coating, (d) As-indented 7.5%SiC-2.6 coating, (e) 50 h heat-treated 7.5%SiC-2.6 coating, (f) 500 h heat-treated 7.5%SiC-2.6 coating, (g) As-indented 2.5%SiC-0.3 coating, (h) 50 h heat-treated 2.5%SiC-0.3 coating, and (i) 500 h heat-treated 2.5%SiC-0.3 coating.
Figure 5 shows a cross-sectional SEM image of a healed crack in a specimen with 5.0% SiC-0.3 coating subjected to an oxidation test at 1300°C for 500 h. Peripheral machining of approximately 20 µm was performed using FIB. The crack extended from the left corner of the rhombic dashed line in Fig. 5(a) owing to the introduction of indentation, and it was confirmed that the crack was healed by high-temperature oxidation. Figure 5(b) shows that the crack healed in the depth direction. This result indicates that the self-healing property is expressed not only on the surface of the coating but also inside the coating. A dense coating with few pores is required to resist from steam oxidation. These results indicate that the self-healing property acts on the entire coating and may favorably affect the steam oxidation resistance.
Cross-sectional SEM observation of the healed crack of 500 h heat-treated 5.0%SiC-0.3 coating by FIB fabrication, (a) Positional relationship between the indentation mark, induced crack, and FIB processing area and (b) Cross-sectional image of the healed crack.
Figure 6 shows surface SEM images of the 7.5% SiC-2.6 atmospheric oxidation specimen with 7.5 wt% SiC of 2.6 µm particle size, and the steam-exposed specimen with indentations introduced and treated at 1300°C for 100 h, respectively. The surface microstructures of the two samples showed no significant differences. Therefore, the oxidation behavior in a high-temperature steam exposure environment may be comparable to that in an atmospheric environment for approximately 100 h. However, it should be noted that the actual equipment will be operated on a 10,000-h scale, which is 100 times longer than the test conditions. As water vapor is a significant accelerator of EBC degradation, it is essential to evaluate its effects by conducting more extended exposure tests to high-temperature water vapor.
Comparison between air and steam oxidation behavior, (a) As-indented 7.5%SiC-2.6 coating, (b) Heat-treated 7.5%SiC-2.6 100 h in air, (c) As-indented 7.5%SiC-2.6 coating, and (d) Heat-treated 7.5%SiC-2.6 100 h in steam.
This study investigated the effects of the SiC particle size and SiC content on the crack self-healing properties of plasma-sprayed Yb2Si2O7/SiC environmental barrier coatings. The findings are summarized below.
