Oxidation of Cr3Si-Cr7C3/SiC/SiC Coated C/SiC Composite in Wet Air

Oxidation behavior of a Cr 3 Si-Cr 7 C 3 /SiC/SiC coated C/SiC composite was investigated in wet air at 700, 900 and 1300 ºC under 1 atm with a gas velocity of 3.0 cm s -1 , and compared with that of a SiC/SiC/SiC coated C/SiC composite. The wet oxidation produced phases on the Cr 3 Si-Cr 7 C 3 /SiC/SiC coating were Cr 2 O 3 at 700 and 900 ºC, and Cr 2 O 3 and SiO 2 at 1300 ºC. The Cr 3 Si-Cr 7 C 3 /SiC/SiC coating showed enhanced protection against wet oxidation compared to the SiC/SiC/SiC coating. After oxidation for 10 h, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composites showed nearly the same failure behavior and residual exural strength as the as-received composites.


Introduction
Continuous carbon ber reinforced silicon carbide (C/SiC) composites exhibit many excellent properties, such as low density, high speci c strength and modulus, high fracture toughness and improved oxidation resistance than carbon/carbon (C/C) composites. Therefore, they are one of the most promising thermostructural materials applied as high temperature structural components such as heat exchangers, hot gas lters, turbine engines, spacecraft reentry thermal protection systems, etc. [1][2][3][4][5]. In certain applications, such as combustor liners, turbine vanes and thrusters for propulsion, the operating environment is a high-temperature oxidizing environment containing water vapor [6]. It has also been demonstrated that water can enhance the oxidation rate of SiC and carbon [7][8][9][10][11][12]. Therefore, protection of C/SiC composites from oxidation, especially in high temperature water vapor containing oxidizing environments is a concern.
Earlier work has demonstrated that a Cr 3 Si-Cr 7 C 3 outer layer prepared on CVD SiC coating [13] showed an enhanced oxidation protection in air as compared to a SiC/SiC/SiC coating. However, the effect of the Cr 3 Si-Cr 7 C 3 outer layer on the wet oxidation of SiC coated C/SiC has not been investigated so far.
In this paper, the high temperature oxidation behavior of a Cr 3 Si-Cr 7 C 3 /SiC/SiC coated C/SiC was investigated in 18 vol. % O 2 + 72 vol. % N 2 + 10 vol. % H 2 O under 1 atm and a gas ow rate of 3.0 cm s − 1 at 700 ºC, 900 ºC and 1300 ºC. The analysis and discussion presented in this paper focus principally on the oxidation mechanisms based on the weight change kinetics, residual exural strength change and microstructure analysis. A comparison with the corresponding properties of a SiC/SiC/SiC coated C/SiC composite is also presented.

Fabrication of samples
A 2D C/SiC composite, used as the substrate, was prepared by low-pressure chemical vapor in ltration (LPCVI). The preform was piled up with polyacrylonitrile (PAN)-based carbon ber clothes (T300™). The volume fraction of the ber preform was controlled in the range of 40-45%. The preform was deposited with a pyrolytic carbon (PyC) and SiC using butane and methyltrichlorosilane (MTS). An LPCVI process was used to deposit the PyC interphase and the SiC matrix for the composite using butane and MTS, respectively. The deposition conditions for the PyC interface layer were as follows: temperature 960 °С, pressure 5 kPa, time 20 h, Ar ow 200 mL min − 1 , C 4 H 10 ow 15 mL min − 1 . The deposition conditions for the SiC matrix were as follows: temperature 1000 °С, pressure 5 kPa, time 120 h, H 2 ow 350 mLmin − 1 , Ar ow 350 mLmin − 1 , and the mole ratio of H 2 to MTS was 10:1. The as-received composite was machined and polished to obtain substrates of dimension of 3.0 mm × 4.0 mm × 30 mm.
The gained composite substrates were initially coated with two layers of SiC by CVD, after which an additional Cr 3 Si-Cr 7 C 3 coating was added using a Powder Immersion Reaction Assisted Coating (PIRAC) method. And this coating was referred to as Cr 3 Si-Cr 7 C 3 /SiC/SiC coating. The conditions for CVD SiC were the same as that of the SiC matrix, except that the deposition time was 30 h per cycle. In the PIRAC processing, samples with two layers of SiC coatings were rst immersed into Cr powders and sealed in a Cr-rich stainless steel container. These were then enclosed in a second stainless steel container with small amounts of titanium and chromium powder that functioned as getters for N 2 and O 2 , respectively.
The Cr 3 Si-Cr 7 C 3 coating was prepared at 1000 °С for 2 h [13]. For comparison, samples with three layers of CVD SiC coating (referred to as SiC/SiC/SiC coating) were prepared.

Oxidation tests
Oxidation tests of the coated C/SiC composites were carried out at 700 °C, 900 °C and 1300 °C under a gas mixture containing 18 vol % O 2 + 72% vol % N 2 + 10 vol % H 2 O at 1 atm pressure. Deionized water was used as water vapor source. Three specimens put in an alumina tube with a purity of 99.99% were used for each test. The samples were introduced into a heating furnace at the desired temperature, and the oxidizing mixture was then admitted into the reactor. It has been demonstrated that silica is volatile under operation conditions of a turbine, where high gas velocity along with a high partial pressure of water vapor is used. However, at low gas velocity and low gas pressure, silica volatilization and the corresponding steady-state recession rate are negligible [14]. In the present experiment, the velocity of the gas mixture was maintained at 3.0 cm s − 1 to minimize the volatility of the SiO 2 scale formed on the SiC coating. The mass of the specimens before and after oxidation were measured using an electronic analytical balance (resolution: 0.01 mg).

Measurements of the composites
The exural strength of the samples before and after the wet oxidation was measured by a three-point bending method, which was carried out on an Instron 1195 machine at room temperature. The span dimension was 20 mm and the loading rate was 0.5 mm min − 1 .
Phase composition and microstructure of the samples were characterized using X-ray diffraction (XRD, BRUKER, D8 ADVANCE A25 X) and scanning electron microscopy (SEM, Hitachi S4800) equipped with EDS. XRD analysis was operated at 40 kV and 40 mA.
Step scans were taken in the range of 2θ = 20-80 °w ith a 0.02-step, 0.1 º s − 1 scan speed and a 2 s exposure.
3. Results Figure 1 shows the XRD patterns of the coated samples before and after the wet oxidation for 10 h. The results showed that the PIRAC prepared Cr 3 Si-Cr 7 C 3 outer layer was mainly composed of Cr 3 Si, Cr 7 C 3 along with a small amount of Cr 3 C 2 . It can be seen that after the wet oxidation at 700 ºC and 900 ºC, Cr 2 O 3 was also detected in the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated samples. However, it should be noted that the ratio of the intensity of the Cr 2 O 3 peak to that of the SiC peak was higher for sample oxidized at 900 °C compared to that oxidized at 700 °C. But no peaks due to oxides were detected for the SiC/SiC/SiC coated samples even after wet oxidation at 900 °C. After the wet oxidation at 1300 ºC, Cr 2 O 3 and SiO 2 were detected in the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated samples. And the detected phases were mainly SiC and SiO 2 for the SiC/SiC/SiC coated samples. Figure 2 shows the weight changes induced by wet oxidation for the coated samples. The SiC/SiC/SiC coated C/SiC composites exhibited continuous weight loss during the wet oxidation process and the weight loss decreased with increasing oxidation temperature. Moreover, during oxidation at 700 ºC and 900 ºC, the weight loss showed a linear relationship with respect to time for SiC/SiC/SiC coated C/SiC, whereas, for oxidation at 1300 °C, this variation followed a power law as shown in Fig. 2 (c) and can be described as follows: ln(∆w loss ) = 0.4782 ln(t) − 2.9132 (1a) i.e., ∆w loss = 0.0543•t 0.4782 (1b) where ∆w loss is the weight loss in mg cm − 2 , and t is the time of oxidation in hours.
During the wet oxidation at 700 ºC and 900 ºC, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples showed remarkably lower weight losses when compared to the SiC/SiC/SiC coated samples. During the wet oxidation at 700 ºC, the weight loss showed a linear relation with respect to time for the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples. During the wet oxidation at 900 ºC, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples showed nonlinear weight loss, with a very rapid weight loss within the rst 2 h followed by a gradual weight loss thereafter. During the wet oxidation at 1300 ºC, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples showed non-linear weight gains, with a very rapid weight gain within the rst 2 h followed by a gradual increase thereafter. Furthermore, during oxidation at 900 ºC and 1300 ºC, weight change vs time curves of the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples was well tted by power function as shown in Fig. 2 (c) and Fig. 2 (d), respectively. Figure 3 shows the surface morphology and area EDS results of the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated samples before and after the wet oxidation for 10 h. It can be seen that the surface morphology of the wet oxidized samples was different to that of the as-received coating. The surface of the as-received Cr 3 Si-Cr 7 C 3 layer exhibited uniform folded ridge morphology as shown in Fig. 3 (a) and (b). After the wet oxidation at 700 ºC, the morphology changed signi cantly with the appearance of rough clusters as shown in Fig. 3 (c). A magni ed view of the surface shown in Fig. 3 (d) indicated that the rough cluster was composed of ne lath-shaped particles. The EDS spectrum showed distinct oxygen and chromium peaks along with a weak carbon peak and the atomic percentages of C, O and Cr were 2.50, 52. 81 and 44.69, respectively. These results indicated the formation of a chromia lm and there was chromium carbide remained. After the wet oxidation at 900 ºC, the surface showed a relatively at morphology as shown in Fig. 3 (e). A magni ed view shown in Fig. 3 (f) indicated that the surface was predominantly consisted of large platelets. The EDS spectrum showed only oxygen and chromium with atomic percentages of 57.05 and 42.95, respectively con rming that the surface was fully covered by chromia.
After the wet oxidation at 1300 ºC, the surface showed an undulated loose morphology with a notable damage in the position of protrusion as shown in Fig. 3 (g). The magni ed surface view showed however, that the sublayer was compact and composed of spherical particles. From the EDS spectrum, the surface composition was determined to be 1.51, 48.90, 5.19 and 44.40 at% of C, O, Si and Cr, respectively. These results revealed that the surface was covered mainly by chromia and silica lms.   Figure 6 shows the typical stress-displacement curves during exural tests of the coated 2D C/SiC before and after the wet oxidation for 10 h. The failure behavior of the as-received 2D C/SiC composite was rather brittle, and exhibited a steep stress drop after the maximum load point. The failure behavior of the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated samples after the wet oxidation for 10 h was similar to that of the as-received samples. After the wet oxidation at 700 ºC and 900 ºC for 10 h, the failure behavior of the SiC/SiC/SiC coated samples showed a gradual drop in stress after the maximum load point. Moreover, it should be noted that, after the wet oxidation at 700 ºC and 900 ºC for 10 h, the slope of the initial linear part of the stress-displacement curve of the SiC/SiC/SiC coated 2D C/SiC showed a clear decrease. This indicated that the elastic modulus of the oxidized SiC/SiC/SiC coated 2D C/SiC was reduced.
For the wet oxidation at 1300 ºC of the SiC/SiC/SiC coated C/SiC, as shown in Eq. 2, the exponent was 0.4782, which was close to 0.5, typical of parabolic kinetics. Moreover, as shown in Fig. 4 (d), the silica surface was smooth without pores, humps or bubbles. Therefore, it can be ascertained that the weight loss of the SiC/SiC/SiC coated C/SiC during the wet oxidation at 1300 ºC follows parabolic kinetics. As a result, under the present experimental conditions, (i.e. gas velocity of 3.0 cm s − 1 , 10 vol. % H 2 O, and at 1 atm), the volatilization of SiO 2 according to the reaction (8) was negligible [14].

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The weight change of the C/SiC composite during oxidation was governed by the overall outcome of the competing reactions, and the crystallography of the oxidized samples depended on the wet oxidation temperature.
As the temperature was increased from 700 ºC to 1000 ºC, the width of the microcracks in the coating decreased as temperature increased [23]. Consequently, the diffusion of the oxidizing gases into the PyC interlayer through microcracks in the coating decreased with increasing oxidation temperature. As a result, the weight loss due to oxidation of carbon phases decreased with increasing oxidation temperature for both the coated samples. For a speci c oxidation condition (i.e. temperature, oxidizing atmosphere and reactant), the oxidation rate was constant. Since the width of the largest microcracks in the top layer of the Cr 3 Si-Cr 7 C 3 /SiC/SiC coating was lower than that in the top layer of the SiC/SiC/SiC coating (as shown in Fig. 7), the inward diffusion of oxidizing gases through the former was much lower. Thus, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples showed signi cantly lower weight loss when compared to the SiC/SiC/SiC coated samples.
At 700 ºC, since the oxidation of both Cr 3 Si and Cr 7 C 3 was very slow, only a small amount of Cr 2 O 3 was produced, and no SiO 2 was detected. Hence, the XRD spectrum showed Cr 3 Si, Cr 7 C 3 and a small amount of Cr 2 O 3 were detected and C, O, and Cr were detected by EDS. On the other hand, the oxidation of carbon phase was faster than the oxidation of Cr 3 Si and Cr 7 C 3 . Therefore, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples showed a nearly linear weight loss. At 900 ºC, the oxidation of Cr 3 Si and Cr 7 C 3 became faster, which led to the increased formation of Cr 2 O 3 and the growth of Cr 2 O 3 whiskers and platelets [24]. The oxide layer became thicker with increasing time of oxidation. Thus, the ratio of the intensity of the Cr 2 O 3 peak to that of the SiC peak was higher for the sample oxidized at 900 ºC when compared to that oxidized at 700 ºC. Correspondingly, the elements detected by EDS were O and Cr. However, due to the thick oxide layer, the inward diffusion of oxidizing gases slowed down rst and then leveled out. As a result, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples showed a nonlinear weight loss behavior, with a very rapid weight loss within the rst 2 h followed by an apparently gradual weight loss thereafter. The very low weight loss indicating the consumption of carbon was limited to the subsurface region. At 1300 ºC, enhanced oxidation of the coating led to the rapid closure of the narrow microcracks in the outer layer of the Cr 3 Si-Cr 7 C 3 /SiC/SiC coating due to thermal expansion of the formed SiO 2 and Cr 2 O 3 [25,26]. Moreover, presence of SiO 2 scale was favorable to inhibit the diffusion of both Cr and O, leading to oxidation process slowed and a relative compact oxide scale formed underneath the external oxide scale [27]. Additionally, water vapor decreased the viscosity of silica [28,29] and thus, the carbon phases were protected from oxidation. On the other hand, the formed Cr 2 O 3 volatilized by further reacting with water vapor to form gaseous Cr(OH) x [19][20][21][22]. Despite the fact that the rapid formation and escape of gases made the oxide lms locally bulged and damaged, the sublayer was a compact oxide layer and retained the Cr 3 Si-Cr 7 C 3 /SiC/SiC coating (shown in Fig. 8). Therefore, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated samples showed non-linear weight gains, with a very rapid weight gains within the rst 2 h folled by an apparently gradual weight gains thereafter. And detected phases were Cr 2 O 3 and SiO 2 . Thus, after the wet oxidation for 10 h, the composite Cr 3 Si-Cr 7 C 3 /SiC/SiC coated composite samples showed nearly the same failure behavior and exural strength as the as-received samples.
For the SiC/SiC/SiC coated C/SiC, high temperature water vapor enhanced the non-uniform oxidation consumption of carbon phases, which led to the formation of pipeline-shaped channels between the carbon ber and the SiC matrix [17]. As a result, the strength of the bers and the bonding between the ber/interphase/matrix would be weakened. Thus the stress-displacement curves showed a gradual stress drop after the maximum load point. Moreover, the non-uniform consumption of carbon phase increased the porosity of the composites increased. It is well acknowledged that the elastic moduli of porous materials decrease with increasing porosity [30,31]. Therefore, the oxidized composite showed a reduced elastic modulus demonstrated by the decrease in the slope of the initial linear part of the stressdisplacement curve. Above 900 ºC, the microcracks in the SiC coating are gradually sealed both by thermal expansion and by the formed SiO 2 , due to surface oxidation. However, even though the oxidation of SiC resulted initially in the formation of a protective SiO 2 layer and a consequent weight gain, the rapid oxidation of carbon resulted in rapid weight loss. Therefore, the weight loss of the SiC/SiC coated composite samples decreased with increasing oxidation temperatures, showing the same trend as the exural strength.
The above results suggest that the Cr 3 Si-Cr 7 C 3 /SiC/SiC coating has enhanced protection against wet oxidation. And this improved oxidation resistance originates from the rapid formation of a protective layer of Cr 2 O 3 by oxidation of Cr 3 Si and Cr 7 C 3 below 1000 ºC,while the formation of more protective layer of SiO 2 by oxidation of Cr 3 Si and SiC at higher temperatures. The work provides support for the preparation of oxidation resistant coatings, which in return are essential to improve the service performance of carbon ber reinforced composites in high temperature oxidizing environments.

Conclusions
In the present work, oxidation behavior of a Cr 3 Si-Cr 7 C 3 /SiC/SiC coated C/SiC was investigated in 18 vol. Compared to the SiC/SiC/SiC coated composite, the Cr 3 Si-Cr 7 C 3 /SiC/SiC coated C/SiC showed remarkably reduced weight loss during the wet oxidation at 700 ºC and 900 ºC. At 1300 ºC, the Cr 3 Si- Weight change of the coated 2D C/SiC composite during the wet oxidation  Flexural strength of the coated 2D C/SiC before and after the wet oxidation for 10h Figure 6 Typical exural stress-displacement curves of the coated 2D C/SiC before and after the wet oxidation for 10h Figure 7 The largest microcracks in the top layer of the coating of (a) SiC/SiC/SiC; (b) Cr3Si-Cr7C3/SiC/SiC Figure 8 Cross-section morphologies of the Cr3Si-Cr7C3/SiC/SiC coating after the wet oxidation at 1300 ºC for 10h.