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SiC composite modified by a chromium silicide–chromium carbide outer layer
Materials Science & Engineering A 644 (2015) 268–274
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Oxidation protective silicon carbide coating for C/SiC composite modified by a chromium silicide–chromium carbide outer layer Shoujun Wu a,n, Yiguang Wang b, Qiang Guo a, Bin Guo a, Lei Luo b a
College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling, Shaanxi 712100, People’s Republic of China Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 May 2015 Received in revised form 26 July 2015 Accepted 27 July 2015 Available online 29 July 2015
A chromium silicide–chromium carbide (Cr3Si–Cr7C3) outer layer mainly composed of Cr3Si, Cr7C3, and a small amount of Cr3C2 was prepared on a SiC coated carbon fiber reinforced silicon carbide (C/SiC) composite using a Powder Immersion Reaction Assisted Coating (PIRAC) method. The Cr3Si–Cr7C3 outer layer showed folded ridge morphology. The width of the largest microcracks on the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating was only one forth of that on the outer layer of the SiC/SiC/SiC coating. The Cr3Si–Cr7C3/SiC/SiC coating showed significantly enhanced oxidation protection compared to the SiC/SiC/ SiC coating. After oxidation for 10 h, the Cr3Si–Cr7C3/SiC/SiC coated composite showed nearly the same failure behavior and residual flexural strength as that of the as-received composite. & 2015 Published by Elsevier B.V.
Keywords: CVD SiC Cr3Si–Cr7C3 C/SiC composite Oxidation Strength
1. Introduction Continuous carbon fiber reinforced silicon carbide (C/SiC) composites are one of the most promising thermostructural materials for applications including turbine engines, spacecraft reentry thermal protection systems and ultra-lightweight mirrors, which involve high temperature oxidizing environments [1–4]. Development of oxidation protective coatings is therefore crucial for maximizing the potential of the C/SiC composites. SiC coatings prepared by chemical vapor deposition (CVD SiC) have been used as the major coating material for oxidation protection of thermal structural composites [5]. Unfortunately, microcracks are unavoidable in CVD SiC coatings, mainly due to mismatch in the coefficient of thermal expansion (CTE) between the composite and the coating [6], which causes rapid inward diffusion of oxygen, and oxidation of the composites. It is well known that above the deposition temperature of CVD SiC, microcracks will be healed due to thermal expansion and the oxidation-produced silica. However, below the deposition temperature, oxygen can diffuse inwards through the microcracks and lead to oxidation of the carbon phases in the composite. The oxidation of CVD SiC coated C/SiC is usually divided into three domains [7,8]: at low temperatures (T o800 °C), the microcracks in the coating are relatively wide, and thus the rate of the reaction n
Corresponding author. Fax: þ86 29 87082901. E-mail address: [email protected] (S. Wu).
http://dx.doi.org/10.1016/j.msea.2015.07.081 0921-5093/& 2015 Published by Elsevier B.V.
between carbon and oxygen controls the oxidation kinetics. At intermediate temperatures (800 °C oT o1100 °C), the oxidation is controlled by gas diffusion through the narrower microcracks in the coating, resulting in a non-uniform degradation of carbon phases. At high temperature (T 41100 °C), the microcracks are sealed by silica, and the oxidation is controlled by oxygen diffusion through the SiO2 scale, so the oxidation mainly takes place on the material surfaces. A lot of effort has therefore been devoted to improving the oxidation resistance of the coatings at low and intermediate temperatures. Research has particularly focused on sealing the coating cracks during oxidation using glass or glaze by means of the viscosity of the formed B2O3 or borosilicate glass [9– 11]. However, B2O3 evaporates above 450 °C, especially in water vapor-containing environments. Therefore, improving the oxidation resistance of SiC-based coatings in the temperature range from 700 to 1100 °C is still a concern. It has been reported that oxidation of Cr3Si starts at 650 °C [12]. Cr3Si has good oxidation resistance, as the formation of Cr2O3 at low temperatures and SiO2 at high temperatures are excellent oxygen barriers [13]. Chromium carbides are good oxidation resistance materials, and begin oxidation above 660 °C [14]. Therefore, it is supposed that oxidation of CVD SiC can be improved with a Cr3Si and chromium carbide outer layer. The Powder Immersion Reaction Assisted Coating (PIRAC) method is a solid state pack cementation method. In PIRAC, substrates are immersed into metal powders with high affinity for C, N, B or Si. Reactive diffusion of metal atoms along the substrate
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surface results in formation of layered coatings containing carbides or nitrides as well as silicides or borides [15–17]. It has been demonstrated that coatings containing Cr3Si and chromium carbides can be obtained by PIRAC treatment of SiC in Cr powder at 900–1200 °C [15]. However, the effect of the Cr3Si–Cr7C3 outer layer on the oxidation SiC coated C/SiC has not been fully investigated. In this paper, a Cr3Si–Cr7C3 outer layer was prepared on a CVD SiC coating using PIRAC. The oxidation behaviors of a two dimensional (2D) C/SiC with a Cr3Si–Cr7C3/SiC/SiC coating and a SiC/ SiC/SiC coating were comparatively studied at 700 °C, 900 °C and 1300 °C in air. We also discuss the effects of the Cr3Si–Cr7C3/SiC/SiC coating on coating cracks, oxidation behavior and the strength degradation of the coated 2D C/SiC composite.
2. Experiment procedure
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Three samples were used for each experimental condition. The samples were placed into an alumina crucible, and the crucible was then placed into a heating furnace at the desired temperature. The mass of the samples before and after oxidation was measured using an electronic analytical balance (resolution: 0.01 mg). 2.3. Measurements of the composites The flexural strength of the samples before and after 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. Phase composition and microstructure of the samples were characterized using X-ray diffraction (XRD, Rigaku D/MAX-2400 with Cu Kα radiation) and scanning electron microscopy (SEM, Hitachi S3400) equipped with EDS. XRD analysis was operated at 40 kV and 40 mA. Step scans were taken in the range of 2θ ¼ 20– 80° with a 0.02-step, 0.01 °/s scan speed and a 2 s exposure.
2.1. Fabrication of samples A 2D C/SiC composite was prepared by low-pressure chemical vapor infiltration (LPCVI). The preform was piled up with polyacrylonitrile (PAN)-based carbon fiber clothes (T300TM). The volume fraction of the fiber 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 flow 200 mL min 1, C4H10 flow 15 mL min 1. The deposition conditions for the SiC matrix were as follows: temperature 1000 °С, pressure 5 kPa, time 120 h, H2 flow 350 mL min 1, Ar flow 350 mL min 1, and the molar ratio of H2 to MTS was 10:1. The asreceived composite was machined and polished to a size of 3.0 mm 4.0 mm 30 mm. Two kinds of coatings were prepared on the obtained samples. The first one was coated with three layers of SiC coating using a CVD process. The other one was initially coated with two layers of SiC by CVD, then an additional Cr3Si–Cr7C3 coating was added using a PIRAC method. 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, the samples with two layers of SiC coatings were initially immersed into Cr powders and sealed in a Cr-rich stainless steel container. They were then placed into an additional stainless steel container with small amounts of titanium and chromium powder, acting as getters for N2 and O2, respectively. The Cr3Si–Cr7C3 coating was prepared at 1000 °С for 2 h. Schematic of samples fabrication processes was shown in Fig. 1. 2.2. Oxidation tests Oxidation tests of the coated C/SiC composites were carried out in an air atmosphere at 700 °C, 900 °C and 1300 °C, respectively. Stacked carbon fiber cloth preform
PyC interface layer By CVI
3. Results and discussion 3.1. Microstructure and phase composition of the PIRAC Cr3Si–Cr7C3 coating Fig. 2 shows the surface morphology of the CVD SiC and the PIRAC Cr3Si–Cr7C3 outer layer. The CVD SiC exhibits a cauliflower morphology, while the Cr3Si–Cr7C3 outer layer has a folded ridge morphology. The width of the largest microcracks in the top layer of the Cr3Si–Cr7C3/SiC/SiC coating was only about one forth of that in the SiC/SiC/SiC coating. It should be noted that a quantity of tenuous microcracks can be observed in the Cr3Si–Cr7C3 outer layer as shown in Fig. 2c. The thermal expansion coefficient (CTE) of Cr3Si and that of Cr7C3 is much large than the CTE of CVD SiC [18–21]. Therefore, the Cr3Si–Cr7C3 outer layer prepared on CVD SiC is prone to cracking. And it is supposed that thermal stress was released by the formation of a quantity of tenuous microcracks and resulted in narrowing of the width of the largest microcracks. Fig. 3 shows the cross section micromorphology of the SiC/SiC/ SiC coating and the Cr3Si–Cr7C3/SiC/SiC coating. It was observed that the PIRAC-prepared Cr3Si–Cr7C3 outer layer was homogeneous and well bonded to the CVD SiC/SiC coatings. The thickness of the PIRAC-prepared Cr3Si–Cr7C3 outer layer was about 6 μm. Fig. 4 shows the surface XRD patterns of the SiC/SiC/SiC coating and the Cr3Si–Cr7C3/SiC/SiC coating. It shows that the PIRAC prepared Cr3Si–Cr7C3 outer layer is mainly composed of Cr3Si, Cr7C3, and a small amount of Cr3C2. According to related thermochemical data [22], Gibbs free energies of the main possible reactions versus temperatures are listed in Table 1. According to thermodynamics calculations, it can be concluded that formation of two kinds of chromium carbides and Cr3Si is thermodynamically favorable in the calculation temperature ranges. However, the formation energy of Cr7C3 was more SiC matrix By CVI
Cut and Machine
Two layers CVD SiC + Cr3Si-Cr7C3 coating
Three layers CVD SiC Coating Fig. 1. Schematic of samples fabrication processes.
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Cr
Fine cracks Cr C
Si
Cr
Fig. 2. Surface morphology of (a), (a*) the CVD SiC and (b), (b*), (c) the PIRAC Cr3Si–Cr7C3 outer layer; (c*) EDS of the PIRAC Cr3Si–Cr7C3 outer layer.
negative. As a result, Cr7C3, Cr3Si, and a small amount of Cr3C2 can be simultaneously formed by the reaction of Cr and SiC. 3.2. Oxidation behavior of the coated C/SiC composite Fig. 5 shows the XRD patterns of the coated samples after oxidation for 10 h. It can be seen that after oxidation at 700 °C, Cr2O3, Cr3Si and Cr7C3 were detected for the Cr3Si–Cr7C3/SiC/SiC coated samples. After oxidation at 900 °C, Cr3C2 was also detected for the Cr3Si–Cr7C3/SiC/SiC coated samples, as well as Cr2O3, Cr3Si and Cr7C3. After oxidation at 1300 °C, the detected phases were mainly Cr2O3 and SiO2 for the Cr3Si–Cr7C3/SiC/SiC coated samples. Fig. 6 shows the surface morphology and EDS spectra of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h. It can be seen that the surface morphology of the oxidized samples is obviously different to that of the as-received coating. After
Fig. 4. XRD patterns of the SiC/SiC/SiC coating and the Cr3Si–Cr7C3/SiC/SiC coating.
oxidation at 700 °C (Fig. 6a), the folded ridge morphology of the as-received Cr3Si–Cr7C3/SiC/SiC had changed into a glassy material
Fig. 3. Cross section morphology of (a) the SiC/SiC/SiC and (b) the Cr3Si–Cr7C3/SiC/SiC coating.
S. Wu et al. / Materials Science & Engineering A 644 (2015) 268–274
Table 1 Gibbs free energy of reaction between silicon carbide and chromium. Related reactions
Gibbs free energy ΔG [kJ/mol] (700– 1300 K)
16/19Cr þ 3/19SiC-3/19Cr3Siþ 1/19Cr7C3 9/11Cr þ2/11SiC-2/11Cr3Siþ 1/11Cr3C2
11.67 to 0.0032 T 11.19 to 0.0029 T
Fig. 5. XRD patterns of the coated samples after oxidation for 10 h. 1. Cr3Si–Cr7C3/SiC/SiC 700 °C; 2. Cr3Si–Cr7C3/SiC/SiC 900 °C; 3. SiC/SiC/SiC 900 °C; 4. Cr3Si–Cr7C3/SiC/SiC 1300 °C.
covered cluster. The EDS spectrum showed a noticeable oxygen peak, indicating the formation of an oxidation film. A magnified view shown in Fig. 6a* indicates that the outer layer is composed of fine particle clusters. After oxidation at 900 °C (Fig. 6b), the surface shows a relatively flat morphology. A magnified view is
shown in Fig. 6b*, indicating that the surface is dominated by hexagonal Cr2O3 platelets. After oxidation at 1300 °C (Fig. 6c), the surface shows a relatively rough loose morphology. The magnified view (Fig. 6c*) shows the rough loose outer layer is dominated by spherical Cr2O3 particles. As Cr3Si and Cr7C3 begin oxidation above 660 °C, Cr2O3 and CO were formed after oxidation at 700 °C. However, since the oxidation was very slow, only a small amount of Cr2O3 was produced, and no SiO2 was detected. The release of CO produced at or near the chromia/Cr3Si–Cr7C3 interface would be baffled, which results in pores inside the oxidation layer (Fig. 7a). At 900 °C, oxidation of Cr3Si and Cr7C3 became faster, which led to more Cr2O3 forming. The oxide layer became increasingly thicker as the oxidation time increased. The thick oxide layer slowed down the oxygen diffusion into the unreacted region, decreasing the weight loss. It is suggested that at the Cr2O3/Cr7C3 interface, formation of Cr2O3 results in carbon released and thus will form a thin layer of Cr3C2 [23]. Therefore, Cr3C2 was also detected in the oxidized Cr3Si–Cr7C3/SiC/SiC coated samples at 900 °C, and less pores were formed inside the oxide layers (Fig. 7b). Since Cr2O3 would evaporate above 950 °C [24], the evaporation of Cr2O3 and release of the CO led to the formation of a porous oxide layer at 1300 °C. The oxidation of SiC at this temperature could result in increased SiO2 production. Fig. 8 shows the weight change of the coated C/SiC composite during oxidation. The SiC/SiC/SiC coated C/SiC composites exhibit consecutive weight loss during oxidation. The weight loss decreases with increasing oxidation temperature. During oxidation at 700 °C and 900 °C, the Cr3Si–Cr7C3/SiC/SiC coated composite
Cr Si O Cr
C
O Cr Cr
C
O
Cr Si
271
Cr
Fig. 6. Surface morphology and EDS spectra of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h at (a) 700 °C; (b) 900 °C; (c) 1300 °C.
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Pores Fig. 7. Cross-section morphologies of the Cr3Si–Cr7C3/SiC/SiC coating after oxidation for 10 h at (a) 700 °C and (b) 900 °C.
Weight Changes (%)
Weight Changes (%)
0.2 0 -0.2 -0.4 -0.6 SiC/SiC/SiC 700°C SiC/SiC/SiC 900°C SiC/SiC/SiC 1300°C CrSi-CrC/SiC/SiC 700°C CrSi-CrC/SiC/SiC 900°C CrSi-CrC/SiC/SiC 1300°C
-0.8 -1 -1.2 -1.4 0
2
4 6 8 Oxidation Time (h)
10
0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.1 -0.15 -0.2
SiC/SiC/SiC 1300°C CrSi-CrC/SiC/SiC 1300°C
y cr =0.13× t 0.33 y sc =-0.066× t 0.417
0
2
4 6 8 Oxidation Time (h)
10
Fig. 8. Weight change of the coated 2D C/SiC composite during oxidation.
450
Flexural Strength (MPa)
400 350 300 250 200 150 100 50 0 As-received
700ºC
Cr3Si-Cr7C3/SiC/SiC
900ºC
1300ºC
SiC/SiC/SiC
Fig. 9. Flexural strength of the coated 2D C/SiC before and after oxidation for 10 h.
samples showed remarkably reduced weight loss compared to that of SiC/SiC/SiC coated samples. During oxidation at 1300 °C, the Cr3Si–Cr7C3/SiC/SiC coated composite samples showed weight gains. Fig. 8b shows the fitted relationship of weight changes versus time during oxidation at 1300 °C using an exponential function. Diffusion-controlled film growth can be generally expressed by the following equation [25]:
Y = k· t n where y is the thickness or weight change of the film, k is the growth rate constant, n is the kinetic exponent, and t is the diffusion time. The result shown in Fig. 8b suggests that at 1300 °C, oxidation of multilayer coated C/SiC was controlled by diffusion. It should be noted that the calculated kinetic exponent for the Cr3Si–Cr7C3/SiC/SiC coated samples was 0.33, while that for the SiC/SiC/SiC coated samples was 0.417 (Fig. 8b). In C/SiC composites, carbon oxidized above 500 °C exhibits absolute weight loss and SiC oxidized above 800 °C shows weight gain. Above 800 °C, the oxidation weight change of the C/SiC composite was the overall result of the competing weight changes. From 700 °C to 1000 °C, the width of the microcracks in the SiC
coatings decreased as temperature increased [26,27]. At a specific oxidation condition (i.e. temperature, oxidizing atmosphere and reactant), the oxidation rate is constant. Consequently, the weight loss decreased with temperature and showed a linear relationship with oxidation time. At 1300 °C, the microcracks in the SiC coating are gradually sealed by thermal expansion and the formed SiO2, due to surface oxidation. However, initially, oxidation of SiC resulted in the formation of a protective SiO2 layer and consequent weight gain, and rapid oxidation of carbon resulted in rapid weight loss. Though carbon fibers are oxidized and evaporate from near-surface, but carbon fibers at deep area may be protected from oxidation. As the oxidation progressed, oxygen diffusion slowed down and then leveled out [21]. As a result, the SiC/SiC/SiC coated samples showed a parabolic weight loss. It is well known that chromia and silica are excellent oxygen barriers [5]. Oxidation of Cr7C3 and Cr3C2 are accompanied with weight gains. For the Cr3Si–Cr7C3/SiC/SiC coated composite samples, below 1000 °C, oxidation of carbon by the inward diffusion oxygen through the cracks in the coatings resulted in weight loss, and oxidation of Cr3Si and Cr7C3 led to weigh gain. Moreover, it is considered the very narrow microcracks in the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating would be narrowed further by the formed chromia and silica. Therefore, it is considered inward oxygen diffusion through the Cr3Si–Cr7C3/SiC/SiC coating is much less than that through the SiC/SiC/SiC coating. As a result, the Cr3Si–Cr7C3/SiC/SiC coated composite samples showed remarkably reduced weight loss than the SiC/SiC/SiC coated samples. At 1300 °C, the narrow microcracks in the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating would be rapidly sealed by thermal expansion of the formed SiO2 and Cr2O3 [28,29]. Therefore, the Cr3Si–Cr7C3/SiC/SiC coated samples showed parabolic weight gains. Fig. 9 shows the flexural strength of the coated samples before and after oxidation for 10 h. After oxidation, the SiC/SiC/SiC coated samples showed lower residual flexural strength compared to the Cr3Si–Cr7C3/SiC/SiC coated samples. The average residual flexural strength of the SiC/SiC/SiC coated samples after 10 h oxidation at
450 400 350 300 250
273
450
200 150 100
Stress (MPa)
Stress (MPa)
S. Wu et al. / Materials Science & Engineering A 644 (2015) 268–274
As-received 900ºC SiC/SiC/SiC 900ºC CrSi-CrC/SiC/SiC
50 0 0
0.1
0.2 0.3 Displacement (mm)
0.4
0.5
400 350 300 250 200 150 100 50 0
As-received 1300ºC SiC/SiC/SiC 1300ºC CrSi-CrC/SiC/SiC 0
0.1
0.2 0.3 Displacement (mm)
0.4
0.5
Fig. 10. Typical flexural stress–displacement curves of the coated 2D C/SiC before and after oxidation for 10 h.
700 °C, 900 °C and 1300 °C was 299 MPa, 321 MPa, and 370 MPa, respectively. While the average residual flexural strength of the Cr3Si–Cr7C3/SiC/SiC coated samples after 10 h oxidation at 700 °C, 900 °C and 1300 °C was 394 MPa, 389 MPa, and 407 MPa, respectively. Considering the strength scatter, the residual flexural strength of the Cr3Si–Cr7C3/SiC/SiC coated samples after 10 h oxidation at 1300 °C was nearly the same as that of the as-received samples (411 MPa). Higher residual flexural strength suggested that the Cr3Si–Cr7C3/SiC/SiC coating had better oxidation resistance than the SiC/SiC/SiC coating. Fig. 10 shows typical stress–displacement curves of the coated 2D C/SiC before and after oxidation for 10 h during flexural tests. It can be seen that the failure behavior of the SiC/SiC/SiC coated samples had altered after oxidation. The failure behavior of the asreceived 2D C/SiC composite was rather brittle, and exhibited a steep stress drop after the maximum load point. After oxidation for 10 h, the failure behavior of the SiC/SiC/SiC coated samples showed a gradual stress drop after the maximum load point. The higher the oxidation temperature was, the more obvious the failure changes were. The failure behavior of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h was similar to that of the as-received samples. For the SiC/SiC/SiC coated C/SiC, when there were microcracks in the coating, the oxidation led to a non-uniform consumption of carbon phases near-surface (Fig. 11). Such non-uniform consumption of carbon phases led to pipeline-shaped channels forming between the carbon fiber and SiC matrix. While carbon fibers at deep area may be protected from oxidation. As a result, the strength of the fibers and the bonding between the fiber/interphase/matrix would be weakened, and thus the stress–displacement curves show a gradual stress drop after the maximum load point. As the width of the microcracks in the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating were remarkably narrow compared to those in the SiC/SiC/SiC coating, inward oxygen diffusion was much lower. Moreover, the Cr3Si–Cr7C3/SiC/SiC coated samples were protected from further oxidation by the presence of oxidation-induced chromia and silica. The load–displacement curve of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation was therefore similar to that of the as-received samples. The above results suggest the Cr3Si–Cr7C3/SiC/SiC has obviously enhanced oxidation protection. The work will provide supports for preparation of oxidation resistant coatings which is favorable to improve service performance of carbon fiber reinforced composites in high temperature oxidizing environments.
Fig. 11. Nonuniform consumption of carbon phases after oxidation at 900 °C for 10 h.
composed of Cr3Si and Cr7C3 plus or minus Cr3C2. The Cr3Si–Cr7C3 outer layer showed folded ridge morphology, and the width of the largest microcracks in the outer layer of Cr3Si–Cr7C3/SiC/SiC coating was nearly one forth of that in the SiC/SiC/SiC coating. The Cr3Si–Cr7C3/SiC/SiC coating showed obviously enhanced oxidation protection. Compared with SiC/SiC/SiC coated composite, at 700 °C and 900 °C, the oxidation-induced weight loss of the Cr3Si–Cr7C3/SiC/SiC coated C/SiC was remarkably reduced. At 1300 °C, the Cr3Si–Cr7C3/SiC/SiC coated C/SiC samples showed parabolic weight gains. After oxidation for 10 h, the residual flexural strength of the Cr3Si–Cr7C3/SiC/SiC coated composite almost maintained the same value as that of the as-received composite. The residual flexural strength of the SiC/SiC/SiC coated composite showed a remarkable reduction after oxidation, especially at 700 °C and 900 °C. Oxidation-induced non-uniform consumption of carbon phases resulted in changes in the failure behavior of the SiC/SiC/SiC coated samples. While the failure behavior of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h was similar to that of the as-received samples.
Acknowledgments The authors gratefully acknowledge the financial support from the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201304) and from the Human Resources Foundation of Northwest A&F University (No. Z111021101).
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Oxidation protective silicon carbide coating for C/SiC composite modified by a chromium silicide–chromium carbide outer layer Shoujun Wu a,n, Yiguang Wang b, Qiang Guo a, Bin Guo a, Lei Luo b a
College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling, Shaanxi 712100, People’s Republic of China Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 May 2015 Received in revised form 26 July 2015 Accepted 27 July 2015 Available online 29 July 2015
A chromium silicide–chromium carbide (Cr3Si–Cr7C3) outer layer mainly composed of Cr3Si, Cr7C3, and a small amount of Cr3C2 was prepared on a SiC coated carbon fiber reinforced silicon carbide (C/SiC) composite using a Powder Immersion Reaction Assisted Coating (PIRAC) method. The Cr3Si–Cr7C3 outer layer showed folded ridge morphology. The width of the largest microcracks on the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating was only one forth of that on the outer layer of the SiC/SiC/SiC coating. The Cr3Si–Cr7C3/SiC/SiC coating showed significantly enhanced oxidation protection compared to the SiC/SiC/ SiC coating. After oxidation for 10 h, the Cr3Si–Cr7C3/SiC/SiC coated composite showed nearly the same failure behavior and residual flexural strength as that of the as-received composite. & 2015 Published by Elsevier B.V.
Keywords: CVD SiC Cr3Si–Cr7C3 C/SiC composite Oxidation Strength
1. Introduction Continuous carbon fiber reinforced silicon carbide (C/SiC) composites are one of the most promising thermostructural materials for applications including turbine engines, spacecraft reentry thermal protection systems and ultra-lightweight mirrors, which involve high temperature oxidizing environments [1–4]. Development of oxidation protective coatings is therefore crucial for maximizing the potential of the C/SiC composites. SiC coatings prepared by chemical vapor deposition (CVD SiC) have been used as the major coating material for oxidation protection of thermal structural composites [5]. Unfortunately, microcracks are unavoidable in CVD SiC coatings, mainly due to mismatch in the coefficient of thermal expansion (CTE) between the composite and the coating [6], which causes rapid inward diffusion of oxygen, and oxidation of the composites. It is well known that above the deposition temperature of CVD SiC, microcracks will be healed due to thermal expansion and the oxidation-produced silica. However, below the deposition temperature, oxygen can diffuse inwards through the microcracks and lead to oxidation of the carbon phases in the composite. The oxidation of CVD SiC coated C/SiC is usually divided into three domains [7,8]: at low temperatures (T o800 °C), the microcracks in the coating are relatively wide, and thus the rate of the reaction n
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http://dx.doi.org/10.1016/j.msea.2015.07.081 0921-5093/& 2015 Published by Elsevier B.V.
between carbon and oxygen controls the oxidation kinetics. At intermediate temperatures (800 °C oT o1100 °C), the oxidation is controlled by gas diffusion through the narrower microcracks in the coating, resulting in a non-uniform degradation of carbon phases. At high temperature (T 41100 °C), the microcracks are sealed by silica, and the oxidation is controlled by oxygen diffusion through the SiO2 scale, so the oxidation mainly takes place on the material surfaces. A lot of effort has therefore been devoted to improving the oxidation resistance of the coatings at low and intermediate temperatures. Research has particularly focused on sealing the coating cracks during oxidation using glass or glaze by means of the viscosity of the formed B2O3 or borosilicate glass [9– 11]. However, B2O3 evaporates above 450 °C, especially in water vapor-containing environments. Therefore, improving the oxidation resistance of SiC-based coatings in the temperature range from 700 to 1100 °C is still a concern. It has been reported that oxidation of Cr3Si starts at 650 °C [12]. Cr3Si has good oxidation resistance, as the formation of Cr2O3 at low temperatures and SiO2 at high temperatures are excellent oxygen barriers [13]. Chromium carbides are good oxidation resistance materials, and begin oxidation above 660 °C [14]. Therefore, it is supposed that oxidation of CVD SiC can be improved with a Cr3Si and chromium carbide outer layer. The Powder Immersion Reaction Assisted Coating (PIRAC) method is a solid state pack cementation method. In PIRAC, substrates are immersed into metal powders with high affinity for C, N, B or Si. Reactive diffusion of metal atoms along the substrate
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surface results in formation of layered coatings containing carbides or nitrides as well as silicides or borides [15–17]. It has been demonstrated that coatings containing Cr3Si and chromium carbides can be obtained by PIRAC treatment of SiC in Cr powder at 900–1200 °C [15]. However, the effect of the Cr3Si–Cr7C3 outer layer on the oxidation SiC coated C/SiC has not been fully investigated. In this paper, a Cr3Si–Cr7C3 outer layer was prepared on a CVD SiC coating using PIRAC. The oxidation behaviors of a two dimensional (2D) C/SiC with a Cr3Si–Cr7C3/SiC/SiC coating and a SiC/ SiC/SiC coating were comparatively studied at 700 °C, 900 °C and 1300 °C in air. We also discuss the effects of the Cr3Si–Cr7C3/SiC/SiC coating on coating cracks, oxidation behavior and the strength degradation of the coated 2D C/SiC composite.
2. Experiment procedure
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Three samples were used for each experimental condition. The samples were placed into an alumina crucible, and the crucible was then placed into a heating furnace at the desired temperature. The mass of the samples before and after oxidation was measured using an electronic analytical balance (resolution: 0.01 mg). 2.3. Measurements of the composites The flexural strength of the samples before and after 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. Phase composition and microstructure of the samples were characterized using X-ray diffraction (XRD, Rigaku D/MAX-2400 with Cu Kα radiation) and scanning electron microscopy (SEM, Hitachi S3400) equipped with EDS. XRD analysis was operated at 40 kV and 40 mA. Step scans were taken in the range of 2θ ¼ 20– 80° with a 0.02-step, 0.01 °/s scan speed and a 2 s exposure.
2.1. Fabrication of samples A 2D C/SiC composite was prepared by low-pressure chemical vapor infiltration (LPCVI). The preform was piled up with polyacrylonitrile (PAN)-based carbon fiber clothes (T300TM). The volume fraction of the fiber 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 flow 200 mL min 1, C4H10 flow 15 mL min 1. The deposition conditions for the SiC matrix were as follows: temperature 1000 °С, pressure 5 kPa, time 120 h, H2 flow 350 mL min 1, Ar flow 350 mL min 1, and the molar ratio of H2 to MTS was 10:1. The asreceived composite was machined and polished to a size of 3.0 mm 4.0 mm 30 mm. Two kinds of coatings were prepared on the obtained samples. The first one was coated with three layers of SiC coating using a CVD process. The other one was initially coated with two layers of SiC by CVD, then an additional Cr3Si–Cr7C3 coating was added using a PIRAC method. 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, the samples with two layers of SiC coatings were initially immersed into Cr powders and sealed in a Cr-rich stainless steel container. They were then placed into an additional stainless steel container with small amounts of titanium and chromium powder, acting as getters for N2 and O2, respectively. The Cr3Si–Cr7C3 coating was prepared at 1000 °С for 2 h. Schematic of samples fabrication processes was shown in Fig. 1. 2.2. Oxidation tests Oxidation tests of the coated C/SiC composites were carried out in an air atmosphere at 700 °C, 900 °C and 1300 °C, respectively. Stacked carbon fiber cloth preform
PyC interface layer By CVI
3. Results and discussion 3.1. Microstructure and phase composition of the PIRAC Cr3Si–Cr7C3 coating Fig. 2 shows the surface morphology of the CVD SiC and the PIRAC Cr3Si–Cr7C3 outer layer. The CVD SiC exhibits a cauliflower morphology, while the Cr3Si–Cr7C3 outer layer has a folded ridge morphology. The width of the largest microcracks in the top layer of the Cr3Si–Cr7C3/SiC/SiC coating was only about one forth of that in the SiC/SiC/SiC coating. It should be noted that a quantity of tenuous microcracks can be observed in the Cr3Si–Cr7C3 outer layer as shown in Fig. 2c. The thermal expansion coefficient (CTE) of Cr3Si and that of Cr7C3 is much large than the CTE of CVD SiC [18–21]. Therefore, the Cr3Si–Cr7C3 outer layer prepared on CVD SiC is prone to cracking. And it is supposed that thermal stress was released by the formation of a quantity of tenuous microcracks and resulted in narrowing of the width of the largest microcracks. Fig. 3 shows the cross section micromorphology of the SiC/SiC/ SiC coating and the Cr3Si–Cr7C3/SiC/SiC coating. It was observed that the PIRAC-prepared Cr3Si–Cr7C3 outer layer was homogeneous and well bonded to the CVD SiC/SiC coatings. The thickness of the PIRAC-prepared Cr3Si–Cr7C3 outer layer was about 6 μm. Fig. 4 shows the surface XRD patterns of the SiC/SiC/SiC coating and the Cr3Si–Cr7C3/SiC/SiC coating. It shows that the PIRAC prepared Cr3Si–Cr7C3 outer layer is mainly composed of Cr3Si, Cr7C3, and a small amount of Cr3C2. According to related thermochemical data [22], Gibbs free energies of the main possible reactions versus temperatures are listed in Table 1. According to thermodynamics calculations, it can be concluded that formation of two kinds of chromium carbides and Cr3Si is thermodynamically favorable in the calculation temperature ranges. However, the formation energy of Cr7C3 was more SiC matrix By CVI
Cut and Machine
Two layers CVD SiC + Cr3Si-Cr7C3 coating
Three layers CVD SiC Coating Fig. 1. Schematic of samples fabrication processes.
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Cr
Fine cracks Cr C
Si
Cr
Fig. 2. Surface morphology of (a), (a*) the CVD SiC and (b), (b*), (c) the PIRAC Cr3Si–Cr7C3 outer layer; (c*) EDS of the PIRAC Cr3Si–Cr7C3 outer layer.
negative. As a result, Cr7C3, Cr3Si, and a small amount of Cr3C2 can be simultaneously formed by the reaction of Cr and SiC. 3.2. Oxidation behavior of the coated C/SiC composite Fig. 5 shows the XRD patterns of the coated samples after oxidation for 10 h. It can be seen that after oxidation at 700 °C, Cr2O3, Cr3Si and Cr7C3 were detected for the Cr3Si–Cr7C3/SiC/SiC coated samples. After oxidation at 900 °C, Cr3C2 was also detected for the Cr3Si–Cr7C3/SiC/SiC coated samples, as well as Cr2O3, Cr3Si and Cr7C3. After oxidation at 1300 °C, the detected phases were mainly Cr2O3 and SiO2 for the Cr3Si–Cr7C3/SiC/SiC coated samples. Fig. 6 shows the surface morphology and EDS spectra of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h. It can be seen that the surface morphology of the oxidized samples is obviously different to that of the as-received coating. After
Fig. 4. XRD patterns of the SiC/SiC/SiC coating and the Cr3Si–Cr7C3/SiC/SiC coating.
oxidation at 700 °C (Fig. 6a), the folded ridge morphology of the as-received Cr3Si–Cr7C3/SiC/SiC had changed into a glassy material
Fig. 3. Cross section morphology of (a) the SiC/SiC/SiC and (b) the Cr3Si–Cr7C3/SiC/SiC coating.
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Table 1 Gibbs free energy of reaction between silicon carbide and chromium. Related reactions
Gibbs free energy ΔG [kJ/mol] (700– 1300 K)
16/19Cr þ 3/19SiC-3/19Cr3Siþ 1/19Cr7C3 9/11Cr þ2/11SiC-2/11Cr3Siþ 1/11Cr3C2
11.67 to 0.0032 T 11.19 to 0.0029 T
Fig. 5. XRD patterns of the coated samples after oxidation for 10 h. 1. Cr3Si–Cr7C3/SiC/SiC 700 °C; 2. Cr3Si–Cr7C3/SiC/SiC 900 °C; 3. SiC/SiC/SiC 900 °C; 4. Cr3Si–Cr7C3/SiC/SiC 1300 °C.
covered cluster. The EDS spectrum showed a noticeable oxygen peak, indicating the formation of an oxidation film. A magnified view shown in Fig. 6a* indicates that the outer layer is composed of fine particle clusters. After oxidation at 900 °C (Fig. 6b), the surface shows a relatively flat morphology. A magnified view is
shown in Fig. 6b*, indicating that the surface is dominated by hexagonal Cr2O3 platelets. After oxidation at 1300 °C (Fig. 6c), the surface shows a relatively rough loose morphology. The magnified view (Fig. 6c*) shows the rough loose outer layer is dominated by spherical Cr2O3 particles. As Cr3Si and Cr7C3 begin oxidation above 660 °C, Cr2O3 and CO were formed after oxidation at 700 °C. However, since the oxidation was very slow, only a small amount of Cr2O3 was produced, and no SiO2 was detected. The release of CO produced at or near the chromia/Cr3Si–Cr7C3 interface would be baffled, which results in pores inside the oxidation layer (Fig. 7a). At 900 °C, oxidation of Cr3Si and Cr7C3 became faster, which led to more Cr2O3 forming. The oxide layer became increasingly thicker as the oxidation time increased. The thick oxide layer slowed down the oxygen diffusion into the unreacted region, decreasing the weight loss. It is suggested that at the Cr2O3/Cr7C3 interface, formation of Cr2O3 results in carbon released and thus will form a thin layer of Cr3C2 [23]. Therefore, Cr3C2 was also detected in the oxidized Cr3Si–Cr7C3/SiC/SiC coated samples at 900 °C, and less pores were formed inside the oxide layers (Fig. 7b). Since Cr2O3 would evaporate above 950 °C [24], the evaporation of Cr2O3 and release of the CO led to the formation of a porous oxide layer at 1300 °C. The oxidation of SiC at this temperature could result in increased SiO2 production. Fig. 8 shows the weight change of the coated C/SiC composite during oxidation. The SiC/SiC/SiC coated C/SiC composites exhibit consecutive weight loss during oxidation. The weight loss decreases with increasing oxidation temperature. During oxidation at 700 °C and 900 °C, the Cr3Si–Cr7C3/SiC/SiC coated composite
Cr Si O Cr
C
O Cr Cr
C
O
Cr Si
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Cr
Fig. 6. Surface morphology and EDS spectra of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h at (a) 700 °C; (b) 900 °C; (c) 1300 °C.
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Pores Fig. 7. Cross-section morphologies of the Cr3Si–Cr7C3/SiC/SiC coating after oxidation for 10 h at (a) 700 °C and (b) 900 °C.
Weight Changes (%)
Weight Changes (%)
0.2 0 -0.2 -0.4 -0.6 SiC/SiC/SiC 700°C SiC/SiC/SiC 900°C SiC/SiC/SiC 1300°C CrSi-CrC/SiC/SiC 700°C CrSi-CrC/SiC/SiC 900°C CrSi-CrC/SiC/SiC 1300°C
-0.8 -1 -1.2 -1.4 0
2
4 6 8 Oxidation Time (h)
10
0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.1 -0.15 -0.2
SiC/SiC/SiC 1300°C CrSi-CrC/SiC/SiC 1300°C
y cr =0.13× t 0.33 y sc =-0.066× t 0.417
0
2
4 6 8 Oxidation Time (h)
10
Fig. 8. Weight change of the coated 2D C/SiC composite during oxidation.
450
Flexural Strength (MPa)
400 350 300 250 200 150 100 50 0 As-received
700ºC
Cr3Si-Cr7C3/SiC/SiC
900ºC
1300ºC
SiC/SiC/SiC
Fig. 9. Flexural strength of the coated 2D C/SiC before and after oxidation for 10 h.
samples showed remarkably reduced weight loss compared to that of SiC/SiC/SiC coated samples. During oxidation at 1300 °C, the Cr3Si–Cr7C3/SiC/SiC coated composite samples showed weight gains. Fig. 8b shows the fitted relationship of weight changes versus time during oxidation at 1300 °C using an exponential function. Diffusion-controlled film growth can be generally expressed by the following equation [25]:
Y = k· t n where y is the thickness or weight change of the film, k is the growth rate constant, n is the kinetic exponent, and t is the diffusion time. The result shown in Fig. 8b suggests that at 1300 °C, oxidation of multilayer coated C/SiC was controlled by diffusion. It should be noted that the calculated kinetic exponent for the Cr3Si–Cr7C3/SiC/SiC coated samples was 0.33, while that for the SiC/SiC/SiC coated samples was 0.417 (Fig. 8b). In C/SiC composites, carbon oxidized above 500 °C exhibits absolute weight loss and SiC oxidized above 800 °C shows weight gain. Above 800 °C, the oxidation weight change of the C/SiC composite was the overall result of the competing weight changes. From 700 °C to 1000 °C, the width of the microcracks in the SiC
coatings decreased as temperature increased [26,27]. At a specific oxidation condition (i.e. temperature, oxidizing atmosphere and reactant), the oxidation rate is constant. Consequently, the weight loss decreased with temperature and showed a linear relationship with oxidation time. At 1300 °C, the microcracks in the SiC coating are gradually sealed by thermal expansion and the formed SiO2, due to surface oxidation. However, initially, oxidation of SiC resulted in the formation of a protective SiO2 layer and consequent weight gain, and rapid oxidation of carbon resulted in rapid weight loss. Though carbon fibers are oxidized and evaporate from near-surface, but carbon fibers at deep area may be protected from oxidation. As the oxidation progressed, oxygen diffusion slowed down and then leveled out [21]. As a result, the SiC/SiC/SiC coated samples showed a parabolic weight loss. It is well known that chromia and silica are excellent oxygen barriers [5]. Oxidation of Cr7C3 and Cr3C2 are accompanied with weight gains. For the Cr3Si–Cr7C3/SiC/SiC coated composite samples, below 1000 °C, oxidation of carbon by the inward diffusion oxygen through the cracks in the coatings resulted in weight loss, and oxidation of Cr3Si and Cr7C3 led to weigh gain. Moreover, it is considered the very narrow microcracks in the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating would be narrowed further by the formed chromia and silica. Therefore, it is considered inward oxygen diffusion through the Cr3Si–Cr7C3/SiC/SiC coating is much less than that through the SiC/SiC/SiC coating. As a result, the Cr3Si–Cr7C3/SiC/SiC coated composite samples showed remarkably reduced weight loss than the SiC/SiC/SiC coated samples. At 1300 °C, the narrow microcracks in the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating would be rapidly sealed by thermal expansion of the formed SiO2 and Cr2O3 [28,29]. Therefore, the Cr3Si–Cr7C3/SiC/SiC coated samples showed parabolic weight gains. Fig. 9 shows the flexural strength of the coated samples before and after oxidation for 10 h. After oxidation, the SiC/SiC/SiC coated samples showed lower residual flexural strength compared to the Cr3Si–Cr7C3/SiC/SiC coated samples. The average residual flexural strength of the SiC/SiC/SiC coated samples after 10 h oxidation at
450 400 350 300 250
273
450
200 150 100
Stress (MPa)
Stress (MPa)
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As-received 900ºC SiC/SiC/SiC 900ºC CrSi-CrC/SiC/SiC
50 0 0
0.1
0.2 0.3 Displacement (mm)
0.4
0.5
400 350 300 250 200 150 100 50 0
As-received 1300ºC SiC/SiC/SiC 1300ºC CrSi-CrC/SiC/SiC 0
0.1
0.2 0.3 Displacement (mm)
0.4
0.5
Fig. 10. Typical flexural stress–displacement curves of the coated 2D C/SiC before and after oxidation for 10 h.
700 °C, 900 °C and 1300 °C was 299 MPa, 321 MPa, and 370 MPa, respectively. While the average residual flexural strength of the Cr3Si–Cr7C3/SiC/SiC coated samples after 10 h oxidation at 700 °C, 900 °C and 1300 °C was 394 MPa, 389 MPa, and 407 MPa, respectively. Considering the strength scatter, the residual flexural strength of the Cr3Si–Cr7C3/SiC/SiC coated samples after 10 h oxidation at 1300 °C was nearly the same as that of the as-received samples (411 MPa). Higher residual flexural strength suggested that the Cr3Si–Cr7C3/SiC/SiC coating had better oxidation resistance than the SiC/SiC/SiC coating. Fig. 10 shows typical stress–displacement curves of the coated 2D C/SiC before and after oxidation for 10 h during flexural tests. It can be seen that the failure behavior of the SiC/SiC/SiC coated samples had altered after oxidation. The failure behavior of the asreceived 2D C/SiC composite was rather brittle, and exhibited a steep stress drop after the maximum load point. After oxidation for 10 h, the failure behavior of the SiC/SiC/SiC coated samples showed a gradual stress drop after the maximum load point. The higher the oxidation temperature was, the more obvious the failure changes were. The failure behavior of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h was similar to that of the as-received samples. For the SiC/SiC/SiC coated C/SiC, when there were microcracks in the coating, the oxidation led to a non-uniform consumption of carbon phases near-surface (Fig. 11). Such non-uniform consumption of carbon phases led to pipeline-shaped channels forming between the carbon fiber and SiC matrix. While carbon fibers at deep area may be protected from oxidation. As a result, the strength of the fibers and the bonding between the fiber/interphase/matrix would be weakened, and thus the stress–displacement curves show a gradual stress drop after the maximum load point. As the width of the microcracks in the outer layer of the Cr3Si–Cr7C3/SiC/SiC coating were remarkably narrow compared to those in the SiC/SiC/SiC coating, inward oxygen diffusion was much lower. Moreover, the Cr3Si–Cr7C3/SiC/SiC coated samples were protected from further oxidation by the presence of oxidation-induced chromia and silica. The load–displacement curve of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation was therefore similar to that of the as-received samples. The above results suggest the Cr3Si–Cr7C3/SiC/SiC has obviously enhanced oxidation protection. The work will provide supports for preparation of oxidation resistant coatings which is favorable to improve service performance of carbon fiber reinforced composites in high temperature oxidizing environments.
Fig. 11. Nonuniform consumption of carbon phases after oxidation at 900 °C for 10 h.
composed of Cr3Si and Cr7C3 plus or minus Cr3C2. The Cr3Si–Cr7C3 outer layer showed folded ridge morphology, and the width of the largest microcracks in the outer layer of Cr3Si–Cr7C3/SiC/SiC coating was nearly one forth of that in the SiC/SiC/SiC coating. The Cr3Si–Cr7C3/SiC/SiC coating showed obviously enhanced oxidation protection. Compared with SiC/SiC/SiC coated composite, at 700 °C and 900 °C, the oxidation-induced weight loss of the Cr3Si–Cr7C3/SiC/SiC coated C/SiC was remarkably reduced. At 1300 °C, the Cr3Si–Cr7C3/SiC/SiC coated C/SiC samples showed parabolic weight gains. After oxidation for 10 h, the residual flexural strength of the Cr3Si–Cr7C3/SiC/SiC coated composite almost maintained the same value as that of the as-received composite. The residual flexural strength of the SiC/SiC/SiC coated composite showed a remarkable reduction after oxidation, especially at 700 °C and 900 °C. Oxidation-induced non-uniform consumption of carbon phases resulted in changes in the failure behavior of the SiC/SiC/SiC coated samples. While the failure behavior of the Cr3Si–Cr7C3/SiC/SiC coated samples after oxidation for 10 h was similar to that of the as-received samples.
Acknowledgments The authors gratefully acknowledge the financial support from the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201304) and from the Human Resources Foundation of Northwest A&F University (No. Z111021101).
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