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carbon composites and in-situ grown SiC nanowires to enhance the thermal cycling performance of Si-Mo-Cr coating under parallel oxyacetylene torch
Corrosion Science 111 (2016) 667–674
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Surface modification of carbon/carbon composites and in-situ grown SiC nanowires to enhance the thermal cycling performance of Si-Mo-Cr coating under parallel oxyacetylene torch Jia-Ping Zhang, Qian-Gang Fu ∗ , Jun-Ling Qu, Hua-Shan Zhou, Ning-Kun Liu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, China
a r t i c l e
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Article history: Received 24 March 2016 Received in revised form 8 June 2016 Accepted 8 June 2016 Available online 9 June 2016 Keywords: A. Ceramic matrix composites B. SEM C. High temperature corrosion
a b s t r a c t Before the preparation of Si-Mo-Cr coating, blasting treatment of C/C composites and in-situ grown SiC nanowires were performed. Compared with the coated C/C composites without SiC nanowires, the bonding strength of Si-Mo-Cr coating was increased by 5.7%, and the mass loss per unit area was reduced by 46.39% after 30 thermal cycles between 1600 ◦ C and room temperature. The performance enhancement can be attributed to the improved bonding strength and the alleviation of mismatch of thermal expansion coefficient between C/C substrate and Si-Mo-Cr coating, thereby decreasing the thermal stress during thermal cycles between high and low temperatures. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Because of their good properties, such as low density, high strength-to-weight ratio and low coefficient of thermal expansion (CTE), carbon/carbon (C/C) composites have attracted extensive interest in many fields, especially in aeronautical and aerospace industry where C/C composites can be used as thermo-structural components [1,2]. In practical engineering applications, the aforementioned structural components are usually exposed to environments with high flow of hot oxidizing gases. Unfortunately, poor oxidation resistance of C/C composites restricts their wide use [3–5]. To address the problem mentioned above, an increasing number of studies have been conducted to improve the oxidation resistance of C/C composites, and the most effective method is to prepare an oxidation protective coating on the surface of C/C composites. To date, a variety of coatings have been developed, such as BN/SiC/Si3 N4 -ZrO2 -SiO2 [5], SiC/B4 C-B2 O3 -SiO2 -Al2 O3 [6], AlPO4 /SiC [7], ZrB2 -SiC/SiC [8], HfC [9] and Si-Mo-Cr [10]. Among them, Si-Mo-Cr alloy proves to be an ideal coating material for C/C composites due to the formation of a compound glass layer of SiO2 and Cr2 O3 , which possesses good resistance to volatilization and oxygen diffusion at high temperature [10–12]. Since the composites undergo temperature gradient and combustion gas erosion in
∗ Corresponding author. E-mail address: [email protected] (Q.-G. Fu). http://dx.doi.org/10.1016/j.corsci.2016.06.007 0010-938X/© 2016 Elsevier Ltd. All rights reserved.
practical use, the CTE mismatch between Si-Mo-Cr coating and C/C composites is the main reason for the crack formation or even the debonding of the coating [10].The introduction of a second phase into the Si-Mo-Cr coating is the mostly taken strategy to tackle this problem so as to improve the toughness of the coating considerably. SiC nanowires are suitable to be used as the reinforcing materials due to their small size, high strength and toughness [13,14]. In particular, SiC nanowires can impede the propagation of the microcracks, and then avoid the formation of penetration cracks [15]. However, the introduction of a second phase proves to be ineffective for the improvement of the interface between Si-Mo-Cr coating and C/C composites In light of this problem, pre-oxidation treatment of C/C composites before the preparation of the coating, as a surface modification method, was proposed to obtain a rough and porous surface [11,16,17]. During the preparation of Si-Mo-Cr coating, the porous structure could provide the entrance channels for the infiltration of the coating materials, resulting in an inlaid structure between coating and C/C substrate [11]. The inlaid interface played a positive role in improving the bonding strength between Si-Mo-Cr coating and C/C composites. Even so, the challenge still exists. The oxidation pre-treatment is time-consuming, and the oxidation treated region of C/C composites is difficult to control, which might lead to the degradation of mechanical properties. Thus, it is of significant importance to seek a simpler, faster and more efficient way to modify the surface of C/C composites. Blasting treatment of C/C composites under oxyacetylene torch (as an alternative method of oxidation treatment) is proposed in the present work in order to construct a porous surface structure on
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Fig. 1. Schematic illustration image of blasting treatment of C/C composites, in-situ grown SiC nanowires and preparation of Si-Mo-Cr coating.
C/C composites quickly and efficiently. During blasting treatment, C/C composites will suffer thermal-physical, thermal-chemical and mechanical erosion under oxyacetylene torch. As a result, a porous surface structure can be achieved more efficiently with blasting treatment. Specifically, it is convenient to choose the region for blasting treatment on C/C composites. After blasting treatment, in-situ grown SiC nanowires are formed on the surface of the astreated C/C composites by chemical vapor deposition (CVD) to improve the toughness of the coating [16]. Previous studies on the thermal cycling test were mainly concentrated on the process where C/C composites underwent static oxidation at high temperature firstly and then cooled down to room temperature in air. When practically used, the coated C/C composites are exposed to high-enthalpy gas erosion, where oxidation, ablation, vaporization or some coupled processes of them might occur. In addition, the angle between the high-enthalpy gas flow and the coated C/C composites also varies from time to time [18,19]. In this work, to obtain a further understanding of the service reliability of the Si-Mo-Cr coating used in complicated environments, the thermal cycling performance is studied under parallel oxyacetylene torch, and the corresponding ablation mechanism is discussed.
torch. The device consisted of gas vessels, pressure gauges and the oxyacetylene gun. The inner diameter of the oxyacetylene gun tip was 2 mm and the distance between the gun and the treated C/C composites was 10 mm. The oxyacetylene gun was perpendicular to the C/C composites. Gas pressures of O2 and C2 H2 were 0.4 and 0.095 MPa, and fluxes of O2 and C2 H2 were 1.12 and 0.83 m3 /h, respectively. The treatment time was 30 s. In the second step, in-situ growth of SiC nanowires was achieved on the surface of the treated C/C composites by CVD. A mixed powder of SiO2 , Si and graphite was placed on the bottom of a crucible. Then, the blasting treated C/C composites were placed above the mixed powders and heated to 1500–1800 ◦ C in an argon atmosphere and held for 1–2 h to form the SiC nanowires. Finally, Si-Mo-Cr coating was prepared by pack cementation. Powder compositions were as follows: 45–60 wt.% Si (300 mesh), 25–30 wt.% MoSi2 (200 mesh), 5–15 wt.% Cr (200 mesh) and 8–15 wt.% graphite (200 mesh). The powder mixtures and C/C specimens were put in a graphite crucible, and then heated to 1800–2300 ◦ C and held for 1–4 h in argon atmosphere to form the Si-Mo-Cr coating.
2.2. Characterization 2. Experimental 2.1. Surface modification, in-situ grown SiC nanowires and preparation of Si-Mo-Cr coating Cylinder specimens (Ø10 mm × 10 mm) used as substrates were cut from 2D C/C composites with a density of 1.70 g/cm3 . They were hand-abraded with 400 grit SiC papers, and then cleaned ultrasonically with water and dried at about 80 ◦ C for 6 h. The procedure of surface modification, in-situ grown SiC nanowires and preparation of Si-Mo-Cr coating is shown in Fig. 1, which can be divided into three steps. The first step was to construct a porous surface, where blasting treatment of C/C composites was conducted by oxyacetylene
Thermal cycling test from 1600 ◦ C to room temperature was carried out under oxyacetylene torch, where an environment of combustion gas erosion was constructed. Gas pressures of O2 and C2 H2 were 0.4 and 0.095 MPa, and gas fluxes of O2 and C2 H2 were 0.88 and 0.65 m3 /h, respectively. Fig. 2 is a schematic of the experimental setup for thermal cycling test under oxyacetylene flame. Firstly, the Si-Mo-Cr coated C/C composites were fixed in the center of the sample stage. Before the thermal cycling test, the angle between the oxyacetylene gun and the Si-Mo-Cr coated C/C composites was adjusted until the mixed gas of O2 and C2 H2 was paralleled to the coated C/C composites (as shown in Fig. 2(a)). Then, the mixed gas of O2 and C2 H2 was ignited and the distance between oxyacetylene gun and the Si-Mo-Cr coated C/C composites was adjusted to 11–13 mm so that the surface temperature of
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Fig. 2. The thermal cycling test of the Si-Mo-Cr coated C/C composites, (a) schematic illustration of the experimental setup before thermal cycling test, (b) experimental setup during thermal cycling test, (c) schematic illustration of (b).
the coated C/C composites was controlled to 1600 ◦ C (as shown in Fig. 2(b)). Infrared radiation thermometer (Raytek MR1SCSF) was used to measure the temperature, and the accuracy of the infrared thermometer was in the range of 0.75%. Finally, the thermal cycling test from 1600 ◦ C to room temperature was carried out (as shown in Fig. 2(c)). After the duration of 5 s, the oxyacetylene flame was extinguished, and the coated C/C composites were cooled down to room temperature before the next thermal cycle. Each sample was tested for 30 cycles. The coated C/C composites with blasting treatment only were marked as S-1. For comparison, the coated C/C composites with blasting treatment and in-situ grown SiC nanowires were marked as S-2. During the thermal cycling test, the mass loss of the C/C composites was measured at room temperature by an electronic balance with a sensitivity of ±0.1 mg. The mass loss per unit area was obtained according to the following Formula (1): W =
m0 − m1 A
(1)
where W is the mass loss per unit area; m0 , m1 are the sample mass before and after thermal cycling test, respectively; A is the area of ablation surface. The final mass loss per unit area is the average of three specimens. The microstructures and morphologies of the coated composites were analyzed by scanning electron microscopy (SEM, JSM-6460, JEOL Ltd., Mitaka, Japan), equipped with energy dispersive spectroscopy (EDS). The phases of the coatings were analyzed by X-ray diffraction (XRD, X’Pert PRO, PANalytical, Almelo, the Netherlands) with a Cu K␣ radiation ( = 0.1542 nm) operating at 40 kV and 35 mA. The analyzed range of the diffraction angle 2 was between 10◦ and 90◦ with a step width of 0.033◦ . CTE of the Si-Mo-Cr coated
C/C composites was measured by a DIL402C Dilatometer (NETZSCH Germany). For comparison, CTE of the bare C/C composites was also provided. The measurement of CTE was conducted in argon atmosphere from 850 to 1800 ◦ C, with a heating rate of 5 ◦ C/min. The bonding strength of the Si-Mo-Cr coated C/C composites before thermal cycling test was measured using an adhesive method. The dimension of the tested sample was Ø10 × 10 mm. Cylindrical stainless steel rods were used as matching parts. Specimens were bonded with the end surface of matching parts by a modified acrylate adhesive. After being positioned for 10–15 min and solidified for 24 h at room temperature, the specimens were measured by a universal test machine (CMT5304-30 kN), and the largest force of each specimen was recorded. The bonding strength () was calculated according to the following Formula (2): =
F S
(2)
where F is the largest force recorded by the universal testing machine and S is the cross-sectional area of the coated specimens. For each group, the bonding strength is the average of three specimens. 3. Results and discussion 3.1. Microstructure of C/C composites with blasting treatment and in-situ grown SiC nanowires Typical surface morphologies of C/C composites before and after blasting treatment are exhibited in Fig. 3. C/C composites are composed of carbon fiber reinforcement and carbon matrix, as shown in Fig. 3(a), from which it can be clearly seen that carbon fibers are
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Fig. 3. Surface morphology of C/C composites before (a) and after (b) blasting treatment.
Fig. 4. Surface morphology (a) and XRD pattern (b) of the SiC nanowires by CVD on the surface of the C/C composites with blasting treatment.
tightly surrounded by carbon matrix. When suffered blasting treatment under oxyacetylene torch, C/C composites were subjected to an oxygen-rich environment accompanied with high speed combustion gas erosion [20]. Therefore, both of the structure and shape of the carbon fiber and carbon matrix have changed. As shown in Fig. 3(b), a rough and porous surface is achieved after blasting treatment. Also, needle-like carbon fibers are found as accompanied with the interface gaps between carbon fibers and carbon matrix. Compared with the surface morphology of C/C composites before blasting treatment (Fig. 3(a)), the formation of gaps implies that the interface between carbon fiber and carbon matrix is oxidized preferentially. The porous surface is expected to provide the entrance channels for the coating materials, leading to the formation of an inlaid interface between the Si-Mo-Cr coating and the C/C substrate. Fig. 4 reveals the surface morphology and XRD pattern of the as-prepared SiC nanowires. It can be seen that SiC nanowires grow randomly and form a porous structure on the blasting treated C/C composites. As exhibited in the insert of Fig. 4(a), the diameter of the prepared nanowires is about 1 m, which is close to that of microwires. From Fig. 4(b), C diffraction peak, corresponding to the C/C substrate, can be observed. The appearance of C peak further indicates that the prepared SiC nanowires layer is porous. During the preparation of the Si-Mo-Cr coating, the porous structure is beneficial for the diffusion of the coating materials into the C/C composites, and the introduction of SiC nanowires in Si-Mo-Cr coating is expected to improve the toughness of the coating, especially to decrease the occurrence tendency of micro-cracks.
Fig. 5. XRD pattern of the Si-Mo-Cr coating prepared by pack cementation.
3.2. Microstructure of the Si-Mo-Cr coating XRD pattern of the as-prepared Si-Mo-Cr coating is shown in Fig. 5, from which it can be observed that the main phase compositions of the coating are SiC, MoSi2 , CrSi2 and Si. Fig. 6 reveals the surface morphology of the Si-Mo-Cr coating. Three phases(as seen in Fig. 6(b)) presenting white (A), dark grey (B) and grey (C)
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Fig. 6. Typical morphology of the Si-Mo-Cr coating prepared by pack cementation, (a) S-1, (b) S-2.
Fig. 7. Cross-section micrograph of the Si-Mo-Cr coating for C/C composites, (a) S-1, (b) S-2.
color can be distinguished as the mixture of MoSi2 and CrSi2 , SiC and Si respectively [10]. In addition, micro-crack is observed on the surface of the coating. When the coated C/C composites cool from the preparation temperature to room temperature, the mismatch of CTE between the Si-Mo-Cr coating and the substrate will lead to the formation of the micro-crack. Comparing the surface morphology of S-1 and S-2 (Fig. 6(a) and (b)), it can be concluded that the size of micro-crack of S-2 is reduced significantly. Fig. 7 shows the cross-section micrograph of the obtained SiMo-Cr coating. The interfaces between the coating and the C/C substrate are illustrated by red lines. It can be seen that an inlaid interface between the coating and the C/C substrate is achieved, which is attributed mainly to the porous structure formed by the blasting treatment. The received inlaid interface is expected to improve the bonding strength and alleviate the CTE mismatch between the coating and the C/C substrate. 3.3. High temperature test of the Si-Mo-Cr coated C/C composites To better investigate the effect of SiC nanowires on the Si-MoCr coating, thermal cycling test of the coated C/C composites was conducted from 1600 ◦ C to room temperature under parallel oxyacetylene torch. The mass loss per unit area is shown in Fig. 8. After 30 thermal cycles between 1600 ◦ C and room temperature, the mass loss per unit area of S-1 is up to 28.56 mg cm−2 . In contrast, the mass loss per unit area of S-2 is only 15.31 mg cm−2 , which is reduced by 46.39%. Therefore, it can be deduced that the combination of blasting treatment and in-situ grown SiC nanowires can
Fig. 8. Mass loss per unit area of the Si-Mo-Cr coated C/C composites during thermal cycling test under oxyacetylene torch.
improve the thermal cycling performance of Si-Mo-Cr coating more effectively. Fig. 9 shows the surface morphology and EDS analysis of the coated C/C composites after thermal cycling test. It can be observed that a glassy layer, deduced as SiO2 + Cr2 O3 (Fig. 9(c)), is formed. In addition, micro-crack is observed on the surface of both S-1 and S-2
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Fig. 9. Surface morphology of the Si-Mo-Cr coated C/C composites after thermal cycling test for 30 times under oxyacetylene torch, (a) S-1, (b) S-2, (c) EDS analysis of Spot1.
Fig. 10. Cross-section micrograph of the Si-Mo-Cr coating for C/C composites after thermal cycling test for 30 times under oxyacetylene torch, (a) S-1, (b) S-2.
after thermal cycling test. The micro-crack can provide the entrance channel for the oxyacetylene torch, and the protective ability of the formed SiO2 + Cr2 O3 glassy protective layer will be degraded due to the shear action of oxyacetylene torch. Compared with the surface morphology of S-1(Fig. 9(a)), the size of micro-crack on the surface of S-2 reduces considerably after thermal cycling test (Fig. 9(b)). Cross-section images of the coated C/C composites after 30 thermal cycles are shown in Fig. 10. It can be found that penetrating crack is formed in the coating of S-1 (Fig. 10(a)). In contrast, the coating of S-2 remains intact and no penetration crack is found (Fig. 10(b)).
3.4. Ablation mechanism The environment under oxyacetylene torch is oxygen-rich. During the thermal cycling test under oxyacetylene torch, the following reactions will occur [21,22]: SiC(s)+2O2 (g) → SiO2 (s)+CO2 (g)
(3)
2SiC (s) + 3O2 (g) → 2SiO2 (s) + 2CO (g)
(4)
Si(s)+O2 (g) → SiO2 (s)
(5)
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Fig. 11. Bonding strengths of Si-Mo-Cr coated C/C composites before thermal cycling test.
4CrSi2 (s) + 11O2 (g) → 8SiO2 (s) + 2Cr2 O3 (s)
(6)
2CrSi2 (s)+7O2 (g) → 4SiO2 (s)+2CrO3 (g)
(7)
CrSi2 (s)+3O2 (g) → 2SiO2 (s)+CrO2 (g)
(8)
2MoSi2 (s) +7O2 (g) → 4SiO2 (s)+2MoO3 (g)
(9)
Based on the above reactions, the oxidation of SiC, Si and CrSi2 (Reaction (3)–(8)) is a mass gain process, while the oxidation of MoSi2 (Reaction (9)) is a mass loss process. As exhibited in Fig. 8, according to the trend of the curve, the mass loss curve can be divided into three stages, marked as S1 , S2 and S3 , respectively. At the initial stage of the thermal cycling test (S1 ), the ablation of the coated C/C composites under oxyacetylene torch gains mass. It is demonstrated that the oxidation of MoSi2 (Reaction (9)) in a short time has little impact on the mass change trend of the coating [21]. As a result, in this stage, the mass gain can be mainly attributed to the formation of SiO2 and Cr2 O3 (Reaction (3)–(8)). Then with the increase of thermal cycles, at the stage of S2 , the mass loss per unit area is relatively stable, indicating that the formed glassy layer of SiO2 and Cr2 O3 plays an effective role in resisting the attack of the oxyacetylene torch. At the stage of S3 , the mass loss per unit
673
area rises quickly. Compared with the morphology before thermal cycling test (Fig. 7), the thickness of both coatings is reduced after the test (Fig. 10). During the conventional thermal cycling test in a high-temperature furnace, the coating thickness usually remains unchanged after thermal cycling test [7,21], and the corresponding mass loss is primarily attributed to the oxidation of C/C substrate. In this work, no obvious oxidation of C/C substrate can be found (as shown in Fig. 10). Hence, at the stage of S3 , the increase of mass loss per unit area can be mainly attributed to the reduction of coating thickness caused by the mechanical erosion of oxyacetylene torch. In addition, it can be found that the mass loss per unit area of S-2 changes slowly (Fig. 8). To illustrate the role of SiC nanowires, bonding strength testing and CTE testing were conducted. Fig. 11 exhibits the bonding strengths of the Si-Mo-Cr coated C/C composites before thermal cycling test, which illustrates that the combination of blasting treatment and in-situ grown SiC nanowires can improve the bonding strength between Si-Mo-Cr coating and C/C substrate. Compared with that of S-1, the bonding strength of S-2 is increased by 5.7% and reaches 27.8 MPa. Fig. 12 exhibits the fracture surface morphology of the coated C/C composites with in-situ grown SiC nanowires after blasting treatment. Some pullout nanowires can be observed. The pullout of SiC nanowires can dissipate fracture energy [21], resulting in the improvement of bonding strength of S-2. Fig. 13 exhibits the CTE of bare C/C composites and the Si-MoCr coated C/C composites. It can be found that the C/C substrates present the similar thermal expansion behavior with the coated C/C composites under the same conditions. Compared with the bare C/C composites, CTE of the C/C composites increases after the preparation of Si-Mo-Cr coating. As a result, the measured results can reflect the CTE difference of the prepared coating. As exhibited in Fig. 13, CTE of S-2 is closer to that of bare C/C composites, indicating that the interface between the Si-Mo-Cr and C/C substrate has been improved [21]. During thermal cycling test, the thermal stress () can be described as follows [23]: = T ˛
E 1−
(10)
where T is the difference between cooling temperature and zero stress temperature, ␣ is the CTE difference between the coating and the substrate, E and are the Young’s modulus and Poisson ratio of the coating, respectively. Zero stress temperature can be
Fig. 12. Fracture surface morphology of the coated C/C composites with in-situ grown SiC nanowires after blasting treatment.
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12-BZ-2014), Program of Introducing Talents of Discipline to Universities, China (111 Project) under Grant no. B08040 and the Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University (Grant No.Z2016068). References
Fig. 13. Coefficients of thermal expansion of C/C composites and Si-Mo-Cr coated C/C composites from 850 to 1800 ◦ C.
assumed as the preparation temperature of Si-Mo-Cr coating. From Eq. (10), it can be found that the CTE mismatch has an obvious effect on the thermal stress during thermal cycling test. Compared with S-2, the weaker bonding strength and the mismatch of CTE have caused higher thermal stress of S-1 during thermal cycles, resulting in the formation of micro-crack or even penetrating cracks (Figs. 9 (a) and 10 (a)). As a result, the protective ability of the formed protective layer of SiO2 and Cr2 O3 will be degraded. The formed cracks can provide the entrance channels for oxyacetylene torch and enhance its mechanical erosion. This might be the primary reason for the rapid mass loss of S-1 at the stage of S3 (Fig. 8). 4. Conclusions Surface modification of C/C composites by blasting treatment and in-situ grown SiC nanowires was proposed to improve the thermal cycling performance of Si-Mo-Cr coating. Compared with the coated C/C composites with blasting treatment only, the bonding strength between Si-Mo-Cr coating and C/C substrate was increased by 5.7% due to the introduction of in-situ grown SiC nanowires. After 30 thermal cycles between 1600 ◦ C and room temperature under parallel oxyacetylene torch, the mass loss per unit area of the coated C/C composites with blasting treatment only was up to 28.56 mg cm−2 and penetrating crack was formed. In contrast, the mass loss per unit area of the coated C/C composites with the combination of blasting treatment and in-situ grown SiC nanowires was 15.31 mg cm−2 . The improvement on performance can be attributed to the enhancement of bonding strength and the efficient alleviation of thermal expansion coefficient mismatch between C/C substrate and Si-Mo-Cr coating, thus decreasing the thermal stress during high-low temperature cycles. Acknowledgments This work has been supported by National Natural Science Foundation of China under Grant nos. 51521061, U1435202 and 51572223, Project supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.
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Surface modification of carbon/carbon composites and in-situ grown SiC nanowires to enhance the thermal cycling performance of Si-Mo-Cr coating under parallel oxyacetylene torch Jia-Ping Zhang, Qian-Gang Fu ∗ , Jun-Ling Qu, Hua-Shan Zhou, Ning-Kun Liu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, China
a r t i c l e
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Article history: Received 24 March 2016 Received in revised form 8 June 2016 Accepted 8 June 2016 Available online 9 June 2016 Keywords: A. Ceramic matrix composites B. SEM C. High temperature corrosion
a b s t r a c t Before the preparation of Si-Mo-Cr coating, blasting treatment of C/C composites and in-situ grown SiC nanowires were performed. Compared with the coated C/C composites without SiC nanowires, the bonding strength of Si-Mo-Cr coating was increased by 5.7%, and the mass loss per unit area was reduced by 46.39% after 30 thermal cycles between 1600 ◦ C and room temperature. The performance enhancement can be attributed to the improved bonding strength and the alleviation of mismatch of thermal expansion coefficient between C/C substrate and Si-Mo-Cr coating, thereby decreasing the thermal stress during thermal cycles between high and low temperatures. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Because of their good properties, such as low density, high strength-to-weight ratio and low coefficient of thermal expansion (CTE), carbon/carbon (C/C) composites have attracted extensive interest in many fields, especially in aeronautical and aerospace industry where C/C composites can be used as thermo-structural components [1,2]. In practical engineering applications, the aforementioned structural components are usually exposed to environments with high flow of hot oxidizing gases. Unfortunately, poor oxidation resistance of C/C composites restricts their wide use [3–5]. To address the problem mentioned above, an increasing number of studies have been conducted to improve the oxidation resistance of C/C composites, and the most effective method is to prepare an oxidation protective coating on the surface of C/C composites. To date, a variety of coatings have been developed, such as BN/SiC/Si3 N4 -ZrO2 -SiO2 [5], SiC/B4 C-B2 O3 -SiO2 -Al2 O3 [6], AlPO4 /SiC [7], ZrB2 -SiC/SiC [8], HfC [9] and Si-Mo-Cr [10]. Among them, Si-Mo-Cr alloy proves to be an ideal coating material for C/C composites due to the formation of a compound glass layer of SiO2 and Cr2 O3 , which possesses good resistance to volatilization and oxygen diffusion at high temperature [10–12]. Since the composites undergo temperature gradient and combustion gas erosion in
∗ Corresponding author. E-mail address: [email protected] (Q.-G. Fu). http://dx.doi.org/10.1016/j.corsci.2016.06.007 0010-938X/© 2016 Elsevier Ltd. All rights reserved.
practical use, the CTE mismatch between Si-Mo-Cr coating and C/C composites is the main reason for the crack formation or even the debonding of the coating [10].The introduction of a second phase into the Si-Mo-Cr coating is the mostly taken strategy to tackle this problem so as to improve the toughness of the coating considerably. SiC nanowires are suitable to be used as the reinforcing materials due to their small size, high strength and toughness [13,14]. In particular, SiC nanowires can impede the propagation of the microcracks, and then avoid the formation of penetration cracks [15]. However, the introduction of a second phase proves to be ineffective for the improvement of the interface between Si-Mo-Cr coating and C/C composites In light of this problem, pre-oxidation treatment of C/C composites before the preparation of the coating, as a surface modification method, was proposed to obtain a rough and porous surface [11,16,17]. During the preparation of Si-Mo-Cr coating, the porous structure could provide the entrance channels for the infiltration of the coating materials, resulting in an inlaid structure between coating and C/C substrate [11]. The inlaid interface played a positive role in improving the bonding strength between Si-Mo-Cr coating and C/C composites. Even so, the challenge still exists. The oxidation pre-treatment is time-consuming, and the oxidation treated region of C/C composites is difficult to control, which might lead to the degradation of mechanical properties. Thus, it is of significant importance to seek a simpler, faster and more efficient way to modify the surface of C/C composites. Blasting treatment of C/C composites under oxyacetylene torch (as an alternative method of oxidation treatment) is proposed in the present work in order to construct a porous surface structure on
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Fig. 1. Schematic illustration image of blasting treatment of C/C composites, in-situ grown SiC nanowires and preparation of Si-Mo-Cr coating.
C/C composites quickly and efficiently. During blasting treatment, C/C composites will suffer thermal-physical, thermal-chemical and mechanical erosion under oxyacetylene torch. As a result, a porous surface structure can be achieved more efficiently with blasting treatment. Specifically, it is convenient to choose the region for blasting treatment on C/C composites. After blasting treatment, in-situ grown SiC nanowires are formed on the surface of the astreated C/C composites by chemical vapor deposition (CVD) to improve the toughness of the coating [16]. Previous studies on the thermal cycling test were mainly concentrated on the process where C/C composites underwent static oxidation at high temperature firstly and then cooled down to room temperature in air. When practically used, the coated C/C composites are exposed to high-enthalpy gas erosion, where oxidation, ablation, vaporization or some coupled processes of them might occur. In addition, the angle between the high-enthalpy gas flow and the coated C/C composites also varies from time to time [18,19]. In this work, to obtain a further understanding of the service reliability of the Si-Mo-Cr coating used in complicated environments, the thermal cycling performance is studied under parallel oxyacetylene torch, and the corresponding ablation mechanism is discussed.
torch. The device consisted of gas vessels, pressure gauges and the oxyacetylene gun. The inner diameter of the oxyacetylene gun tip was 2 mm and the distance between the gun and the treated C/C composites was 10 mm. The oxyacetylene gun was perpendicular to the C/C composites. Gas pressures of O2 and C2 H2 were 0.4 and 0.095 MPa, and fluxes of O2 and C2 H2 were 1.12 and 0.83 m3 /h, respectively. The treatment time was 30 s. In the second step, in-situ growth of SiC nanowires was achieved on the surface of the treated C/C composites by CVD. A mixed powder of SiO2 , Si and graphite was placed on the bottom of a crucible. Then, the blasting treated C/C composites were placed above the mixed powders and heated to 1500–1800 ◦ C in an argon atmosphere and held for 1–2 h to form the SiC nanowires. Finally, Si-Mo-Cr coating was prepared by pack cementation. Powder compositions were as follows: 45–60 wt.% Si (300 mesh), 25–30 wt.% MoSi2 (200 mesh), 5–15 wt.% Cr (200 mesh) and 8–15 wt.% graphite (200 mesh). The powder mixtures and C/C specimens were put in a graphite crucible, and then heated to 1800–2300 ◦ C and held for 1–4 h in argon atmosphere to form the Si-Mo-Cr coating.
2.2. Characterization 2. Experimental 2.1. Surface modification, in-situ grown SiC nanowires and preparation of Si-Mo-Cr coating Cylinder specimens (Ø10 mm × 10 mm) used as substrates were cut from 2D C/C composites with a density of 1.70 g/cm3 . They were hand-abraded with 400 grit SiC papers, and then cleaned ultrasonically with water and dried at about 80 ◦ C for 6 h. The procedure of surface modification, in-situ grown SiC nanowires and preparation of Si-Mo-Cr coating is shown in Fig. 1, which can be divided into three steps. The first step was to construct a porous surface, where blasting treatment of C/C composites was conducted by oxyacetylene
Thermal cycling test from 1600 ◦ C to room temperature was carried out under oxyacetylene torch, where an environment of combustion gas erosion was constructed. Gas pressures of O2 and C2 H2 were 0.4 and 0.095 MPa, and gas fluxes of O2 and C2 H2 were 0.88 and 0.65 m3 /h, respectively. Fig. 2 is a schematic of the experimental setup for thermal cycling test under oxyacetylene flame. Firstly, the Si-Mo-Cr coated C/C composites were fixed in the center of the sample stage. Before the thermal cycling test, the angle between the oxyacetylene gun and the Si-Mo-Cr coated C/C composites was adjusted until the mixed gas of O2 and C2 H2 was paralleled to the coated C/C composites (as shown in Fig. 2(a)). Then, the mixed gas of O2 and C2 H2 was ignited and the distance between oxyacetylene gun and the Si-Mo-Cr coated C/C composites was adjusted to 11–13 mm so that the surface temperature of
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Fig. 2. The thermal cycling test of the Si-Mo-Cr coated C/C composites, (a) schematic illustration of the experimental setup before thermal cycling test, (b) experimental setup during thermal cycling test, (c) schematic illustration of (b).
the coated C/C composites was controlled to 1600 ◦ C (as shown in Fig. 2(b)). Infrared radiation thermometer (Raytek MR1SCSF) was used to measure the temperature, and the accuracy of the infrared thermometer was in the range of 0.75%. Finally, the thermal cycling test from 1600 ◦ C to room temperature was carried out (as shown in Fig. 2(c)). After the duration of 5 s, the oxyacetylene flame was extinguished, and the coated C/C composites were cooled down to room temperature before the next thermal cycle. Each sample was tested for 30 cycles. The coated C/C composites with blasting treatment only were marked as S-1. For comparison, the coated C/C composites with blasting treatment and in-situ grown SiC nanowires were marked as S-2. During the thermal cycling test, the mass loss of the C/C composites was measured at room temperature by an electronic balance with a sensitivity of ±0.1 mg. The mass loss per unit area was obtained according to the following Formula (1): W =
m0 − m1 A
(1)
where W is the mass loss per unit area; m0 , m1 are the sample mass before and after thermal cycling test, respectively; A is the area of ablation surface. The final mass loss per unit area is the average of three specimens. The microstructures and morphologies of the coated composites were analyzed by scanning electron microscopy (SEM, JSM-6460, JEOL Ltd., Mitaka, Japan), equipped with energy dispersive spectroscopy (EDS). The phases of the coatings were analyzed by X-ray diffraction (XRD, X’Pert PRO, PANalytical, Almelo, the Netherlands) with a Cu K␣ radiation ( = 0.1542 nm) operating at 40 kV and 35 mA. The analyzed range of the diffraction angle 2 was between 10◦ and 90◦ with a step width of 0.033◦ . CTE of the Si-Mo-Cr coated
C/C composites was measured by a DIL402C Dilatometer (NETZSCH Germany). For comparison, CTE of the bare C/C composites was also provided. The measurement of CTE was conducted in argon atmosphere from 850 to 1800 ◦ C, with a heating rate of 5 ◦ C/min. The bonding strength of the Si-Mo-Cr coated C/C composites before thermal cycling test was measured using an adhesive method. The dimension of the tested sample was Ø10 × 10 mm. Cylindrical stainless steel rods were used as matching parts. Specimens were bonded with the end surface of matching parts by a modified acrylate adhesive. After being positioned for 10–15 min and solidified for 24 h at room temperature, the specimens were measured by a universal test machine (CMT5304-30 kN), and the largest force of each specimen was recorded. The bonding strength () was calculated according to the following Formula (2): =
F S
(2)
where F is the largest force recorded by the universal testing machine and S is the cross-sectional area of the coated specimens. For each group, the bonding strength is the average of three specimens. 3. Results and discussion 3.1. Microstructure of C/C composites with blasting treatment and in-situ grown SiC nanowires Typical surface morphologies of C/C composites before and after blasting treatment are exhibited in Fig. 3. C/C composites are composed of carbon fiber reinforcement and carbon matrix, as shown in Fig. 3(a), from which it can be clearly seen that carbon fibers are
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Fig. 3. Surface morphology of C/C composites before (a) and after (b) blasting treatment.
Fig. 4. Surface morphology (a) and XRD pattern (b) of the SiC nanowires by CVD on the surface of the C/C composites with blasting treatment.
tightly surrounded by carbon matrix. When suffered blasting treatment under oxyacetylene torch, C/C composites were subjected to an oxygen-rich environment accompanied with high speed combustion gas erosion [20]. Therefore, both of the structure and shape of the carbon fiber and carbon matrix have changed. As shown in Fig. 3(b), a rough and porous surface is achieved after blasting treatment. Also, needle-like carbon fibers are found as accompanied with the interface gaps between carbon fibers and carbon matrix. Compared with the surface morphology of C/C composites before blasting treatment (Fig. 3(a)), the formation of gaps implies that the interface between carbon fiber and carbon matrix is oxidized preferentially. The porous surface is expected to provide the entrance channels for the coating materials, leading to the formation of an inlaid interface between the Si-Mo-Cr coating and the C/C substrate. Fig. 4 reveals the surface morphology and XRD pattern of the as-prepared SiC nanowires. It can be seen that SiC nanowires grow randomly and form a porous structure on the blasting treated C/C composites. As exhibited in the insert of Fig. 4(a), the diameter of the prepared nanowires is about 1 m, which is close to that of microwires. From Fig. 4(b), C diffraction peak, corresponding to the C/C substrate, can be observed. The appearance of C peak further indicates that the prepared SiC nanowires layer is porous. During the preparation of the Si-Mo-Cr coating, the porous structure is beneficial for the diffusion of the coating materials into the C/C composites, and the introduction of SiC nanowires in Si-Mo-Cr coating is expected to improve the toughness of the coating, especially to decrease the occurrence tendency of micro-cracks.
Fig. 5. XRD pattern of the Si-Mo-Cr coating prepared by pack cementation.
3.2. Microstructure of the Si-Mo-Cr coating XRD pattern of the as-prepared Si-Mo-Cr coating is shown in Fig. 5, from which it can be observed that the main phase compositions of the coating are SiC, MoSi2 , CrSi2 and Si. Fig. 6 reveals the surface morphology of the Si-Mo-Cr coating. Three phases(as seen in Fig. 6(b)) presenting white (A), dark grey (B) and grey (C)
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Fig. 6. Typical morphology of the Si-Mo-Cr coating prepared by pack cementation, (a) S-1, (b) S-2.
Fig. 7. Cross-section micrograph of the Si-Mo-Cr coating for C/C composites, (a) S-1, (b) S-2.
color can be distinguished as the mixture of MoSi2 and CrSi2 , SiC and Si respectively [10]. In addition, micro-crack is observed on the surface of the coating. When the coated C/C composites cool from the preparation temperature to room temperature, the mismatch of CTE between the Si-Mo-Cr coating and the substrate will lead to the formation of the micro-crack. Comparing the surface morphology of S-1 and S-2 (Fig. 6(a) and (b)), it can be concluded that the size of micro-crack of S-2 is reduced significantly. Fig. 7 shows the cross-section micrograph of the obtained SiMo-Cr coating. The interfaces between the coating and the C/C substrate are illustrated by red lines. It can be seen that an inlaid interface between the coating and the C/C substrate is achieved, which is attributed mainly to the porous structure formed by the blasting treatment. The received inlaid interface is expected to improve the bonding strength and alleviate the CTE mismatch between the coating and the C/C substrate. 3.3. High temperature test of the Si-Mo-Cr coated C/C composites To better investigate the effect of SiC nanowires on the Si-MoCr coating, thermal cycling test of the coated C/C composites was conducted from 1600 ◦ C to room temperature under parallel oxyacetylene torch. The mass loss per unit area is shown in Fig. 8. After 30 thermal cycles between 1600 ◦ C and room temperature, the mass loss per unit area of S-1 is up to 28.56 mg cm−2 . In contrast, the mass loss per unit area of S-2 is only 15.31 mg cm−2 , which is reduced by 46.39%. Therefore, it can be deduced that the combination of blasting treatment and in-situ grown SiC nanowires can
Fig. 8. Mass loss per unit area of the Si-Mo-Cr coated C/C composites during thermal cycling test under oxyacetylene torch.
improve the thermal cycling performance of Si-Mo-Cr coating more effectively. Fig. 9 shows the surface morphology and EDS analysis of the coated C/C composites after thermal cycling test. It can be observed that a glassy layer, deduced as SiO2 + Cr2 O3 (Fig. 9(c)), is formed. In addition, micro-crack is observed on the surface of both S-1 and S-2
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Fig. 9. Surface morphology of the Si-Mo-Cr coated C/C composites after thermal cycling test for 30 times under oxyacetylene torch, (a) S-1, (b) S-2, (c) EDS analysis of Spot1.
Fig. 10. Cross-section micrograph of the Si-Mo-Cr coating for C/C composites after thermal cycling test for 30 times under oxyacetylene torch, (a) S-1, (b) S-2.
after thermal cycling test. The micro-crack can provide the entrance channel for the oxyacetylene torch, and the protective ability of the formed SiO2 + Cr2 O3 glassy protective layer will be degraded due to the shear action of oxyacetylene torch. Compared with the surface morphology of S-1(Fig. 9(a)), the size of micro-crack on the surface of S-2 reduces considerably after thermal cycling test (Fig. 9(b)). Cross-section images of the coated C/C composites after 30 thermal cycles are shown in Fig. 10. It can be found that penetrating crack is formed in the coating of S-1 (Fig. 10(a)). In contrast, the coating of S-2 remains intact and no penetration crack is found (Fig. 10(b)).
3.4. Ablation mechanism The environment under oxyacetylene torch is oxygen-rich. During the thermal cycling test under oxyacetylene torch, the following reactions will occur [21,22]: SiC(s)+2O2 (g) → SiO2 (s)+CO2 (g)
(3)
2SiC (s) + 3O2 (g) → 2SiO2 (s) + 2CO (g)
(4)
Si(s)+O2 (g) → SiO2 (s)
(5)
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Fig. 11. Bonding strengths of Si-Mo-Cr coated C/C composites before thermal cycling test.
4CrSi2 (s) + 11O2 (g) → 8SiO2 (s) + 2Cr2 O3 (s)
(6)
2CrSi2 (s)+7O2 (g) → 4SiO2 (s)+2CrO3 (g)
(7)
CrSi2 (s)+3O2 (g) → 2SiO2 (s)+CrO2 (g)
(8)
2MoSi2 (s) +7O2 (g) → 4SiO2 (s)+2MoO3 (g)
(9)
Based on the above reactions, the oxidation of SiC, Si and CrSi2 (Reaction (3)–(8)) is a mass gain process, while the oxidation of MoSi2 (Reaction (9)) is a mass loss process. As exhibited in Fig. 8, according to the trend of the curve, the mass loss curve can be divided into three stages, marked as S1 , S2 and S3 , respectively. At the initial stage of the thermal cycling test (S1 ), the ablation of the coated C/C composites under oxyacetylene torch gains mass. It is demonstrated that the oxidation of MoSi2 (Reaction (9)) in a short time has little impact on the mass change trend of the coating [21]. As a result, in this stage, the mass gain can be mainly attributed to the formation of SiO2 and Cr2 O3 (Reaction (3)–(8)). Then with the increase of thermal cycles, at the stage of S2 , the mass loss per unit area is relatively stable, indicating that the formed glassy layer of SiO2 and Cr2 O3 plays an effective role in resisting the attack of the oxyacetylene torch. At the stage of S3 , the mass loss per unit
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area rises quickly. Compared with the morphology before thermal cycling test (Fig. 7), the thickness of both coatings is reduced after the test (Fig. 10). During the conventional thermal cycling test in a high-temperature furnace, the coating thickness usually remains unchanged after thermal cycling test [7,21], and the corresponding mass loss is primarily attributed to the oxidation of C/C substrate. In this work, no obvious oxidation of C/C substrate can be found (as shown in Fig. 10). Hence, at the stage of S3 , the increase of mass loss per unit area can be mainly attributed to the reduction of coating thickness caused by the mechanical erosion of oxyacetylene torch. In addition, it can be found that the mass loss per unit area of S-2 changes slowly (Fig. 8). To illustrate the role of SiC nanowires, bonding strength testing and CTE testing were conducted. Fig. 11 exhibits the bonding strengths of the Si-Mo-Cr coated C/C composites before thermal cycling test, which illustrates that the combination of blasting treatment and in-situ grown SiC nanowires can improve the bonding strength between Si-Mo-Cr coating and C/C substrate. Compared with that of S-1, the bonding strength of S-2 is increased by 5.7% and reaches 27.8 MPa. Fig. 12 exhibits the fracture surface morphology of the coated C/C composites with in-situ grown SiC nanowires after blasting treatment. Some pullout nanowires can be observed. The pullout of SiC nanowires can dissipate fracture energy [21], resulting in the improvement of bonding strength of S-2. Fig. 13 exhibits the CTE of bare C/C composites and the Si-MoCr coated C/C composites. It can be found that the C/C substrates present the similar thermal expansion behavior with the coated C/C composites under the same conditions. Compared with the bare C/C composites, CTE of the C/C composites increases after the preparation of Si-Mo-Cr coating. As a result, the measured results can reflect the CTE difference of the prepared coating. As exhibited in Fig. 13, CTE of S-2 is closer to that of bare C/C composites, indicating that the interface between the Si-Mo-Cr and C/C substrate has been improved [21]. During thermal cycling test, the thermal stress () can be described as follows [23]: = T ˛
E 1−
(10)
where T is the difference between cooling temperature and zero stress temperature, ␣ is the CTE difference between the coating and the substrate, E and are the Young’s modulus and Poisson ratio of the coating, respectively. Zero stress temperature can be
Fig. 12. Fracture surface morphology of the coated C/C composites with in-situ grown SiC nanowires after blasting treatment.
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12-BZ-2014), Program of Introducing Talents of Discipline to Universities, China (111 Project) under Grant no. B08040 and the Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University (Grant No.Z2016068). References
Fig. 13. Coefficients of thermal expansion of C/C composites and Si-Mo-Cr coated C/C composites from 850 to 1800 ◦ C.
assumed as the preparation temperature of Si-Mo-Cr coating. From Eq. (10), it can be found that the CTE mismatch has an obvious effect on the thermal stress during thermal cycling test. Compared with S-2, the weaker bonding strength and the mismatch of CTE have caused higher thermal stress of S-1 during thermal cycles, resulting in the formation of micro-crack or even penetrating cracks (Figs. 9 (a) and 10 (a)). As a result, the protective ability of the formed protective layer of SiO2 and Cr2 O3 will be degraded. The formed cracks can provide the entrance channels for oxyacetylene torch and enhance its mechanical erosion. This might be the primary reason for the rapid mass loss of S-1 at the stage of S3 (Fig. 8). 4. Conclusions Surface modification of C/C composites by blasting treatment and in-situ grown SiC nanowires was proposed to improve the thermal cycling performance of Si-Mo-Cr coating. Compared with the coated C/C composites with blasting treatment only, the bonding strength between Si-Mo-Cr coating and C/C substrate was increased by 5.7% due to the introduction of in-situ grown SiC nanowires. After 30 thermal cycles between 1600 ◦ C and room temperature under parallel oxyacetylene torch, the mass loss per unit area of the coated C/C composites with blasting treatment only was up to 28.56 mg cm−2 and penetrating crack was formed. In contrast, the mass loss per unit area of the coated C/C composites with the combination of blasting treatment and in-situ grown SiC nanowires was 15.31 mg cm−2 . The improvement on performance can be attributed to the enhancement of bonding strength and the efficient alleviation of thermal expansion coefficient mismatch between C/C substrate and Si-Mo-Cr coating, thus decreasing the thermal stress during high-low temperature cycles. Acknowledgments This work has been supported by National Natural Science Foundation of China under Grant nos. 51521061, U1435202 and 51572223, Project supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.
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