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carbon composites against ablation
Surface & Coatings Technology 300 (2016) 1–9
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SiC/ZrB2–SiC–ZrC multilayer coating for carbon/carbon composites against ablation Yulei Zhang ⁎, Heng Hu, Pengfei Zhang, Zhixiong Hu, Hejun Li, Leilei Zhang State Key Laboratory of Solidification Processing, C/C Composites Research Center, Northwestern Polytechnical University, Xi'an, 710072, PR China
a r t i c l e
i n f o
Article history: Received 12 January 2016 Revised 29 April 2016 Accepted in revised form 3 May 2016 Available online 12 May 2016 Keywords: Carbon/carbon composites ZrB2–SiC–ZrC coating Supersonic atmosphere plasma spraying Ablation Layered structure
a b s t r a c t To improve the ablation resistance of carbon/carbon (C/C) composites at high temperature, a ZrB2–SiC–ZrC coating was prepared on the surface of SiC-coated C/C composites by supersonic atmosphere plasma spray (SAPS). Ablation resistance of the coated C/C composites was tested in an oxyacetylene torch environment with a heat flux of 2400 kW/m2 for 120, 150 and 200 s, respectively. After ablation for 120 s, the coating exhibited a threelayered structure consisting of a porous ZrO2 layer, a ZrO2–SiO2 thin layer and a SiC-depleted layer, whereas it transformed into a two-layered structure (ZrO2 recrystallized layer and ZrO2–SiO2 layer) after ablation for 150 s. With the increase of ablation time, the SiC inner coating was almost consumed and a gap emerged between the ZrO2 layer and the C/C matrix after ablation for 200 s. The good ablation resistance of the sprayed coating is mainly attributed to the layered structure acting as a thermal barrier and inhibiting inward diffusion of oxygen. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites possess many attractive properties, such as high strength-to-weight ratio, low coefficient of thermal expansion (CTE) and good strength retention at high temperatures in a reducing environment [1]. Therefore, they are considered as one of the most promising candidate materials for thermal–structural components in turbine engines and in aerospace and re-entry vehicles [2]. However, the oxidation of C/C composites above 450 °C in air limits their application in an oxidizing environment, especially for high-temperature associated with high-speed combustion gas flow [3–5]. So, it is necessary to improve the ablation resistance of C/C composites to meet the application requirements in these extreme environments. Several approaches have been developed to protect C/C composites against oxidation at high temperatures. Applying a coating on C/C composites has been proved to be an effective method [6–10]. In recent years, many ultra-high temperature ceramics (UHTCs), such as ZrC, HfC, TaC and ZrB2, have been used as ablation resistance coatings for C/C composites [11–15]. Zirconium diboride (ZrB2) and zirconium carbide (ZrC) based ceramics have a high melting point, high thermal and electrical conductivities, and good thermal shock resistance [16– ⁎ Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.surfcoat.2016.05.028 0257-8972/© 2016 Elsevier B.V. All rights reserved.
19]. So, they have been applied in a high-temperature environment to resist oxidation, corrosion and wear. It is well known that the addition of a SiC ceramic can significantly improve the oxidation behavior of ZrB2-based ceramics in the moderately high temperature range and the optimum amount of SiC is between 15 and 20 vol.% [20–22]. In recent years, oxidation resistance of ZrB2-based ceramics at high temperature has been investigated by several researchers [21,23,24], and it has been reported that the typical scale after oxidation of a ZrB2–SiC ceramic is composed of four layers: a SiO2 rich glassy layer, a ZrO2–SiO2 layer, a SiC-depleted layer and an unaffected layer [25]. The typical layers could provide good oxidation protection for the substrates. According to previous works, ZrB2–SiC coatings on C/C composites could improve ablation resistance [26,27]. Furthermore, Wang et al. have reported that a ZrB2–SiC–ZrC ceramic had good static oxidation resistance at 1750 °C [28], and the introduction of the ZrC phase improved the fracture toughness of the ZrB2–SiC ceramic from 4.1 MPa to 6.7 MPa [29]. Thus, the ZrB2–SiC–ZrC ceramic is a promising material to resist oxidation at high temperature and erosion by high-speed gas flow. However, little work has been focused on the ablation resistance of the ZrB2–SiC–ZrC coating for C/C composites up to now [28–30]. In this work, the ZrB2–SiC–ZrC ceramic was selected as the ablation coating for C/C composites and supersonic atmosphere plasma spraying (SAPS) was employed to prepare this coating due to the high temperature of a plasma arc and the high velocity of particles. Between the C/C
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substrate and the UHTC coating, a SiC buffer layer was sprayed to alleviate the mismatch. The purpose of this work is to describe the microstructure evolution of the coating as a function of different ablation time. The ablation mechanism of C/C composites with the multilayer coating is also discussed. 2. Experimental 2.1. Preparation of the multilayer coating The substrate samples (Ф 30 × 10 mm) were cut from bulk two-dimensional (2D) C/C composites with a density of about 1700 kg/m3. 2D C/C composites were prepared by thermal gradient chemical vapor deposition and details can be found in our previous work [31]. Then these samples were cleaned in an ultrasonic bath with ethanol and dried at 100–150 °C for 2 h. The SiC inner layer was prepared by pack cementation with Si (60–80 wt.%) (Jiuling Smelting Co., Ltd., Shanghai, China), graphite (10–25 wt.%) (Carbon Plant, Xi'an, China) and Al2O3 (5– 15 wt.%) (Guoyao Chemical Reagent Co., Ltd., Shanghai, China) powders in inert atmosphere at 1700–1900 °C for 2 h. [32]. SAPS was applied to prepare the ZrB2–SiC–ZrC coating on the surface of SiC-coated C/C composites. The spraying apparatus is schematically shown in Fig. 1, and the spraying parameters are summarized in Table 1. The spraying system consists of a plasma torch, powder feeder, gas supply system, water-cooling circulator, control unit with PC interface and power supply unit. The ZrB2 particles (800 mesh, N99.9% purity, DanDong Research Insitute of Chemical Industry, China), SiC particles (800 mesh, N99.9% purity, Linyi Jin meng Carborundum Co. Ltd) and ZrC particles (400 mesh, N 98% purity, Jinzhou Metal Material Research Institute, Liaoning, China) were selected as the raw material of spraying powders. Before spraying, the mixture particles containing 60 vol.% ZrB2, 20 vol.% SiC and 20 vol.% ZrC were preliminary prepared by attrition milling with ZrO2 milling media for 2 h, then they were agglomerated by a spray drier and details have been reported in Ref. [33]. 2.2. Ablation test and microstructure analysis An oxyacetylene torch test was used to evaluate the ablation resistance of the coated samples, according to the National Standard of Ablation Test method of ablative materials (GJB 323A-96) with heat flux of 2400 kW/m2 [34]. The pressure of oxygen and acetylene was 0.4 MPa and 0.095 MPa, and the flux of oxygen and acetylene 0.244 l/s and 0.167 l/s, respectively. The oxyacetylene gun has an inner diameter of 2 mm, and the distance between the gun tip and the sample was set as 10 mm. After the flame of ablation gun had reached stability, the samples were vertically placed to the flame and maintained for 120, 150 and 200 s, respectively. The surface temperature of the sample was measured by an infrared thermometer. A picture of the oxyacetylene torch test can be found in Ref. [35]. The linear and mass ablation rates (Rl and Rm, respectively) of the samples were calculated according to
Table 1 Details of the spraying parameters for ZrB2–SiC–ZrC multilayer coating. Content
Parameters
Spraying current, A Spraying voltage, V Primary gas Ar, l/min Carrier gas Ar, l/min Second gas H2, l/min Powder feed rate, g/min Spraying distance, mm Injector internal diameter, mm Injector position
385–425 100–130 70 15 5 30 100 5.5 Perpendicular to samples
following equations: Rl ¼ Δd=t
ð1Þ
Rm ¼ Δm=t
ð2Þ
where Δd is the change of thickness before and after ablation, Δm is the change of mass before and after ablation, and t is ablation time. The ultimate ablation rates of the composites were the average ablation rates of three samples. The crystalline structure of the coatings was investigated by X-ray diffraction (XRD, X'Pert Pro MPD) with a Cu Kα radiation having a wavelength of 0.154 nm. The morphology and element composition of the coatings were investigated by scanning electron microscopy (SEM, JSM-6460) equipped with energy dispersion spectroscopy (EDS). 3. Results and discussion 3.1. Phase composition and microstructure of as-sprayed coating Fig. 2 shows the XRD pattern of the as-sprayed coating. The predominant phases in the ZrB2–SiC–ZrC coating are ZrB2, SiC, ZrC and a small quantity of ZrO2 according to the JCPDS cards: No. (00-034-0423) for ZrB2, (01-073-2084) for SiC, (01-089-3829) for ZrC, (01-088-1007) for tetragonal ZrO2 (t-ZrO2) and (00-007-0343) for monoclinic ZrO2 (mZrO2). The ZrB2–SiC–ZrC coating was prepared by SAPS in air, so the oxidation of spraying powders was unavoidable, which would result in the formation of ZrO2, SiO2 and gaseous byproducts (such as B2O3, CO, CO2, and SiO). In the sprayed coating, t-ZrO2 was detected because part of tZrO2 could not be transformed to m-ZrO2 during rapid cooling from high temperature to room temperature. Due to the low content and amorphous structure of SiO2, it could not be detected by X-ray diffraction technique. In addition, the lower intensity of SiC may result from the oxidation and decomposition of the SiC ceramic at high temperature. Fig. 3 shows SEM images of the surface of the ZrB2–SiC–ZrC coating. It can be found that the coating is composed of the fully molten area and insufficiently molten area, and there is no obvious crack or void in the coating. According to the results of XRD and EDS analysis (Fig. 3c), the
Fig. 1. Sketch of the spraying apparatus.
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Table 2 shows the mass and liner ablation rates of the coated C/C composites after ablation for different time. The results show that the mass and liner ablation rates of the specimens are both negative, which mean that the weight and volume of the specimens both increased after ablation. While the mass ablation rate, Rm, can be considered not varying between the three times, within the error range, the linear ablation rate, Rl, tends to decrease with prolonged times, indicating the possible achievement of a steady state. Fig. 6 shows a typical temperature curve of the coated C/C composites during the ablation test. It can be found that the surface temperature of the coating increases quickly in the initial stage (about 50 s) with a linear rate. 3.3. Morphology of the multilayer coating upon ablation tests
Fig. 2. XRD pattern of the ZrB2–SiC–ZrC coating prepared by SAPS.
fully molten area is mainly composed of ZrO2 and SiO2. Fig. 3b is a high magnification of Fig. 3a. As shown in Fig. 3b, small voids are also found in the insufficiently molten area, which are mainly attributed to the gap among unmelted particles and the evaporation of gaseous byproducts. Fig. 4 shows backscatter SEM image and EDS line analysis of the cross-section of the coated C/C composites. As shown in Fig. 4a, the multilayer coating exhibits a two-layered structure: a ZrB2–SiC–ZrC outer layer (~140 μm) and a SiC inner layer (~60 μm). There is no macro defect or crack found at the interface between the SiC inner layer and the ZrB2–SiC–ZrC outer layer, which indicates a good interface bonding between them. Fig. 4c shows that no penetrating crack is found in the as-sprayed coating, but some micro voids were observed in the coating resulting from the evaporation of gaseous byproducts and insufficiently melted particles. In addition, from the EDS line analysis result, the content of oxygen is very low in the ZrB2–SiC–ZrC coating, confirming that only little oxidation of the powders occurred during spraying. 3.2. Ablation resistance of the multilayer coating To investigate the ablation resistance of the multilayer coating, the coated C/C composites were tested in an oxyacetylene torch environment with a heat flux of 2400 kW/m2 for 120 s, 150 s, and 200 s, respectively. Fig. 5 shows the pictures of the coated samples before and after ablation. As shown in Fig. 5a, the powders spread out homogeneously on the surface of the SiC inner layer and a compact coating was formed after the spraying process. After ablation, the coatings covered with a white phase in the ablation center retained integrity; besides, no macroscopic spallation or hole occurred in the ablation center (Fig. 5b,c,d), indicating that the multilayer coating could provide good ablation resistance for C/C composites for 200 s.
Fig. 7 shows the XRD patterns of the ZrB2–SiC–ZrC coating after ablation for different time. It can be found that the ablated samples are mainly composed of m-ZrO2 (JCPDS cards No. 00-007-0343) derived from the oxidation of ZrB2 and ZrC during the ablation process, besides, the peaks of ZrB2 (JCPDS cards No. 00-006-0610), SiC (JCPDS cards No. 01-089-2230) and ZrC (JCPDS cards No. 03-065-8834) phases still present in the XRD patterns. The detection of these non-oxide phases is probably due to the unoxidized edges of the samples (see Fig. 5). Fig. 8 shows SEM images of the surface in the ablation center after ablation for different time. During the ablation test, ZrB2, SiC and ZrC were oxidized to ZrO2 and gas byproducts (B2O3, CO and SiO). Evaporation of these gases might lead to the formation of some holes at high temperature [36]. SEM image (Fig. 8a) of the ZrB2–SiC–ZrC coating after ablation for 120 s shows a compact oxide scale. EDS results (not shown) reveal that the oxide scale is primarily composed of Zr, Si, and O, so the glassy phase is confirmed as ZrO2 and SiO2. In addition, there is no obvious crack or hole in the ablation center, which may be attributed to the possibility that the glassy phase effectively sealed the microholes and microcracks in the coating at high temperature. The dense glassy scale can prevent the infiltration of oxygen as an effective barrier and provide good ablation resistance for C/C composites. As shown in Fig. 8b, some microholes and microcracks are found in the ablation center of the coating after ablation for 150 s. With the increase of ablation time, the vapor pressure of gas byproducts increased gradually. As a result, the gases would escape through the oxide scale and leave some microholes in the coating. The volume expansion upon the conversion of ZrB2 and ZrC to ZrO2 is 13.0% and 36.5%, respectively, based on the density calculations [23]. Furthermore, when the sample is cooled from high temperature to room temperature, the t– m phase transformation could result in the volume expansion of the coating, and lead to the formation of microcracks. Due to the evaporation of a great quantity of SiO2 at high temperature, the amount of SiO2 on the surface of the coating decreased. In these conditions, the glassy phase was not sufficient to fill all of these defects. After ablation for 200 s, the diameter of the bubbles and cracks increase significantly
Fig. 3. SEM images and EDS analysis of the surface of the as-deposited ZrB2–SiC–ZrC coating: (a) low magnification; (b) high magnification of (a); (c) EDS analysis.
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Fig. 4. Cross-section BSE image and EDS line analysis of the cross section of the coated C/C composites: (a) BSE image; (b) EDS line analysis; (c) high magnification of (a).
(Fig. 8c). The increase of bubbles' size indicates that more gases evaporated and the coating suffered great shearing force during the ablation process. In addition, the long-term mechanical denudation and the volume expansion might also result in the increase of the size of the microcrack during the ablation process.
BSE images of the cross-section of the multilayer coating after ablation for 120 s are shown in Fig. 9. The analysis of the BSE images combined with the EDS result indicates that the layered structure consists of a porous ZrO2 layer (Layer (1)), a thin layer of ZrO2–SiO2 (Layer (2)) and a SiC-depleted layer (Layer (3)). Because the temperature in
Fig. 5. Morphology of the coated C/C composites before and after ablation: (a) as sprayed composites; (b), (c) and (d) coated composite ablation for 120 s, 150 s, and 200 s, respectively.
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Table 2 Mass and liner ablation rates of the coated C/C composites after ablation for different time. Ablation time (s) 120 150 200
Rm (×10−3g/s)
Rl (μm/s)
−0.25 ± 0.01 −0.27 ± 0.02 −0.23 ± 0.01
−0.258 ± 0.012 −0.160 ± 0.009 −0.065 ± 0.008
the present work is much higher than 1500 °C that has been reported in a previous study [25], as a result, most of the borosilicate and silica evaporated. The voids in Layer (1) were the main channel for the transportation of oxygen from the surface to the inner layer, hence, the oxidation of ZrB2, SiC and ZrC continued at high temperature. The evaporation of gas byproducts (B2O3, CO and SiO) could also further promote the formation of a porous oxide scale. This porous ZrO2 layer could act as thermal barrier during the ablation process, so, the temperature beneath Layer (1) was lower than that of the outer layer, which made it possible for the SiO2 phase to be preserved in the subsurface layer. Since the SiO2 phase could seal the porous scale, a dense ZrO2–SiO2 layer was formed. The dense ZrO2–SiO2 layer could act as an oxygen obstacle against the inward diffusion of oxygen, and as a result, the partial pressure of oxygen under the ZrO2–SiO2 layer was lower and the active oxidation of SiC occurred to generate SiO, the evaporation of which led to the appearance of a SiC-depleted layer under this dense ZrO2–SiO2 layer. Remarkably, there is no gap between the outer coating and SiC inner coating, indicating that the ZrB2–SiC–ZrC coating can prevent C/C composites from ablation for 120 s. The layered structure and EDS maps of the coating after ablation for 150 s are shown in Fig. 10. According to the BSE images and EDS analysis results, the outermost layer is a recrystallized layer, characterized by ZrO2 grains as coarse as 10 μm. Zhang et al. [37] reported that a notable coarsening of ZrO2 grains occurs upon exposition to a temperature higher than 1900 °C. Thus, the ZrO2 recrystallized layer (Layer (1)) formed after ablation for 150 s. EDS maps (Fig. 10c, d, e) of the reaction layer (Layer (2)) formed by the ablation of the ZrB2–SiC–ZrC coating for 150 s demonstrate that the layer mainly contains Zr, O, and Si. According to the analysis of Fig. 9, silicon should not appear in the reaction layer with the increase of the ablation time, because the evaporation of SiO2 could consume the remaining silicon in Layer (2) from Fig. 9(b). However, Fig. 10(d) shows that there is still silicon present and mainly distributed in Layer (2), which can be named as the ZrO2–SiO2 layer. It can be inferred that the appearance of silicon in the reaction layer is due to the passive and active oxidation of the SiC inner coating. As shown in Fig. 10(a), big holes covered with a glass phase emerge at the interface between the outer coating and SiC inner coating. By EDS analysis, the
Fig. 6. Typical time–surface temperature curve of the coated C/C composites during the ablation test.
Fig. 7. XRD patterns of the ZrB2–SiC–ZrC coating after ablation for different time (120, 150 and 200 s).
glass phase is mainly composed of Si and O, indicating that oxidation of the SiC inner coating occurred, which indicates that the multiphase external coating was not effective in protecting the inner SiC coating. With the transportation of silicon from the interface to the outer layer, a dense ZrO2–SiO2 layer formed in the reaction layer at high temperature. Although the SiC inner coating has been oxidized, C/C composites still retained an integrity scale. As the ablation time was further increased to 200 s, the oxidation of SiC accelerated the consumption of the SiC inner coating. As shown in Fig. 11, the SiC inner coating has almost been consumed and the glass phase is left behind after ablation for 200 s. Simultaneously, a gap between the ZrO2 external layer and SiC inner layer presents due to the quick consumption of the SiC inner coating. Thus, a subsequent research on how to avoid the gap and further improve the long-term ablation resistance of the coating is necessary. During the test, oxygen diffused to the reaction interface, and reacted with ZrB2, SiC and ZrC to produce ZrO2 and gas byproducts. The main reactions that might occur during the ablation process are described as follows [35]: 2ZrB2 ðsÞ þ 5O2 ðgÞ→2ZrO2 ðsÞ þ 2B2 O3 ðgÞ
ð1Þ
2SiCðsÞ þ 3O2 ðgÞ→2SiO2 ðlÞ þ 2COðgÞ
ð2Þ
SiCðsÞ þ O2 ðgÞ→SiOðgÞ þ COðgÞ
ð3Þ
ZrCðsÞ þ 2O2 ðgÞ→ZrO2 ðsÞ þ CO2 ðgÞ
ð4Þ
2ZrCðsÞ þ 3O2 ðgÞ→2ZrO2 ðsÞ þ 2COðgÞ
ð5Þ
SiO2 ðlÞ→SiO2 ðgÞ:
ð6Þ
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Fig. 8. SEM images of the ablation center after ablation for different times of coated specimens: (a) 120 s; (b) 150 s; (c) 200 s.
Fig. 9. BSE image and EDS line analysis of the multilayer coating after ablation for 120 s: (a), (b) BSE image of the cross-section; (c) EDS line analysis (A and B indicate the start and end of the line scan).
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Fig. 10. BSE image and EDS maps of the multilayer coating after ablation for 150 s: (a) BSE image of the cross-section; (b) high magnification of the boxed area in (a); (c) O, (d) Si and (e) Zr EDS maps for (b).
3.4. Ablation mechanism of the coated C/C composites There are mainly two kinds of ablation mechanisms active in C/C composites under an oxyacetylene torch environment: chemical erosion and mechanical denudation [12,13]. Fig. 5 shows that no spallation or ablation hole is found in the ablation center, implying that ablation mechanism is mainly controlled by chemical erosion. As shown in Fig. 12, the oxidation process of the coating is generally composed of two stages: (1) the oxygen molecules in the gas phase diffuse through the gas boundary layer to the reaction interface; and (2) the oxygen molecules diffuse through the oxide layer to the reaction interface. From the illustration, the diffusion of oxygen can be inhibited by the oxide scale. Therefore, a dense adherent scale formed by the oxidation products is vital to improve the ablation resistance of the coating. As shown in Fig. 6, the surface temperature of the coated specimen increased quickly in the initial stage during the ablation test. Oxygen diffused to the reaction interface and reacted with the ZrB2–SiC–ZrC
coating, and as a result the oxide layer including ZrO2 and SiO2 formed on the surface of the coating. With the increase of the amount of SiO2, the melt of ZrO2 might form the ZrO2 layer at high temperature. Both the evaporation of gases and the formation of molten ZrO2 consumed much heat, which led to the decrease of the raise velocity of temperature. As the ablation went on, the thickness of the ZrO2 layer increased. The oxide layer could limit the diffusion of oxygen and resulted in the lower partial pressure of oxygen in the inner layer. Therefore, the active oxidation of SiC could take place in the inner layer and produce a large amount of gases. The evaporation of these gases would lead to the decrease of the surface temperature of the coated specimens, and it reached the lowest level of 2240 °C at 117 s (Fig. 6). As shown in Fig. 9(b), the ablated coating shows a dense layer structure after ablation for 120 s. With the increase of ablation time, the molten ZrO2 phase coarsened, so the temperature increased rapidly. After ablation for 150 s, the ZrO2 recrystallized layer and dense ZrO2–SiO2 layer formed, meanwhile, the oxygen diffused to the interface between the outer
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layer. After ablation for 120 s, the coating exhibited a three-layered structure consisting of a porous ZrO2 layer, a ZrO2–SiO2 thin layer and a SiC-depleted layer. The layered structure could reduce thermal and oxygen diffusion into the inner layer. The molten ZrO2 layer grew up to crystal grains, and resulted in the formation of a two-layered structure consisting of a ZrO2 layer and a ZrO2–SiO2 layer after ablation for 150 s. However, the active oxidation of SiC led to the quick consumption of the SiC inner coating at high temperature, and as a result, the gap appeared between the ZrO2 layer and C/C matrix after ablation for 200 s.
Acknowledgements This work has been supported by the National Natural Science Foundation of China under Grant Nos. 51272213 and 51521061, and supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 98-QZ-2014).
References Fig. 11. Cross-section BSE image of the multilayer coating after ablation for 200 s.
layer and the SiC inner coating, and resulted in the oxidation of the SiC inner coating (Fig. 10). As shown in Fig. 12, the SiC inner coating was almost worn out and the gap between the ZrO2 layer and C/C composites formed after ablation for 200 s. 4. Conclusions A ZrB2–SiC–ZrC coating was prepared on the surface of SiC-coated C/ C composites by SAPS. The multilayer coating could effectively protect C/C composites from ablation for 200 s with the heat flux of 2400 kW/ m2, which was mainly attributed to the formation of a dense oxides scale. The ablation mechanism of the coated C/C composites was mainly controlled by chemical erosion. The ablation resistance of the coating was closely related to the structure and composition of the oxides
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Fig. 12. Sketch of oxygen diffusion during the ablation process of the multilayer coating.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
SiC/ZrB2–SiC–ZrC multilayer coating for carbon/carbon composites against ablation Yulei Zhang ⁎, Heng Hu, Pengfei Zhang, Zhixiong Hu, Hejun Li, Leilei Zhang State Key Laboratory of Solidification Processing, C/C Composites Research Center, Northwestern Polytechnical University, Xi'an, 710072, PR China
a r t i c l e
i n f o
Article history: Received 12 January 2016 Revised 29 April 2016 Accepted in revised form 3 May 2016 Available online 12 May 2016 Keywords: Carbon/carbon composites ZrB2–SiC–ZrC coating Supersonic atmosphere plasma spraying Ablation Layered structure
a b s t r a c t To improve the ablation resistance of carbon/carbon (C/C) composites at high temperature, a ZrB2–SiC–ZrC coating was prepared on the surface of SiC-coated C/C composites by supersonic atmosphere plasma spray (SAPS). Ablation resistance of the coated C/C composites was tested in an oxyacetylene torch environment with a heat flux of 2400 kW/m2 for 120, 150 and 200 s, respectively. After ablation for 120 s, the coating exhibited a threelayered structure consisting of a porous ZrO2 layer, a ZrO2–SiO2 thin layer and a SiC-depleted layer, whereas it transformed into a two-layered structure (ZrO2 recrystallized layer and ZrO2–SiO2 layer) after ablation for 150 s. With the increase of ablation time, the SiC inner coating was almost consumed and a gap emerged between the ZrO2 layer and the C/C matrix after ablation for 200 s. The good ablation resistance of the sprayed coating is mainly attributed to the layered structure acting as a thermal barrier and inhibiting inward diffusion of oxygen. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites possess many attractive properties, such as high strength-to-weight ratio, low coefficient of thermal expansion (CTE) and good strength retention at high temperatures in a reducing environment [1]. Therefore, they are considered as one of the most promising candidate materials for thermal–structural components in turbine engines and in aerospace and re-entry vehicles [2]. However, the oxidation of C/C composites above 450 °C in air limits their application in an oxidizing environment, especially for high-temperature associated with high-speed combustion gas flow [3–5]. So, it is necessary to improve the ablation resistance of C/C composites to meet the application requirements in these extreme environments. Several approaches have been developed to protect C/C composites against oxidation at high temperatures. Applying a coating on C/C composites has been proved to be an effective method [6–10]. In recent years, many ultra-high temperature ceramics (UHTCs), such as ZrC, HfC, TaC and ZrB2, have been used as ablation resistance coatings for C/C composites [11–15]. Zirconium diboride (ZrB2) and zirconium carbide (ZrC) based ceramics have a high melting point, high thermal and electrical conductivities, and good thermal shock resistance [16– ⁎ Corresponding author. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.surfcoat.2016.05.028 0257-8972/© 2016 Elsevier B.V. All rights reserved.
19]. So, they have been applied in a high-temperature environment to resist oxidation, corrosion and wear. It is well known that the addition of a SiC ceramic can significantly improve the oxidation behavior of ZrB2-based ceramics in the moderately high temperature range and the optimum amount of SiC is between 15 and 20 vol.% [20–22]. In recent years, oxidation resistance of ZrB2-based ceramics at high temperature has been investigated by several researchers [21,23,24], and it has been reported that the typical scale after oxidation of a ZrB2–SiC ceramic is composed of four layers: a SiO2 rich glassy layer, a ZrO2–SiO2 layer, a SiC-depleted layer and an unaffected layer [25]. The typical layers could provide good oxidation protection for the substrates. According to previous works, ZrB2–SiC coatings on C/C composites could improve ablation resistance [26,27]. Furthermore, Wang et al. have reported that a ZrB2–SiC–ZrC ceramic had good static oxidation resistance at 1750 °C [28], and the introduction of the ZrC phase improved the fracture toughness of the ZrB2–SiC ceramic from 4.1 MPa to 6.7 MPa [29]. Thus, the ZrB2–SiC–ZrC ceramic is a promising material to resist oxidation at high temperature and erosion by high-speed gas flow. However, little work has been focused on the ablation resistance of the ZrB2–SiC–ZrC coating for C/C composites up to now [28–30]. In this work, the ZrB2–SiC–ZrC ceramic was selected as the ablation coating for C/C composites and supersonic atmosphere plasma spraying (SAPS) was employed to prepare this coating due to the high temperature of a plasma arc and the high velocity of particles. Between the C/C
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substrate and the UHTC coating, a SiC buffer layer was sprayed to alleviate the mismatch. The purpose of this work is to describe the microstructure evolution of the coating as a function of different ablation time. The ablation mechanism of C/C composites with the multilayer coating is also discussed. 2. Experimental 2.1. Preparation of the multilayer coating The substrate samples (Ф 30 × 10 mm) were cut from bulk two-dimensional (2D) C/C composites with a density of about 1700 kg/m3. 2D C/C composites were prepared by thermal gradient chemical vapor deposition and details can be found in our previous work [31]. Then these samples were cleaned in an ultrasonic bath with ethanol and dried at 100–150 °C for 2 h. The SiC inner layer was prepared by pack cementation with Si (60–80 wt.%) (Jiuling Smelting Co., Ltd., Shanghai, China), graphite (10–25 wt.%) (Carbon Plant, Xi'an, China) and Al2O3 (5– 15 wt.%) (Guoyao Chemical Reagent Co., Ltd., Shanghai, China) powders in inert atmosphere at 1700–1900 °C for 2 h. [32]. SAPS was applied to prepare the ZrB2–SiC–ZrC coating on the surface of SiC-coated C/C composites. The spraying apparatus is schematically shown in Fig. 1, and the spraying parameters are summarized in Table 1. The spraying system consists of a plasma torch, powder feeder, gas supply system, water-cooling circulator, control unit with PC interface and power supply unit. The ZrB2 particles (800 mesh, N99.9% purity, DanDong Research Insitute of Chemical Industry, China), SiC particles (800 mesh, N99.9% purity, Linyi Jin meng Carborundum Co. Ltd) and ZrC particles (400 mesh, N 98% purity, Jinzhou Metal Material Research Institute, Liaoning, China) were selected as the raw material of spraying powders. Before spraying, the mixture particles containing 60 vol.% ZrB2, 20 vol.% SiC and 20 vol.% ZrC were preliminary prepared by attrition milling with ZrO2 milling media for 2 h, then they were agglomerated by a spray drier and details have been reported in Ref. [33]. 2.2. Ablation test and microstructure analysis An oxyacetylene torch test was used to evaluate the ablation resistance of the coated samples, according to the National Standard of Ablation Test method of ablative materials (GJB 323A-96) with heat flux of 2400 kW/m2 [34]. The pressure of oxygen and acetylene was 0.4 MPa and 0.095 MPa, and the flux of oxygen and acetylene 0.244 l/s and 0.167 l/s, respectively. The oxyacetylene gun has an inner diameter of 2 mm, and the distance between the gun tip and the sample was set as 10 mm. After the flame of ablation gun had reached stability, the samples were vertically placed to the flame and maintained for 120, 150 and 200 s, respectively. The surface temperature of the sample was measured by an infrared thermometer. A picture of the oxyacetylene torch test can be found in Ref. [35]. The linear and mass ablation rates (Rl and Rm, respectively) of the samples were calculated according to
Table 1 Details of the spraying parameters for ZrB2–SiC–ZrC multilayer coating. Content
Parameters
Spraying current, A Spraying voltage, V Primary gas Ar, l/min Carrier gas Ar, l/min Second gas H2, l/min Powder feed rate, g/min Spraying distance, mm Injector internal diameter, mm Injector position
385–425 100–130 70 15 5 30 100 5.5 Perpendicular to samples
following equations: Rl ¼ Δd=t
ð1Þ
Rm ¼ Δm=t
ð2Þ
where Δd is the change of thickness before and after ablation, Δm is the change of mass before and after ablation, and t is ablation time. The ultimate ablation rates of the composites were the average ablation rates of three samples. The crystalline structure of the coatings was investigated by X-ray diffraction (XRD, X'Pert Pro MPD) with a Cu Kα radiation having a wavelength of 0.154 nm. The morphology and element composition of the coatings were investigated by scanning electron microscopy (SEM, JSM-6460) equipped with energy dispersion spectroscopy (EDS). 3. Results and discussion 3.1. Phase composition and microstructure of as-sprayed coating Fig. 2 shows the XRD pattern of the as-sprayed coating. The predominant phases in the ZrB2–SiC–ZrC coating are ZrB2, SiC, ZrC and a small quantity of ZrO2 according to the JCPDS cards: No. (00-034-0423) for ZrB2, (01-073-2084) for SiC, (01-089-3829) for ZrC, (01-088-1007) for tetragonal ZrO2 (t-ZrO2) and (00-007-0343) for monoclinic ZrO2 (mZrO2). The ZrB2–SiC–ZrC coating was prepared by SAPS in air, so the oxidation of spraying powders was unavoidable, which would result in the formation of ZrO2, SiO2 and gaseous byproducts (such as B2O3, CO, CO2, and SiO). In the sprayed coating, t-ZrO2 was detected because part of tZrO2 could not be transformed to m-ZrO2 during rapid cooling from high temperature to room temperature. Due to the low content and amorphous structure of SiO2, it could not be detected by X-ray diffraction technique. In addition, the lower intensity of SiC may result from the oxidation and decomposition of the SiC ceramic at high temperature. Fig. 3 shows SEM images of the surface of the ZrB2–SiC–ZrC coating. It can be found that the coating is composed of the fully molten area and insufficiently molten area, and there is no obvious crack or void in the coating. According to the results of XRD and EDS analysis (Fig. 3c), the
Fig. 1. Sketch of the spraying apparatus.
Y. Zhang et al. / Surface & Coatings Technology 300 (2016) 1–9
3
Table 2 shows the mass and liner ablation rates of the coated C/C composites after ablation for different time. The results show that the mass and liner ablation rates of the specimens are both negative, which mean that the weight and volume of the specimens both increased after ablation. While the mass ablation rate, Rm, can be considered not varying between the three times, within the error range, the linear ablation rate, Rl, tends to decrease with prolonged times, indicating the possible achievement of a steady state. Fig. 6 shows a typical temperature curve of the coated C/C composites during the ablation test. It can be found that the surface temperature of the coating increases quickly in the initial stage (about 50 s) with a linear rate. 3.3. Morphology of the multilayer coating upon ablation tests
Fig. 2. XRD pattern of the ZrB2–SiC–ZrC coating prepared by SAPS.
fully molten area is mainly composed of ZrO2 and SiO2. Fig. 3b is a high magnification of Fig. 3a. As shown in Fig. 3b, small voids are also found in the insufficiently molten area, which are mainly attributed to the gap among unmelted particles and the evaporation of gaseous byproducts. Fig. 4 shows backscatter SEM image and EDS line analysis of the cross-section of the coated C/C composites. As shown in Fig. 4a, the multilayer coating exhibits a two-layered structure: a ZrB2–SiC–ZrC outer layer (~140 μm) and a SiC inner layer (~60 μm). There is no macro defect or crack found at the interface between the SiC inner layer and the ZrB2–SiC–ZrC outer layer, which indicates a good interface bonding between them. Fig. 4c shows that no penetrating crack is found in the as-sprayed coating, but some micro voids were observed in the coating resulting from the evaporation of gaseous byproducts and insufficiently melted particles. In addition, from the EDS line analysis result, the content of oxygen is very low in the ZrB2–SiC–ZrC coating, confirming that only little oxidation of the powders occurred during spraying. 3.2. Ablation resistance of the multilayer coating To investigate the ablation resistance of the multilayer coating, the coated C/C composites were tested in an oxyacetylene torch environment with a heat flux of 2400 kW/m2 for 120 s, 150 s, and 200 s, respectively. Fig. 5 shows the pictures of the coated samples before and after ablation. As shown in Fig. 5a, the powders spread out homogeneously on the surface of the SiC inner layer and a compact coating was formed after the spraying process. After ablation, the coatings covered with a white phase in the ablation center retained integrity; besides, no macroscopic spallation or hole occurred in the ablation center (Fig. 5b,c,d), indicating that the multilayer coating could provide good ablation resistance for C/C composites for 200 s.
Fig. 7 shows the XRD patterns of the ZrB2–SiC–ZrC coating after ablation for different time. It can be found that the ablated samples are mainly composed of m-ZrO2 (JCPDS cards No. 00-007-0343) derived from the oxidation of ZrB2 and ZrC during the ablation process, besides, the peaks of ZrB2 (JCPDS cards No. 00-006-0610), SiC (JCPDS cards No. 01-089-2230) and ZrC (JCPDS cards No. 03-065-8834) phases still present in the XRD patterns. The detection of these non-oxide phases is probably due to the unoxidized edges of the samples (see Fig. 5). Fig. 8 shows SEM images of the surface in the ablation center after ablation for different time. During the ablation test, ZrB2, SiC and ZrC were oxidized to ZrO2 and gas byproducts (B2O3, CO and SiO). Evaporation of these gases might lead to the formation of some holes at high temperature [36]. SEM image (Fig. 8a) of the ZrB2–SiC–ZrC coating after ablation for 120 s shows a compact oxide scale. EDS results (not shown) reveal that the oxide scale is primarily composed of Zr, Si, and O, so the glassy phase is confirmed as ZrO2 and SiO2. In addition, there is no obvious crack or hole in the ablation center, which may be attributed to the possibility that the glassy phase effectively sealed the microholes and microcracks in the coating at high temperature. The dense glassy scale can prevent the infiltration of oxygen as an effective barrier and provide good ablation resistance for C/C composites. As shown in Fig. 8b, some microholes and microcracks are found in the ablation center of the coating after ablation for 150 s. With the increase of ablation time, the vapor pressure of gas byproducts increased gradually. As a result, the gases would escape through the oxide scale and leave some microholes in the coating. The volume expansion upon the conversion of ZrB2 and ZrC to ZrO2 is 13.0% and 36.5%, respectively, based on the density calculations [23]. Furthermore, when the sample is cooled from high temperature to room temperature, the t– m phase transformation could result in the volume expansion of the coating, and lead to the formation of microcracks. Due to the evaporation of a great quantity of SiO2 at high temperature, the amount of SiO2 on the surface of the coating decreased. In these conditions, the glassy phase was not sufficient to fill all of these defects. After ablation for 200 s, the diameter of the bubbles and cracks increase significantly
Fig. 3. SEM images and EDS analysis of the surface of the as-deposited ZrB2–SiC–ZrC coating: (a) low magnification; (b) high magnification of (a); (c) EDS analysis.
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Fig. 4. Cross-section BSE image and EDS line analysis of the cross section of the coated C/C composites: (a) BSE image; (b) EDS line analysis; (c) high magnification of (a).
(Fig. 8c). The increase of bubbles' size indicates that more gases evaporated and the coating suffered great shearing force during the ablation process. In addition, the long-term mechanical denudation and the volume expansion might also result in the increase of the size of the microcrack during the ablation process.
BSE images of the cross-section of the multilayer coating after ablation for 120 s are shown in Fig. 9. The analysis of the BSE images combined with the EDS result indicates that the layered structure consists of a porous ZrO2 layer (Layer (1)), a thin layer of ZrO2–SiO2 (Layer (2)) and a SiC-depleted layer (Layer (3)). Because the temperature in
Fig. 5. Morphology of the coated C/C composites before and after ablation: (a) as sprayed composites; (b), (c) and (d) coated composite ablation for 120 s, 150 s, and 200 s, respectively.
Y. Zhang et al. / Surface & Coatings Technology 300 (2016) 1–9
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Table 2 Mass and liner ablation rates of the coated C/C composites after ablation for different time. Ablation time (s) 120 150 200
Rm (×10−3g/s)
Rl (μm/s)
−0.25 ± 0.01 −0.27 ± 0.02 −0.23 ± 0.01
−0.258 ± 0.012 −0.160 ± 0.009 −0.065 ± 0.008
the present work is much higher than 1500 °C that has been reported in a previous study [25], as a result, most of the borosilicate and silica evaporated. The voids in Layer (1) were the main channel for the transportation of oxygen from the surface to the inner layer, hence, the oxidation of ZrB2, SiC and ZrC continued at high temperature. The evaporation of gas byproducts (B2O3, CO and SiO) could also further promote the formation of a porous oxide scale. This porous ZrO2 layer could act as thermal barrier during the ablation process, so, the temperature beneath Layer (1) was lower than that of the outer layer, which made it possible for the SiO2 phase to be preserved in the subsurface layer. Since the SiO2 phase could seal the porous scale, a dense ZrO2–SiO2 layer was formed. The dense ZrO2–SiO2 layer could act as an oxygen obstacle against the inward diffusion of oxygen, and as a result, the partial pressure of oxygen under the ZrO2–SiO2 layer was lower and the active oxidation of SiC occurred to generate SiO, the evaporation of which led to the appearance of a SiC-depleted layer under this dense ZrO2–SiO2 layer. Remarkably, there is no gap between the outer coating and SiC inner coating, indicating that the ZrB2–SiC–ZrC coating can prevent C/C composites from ablation for 120 s. The layered structure and EDS maps of the coating after ablation for 150 s are shown in Fig. 10. According to the BSE images and EDS analysis results, the outermost layer is a recrystallized layer, characterized by ZrO2 grains as coarse as 10 μm. Zhang et al. [37] reported that a notable coarsening of ZrO2 grains occurs upon exposition to a temperature higher than 1900 °C. Thus, the ZrO2 recrystallized layer (Layer (1)) formed after ablation for 150 s. EDS maps (Fig. 10c, d, e) of the reaction layer (Layer (2)) formed by the ablation of the ZrB2–SiC–ZrC coating for 150 s demonstrate that the layer mainly contains Zr, O, and Si. According to the analysis of Fig. 9, silicon should not appear in the reaction layer with the increase of the ablation time, because the evaporation of SiO2 could consume the remaining silicon in Layer (2) from Fig. 9(b). However, Fig. 10(d) shows that there is still silicon present and mainly distributed in Layer (2), which can be named as the ZrO2–SiO2 layer. It can be inferred that the appearance of silicon in the reaction layer is due to the passive and active oxidation of the SiC inner coating. As shown in Fig. 10(a), big holes covered with a glass phase emerge at the interface between the outer coating and SiC inner coating. By EDS analysis, the
Fig. 6. Typical time–surface temperature curve of the coated C/C composites during the ablation test.
Fig. 7. XRD patterns of the ZrB2–SiC–ZrC coating after ablation for different time (120, 150 and 200 s).
glass phase is mainly composed of Si and O, indicating that oxidation of the SiC inner coating occurred, which indicates that the multiphase external coating was not effective in protecting the inner SiC coating. With the transportation of silicon from the interface to the outer layer, a dense ZrO2–SiO2 layer formed in the reaction layer at high temperature. Although the SiC inner coating has been oxidized, C/C composites still retained an integrity scale. As the ablation time was further increased to 200 s, the oxidation of SiC accelerated the consumption of the SiC inner coating. As shown in Fig. 11, the SiC inner coating has almost been consumed and the glass phase is left behind after ablation for 200 s. Simultaneously, a gap between the ZrO2 external layer and SiC inner layer presents due to the quick consumption of the SiC inner coating. Thus, a subsequent research on how to avoid the gap and further improve the long-term ablation resistance of the coating is necessary. During the test, oxygen diffused to the reaction interface, and reacted with ZrB2, SiC and ZrC to produce ZrO2 and gas byproducts. The main reactions that might occur during the ablation process are described as follows [35]: 2ZrB2 ðsÞ þ 5O2 ðgÞ→2ZrO2 ðsÞ þ 2B2 O3 ðgÞ
ð1Þ
2SiCðsÞ þ 3O2 ðgÞ→2SiO2 ðlÞ þ 2COðgÞ
ð2Þ
SiCðsÞ þ O2 ðgÞ→SiOðgÞ þ COðgÞ
ð3Þ
ZrCðsÞ þ 2O2 ðgÞ→ZrO2 ðsÞ þ CO2 ðgÞ
ð4Þ
2ZrCðsÞ þ 3O2 ðgÞ→2ZrO2 ðsÞ þ 2COðgÞ
ð5Þ
SiO2 ðlÞ→SiO2 ðgÞ:
ð6Þ
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Fig. 8. SEM images of the ablation center after ablation for different times of coated specimens: (a) 120 s; (b) 150 s; (c) 200 s.
Fig. 9. BSE image and EDS line analysis of the multilayer coating after ablation for 120 s: (a), (b) BSE image of the cross-section; (c) EDS line analysis (A and B indicate the start and end of the line scan).
Y. Zhang et al. / Surface & Coatings Technology 300 (2016) 1–9
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Fig. 10. BSE image and EDS maps of the multilayer coating after ablation for 150 s: (a) BSE image of the cross-section; (b) high magnification of the boxed area in (a); (c) O, (d) Si and (e) Zr EDS maps for (b).
3.4. Ablation mechanism of the coated C/C composites There are mainly two kinds of ablation mechanisms active in C/C composites under an oxyacetylene torch environment: chemical erosion and mechanical denudation [12,13]. Fig. 5 shows that no spallation or ablation hole is found in the ablation center, implying that ablation mechanism is mainly controlled by chemical erosion. As shown in Fig. 12, the oxidation process of the coating is generally composed of two stages: (1) the oxygen molecules in the gas phase diffuse through the gas boundary layer to the reaction interface; and (2) the oxygen molecules diffuse through the oxide layer to the reaction interface. From the illustration, the diffusion of oxygen can be inhibited by the oxide scale. Therefore, a dense adherent scale formed by the oxidation products is vital to improve the ablation resistance of the coating. As shown in Fig. 6, the surface temperature of the coated specimen increased quickly in the initial stage during the ablation test. Oxygen diffused to the reaction interface and reacted with the ZrB2–SiC–ZrC
coating, and as a result the oxide layer including ZrO2 and SiO2 formed on the surface of the coating. With the increase of the amount of SiO2, the melt of ZrO2 might form the ZrO2 layer at high temperature. Both the evaporation of gases and the formation of molten ZrO2 consumed much heat, which led to the decrease of the raise velocity of temperature. As the ablation went on, the thickness of the ZrO2 layer increased. The oxide layer could limit the diffusion of oxygen and resulted in the lower partial pressure of oxygen in the inner layer. Therefore, the active oxidation of SiC could take place in the inner layer and produce a large amount of gases. The evaporation of these gases would lead to the decrease of the surface temperature of the coated specimens, and it reached the lowest level of 2240 °C at 117 s (Fig. 6). As shown in Fig. 9(b), the ablated coating shows a dense layer structure after ablation for 120 s. With the increase of ablation time, the molten ZrO2 phase coarsened, so the temperature increased rapidly. After ablation for 150 s, the ZrO2 recrystallized layer and dense ZrO2–SiO2 layer formed, meanwhile, the oxygen diffused to the interface between the outer
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layer. After ablation for 120 s, the coating exhibited a three-layered structure consisting of a porous ZrO2 layer, a ZrO2–SiO2 thin layer and a SiC-depleted layer. The layered structure could reduce thermal and oxygen diffusion into the inner layer. The molten ZrO2 layer grew up to crystal grains, and resulted in the formation of a two-layered structure consisting of a ZrO2 layer and a ZrO2–SiO2 layer after ablation for 150 s. However, the active oxidation of SiC led to the quick consumption of the SiC inner coating at high temperature, and as a result, the gap appeared between the ZrO2 layer and C/C matrix after ablation for 200 s.
Acknowledgements This work has been supported by the National Natural Science Foundation of China under Grant Nos. 51272213 and 51521061, and supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 98-QZ-2014).
References Fig. 11. Cross-section BSE image of the multilayer coating after ablation for 200 s.
layer and the SiC inner coating, and resulted in the oxidation of the SiC inner coating (Fig. 10). As shown in Fig. 12, the SiC inner coating was almost worn out and the gap between the ZrO2 layer and C/C composites formed after ablation for 200 s. 4. Conclusions A ZrB2–SiC–ZrC coating was prepared on the surface of SiC-coated C/ C composites by SAPS. The multilayer coating could effectively protect C/C composites from ablation for 200 s with the heat flux of 2400 kW/ m2, which was mainly attributed to the formation of a dense oxides scale. The ablation mechanism of the coated C/C composites was mainly controlled by chemical erosion. The ablation resistance of the coating was closely related to the structure and composition of the oxides
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