C composites by supersonic atmosphere plasma spraying

C composites by supersonic atmosphere plasma spraying

Surface & Coatings Technology 374 (2019) 966–974 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsev...

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Surface & Coatings Technology 374 (2019) 966–974

Contents lists available at ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Microstructure and oxidation property of CrSi2-ZrSi2-Y2O3/SiC coating prepared on C/C composites by supersonic atmosphere plasma spraying ⁎

T



Fei Liua,b, Hejun Lia, , Shengyue Gua, Xiyuan Yaoa, , Qiangang Fua a b

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China College of Mechanical and Electrical Engineering, Xi'an Polytechnic University, Xi'an 710048, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Double silicon-based ceramic coatings CrSi2-ZrSi2-Y2O3/SiC coating C/C composites Oxidation resistance Supersonic atmosphere plasma spraying (SAPS)

Double silicon-based ceramic coatings of CrSi2-ZrSi2-Y2O3/SiC were deposited on the surface of C/C composites to improve their oxidation resistance. The SiC inner coating was prepared by pack cementation while the CrSi2ZrSi2-Y2O3 outer coatings with different CrSi2 contents were deposited on the surface of the SiC coating by supersonic atmosphere plasma spraying (SAPS). Microstructure feature, phase composition and oxidation resistance of the coatings were studied. The coatings prepared by the two steps had a dense microstructure with few pores and microcracks, and the interface exhibited good coherence. Oxidation experimental results showed that the CrSi2-ZrSi2-Y2O3/SiC coating with 30 wt% CrSi2 effectively protect C/C composites from being oxidized at 1500 °C in air, the mass change of which decreased by 1.2% after the oxidation time of 288 h. Meanwhile, it could also withstand 15 times thermal shock cycles from 1500 °C to room temperature (RT). A glass layer including SiO2 and Cr2O3 could effectively heal the microcracks and pores of the coating and thus reduce oxygen permeation. Meanwhile, the oxides such as ZrO2, ZrSiO4, Y2Si2O7 and Y2SiO5 could inhibit the growth and propagation of the coating microcracks and improve the oxidation resistance of the specimen.

1. Introduction Carbon and carbon (C/C) composites have a series of outstanding physical, chemical and mechanical properties, such as light weight, low density, low coefficient of thermal expansion (CTE), high thermal conductivity and so on [1–4], compared with resin-based, metal-based and ceramic-based composites. They are also able to retain their strength, modulus and other mechanical properties at high temperature (usually above 3000 °C). However, the disadvantage of rapidly being oxidized above 450 °C in oxygen-containing atmosphere limits their wider application in the various fields of aerospace [5,6]. Actually, the most mature and effective method to tackle above problem is to use surface coating technology [7–10]. It mainly consists of inner coating and outer coating to form multi-layer coating system. SiC ceramic used as inner coating has various excellent properties, including outstanding oxidation resistance, good physical and chemical compatibility with C/C composites. In addition, SiC could be oxidized at high temperature in air to form a glassy silica layer on the surface of coating, which is capable of preventing oxygen from diffusing into substrate [11,24]. Ultra-high temperature ceramics (UHTCs), especially silicon-based ceramics, are most widely used as outer coating materials of C/C composites. There are two reasons. First, they can produce high



melt point oxides and have excellent anti-oxidation property at high temperature [12,13]. Second, according to the oxidation mechanism of C/C composites, the oxidation was mainly controlled by the concentration and diffusion velocity of oxygen [14]. At high temperature (above 1000 °C), the oxidation resistance of C/C composites is governed by oxygen diffusion through a stagnant layer of oxidation products. The very low permeability of silica (stagnant layer) produced from siliconbased ceramics provides good oxidation protection, and silica could heal microcracks which exist in the coating. The commonly used silicon-based ceramics are MoSi2, HfSi2, CrSi2, TaSi2 and ZrSi2 [15]. Up till now, many silicon-based ceramics oxidation resistance coating systems have been studied, such as MoSi2/SiC [16], ZrB2-MoSi2/SiC [17], ZrSi2-SiC-ZrC/SiC [18], ZrSi2-CrSi2-Si/SiC [19] and ZrSi2-Y2O3/ SiC [20]. Among them, ZrSi2-Y2O3/SiC coating presents good oxidation resistance, due to form SiO2 glass layer and ZrO2 ceramic phase (Tm = 2700 °C) at high temperature [20]. However, because of the difference of CTE between outer coating and SiC coated C/C composites, the coating presents a lot of microcracks and pores with increasing oxidation time in air at high temperature, which aggravates the failure of the coating. Some researchers have introduced CrSi2 into coating systems to protect C/C composites from being oxidized, such as MoSi2CrSi2/SiC [21], MoSi2-CrSi2-Si/SiC [22,23], MoSi2-CrSi2-Si/B-modified

Corresponding author. E-mail addresses: [email protected] (H. Li), [email protected] (X. Yao).

https://doi.org/10.1016/j.surfcoat.2019.06.087 Received 23 December 2018; Received in revised form 20 May 2019; Accepted 28 June 2019 Available online 30 June 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

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SiC [24] and SiC NWs toughed CrSi2-Cr3C2-MoSi2-Mo2C/SiC [25]. The first reason, CrSi2 can be oxidized at high temperature and form a stable SiO2 glass layer. The second reason, it has low melting point (Tm = 1490 °C) and can fill the microcracks of coating and reduce the porosity of coating. In addition, Cr2O3 (formed by CrSi2 at high temperature) with high melting point (Tm = 2435 °C), can served as “pinning phase” and restrain the generation and propagation of the microcracks. Therefore, CrSi2 ceramic used as additive could be introduced into ZrSi2-Y2O3 outer coating to inhibit the growth of the coating microcracks and decrease the porosity of the coating. For coating preparation, supersonic atmosphere plasma spraying (SAPS) technique has been widely used to prepare coatings, because its plasma arc temperature is around 9700 °C, and jet velocity is up to 600 m/s, which can melt all high melting point ceramic materials, and is especially suitable for the preparation of UHTCs coatings. Coating produced by SAPS will also have good bonding strength with substrates [26,27]. Up to now, little research has been reported whether CrSi2-ZrSi2Y2O3/SiC coating system has long time oxidation resistance for C/C composites. In this paper, the CrSi2-ZrSi2-Y2O3/SiC coatings with different CrSi2 contents were prepared by pack cementation and SAPS. Their microstructures, phase compositions and oxidation resistance were studied.

Table 1 Spraying parameters of the CrSi2-ZrSi2-Y2O3 coatings prepared by SAPS. Parameters

Values

Spraying power (kW) Main gas flow (Ar), L/min Carrier gas (Ar), L/min The second gas (H2), L/min Power feed rate (g/min) Spraying distance (mm) Nozzle diameter (mm)

40 75 10 5 20 100 5.5

specimens were put into the furnace and then were taken out directly after a given period of time to cool down to RT in air. In this process, the furnace maintained constant temperature at 1500 °C and specimens experienced thermal shock test simultaneously. Mass change of the specimens was measured by an electronic balance with a sensitivity of ± 0.1 mg. Cumulative weight change percentages (Δw%) of the specimens were calculated by the Eq. (1).

∆w% =

m 0 − m1 × 100 m0

(1)

where m0 and m1 were the weights of specimens before and after oxidation, respectively.

2. Experimental 2.3. Bonding strength test 2.1. Coating preparation The bonding strength was measured using ASTM C663 which was especially designed for plasma sprayed coatings. Fig. 1 shows the schematic of the test device. Cylindrical stainless steel rods were used as matching parts, and their dimensions were ∅25 mm ×∅25 mm. Specimens were divided into four groups according the different CrSi2 contents, and bonded with the end surface of the matching parts by a modified acrylate adhesive. They were positioned for 10–15 min and solidified for 24 h at room temperature. Five specimens per group were measured by a universal testing machine (Type: CMT5304-30kN), and the largest force of each specimen was recorded. The bonding strength (σ) was calculated by the Eq. (2) [30].

Small cubic specimens (10 mm × 10 mm × 10 mm) used as substrates were cut from 2D C/C composites bulk with density of 1.73 g/ cm3. Subsequently, all cubic specimens were hand-abraded using 200 and 400 grit SiC papers, then cleaned ultrasonically in ethanol and dried in air at 100 °C for 1–2 h [20]. First, SiC inner coating was prepared by pack cementation, the original powders were mixed up, and proportions of ingredients were as follows: 65–85% Si, 10–20% graphite, 5–15% Al2O3. The mixture and cubic C/C composites were then directly placed in a graphite crucible, and every face of specimens was completely packed with mixtures. After that, the graphite crucible was put into electrical furnace and heated to 1800 °C for 2 h in argon to form required SiC inner layer, more detailed experimental operations please refer to [28]. Second, the outer ZrSi2-Y2O3 coatings with different mass percentages of CrSi2 were deposited on the SiC coated C/C composites by SAPS. CrSi2 (purity: ≥99%), ZrSi2 (purity: ≥99%) and Y2O3 (purity: ≥99.99%) original powders were all supplied by Hua Wei Rui Ke chemical co., Ltd., Beijing, China, and the particle sizes of which were about 20 μm, 1–3 μm and 2–5 μm, respectively. Previous research results had proved that added 10 wt% Y2O3 into ZrSi2/SiC coating could obviously improve the oxidation resistance of C/C composites, so in this paper the amount of Y2O3 has been kept constant [20]. The CrSi2-ZrSi2-Y2O3 mixed powders with different mass percentages of CrSi2 (10 wt%, 20 wt%, 30 wt% and 40 wt%) were mixed with 3 wt% water and 97 wt% polyvinyl alcohol (PVA) to form slurry. The powders for spraying were obtained by centrifugal spray dryer in order to make them good flowability, and the equipment was provided by Wuxi Dongsheng Spray Granulating and Drying Equipment Plant, Jiangsu, China. Its inlet and outlet temperatures were 300–350 °C and 100–150 °C, respectively. The rotation speed of the nozzle was 40–45 r/ min. Then four kinds of coatings were prepared in each group with at least five specimens by SAPS in order to reduce experimental error. Other spraying parameters were listed in Table 1 [29].

σ=

F S

(2)

where F was the largest force recorded by the universal testing machine and S was the cross sectional area of the specimens. 2.4. Characterization The crystalline structure of the coatings was measured by X-ray diffraction (XRD, Rigaku D/max-3C) with a Cu Kα radiation (λ = 0.1542 mm) produced at 40 kV and 35 mA. Their morphologies and the element distributions were studied by scanning electron microscope (SEM, Type: Supra55) and energy dispersive spectroscopy (EDS, Type: Supra55). Porosity of coatings was measured by Archimedes method. The bonding strength of coatings was obtained through the scratch tester (WS-2005 multi-functional tester, China) equipped with a diamond cone (cone apex angle 120°, tip radius 0.2 mm). Scratch test was carried out by applying a constantly changing load which ranged from 0 to 50 N during sliding on the 5 mm path at the loading rate of 50 N·min−1. Roughness (Ra) is obtained by a confocal laser scanning microscope (Optelics C130, Lasertec Corp., Yokohama, Japan).

2.2. Oxidation test

3. Results and discussion

The isothermal oxidation resistance of the specimens was tested in air at 1500 °C in an electrical furnace. They were divided into four groups to test respectively the different contents of CrSi2. The

3.1. Microstructure and composition of the CrSi2-ZrSi2-Y2O3 coatings Fig. 2 shows the surface morphologies of CrSi2-ZrSi2-Y2O3 mixture 967

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Fig. 1. The schematic of the bonding strength test device.

powders used for SAPS at the CrSi2 ratio of 30 wt%. It can be found that the average diameter of the particles is about 35–55 μm. The shapes of the powders are close to ovoid or globular structures, which could present good flowability to enhance the deposition efficiency of the coating prepared by SAPS. Fig. 3 shows the distributions of the elements of the CrSi2-ZrSi2-Y2O3 mixture powders used for SAPS at the CrSi2 ratio of 30 wt%. It can be seen that there are five elements, including O, Si, Zr, Y and Cr, which are in agreement with the mixture powders composition. Meanwhile, the five elements are uniformly dispersed in the particles, which are expected to form homogeneous outer coating prepared by SAPS. Fig. 4 shows the SEM images of surface morphologies of the CrSi2ZrSi2-Y2O3 outer coatings with different CrSi2 mass percentages prepared by SAPS. All the coatings exhibit a relative dense structure, some microcracks and pores. The surface roughness is due to randomly the existence of unmelted particles, partially melted particles and completely melted particles, and the values of which are shown in Table 2. Results show that the coating with 30 wt% CrSi2 owns the lowest Ra value (8.63 μm). Fig. 5 shows the XRD patterns of the CrSi2-ZrSi2-Y2O3 coatings surface prepared by SAPS with different CrSi2 mass percentages. It can be found that there are four phases existing in the coatings after spraying. In the coatings appear oxidation products c-ZrO2, which

Fig. 2. Surface morphologies of CrSi2-ZrSi2-Y2O3 mixture powders used for SAPS at the CrSi2 ratio of 30 wt%.

Fig. 3. Distributions of the elements of the CrSi2-ZrSi2-Y2O3 mixture powders used for SAPS at the CrSi2 ratio of 30 wt%. (a). surface morphology of powders; (b). O element distr0069bution; (c). Si element distribution; (d) Zr element distribution; (e) Y element distribution; (f). Cr element distribution. 968

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Fig. 4. Surface morphology images of the CrSi2-ZrSi2-Y2O3 outer coatings with different CrSi2 mass percentages prepared by SAPS. (a) 10 wt% CrSi2; (b) 20 wt% CrSi2; (c) 30 wt% CrSi2; (d) 40 wt% CrSi2;

indicate that the coatings are slightly oxidized during spraying. The corresponding reaction equation is as follows:

ZrSi2 (s) + 3O2 (g) = ZrO2 (s) + 2SiO2 (s)

(1)

Crystalline SiO2 peak is not detected by XRD, the possible reason may be rapid solidification of melted particles in the process of spraying, SiO2 does not have sufficient time to crystallize, so it forms amorphous state. The main reason of the appearance of c-ZrO2 phase is that Y2O3 inhibits the phase transformation of ZrO2 at high temperature. Fig. 6 shows the cross-section backscattering electron images of the CrSi2-ZrSi2-Y2O3/SiC coatings with different CrSi2 mass percentages prepared by SAPS. It can be found that the average width of CrSi2-ZrSi2Y2O3 outer coating is about 100 μm. The cross-sections present few pores and microcracks for all the different CrSi2 contents. The formed microcracks can be ascribed to the coating thermal stress during the spraying and the CTE difference between inner coating and outer coating. During the spraying process, the powders are heated around 9700 °C by plasma jet and the melted particles can collide each other to produce heat exchange, and rapidly cool and shrink at a rate about 106 K·s−1 [31]. Moreover, the CTE difference between inner coating (αSiC = 4.5 × 10−6 K−1) and outer coating [Mixed ZrSi2 (αZrSi2 = 8.5 × 10−6 K−1) with CrSi2 (αCrSi2 = 10.5 × 10−6 K−1)] results in large thermal stress on the coatings under temperature gradient [32,33]. The interfacial characteristics between the outer coating and

Fig. 5. XRD patterns of the CrSi2-ZrSi2-Y2O3 coating surfaces prepared by SAPS with different CrSi2 mass percentages.

SiC inner coating in four pictures are mechanical interlocking, exhibiting good bonding coherence. The main principle is that the flattened particles can produce volume shrink force and bond tightly to

Table 2 The data of roughness, porosity and bonding strength of the coatings with different CrSi2 mass percentages. Subject

Ra (μm) Porosity (%) Bonding strength (N)

CrSi2 (wt%) 0

10

20

30

40

10.80 3 ± 0.3 10.8 ± 0.4

9.68 2.4 ± 0.2 11.6 ± 0.4

9.17 1.6 ± 0.1 12.9 ± 0.6

8.63 1.1 ± 0.2 15.1 ± 1.3

9.72 1.1 ± 0.2 12.3 ± 0.6

969

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Fig. 6. Cross-section backscattering electron images of the CrSi2-ZrSi2-Y2O3/SiC coatings with different CrSi2 mass percentages. (a) 10 wt% CrSi2. (b) 20 wt% CrSi2. (c) 30 wt% CrSi2. (d) 40 wt% CrSi2.

Fig. 7. The schematic of the mechanical interlocking between flattened particles and substrate.

3.2. Oxidation resistance of the CrSi2-ZrSi2-Y2O3/SiC coatings

substrate [34], which can be seen in Fig. 7. Bonding strength values are tested and the results are listed in the Table 2; porosity of the coatings is also reported. Increasing the CrSi2 content, the bonding strength values first increase and then decrease, while the change trend of porosity is opposite. Because of the low melting point of CrSi2, a small amount of CrSi2 will promote the sintering of ZrSi2, and it used as filling pores can reduce the porosity of the coating and improve the bonding strength value. Therefore, the porosity tends to be stable after the addition of 30 wt% CrSi2. However, the excessive CrSi2 may increase the interface of heterogeneous coatings and inhibit the sintering of the coatings. In addition, the high content of CrSi2 also enhances thermal mismatch of coatings, thereby reducing bonding strength values. Thus, the coating with 30 wt% CrSi2 possesses less porosity and higher bonding strength value. By comparing the data of four kinds of the coatings, it can be confirmed that the coating with 30 wt% CrSi2 has optimum structure.

Fig. 8 shows the isothermal oxidation curves of the CrSi2-ZrSi2Y2O3/SiC coatings with different CrSi2 mass percentages in air at 1500 °C. It can be seen that all the four kinds of coatings have good oxidation resistance to protect C/C composites from being oxidized for a long time. However, there are some subtle differences among them in terms of oxidation resistance time. The coating with 30 wt% CrSi2 exhibits outstanding oxidation resistance, and the mass change decreases by 1.2% after 288 h oxidation at 1500 °C in air and can withstand 15 times thermal shock cycles. Meanwhile, the oxidation characteristics of the coatings can be divided into three stages marked as A, B and C. For the coating with 30 wt % CrSi2, at the early stage of 0–48 h (A), the oxidation curve declines rapidly, suggesting the mass change of coating increases by 9.58% at 48 h. As pores and microcracks existing in the coating provide channels 970

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times, which allows oxygen to contact and react with C/C composites to form CO2 and CO gases. The gases need to escape from the coating through glass layer, leading to damage the glass layer (picture (d)). Fig. 10 shows the XRD pattern of the CrSi2-ZrSi2-Y2O3/SiC coating at the CrSi2 ratio of 30 wt% after oxidation at 1500 °C for 288 h in air. It can be observed that the coating has seven phases after oxidation. The impact of SiO2 and Cr2O3 on the oxidation performance of the coating has been discussed in the previous section. The XRD pattern shows that strong peaks mostly belong to ZrSiO4; it means that this phase is present in the coating. It has low CTE value (ZrSiO4 = 4.9 × 10−6 K−1), approaching to the CTE of SiC (SiC = 4.5 × 10−6 K−1), which could inhibit the formation and propagation of the coating microcracks [39,40]. Moreover, XRD do not detect m-ZrO2 and t-ZrO2 phases, while only cZrO2 phase appears in the coating. The results confirm that part of Y2O3 phase acts on the ZrO2 lattice to inhibit phase transformation. The other part of Y2O3 reacts with SiO2 to form Y2Si2O7 and Y2SiO5 phases. The relevant reaction equations are as follows:

Fig. 8. Isothermal oxidation curves of the CrSi2-ZrSi2-Y2O3/SiC coatings with different CrSi2 mass percentages in air at 1500 °C.

for oxygen to diffuse into the coating and react with it, some oxides could be formed to increase the samples weight, which can be proved by Fig. 10. The CrSi2 and oxides can fill microcracks and pores of the coating to improve the densification degree. At the second stage of 48–216 h (B), the curve is approximately parallel to X-axis. The above appearance indicates that the coating possesses excellent oxidation resistance during this period. The main reason is that a compact SiO2 glass layer may be formed on the surface of the CrSi2-ZrSi2-Y2O3 outer coating [35]. As the SiO2 glass layer has self-healing capacity and low oxygen permeability, oxygen is not able to come into the coating to react with the substrate [6]. Furthermore, Cr2O3 generated in the process of oxidation test could form a stable glass layer with SiO2, effectively reducing the evaporation of SiO2 and improving the service life of the coating [23,36,37]. At the final stage of 216–288 h (C), the curve starts to go up rapidly, which implies that the mass change of the specimens begins to decrease, compared to the second stage. During the oxidation test, the specimens must be continually taken out from the furnace to measure their weights at room temperature and put them into furnace again. The repeated operations could cause thermal stress on the coating to form microcracks, as the CTE difference between the ceramic coating and C/C composites. With the increase of oxidation time, oxygen continues to come into the substrate through the microcracks of the coating to react with it and form CO and CO2 gases [38], resulting in decrease of the mass change of the specimens. These gases escape from the coating defects and the glass layer, resulting in creating pores in the coating surfaces, which accelerates the performance failure of the coating systems. In order to further explain oxidation behaviors of CrSi2-ZrSi2-Y2O3/ SiC coated C/C composites, the oxidation model is shown in Fig. 9. Picture (a) exhibits the cross-section original structure of the coating and substrate after spraying, consisting of CrSi2-ZrSi2-Y2O3 outer coating, SiC inner coating and C/C composites. The interface between outer and inner coating presents mechanical bond, and the outer coating exists some microcracks and pores after spraying, according to Fig. 5. When the specimens are put into the furnace to experiment the static oxidation resistance, oxygen can penetrate into the coating through these microcracks and pores to react with them. As the time goes on, CrSi2 and oxides could gradually fill into microcracks and pores of the coating, and form a slippery glass layer on the coating surface, which can be seen in picture (b). In the process of oxidation, the specimens have to be directly taken out from furnace to obtain its weight change, which could generate some microcracks in the outer and inner coating due to CTE difference, as shown in picture (c). These microcracks are growing and propagating with increased weighting

ZrO2 (s) + SiO2 (s) = ZrSiO4 (s)

(2)

Y2O3 (s) + 2SiO2 (s) = Y2Si2 O7 (s)

(3)

Y2O3 (s) + SiO2 (s) = Y2SiO5 (s)

(4)

Therefore, Y2Si2O7 and Y2SiO5 phases can be found in the coating after oxidation at 1500 °C in air. Y2Si2O7 (Tm = 1775 °C) and Y2SiO5 (Tm = 1980 °C) have good oxidation resistance [41], because they have higher melting point than SiO2 (Tm = 1650 °C). Y2SiO5 is one of the significant rare-earth silicates to be able to improve the coating oxidation resistance, as it can be formed in glassy grain boundary of coating surface at high temperature, which can decrease the volatile rate of the glass layer and the growth of the microcracks [42]. Meanwhile, Y2Si2O7 (close to the CTE value of SiC) and Y2SiO5 (αY2SiO5 = 8.36 × 10−6 K−1) have low oxygen permeability, although they are in tiny amounts in the coating, they still provide positive effect for the coating oxidation resistance [43]. In addition, ZrO2 has high melt pointing (Tm = 2700 °C) to be able to improve the stability of the coating at high temperature. Fig. 11 shows the backscattered electron images of the CrSi2-ZrSi2Y2O3/SiC coating surface at the CrSi2 ratio of 30 wt% after oxidation at 1500 °C for 288 h in air. A dense and smooth glass layer is formed on the coating surface seen from Fig. 11(a). It can be clearly found that the coating has no obvious microcracks, but some pores appear. The reason of their formation is that gases produced by the C/C composites reacting with oxygen escape from the glass layer. Meanwhile, oxygen comes into the C/C composites through these pores to react with it again, resulting in generating more unhealed pores on the coating surface until the coating fails. Many irregular granular phases are embedded in the glass layer, which could effectively reduce the evaporation rate of the glass layer to improve the oxidation resistance of the coating. It can be further observed in Fig. 11(b) that there are two distinct grain shapes, including a large number of small spherical particles with the average grain size of 1–5 μm and a fraction of columnar particles with the average grain size of 10–20 μm, as shown in the dashed box in Fig. 11(b). These columnar crystalline grains could decrease the formation of the coating microcracks due to their larger volumes and disordered orientations. The coating has light grey phase, black phase and grey phase marked as Spot A, Spot B and Spot C in Fig. 11(c), respectively. The EDS results show that Spot A consists of Si, O, Y and Au elements. It can be deduced that the columnar crystalline grains are Y2Si2O7 and Y2SiO5 phases. Au element is carried into the coating in the process of experiments and preparation, and can be ignored. Spot B is composed of O, Si, Cr and Au elements, which illustrates it includes SiO2 and Cr2O3 phases. Spot C has O, Si, Zr and Au elements, suggesting the small spherical grains are mainly made up of ZrSiO4 and ZrO2 phases. Combined the backscattered electron cross-section images of the 971

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Fig. 9. The model of oxidation process of CrSi2-ZrSi2-Y2O3/SiC coated C/C composites at the CrSi2 ratio of 30 wt%.

Fig. 10. XRD pattern of the CrSi2-ZrSi2-Y2O3/SiC coating prepared by SAPS at the CrSi2 ratio of 30 wt% after oxidation at 1500 °C for 288 h in air.

change of the specimens. There are many big pores in the SiC inner coating, explaining that the SiC coating occurs local oxidation. The reaction equations are as follows:

coating at the CrSi2 ratio of 30 wt% after oxidation at 1500 °C for 288 h in Fig. 12, the interface between C/C composites and inner coating exists no obvious microcracks and pores. Some pores appear in the inner coating and outer coating and the interface between them, which are the major reasons for the coating failure and decrease of the mass

SiC (s) + 2O2 (g) = SiO2 (s) + CO2 (g) 972

(5)

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Fig. 11. Backscattered electron images of the CrSi2-ZrSi2-Y2O3/SiC coating surface at the CrSi2 ratio of 30 wt% after oxidation at 1500 °C for 288 h in air. (b) is the magnified images of (a). (c) is the local magnified images of (b).

Fig. 12. Backscattered electron cross-section images of the CrSi2-ZrSi2-Y2O3/SiC coating at the CrSi2 ratio of 30 wt% after oxidation at 1500 °C for 288 h. (b) is the magnified images of (a). (c) is the local magnified images of (b).

2SiC (s) + 3O2 (g) = 2SiO2 (s) + 2CO (g)

(6)

Al elements, in agreement with EDS results of Spot B in Fig. 11. A little of Al element comes from the inner coating prepared by pack cementation, which can be ignored. The light grey phase (Spot E) has O, Si, Cr, Zr and Au elements, and the white phase (Spot F) includes O, Si, Cr, Y and Zr elements. Combining with the XRD results in Fig. 10, the grey phase mainly consists of Cr2O3 and SiO2, the light grey phase mainly exists ZrSiO4, Cr2O3 and SiO2 phases, and the white phase mainly contains ZrO2, Y2Si2O7 and Y2SiO5 phases.

Meanwhile, some smaller and dense pores appear in the middle of the outer coating, as the slight volatilation of the outer coating. A bit of tiny microcracks can be found in this area seen in Fig. 12(c). These microcracks, generated from coating thermal stress, allow oxygen penetrate into the coating and reach C/C composites to react with them. The coating has three phases, including the grey phase, light grey phase and white phase marked as Spot D, Spot E and Spot F, respectively. Analyzed by EDS, the grey phase (Spot D) consists of O, Si, Cr, Au and 973

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4. Conclusions

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In this paper, four kinds of CrSi2-ZrSi2-Y2O3 outer coatings with distinct CrSi2 contents (10 wt%, 20 wt%, 30 wt% and 40 wt%) were deposited on the surface of SiC coated C/C composites by supersonic atmosphere plasma spraying technique. It could be found that all the coating systems present a dense structure with a small number of microcracks and pores and good bonding state. ZrO2, SiO2, Cr2O3, ZrSiO4, Y2Si2O7 and Y2SiO5 phases were generated during the oxidation test. Y2O3 phase could inhibit the phase transformation of ZrO2 to relieve partial coating thermal stress. The crystalline phases including ZrSiO4, ZrO2, Y2Si2O7 and Y2SiO5 might restrain the crack propagation and growth. Meanwhile, the glass phases including SiO2 and Cr2O3 could improve self-healing ability of the coating. The combined action of the glass phases and crystalline phases improve the oxidation resistance of the coating with 30 wt% CrSi2. The mass of the specimen decrease by 1.2% after the oxidation test for 288 h in air at 1500 °C, and could be able to withstand 15 times thermal shock cycles from 1500 °C to RT. Some big pores appeared in the inner and outer coating, resulting in the failure of the coating. This coating system prepared by pack cementation and SAPS can produce complicated aerospace parts with high efficiency (short preparation time) and high quality, such as nose-tip of missile and rocket motor nozzle. In aerospace field, it can be expected to use for a long time at 1500 °C and for a short time at higher temperature (about 1600 °C–2000 °C). Acknowledgements This work has been supported by the National Natural Science Foundation of China (Grant Nos. 51521061, 51502245), Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2017JM5009), Creative Research Foundation of Science and Technology on Thermostructural Composite Materials Laboratory (Grant No. 6142911020207). References [1] X. Yang, Z.A. Su, Q.Z. Huang, X. Fang, L.Y. Chai, Microstructure and mechanical properties of C/C-ZrC-SiC composites fabricated by reactive melt infiltration with Zr, Si mixed powders, J. Mater. Sci. Technol. 29 (2013) 702–710. [2] Z.Q. Yan, X. Xiong, P. Xiao, Si-Mo-SiO2 oxidation protective coatings prepared by slurry painting for C/C-SiC composites, Surf. Coat. Tech. 202 (2008) 4734–4740. [3] P. Chowdhury, H. Sehitoglu, R. Rateick, Damage tolerance of carbon-carbon composites in aerospace application, Carbon 126 (2017) 382–393. [4] T. Deak, C. Bellamy, L.G. D'Agostino, Influence of fibre-matrix interface on the fracture behaviour of carbon-carbon composites, J. Eur. Ceram. Soc. 23 (2003) 2857–2866. [5] H. Wu, H.J. Li, C. Ma, Q.G. Fu, Y.J. Wang, MoSi2-based oxidation protective coatings for SiC-coated carbon/carbon composites prepared by supersonic plasma spraying, J. Eur. Ceram. Soc. 30 (2010) 3267–3270. [6] J.R. Strife, J.E. Sheehan, Ceramic coatings for carbon-carbon composites, Am. Ceram. Soc. Bull. 67 (1988) 369–374. [7] J. Sun, Q.G. Fu, L.P. Guo, Y. Liu, C.X. Huo, H.J. Li, Effect of filler on the oxidation protective ability of MoSi2 coating for Mo substrate by halide activated pack cementation, Mater. Des. 92 (2016) 602–609. [8] P.P. Wang, H.J. Li, R.M. Yuan, Y.L. Zhang, Z.G. Zhao, The oxidation resistance of two-temperature synthetic HfB2-SiC coating for the SiC coated C/C composites, J. Alloy. Compd. 747 (2018) 438–446. [9] H.J. Zhou, L. Gao, Z. Wang, S.M. Dong, ZrB2-SiC oxidation protective coating on C/ C composites prepared by vapor silicon infiltration process, J. Am. Ceram. Soc. 93 (2010) 915–919. [10] A. Joshi, J.S. Lee, Coatings with particulate dispersions for high temperature oxidation protection of carbon and C/C composites, Compos. Part A-Appl. 28 (1997) 181–189. [11] C. Sun, H.J. Li, Q.G. Fu, J.P. Zhang, H. Peng, Double SiC coating on carbon/carbon composites against oxidation by a two-step method, T. Nonferr. Metal. Soc. 23 (2013) 2107–2112. [12] E. Zapata-Solvas, D.D. Jayaseelan, H.T. Lin, P. Brown, W.E. Lee, Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering, J. Eur. Ceram. Soc. 33 (2013) 1373–1386. [13] C.J. Leslie, E.E. Boakye, K.A. Keller, M.K. Cinibulk, Development and characterization of continuous SiC fiber-reinforced HfB2-based UHTC matrix composites using polymer impregnation and slurry infiltration techniques, Int. J. Appl. Ceram. Technol. 12 (2015) 235–244.

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