1 Introduction

Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiC) are potential candidates for a variety of applications in the aerospace field including rocket nozzles, aeronautic jet engines and aircraft braking systems [1–5]. However, the matrix cracks resulted from the mismatch of coefficient of thermal expansion (CTE) between the fiber and matrix are unavoidable [6]. These cracks remain open below matrix depositing temperature (usually 1,000°C), through which, the carbon fibers and pyrolytic carbon (PyC) interphase will be consumed by the oxidizing medium. This drawback limits long-term applications of C/SiC in high-temperature oxidizing environments [3, 4]. As a result, the development of reliable oxidation-resistant coating is crucial to utilizing the full potential of the composites. The oxidation of CVD SiC is passive up to 1,700°C and the formed SiO2 film has a low oxygen diffusion coefficient. Thus CVD SiC is the fundamental coating material for oxidation protection of thermal structural composites [7]. While, researches on CVD SiC coating showed that cracks resulted from CTE mismatch between composite and coating is also unavoidable [6]. During oxidation, boron bearing materials, such as boron [8], boron doped carbon (BC x ) [3, 9–12], and B4C [13–15] formed fluid oxide phases (B2O3 or B–Si–O ternary phase) which can fill cracks and slow down the in-depth diffusion of moisture and oxygen [3, 9]. So, boron-containing materials were widely used as internal layer together with CVD SiC as external layer to form a self healing multilayered coating.

Some studies on oxidation behavior of C/SiC coated with boron-containing hybrid coating have been performed. It is proved that the SiC-B-SiC coating was not suitable as oxidation protection coating for C/SiC composite up to 1,300°С in static air [8]. The results of oxidation of 3D C/SiC composites coated with SiC-graphitic (B-C)-SiC coating showed that this coating could provide oxidation protection in air up to 1,300°С for 15 h [11]. The oxidation behavior of SiC–B4C–SiC coating protected C/C composite was in static air at 1,300°С for 200 h and showed a residual strength of about 33% [16]. These researches focused on with the oxidation in air. While, previous studies showed that water vapor can not only remarkably accelerate the volatilization of B2O3, but also reduce its viscosity [17–19]. This phenomenon will reduces the self-sealing ability of the boron containing self healing multilayered coating system in water vapor containing atmosphere. In the combustion process, substantial amounts of water vapor are produced from burning hydrocarbon fuels in air. Calculation showed that under equilibrium conditions 5–10% of the combustion gas is water vapor [20]. Oxidation behaviors of boron containing self healing multilayered coating in wet oxygen atmosphere are therefore a concern.

In this paper, an amorphous boron carbide coating was synthesized by CVD. The atom ratio of boron to carbon is about 4, so the deposited boron carbon material is denoted by B4C. The CVD B4C was used as internal layer of a SiC-B4C multilayered coating. Firstly, the morphology and composition of the SiC-B4C coating was reported. Then the oxidation behavior of the SiC-B4C coating protected 3D C/SiC composite was investigated in environment of \({{P_{{\text{H}}_{\text{2}} {\text{O}}} } \mathord{\left/ {\vphantom {{P_{{\text{H}}_{\text{2}} {\text{O}}} } {{{P_{{\text{O}}_{\text{2}} } } \mathord{\left/ {\vphantom {{P_{{\text{O}}_{\text{2}} } } {P_{{\text{Ar}}} }}} \right. \kern-\nulldelimiterspace} {P_{{\text{Ar}}} }}}}} \right. \kern-\nulldelimiterspace} {{{P_{{\text{O}}_{\text{2}} } } \mathord{\left/ {\vphantom {{P_{{\text{O}}_{\text{2}} } } {P_{{\text{Ar}}} }}} \right. \kern-\nulldelimiterspace} {P_{{\text{Ar}}} }}}} = {{14} \mathord{\left/ {\vphantom {{14} {{8 \mathord{\left/ {\vphantom {8 {78}}} \right. \kern-\nulldelimiterspace} {78}}}}} \right. \kern-\nulldelimiterspace} {{8 \mathord{\left/ {\vphantom {8 {78}}} \right. \kern-\nulldelimiterspace} {78}}}}\) with a slow gas flow rate of 2.8 cm s−1 at 700°C, 1,000°C and 1,300°C for 100 h, respectively.

2 Experiment Procedure

2.1 Fabrication of Specimens

The detailed fabrication processes of the substrates were described in reference [11]. Two kinds of specimens were prepared. The first kind was coated with two layers of CVD SiC, denoted by SiC-SiC coating. The second kind was coated with a multilayered coating which consisted of a layer of CVD SiC as external coating and a layer of B4C as internal coating, denoted as SiC-B4C coating. The conditions for CVD SiC were the same as that of the SiC matrix except for the deposition time, which was 30 h each time. The CVD B4C layer was deposited from BCl3–CH4–H2 mixtures, the deposition temperature was 980°С, the pressure was 3 KPa, and the molar ratio of BCl3 to CH4 to H2 was 5/1/5. During the deposition of B4C, argon with the same velocity of flow as H2 was used as the dilute gas to get more uniform deposition.

2.2 Oxidation Tests

The oxidation tests were conducted in a MoSi2 furnace in a wet oxygen environment of \({{P_{{\text{H}}_{\text{2}} {\text{O}}} } \mathord{\left/ {\vphantom {{P_{{\text{H}}_{\text{2}} {\text{O}}} } {{{P_{{\text{O}}_{\text{2}} } } \mathord{\left/ {\vphantom {{P_{{\text{O}}_{\text{2}} } } {P_{{\text{Ar}}} }}} \right. \kern-\nulldelimiterspace} {P_{{\text{Ar}}} }}}}} \right. \kern-\nulldelimiterspace} {{{P_{{\text{O}}_{\text{2}} } } \mathord{\left/ {\vphantom {{P_{{\text{O}}_{\text{2}} } } {P_{{\text{Ar}}} }}} \right. \kern-\nulldelimiterspace} {P_{{\text{Ar}}} }}}} = {{14} \mathord{\left/ {\vphantom {{14} {{8 \mathord{\left/ {\vphantom {8 {78}}} \right. \kern-\nulldelimiterspace} {78}}}}} \right. \kern-\nulldelimiterspace} {{8 \mathord{\left/ {\vphantom {8 {78}}} \right. \kern-\nulldelimiterspace} {78}}}}\), the flowing velocity of gaseous mixtures is about 2.8 cm s−1. The oxidation tests were performed at 700°С, 1,000°С, and 1,200°С for 100 h, respectively. For each experimental condition, five specimens were tested. The specimens were introduced in the furnace at desired temperature. After oxidized for a certain time, the specimens were picked out and weighted. The weight were recorded after 1, 3, 6 and 10 h for the initial 10 h, then the interval record time was 10 h until oxidized for 100 h. The specimens were measured using an electronic balance with a sensitivity of 0.01 mg (METTLER TOLEDO AG 135).

2.3 Measurements of the Composites

The surface and fracture section morphologies of the coatings were observed with a scanning electron microscope (SEM, SMJ-6700). An energy dispersive X-ray spectrum (EDS, EDXA) was performed to identify the element content of boron and carbon in the multilayereded coating. The EDS was calibrated with a B4C standard (99.5%, CERAC, USA) before the quantitative analysis of the deposited B4C coating. Phase identification was obtained with an X-ray diffraction device (XRD, Rigaku D/MAX-2400 with Cu Kа radiation) in grazing incidence (GIXRD) mode.

Flexural strength of the specimens before and after oxidation was measured by a three-point bending method at room temperature. The span dimension was 20 mm and the loading rate was 0.5 mm·min−1.

3 Results and Discussion

3.1 Characterization of the as Received SiC-B4C Coating

Figure 1 shows the surface morphology and EDS result of as received B4C layer. It is clear that the B4C layer has typical cauliflower-like surface which indicates that the B4C is deposited by direct nucleation from liquid phase. The EDS analysis exhibits only boron and carbon peaks, which contents are given in Table 1. It can be seen that the atom ratio of boron to carbon is about 4, so the deposited boron carbon materials shall be denoted by B4C. The XRD analysis found no evidence of either boron carbide crystal or graphite phase. This means that the deposited B4C is amorphous, which is consistent with other previous results [21, 22].

Fig. 1
figure 1

Characterizations of the CVD B4C layer showing, a surface morphology; b EDS pattern

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Table 1 The elements and content (EDS) of the CVD B4C layer
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Figure 2 shows the fracture section of the SiC-B4C coating, in which, the B4C layer exhibits dense glass-like fracture section morphology. The thickness of B4C layer is about 18 μm, and that of the external SiC layer is about 25 μm. Figure 3 shows that the cracks are deflected at the interfaces of the SiC-B4C coating. This phenomenon will prolong the diffusion path of the corrosive atmosphere before it reaches the carbon fiber.

Fig. 2
figure 2

Fracture section morphology and EDS line scan of the SiC-B4C coating

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Fig. 3
figure 3

Crack deflections at interfaces of the SiC-B4C coating

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Figure 4 shows the surface crack density of SiC-SiC coating and SiC-B4C coating, it is clear that the crack density of SiC coating is about twice that of SiC-B4C coating. This may due to the less adhesion at the heterogeneous interfaces of the SiC-B4C coating, shown in Fig. 3, which not only released thermal stress but also weakened transfer of the thermal stress.

Fig. 4
figure 4

Surface crack density of a SiC-SiC coating and b SiC-B4C coating

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3.2 Morphologies and Compositions of Hybrid Coating after Oxidation

Figure 5 shows the morphologies of the SiC-B4C coatings after oxidation at 700°С for 100 h. It is clear that the external SiC layer of the specimen keeps intact and there is no evidence of liquid phase around crack. After oxidation at 1000°С, as shown in Fig. 6, the outer SiC layer is homogeneously covered with a layer of dense but rimous glass with a thickness of about 1.7 μm. There are two kinds of cracks in the glass layer, namely the long straight crack partly healed by liquid phase balls, and the direction free net cracks. The EDS result of the ramous glass layer showed that this layer consisted of Si, O, and C. Therefore, it can be deduced that the glass layer is SiO2, and the carbon signal is from SiC beneath the SiO2 layer. While the EDS result of liquid balls showed Si, O, and B, which indicated that the liquid balls consisted of B2O3.

Fig. 5
figure 5

Characterization of the SiC-B4C coating oxidized at 700°С for 100 h showing, a surface morphology, b EDS pattern

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Fig. 6
figure 6

Characterization of the SiC-B4C coating oxidized at 1,000°С for 100 h showing, a surface morphology, b fracture section morphology, c EDS pattern of spectrum 1 in (a), d EDS pattern of spectrum 2 in (a)

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Based on the microstructure and EDS analysis, it can be deduced that the long straight crack is a coating crack which deduced by the CET mismatch between coating and composite. During oxidation at 1000°С, the coating crack was nearly closed, however, the wet oxygen gas still can reach the internal B4C layer. The liquid B2O3 was formed by oxidation of B4C accompanied by a remarkable volume increase [23], which makes it overflowed from the crack of external SiC layer and formed a series of liquid balls above the crack. During cooling stage, the crack was re-ruptured and the tension stress also torn the glass phase balls. On the same time, accelerated oxidation of SiC in wet oxygen atmosphere leads to the surface of coating was covered by a thin layer of SiO2 [24, 25].During cooling-down, volume contraction of SiO2 resulted in the net cracks produced all over the glass layer.

Figure 7 shows the surface morphologies and EDS analysis of the SiC-B4C coatings after oxidation at 1,200°С for 100 h. It is clear that the specimen is covered by a layer of rimous SiO2. Bubbles and partial peel-off can be observed in the SiO2 layer and cracks also can be found in the exposed underlayer. The EDS analysis of the underlayer shows Si, O, and C, which indicates that beneath the surface rimous SiO2 layer, there is still a layer of SiO2. Magnified image of the surface SiO2 is shown in Fig. 7b, it is clear that the surface SiO2 layer exhibits a distinct dendrite morphology, which indicates the crystallization of the SiO2 layer. The GIXRD patterns of the SiC- B4C coatings after oxidation for 100 h are shown in Fig. 8. It is clear that the SiO2 crystallized after oxidation at 1,000°С and the crystallization was enhanced after oxidation at 1,200°С.

Fig. 7
figure 7

Characterization of the SiC-B4C coating oxidized at 1,200°С for 100 h showing, a surface morphology with low magnification, b surface morphology with high magnification, c EDS pattern of spectrum 1 in (a), d EDS pattern of spectrum 2 in (a)

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Fig. 8
figure 8

GIXRD patterns of the SiC/B4C coatings after oxidation for 100 h

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Figure 9 shows the cross section of the SiC-B4C coatings after oxidation at 1,200°С for 100 h. The partial enlarged detail of the glass layer exhibited distinct two kinds of structures. The external layer is about 1.5 μm and shows a dense fracture structure, while the internal layer shows a porous structure. This may due to the diffusion coefficient of water in silica is an order of magnitude smaller than that of oxygen. However, the solubility of water in vitreous silica is much larger than that of oxygen [26], these will result in a gradient distribution of water in the silica. The solution of water will significantly decrease the viscosity of silica. Then, it can be deduced that the external layer of silica has a bigger solution of water and a low viscosity, while the internal layer of silica has a smaller solution of water and a high viscosity. Under oxidation at 1,200°С in wet environment, gaseous species will be produced. These gaseous products diffuse into the SiO2 layer and form bubbles. The internal layer of silica has a high viscosity, and the bubbles are hard to escape, so the internal part of silica layer is porous. While the viscosity of external silica layer is low, bubbles will easily escape, thus the structure of external part of silica layer is dense.

Fig. 9
figure 9

Fracture section morphology of the SiC-B4C coating after oxidation at 1,200°С for 100 h a morphology with low magnification, b morphology with high magnification

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3.3 Weight Change of SiC-B4C Coated C/SiC Composite

Figure 10 shows the weight changes of SiC-B4C coating coated C/SiC composite after oxidation for 100 h. The weight changes of SiC-SiC coated C/SiC composite were also given for comparison. It can be seen that the SiC-B4C coated C/SiC showed weight loss at 700°С, and weight gain above 1,000°С.

Fig. 10
figure 10

Weight change of the coated 3D C/SiC composite a oxidized for 100 h, b detailed information for initial 10 h of (a)

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At 700°С, the SiC-B4C coated C/SiC composite showed a parabolic weight loss for the initial 5 h, and then the weight loss got steadied for the next 20 h. After that the weight loss gradually accelerated, especially after oxidation for about 50 h. It can be deduced that during oxidation at 700°С, the liquid B2O3 can nearly seal the cracks within the initial 5 h. During the following 20 h, the oxidation is mainly controlled by diffusion of oxygen and water in liquid B2O3. The steadied weight change indicates that weight gain caused by oxidation of B4C and weight loss caused by the evaporation of B2O3 and oxidation of carbon phase got balanced. While, with the evaporation of B2O3 carrying on, the seal of cracks got abated, and the oxidation of carbon phase was aggravated. So the weight loss gradually accelerated with time prolongs. Even though, the weight loss of SiC-B4C coated C/SiC after oxidized at 700°С for 100 h is about 0.45%, which is much smaller than that of SiC-SiC coated C/SiC.

At 1,000°С, the SiC-B4C coated C/SiC showed that the weight increased rapidly for the initial 3 h, and then the weight almost kept steady. It can be deduced that at 1,000°С, the breadth of cracks was reduced and the formation of B2O3 was accelerated, so it is easier to seal cracks. Meanwhile the oxidation of SiC resulted in a thin layer SiO2 and the formation of SiO2 would be significantly increase by B2O3 [27]. While, the SiC-SiC coated C/SiC can only seal cracks by the formation of SiO2, which is slow at this condition.

At 1,200°С, the SiC-B4C coated C/SiC showed approximate linear weight gain. At this temperature, the oxidation of SiC was enhanced by B2O3 and water [26, 27]. Also, B2O3 and SiO2 would combine to form boron silicate, and the saturated vapor pressure of boron silicate is much lower than that of B2O3, which means that the evaporation of B2O3 was slowed down. The enhanced oxidation of SiC and the weakened evaporation of B2O3 resulted in the linear weight gain of SiC-B4C coated C/SiC at 1,200°С.

3.4 Residual Flexural Strength of SiC-B4C Coated C/SiC Composite

The residual flexural strength of coated C/SiC after oxidation at 700°С, 1,000°С and 1,200°С for 100 h was compared as shown in Fig. 11. The residual strength of the SiC-B4C coated C/SiC is higher than that of SiC-SiC coated C/SiC below 1,000°С, especially at 700°С, the residual strength of the SiC-SiC coated C/SiC is about 64%, while that of SiC B4C coated C/SiC is about 86%. After oxidized at 1,000°С, the residual strength of SiC-B4C coated C/SiC was almost constant, while a small strength loss (94% retained strength) was achieved by SiC-SiC coated C/SiC. After oxidized at 1200°С, the residual strength of the SiC-B4C coated C/SiC (86%) is slightly lower than that of SiC coated C/SiC (89%). These results indicated that compared with SiC-SiC coating, the SiC-B4C hybrid coating could provide better protection for C/SiC composite in \(P_{{\text{H}}_{\text{2}} {\text{O}}} /P_{{\text{O}}_{\text{2}} } /P_{{\text{Ar}}} = 14/8/78\) atmosphere for 100 h up to 1,000°C.and the protection of SiC-B4C coating is still efficient at 1,200°С.

Fig. 11
figure 11

Residual flexural strength of the coated C/SiC before and after oxidation for 100 h

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4 Conclusions

  1. 1.

    The crack density of SiC-B4C hybrid coating was about half of that of SiC coating. Due to the less adhesion at the heterogeneous interfaces of the SiC-B4C coating, which not only release thermal stress but also weaken transfer of the thermal stress.

  2. 2.

    Wet oxidation behaviors of C/SiC composite protected by SiC-B4C coating and SiC-SiC coating were investigated in an environment of \({{P_{{\text{H}}_{\text{2}} {\text{O}}} } \mathord{\left/ {\vphantom {{P_{{\text{H}}_{\text{2}} {\text{O}}} } {{{P_{{\text{O}}_{\text{2}} } } \mathord{\left/ {\vphantom {{P_{{\text{O}}_{\text{2}} } } {P_{{\text{Ar}}} }}} \right. \kern-\nulldelimiterspace} {P_{{\text{Ar}}} }}}}} \right. \kern-\nulldelimiterspace} {{{P_{{\text{O}}_{\text{2}} } } \mathord{\left/ {\vphantom {{P_{{\text{O}}_{\text{2}} } } {P_{{\text{Ar}}} }}} \right. \kern-\nulldelimiterspace} {P_{{\text{Ar}}} }}}} = {{14} \mathord{\left/ {\vphantom {{14} {{8 \mathord{\left/ {\vphantom {8 {78}}} \right. \kern-\nulldelimiterspace} {78}}}}} \right. \kern-\nulldelimiterspace} {{8 \mathord{\left/ {\vphantom {8 {78}}} \right. \kern-\nulldelimiterspace} {78}}}}\) at 700°C, 1,000°C and 1,200°C for 100 h. The SiC-B4C coated C/SiC showed much smaller weight loss at 700°C than that of SiC-SiC coated C/SiC. Above 1,000°C, the SiC-B4C coated C/SiC showed a slight weight gain, while SiC-SiC coated C/SiC showed weight loss.

  3. 3.

    The residual strength analysis indicated that the SiC-B4C coating was more efficient to protect C/SiC than that of SiC-SiC coating below 1,000°C, especially at 700°С, the residual strength of SiC-B4C coated C/SiC is about 86%., while that of the SiC-SiC coated C/SiC is about 64%. The residual strength of the SiC-B4C coated C/SiC (86%) is slightly lower than that of SiC coated C/SiC (89%) after oxidation at 1,200°C. The SiC-B4C coating can provide C/SiC composite in wet oxygen atmosphere up to 1,200°C for 100 h.