alumina–borosilicate coating for carbon–carbon composites

Applied Surface Science 255 (2008) 1967–1974

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Oxidation behavior and kinetics of SiC/alumina–borosilicate coating for carbon–carbon composites Jun Li a, Ruiying Luo a,*, Yaping Chen a,b, Qiao Xiang a, Chen Lin a, Peng Ding a, Na An a, Jiwei Cheng a a b

College of Science, Beijing University of Aeronautics & Astronautics, Beijing 100083, PR China Equipment Academy of Second Artillery, Beijing 100085, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 November 2007 Received in revised form 16 June 2008 Accepted 18 June 2008 Available online 3 July 2008

A SiC/alumina–borosilicate double-layer coating was prepared by a two-step slurry technique for the protection of carbon–carbon composites from oxidation. SEM, XRD and EDX analysis showed that the inner layer coating obtained from the first step consisted of b-SiC, Si and SiO2. The outer layer consisted of B4C, Al, amorphous SiO2 and quartz. The oxidation behavior and kinetics of the SiC/alumina– borosilicate coating were investigated. It was found that the as-prepared coating could effectively protect carbon–carbon composites from oxidation at 1300 8C for 100 h. The isothermally oxidation kinetics could be described by Microflaws Model and Diffusion Model. During short-time oxidation, the overall oxidation kinetics was determined by the transport processes through microflaws, which resulted in a linear growth law. While during oxidation of longer time, the weight loss rate was controlled by the diffusion processes in the coating with growing oxide layer, which led to a parabolic growth law. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Carbon/carbon composites Double-layer structure Coating Oxidation

1. Introduction Carbon/carbon (C/C) composites are widely used as aeronautic and astronautic materials for their excellent performance in mechanical properties at high temperature, such as heat shields, nose-tips for reentry vehicles, leading edges and airplane brakes [1–4]. However, several technological barriers limit a wider application of C/C composites. One of the major technological obstacles is the susceptibility of these composites to air oxidation at temperature above 460 8C, which would result in the erosion of the structure and eventually the degradation of the properties that the material originally possesses [5–7]. Over the past 60 years, many researches have been conducted on oxidation resistance for C/C composites, and the application of refractory materials coated on the surface of C/C composites is considered to be the best choice. Consequently, most of the researchers focus on the silicon-containing ceramic coatings [8]. However, the cracks formed by the coefficient thermal expansion (CTE) mismatch of the coatings and the C/C substrate is an inevitable problem, which results in limited oxidation protection. Recently, the advanced coatings, with a functionally gradient layer

* Corresponding author. Tel.: +86 10 8233 8267; fax: +86 10 8233 8267. E-mail addresses: [email protected] (J. Li), [email protected] (R. Luo). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.06.169

to minimize the influence of the CTE mismatch, are designed to be multilayer coatings [3,8]. Most of these coatings are prepared by Chemical Vapor Deposition (CVD) [3,9], Chemical Vapor Reaction (CVR) [9], Electrophoretic Deposition (EPD) [10] and pack cementation [11]. However, the interfacial cracks and the high cost still limit the application of such coatings [12]. On the other hand, the oxidation kinetics of multilayer coatings has not yet been discussed in depth. This paper demonstrates a SiC/alumina–borosilicate coating system. The double-layer coating, with much lower volatility and oxygen permeability, could inhibit C/C composites from oxidation at 1300 8C for 100 h. The oxidation kinetics and anti-oxidation mechanics are studied in order to analyze the effects of the coating system on oxidation resistance property. 2. Experimental Specimens in size of 8 mm  8 mm  8 mm were cut from twodimensional C/C composites prepared by the rapid direction diffused chemical vapor infiltration (RDD CVI), and their bulk density was 1.72 g/cm3. The coating consisted of two layers. The inner layer was prepared with a slurry method. Silicon powder (325 mesh) was used to prepare slurry with phenol–formaldehyde (PF) resin. The specimens were pasted with the slurry and dried at 80 8C for 1 h.

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Then, they were heat-treated in an electric furnace at temperature of 1520 8C for 2 h with protection of argon. The outer layer was obtained by slurry method also. The precursors for preparing the outer layer consisted of boron carbide (800 mesh), low melting point glass powder (425 mesh, with soften temperature from 600 8C to 900 8C), quartz (425 mesh), silicone resin and aluminum (325 mesh). All the chemicals are analytical grade except that the glass powder is industrial grade. The precursors were evenly mixed and pasted on anterior specimens. Then, they were heat-treated in the electric furnace at 800 8C for 3 h in a nitrogen atmosphere. Isothermal oxidation tests of the as-prepared specimens were carried out in static air, and the specimens were oxidized at a constant temperature of 500 8C, 700 8C, 900 8C, 1100 8C, 1300 8C and 1400 8C, respectively. The mass of the specimens was determined on an electronic balance, whose accuracy was 0.1 mg. The weight change rate (W) of the specimens at any temperature was calculated by: W¼

Dm m0

¼

m0  mi  100% m0

(1)

where m0 and mi are the weight of the specimens before and after oxidation tests. The morphologies of the as-prepared coating were analyzed by using Scanning Electron Microscopy (SEM). The crystalline structures of the coating were studied with X-ray Diffraction (XRD) and Energy Dispersive X-ray (EDX). 3. Results and discussion 3.1. Microstructure of the coating Fig. 1(a) and (b) shows the SEM micrograph surface for inner layer and outer layer, as can be seen that the surfaces of both layers

were very smooth, shapely and integrated. The SEM micrograph cross-section of the coating is shown in Fig. 1(c). It can be seen that the coating with a thickness of around 150 mm was well compact, and there were no penetration cracks or holes in the coating; moreover, there were no obvious interfaces between the two layers. Fig. 2(a) shows the X-ray analysis confirmed that this layer consisted of b-SiC, Si and small amount of SiO2. As reported in Ref. [13], the contact angle of liquid Si on C is near 08, so that the molten Si could impregnate into porosities on the surface of the C/C substrate at the heat retaining process at 1520 8C, and reacted with C/C substrate to form b-SiC [14], and a gradient buffer layer based on b-SiC and Si was formed on the surface of C/C substrate. Fig. 2(b) illustrated the X-ray diffraction pattern of the specimens with this layer. The identified phases of the coating were B4C, b-SiC, Al and SiO2 (quartz). Meanwhile, there was some amorphous phase. B4C were detected together with the presence of an amorphous halo evidence of a residual glassy phase, and the identified b-SiC was the inner layer. Moreover, when heated to the second heat retaining process (800 8C), silicone resin with thermolysing temperature of 600–800 8C was condensed to be amorphous SiO2 [15]. Meanwhile, the glass powder has a low softening temperature range of 700–900 8C. Therefore, the indicated amorphous phase consisted of amorphous SiO2, which had a low viscosity at 800 8C so that they could flow into the interspaces between each B4C and quartz particles [11], and then closed down the interspaces. Fig. 3 presents EDX element analysis for cross-section of the double-layer coating. From EDX elements analysis for carbon in Fig. 3(a) and silicon in Fig. 3(b), it was found that silicon infiltrated into C/C substrate along its microflaws. On the other hand, the concentration of carbon changed gradually from C/C substrate to coating. Therefore, a functionally gradient SiC/C conversion layer was formed, and at the same time, with the progress of silicon

Fig. 1. (a) SEM micrograph surface for inner layer; (b) SEM micrograph surface for outer layer and (c) SEM micrograph for cross-section of the double-layer coating.

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Fig. 2. (a) X-ray diffraction patterns inner layer and (b) X-ray diffraction patterns for outer layer.

Fig. 3. EDX element analysis from cross-section of the coating for (a) C, (b) Si, (c) O, and (d) Al.

infiltration process, silicon could diffuse into deeper region through microflaws and the reaction with carbon then processed, and then the formed SiC could play the role of staples to reinforce the bonding strength of coating as reported by Zhu et al. [3]. Fig. 3(c) and (d) shows the distributions of oxygen and aluminum element, respectively, as can be seen that both elements were uniformly dispersed in the coating. 3.2. Oxidation kinetics analysis for the coating All oxidation reactions of the coating are as follows: 2B4 CðsÞ þ 7O2 ðgÞ ¼ 4B2 O3 ðsÞ þ 2COðgÞ

(2)

4AlðsÞ þ 3O2 ðgÞ ¼ 2Al2 O3 ðsÞ

(3)

2SiCðsÞ þ 3O2 ðgÞ ¼ 2SiO2 ðsÞ þ 2COðgÞ

(4)

SiðsÞ þ O2 ðgÞ ¼ SiO2 ðsÞ

(5)

CðsÞ þ O2 ðgÞ ¼ CO2 ðgÞ

(6)

2CðsÞ þ O2 ðgÞ ¼ 2COðgÞ

(7)

where each reaction is zero order irreversible reaction. Eqs. (2)–(5) are the oxidation reactions in the coating. All the reactions result in weight gain, and we treat them as a whole mass weight gain (DWc) as

DW c ¼ A0 M c qc Dt

(8)

where A0 is the surface area, Mc is the weight gain of non-oxide layer for each mole of oxygen consumed, and qc is the total seepage discharge of O2 to surface of non-oxide layer. Meanwhile, Eqs. (6) and (7), as oxidation reactions of C/C substrate, are also treated as a

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whole reaction, which cause the weight loss (DWs) as

DW s ¼ A0 M s qs Dt

(9)

here Ms is the weight loss of C/C substrate for each mole of oxygen consumed, and qs is the total seepage discharge of O2 to surface of C/C. Additionally, we considered that all of infiltrated O2 would react with C/C substrate for its low quantity. The diffusion behavior of O2 can be expressed by Fick’s first and second laws [16,17]: J ¼ D

dC dt

@Cðx; tÞ @2 Cðx; tÞ ¼D @t @x2

(10)

(11)

where J is the flux density of O2, D is the diffusion coefficient, and C is concentration of O2. At low concentrations the solubility coefficient (S) of oxygen depends on its partial pressure P, and then the concentrations of oxygen on both sides of coating could be expressed according to Sievert’s law [18]: pffiffiffiffiffi C 1 ¼ S P1 (12) pffiffiffiffiffi C 2 ¼ S P2

(13)

We assume the thickness of the coating is h, thus the inner side of oxygen diffusion flux is given by the Fick’s first and second laws:   @C J t ¼ D @t x¼h !) pffiffiffiffiffi pffiffiffiffiffi ( 1 X DSð P1  P 2 Þ i2 p2 1þ2 ¼ ð1Þi exp  2 Dt (14) h h i¼1 On the steady-state condition: pffiffiffiffiffi pffiffiffiffiffi DSð P1  P 2 Þ J t¼1 ¼ h

(15)

Generally, P1  P2, C2  0, thus we set P = P1, and then we could obtain the seepage discharge of O2: pffiffiffi pffiffiffi DS P F P (16) ¼ q ¼ J1 ¼ h h where F is the diffusion rate:

F ¼ DS and F depends on:   H F ¼ F0 exp  F 2RT

(17)

(18)

Fig. 4. Schematic description of Microflaws Model for the double-layer coating (I C/ C substrate and II non-oxide layer).

The oxidation step model was applied to two situations: Microflaws Model and Diffusion Model as the following: (1) Microflaws Model: at the beginning step of oxidation, there are some microflaws as can be seen from Figs. 1(b) and 4 show the Microflaws Model for the double-layer coating. Here we assume the coating as a non-oxide layer, although there are some oxide components. Thus, oxygen can react with non-oxide on the surface of the coating, and the microflaws allow oxygen transport to C/C substrate, and all reactions were interface reaction. Meanwhile, the oxygen transported by solid-state diffusion was rather slower than that of microflaws. Thus, the seepage discharge of O2 through microflaws to the C/C substrate can be expressed by [19]: pffiffiffi Fm Ad P qM ¼ (19) heff where Fm is the O2 diffusion rate in the microflaws, Ad is the effective square, and heff is the effective thickness. Then, according to Eq. (8), the weight change from C/C substrate is: pffiffiffi F A P DW s ¼ Ms A0 qM Dt ¼ Ms A0 m d Dt (20) heff The reaction rate at the oxygen-coating interface was reported by Fritze et al. [17] as follows:   E DW c ¼ kI exp  I Dt (21) RT where kI and EI are the pre-exponential factor and the activation energy, respectively. Then the weight change rate could be expressed by: pffiffiffi  ! DW s þ DW c Fm Ad P kI EI W¼ ¼ M s A0  exp  Dt (22) m0 heff m0 m0 RT where we defined a parameter of P0 as follows: pffiffiffi   Fm Ad P kI EI P 0 ¼ M s A0  exp  m0 heff m0 RT

(23)

Then, we could obtain the weight change rate: W ¼ P0 t

(24)

where P0 depends on temperature, and on the condition of constant temperature, the oxidation function is a linear model. (2) Diffusion Model: at the upper step of oxidation, most microflaws have been closed as shown in Fig. 5, and O2 basically diffused through the solid coating. According to Eq. (16), the seepage discharge of O2 (qg) through layer I can be expressed by: pffiffiffi Fg P (25) qg ¼ x

Fig. 5. Schematic description of Diffusion Model for the double-layer coating (I C/C substrate, II non-oxide layer and III oxide layer).

J. Li et al. / Applied Surface Science 255 (2008) 1967–1974

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where Fg is the diffusion rate of O2 (qg) through layer III, x is the thickness of layer III. Then, according to Eq. (7), the oxidation of layer II can lead to weight gain: pffiffiffi F P DW c ¼ Mc A0 qg Dt ¼ Mc A0 g Dt (26) x The reaction rate for the oxygen-layer II interface can be expressed as dx ¼k dt

(27)

x ¼ kt þ x0

(28)

where x0 is initial thickness of oxide layer, and k is the weight loss rate constant, which follows Arrhenius equation [5]:   Ea (29) k ¼ k0 exp  RT Meanwhile, the seepage discharge of O2 (qeff) through multilayer is given by the following expression: pffiffiffi F P (30) qeff ¼ eff h where Feff, the effective O2 diffusion rate in the multilayer, is defined as follows: 8 x þ ðh  xÞ h > > Feff ¼ ¼ > > > Rg þ Rc Rg þ Rc > < x Rg ¼ (31) F > g > > > h  x > > : Rc ¼

Fc

Then qeff can be expressed as pffiffiffi Fg Fc P qeff ¼ ðFc  Fg Þx þ hFg

(32)

According to Eq. (8), the oxidation of C/C substrate can lead to weight loss: pffiffiffi Fg Fc P DW s ¼ Ms A0 qeff Dt ¼ Ms A0 Dt (33) ðFc  Fg Þx þ hFg Thus, together with Eq. (28), we obtain the total weight change of Diffusion Model: W¼

DW s þ DW c m0

¼ M s A0

pffiffiffi

pffiffiffi

Fg Fc P F P Dt  M c A 0 g Dt m0 ½ðFc  Fg Þx þ hFg  m0 x

(34)

where we define the parameters of P1, P2, P3 and P4 as follows: 8 pffiffiffi > M s A0 P > > ¼ P > 1 > m0 ðððh  x0 Þ=Fc Þ þ ðx0 =Fg ÞÞ > > > > k > > < P2 ¼ x0 þ ðh=ðFc =Fg Þ  1Þ (35) pffiffiffi > 1 > > P M ¼ A F P > > > 3 m0 k c 0 g > > > k > > : P4 ¼ x0 Then, we could get the weight loss rate as: W ¼ P1 ln ðP 2 t þ 1Þ  P3 ln ðP 4 t þ 1Þ

(36)

On the condition of constant temperature, this oxidation function is a parabolic model.

Fig. 6. Raw data for the mass weight loss of reference samples. The solid curves are linear and parabolic fits of the experimental data.

Comparatively, the oxidation behavior of the real RDD CVI C/C composites was described in our previous paper as Ref. [5]. We found that the weight loss of C/C is linearly increasing with time at the oxidation temperatures from 600 8C to 800 8C as the following equation: W ¼ k0 m0 t

(37)

where k0 is the oxidation rate constant for the real C/C composites, which depends on temperature and structure of these composites. According to the oxidation model developed by Lachaud et al. [20], the complex oxidation behavior of C/C composites could be mainly attributed to a weakest-link process due to the presence of a weak interphase lying between the carbon fibers and the carbon matrices. 3.3. Oxidation behavior of the coating and fit results According to the Microflaws and Diffusion Models (see Eqs. (24) and (36)), the isothermally oxidation results are fitted as shown in Fig. 6, as can be seen that the oxidation kinetics for the isothermal oxidation data at temperatures higher than 500 8C are indeed linear at the initial steps and parabolic above these steps. The fit parameters for Eqs. (24) and (36) are shown in Table 1, wherein R is the coefficient of association, all of which are close to 0.99. Therefore, we could conclude that the oxidation models basically met the oxidation test results. On the other hand, as can been seen from the isothermally oxidation results that there were three oxidation processes: in the relatively low temperatures range, from 500 8C to 700 8C, a quite small weight loss rate was obtained at 500 8C, and the weight loss rate with time curve was basically linear. However, the rate increased fast from 0.21% at 500 8C to about 2.86% at 700 8C after oxidation for 100 h. Fig. 7(a) illustrates the SEM microphotograph of the double-layer coating that has been oxidized for 70 h at 700 8C in air. As can be seen that some oxide glass was formed on the surface, but it was not a continuous layer and there were some micro-pores on the coating. In the intermediate temperature range (800–1100 8C), the weight loss rate was less than the rate at 700 8C, the best oxidation resistance presented at 1100 8C (the weight loss rate was 0.96% with oxidation time of 100 h). Fig. 7(b) shows the SEM microphotograph of the double-layer coating after oxidation for 100 h at 1100 8C, it was found that a continuous oxide glass layer was formed on the surface. However, there was a micro-

J. Li et al. / Applied Surface Science 255 (2008) 1967–1974

1972 Table 1 The fit parameters for reference samples Temperature (8C)

500 700 900 1100 1300 1400

Linear step

Parabolic step

P0

R

P1

P2

P3

P4

R2

0.00255 0.05989 0.03337 0.01449 0.07956 0.22285

0.99057 0.99547 0.99196 0.98780 0.99550 0.99168

– 1.61735 1.38482 1.41653 2.19570 1.36606

– 0.06263 0.39604 0.02148 0.07994 0.11702

– 0.16066 0.51355 0.56155 0.22384 1.00362

– 0.06269 4.11448 0.02169 1.12024 0.11700

– 0.99230 0.99529 0.99792 0.99022 0.99667

Fig. 7. SEM micrographs for the double-layer coating after oxidation for 100 h at (a) 700 8C, (b) 900 8C and (c) 1300 8C.

crack. At high temperature range, from 1200 8C to 1400 8C, the rate of weight loss increased over again like that in the low temperature range, but it higher than the rate in the low temperature range. When oxidized for 100 h at 1300 8C, the weight loss rate increased to be 3.78%. Fig. 7(c) shows the SEM microphotograph for specimen oxidized for 70 h at 1300 8C. As can been seen, more micro-cracks and micro-pores were formed. In addition, the cross-section view for this specimen is shown in Fig. 8. It can be observed that some pores and crack presented in the interface between the coating and C/C substrate, which would be the failure areas of the coating. By Xray analysis for the coating after oxidation for 70 h at 1300 8C (Fig. 9), there are three different crystal phases other than that of coating before oxidation (Fig. 2(b)): B2O3, Al2O3 and Al2SiO5. Moreover, the diffraction peaks of B4C and Al were too low to be discerned. As the oxidation temperature elevated to be 1400 8C, the weight loss rate increased to be 4.81% after 59 h, thus, we could conclude that the C/C substrate had begun to oxidize. 3.4. Mechanism of anti-oxidation for the coating According to the isothermally oxidation curves at Fig. 6, the different oxidative processes at different temperature ranges may proceed through the following basis:

Zone I (500–700 8C): the weight changes strongly depended on temperatures. At around 500 8C, the sample showed a slight weight loss, because Fm was small and the oxidation reaction was very slow, furthermore the air oxidation could only occur at some active sites. However, when temperature was raised to 700 8C, oxidation of B4C on the surface of coating would quickly form B2O3 according to Eq. (2), which filled in micro-pores and inhibited oxidation of C/ C substrate just like that of C/C composites coated with B4C [21], and then the P4 at this temperature depended on Eq. (2). On the other hand, the formation of B2O3 was accompanied by a 250% volume expansion [5] and filled in micro-pores also. Furthermore, Al2O2 formed from oxidation of Al could restrict the volatilization of B2O3. Nevertheless, the b-SiC inner layer could not form a compact oxide glass for its high oxidation temperature, so that some oxygen molecules could diffuse to the C/C substrate and oxidize. Therefore, the coating showed unsatisfying oxidation resistance for long-time as shown in Fig. 6. Zone II (800–1100 8C): more B2O3 was formed from B4C, and the softening temperature of SiO2 glass powder was from 700 8C to 900 8C, meanwhile, some other amorphous SiO2 formed by thermolysis of remaining silicone resin with low softening point. Therefore, when B2O3 flowed to the surface of the amorphous SiO2, a kind of borosilicate glass was evolved in the form of B2O3–SiO2

J. Li et al. / Applied Surface Science 255 (2008) 1967–1974

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Fig. 9. X-ray diffraction patterns of the coating after oxidation for 70 h at 1300 8C. Fig. 8. SEM micrograph for cross-section of the coating after oxidation for 70 h at 1300 8C.

eutectic solutions [21], which rapidly covered the surface of the composite due to the fast oxidation rate of fine-grained B4C and Al dispersed uniformly in the coating. As reported in Refs. [21,22], the borosilicate glass with low volatility and oxygen permeability was a B2O3-rich phase for the relatively lower content of SiO2, and it could close the cracks and inhibit volatilization of B2O3 and oxidation of C/C substrate. Fig. 10 shows SEM micrographs for

crack on the surface of the coating oxidized at 1100 8C for 43 h, as can be seen that the crack with width of 2 mm was filled in glass, whose elements were Si, O, B and small amount of Al according to the EDX analysis. Thus, we could infer that the glass filled in cracks consisted of SiO2, B2O3 and small amount of Al2O3. Meanwhile, with presence of Al in this coating, an alumina–borosilicate glass was evolved, and this kind of glass had much lower volatility and

Fig. 10. SEM micrographs and EDX analysis for crack on the surface of the coating oxidized at 1100 8C for 43 h.

Fig. 11. SEM micrographs and EDX analysis for crack on the surface of the coating oxidized at 1300 8C for 57 h.

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J. Li et al. / Applied Surface Science 255 (2008) 1967–1974

oxygen permeability than the borosilicate and alumina-B2O3 glass, therefore much less weight loss rate resulted at this temperature region than that of 700 8C as shown in Fig. 6. Zone III (1200–1400 8C): Fig. 11 shows the SEM micrographs for crack with oxidation temperature of 1300 8C, as can be found that some glass in crack overflowed, and EDX analysis shows that glass in cracks consisted of SiO2, Al2O3 and B2O3 too. However, by contrast with that of 1100 8C, this kind of glass had much lower concentration of B2O3 because of the evaporation of some B2O3 and some additive SiO2 formed from oxidation of b-SiC inner layer, therefore the borosilicate glass was a SiO2-rich alumina– borosilicate, whose volatile was much less than B2O3-rich alumina–borosilicate. However, the glass was less compact than B2O3-rich alumina–borosilicate. Moreover, the thickness of this glass layer was reduced for evaporation of some B2O3, so that the oxygen permeability was higher than that of the B2O3-rich borosilicate glass and lager weight loss rates resulted at this temperature region. 4. Conclusions A SiC/aluminum–borosilicate double-layer coating was prepared by slurry pasted and heat-treated process to protect C/C composites. The isothermally oxidation curves of specimens with the double-layer coatings consisted of linear and parabolic steps, which could be described by Microflaws Model and Diffusion Model, respectively. And the parameters for both of models strongly depended on oxidation temperature and mechanics. There were three oxidation processes and oxidation mechanisms for the coated specimens: from 500 8C to 700 8C, oxidation of B4C and Al could form an aluminum–B2O3 glass layer to inhibit oxidation, but it could not operate for long time. From 800 8C to 1100 8C, the excellent anti-oxidation property of the coating at this temperature region was attributed to the formation of B2O3-rich aluminum–borosilicate glass. Finally, 1200–1400 8C, with evaporation of some B2O3, a kind of SiO2-rich aluminum–borosilicate glass formed, and it could protect the C/C composites up to 1300 8C. Acknowledgments This investigation was supported by the Program for New Century Excellent Talents in University (NCET 05-0195) and Beijing Municipal Science & Technology Program (Y040600 1040211). The authors would like to thank Dr. Jean Lachaud from the NASA Ames Research Center for giving us advice on the oxidation models. And also thank Dr. Gamal Mahmoud from

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