carbon composites

Corrosion Science 67 (2013) 292–297

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Short Communication

Oxidation and ablation resistance of ZrB2–SiC–Si/B-modified SiC coating for carbon/carbon composites Feng Tao, Li He-Jun ⇑, Shi Xiao-Hong, Yang Xi, Wang Shao-Long State Key Laboratory of Solidification Processing, C/C Composites Technology Research Center, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China

a r t i c l e

i n f o

Article history: Received 29 June 2012 Accepted 24 October 2012 Available online 10 November 2012 Keywords: A. Ceramic matrix composites B. SEM B. X-ray diffraction C. Oxidation

a b s t r a c t ZrB2–SiC–Si/B-modified SiC coating was prepared on the surface of carbon/carbon (C/C) composites by two-step pack cementation. The coating could efficiently provide protection for C/C composites from oxidation and ablation. The improvement of oxidation resistance was attributed to the self-sealing property of the multilayer coating. A dense glassy oxide layer could afford the high temperature up to 2573 K and efficiently protect C/C composites from ablation. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Carbon/carbon (C/C) composites are attractive materials for high-temperature applications in aeronautical and aerospace fields because of their desirable properties at high temperatures [1,2]. These composites are required to undergo in extreme environments with high heat fluxes or high-temperature combustion gas. The work temperature usually ranges from room temperature to high temperatures [3,4]. Unfortunately, rapid oxidation above 723 K limits their application as structural materials in oxygencontaining environment [5]. Therefore, oxidation and ablation resistance of C/C composites should be improved to adapt such rigorous conditions. Applying coatings is considered as an effective method to prevent oxidation and ablation under such conditions [6–8]. At present, many coating systems, especially the silicide coatings have been explored to protect C/C composites against oxidation due to their inherent oxidation resistance and chemical compatibility with C/C composites [9–11]. Due to the mismatch of thermal expansion coefficient (CTE) between silicide coating and C/C composites, cracking is almost unavoidable in the coating during oxidation or preparation period [12]. These cracks can be sealed by SiO2 at high temperatures. However, the worse fluidity of SiO2 at intermediate temperatures cannot seal these cracks, resulting in the failure of the coating [13]. In addition, some holes and bubbles are generated in the SiO2 film as the service time prolongs or the work temperature increases, resulting in the failure of ⇑ Corresponding author. Tel.: +86 29 88495004; fax: +86 29 88492642. E-mail address: [email protected] (H.-J. Li). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.10.041

the coating [14]. Therefore, the protective temperature range of silicide coating is too narrow, which limits its application for protection of C/C composites under real environment. Multi-phase Boride and silicide ceramic, especially ZrB2–SiC–Si ceramic is the most promising coating material for the protection of C/C composites because of its high melting temperature, strength retention and superior oxidation and ablation resistance [15–17]. At temperature less than 1273 K, B2O3 oxidized from boride has low viscosity and high wettability to seal cracks in the coating, which can limit the oxygen diffusion. Moreover, ZrO2 from the oxidation of ZrB2 can improve the thermal stability of the SiO2 at higher temperatures, which can decrease the consumption of coating materials and increase the service life of the coating [18,19]. Moreover, free Si can increase the content of phase interface, which can relax the thermal stress and decrease the frequency of the cracks in the coating [20]. In order to further relax the thermal mismatch generated by the coatings for the C/C composites, multilayer coatings are usually designed and applied [6,7,14,19]. In our previous research, B-modified SiC coating has been explored and used as a bonding coating and secondary oxidation barrier beneath the outer coating, which can improve significantly the oxidation protective ability of the coating [21]. Up to now, oxidation or ablation protective coatings are usually explored separately in references. Limited literature is reported about a coating system providing good both oxidation and ablation protection for C/C composites. In this paper, the proposed ZrB2– SiC–Si/B-modified SiC coating was prepared on the surface of C/C composites by two-step pack cementation. The microstructures and oxidation resistance of the coating were investigated. The ablation resistance of the coating was also studied.

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2. Experimental

2.4. Characterization

2.1. Preparation of coated C/C composites

The crystalline structure of the coating was measured with Xray diffraction (XRD, X’Pert Pro MPD). The morphology and the element distribution of the coating were also analyzed by scanning electron microscope (SEM, JSM6460), equipped with energy dispersive spectroscopy (EDS).

Cubic specimens (15  15  15 mm) and plate specimens (U 30  10 mm) used as substrates were cut from a 2D C/C composite bulk with density of 1.7 g/cm3. All specimens were hand-abraded using 340 grit SiC paper, then watered by distilled water and dried at 283 K for 3 h. The B-modified SiC coating was prepared on the surface of C/C specimens by pack cementation in argon. The preparation details of the B-modified SiC coating were reported in reference [21]. The precursor powder of the ZrB2–SiC–Si outer coating for the second step pack cementation were mixed as follows: ZrB2 10–25 wt.%, Si 55–75 wt.%, graphite 5–10 wt.% and some additives. The as-prepared B-modified SiC coated C/C specimens and the second step mixtures were put in a graphite crucible, and then were heat-treated in argon at 2173–2273 K for 2–3 h.

3. Results and discussion 3.1. Microstructure of the coating Fig. 1 illustrates the SEM micrograph and X-ray pattern of the ZrB2–SiC–Si/B-modified SiC coating surface obtained from the two-step pack cementation. It is clear that the coating (Fig. 1(a)) is dense and no visible cracks appear. The corresponding XRD pattern (Fig. 1(b)) shows that the coating is composed of ZrB2, SiC and Si. In addition, the B-modified SiC bonding coating is composed of SiC, Si and B4C [21]. After the two-step coating preparation, ZrB2– SiC–Si outer layer covers the B-modified SiC inter layer, resulting in that B4C phase cannot be measured. Fig. 2 displays backscattering electron images along the crosssection of the ZrB2–SiC–Si/B-modified SiC coating. As shown in Fig. 2(b), it can be seen that there are three kinds of compositions in white, dark grey and grey color, respectively. By XRD and EDS analysis, the white, dark grey and grey are distinguished as ZrB2 (A), SiC (B) and Si (C), respectively. During the second step pack cementation, Si melts and penetrates easily into the B-modified SiC internal coating. The ZrB2 grains also penetrate into the B-modified SiC with the liquid Si. Therefore, the ZrB2 and Si are embedded into the B-modified SiC layer. Moreover, the average thickness of the coating is about 200 lm. No obvious gaps are found between layer-to-layer and coating-to-substrate, which indicates good interaction between them.

2.2. Oxidation test The oxidation test was carried out in air in an electrical furnace at 1173 and 1773 K, respectively. The cubic specimens were moved into the hot zone of the electric furnace, whereafter they were moved out and cooled to room temperature. Mass of the specimens was measured by an electronic precision balance with sensitivity of ±0.1 mg (Sartorious CP224S), and then they were moved into the furnace again for the next oxidation period. The final mass change rates of the specimens were the average oxidation rates of three specimens. 2.3. Ablation test The ablation test was tested in oxyacetylene torch with plate specimens, and the oxyacetylene torch flame was parallel to the axial orientation of specimens. The inner diameter of the oxyacetylene gun tip was 2 mm, and the distance between the gun tip and the specimens was 10 mm. The flux and pressure of O2 were 0.4 L/S and 0.4 MPa, and those of C2H2 were 0.4 L/S and 0.095 MPa, respectively. The heat flux on the specimens received from the flame was 4.2 MW/m2. The oxyacetylene flame was above 2573 K. The specimens were fixed in a water cooled copper concave device and exposed to the flame for 30 s in atmosphere. Both linear and mass ablation rate of the specimens were calculated by thickness and mass changes before and after ablation test of each specimen. The final ablation rates of the specimens were the average ablation rates of three specimens.

3.2. Oxidation resistance of the coated C/C composites The oxidation curve of the coated C/C composites at 1173 K in air is shown in Fig. 3. The mass loss of the SiC coated C/C specimens is 14% after oxidation for at 1173 K 10 h. After preparing ZrB2–SiC– Si external coating on the surface of the B-modified SiC coating, it is clear that the multi-layer ZrB2–SiC–Si/B-modified SiC coating can provide good oxidation protection for C/C composites. The mass loss of the coated specimens is less than 1.9% after oxidation at 1173 K for 50 h. In oxygen-containing environment, the coated C/C specimens will react with oxygen as follows:

a:ZrB2 b:SiC c:B4C b d:Si

b

Intensity

(b) b b

bb

(2) a

(a)

(1)

d

d

d

a b d

b

b b bc c

d

d

b a

b b

b

b

b

20 25 30 35 40 45 50 55 60 65 70 75 80 2θ/degree Fig. 1. SEM micrograph and X-ray patterns of the coating surface by two-step pack cementation: (a) surface micrograph; (b) X-ray pattern of the surface: (1) B-Modified SiC; (2) ZrB2–SiC–Si/B-modified SiC.

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(a)

(b)

Fig. 2. Cross-section backscattering electron image of the coating: (b) magnified the white rectangle area in (a).

15

SiC ZrB2-SiC-Si/B-modified SiC

14 13 12 11

Mass loss/%

10 9 8 7 6 5 4 3 2 1 0 0

5

10

15

20

25

30

35

40

45

50

55

Oxidation time/h Fig. 3. Isothermal oxidation curve of the coated C/C specimens in air at 1173 K.

2ZrB2 ðsÞ þ 5O2 ðgÞ ! 2ZrO2 ðsÞ þ 2B2 O3 ðlÞ

ð1Þ

SiCðsÞ þ 2O2 ðgÞ ! SiO2 ðsÞ þ CO2 ðgÞ

ð2Þ

B4 CðsÞ þ 4O2 ðgÞ ! 2B2 O3 ðlÞ þ CO2 ðgÞ

ð3Þ

SiðlÞ þ O2 ðgÞ ! SiO2 ðsÞ

ð4Þ

ZrO2 ðsÞ þ SiO2 ðsÞ ! ZrSiO4 ðsÞ

ð5Þ

2CðsÞ þ O2 ðgÞ ! 2COðgÞ

ð6Þ

CðsÞ þ O2 ðgÞ ! CO2 ðgÞ

ð7Þ

Reactions (1)–(5) lead to a net mass gain, and reactions (6) and (7) would cause a net mass loss. Therefore, the mass loss of the coated specimens during oxidation mainly results from the oxidation of the C/C substrate and the volatilization of oxidation production. The melting temperature of SiO2 from the oxidation product of SiC and Si is in range of 1273–1923 K [14,22], which results in that SiO2 cannot seal cracks in the coating. The oxidation of ZrB2 and B4C can produce molten B2O3. The molten B2O3 is primarily effective as a crack sealant from 873 to 1273 K [23]. Therefore, it indicates that the oxidation resistance of ZrB2–SiC–Si/B-modified SiC coating is significantly improved at 1173 K, due to the oxidation ability of ZrB2 and B4C. Compared with that of the coated C/C composites before oxidation (Fig. 1(a)), the crystalline particles become smaller as shown in Fig. 4(a), which can be inferred that the coating material is consumed gradually with extending oxidation time. Meanwhile, a continuous glass phase with some microcracks can be detected significantly on the surface of the crystalline particles and at the interface between crystalline particles. These microcracks are generated in the stage of quick cooling from 1173 K to room temperature, and can be sealed by the glass phase when the coating is heated to 1173 K again. Therefore, this continuous glass phase can efficiently prevent oxygen from diffusing in the coating during oxidation. Compared with the XRD patterns of the coating after oxidation (Fig. 4(b)), amorphous characteristics of B2O3 can be observed on the surface of the coating. It can be inferred that the B2O3 glass plays

a:ZrB2 b:SiC c:Si d: B2O3

b

Intensity

(b) b b b b a d

(a)

b

c b ab

b

bc

20 25 30 35 40 45 50 55 60 65 70 75 80 2θ/degree Fig. 4. SEM micrograph and X-ray pattern of the coating surface after isothermal oxidation at 1173 K for 50 h: (a) surface; (b) X-ray pattern.

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Fig. 5. Cross-section electron image of the coated C/C specimens after isothermal oxidation at 1173 K for 50 h.

a decisive role in the oxidation resistance of the coating at 1173 K. However, due to the mismatch of the CTE between the ceramic coating and C/C composites, some cracks especially penetrating cracks can be generated in the stage of quick cooling from 1173 K to room temperature. C/C matrix is oxidized by oxygen diffusing through these penetrating cracks in the coating before these cracks are sealed entirely by the glass phase. Oxidation mark can be found as shown in Fig. 5. So the failure of the coating is considered due to the formation of penetrating cracks in the stage of quick cooling. The oxidation curve of the coated C/C composites at 1773 K in air is shown in Fig. 6. It is clear that the mass loss of the SiC coated C/C specimens is up to 7.2% after oxidation for at 1773 K for only 20 h. Whereas the ZrB2–SiC–Si/B-modified SiC coating can effectively protect C/C composites from oxidation at 1773 K for 150 h, and no mass loss but slight mass gain of the coated specimens is 0.89%. A smooth and dense glass layer can be found on the coating surface after oxidation at 1773 K for 150 h, as shown in Fig. 7(a). In addition, some microcracks can also be found on the surface of the glass layer. These microcracks are generated when the specimens suffer thermal shock, and can be sealed by the glass at the oxidation temperature. From the XRD pattern of the coating after oxidation (Fig. 7(b)), characteristics of SiO2 and ZrSiO4 can be observed obviously. The ZrSiO4 (Fig. 5) dispersants can improve the stability of SiO2 glass and lower the oxygen diffusion in the glass [18,19],

8 7

SiC ZrB2-SiC-Si/B-modified SiC

Mass change/%

6 5 4 3 2 1 0 -1 -2

0

20

40

60 80 100 Oxidation time/h

120

140

160

Fig. 6. Isothermal oxidation curve of the coated C/C specimens in air at 1773 K (minus mass change means mass gain).

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which can improve the oxidation resistance of the coating. After oxidation at 1773 K for 150 h, the glass layer covers the whole coating surface and ZrB2 on the coating surface has been exhausted (Reaction (1)), resulting in that ZrB2 phase cannot be measured. Fig. 8 shows the cross-section electron images of the ZrB2–SiC– Si/B-modified SiC coating after oxidation at 1773 K for 150 h. As shown in Fig. 8(a), the thickness of the coating has no obvious change compared to that of the specimens before oxidation (Fig. 2), which can be inferred that the depletion of the coating is slight due to the protection of the glass layer. Moreover, no obvious oxidation mark is found, which can be inferred that the depletion of the coating materials results in the mass loss of the coated specimens. No debonding or large cracks are detected, which illuminates high bonding strength and good thermal compatibility between layer-to-layer and coating-to-substrate. As shown in Fig. 8(b), it is clear that three kinds of components in white, grey and dark grey color still exit, which can be inferred that the components of the coating have good chemical stability due to the protection of the glass layer during oxidation. 3.3. Ablation resistance of the coated C/C composites Without protection, the linear ablation and mass ablation rates of the pure C/C specimens are 2.79 lm/s and 3.55 mg/s, respectively. It can be seen that the pure C/C composites suffer severe ablation from oxyacetylene torch and are consumed rapidly, resulting in damages to the composite properties. With the protection of the ZrB2–SiC–Si/B-modified SiC coating, the ablation resistance of the C/C composites is improved significantly. The linear ablation and mass ablation rates of the coated C/C specimens can be lowered to 0.21 lm/s and 1.5 mg/s, respectively. From Fig. 9(a1), the glass layer with some micro-holes and cracks covers the coating surface and can efficiently protect C/C substrate in ablation center region. From the XRD pattern of the coating in ablation center region (Fig. 10), characteristics of SiO2 and ZrO2 can be observed obviously. It is reveals that the glass layer consists of a mixture of SiO2 and ZrO2. These molten oxidation products can absorb a large amount of heat to reduce temperature of the coating surface, which can weaken the oxyacetylene ablation. Cracks are resulted in from the rapid cooling from high temperature to room temperature. Micro-holes are formed due to the volatilization of B2O3. In addition, the ablative surface is very smooth and glassy and no pits or grooves are formed under the impact of the oxyacetylene flame, which can be referred that the coating materials are dense and can suffer the mechanical denudation from the flame. The substrate is not exposed in ablation center region. No flaking coating is found because of good thermal compatibility of the coating and C/C substrate and high bonding between the coating and substrate. Fig. 9(a2), there are two kinds of compositions in white and grey white phases, respectively. According to XRD and EDS analysis, the white and grey is distinguished as ZrO2 and SiO2. The formation of oxide layer can effectively suppress the depletion of the coating material and oxygen diffusion into the coating, which can be inferred that ZrO2 can improve the thermal stability of the SiO2 at higher temperatures. In addition, there is a large amount of white phases after ablation, which can be deduced that the depletion of the coating in ablation center region after ablation is more serious than the one after oxidation at 1773 K for 150 h. Compared with the morphology of the coating in ablation center region, the effect of ablation behavior has been weakened in ablation transition region, as shown in Fig. 9(b1). Moreover, the content of the white phase in ablation transition region is less than that in ablation center region, as shown in Fig. 9(b2). It reveals that the transition region of the material has lower temperature, pressure and flow rate because of the deviation from oxyacetylene flame center. Glass from oxidation of the

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a:SiO2 b:SiC c:ZrSiO4

b

Intensity

(b) a c

(a)

b b a a b cb

b b aa

caa

b

bb

a

20 25 30 35 40 45 50 55 60 65 70 75 80 2θ/degree Fig. 7. SEM micrograph and X-ray pattern of the coating surface after isothermal oxidation at 1773 K for 150 h: (a) surface; (b) X-ray pattern.

(a)

(b)

Fig. 8. Cross-section electron images of the coated C/C specimens after isothermal oxidation at 1773 K for 150 h: (a) scanning electron image; (b) backscattering electron image.

(a1)

(a2)

(b1)

(b2)

Fig. 9. SEM micrographs of the coating surface after ablation in different region: ablation center region: (a1) SEM; (a2) the corresponding backscattering image; ablation transition region: (b1) SEM; (b2) the corresponding backscattering image.

coating materials forms on the surface of the crystalline particles and at the interface between crystalline particles in ablation tran-

sition region, which can effectively prevent the further ablation into the internal coating.

T. Feng et al. / Corrosion Science 67 (2013) 292–297

a:SiO2 b:ZrO2 c:SiC

c

Intensity

a

c

b b

c

c ba a 20

25

30

35

40

45

b

bb a 50

55

60

c

c

b b 65

a 70

75

80

2 θ /degree Fig. 10. X-ray pattern of coating surface after ablation in ablation center region.

4. Conclusions Multi-layer ZrB2–SiC–Si/B-modified SiC coating has been prepared on the surface of carbon/carbon (C/C) composites by a two-step pack cementation method. The coating can protect C/C composites from oxidation at 1173 K for 50 h and at 1773 K for 150 h and from ablation at 2573 K for 60 s, respectively. The oxidation resistance of ZrB2–SiC–Si/B-modified SiC coating is improved at 1173 K, due to the oxidation ability of ZrB2 and B4C. The ZrSiO4 dispersants can improve the stability of SiO2 glass, which can improve the oxidation resistance of the coating at 1773 K. A dense glassy oxide layer can afford the high temperature up to 2573 K and efficiently protect C/C composites from ablation. Acknowledgements This work has been supported by the National Natural Science Foundation of China under Grant No. 50832004 and the ‘‘111’’ Project under Grant No. B08040. References [1] N.S. Jacobson, D.M. Curry, Oxidation microstructure studies of reinforced carbon/carbon, Carbon 44 (2006) 1142–1150.

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