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carbon composites under an oxyacetylene flame
Corrosion Science 53 (2011) 105–112
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The effect of zirconium carbide on ablation of carbon/carbon composites under an oxyacetylene flame Xue-Tao Shen, Ke-Zhi Li ⇑, He-Jun Li, Qian-Gang Fu, Shu-Ping Li, Fei Deng C/C Composites Technology Research Centre, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an Shaanxi 710072, PR China
a r t i c l e
i n f o
Article history: Received 23 April 2010 Accepted 2 September 2010 Available online 16 September 2010 Keywords: A. Ceramic matrix composites B. SEM C. Oxidation
a b s t r a c t Ablation of zirconium carbide (ZrC) modified carbon/carbon (C/C) composites was tested by an oxyacetylene torch. The formation of zirconia from the oxidation of ZrC improves the ablation resistance of the C/C composites because of the evaporation at elevated temperature, which absorbs heat from the flame and reduces the erosive attack to carbon. Zirconia also acts as an accelerator for carbon oxidation as it reacts with carbon during the ablation, increasing the mechanical breakage rate of the fibres. However, the effect of mechanical breakage is inferior in the ablation of the composites. The heterogeneous reactions control the ablation of the composites. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites are ideal candidates as aerospace materials in nozzles of solid rocket motor [1,2], and leading edges, heat shields and nose tips for hypersonic reentry vehicles [2,3] due to their unique properties such as low density, low coefficient of thermal expansion, high specific strength/modulus, excellent ablation and thermal shock resistance [4,5]. However, currently used C/C composites cannot meet the demand of the components (nozzles and nose tips, etc.) for new spacecrafts. Therefore, C/C composites with better ablation resistance are needed. The ablation resistance of C/C composites can be improved by infiltrating them with ultra-refractory materials [6], such as carbides/borides. Zirconium carbide (ZrC) is an advanced ceramic with a high melting point (3693 K), great hardness (25.5 GPa) and excellent mechanical stability [7]. ZrC modified C/C composites, which combine the advantages of both C/C and ZrC materials, is a possible solution to be used as high-temperature structural materials. Although many studies on refractory carbides or borides modified carbon-based composites have been reported, there is limited work on carbides/borides modified C/C composites. ZrC/C [8] and zirconium diboride/carbon (ZrB2/C) [9] composites were prepared from petroleum coke, coal tar pitch and zirconia (ZrO2)/ZrB2 powder by hot-pressing. Tang et al. [10] prepared ZrB2 based particles, combined with silicon carbide (SiC), hafnium carbide (HfC) and tantalum carbide (TaC), doped C/C composites by powder infiltration and isothermal chemical vapour infiltration (CVI). These results revealed that dense and uniform ZrO2 coatings formed on ⇑ Corresponding author. Tel./fax: +86 29 88495764. E-mail addresses: [email protected] (X.-T. Shen), [email protected] (K.-Z. Li). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.09.028
the substrate surface could act as effective oxygen barriers to hinder the diffusion of oxygen. Coatings such as ZrC [11–13] on carbon/graphite, TaC [14,15] on C/C or carbon fibres and HfC [16] on carbon fibres, were also prepared by chemical vapour deposition (CVD), but the correlative ablation behaviour of the coated carbon materials had not been reported. Sun et al. [17] prepared ZrC coating on C/C composites by CVD and tested its ablation resistance under an oxyacetylene torch. It indicated that a dense ZrO2 layer generated from the oxidation of the top ZrC coating acted as a thermal barrier and reduced the diffusion of oxygen. Although it is feasible to prepare refractory coatings on complex C/C composite components by CVD, the cost is very high and thermal expansion coefficients of the coatings and nozzle materials do not match. Moreover, it is not practical to manufacture complex-shape or large-size components by hot-pressing. So a new method is needed to prepare refractory materials modified C/C composites for large and complex components. It is also necessary to investigate the ablation mechanism for further optimizing the fabrication and structural design of the modified composites. In our previous work, HfC [18] and ZrC [19,20] modified C/C composites were developed using a novel, simple and mass-produced Hf/Zr precursor infiltration process. The preparation technology, microstructure and ablation properties of the modified C/C composites with low ZrC content (less than 4.1 wt.%) were studied. But the study is limited in ablation behaviour/mechanism of the composites. In the present study, more ZrC (the largest contents 14.1 wt.%) were infiltrated into C/C composites and the effect of ZrC on the ablation of C/C composites was investigated. The ablation behaviour/mechanism and microstructure of the as-produced materials were further discussed. In addition, for better understanding the ablation mechanism of these modified composites, the combustion gas composition
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of the oxyacetylene flame was estimated by the principle of free energy minimum, and the heterogeneous chemical reactions involved during the ablation tests were also studied. 2. Experimental 2.1. Material preparation T300 PAN-based carbon fibres (Toray Inc., Japan) were used to fabricate the integral felts. The integral felts were formed by alternatively stacking the carbon fabric, short-cut-fibre web as 0°/90°/ 0°/90° pierced with carbon fibre bundles in Z direction. The as-prepared integral felts (with apparent density of 0.20 g/cm3) were employed as the preforms of C/C composites. Zirconium oxychloride octahydrate (ZrOCl28H2O, analytical purity, manufactured by Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) was dissolved in water. ZrC modified C/C composites were prepared by the subsequent steps: immersing the performs into ZrOCl2 aqueous solution, heat treatment, densification via thermal gradient CVI with methane and graphitization. Details of the preparation were given elsewhere [19]. Zr compounds in the carbon felts were first decomposed to ZrO2 after heat treatment. Then ZrO2 was completely converted into ZrC after graphitization, via an in situ synthesis by carbothermal reduction reaction of ZrO2 and carbon (Eq. (1)). Unmodified C/C composites were also fabricated using the same process steps without the immersion of ZrOCl2 solution, for the sake of comparative analysis. The mass of ZrC was calculated from the mass of ZrO2 in C/C composites based on Eq. (1), and the content of ZrC in C/C composites was calculated by the ratio of the mass of ZrC and C/C composites (Eq. (2)). The as-produced samples were marked ZW-x, in which x indicates the mass content (wt.%) of ZrC in C/C composites. The apparent density (overall mass-to-volume ratio) of the samples is about 1.81–1.82 g/cm3. Some details of the samples are listed in Table 1.
ZrO2 ðsÞ þ 3CðsÞ ¼ ZrCðsÞ þ 2COðgÞ ZrCðwt:Þ% ¼ 0:838
ð1Þ
m1 m0 100% m
ð2Þ
where m1 is the mass of ZrO2-containing carbon felt, m0 is the mass of carbon felt before immersion, m1 m0 refers to the mass of ZrO2 in the carbon felt, 0.838 is the mass ratio of ZrO2 converting to ZrC, and m is the final mass of ZrC-containing C/C composites after graphitization.
oxyacetylene flame was parallel to the axial orientation of samples. The pressure and flux of O2 were 0.4 MPa and 0.42 L/s, and those of C2H2 were 0.095 MPa and 0.31 L/s, respectively. The heat flux on the samples received from the flame was 4.2 MW/m2. The inner diameter of the nozzle was 2 mm. The distance between the nozzle tip and the sample was 10 mm. The sample was fixed in a watercooled copper concave fixture and exposed to the flame with estimated temperature about 3273 K for 60 s. The erosion direction of the flame was parallel to Z direction of carbon felts. Both the linear and mass ablation rates were calculated by thickness and weight changes per unit time. The final ablation rates of the samples were the average ablation rates of three samples. 2.3. Characterization The phase and composition of the samples before and after ablation were identified by X-ray diffraction (XRD, X’Pert Pro MPD). The microstructure and morphology of the samples were analysed by a field emission scanning electron microscopy (FESEM, Zeiss-Supra 55) combined with energy dispersive spectroscopy (EDS, Oxford-INCAPentaFET3). 3. Results 3.1. Composition of combustion gas The equilibrium composition of oxyacetylene combustion products was calculated by the Chemical Equilibrium with Applications Code from NASA, which is a program calculating chemical thermodynamic equilibrium compositions by the principle of free energy minimum [21,22]. The combustion products of the oxyacetylene flame are listed in Table 2. It indicates that O2, CO2, O, OH and H2O are the main oxidising species for chemical attack to carbon. 3.2. Microstructure Fig. 2 shows backscattered electron images and EDS patterns of the cross sections of C/C samples with different ZrC contents before ablation. Fig. 3 shows XRD patterns of ZrC modified C/C composites before and after ablation. Lots of white spots in Fig. 2 are dispersed in the C/C composites and they increase with increasing ZrC contents. The white spots are ZrC particles according to Fig. 3 and the previous work [19]. The samples are mainly composed of ZrC
2.2. Ablation tests Cylindrical samples (U30 mm 10 mm) for ablation tests were machined from the as-produced composites. The axial orientation of samples was parallel to Z direction of carbon felts (needlepunching direction), while the radial direction was parallel to the XY plane of carbon felts, as shown in Fig. 1. The ablation properties of the samples were tested with an oxyacetylene torch, and the
Table 1 Density and porosity of the samples. Samples
The content of ZrC (wt.%)
Sample density (g/cm3)
Porosity (%)
ZW-0 ZW-3.2 ZW-4.1 ZW-5.0 ZW-8.7 ZW-12.1 ZW-14.1
0 3.2 4.1 5.0 8.7 12.1 14.1
1.80 1.81 1.81 1.82 1.82 1.82 1.82
7.5 7.4 7.3 7.2 7.3 7.3 7.4
Fig. 1. Schematic of C/C composite sample in the ablation test.
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Table 2 Chemical composition of the equilibrium combustion products. Species
Mole fractions (mol%)
O2 CO2 CO O OH H2O H H2
43.02 15.38 12.04 10.75 8.45 7.83 1.77 0.76
and carbon before ablation, while ZrO2 phase is formed after ablation. It indicates that ZrC is oxidised into ZrO2 during ablation. 3.3. Ablation properties
Fig. 3. XRD patterns of ZrC modified C/C composites before and after ablation.
The ablation properties of the samples are shown in Fig. 4. A similar tendency of decrease in ablation rates of the samples with more ZrC are found. At the beginning, the linear and mass ablation rates of the unmodified sample are 2.2 103 mm/s and 2.0 103 g/s, respectively. Then they decrease rapidly when ZrC contents are below 4.1 wt.%, followed by a slow decrease when ZrC contents are less than 12.1 wt.%, and finally keep close to constant with more ZrC. The linear and mass ablation rates of ZW-12.1 are 4.2 104 mm/s and 2.8 104 g/s, respectively. The linear and mass ablation rates of C/C composites are decreased after adding 12.1 wt.% ZrC by 80.6% and 85.9%, respectively. 3.4. Ablation morphology Fig. 5 shows the ablation morphology in the carbon fibre fabric area (where the fibres are perpendicular to the flame) and the EDS pattern of the samples. The micro particles with relatively low gray scale on the ablated surface are ZrO2 according to XRD (Fig. 3) and EDS (Fig. 5h) analysis. The fibres of the samples with ZrC contents blow 4.1 wt.% are ablated into sharp needle shapes (Fig. 5a–c), which are similar to those of carbon-based composites during oxidising ablation [23,24] or oxidation/sublimation process [25,26]. The sublimation temperature of C/C composites at 1 atm is about
Fig. 4. Ablation rate as a function of the content of ZrC.
4000 K and it increases with increasing pressure [27]. Sublimation is neglected as an ablation mechanism under the oxyacetylene flame, so that the formation of needle-shaped fibres is attributed
Fig. 2. Backscattered electron images and EDS patterns of the cross sections of ZrC modified C/C composites before ablation (a) ZW-4.1, (b) ZW-8.7 and (c) ZW-12.1.
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Fig. 5. Ablation morphology in the carbon fibre fabric area, where the fibres were perpendicular to the flame and EDS pattern of ZrC modified C/C composites after ablation (a) ZW-0, (b) ZW-3.2, (c) ZW-4.1, (d) ZW-5.0, (e) ZW-8.7, (f) ZW-12.1, (g) ZW-14.1 and (h) EDS pattern corresponding to 1, marked in (g).
to oxidation. Some fractured fibres with flat ends (Fig. 5a upper part) among the needle-shaped fibres are caused by the mechanical breakage of the flame. The fibres of the samples with ZrC P 5.0 wt.% have flat tips with little needle-shaped characteristic, along with a number of pits and grooves on their surfaces (Fig. 5d–g). When the fibres fracture at the pit positions, related grooves would be left on the matrix surface (Fig. 5g). Fig. 6 shows the ablation morphology of needling fibre bundle, parallel to the flame. The remaining fibres, matrices and ablated defects (cracks, holes) comprise the ablative surfaces of all the samples. The gaps between the fibres and matrix can be clearly observed, which indicates that the fibre–matrix interface has weak ablation resistance and apt to be oxidised. The carbon fibres in the unmodified sample (Fig. 6a) are ablated into cone shapes and
the matrix is ablated into a shell shape due to oxidation at the fibre–matrix interface and the contact surface between matrices, while the modified samples show different features. The tips of fibres in ZW-3.2 are ablated into obtuse shapes (Fig. 6b) and those in ZW-4.1 are flat. The fibre tips are not observed clearly and many fibres are detached from the matrix, with holes left on the surface of the matrix when ZrC contents are greater than 5.0 wt.% (Fig. 6d– f). The ablated matrix of the modified samples with ZrC blow 4.1 wt.% have the similar morphology (Fig. 6b and c) to those of the unmodified sample. More carbon matrix of the samples with ZrC above 5.0 wt.% are remained after ablation (Fig. 6d–f) because of the lower ablation rates of the carbon matrix and contact surface between matrices. Those phenomena, combined with the ablation rate curves (Fig. 4), indicate that the ablation resistance of the
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Fig. 6. Ablation morphology of the samples in needling fibre area parallel to the flame (a) ZW-0, (b) ZW-3.2, (c) ZW-4.1, (d) ZW-5.0, (e) ZW-8.7 and (f) ZW-12.1.
fibres decreases and that of the matrix obviously increases with the addition of ZrC, especially with ZrC content more than 5.0 wt.%. 4. Discussion 4.1. The role of ZrO2 in the ablation of ZrC modified C/C composites If ZrO2 accelerates the oxidation of carbon in some way, ZrO2 grains or frozen droplets may be found in the bottom of the pits. However, if ZrO2 would increase the ablation resistance of the carbon matrix, the grains or frozen droplets of ZrO2 may appear on the top of the pits. In our ablated samples, a large number of ZrO2 grains have been observed on the top of the pits (Fig. 5d–f) and some ZrO2 can be found at the bottom areas of the pits (Fig. 5c and e lower right part), which indicates that ZrO2 is more resistant to ablation than carbon under the oxyacetylene flame. It can be inferred that ZrO2 plays double roles during ablation, and the ablation resistance of adding ZrO2 is much superior to accelerating the oxidation of carbon. The evaporation of ZrO2 can absorb a large amount of heat and reduce the erosion of the flame at the surface of the sample, attributing to the ablation resistance of adding ZrO2. In addition, the particle size of ZrO2 lying at the bottom of the pits is much bigger. Moreover, almost all ZrO2 particles are embedded in the pits on the ablated surface when ZrC contents achieve 14.1 wt.% (Fig. 5g). Because the ZW-14.1 sample has the highest
ZrC content, more ZrO2 are produced from ZrC with the oxidising species of the flame during ablation. Fig 7 shows the high magnification SEM images of the matrix and fibres after ablation. The pits appear clearly on the surfaces of the fibres and matrix as well as contact surface between matrices. Smaller ZrO2 particles on the pit wall of the contact surface between matrices (Fig. 7a) can also be observed. Nano-size sub-pits inside the micro-size pits are embedded in the carbon matrix (Fig. 7b and 7c) and fibres (Fig. 7b). Most of ZrO2 grains lie on the top of the pits, while a few lie on the bottom of the pits, indicating again that ZrO2 plays a more important role in improving the ablation resistance than accelerating the oxidation of carbon. The interface of pyrocarbon and ZrC/ZrO2 is prone to be oxidised preferentially during ablation. With further ablation, the gaps between the pyrocarbon and ZrO2 grains become bigger. 4.2. Ablation mechanism of ZrC modified C/C composites Ablation is an erosive phenomenon with a material removal via thermal chemical, physical and mechanical actions by high-temperature, high-pressure and high-velocity combustion gases. The heterogeneous reactions (Eqs. (3)–(14)) between these oxidising species (O2, O, CO2, H2O and OH) and ZrC in C/C composites are calculated by FactSage (one of the largest fully integrated database computing systems in chemical thermodynamics), as follows.
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Fig. 7. High magnification SEM images of the matrix and fibres after ablation (a) matrix interface of ZW-5.0 around the needling fibres, (b) the fibres of ZW-5.0 in the carbon fibre fabric area, (c) matrix of ZW-8.7 around the needling fibres and (d) matrix around the carbon fibre fabric of ZW-12.1.
3 O2 ðgÞ þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ 2
ð3Þ
3O þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ
ð4Þ
3CO2 ðgÞ þ ZrCðsÞ ¼ ZrO2 ðlÞ þ 4COðgÞ
ð5Þ
3H2 OðgÞ þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ þ 3H2 ðgÞ
ð6Þ
3 3OH þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ þ H2 ðgÞ 2
ð7Þ
ZrO2 ðlÞ ¼ ZrO2 ðgÞ
ð8Þ
1 O2 ðgÞ þ CðsÞ ¼ COðgÞ 2
ð9Þ
O þ CðsÞ ¼ COðgÞ
ð10Þ
CO2 ðgÞ þ CðsÞ ¼ 2COðgÞ
ð11Þ
H2 OðgÞ þ CðsÞ ¼ COðgÞ þ H2 ðgÞ
ð12Þ
1 OH þ CðsÞ ¼ COðgÞ þ H2 ðgÞ 2
ð13Þ
CðsÞ þ O2 ðgÞ ¼ CO2 ðgÞ
ð14Þ
(14)). The calculated results are rightly accordant with those of XRD analysis (Fig. 3). It can be found that the oxidation of ZrC is an endothermic process with weight increasing, while carbon oxidation is an exothermal reaction and a weight loss process. The preferential oxidation of ZrC would absorb heat from the flame and decrease the thermal erosion of carbon materials. So the ablation resistance of C/C composites can be improved by the incorporation of ZrC. The evaporation of ZrO2 (Eq. (8)) from ZrC oxidation absorbs lots of heat from the flame and also reduces the erosive attack to carbon, which provides the improvement on the ablation resistance. Changes in Gibbs free energy of the reaction between the ZrO2 and carbon (Eq. (15)) are calculated as 0.23 kJ mol1 at 1930 K, 3.12 kJ mol1 at 1940 K, 437.72 kJ mol1 at 3300 K and always decrease with the increasing temperature. So ZrO2 can act as an accelerator for carbon oxidation at temperature above 1940 K. In addition, the products from these reactions are calculated in the presence of oxygen. We assume that: ZrO2 = 1 mol, C = 3 mol. Different oxygen contents (0.5, 1 and 1.5 mol) are given when keeping the contents of ZrO2 and carbon constant. The products were calculated in the presence of oxygen, as shown in Table 4. The amount of ZrO2, CO and CO2 increases and carbide of zirconium decreases with increasing oxygen contents. It indicates that, with the increase of oxygen content, the reaction rate of carbon and ZrO2 decreases, while that of carbon and oxygen increases.
ZrO2 þ 3C ¼ ZrC þ 2CO Both the changes of Gibbs free energy and absorbed heat of the above heterogeneous reactions are also calculated by FactSage software, as listed in Table 3. The feasibility of reactions is verified from the negative values of the changes of Gibbs free energy. It is obvious that changes of Gibbs free energy of the reactions between the oxidising species and ZrC are lower than those between the oxidising species and carbon, which indicates that ZrC preferentially reacts with the oxidising species. The solid ZrC is oxidised into liquid ZrO2 (Eqs. (3)–(7)) with a few gaseous ZrO2 (Eq. (8)), while carbon is changed into CO (Eqs. (9)–(13)) and CO2 (Eq.
ð15Þ
The formation of pits on the ablation surface is caused by adding ZrC in the C/C composites, which is very important and helpful to understand the ablation of ZrC modified C/C composites. The preferential oxidation of ZrC reduces the consumption of carbon and simultaneously create a new interface between carbon matrix and fibres which is an active area of oxidation. The active interface of the carbon matrix/fibre would be oxidised rapidly, resulting in a large consumption of carbon instantaneously. The accelerated oxidation of carbon may form on the ablation surface. Moreover, ZrO2 can react with carbon (Eq. (15)), also accelerating the oxidation of carbon, which is the main mechanism of accelerating oxidation of
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X.-T. Shen et al. / Corrosion Science 53 (2011) 105–112 Table 3 Changes in Gibbs free energy and absorbed heat of oxidation reactions between the oxidising species and ZrC as well as carbon at 3300 K. Oxidising species
Change in Gibbs free energy (DG(T)/kJ mol1)
Absorbed heat ((DH(T) DH(298.15))/kJ mol1)
ZrC
Carbon
ZrC
Carbon
O2 O CO2 H2O OH
736.91 839.68 725.66 557.68 712.42
391.55 425.80 387.79 331.80 383.38
102.38 78.87 66.94 5.16 115.40
19.76 27.59 31.57 52.16 15.42
Table 4 The effect of oxygen on the reaction of ZrO2 and carbon at 3300 K. Products
CO CO2 ZrO2 ZrC4
Equal amount of the products (mol) *ZrO + 3C 2
ZrO2 + 3C + 0.5O2
ZrO2 + 3C + 1O2
ZrO2 + 3C + 1.5O2
0.859 0.082 0.485 0.515
1.431 0.137 0.641 0.358
2.003 0.191 0.798 0.201
2.576 0.246 0.954 0.045
*ZrO = 1 mol, C = 3 mol, O = 0.5 mol, 1 mol and 1.5 mol for product calculation by 2 2 FactSage.
carbon fibres based on their ablated morphology of carbon fibres (Fig. 5d–g and Fig. 7b). With further oxidation, the interface between carbon and ZrC/ZrO2 becomes larger, and ZrO2 particles become smaller (Fig. 7d) because of the vapourization (Eq. (8)). Consequently, the honeycombed pits are formed on the ablation surface. However, some pits with different sizes can also be observed in the undoped carbon matrix, as shown in Fig. 5a. Some impurities (Na, K elements, etc.) may be introduced into the fibre felts during the preparation procedure, so that a few impurities or related active points would exist in the C/C composites after CVI and graphitization. This will lead to the formation of pits in the undoped carbon matrix. In summary, the formation of ZrO2 from ZrC oxidation improves the ablation resistance of C/C composites, but accelerates the oxidation carbon matrix during the ablation. The former is dominant. The evaporation of ZrO2 absorbs lots of heat from the flame and reduces the erosive attack to carbon. The formation of pits in the fibres reduces the fibre strength. So the fibres fracture in the pit positions by the shearing force of the flame and the mechanical breakage rate of the fibres increases after adding ZrC, which has negative influence to the ablation properties of the composites. However, the total ablation rates of C/C composites decrease after adding ZrC (Fig. 4), indicating the effect of mechanical breakage is inferior in the ablation of the C/C composites. The ablation of the composites are mainly controlled by the heterogeneous reactions. 5. Conclusions ZrC modified C/C composites were fabricated by subsequent steps: immersing the performs into ZrOCl2 aqueous solution, heat treatment, rapid densification via thermal gradient CVI with methane and graphitization. The ablation properties were tested by an oxyacetylene torch. The linear and mass ablation rates of ZW12.1 are 4.2 104 mm/s and 2.8 104 g/s, respectively. The linear and mass ablation rates of C/C composites are decreased by 80.6% and 85.9%, respectively after adding 12.1 wt.% ZrC. It is concluded that the samples with ZrC above 12.1 wt.% exhibit better ablation resistance. ZrO2 produced from ZrC oxidation plays double roles including improving the ablation resistance of C/C composites and accelerating the oxidation of carbon matrix during ablation. The former is attributed to the evaporation of ZrO2, which is superior to the effect of accelerating carbon oxidation. The evaporation of ZrO2
at elevated temperature absorbs lots of heat from the flame and reduces the erosive attack to carbon. The latter is mainly caused by the reaction between ZrO2 and carbon during ablation, which results in the formation of pits and increase of the mechanical breakage rate of the fibres. However, the effect of mechanical breakage is inferior in the ablation of the C/C composites, and the heterogeneous reactions mainly control the ablation of the composites. Acknowledgements This work has been supported by the National Natural Science Foundation of China under Grant Nos. 50902111, 50972120 and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. G8QT0222). References [1] M. Berdoyes, Snecma propulsion solide advanced technology SRM nozzles. History and future, in: 42nd Joint Propulsion Conference and Exhibit, Sacramento, CA, US, AIAA-2006-4596, 2006. [2] G. Savage, Carbon–carbon Composites, Chapman & Hall, London, 1993. [3] C.P. Leonard, R.M. Amundsen, W.E. Bruce III, Hyper-X hot structures design and comparison with flight data, in: AIAA/CIRA 13th International Space Planes and Hypersonics Systems and Technologies Conference, Capua, Italy, AIAA-20053438, 2005. [4] E. Fitzer, L.M. Manocha, Carbon Reinforcements and Carbon/Carbon Composites, Springer, Berlin, 1998. [5] J.D. Buckley, D.D. Edie, Carbon/Carbon Materials and Composites, Noyes Publications, New Jersey, 1993. [6] J.J. Choury, Carbon–carbon materials for nozzles of solid propellant rocket motors, in: 12th Propulsion Conference, Palo Alto, CA, AIAA-76-609, 1976. [7] H.O. Pierson, Handbook of Refractory Carbides and Nitride, Noyes Publications, New Jersey, 1996. p. 68. [8] Q.F. Tong, J.L. Shi, Y.Z. Song, Q.G. Guo, L. Liu, Resistance to ablation of pitchderived ZrC/C composites, Carbon 42 (2004) 2495–2500. [9] X.T. Li, J.L. Shi, G.B. Zhang, H. Zhang, Q.G. Guo, L. Liu, Effect of ZrB2 on the ablation properties of carbon composites, Mater. Lett. 60 (2006) 892–896. [10] S.F. Tang, J.Y. Deng, S.J. Wang, W.C. Liu, K. Yang, Ablation behaviors of ultrahigh temperature ceramic composites, Mater. Sci. Eng. A 465 (2007) 1–7. [11] Y.G. Wang, Q.M. Liu, J.L. Liu, L.T. Zhang, L.F. Cheng, Deposition mechanism for chemical vapor deposition of zirconium carbide coatings, J. Am. Ceram. Soc. 91 (2008) 1249–1252. [12] J.H. Park, C.H. Jung, D.J. Kim, J.Y. Park, Temperature dependency of the LPCVD growth of ZrC with the ZrCl4–CH4–H2 system, Surf. Coat. Technol. 203 (2008) 324–328. [13] J.H. Park, C.H. Jung, D.J. Kim, J.Y. Park, Effect of H2 dilution gas on the growth of ZrC during low pressure chemical vapor deposition in the ZrCl4–CH4–Ar system, Surf. Coat. Technol. 203 (2008) 87–90. [14] H.W. He, K.C. Zhou, X. Xiong, B.Y. Huang, Investigation on decomposition mechanism of tantalum ethylate precursor during formation of TaC on C/C composite material, Mater. Lett. 60 (2006) 3409–3412. [15] Z.K. Chen, X. Xiong, B.Y. Huang, G.D. Li, F. Zheng, P. Xiao, et al., Phase composition and morphology of TaC coating on carbon fibers by chemical vapor infiltration, Thin Solid Films 516 (2008) 8248–8254. [16] N.I. Baklanova, T.M. Zima, A.I. Boronin, S.V. Kosheev, A.Y. Titov, N.V. Isaeva, et al., Protective ceramic multilayer coatings for carbon fibers, Surf. Coat. Technol. 201 (2006) 2313–2319. [17] W. Sun, X. Xiong, B.Y. Huang, G.D. Li, H.B. Zhang, Z.K. Chen, et al., ZrC ablation protective coating for carbon/carbon composites, Carbon 47 (2009) 3368–3371. [18] S.P. Li, K.Z. Li, H.J. Li, Y.L. Li, Q.L. Yuan, Effect of HfC on the ablative and mechanical properties of C/C composites, Mater. Sci. Eng. A 517 (2009) 61–67. [19] X.T. Shen, K.Z. Li, H.J. Li, H.Y. Du, W.F. Cao, F.T. Lan, Microstructure and ablation properties of zirconium carbide doped carbon/carbon composites, Carbon 48 (2010) 344–351. [20] X.T. Shen, K.Z. Li, H.J. Li, F.T. Lan, T. Feng, Effect of ZrO2 from the oxidation of ZrC on ablation of ZrC modified carbon/carbon composites, J. Inorg. Mater. 24 (2009) 943–947.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
The effect of zirconium carbide on ablation of carbon/carbon composites under an oxyacetylene flame Xue-Tao Shen, Ke-Zhi Li ⇑, He-Jun Li, Qian-Gang Fu, Shu-Ping Li, Fei Deng C/C Composites Technology Research Centre, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an Shaanxi 710072, PR China
a r t i c l e
i n f o
Article history: Received 23 April 2010 Accepted 2 September 2010 Available online 16 September 2010 Keywords: A. Ceramic matrix composites B. SEM C. Oxidation
a b s t r a c t Ablation of zirconium carbide (ZrC) modified carbon/carbon (C/C) composites was tested by an oxyacetylene torch. The formation of zirconia from the oxidation of ZrC improves the ablation resistance of the C/C composites because of the evaporation at elevated temperature, which absorbs heat from the flame and reduces the erosive attack to carbon. Zirconia also acts as an accelerator for carbon oxidation as it reacts with carbon during the ablation, increasing the mechanical breakage rate of the fibres. However, the effect of mechanical breakage is inferior in the ablation of the composites. The heterogeneous reactions control the ablation of the composites. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Carbon/carbon (C/C) composites are ideal candidates as aerospace materials in nozzles of solid rocket motor [1,2], and leading edges, heat shields and nose tips for hypersonic reentry vehicles [2,3] due to their unique properties such as low density, low coefficient of thermal expansion, high specific strength/modulus, excellent ablation and thermal shock resistance [4,5]. However, currently used C/C composites cannot meet the demand of the components (nozzles and nose tips, etc.) for new spacecrafts. Therefore, C/C composites with better ablation resistance are needed. The ablation resistance of C/C composites can be improved by infiltrating them with ultra-refractory materials [6], such as carbides/borides. Zirconium carbide (ZrC) is an advanced ceramic with a high melting point (3693 K), great hardness (25.5 GPa) and excellent mechanical stability [7]. ZrC modified C/C composites, which combine the advantages of both C/C and ZrC materials, is a possible solution to be used as high-temperature structural materials. Although many studies on refractory carbides or borides modified carbon-based composites have been reported, there is limited work on carbides/borides modified C/C composites. ZrC/C [8] and zirconium diboride/carbon (ZrB2/C) [9] composites were prepared from petroleum coke, coal tar pitch and zirconia (ZrO2)/ZrB2 powder by hot-pressing. Tang et al. [10] prepared ZrB2 based particles, combined with silicon carbide (SiC), hafnium carbide (HfC) and tantalum carbide (TaC), doped C/C composites by powder infiltration and isothermal chemical vapour infiltration (CVI). These results revealed that dense and uniform ZrO2 coatings formed on ⇑ Corresponding author. Tel./fax: +86 29 88495764. E-mail addresses: [email protected] (X.-T. Shen), [email protected] (K.-Z. Li). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.09.028
the substrate surface could act as effective oxygen barriers to hinder the diffusion of oxygen. Coatings such as ZrC [11–13] on carbon/graphite, TaC [14,15] on C/C or carbon fibres and HfC [16] on carbon fibres, were also prepared by chemical vapour deposition (CVD), but the correlative ablation behaviour of the coated carbon materials had not been reported. Sun et al. [17] prepared ZrC coating on C/C composites by CVD and tested its ablation resistance under an oxyacetylene torch. It indicated that a dense ZrO2 layer generated from the oxidation of the top ZrC coating acted as a thermal barrier and reduced the diffusion of oxygen. Although it is feasible to prepare refractory coatings on complex C/C composite components by CVD, the cost is very high and thermal expansion coefficients of the coatings and nozzle materials do not match. Moreover, it is not practical to manufacture complex-shape or large-size components by hot-pressing. So a new method is needed to prepare refractory materials modified C/C composites for large and complex components. It is also necessary to investigate the ablation mechanism for further optimizing the fabrication and structural design of the modified composites. In our previous work, HfC [18] and ZrC [19,20] modified C/C composites were developed using a novel, simple and mass-produced Hf/Zr precursor infiltration process. The preparation technology, microstructure and ablation properties of the modified C/C composites with low ZrC content (less than 4.1 wt.%) were studied. But the study is limited in ablation behaviour/mechanism of the composites. In the present study, more ZrC (the largest contents 14.1 wt.%) were infiltrated into C/C composites and the effect of ZrC on the ablation of C/C composites was investigated. The ablation behaviour/mechanism and microstructure of the as-produced materials were further discussed. In addition, for better understanding the ablation mechanism of these modified composites, the combustion gas composition
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of the oxyacetylene flame was estimated by the principle of free energy minimum, and the heterogeneous chemical reactions involved during the ablation tests were also studied. 2. Experimental 2.1. Material preparation T300 PAN-based carbon fibres (Toray Inc., Japan) were used to fabricate the integral felts. The integral felts were formed by alternatively stacking the carbon fabric, short-cut-fibre web as 0°/90°/ 0°/90° pierced with carbon fibre bundles in Z direction. The as-prepared integral felts (with apparent density of 0.20 g/cm3) were employed as the preforms of C/C composites. Zirconium oxychloride octahydrate (ZrOCl28H2O, analytical purity, manufactured by Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) was dissolved in water. ZrC modified C/C composites were prepared by the subsequent steps: immersing the performs into ZrOCl2 aqueous solution, heat treatment, densification via thermal gradient CVI with methane and graphitization. Details of the preparation were given elsewhere [19]. Zr compounds in the carbon felts were first decomposed to ZrO2 after heat treatment. Then ZrO2 was completely converted into ZrC after graphitization, via an in situ synthesis by carbothermal reduction reaction of ZrO2 and carbon (Eq. (1)). Unmodified C/C composites were also fabricated using the same process steps without the immersion of ZrOCl2 solution, for the sake of comparative analysis. The mass of ZrC was calculated from the mass of ZrO2 in C/C composites based on Eq. (1), and the content of ZrC in C/C composites was calculated by the ratio of the mass of ZrC and C/C composites (Eq. (2)). The as-produced samples were marked ZW-x, in which x indicates the mass content (wt.%) of ZrC in C/C composites. The apparent density (overall mass-to-volume ratio) of the samples is about 1.81–1.82 g/cm3. Some details of the samples are listed in Table 1.
ZrO2 ðsÞ þ 3CðsÞ ¼ ZrCðsÞ þ 2COðgÞ ZrCðwt:Þ% ¼ 0:838
ð1Þ
m1 m0 100% m
ð2Þ
where m1 is the mass of ZrO2-containing carbon felt, m0 is the mass of carbon felt before immersion, m1 m0 refers to the mass of ZrO2 in the carbon felt, 0.838 is the mass ratio of ZrO2 converting to ZrC, and m is the final mass of ZrC-containing C/C composites after graphitization.
oxyacetylene flame was parallel to the axial orientation of samples. The pressure and flux of O2 were 0.4 MPa and 0.42 L/s, and those of C2H2 were 0.095 MPa and 0.31 L/s, respectively. The heat flux on the samples received from the flame was 4.2 MW/m2. The inner diameter of the nozzle was 2 mm. The distance between the nozzle tip and the sample was 10 mm. The sample was fixed in a watercooled copper concave fixture and exposed to the flame with estimated temperature about 3273 K for 60 s. The erosion direction of the flame was parallel to Z direction of carbon felts. Both the linear and mass ablation rates were calculated by thickness and weight changes per unit time. The final ablation rates of the samples were the average ablation rates of three samples. 2.3. Characterization The phase and composition of the samples before and after ablation were identified by X-ray diffraction (XRD, X’Pert Pro MPD). The microstructure and morphology of the samples were analysed by a field emission scanning electron microscopy (FESEM, Zeiss-Supra 55) combined with energy dispersive spectroscopy (EDS, Oxford-INCAPentaFET3). 3. Results 3.1. Composition of combustion gas The equilibrium composition of oxyacetylene combustion products was calculated by the Chemical Equilibrium with Applications Code from NASA, which is a program calculating chemical thermodynamic equilibrium compositions by the principle of free energy minimum [21,22]. The combustion products of the oxyacetylene flame are listed in Table 2. It indicates that O2, CO2, O, OH and H2O are the main oxidising species for chemical attack to carbon. 3.2. Microstructure Fig. 2 shows backscattered electron images and EDS patterns of the cross sections of C/C samples with different ZrC contents before ablation. Fig. 3 shows XRD patterns of ZrC modified C/C composites before and after ablation. Lots of white spots in Fig. 2 are dispersed in the C/C composites and they increase with increasing ZrC contents. The white spots are ZrC particles according to Fig. 3 and the previous work [19]. The samples are mainly composed of ZrC
2.2. Ablation tests Cylindrical samples (U30 mm 10 mm) for ablation tests were machined from the as-produced composites. The axial orientation of samples was parallel to Z direction of carbon felts (needlepunching direction), while the radial direction was parallel to the XY plane of carbon felts, as shown in Fig. 1. The ablation properties of the samples were tested with an oxyacetylene torch, and the
Table 1 Density and porosity of the samples. Samples
The content of ZrC (wt.%)
Sample density (g/cm3)
Porosity (%)
ZW-0 ZW-3.2 ZW-4.1 ZW-5.0 ZW-8.7 ZW-12.1 ZW-14.1
0 3.2 4.1 5.0 8.7 12.1 14.1
1.80 1.81 1.81 1.82 1.82 1.82 1.82
7.5 7.4 7.3 7.2 7.3 7.3 7.4
Fig. 1. Schematic of C/C composite sample in the ablation test.
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Table 2 Chemical composition of the equilibrium combustion products. Species
Mole fractions (mol%)
O2 CO2 CO O OH H2O H H2
43.02 15.38 12.04 10.75 8.45 7.83 1.77 0.76
and carbon before ablation, while ZrO2 phase is formed after ablation. It indicates that ZrC is oxidised into ZrO2 during ablation. 3.3. Ablation properties
Fig. 3. XRD patterns of ZrC modified C/C composites before and after ablation.
The ablation properties of the samples are shown in Fig. 4. A similar tendency of decrease in ablation rates of the samples with more ZrC are found. At the beginning, the linear and mass ablation rates of the unmodified sample are 2.2 103 mm/s and 2.0 103 g/s, respectively. Then they decrease rapidly when ZrC contents are below 4.1 wt.%, followed by a slow decrease when ZrC contents are less than 12.1 wt.%, and finally keep close to constant with more ZrC. The linear and mass ablation rates of ZW-12.1 are 4.2 104 mm/s and 2.8 104 g/s, respectively. The linear and mass ablation rates of C/C composites are decreased after adding 12.1 wt.% ZrC by 80.6% and 85.9%, respectively. 3.4. Ablation morphology Fig. 5 shows the ablation morphology in the carbon fibre fabric area (where the fibres are perpendicular to the flame) and the EDS pattern of the samples. The micro particles with relatively low gray scale on the ablated surface are ZrO2 according to XRD (Fig. 3) and EDS (Fig. 5h) analysis. The fibres of the samples with ZrC contents blow 4.1 wt.% are ablated into sharp needle shapes (Fig. 5a–c), which are similar to those of carbon-based composites during oxidising ablation [23,24] or oxidation/sublimation process [25,26]. The sublimation temperature of C/C composites at 1 atm is about
Fig. 4. Ablation rate as a function of the content of ZrC.
4000 K and it increases with increasing pressure [27]. Sublimation is neglected as an ablation mechanism under the oxyacetylene flame, so that the formation of needle-shaped fibres is attributed
Fig. 2. Backscattered electron images and EDS patterns of the cross sections of ZrC modified C/C composites before ablation (a) ZW-4.1, (b) ZW-8.7 and (c) ZW-12.1.
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Fig. 5. Ablation morphology in the carbon fibre fabric area, where the fibres were perpendicular to the flame and EDS pattern of ZrC modified C/C composites after ablation (a) ZW-0, (b) ZW-3.2, (c) ZW-4.1, (d) ZW-5.0, (e) ZW-8.7, (f) ZW-12.1, (g) ZW-14.1 and (h) EDS pattern corresponding to 1, marked in (g).
to oxidation. Some fractured fibres with flat ends (Fig. 5a upper part) among the needle-shaped fibres are caused by the mechanical breakage of the flame. The fibres of the samples with ZrC P 5.0 wt.% have flat tips with little needle-shaped characteristic, along with a number of pits and grooves on their surfaces (Fig. 5d–g). When the fibres fracture at the pit positions, related grooves would be left on the matrix surface (Fig. 5g). Fig. 6 shows the ablation morphology of needling fibre bundle, parallel to the flame. The remaining fibres, matrices and ablated defects (cracks, holes) comprise the ablative surfaces of all the samples. The gaps between the fibres and matrix can be clearly observed, which indicates that the fibre–matrix interface has weak ablation resistance and apt to be oxidised. The carbon fibres in the unmodified sample (Fig. 6a) are ablated into cone shapes and
the matrix is ablated into a shell shape due to oxidation at the fibre–matrix interface and the contact surface between matrices, while the modified samples show different features. The tips of fibres in ZW-3.2 are ablated into obtuse shapes (Fig. 6b) and those in ZW-4.1 are flat. The fibre tips are not observed clearly and many fibres are detached from the matrix, with holes left on the surface of the matrix when ZrC contents are greater than 5.0 wt.% (Fig. 6d– f). The ablated matrix of the modified samples with ZrC blow 4.1 wt.% have the similar morphology (Fig. 6b and c) to those of the unmodified sample. More carbon matrix of the samples with ZrC above 5.0 wt.% are remained after ablation (Fig. 6d–f) because of the lower ablation rates of the carbon matrix and contact surface between matrices. Those phenomena, combined with the ablation rate curves (Fig. 4), indicate that the ablation resistance of the
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Fig. 6. Ablation morphology of the samples in needling fibre area parallel to the flame (a) ZW-0, (b) ZW-3.2, (c) ZW-4.1, (d) ZW-5.0, (e) ZW-8.7 and (f) ZW-12.1.
fibres decreases and that of the matrix obviously increases with the addition of ZrC, especially with ZrC content more than 5.0 wt.%. 4. Discussion 4.1. The role of ZrO2 in the ablation of ZrC modified C/C composites If ZrO2 accelerates the oxidation of carbon in some way, ZrO2 grains or frozen droplets may be found in the bottom of the pits. However, if ZrO2 would increase the ablation resistance of the carbon matrix, the grains or frozen droplets of ZrO2 may appear on the top of the pits. In our ablated samples, a large number of ZrO2 grains have been observed on the top of the pits (Fig. 5d–f) and some ZrO2 can be found at the bottom areas of the pits (Fig. 5c and e lower right part), which indicates that ZrO2 is more resistant to ablation than carbon under the oxyacetylene flame. It can be inferred that ZrO2 plays double roles during ablation, and the ablation resistance of adding ZrO2 is much superior to accelerating the oxidation of carbon. The evaporation of ZrO2 can absorb a large amount of heat and reduce the erosion of the flame at the surface of the sample, attributing to the ablation resistance of adding ZrO2. In addition, the particle size of ZrO2 lying at the bottom of the pits is much bigger. Moreover, almost all ZrO2 particles are embedded in the pits on the ablated surface when ZrC contents achieve 14.1 wt.% (Fig. 5g). Because the ZW-14.1 sample has the highest
ZrC content, more ZrO2 are produced from ZrC with the oxidising species of the flame during ablation. Fig 7 shows the high magnification SEM images of the matrix and fibres after ablation. The pits appear clearly on the surfaces of the fibres and matrix as well as contact surface between matrices. Smaller ZrO2 particles on the pit wall of the contact surface between matrices (Fig. 7a) can also be observed. Nano-size sub-pits inside the micro-size pits are embedded in the carbon matrix (Fig. 7b and 7c) and fibres (Fig. 7b). Most of ZrO2 grains lie on the top of the pits, while a few lie on the bottom of the pits, indicating again that ZrO2 plays a more important role in improving the ablation resistance than accelerating the oxidation of carbon. The interface of pyrocarbon and ZrC/ZrO2 is prone to be oxidised preferentially during ablation. With further ablation, the gaps between the pyrocarbon and ZrO2 grains become bigger. 4.2. Ablation mechanism of ZrC modified C/C composites Ablation is an erosive phenomenon with a material removal via thermal chemical, physical and mechanical actions by high-temperature, high-pressure and high-velocity combustion gases. The heterogeneous reactions (Eqs. (3)–(14)) between these oxidising species (O2, O, CO2, H2O and OH) and ZrC in C/C composites are calculated by FactSage (one of the largest fully integrated database computing systems in chemical thermodynamics), as follows.
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Fig. 7. High magnification SEM images of the matrix and fibres after ablation (a) matrix interface of ZW-5.0 around the needling fibres, (b) the fibres of ZW-5.0 in the carbon fibre fabric area, (c) matrix of ZW-8.7 around the needling fibres and (d) matrix around the carbon fibre fabric of ZW-12.1.
3 O2 ðgÞ þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ 2
ð3Þ
3O þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ
ð4Þ
3CO2 ðgÞ þ ZrCðsÞ ¼ ZrO2 ðlÞ þ 4COðgÞ
ð5Þ
3H2 OðgÞ þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ þ 3H2 ðgÞ
ð6Þ
3 3OH þ ZrCðsÞ ¼ ZrO2 ðlÞ þ COðgÞ þ H2 ðgÞ 2
ð7Þ
ZrO2 ðlÞ ¼ ZrO2 ðgÞ
ð8Þ
1 O2 ðgÞ þ CðsÞ ¼ COðgÞ 2
ð9Þ
O þ CðsÞ ¼ COðgÞ
ð10Þ
CO2 ðgÞ þ CðsÞ ¼ 2COðgÞ
ð11Þ
H2 OðgÞ þ CðsÞ ¼ COðgÞ þ H2 ðgÞ
ð12Þ
1 OH þ CðsÞ ¼ COðgÞ þ H2 ðgÞ 2
ð13Þ
CðsÞ þ O2 ðgÞ ¼ CO2 ðgÞ
ð14Þ
(14)). The calculated results are rightly accordant with those of XRD analysis (Fig. 3). It can be found that the oxidation of ZrC is an endothermic process with weight increasing, while carbon oxidation is an exothermal reaction and a weight loss process. The preferential oxidation of ZrC would absorb heat from the flame and decrease the thermal erosion of carbon materials. So the ablation resistance of C/C composites can be improved by the incorporation of ZrC. The evaporation of ZrO2 (Eq. (8)) from ZrC oxidation absorbs lots of heat from the flame and also reduces the erosive attack to carbon, which provides the improvement on the ablation resistance. Changes in Gibbs free energy of the reaction between the ZrO2 and carbon (Eq. (15)) are calculated as 0.23 kJ mol1 at 1930 K, 3.12 kJ mol1 at 1940 K, 437.72 kJ mol1 at 3300 K and always decrease with the increasing temperature. So ZrO2 can act as an accelerator for carbon oxidation at temperature above 1940 K. In addition, the products from these reactions are calculated in the presence of oxygen. We assume that: ZrO2 = 1 mol, C = 3 mol. Different oxygen contents (0.5, 1 and 1.5 mol) are given when keeping the contents of ZrO2 and carbon constant. The products were calculated in the presence of oxygen, as shown in Table 4. The amount of ZrO2, CO and CO2 increases and carbide of zirconium decreases with increasing oxygen contents. It indicates that, with the increase of oxygen content, the reaction rate of carbon and ZrO2 decreases, while that of carbon and oxygen increases.
ZrO2 þ 3C ¼ ZrC þ 2CO Both the changes of Gibbs free energy and absorbed heat of the above heterogeneous reactions are also calculated by FactSage software, as listed in Table 3. The feasibility of reactions is verified from the negative values of the changes of Gibbs free energy. It is obvious that changes of Gibbs free energy of the reactions between the oxidising species and ZrC are lower than those between the oxidising species and carbon, which indicates that ZrC preferentially reacts with the oxidising species. The solid ZrC is oxidised into liquid ZrO2 (Eqs. (3)–(7)) with a few gaseous ZrO2 (Eq. (8)), while carbon is changed into CO (Eqs. (9)–(13)) and CO2 (Eq.
ð15Þ
The formation of pits on the ablation surface is caused by adding ZrC in the C/C composites, which is very important and helpful to understand the ablation of ZrC modified C/C composites. The preferential oxidation of ZrC reduces the consumption of carbon and simultaneously create a new interface between carbon matrix and fibres which is an active area of oxidation. The active interface of the carbon matrix/fibre would be oxidised rapidly, resulting in a large consumption of carbon instantaneously. The accelerated oxidation of carbon may form on the ablation surface. Moreover, ZrO2 can react with carbon (Eq. (15)), also accelerating the oxidation of carbon, which is the main mechanism of accelerating oxidation of
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X.-T. Shen et al. / Corrosion Science 53 (2011) 105–112 Table 3 Changes in Gibbs free energy and absorbed heat of oxidation reactions between the oxidising species and ZrC as well as carbon at 3300 K. Oxidising species
Change in Gibbs free energy (DG(T)/kJ mol1)
Absorbed heat ((DH(T) DH(298.15))/kJ mol1)
ZrC
Carbon
ZrC
Carbon
O2 O CO2 H2O OH
736.91 839.68 725.66 557.68 712.42
391.55 425.80 387.79 331.80 383.38
102.38 78.87 66.94 5.16 115.40
19.76 27.59 31.57 52.16 15.42
Table 4 The effect of oxygen on the reaction of ZrO2 and carbon at 3300 K. Products
CO CO2 ZrO2 ZrC4
Equal amount of the products (mol) *ZrO + 3C 2
ZrO2 + 3C + 0.5O2
ZrO2 + 3C + 1O2
ZrO2 + 3C + 1.5O2
0.859 0.082 0.485 0.515
1.431 0.137 0.641 0.358
2.003 0.191 0.798 0.201
2.576 0.246 0.954 0.045
*ZrO = 1 mol, C = 3 mol, O = 0.5 mol, 1 mol and 1.5 mol for product calculation by 2 2 FactSage.
carbon fibres based on their ablated morphology of carbon fibres (Fig. 5d–g and Fig. 7b). With further oxidation, the interface between carbon and ZrC/ZrO2 becomes larger, and ZrO2 particles become smaller (Fig. 7d) because of the vapourization (Eq. (8)). Consequently, the honeycombed pits are formed on the ablation surface. However, some pits with different sizes can also be observed in the undoped carbon matrix, as shown in Fig. 5a. Some impurities (Na, K elements, etc.) may be introduced into the fibre felts during the preparation procedure, so that a few impurities or related active points would exist in the C/C composites after CVI and graphitization. This will lead to the formation of pits in the undoped carbon matrix. In summary, the formation of ZrO2 from ZrC oxidation improves the ablation resistance of C/C composites, but accelerates the oxidation carbon matrix during the ablation. The former is dominant. The evaporation of ZrO2 absorbs lots of heat from the flame and reduces the erosive attack to carbon. The formation of pits in the fibres reduces the fibre strength. So the fibres fracture in the pit positions by the shearing force of the flame and the mechanical breakage rate of the fibres increases after adding ZrC, which has negative influence to the ablation properties of the composites. However, the total ablation rates of C/C composites decrease after adding ZrC (Fig. 4), indicating the effect of mechanical breakage is inferior in the ablation of the C/C composites. The ablation of the composites are mainly controlled by the heterogeneous reactions. 5. Conclusions ZrC modified C/C composites were fabricated by subsequent steps: immersing the performs into ZrOCl2 aqueous solution, heat treatment, rapid densification via thermal gradient CVI with methane and graphitization. The ablation properties were tested by an oxyacetylene torch. The linear and mass ablation rates of ZW12.1 are 4.2 104 mm/s and 2.8 104 g/s, respectively. The linear and mass ablation rates of C/C composites are decreased by 80.6% and 85.9%, respectively after adding 12.1 wt.% ZrC. It is concluded that the samples with ZrC above 12.1 wt.% exhibit better ablation resistance. ZrO2 produced from ZrC oxidation plays double roles including improving the ablation resistance of C/C composites and accelerating the oxidation of carbon matrix during ablation. The former is attributed to the evaporation of ZrO2, which is superior to the effect of accelerating carbon oxidation. The evaporation of ZrO2
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