carbon composites for oxidation protection

NEW CARBON MATERIALS Volume 27, Issue 2, Apr 2012 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2012, 27(2):105–109.

RESEARCH PAPER

C/SiC/Si-Mo-Cr multilayer coating for carbon/carbon composites for oxidation protection ZHANG Yu-lei1,*, LI He-jun1, LI Ke-zhi1, FEI Jie1, ZENG Xie-rong2,3 1

C/C Composites Research Center, National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, China;

2

College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China;

3

Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, China

Abstract: To improve the oxidation resistance of carbon/carbon (C/C) composites at high temperature, the C/C composites were first slurry-coated by carbon, then pack cemented by mixtures of Si and graphite powder (60-80:10-25 wt/wt), followed by heat treatment to form C/C composites coated with a C/SiC layer, and finally a Si-Mo-Cr oxidation protective coating was prepared on the surface of the these composites by pack cementation. Scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy were used to characterize the compositions and microstructure of the coating. The oxidation behavior of the coated C/C composites at 1873 and 1 973 K in air was investigated. The coating showed excellent oxidation resistance at 1 873 K due to the formation of a glassy layer of SiO2 and Cr2O3 during the oxidation test, which could effectively protect C/C composites from oxidation for 135 h. The coating failed after oxidation for 30 h at 1973 K because the protective glass layer was disrupted. Key Words: Carbon/carbon composites; Coating; Oxidation

1

Introduction

Carbon/carbon (C/C) composites have attracted extensive attentions because of their excellent mechanical properties at high temperature, such as high specific strength and modulus, high creep resistance, and retention of mechanical properties at temperature above 2 473 K[1-2]. Therefore, they are one of the most promising thermal structural materials applied in aircraft and aerospace field. These applications require C/C composites to expose to an oxidizing environment at elevated temperature. However, C/C composites are susceptible of oxidation in the oxygen-containing environment with the formation of gaseous CO and CO2, which restricts them for wider applications[3-5]. Oxidation-resistant coatings, especially multilayer coatings, are considered the logical choice to protect C/C composites from oxidation at high temperature. Up to now, many multilayer coatings such as SiC/yttrium silicate/glass coating[6], SiC/mullite-Al2O3 coating[7] , and SiC/borosilicate glass coating[8] have been achieved by several researchers. Among these multilayer coatings, silicon carbide (SiC) was widely used as bonding layers between C/C composites and the outer layer because of its good physical and chemical adaptability of coating-to-matrix and bonding layer-to-outer layer [9]. Si-Mo-Cr alloy is a promising coating material for high tem-

perature application, which provided an outstanding oxidation resistance at high temperature, resulting from the formation of vitreous SiO2 and Cr2O3 glass layer during the oxidation. This kind of glass layer possesses high resistance of both volatilization and oxygen diffusion at high temperature[10-11]. In the previous research, Si-Mo-Cr coating was developed to protect C/C composites from oxidation by using slurry method [11]. However, the coating prepared by slurry method was not dense and could protect only C/C composites from oxidation at 1 773 K, and the bonding strength of the coating and C/C matrix was poor. Compared with slurry method, the pack cementation technique was easier to operate in one process, and the coatings applied by this approach exhibited a better adhesion to the matrix and a gradient coating with excellent thermal shock resistance can be easily provided [12-13]. The aim of this work is to develop a multilayer coating based on this Si-Mo-Cr alloy, which is expected to protect C/C composites above 1 773 K in air for long-term application. The C/SiC gradient layer acting as bonding layer was first prepared by the slurry and pack cementation and then the Si-Mo-Cr alloy outer coating was produced on the surface of the C/SiC-coated specimens by pack cementation. The microstructure and oxidation behavior of the as-prepared coating at 1 873 K and 1 973 K in air were primarily investigated.

Received date: 19 September 2011; Revised date: 1 April 2011 *Corresponding author. E-mail: [email protected] Copyright©2012, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(12)60006-7

ZHANG Yu-lei et al. / New Carbon Materials, 2012, 27(2): 105–110

2

Experimental

The bulk two-dimensional C/C composites with a density of 1.70 g/cm3 as substrates were cut into small specimens with the size of 10 mm×10 mm×10 mm. Before the coating process, the specimens were polished with 340 grit SiC paper and then were ultrasonically cleaned with ethanol and dried at 373 K for several hours. The C/SiC gradient inner layer was prepared by two step technique [14]. Firstly, the pre-coated carbon layer was prepared by slurry and high-temperature treatment method with phenol formaldehyde resin and graphite as raw materials. The powder composition (mass fraction) for preparing the C/SiC coating by pack cementation was as follows: 60-80% Si, 10-25% graphite and 5-15% Al2O3. The as-prepared carbon layer coated C/C specimens and the pack mixtures were heat-treated at 2 173 K for 2 h in an argon protective atmosphere to form the C/SiC inner coating. The pack cementation technique was used to prepare the Si-Mo-Cr outer coating on the surface of C/SiC-coated C/C composites. The commercially available powders of Si (300 mesh), Mo (500 mesh), Cr (500 mesh), and graphite (350 mesh) were weighed to the following composition (by mass fraction) 50-70% Si, 10-30% Mo, 5-20% Cr, and 10-30% graphite and then mixed by tumbling in a ball mill up to 30 h. The as-prepared C/SiC-coated C/C specimens and the above pack mixtures were put in a graphite crucible and then were heat-treated at 2 073-2 273 K for 2 h in an argon atmosphere to form Si-Mo-Cr outer coating. To investigate the isothermal oxidation behavior of the as-coated specimens, the oxidation resistance test was performed at 1 873 K and 1 973 K in air in an electrical furnace. The samples were put inside or taken out of the electrical furnace directly at certain period. Cumulative weight change of the samples after every thermal cycle from high temperature to room temperature was measured by a precision balance with a sensitivity of ±0.1 mg and was recorded as a function of time.

grey, and white were found in the coating. By EDS and XRD analysis, the brown, white, and grey phases could be distinguished as the SiC matrix, the mixture of MoSi2 and CrSi2, and residual Si, respectively. In addition, the as-obtained coating was very dense due to the effective filling of Si, MoSi2, and CrSi2 in the pinholes of the C/SiC gradient inner layer. During the heat treatment of pack cementation, it was easy for molten silicon to infiltrate into pinholes as a result of the increasing wettability and the decreasing viscosity of molten silicon with increasing temperature, and the molten silicon would react with Mo and Cr powder to form MoSi2 and CrSi2. The cross-section SEM image of the as-prepared coating is exhibited in Fig. 3. It is revealed that the multilayer coating was uneven in thickness, from 120 to 200 µm, and there were no penetration crack or big hole in the coating. In addition, there are no obvious interface between the Si-Mo-Cr outer layer and the C/SiC inner layer because of the effective infiltration

Fig.1 XRD pattern of the Si-Mo-Cr coating

The crystalline structures of the coated specimens were measured with X’Pert PRO X-ray diffraction (XRD). The morphologies and element distribution of the as-prepared coatings were analyzed by JSM-6460 scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

3

Results and discussion

3.1

Microstructure of the coating

Fig. 1 exhibits the XRD pattern of the Si-Mo-Cr coating surface. It could be seen that the phase compositions of the coating were composed of SiC, Si, MoSi2, and CrSi2. The MoSi2 and CrSi2 phases were the reacting products of Si, Mo, and Cr during the heat treatment of pack cementation, and the SiC phase mainly came from the C/SiC inner layer. Fig. 2 displays the backscattering electron image and spot EDS analyses of the Si-Mo-Cr coating surface. It was found that three kinds of crystalline particles characterized as brown,

Fig. 2 Surface backscattering electron SEM image and EDS analysis of the Si-Mo-Cr coating

ZHANG Yu-lei et al. / New Carbon Materials, 2012, 27(2): 105–110

formed because of the quick cooling from 1 873 K to room temperature during the oxidation test. And the escape of gases such as CO and CO2 resulted in the formation of the holes. These cracks and holes are disadvantageous to the oxidation protection of the coating[17]. Fig. 6b shows the cross-section SEM image of the coated specimen after oxidation at 1873 K for 135 h. It can be found that the C/C matrix was oxidized beneath the coating and a cavity was formed in C/C matrix below a penetration crack. Consequently, the oxidation of C/C sample at 1 873 K is primarily attributed to the formation of the penetration cracks in the coating. Further research on how to eliminate the penetration cracks is needed.

Fig. 3 Cross-section SEM micrograph of the C/SiC/Si-Mo-Cr multilayer coating

of outer coating powder into the inner layer. Thus, the dense Si-Mo-Cr exterior coating could be prepared by the pack cementation technique, which was expected to have good oxidation protective ability at elevated temperatures. 3.2

Oxidation protective ability of the coating

The isothermal oxidation curve of the coated C/C composites with Si-Mo-Cr coating in air at 1 773 K is illustrated in Fig. 4a. It could be found that the Si-Mo-Cr-coated specimens were characterized by an excellent oxidation-protective ability and could efficiently protect C/C composites from oxidation for 186 h. The corresponding weight loss and weight loss rate of the coated specimens was only 2.11% and 0.57×10-4 g/(cm2·h), respectively. The as-prepared Si-Mo-Cr alloy coating exhibited better oxidation-protective ability than the Si-Mo coating (2.47% after oxidation in 1 773 K in air for 103 h)[15]. Therefore, the introduction of Cr into the Si-Mo coating could improve the oxidation-protective ability of the Si-Mo coating. When the oxidation temperature was increased to 1 973 K, the coated specimens exhibited a less weight loss of 1.23% after oxidation for 12 h. With the extension in the oxidation time, the weight loss of the coated specimens increased quickly and reached 8.49% after oxidation for 30 h, the corresponding weight loss rate reached 10.28×10-4 g/cm2·h (Fig. 4b).

Fig. 4 The isothermal oxidation curves of the coated specimens at (a) 1 873 K and (b) 1 973 K in air

Fig. 5 shows the XRD pattern of the coating surface after oxidation at 1 873 K for 135 h. Compared with the phase composition of the coating before oxidation, two new phases of SiO2 and Cr2O3 appeared in the coating. The new stable compound oxide film including Cr2O3 and SiO2 is stable even at 1 873 K, indicating that the formation of the Cr2O3 improves the stability of SiO2 glass [16]. From the surface micrograph of the Si-Mo-Cr coating after oxidation at 1 873 K for 135 h (Fig. 6a), it can be found that the coating surface has changed into glass due to the formation of SiO2 and Cr2O3, and some microcracks and pinholes are also found from the coated surface. The cracks might have

Fig. 5 XRD pattern of as-prepared coating after oxidation for 135 h at 1 873 K

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973 K. In addition, at or above this temperature, the gas pressures of CO (g) produced by the reduction of SiO2 by carbon becomes more than one atmosphere. It would disrupt the protective glass coating[18-19] and result in the failure of the coating. In addition, the crystallization of the amorphous silica was also a main reason for the weight loss of the coated specimens at 1 973 K. It was accompanied by a volume contraction, which led to cracking and spalling of the glass layer[20].

4

Conclusions

A novel and effective Si-Mo-Cr oxidation-protective coating was produced on the surface of C/SiC-coated C/C composites by pack cementation. The as-prepared coating was characterized by an excellent oxidation-protective ability. It can effectively protect C/C composites for more than 135 h at 1 873 K and 12 h at 1 973 K in air. The oxidation-protective ability of the coating can be improved by introducing the Cr element. The failure of the coating is likely to be caused by the formation of the penetration cracks in it below 1 873 K and the disruption and crystallization of the compound glass layer at 1 973 K; the coating cannot provide effective protection for the C/C composites for a long time at temperature above 1 973 K. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 50902111, the NPU Foundation for Fundamental Research, the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.73-QP-2010), and the Shenzhen Key Laboratory of Special Functional Materials in Shenzhen University under Grant No. T201010.

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