Carbon 41 (2003) 2825–2829 Mullite-Al 2 O 3 -SiC oxidation protective coating for carbon / carbon composites Huang Jian-Feng a , Zeng Xie-Rong b , Li He-Jun a , *, Xiong Xin-Bo a , Huang Min a a C /C Composites Technology Research Center, Northwestern Polytechnical University, Xi’ an, Shannxi 710072, PR China b Department of Materials Science, Shenzhen University, Shenzhen 518060, PR China Received 6 June 2003; accepted 18 August 2003 Abstract In order to exploit the unique high temperature mechanical properties of carbon / carbon (C / C) composites, a new type of oxidation protective coating has been produced by a two-step pack cementation technique in an argon atmosphere. XRD analysis showed that the internal coating obtained from the ﬁrst step was a gradient SiC layer that acts as a buffer layer, and the multi-layer coating formed in the second step was an Al 2 O 3 -mullite layer. It was found that the as-received coating characterized by excellent thermal shock resistance on the surface of C / C composites during exposure to an oxidizing atmosphere at 1873 K, could effectively protect the C / C composites from oxidation for 45 h. The failure of the coating is due to the formation of bubble holes on the coating surface. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Carbon / carbon composites; B. Coating; Oxidation 1. Introduction Si 3 N 4 ceramics from oxidation by employing pulsed-laser deposition (PLD), chemical vapor deposition (CVD) and As an important high temperature structural material, plasma spraying technique [4–8]. carbon / carbon (C / C) composites offer many advantages Pack cementation is a good method to deposit coatings over traditional materials, such as low density, high on all surfaces of the sample, such as to deposit a SiC strength-to-weight ratio and retention of mechanical prop- coating on the surface of a carbon material , and it is erties at high temperature. Therefore, they can be used in widely used in producing oxidation protective coatings on aircraft and aerospace industries. These applications re- metal surfaces . But up to now, we could ﬁnd no report quire the C / C composites to operate in an oxidizing about using the pack cementation technique to produce a atmosphere. Unfortunately, C / C composites are prone to mullite-containing coating, especially on C / C surfaces. oxidize at ambient environment with the temperature In the present work, a new kind of multi-coating above 723 K and the oxidation rate increases quickly with including mullite, Al 2 O 3 and SiC on C / C composites was the temperature increase [1,2]. produced by a two-step pack cementation method. The Oxidation resistant coatings are the logical choice for structure and oxidation protection properties of the coating protecting C / C composites at high temperatures. Mullite were primarily investigated. ceramic (3Al 2 O 3 ?2SiO 2 ) coatings are promising candidate materials for high temperature applications. Mullite coated SiC exhibits excellent oxidizing resistance in an oxidizing 2. Experimental atmosphere by forming a slowly growing SiO 2 scale at the mullite–SiC interface . In recent papers, mullite coat- Small specimens (14310310 mm 3 ) used as substrates ings were developed to protect carbon materials, SiC and were cut from bulk 2D-C / C composites with a density of 1.8 g / cm 3 . Before carrying out the pack cementation *Corresponding author. Tel.: 186-298-495-004; fax: 186- procedure, the specimens were hand-polished using 340 298-495-240. grit SiC paper, then cleaned with distilled water and dried E-mail address: firstname.lastname@example.org (L. He-Jun). at 353 K for 2 h. Pack compositions for the pack 0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00397-X 2826 H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829 cementation process were as follow: 40–45 wt.% Si (300 formation of a b-SiC layer. Fig. 1b shows the XRD pattern mesh), 38–42 wt.% SiC (325 mesh), 3–6 wt.% graphite of the multi-coating surface on C / C achieved by the (300 mesh) and 5–9 wt.% Al 2 O 3 (325 mesh) for the ﬁrst two-step pack cementation. It is concluded that new phases step, and 22–29 wt.% SiO 2 (400 mesh), 50–60 wt.% containing corundum (aluminum oxide) and mullite Al 2 O 3 (325 mesh), 3–9 wt.% Si (300 mesh) and 4–8 wt.% (3Al 2 O 3 ?2SiO 2 ) were generated in the second process. graphite (325 mesh) for the second process. All the above Fig. 2 shows the microstructure and the spot EDS powders were reagent grade. The pack mixtures were analyses of the multi-coating surface on C / C composites. weighed and then mixed by tumbling in a ball mill for up There are two kinds of crystalline particles (characterized to 4 h. The C / C composites and the ﬁrst pack mixtures as grey and white) found on the surface. By EDS analysis, were placed in a graphite crucible with a graphite lid. The the grey and white phases could be discriminated as crucible was placed into an electric furnace, which was corundum and mullite, respectively. The corundum par- then heated in an argon atmosphere to 1875 K and held at ticles are bigger than the mullite ones and most of the that temperature for 2 h to form SiC coating. On comple- tion of the coating deposition, the pack was allowed to furnace cool, and the samples were removed from the pack and ultrasonically cleaned to remove any loosely embed- ded pack material. Then the SiC-coated samples were immersed in the second pack mixture. The second pack cementation process was conducted at 2073 K for 2 h to achieve the multi-layer coatings, and the other operations in the second process are as same as for the ﬁrst one. The as-coated specimens were heated at 1873 K in air in an electrical furnace to investigate the isothermal and thermal cycling oxidation behavior. The morphology and crystalline structure of the multi-coatings were analyzed using scanning electron microscopy (SEM), X-ray diffrac- tion (XRD) and energy dispersive spectroscopy (EDS). 3. Results and discussion 3.1. Structure of the multi-coating Fig. 1a shows the XRD pattern of the coating surface obtained from the ﬁrst pack cementation step. It shows the Fig. 1. XRD patterns of the coatings: (a) the SiC coating on C / C composites obtained in the ﬁrst pack cementation process; (b) the multi-coating on C / C composites prepared by two-step pack Fig. 2. SEM micrograph and the spot EDS analyses of the surface cementation. of the as-prepared multi-coating on C / C composites. H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829 2827 space among the corundum particles is ﬁlled with small layer may consist of Al 2 O 3 and a little SiC. From the mullite crystallites. transition layer to the coating surface, the concentrations of It is clear from the cross-section (Fig. 3) of the multi- Si, C, Al and O are almost constant, which means that the coating that the as-received coating with a thickness of pores of porous SiC may be ﬁlled with mullite and Al 2 O 3 around 200 mm is perfectly dense, and no obvious crystallites. interface is found between the SiC bonding layer and the mullite-Al 2 O 3 outer layer. By element line scanning analyses, the concentrations of C, O, Si and Al are also 3.2. Oxidation test shown in Fig. 3. Near the interface between coating and C / C, there is a transition layer with little Si and C but The results of the isothermal oxidation test in air at 1873 enriched with Al and O, which indicates that the transition K are shown in Fig. 4. After oxidation in air for 45 h, the weight loss of the coated C / C composites is only 1.86%, and the corresponding weight loss rate is 1.51310 24 g / cm 2 ?h. The protective temperature is higher than that of the mullite coating for SiC ﬁber, SiC and Si 3 N 4 ceramics (reported to 1573 K) produced by CVD and plasma spraying methods [2,5–7]. The effective protecting time of the as-prepared coating (1873 K for around 45 h) for C / C composites is longer than that of a mullite / Si–SiC multi- coating (1873 K for about 32 h) reported by Fritze et al. . In addition, in comparison with other reported methods [3–8], the two-step pack cementation technique is easier to operate, and all the surfaces of the C / C sample could be coated in one process, while the PLD, CVD and plasma spraying techniques need to coat each surface of the sample separately. Therefore, the repeated deposition processing is avoided using the pack cementation method. According to the oxidation curve shown in Fig. 4, the oxidation behavior of the coated C / C composites could be divided into three processes marked as A, B and C. Under 10 h (process A), the sample gains weight due to the formation of SiO 2 glass . From 10 to 22 h (process B), the weight loss rate of the sample is almost constant. Above 22 h (process C), the weight loss rate increases linearly with time. Additionally, the sample has endured thermal cycling between 1873 K and room temperature for nine times, and no cracks and destruction were found, from which it can be inferred that the coating has excellent thermal shock resistance. Fig. 4. The isothermal oxidation curves of the C / C composites Fig. 3. SEM cross-section image and the element line scanning with a multi-coating in air at 1873 K (minus weight loss means results of the as-prepared coating on C / C composite. weight gain). 2828 H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829 3.3. Analyses of the failure of the multi-coating After 45 h oxidation at 1873 K in air, the coating surface was observed with the scanning electron microscope, and it was found that a smooth glass layer was formed on the sample’s surface (Fig. 5), which may change from mullite and SiC. Fig. 6 shows the XRD pattern of the coating after oxidation. This shows that the peak intensity of SiC was weakened and mullite and Al 2 O 3 phases disappeared, while a weak SiO 2 phase appeared, which indicates that mullite, Al 2 O 3 and some of the SiC have been transformed into a glass phase during the oxidation at 1873 K. The amorphous nature of silicate glass between 18 and 238 of 2u in Fig. 6 also veriﬁes this deduction. From the isothermal oxidation results, it is proposed that the glass layer could effectively seal the pores of the coating layer at Fig. 6. XRD pattern of the as-prepared coating surface after the initial oxidation stage, which leads to a low weight loss oxidation at 1873 K for 45 h. rate for 22 h. After 22 h oxidation in air at 1873 K, the thickness of the glass may decrease gradually by gasiﬁca- tion. Subsequently, the residual glass layer cannot fully prevent oxygen from penetrating the coating to attack the C / C composites. Then gases generated during the reaction between the C / C and oxygen would get out through the coating, which produces holes and bubbles on the coating surface. From Fig. 5a, we can ﬁnd some pinholes and some big gas bubbles from the surface. After the gas bubbles break up, they will leave big holes with diameter of almost 30 mm on the surface (Fig. 5b). The formation of this kind of hole offers entrance channels for air and results in the linear increase of weight loss rate. Additionally, some microcracks are also found near the bubble hole in Fig. 5b. We consider that these microcracks may be generated during quick cooling from 1873 K to the room temperature during the isothermal oxidation test, and they could self- seal when the coating is heated to 1873 K again by the observation of the white traces of the sealed cracks in Fig. 5b. Therefore, the microcracks are not the main cause of the efﬁciency loss of the coating at 1873 K, which is rather due to the formation of holes in the coating, though the convincing reason for the formation of these holes needs further research. 4. Conclusions Based on this work, it can be concluded that the Al 2 O 3 - mullite-SiC multi-coatings with a thickness of around 200 mm could be produced by a two-step pack cementation process. The multi-coating could effectively protect the C / C composites from oxidation at 1873 K for 45 h, and the weight loss rate of the coated C / C composites is only 1.51310 24 g / cm 2 ?h. The failure of the coating is thought Fig. 5. Morphologies of the as-prepared coating on C / C compos- to occur due to the formation of bubble holes on the ites after oxidation at 1873 K for 45 h. coating surface. H. Jian-Feng et al. / Carbon 41 (2003) 2825–2829 2829 Acknowledgements  Varadarajan S, Pattanaik AK, Sarin VK. Mullite interfacial coatings for SiC ﬁbers. Surf Coat Technol 2001;139:153–60. This work was supported by the National Natural  Lee KN, Miller RA. Development and environmental durability of mullite and mullite / YSZ dual layer coatings for Science Foundation of China (Distinguished Young Scho- SiC and Si 3 N 4 ceramics. Surf Coat Technol 1996;86– lar Fund) under grant No. 50225210 87:142–8.  Basu SN, Hou P, Sarin VK. Formation of mullite coatings on silicon-based ceramics by chemical vapor deposition. Int J References Refract Metals Hard Mater 1998;16:343–52.  Auqer ML, Sarin VK. The development of CVD mullite  Sheehan JE, Buesking KW, Sullivan BJ. 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