MATERIALS HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 466(2007)172-177 www.elseviercom/locate/msea Oxidation protection of multilayer CVD Sic/B/Sic coatings for 3D C/SiC composite Yongsheng Liu", Laifei Cheng, Litong Zhang, Shoujun Wu, Duo Li, Yongdong Xu National Key Laboratory of Thermostricture Composite Materials, Northwestern Polytechnical University. Xi'an Shaanxi 710072, People's Republic of China Received 2 December 2006; received in revised form 10 February 2007; accepted 12 February 2007 Abstract A CVD boron coating was introduced between two CVD SiC coating layers. EDS and XRD results showed that the Cvd B coating was a boron crystal without other impurity elements. SEM results indicated that the Cvd B coating was a flake-like or column-like crystal with a compact cross-section. The crack width in the CVd SiC coating deposited on CVD B is smaller than that in a CVD SiC coating deposited on CVD SiC oating. After oxidation at 700C and 1000"C, XRD results indicated that the coating was covered by product B2O3 or B2O3-xSiO2 film. The cracks were sealed as observed by sEM. There was a large amount of fake-like material on hybrid coating surface after oxidation at 1300C Oxidation weight loss and residual flexural strength results showed that hybrid SiC/B/SiC multilayer coating provided better oxidation protection for C/SiC composite than a three layer CVD SiC coating at temperatures from 700 C to 1000C for 600 min, but worse oxidation protection above 1000 C due to the large amount of volatilization of B2 O3 or B,O3xSiOz o 2007 Elsevier B. v. All rights reserved Keywords: CVD SiC; CVD B: C/SiC composite; Oxidation protection Introduction face and the fiber in the composites. Below the CVD SiC coating deposition temperature(usually 1000C), the cracks in the CVD Continuous carbon fiber reinforced silicon carbide(C/SiC) coating are not sealing because of the thermal expansion mis- composites are potential candidates for a variety of applications match between the carbon fiber and SiC matrix as well as the in the aerospace field including rocket nozzles, aeronautic jet limited Sioz from surface oxidation. Above the CVD Sic coat- engines, heat shields and aircraft braking systems [1-3. How- ing deposition temperature(usually 1000oC), the cracks in the ever the oxidation of carbon fiber and interface limits long-term CVD SiC coating could be sealed by thermal expansion and a applications of C/Sic composite in high-temperature oxidizing layer of SiOz film due to surface oxidation. However this effect environments. Therefore the development of reliable oxidation is minor below 1300C. Therefore, it is important that the com- protection by coating is crucial to the composites. Because the posites to be protected by a modified coating from 700C to oxidation of CVD Sic is passive up to 1700C and the formed 1300C SiO2 film has a low oxygen diffusion coefficient, CVD In order to protect carbon articles in the oxidation environ- coating is fundamental coating material for oxidation protection ment, the National Carbon Co has developed a hybrid coating of carbon-based materials [4]. Research on CVD SiC coating which is composed of an inner SiC layer and an outer glaze showed that defects, such as cracks due to the coefficient of based on B2O3 [4]. Since that time, borate glassy materials thermal expansion(CTE)mismatch between the composite and became important constituents of oxidation protection systems the coating are unavoidable in a CVD SiC coating 5]. These for carbon [6-10]. This research was focused on sealing the cracks lead to oxygen diffusion inward and oxidation of inter- coating cracks during oxidation by formation of low viscosity B2O or borosilicate glass. The developed coatings were mainly B2O3 [11], B4C[12]or SiBC [13]. An outer coating of B2 O3 is Corresponding author. Tel : +862988486068x823: fax: +8629. not feasible due to B2O3 volatilization. Multilayer SiC/B4C/SiC E-mail addresses: liuys99067(@ mail nwpu.edu. c or SiC/SiBC/SiC coatings can be fabricated expediently by Liuys99067(@163.com(YLiu) CVD process [12, 13], but the escape of CO and CO2 makes 0921-5093/S-see front matter O 2007 Elsevier B v. All rights reserved doi:10.1016/msea.2007.02059Materials Science and Engineering A 466 (2007) 172–177 Oxidation protection of multilayer CVD SiC/B/SiC coatings for 3D C/SiC composite Yongsheng Liu ∗, Laifei Cheng, Litong Zhang, Shoujun Wu, Duo Li, Yongdong Xu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China Received 2 December 2006; received in revised form 10 February 2007; accepted 12 February 2007 Abstract A CVD boron coating was introduced between two CVD SiC coating layers. EDS and XRD results showed that the CVD B coating was a boron crystal without other impurity elements. SEM results indicated that the CVD B coating was a flake-like or column-like crystal with a compact cross-section. The crack width in the CVD SiC coating deposited on CVD B is smaller than that in a CVD SiC coating deposited on CVD SiC coating. After oxidation at 700 ◦C and 1000 ◦C, XRD results indicated that the coating was covered by product B2O3 or B2O3·xSiO2 film. The cracks were sealed as observed by SEM. There was a large amount of flake-like material on hybrid coating surface after oxidation at 1300 ◦C. Oxidation weight loss and residual flexural strength results showed that hybrid SiC/B/SiC multilayer coating provided better oxidation protection for C/SiC composite than a three layer CVD SiC coating at temperatures from 700 ◦C to 1000 ◦C for 600 min, but worse oxidation protection above 1000 ◦C due to the large amount of volatilization of B2O3 or B2O3·xSiO2. © 2007 Elsevier B.V. All rights reserved. Keywords: CVD SiC; CVD B; C/SiC composite; Oxidation protection 1. Introduction Continuous carbon fiber reinforced silicon carbide (C/SiC) composites are potential candidates for a variety of applications in the aerospace field including rocket nozzles, aeronautic jet engines, heat shields and aircraft braking systems [1–3]. How￾ever the oxidation of carbon fiber and interface limits long-term applications of C/SiC composite in high-temperature oxidizing environments. Therefore the development of reliable oxidation protection by coating is crucial to the composites. Because the oxidation of CVD SiC is passive up to 1700 ◦C and the formed SiO2 film has a low oxygen diffusion coefficient, CVD SiC coating is fundamental coating material for oxidation protection of carbon-based materials [4]. Research on CVD SiC coating showed that defects, such as cracks due to the coefficient of thermal expansion (CTE) mismatch between the composite and the coating are unavoidable in a CVD SiC coating [5]. These cracks lead to oxygen diffusion inward and oxidation of inter- ∗ Corresponding author. Tel.: +86 29 8848 6068x823; fax: +86 29 8849 4620. E-mail addresses: liuys99067@mail.nwpu.edu.cn, Liuys99067@163.com (Y. Liu). face and the fiber in the composites. Below the CVD SiC coating deposition temperature (usually 1000 ◦C), the cracks in the CVD coating are not sealing because of the thermal expansion mis￾match between the carbon fiber and SiC matrix as well as the limited SiO2 from surface oxidation. Above the CVD SiC coat￾ing deposition temperature (usually 1000 ◦C), the cracks in the CVD SiC coating could be sealed by thermal expansion and a layer of SiO2 film due to surface oxidation. However this effect is minor below 1300 ◦C. Therefore, it is important that the com￾posites to be protected by a modified coating from 700 ◦C to 1300 ◦C. In order to protect carbon articles in the oxidation environ￾ment, the National Carbon Co. has developed a hybrid coating which is composed of an inner SiC layer and an outer glaze based on B2O3 [4]. Since that time, borate glassy materials became important constituents of oxidation protection systems for carbon [6–10]. This research was focused on sealing the coating cracks during oxidation by formation of low viscosity B2O3 or borosilicate glass. The developed coatings were mainly B2O3 [11], B4C [12] or SiBC [13]. An outer coating of B2O3 is not feasible due to B2O3 volatilization. Multilayer SiC/B4C/SiC or SiC/SiBC/SiC coatings can be fabricated expediently by CVD process [12,13], but the escape of CO and CO2 makes 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.02.059