Tasi2 Oxidation Protective Coating for Sic Coated Carbon/Carbon Composites

第十三届全国博士生学术年会 新材料专题

TaSi2 Oxidation Protective Coating for SiC Coated Carbon/Carbon Composites

Rare Metal1, 2, Rare Metal 2, Rare Metal 1, Rare Metal 1, Rare Metal 2

1 Rare Metal Material and Engineering, Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China; 2 Rare Metal Material and Engineering, Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China

Abstract: Mode mode mode mode mode..

Key words: rare metal; material; engineering

第十三届全国博士生学术年会 新材料专题

Carbon/carbon (C/C) composites have been used as attractive materials for high-temperature applications, such as reentry shields of space vehicles, aircraft brake and rocket noses etc. In an inert atmosphere or in vacuum, C/C composites retain their strength, modulus and other mechanical properties to temperatures higher than those tolerated by other materials. However, the most important high-temperature structural applications for C/C composites occur in oxidizing environments, and the carbon in any form will react with oxygen, oxidize at temperatures as low as 723 K[1]. So, the broad use of C/C composites as high-temperature structural materials requires the development of effective oxidation protection. The coating technology has been considered as the effective way to protect C/C composites[2-8].

1 Experiment

Small specimens (10 mm×10 mm×10 mm) used as substrates were cut from 2D-C/C composites with a density of 1.78 g/cm3. The specimens were hand-polished using 400 grit sand paper, then cleaned with alcohol and dried in air. Powder compositions in mass fraction for pack cementation process were as follows: 75%-80% Si (45 µm) and 20%-25% graphite (45 µm) for SiC coating, 60%-75% Si (45 µm), 20%-30% Ta (45 µm) and 5%-10% graphite (45 µm) for TaSi2 coating. The pack mixtures were weighed and mixed by tumbling in a ball mill up to 10 h.

TaSi2/SiC coated C/C samples is only 1.41% after oxidation for 233 h. It is because that the outer TaSi2 coating properly repairs the cracks and the holes in SiC coating. At high temperature, these holes could shrink to destroy the coating. So, this structure of SiC coating is not beneficial to the anti-oxidation of C/C composites. Many holes exist in SiC coating simultaneously. At high temperature, these holes could shrink to destroy the coating. So, this structure of SiC coating is not beneficial to the anti-oxidation of C/C composites.

2 Results and Discussion

2.1 Microstructure of the as-received coating

Fig.1 shows the microstructures of the surface and cross section of SiC coated C/C composites. Fig.1a reveals that the surface of SiC coating has large cracks. The cracks have turned into the penetrating channel (shown in Fig.1b) through which oxygen can diffuse into C/C matrix and oxidize the matrix. Many holes exist in SiC coating simultaneously.

Fig.2 reveals the XRD patterns of SiC coating and TaSi2 coating on the C/C composites. The new phases, TaSi2 and Si, appear in the TaSi2 coating comparing with the SiC coating. The rich Si phase in the coating plays the role of filling cracks due to its liquid state at the high temperature. According to the SEM image (Fig.3), no cracks exist on the surface of TaSi2 coating.

Fig.1 SEM images of surface (a) and cross section (b) of SiC coated C/C composites

Fig.2 XRD patterns of SiC coating and TaSi2 coating on C/C composites

2.2 Oxidation-protective property of the as-received coating

Fig.5 shows the isothermal oxidation curves of the coated C/C samples in air at 1773 K. The sample with SiC coating has 42.7% mass loss after oxidation for 5 h. However, the mass loss of TaSi2/SiC coated C/C samples is only 1.41% after oxidation for 233 h. It is because that the outer TaSi2 coating properly repairs the cracks and the holes in SiC coating. So the oxidation-protective ability of the whole coating is greatly improved.

(1)

The XRD pattern (Fig.6) of TaSi2/SiC coated C/C composites indicates that the coating surface contains SiC, Si, TaSi2, SiO2 and Ta2O5 after oxidation in air at 1773 K. At high temperature, a few of reactions have happened as follows:

(1)

(2)

SiO2 and Ta2O5 have excellent anti-oxidation property that can protect C/C matrix from oxidation. At 1773 K, SiO2 present the melting state that could fill the cracks formed in coating due to the thermal stress.

Small specimens (10 mm×10 mm×10 mm) used as substrates were cut from 2D-C/C composites with a density of 1.78 g/cm3. The specimens were hand-polished using 400 grit sand paper, then cleaned with alcohol and dried in air. Powder compositions in mass fraction for pack cementation process were as follows: 75%-80% Si (45 µm) and 20%-25% graphite (45 µm) for SiC coating, 60%-75% Si (45 µm), 20%-30% Ta (45 µm) and 5%-10% graphite (45 µm) for TaSi2 coating. The pack mixtures were weighed and mixed by tumbling in a ball mill up to 10 h.

TaSi2/SiC coated C/C samples is only 1.41% after oxidation for 233 h. It is because that the outer TaSi2 coating properly repairs the cracks and the holes in SiC coating. At high temperature, these holes could shrink to destroy the coating. So, this structure of SiC coating is not beneficial to the anti-oxidation of C/C composites. Many holes exist in SiC coating simultaneously. At high temperature, these holes could shrink to destroy the coating. So, this structure of SiC coating is not beneficial to the anti-oxidation of C/C composites.

Table 1 Chemical composition and phase transformation

Temperatures[9]

Fig.5 Isothermal oxidation curves of coated C/C samples in air at

1773 K

3 Conclusions

1) TaSi2 coating can be obtained on the surface of SiC coated C/C composites by pack cementation method. TaSi2 can effectively fill the cracks and holes in the SiC coating.

2) The double-layer TaSi2/SiC coating can protect C/C composites more than 233 h at 1773 K.

3) The cracks and holes in the coating should be the main reason of decrease of oxidation protective ability of TaSi2/SiC coating.

References

[1] Zeng Dawen. Rare Metal Materials and Engineering[J], 1998, 27(1): 87.

[2] Zhou Lian. 98’ Symposia Proceedings of C-MRS, Advances in Materials Research and Application[C]. Beijing: Chemical Industry Press, 1999: 25.

[3] Huang Boyun, Li Chenggong, Shi Likai et al. Chinese Material Engineering Dictionary[M]. Beijing: Chemical Industry Press, 2006: 471.

[4] Geiger M, Kleiner M, Eckstein R. Materials Science and Engineering A[J], 2008, 475: 279.

[5] Li Hejun, Jiao Gengsheng, Li Kezhi et al. Materials Science and Engineering A[J], 2008, 475: 279.

[6] Chakraborty N, Ficher W, Gupta A D. Surface and Coating Technology[J], 2006, 201: 1152.

第十三届全国博士生学术年会 新材料专题

碳/碳复合材料TaSi2/SiC氧化保护涂层

稀有金属1，2，稀有金属2，稀有金属1，稀有金属1，稀有金属2

(1. 西北有色金属研究院《稀有金属材料与工程》杂志社，陕西 西安 710016)

(2. 西北有色金属研究院《稀有金属材料与工程》杂志社，陕西 西安 710016)

摘 要：模板

关键词：碳/碳复合材料; 涂层; TaSi2; SiC

作者简介：xxx，男，19xx年生，博士研究生，北京科技大学新金属材料国家重点实验室，北京市海淀区学院路30号 100083，电话：xxx，E-mail: xxx