Zeng Xierong

Mullite-Al2O3-SiC oxidation protective coating for carbon/carbon composites

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 Al2O3-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.

Strong bonding strength between HA and (NH4)2S2O8-treated carbon/carbon composite by hydrothermal treatment and induction heating

Carbon/carbon composite with hydroxyapatite (HA) coating is an attractive material in the dental and orthopedic fields, but the reported bonding strength between them was very poor. In this study, a compact crystalline HA coating on (NH(4))(2)S(2)O(8)-treated C/C substrate about 10 microm in width was obtained by hydrothermal treatment and induction heating. The microstructure, composition and morphologies of the as-prepared coatings were identified by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. A strong shear strength averaging 74.2 MPa between C/C substrate and HA was achieved and adhesion failures were observed more frequently than cohesion failures. The coating adhesion measured using a scratch test was 23 N and the reasons for this are discussed.

TaSi2 Oxidation Protective Coating for SiC Coated Carbon/Carbon Composites

Abstract In order to improve the oxidation protective property of carbon/carbon composites at high temperature, TaSi 2 coating was prepared on the surface of SiC coated carbon/carbon composites by pack cementation technique. The crystal structure and morphology of the as-obtained coating was analyzed by XRD and SEM with EDS. The isothermal oxidation test was carried out in air at 1773 K. The results show that the as-prepared coating is composed of SiC, Si and TaSi 2 . The multilayer coating is about 200 μm in thickness, and there are no cracks in it. The results of EDS indicate that TaSi 2 can effectively fill the holes of SiC coating. There is no visible interface between the TaSi 2 outer coating and the SiC inner coating. The isothermal oxidation curve of the TaSi 2 /SiC coated carbon/carbon composites reveals that the coating can effectively protect carbon/carbon composites for 233 h at 1773 K.

A Novel Strategy for Preparation of Si-HA Coatings on C/C Composites by Chemical Liquid Vaporization Deposition/Hydrothermal Treatments.

A novel strategy for the preparation of Si-doped hydroxyapatite (Si-HA) coatings on H2O2-treated carbon/carbon composites (C/C) was developed. HA coating was prepared on C/C through chemical liquid vaporization deposition (CLVD)/hydrothermal treatment. HA coating was immersed in an H2SiO3 solution at an autoclave at 413 K for transformation into Si-HA coating. The effects of H2SiO3 mass contents on the phase, morphology, and composition of the Si-HA coatings were studied through SEM, EDS,XRD, and FTIR. Their bonding performance to C/C was measured through a scratch test. Under the optimal content condition, the in vitro skull osteoblast response behaviors of the Si-HA coating were evaluated. Results showed that SiO32− could enter into the HA lattice and occupy the PO43− sites. Doped SiO32− significantly improved the bonding performance of the HA coating to C/C in comparison with the untreated HA. The adhesive strength of the coatings initially increased and then decreased with increasing H2SiO3 content. Meanwhile, the cohesive strength of the Si-HA coatings was almost nearly identical. The Si-HA coating achieved at a content of 90% H2SiO3 exhibited the best bonding performance, and its osteoblast compatibility in vitro was superior to that of the untreated HA coating on C/C through CLVD/hydrothermal treatment.

Structure and oxidation behavior of a plasma sprayed yttrium silicates coated SiC-C/C with a glass outer sealant from 1 573K to 1 873K

Different compositions of yttrium silicates coatings were deposited on SiC-C/C by plasma spraying and an outer borosilicate glass was applied on the yttrium silicates coatings surfaces. The structure of the multi-layer coatings was characterized by XRD and SEM analyses. High temperature oxidation behavior of the multi-layer coatings coated C/C composites was investigated. Results show that SiC/2SiO2·Y2O3/1.5SiO2·Y2O3/ SiO2·Y2O3/glass multi-layer coating has better high temperature oxidation resistance, protecting carbon/ carbon composites from oxidation at 1 773 K in air for 164 h with the weight loss of 1.65%. The oxidation weight loss of the coated C/C with time accorded with parabolic rule in the temperature range 1 573 K–1 873 K; and the corresponding oxidation activation energy of the coated carbon/carbon composites is 132.2 kJ/mol.