Effects of PyC shell thickness on the microstructure, ablation resistance of SiCnws/PyC-C/C-ZrC-SiC composites

doi:10.1016/j.jmst.2020.08.047

Abstract

Abstract:

SiC nanowires/pyrocarbon (SiCnws/PyC) core-shell structure toughened C/C-ZrC-SiC composites were fabricated by CLVD process, and the influences of PyC shell thickness on the microstructure and ablation resistance of the composites were researched. The results presented that SiCnws/PyC core-shell structure had a linear shape, and the composites became dense with the increasing PyC thickness. When the thickness of PyC shell increased from 0 to 2.4 μm, the density and thermal conductivity of the composites was improved gradually, but the coefficient of thermal expansion (CTE) decreased firstly and then increased. After the ablation test for 90 s, the ablation rates of the composites decreased continuously as the PyC thickness increased from 0 to 1.4 μm, but increased when the PyC thickness was up to 2.4 μm. Especially when the PyC thickness was 1.4 μm, the linear and mass ablation rates of the composites were 71.25 % and 63.01 % lower than those of the composites without PyC shell. The reasons behind the remarkable improvement of anti-ablation property were that the proper PyC thickness could alleviate the CTE mismatch to promote the formation of complete oxide coating, improve the thermal conductivity to reduce heat corrosion and enhance the capability to limit the mechanical erosion.

Key words: SiCnws/PyC core-shell structure, C/C-ZrC-SiC composites, Microstructure, Ablation resistance, CLVD

Qinchuan He, Hejun Li, Xuemin Yin, Jinhua Lu. Effects of PyC shell thickness on the microstructure, ablation resistance of SiCnws/PyC-C/C-ZrC-SiC composites[J]. J. Mater. Sci. Technol., 2021, 71: 55-66.

Figures/Tables 16

Fig. 1. The morphology and microstructure of SiCnws-C/C-SiC composites and SiCnws: (a), (b) SiCnws-C/C-SiC composites; (c) SiCnws and corresponding EDS analysis; (d) XRD pattern of SiCnws; (e) low-resolution TEM image of SiCnws; (f) HRTEM and SAED image of SiCnws.

Fig. 2. XRD results of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites before the ablation.

Fig. 3. The SEM images of S/P-C-0, S/P-C-0.4, S/P-C-1.4, S/P-C-2.4 composites and the schematic microstructure of all the composites: (a) S/P-C-0; (b) S/P-C-0.4; (c) S/P-C-1.4; (d) S/P-C-2.4; (e) the schematic microstructure of all the composites.

Table 1 The density, porosity and PyC thickness of four composites.

Composites	Density (g/cm³)	Porosity (%)	PyC thickness (μm)
S/P-C-0	1.88	12.34	0
S/P-C-0.4	2.01	10.96	0.4
S/P-C-1.4	2.1	9.94	1.4
S/P-C-2.4	2.19	8.79	2.4

Fig. 4. High magnified SEM and TEM images of SiCnws/PyC core-shell structure of S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites: (a) S/P-C-0.4; (b) TEM image of S/P-C-0.4; (c) S/P-C-1.4; (d) S/P-C-2.4.

Fig. 5. The thermal conductivity and CTE of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites: (a) thermal conductivity; (b) CTE.

Fig. 6. The ablation rates of linear and mass of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites.

Fig. 7. The ablation surface appearance of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites: (a) S/P-C-0; (b) S/P-C-0.4; (c) S/P-C-1.4; (d) S/P-C-2.4.

Fig. 8. XRD results of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites after the ablation.

Fig. 9. The surface temperature of ablation center of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites.

Fig. 10. The SEM images of ablation center morphologies of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites: (a) S/P-C-0; (b) S/P-C-0.4; (c) S/P-C-1.4; (d) S/P-C-2.4.

Fig. 11. The cross-section of ablation center of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites: (a) S/P-C-0; (b) S/P-C-0.4; (c) S/P-C-1.4; (d) S/P-C-2.4.

Fig. 12. The morphologies and EDS patterns of ablation center under ZrO2 coating of S/P-C-0, S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites: (a) S/P-C-0; (b) S/P-C-0.4; (c) high magnification of S/P-C-0.4; (d) corresponding EDS pattern of Fig.13 (c); (e) S/P-C-1.4; (f) S/P-C-2.4.

Fig. 13. The ablation morphologies of SiCnw/PyC core-shell structure of S/P-C-0.4, S/P-C-1.4 and S/P-C-2.4 composites: (a), (b) S/P-C-0.4; (c), (d) S/P-C-1.4; (e), (f) S/P-C-2.4.

Fig. 14. The variation of Gibbs free energy (ΔG) of the reactions between carbon and oxygen as well as SiC at the temperature from 2000 to 2500 °C.

Fig. 15. The diagrammatic sketch of ablation behavior of SiCnws/PyC core-shell structure.

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