Effect of heat treatment on the microstructure and properties of CVD SiC fiber

doi:10.1016/j.jmst.2017.02.009

Abstract

Abstract:

In this study, the effect of heat treatment on the room temperature strength of W-core SiC fiber produced by chemical vapor deposition (CVD) was investigated. Thermal exposure in the temperature range of 900-1000 °C decreases the strength of the SiC fiber. Fracture morphology analysis indicates that failure initiations predominantly take place at the W-core/SiC interface. A reaction layer that formed at the W-core/SiC interface during thermal exposure degraded the fiber strength and an empirical linear relationship of strength vs thickness of the reaction layer can be obtained. The kinetics of the growth of the W-core/SiC reaction layer were determined.

Key words: Silicon carbide fiber, Heat treatment, Reaction layer, Strength degradation

Zhao Chuanbao, Wang Yumin, Zhang Guoxing, Yang Qing, Zhang Xu, Yang Li_na, Yang Rui. Effect of heat treatment on the microstructure and properties of CVD SiC fiber[J]. J. Mater. Sci. Technol., 2017, 33(11): 1378-1385.

Figures/Tables 13

Table 1 Heat-treatment schemes for SiC fibers.

Temperature (°C)	Duration (h)
900	100	200	500	1000
1000	10	20	50	120
1100	0.5	1	1.5	2	2.5	4	5

Fig. 1. Weibull analysis of the tensile strength data of as-produced fibers: (a) probability of survival at different applied stress, (b) linear regression with a Weibull modulus.

Table 2 Weibull statistical analysis of the strength and thickness of W/SiC reaction layer for SiC fibers after different thermal exposure.

	Thermal exposure time (h)	Weibull characteristic strength, σ₀ (MPa)	Weibull modulus, m	R²	Thickness of W/SiC reaction layer (nm)
As-produced	-	3801	25.8	0.93	306
900 °C	100	3839	15.4	0.85	340
	200	3748	21.8	0.83	348
	500	3743	35.7	0.96	370
	1000	3709	23.9	0.96	397
1000 °C	10	3655	26.4	0.96	403
	20	3537	28.2	0.96	458
	50	3136	30.3	0.90	552
	120	2460	18.8	0.93	970
1100 °C	0.5	3336	29.4	0.95	490
	1	3175	28.5	0.93	607
	1.5	2842	21.7	0.97	713
	2	2655	24.6	0.93	795
	2.5	2475	24.9	0.95	903
	4	2084	28.6	0.96	1120
	5	1885	24.4	0.96	1177

Fig. 2. Summary of effect of thermal exposure time on tensile strength of SiC fibers, the inset showing details for exposure at 1100 °C.

Fig. 3. SEM images of a typical fracture surface: (a) overview, (b) magnification of the tungsten core/SiC interfacial reaction zone.

Fig. 4. Backscattered electron images of carbon coatings of SiC fibers after different thermal exposures. The carbon coating thickness is around 1.7 μm for all cases: (a) 900 °C/1000 h, (b) 1000 °C/120 h, (c) 1100 °C/2.5 h, (d) 1100 °C/5 h.

Fig. 5. Raman spectra obtained from carbon coatings of SiC fiber after different thermal exposures: (a) as-produced, (b) 900 °C/1000 h, (c) 1000 °C/120 h, (d) 1100 °C/5 h.

Fig. 6. XRD patterns of the SiC fibers after different thermal exposures: (a) as-produced, (b) 900 °C/1000 h, (c) 1000 °C/120 h, (d)1100 °C/5 h.

Fig. 7. SEM images of the fracture surfaces of the tungsten core showing two cases with different types of crack initiation and propagation correlated with the thickness of the W/SiC reaction layer: (a) and (b) thin, (c) and (d) thick.

Fig. 8. Morphology of tungsten core/SiC interfacial reaction layer with various degrees of growth after thermal exposure: (a) as-produced, (b) 900 °C/500 h, (c) 900 °C/1000 h, (d) 1000 °C/50 h, (e) 1000 °C/120 h, (f) 1100 °C/0.5 h, (g) 1100 °C/2 h, (h) 1100 °C/5 h.

Fig. 9. Plot of strength vs thickness of W/SiC reaction layer, the inset showing a summary of the work of Gambone et al. using Trimarc 1? SiC fiber [10,21].

Fig. 10. Linear fit of thickness x vs T1/2 and Arrhennius plot of lnk vs 1/T (inset).

Table 3 Growth parameters of W/SiC interfacial reaction layer.

Temperature, T (°C)	Growth rate constant, k (nm h^-1/2)	Activation energy, Q (kJ/mol)
900	2.8	336
1000	59
1100	414

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