Influence of Sintering Conditions on Preparation of Nearly Stoichiometric SiC Fibers with Highly Crystalline Microstructure

doi:10.15541/jim20240439

Abstract

Abstract:

Fine-diameter continuous SiC fibers are considered one of the most effective reinforcing fibers for advanced ceramic matrix composites, possessing significant application potential in aerospace and nuclear industries. Among them, near-stoichiometric SiC fibers characterized by a highly crystalline microstructure have garnered considerable attention due to their exceptional high-temperature resistance. However, influence of high-temperature sintering conditions on composition and microstructure of the fibers is still unclear. Here, influences of different sintering temperatures and durations on decomposition of the SiC_xO_y phase, grain growth and densification of the fibers were systematically investigated. It was found that decomposition of SiC_xO_y and densification of fibers occur progressively from surface to core. Notably, a specific sintering temperature of 1800 ℃ was identified as optimal, wherein growth of β-SiC grains effectively compensated for the pore defects resulting from the decomposition of SiC_xO_yphase, thereby achieving fiber densification. Conversely, excessively high sintering temperature might result in decomposition of β-SiC grains. Although extending sintering duration facilitated removal of residual oxygen within the fibers, it could cause accumulation of graphite phases at β-SiC grain boundaries, leading to an increase in pore defects within the fiber core. Finally, near-stoichiometric SiC fibers with highly crystalline microstructure were successfully fabricated through optimization of sintering conditions, possessing a composition of SiC_1.04O_0.02Al_<0.01. The β-SiC grains within the fibers were uniformly distributed with sizes ranging from 100 to 200 nm. The fibers exhibited a tensile strength of 1.88 GPa and a Young’s modulus of 373 GPa, accompanied by a high density of 3.1 g/cm³. The findings of this research provide a robust foundation for further improving comprehensive properties of SiC fibers.

Key words: SiC fiber, sintering, nearly stoichiometric, density, highly crystalline

CLC Number:

TQ343

GOU Yanzi, KANG Weifeng, WANG Pengren. Influence of Sintering Conditions on Preparation of Nearly Stoichiometric SiC Fibers with Highly Crystalline Microstructure[J]. Journal of Inorganic Materials, 2025, 40(4): 405-414.

Figures/Tables 24

Table 1 Chemical composition and mechanical properties of Si-C-O-Al fibers and intermediate fibers

Sample	O content/(%, in mass)	C/Si atomic ratio	Tensile strength/GPa	Young’s modulus/GPa	Diameter/μm
Si-C-O-Al fiber	8.93	1.33	2.27	204	11.6
Intermediate SiC fiber	0.98	1.06	1.54	283	10.8

Fig. 1 SEM images of the intermediate fibers (a) Fiber surface; (b) Magnified region 1; (c) Fiber cross section; (d) Magnified region 2; (e) Magnified region 3

Fig. 2 Surface morphologies of the sintered SiC (SF) fibers (a, b) SiC (SF)-1600-1 h; (c, d) SiC (SF)-1800-1 h; (e, f) SiC (SF)-2000-1 h

Fig. 3 Cross-sectional morphologies of the sintered SiC (SF) fibers (a-c) SiC (SF)-1600-1 h; (d-f) SiC (SF)-1800-1 h; (g-i) SiC (SF)-2000-1 h

Table 2 Chemical composition (mass fraction) of the SiC (SF) fibers sintered at different temperatures

Sample	C/%	Si/%	O/%	Al/%	C/Si atomic ratio
SiC (SF)-1600-1 h	31.88	67.50	0.80	<1.00	1.10
SiC (SF)-1700-1 h	32.37	66.31	0.82	<1.00	1.14
SiC (SF)-1800-1 h	31.48	67.41	0.61	<1.00	1.09
SiC (SF)-1900-1 h	33.04	66.14	0.32	<1.00	1.17
SiC (SF)-2000-1 h	38.27	61.13	0.10	<1.00	1.46

Fig. 4 Raman spectra (a) and XRD patterns (b) of the SiC (SF) fibers sintered at different temperatures

Fig. 5 HRTEM images of the SiC (SF) fibers (a, b) SiC (SF)-1600-1 h; (c, d) SiC (SF)-1800-1 h; (e, f) SiC (SF)-2000-1 h

Fig. 6 Schematic diagram of the sintering densification process of nearly stoichiometric polycrystalline SiC fibers

Fig. 7 Cross-sectional morphologies of the SiC (SF) fibers sintered at 1800 ℃ for different durations (a-c) SiC (SF)-1800-10 min; (d-f) SiC (SF)-1800-30 min; (g-i) SiC (SF)-1800-2 h

Table 3 Chemical composition (mass fraction) of the SiC (SF) fibers sintered at 1800 ℃ for different durations

Sample	C/%	Si/%	O/%	Al/%	C/Si atomic ratio
SiC (SF)-1800-10 min	31.82	66.93	0.75	<1.00	1.11
SiC (SF)-1800-1 h	31.48	67.41	0.61	<1.00	1.09
SiC (SF)-1800-2 h	31.54	67.96	0	<1.00	1.08

Fig. 8 HRTEM images of the SiC (SF) fibers sintered at 1800 ℃ for different durations (a, b) SiC (SF)-1800-10 min; (c, d) SiC (SF)-1800-30 min; (e, f) SiC (SF)-1800-2 h

Table 4 Mechanical properties of SiC (SF) and SiC (SG) fibers

Sample	Tensile strength/ GPa	Young’s modulus/ GPa	Fiber diameter/ μm	Chemical formula
SiC (SF)	1.88	373	11.2	SiC_1.04O_0.02Al_<0.01
SiC (SG)	2.96	268	11.6	SiC_1.31O_0.05

Fig. 9 Surface SEM images of SiC (SF) and SiC (SG) fibers (a, b) SiC (SF) fiber; (c, d) SiC (SG) fiber

Fig. 10 Cross-sectional SEM images of SiC (SF) and SiC (SG) fibers (a-c) SiC (SF) fiber; (d-f) SiC (SG) fiber

Fig. 11 SAED patterns, TEM and HRTEM images of SiC (SF) and SiC (SG) fibers (a-c) SiC (SF) fiber; (d-f) SiC (SG) fiber

Fig. S1 XRD patterns of the intermediate fibers (a) and the Si-C-O-Al fibers (b)

Table S1 Raman results of the SiC (SF) fibers sintered at 1600-2000 ℃ for 1 h

Sample	D peak/cm^-1		G peak/cm^-1		I_D/I_G
Sample	Position	FWHM	Position	FWHM	I_D/I_G
SiC (SF)-1600-1 h	1357	56.0	1590	47.0	1.41
SiC (SF)-1800-1 h	1358	51.4	1593	42.2	1.01
SiC (SF)-2000-1 h	1357	49.3	1585	28.8	0.31

Fig. S2 Changes of mechanical properties of the SiC (SF) fibers with sintering temperature

Fig. S3 Tensile strength of SiC (SF) fibers after sintered at different temperatures (a) 1500 ℃; (b) 1600 ℃; (c) 1700 ℃; (d) 1800 ℃; (e) 1900 ℃

Fig. S4 Surface morphologies of the SiC (SF) fibers sintered at 1800 ℃ for different durations (a, b) SiC (SF)-1800-10 min; (c, d) SiC (SF)-1800-30 min; (e, f) SiC (SF)-1800-2 h

Fig. S5 XRD patterns of the SiC (SF) fibers sintered at 1800 ℃ for different durations

Fig. S6 Changes of mechanical properties of the SiC (SF) fibers with sintering time at 1800 ℃

Fig. S7 Tensile strength of SiC (SF) fibers sintered at 1800 ℃ for different durations (a) 10 min; (b) 30 min; (c) 1 h; (d) 2 h

Fig. S8 SEM images of fracture morphologies for the ﬁbers F-1 (a-c), F-2 (d-f) and F-3 (g-i) More details of the fracture surfaces can be found in the enlarged images for the outer parts (b, e, h) as indicated by red square 1 and the core parts (c, f, i) as indicated by red square 2[25]

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