Microstructures and mechanical behaviors of CVI-based C/C composites containing h-BN powdered additives

doi:10.1016/j.jmst.2019.07.003

Abstract

Abstract:

Hexagonal boron nitride (h-BN) powders were introduced into carbon fiber preform by powder addition and subsequent combined with chemical vapor infiltration (CVI) for densification to prepare carbon fiber reinforced/carbon and boron nitride dual matrix composites (C/C-BN). Microstructures and mechanical properties of C/C composites with three different volume contents of h-BN powders were investigated in comparison to pure C/C composites. Results indicated that the introduction of h-BN powders into C/C composites significantly reduced the size of PyC and the anisotropy of thermal contraction in matrix, leading to a gradual disappearance of ring defects as the h-BN content increased. In addition, an enhanced interfacial bonding between fiber and matrix obtained due to higher-textured PyC and rougher fiber surface. Thereby, the flexural strengths and modulus of as-prepared composites decreased firstly and then increased, while the impact toughness presented a decreasing tendency as the content of BN powders increased. Furthermore, with the increasing of h-BN content, anisotropies of compressive properties were weakened, and the compressive strength of C/C-BN composites were always higher than that of pure C/C composit. However, when C/C composites modified by 13.5?vol% content of h-BN, excessive loose BN aggregates appeared in C/C-BN composites, leading to a relatively slight reduction of compressive strength.

Key words: C/C composites, h-BN, Pyrolytic carbon, Interfacial bonding, Mechanical properties

Peng Xiao, Zhichao Li, Zeyan Liu, Zhuan Li, Bengu Zhang, Jinwei Li, Yang Li. Microstructures and mechanical behaviors of CVI-based C/C composites containing h-BN powdered additives[J]. J. Mater. Sci. Technol., 2019, 35(12): 2824-2831.

Figures/Tables 10

Fig. 1. Schematic diagram for C/C-BN composites fabrication [20].

Table 1 General properties of C/C and C/C-BN composites.

Composites	BN volume fraction (vol%)	Bulk density (g/cm³)	Open porosity (%)
C/C	0	1.68	9.87
C/C-BN4.5	4.5	1.70	4.76
C/C-BN9	9	1.71	3.59
C/C-BN13.5	13.5	1.72	3.35

Fig. 2. PLM photographs of C/C and C/C-BN composites: (a) C/C composite; (b) C/C-BN4.5 composite; (c) C/C-BN9 composite; (d) C/C-BN13.5 composite.

Fig. 3. SEM images of C/C composite and C/C-BN4.5 composite. (a) whole view of C/C composites; (b) whole view of C/C-BN4.5 composites; (c) BSED images of C/C-BN4.5 composites; (d) magnified view in the region of BN&PyC; (e) magnified view of fibers in C/C-BN4.5 composites.

Fig. 4. TEM images of C/C-BN composites. (a) the region of small-sized (1-5?μm) BN powder aggregates; (b) the region closed to fiber (insert: the electron diffraction pattern) [20].

Table 2 Flexural behavior and impact toughness of C/C and C/C-BN composites.

	C/C	C/C-BN4.5	C/C-BN9	C/C-BN13.5
Flexural strength[MPa]	90.9?±?10	72.5?±?6	87.8?±?9	105.4?±?11
Flexural modulus[GPa]	21.5?±?7	12.1?±?4	14.3?±?4	18.9?±?5
Impact toughness[kJ/m²]	2.8?±?0.2	2.4?±?0.3	2.3?±?0.2	1.7?±?0.1

Fig. 5. The SEM images of fracture surface of specimens after bending tests: (a) C/C composites; (b) C/C-BN4.5 composites; (d) C/C-BN9 composites; (f) C/C-BN13.5 composites. The areas within square in (b), (d) and (f) were shown at a higher magnification in (c), (e) and (g), respectively.

Fig. 6. SEM micrograph of impact fracture surfaces of as-prepared composites. (a) C/C-composites; (b) C/C-BN9 composites.

Table 3 Compressive properties of C/C and C/C-BN composites.

		C/C	C/C-BN4.5	C/C-BN9	C/C-BN13.5
Compressive	⊥	238.8?±?19.1	330.8?±?28.5	406.4?±?35.4	368.4?±?30.3
strength[MPa]	//	157.3?±?12.2	289.1?±?29.4	364.7?±?25.3	328.4?±?29.9
Strength ratio	⊥///	1.51	1.14	1.12	1.12
Compressive	⊥	2.35?±?0.48	2.65?±?0.56	2.94?±?0.44	2.85?±?0.70
modulus[GPa]	//	2.97?±?0.36	2.90?±?0.66	2.90?±?0.39	2.81?±?0.53
Modulus ratio	⊥///	0.79	0.91	1.01	1.01

Fig. 7. SEM images of fracture surfaces of C/C and C/C-BN9 composites after compression tests. (a) and (b) C/C composites after perpendicular and parallel compression tests, (c) and (d) C/C-BN9 composites after perpendicular and parallel compression tests.

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