CC BY
41ÏSHS2019
Moscow, Russia
ULTRAFINE-GRAINED BORON CARBIDE CERAMICS FABRICATED VIA ULTRAFAST SHS-QP SINTERING ASSISTED BY HIGH-ENERGY
BALL MILLING
W. Wang*", Z. Zhangb Z. Fua, H. Wang", J. Zhang", and W. Jia
aState Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology, Wuhan 430070, China bCollege of Materials Science and Engineering, Hebei University of Engineering,
Handan 056038, China *e-mail: wangwm@hotmail.com
DOI: 10.24411/9999-0014A-2019-10189
Boron carbide (B4C) is a structural material with outstanding physical and chemical properties, including high melting point, good wear resistance, excellent chemical stability and good neutron absorption capability [1, 2]. The combined low density (2.52 g/cm3) and high hardness properties of B4C make it distinct from other structural materials. Researchers aim to fabricate dense B4C ceramics with a fine-grained structure because these characteristics maintain the excellent properties of B4C materials [3, 4]. However, due to the low self-diffusion coefficient results from the strong covalent bonding, low plasticity and high resistance to grain boundary sliding, high sintering temperature (> 2100°C) and long holding time (> 40 min) are necessary to obtain dense B4C ceramics using mainstream pressureless or hot-pressing sintering. High sintering temperature, long holding time and slow heating rate not only lead to considerable time and energy cost but also serious grain growth. Therefore, fabrication of dense B4C with fine-grained microstructure is considerably difficult.
In this paper, ultrafine-grained boron carbide ceramics with ultrahigh hardness were fabricated via a unique combination of high-energy ball milling and self-propagating high-temperature synthesis plus quick pressing (SHS/QP) method (Fig.1a).
Fig. 1. (a) Schematic representation of the SHS/QP process and temperature curve versus time during the SHS/QP process. (b)-(f) Characteristics of B4C powders before and after milling. (b) SEM image of commercial B4C powders. (c) SEM image of milled B4C powders. (d) XRD patterns of B4C powders before and after milling. (e) TEM image with SAED pattern of milled powders. (f) HRTEM image of the white rectangle in (e).
XV International Symposium on Self-Propagating High-Temperature Synthesis
Ultrafine powders obtained by milling possess a disordered character (Figs. 2b-2f). During the SHS/QP, the tiny particles in the milled powders are integrated into neighbouring large particles to form ceramic grains, most of the grain sizes in the ceramics cluster range from 100 to 400 nm, and only approximately 2.2% grains with a size less than 100 nm are found, as shown in Fig. 2c. The average grain size of the ceramics is approximately 290 nm.
Fig. 2. Microstructure and grain size distribution of B4C ceramics. (a) SEM image of fracture surface. (b) SEM image of polished and etched surface. (c) statistical chart of grain size distribution.
After SHS/QP, a few amorphous areas and high-density defects were still observed in ceramics (Fig. 3). The ceramics possess high relative density (approximately 99.2%) and ultrafine grain, but with a weak degree of crystallinity which weakens hardness. Heat treatment can improve crystallinity while maintaining the ultrafine grain of the B4C ceramics, thereby increasing the hardness of the ceramics from 31 GPa to 40 GPa (Table 1).
Fig. 3. TEM images with EDS of B4C ceramics. (a) ordinary dislocations. (b) high-density short dislocations. (c) amorphous region. (d) HRTEM image of the white rectangle in (c).
Table 1. Characteristics of B4C ceramics.
B4C samples Before heat treatment After heat treatment
Relative density (%) 99.2 ± 0.4 99.1 ± 0.5
Average grain size (nm) 287 294
Vickers hardness (GPa) 31.3 ± 1.6 39.8 ± 1.3
518
W. Wang et al.
ISHS 2019 Moscow, Russia
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