Fabrication and characterization of B4C nanowires and dense B4C ceramic/nanowires composite
Chua, Miao Jun.
Date of Issue2012
School of Materials Science and Engineering
The superior properties of boron carbide (B4C), such as high melting point, extreme hardness (ranking third after diamond and cBN), low density, good thermal and chemical resistance properties, have made it a promising material for ceramic armor applications. However, the widespread use of boron carbide is restricted due to its inferior fracture toughness, low mechanical strength and poor sinterability. Recently, it was found that boron carbide nanowires (BCNWs) can be used as reinforcement agent to improve the mechanical properties of bulk boron carbides. As a result, in the present study, boron nanowires were synthesized through vapor solid (VS) for catalyst-free BCNWs and vapor-liquid-solid (VLS) mechanism for catalyzed BCNWs. Catalyst materials, like iron particles and nickel nitrates, were used with different contents (5 and 10 wt.%) to study their effects on the growth of BCNWs. Morphology and phase composition were examined by using high resolution scanning electron microscope (SEM) and X-ray diffraction (XRD). Catalyst-free BCNWs with diameter less than 100 nm were obtained at 1400ºC. Smaller nanowires with diameter of less than 50 nm were obtained when 10 wt.% Fe was used as catalyst. Droplets were observed on the nanowire tips by using transmission electron microscopy (TEM), revealing the growth mechanism of the nanowires. The addition of 10 wt.% nickel nitrate was noted to significantly increase the yield of the nanowires. Nickel catalyst was also found to facilitate the growth of nanowires at lower temperatures. The Ni-catalyzed BCNWs after acid treated were used as reinforcement to fabricate B4C matrix ceramics with high mechanical performances. Monolithic B4C ceramics with relative density of 96.1% were fabricated by using spark plasma sintering (SPS) technology, at 1900°C for 3 min at 100 MPa. Vickers hardness and fracture toughness of the samples were measured to be 38.4 ± 0.8 GPa and 2.19 ± 0.14 MPa•m1/2, respectively. B4C composite ceramics with the synthesized BCNWs, with different weight percentages (1, 3, 5 and 8 wt.%), as reinforcement agent were produced by using SPS at similar conditions as those used to make monolithic B4C ceramics. Microstructures and mechanical properties, including hardness and fracture toughness, of the materials were studied and discussed. Optimum fracture toughness of the materials with nanowires reinforcement was found in the sample with 5 wt.% of BCNWs, which was 3.66 ± 0.05 MPa•m1/2 (67.1% improvement compared to fracture toughness of monolithic B4C). Boron carbide-titanium diboride (B4C-TiB2) composite ceramics with various TiB2 contents (3, 8 and 15 wt.%) were fabricated by using SPS. Optimum fracture toughness was obtained in the sample with 15 wt.% TiB2 sintered at 1900°C. It had a KIc value of 3.90 ± 0.10 MPa•m1/2, which showed a 78.0% improvement as compared to the pure B4C ceramics (2.1-2.2 MPa•m1/2). Due to reduction in the content of hard B4C phase, compared to monolithic B4C samples, the hardness of both the B4C-TiB2 and B4C-BCNWs composites was decreased with increasing contents of TiB2 and BCNWs. With the conclusions drawn from the current study, future works was proposed in order to develop B4C based ceramic materials with improved performances.