B4C Grains Research Articles

Thermoelectric properties of boron carbide-titanium boride composites

Boron carbide (B4C) composites containing 0–10 wt.% graphene nanoplatelets (GNPs) were consolidated using hot pressing at 1950 °C for 60 min under 30 MPa in an argon atmosphere. Their electrical discharge machining (EDM) characteristics were evaluated for the first time. Additionally, the effects of GNPs on the microstructure, electrical conductivity, material removal rate (MRR), and surface roughness (Ra) of B4C composites were investigated. The results show that the B4C composite containing 10 wt.% GNPs exhibited the highest electrical conductivity of 4997 S·m−1 because of the formation of GNPs conductive networks. Under a fine machining condition, the MRR of the B4C composite was enhanced by 43.5 % (from 6.55 to 9.40 mm3 min−1) compared with that of monolithic B4C. Furthermore, the Ra decreased to 1.12 μm, which was significantly lower than that of pure B4C (2.44 μm). Scanning electron microscope and energy disperse spectroscopy analysis of the EDM surfaces were used to determine that the main material removal mechanisms for B4C composites with less than 2 wt.% GNPs were spalling and melting. As the GNPs content increased, B4C grain fallout, melting, and evaporation became more dominant. The other mechanisms, including thermal shock and oxidation, are also discussed in detail.

Fabrication of silicon carbide ceramics

Pressureless sintering process to obtain high density silicon carbide–boron carbide (5% vol) composite (BCSiC) was studied. Sintering behaviour of this material was investigated between 1950 and 2200°C and it was compared to SiC material doped with boron and carbon. Density up to 96% T.D. was reached at 2150°C. Microstructural investigation revealed that B4C grains, having an average size of 2 μm, were well dispersed in the SiC matrix. Mechanical properties like fracture toughness, hardness, flexural strength were determined in the range RT-1500°C and compared to other SiC-based material. BCSiC material showed same fracture toughness of SSiC, higher hardness and higher flexural strength than SSiC also at high temperature; these properties make BCSiC a very interesting material for application in which high oxidation resistance is requested, e.g. ceramic heat exchanger.

Processing, Microstructure, and Mechanical Properties of B4C ― TiB2 Particulate Sintered Composites. Part I. Pressureless Sintering and Microstructure Evolution

Pressureless sintering of boron carbide ceramics containing 0-25 vol. % TiB2 phase, produced via an in-situ chemical reaction between B4C, TiO2, and elemental carbon, was studied in the isothermal and constant-heating-rate regimes. The presence of TiB2 results in a decrease in activation energy for sintering from 717 kJ/mol at 0 vol. % TiB2 to 266 kJ/mol at 25 vol. % TiB2. Ceramic bodies of B4C ― TiB2 particulate composites with relative densities of up to 99% were sintered without pressure at temperatures of 2050-2100°C. Grain boundary diffusion is the primary mechanism of TiB2 particle coarsening. TiB2 particle size is bimodal depending on whether the particle is confined within a B4C grain or located on the grain boundary. Densification behavior of the B4C ― TiB2 system is identical at different heating rates in the temperature range of 1800-2150°C.

Response of aluminium-infiltrated boron carbide cermets to shock wave loading

Shock-recovery and shock-spallation experiments were performed on two compositions of aluminium-infiltrated B4C cermets as a function of shock pressure. Sixty-five per cent volume B4C-Al cermets were recovered largely intact after shock loading up to pressures of ca. 12 GPa which permitted a critical study of the microstructural changes produced by the shock. Significantly, shock loading to between 12 and 13 GPa produced a combination of dislocation debris, stacking faults and deformation twins in a small fraction of the B4C grains. Fragmentation of shock-loaded 80% B4C-Al samples prevented meaningful microstructural investigation. Spall-strength testing also provided indirect evidence for the Hugoniot elastic limits (HEL) of these composites. Spall-strength calculations based on an elastic equation of state for 65% B4C-Al indicated that the elastic regime extended up to shock pressures of ca. 10 GPa, or approximately 65% of the HEL of polycrystalline B4C. A complete loss of spall strength was then observed at the transition to a plastic equation of state at a pressure of 12 GPa which coincided with observations of plasticity within the B4C-substructure. This study demonstrated that composites containing a highly ductile phase combined with a high compressive strength ceramic phase could support high dynamic tensile stresses by resisting the propagation of catastrophic cracks through the brittle ceramic substructure.

Modeling of the Formation of Boron Carbide Particles in an Aerosol Flow Reactor

ABSTRACTThe formation of submicron crystals of boron carbide (B4C) by rapid carbothermal reduction of intimately mixed carbon-boron oxide precursor powders in an aerosol flow reactor at temperatures above the boiling point of boron oxide is investigated. The employed high heating rates (105 K/s) of the process force release of gaseous boron oxide and suboxides and rupture of the precursor particles resulting in formation of boron carbide molecular clusters that grow to macroscopic particles by coagulation. Consequently, the formation and growth of B4C particles is described by simultaneous interparticle collision and coalescence using a two-dimensional distribution model that traces the evolution of both size and shape characteristics of the particles through their volume and surface area. Here, in addition to the coagulation term, the governing population balance equation includes a coalescence contribution based on B4C sintering law. The predicted evolution of the two-dimensional particle size distribution leads to a direct characterization of morphology as well as the average size and polydispersity of the powders. Furthermore, model predictions of the volume and surface area of boron carbide particles can be directly compared with experimental data of B4C specific surface area and grain size.

Effect of Iron and Boron Carbide on the Densification and Mechanical Properties of Titanium Diboride Ceramics

The effect of Fe and B4C on the sintering behavior and mechanical properties of TiB2 ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition‐milled TiB2 powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B4C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B4C grains did not act as a grain growth inhibitor, and B4C grains were frequently trapped in large TiB2 grains. The best mechanical properties were obtained for the TiB2–10 wt% B4C–0.5 wt% Fe ceramics, which exhibited a three‐point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m1/2.