Sintering Of Boron Carbide Research Articles

Demonstrating low‐temperature sintering of boron carbide powders

AbstractCapability to sinter ceramics, especially carbides, at low temperature, is desired but is a challenge. Recently, cold sintering has been successfully demonstrated with oxides and other ceramics, but not with carbides due to high thermodynamic and chemical stability. In this work, low‐temperature sintering of boron carbide (B4C) was attempted at 1000‐1400°C; the proposed sintering is to form B4C nanoparticles with in situ carbothermal reduction from organic liquid‐phase precursors (boric acid and glycerin), and then to sinter the formed B4C nanoparticles, between B4C micropowders, which are expected to increase packing density and enhance diffusion and densification. Formation of B4C nanoparticles by in situ carbothermal reduction was confirmed, but only limited densification was observed due to material loss from chemical reactions and lack of pressurization. The achieved relative densities are low (57.7%‐62.8%) but comparable with those achieved with pressureless sintering at >2000℃.

Synthesis of Polydisperse Boron Carbide and Synthesis of a Ceramic on Its Basis

Results obtained in a study of the process of synthesis of a polydisperse boron carbide powder (average particle size 2.10 µm) characterized by a broad particle size distribution are presented. The process in which a ceramic is produced from the thus synthesized boron carbide by hot compaction was also analyzed. In some cases, a sintering additive, highly dispersed chromium carbide powder (average particle size 7.13 µm), was used. The hot compaction was performed in argon at moderate parameters: pressure 35 MPa and temperature 1950°C. The porosity and water-absorption capacity of the samples obtained are very low and do not exceed 0.02%. The average values of the bending and compression strengths were 406 and 1553 MPa, respectively. A microhardness of about 42 GPa was reached in sintering of boron carbide. The microhardness in sintering of boron carbide with addition of chromium carbide was 45–46 GPa.

Modeling of Gas-Phase Transport and Composition Evolution during the Initial-Stage Sintering of Boron Carbide with Carbon Additions

Densification of 4 B C during sintering can be aided by removing the native 23 BO (condensed) layer present on the starting 4 B C powder. B2O3 can be removed by adding excess C and holding the powder compact at an intermediate temperature below the normal sintering temperature. This allows time for CO and minor boron gases to diffuse out from the porous compact before the pores close. This process is examined using a computational model based on co-diffusion of multiple gas species, which enables prediction of the gas and condensed phase composition as a function of time and position in the specimen. The model, previously described elsewhere, was originally applied to the 2

Microstructural and Mechanical Properties of Boron Carbide Ceramics by Methanol Washed Powder

AbstractBoron carbide (B4C) is currently used in lightweight armors and high temperature materials, because it has high meting point, good hardness, low specific gravity and good mechanical properties. The sintering of boron carbide, however, is restricted by its high covalent bonding and B2O3coatings on B4C particles surface which can cause a microstructural coarsening during sintering. Therefore, it is necessary to remove B2O3film of By4C particles surface to restrict microstructural coarsening and densification of B4C. B4C ceramics were fabricated by a hot-press sintering and its sintering behavior, microstructure and mechanical properties were evaluated. The relative density of B4C ceramics were obtained by a hot-press sintering reached as high as 99% without any sintering additives. The mechanical properties of B4C ceramics was improved by a methanol washing which can remove B2O3phase from a B4C powder surface. This improvement is resulted from the formation of homogeneous microstructure because the grain coarsening was suppressed by the elimination of B2O3phase. Particularly, the mechanical properties of the sintered samples using a methanol washed powder improved compared with the samples using an as-received commercial powder.

Simultaneous Synthesis and Sintering of Boron Carbide by SPS method.

Simultaneous Synthesis and Sintering of Boron Carbide by SPS method.

Effects of addition of chromium oxide on sintering of boron carbide : N. Frage et al. (Ben-Gurion University of the Negev, Beer-Sheva, Israel.)

Effects of addition of chromium oxide on sintering of boron carbide : N. Frage et al. (Ben-Gurion University of the Negev, Beer-Sheva, Israel.)

Sintering of Boron Carbide Heat‐Treated with Hydrogen

Hydrogen gas (H2) was used to extract B2O3 coatings from boron carbide (B4C) particles, permitting a lower temperature onset of sintering and restricting coarsening via solution and precipitation of B4C in B2O3 liquid. Remnant H2 had to be removed from the furnace before specimens were heated through temperature ranges in which evaporation‐condensation coarsening competed with sintering (2010°–2140°C), because the presence of H2 increased the B4C vapor pressure. Heat treatment of B4C compacts in a 50:50 H2‐He mixture at 1350°C, followed by a purge of the H2 gas and then rapid heating to 2230°C, resulted in a percentage of theoretical density of 94.7%. This is higher than the value of 92.8%, which was the highest achieved without the use of H2.

Microstructural Coarsening During Sintering of Boron Carbide

The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot‐pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot‐pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density.

Sintering of boron carbide containing small amounts of free carbon

A study was made of the sintering kinetics of grade “ch.” boron carbide, technical boron carbide cleaned of impurities, and boron carbide synthesized from the elements. An investigation was carried out also into the reactive sintering of a mixture of boron and carbon black. Reactive sintering fails to yield dense parts in boron carbide. The best sinterability is exhibited by a fine technical boron carbide powder cleaned of free carbon and other contaminants. Parts with a porosity of less than 5% can be produced from such a powder by pressing and subsequent sintering. However, it is not the presence of free carbon that controls the sintering behavior of boron carbide, since pure and synthesized boron carbide powders containing no free carbon are characterized by poor sinterability. The high activity of the technical powder is probably linked with the presence of structural defects and stresses generated in the course of its manufacture, during milling and quenching. The annealing spectrum of these defects covers a wide temperature range, and consequently the energy of activation for the densification of boron carbide steadily grows with increase in shrinkage.

Formation and reactivity of borides, carbides and silicides. II. Production and sintering of boron carbide

AbstractThe principal methods of boron carbide production are compared and their thermodynamics are examined. Mechanism of carbide formation and sintering of the products are discussed and investigated further. High‐purity stoicheiometric B4C of submicron size was produced on a semi‐technical scale by reduction of diboron trioxide with carbon and magnesium. Rates of sintering were determined from changes in surface areas and average crystallite and aggreagte sizes.Sintering of boron carbide was enhanced by increased temperature and time of calcination. Addition of chromium accelerated sintering at temperatures above 1600° and especially at 1800°. For more extensive sintering, submicron powders from the magnesium reduction process were more suitable than the coarser samples given by the electro‐thermal carbon reduction; the latter required ballmilling to provide suitable grain size composition for effective hot pressing.