The densification behavior of 20–40-nm graphite-coated B4C nano-particles was studied using dilat... more The densification behavior of 20–40-nm graphite-coated B4C nano-particles was studied using dilatometry, x-ray diffraction, and electron microscopy. The sintering onset temperature was higher than expected from a nanoscale powder (∼1500 °C); remnant B2O3 kept particles separated until B2O3 volatilization, and the graphite coatings imposed particle-to-particle contact of a substance more refractory than B4C. Solid-state sintering (1500–1850 °C) was followed by a substantial slowing of contraction rate attributed to the formation of eutectic liquid droplets more than 10× the size of the original nano-particles. These droplets were induced to form well below the B4C-graphite eutectic temperature by the high surface energy of nanoparticles. They were interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded detection of continued solid-state sintering. Starting at 2200 °C, a permanent and interconnected liquid phase formed, which facilitated rapid contraction by liquid phase sintering and/or compact slumping.
Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soa... more Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soak temperature, and holding at that temperature for the minimum time required to reach terminal density. Although the relative densities of pressureless sintered specimens were lower than that of commercially produced hot-pressed B4C, their (Vickers) hardness values were comparable. For 4.45-cm-diameter, 1.35-cm-high disk-shaped specimens, pressureless sintered to at least 93.0% relative density, post-hot isostatic pressing resulted in vast increases in relative densities (e.g., 100.0%) and hardness values significantly greater than that of commercially produced hot-pressed B4C.
Boron carbide, most often used in personnel armor systems, is rather difficult to form and is alm... more Boron carbide, most often used in personnel armor systems, is rather difficult to form and is almost always densified from ceramic powders under heat and pressure. Recent developments by PPG and Georgia Tech are allowing for the full densification of nano-sized powders through a pressureless sintering route. PPG is manufacturing these powders in a plasma processing technique, and they are less expensive than traditionally produced powders. The powders are fonned directly in the plasma, and they can be doped with densification aids. Pressureless sintering teclmiques developed by Georgia Tech offers a low-cost processing route for B4C that was previously unavailable. Together both efforts will lead to boron carbide with shapes and sizes unavailable today and using a fine-grained domestically produced starting powder.
The processing of boron carbide powder including sintering optimization, green body optimization ... more The processing of boron carbide powder including sintering optimization, green body optimization and sintering behavior of nano-sized boron carbide was investigated for the development of complex shaped body armor. Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soak temperature, and holding at that temperature for the minimum time required to reach terminal density. Although the relative densities of pressureless sintered specimens were lower than that of commercially produced hot-pressed B4C, their (Vickers) hardness values were comparable. For 4.45 cm dia. 1.35 cm height disk-shaped specimens, pressureless sintered to at least 93.0% relative density, post-hot isostatic pressing resulted in vast increases in relative densities (e.g. 100.0%) and hardness values significantly greater than that of commercially produced hot-pressed B 4C. The densification behavior of 20-40nm graphite-coated B4C nano-particles was studied using dilatometry, x-ray diffraction and electron microscopy. The higher than expected sintering onset from a nano-scale powder (˜1500°C) was caused by remnant B2O3 not removed by methanol washing, keeping particles separated until volatilization, and the carbon coatings, which imposed particle to particle contact of a substance more refractory than B4C. Solid state sintering (1500-1850°C) was followed by an arrest in contraction attributed to formation of eutectic liquid droplets of size more than 10X the original nano-particles. These droplets, induced to form well below known B4C-graphite eutectic temperatures by the high surface energy of nanoparticles, are interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded continued solid state sintering. Starting at 2200°C, a permanent liquid phase formed which facilitated a rapid measured contraction by liquid phase sintering and/or compact slumping.
The densification behavior of 20–40-nm graphite-coated B4C nano-particles was studied using dilat... more The densification behavior of 20–40-nm graphite-coated B4C nano-particles was studied using dilatometry, x-ray diffraction, and electron microscopy. The sintering onset temperature was higher than expected from a nanoscale powder (∼1500 °C); remnant B2O3 kept particles separated until B2O3 volatilization, and the graphite coatings imposed particle-to-particle contact of a substance more refractory than B4C. Solid-state sintering (1500–1850 °C) was followed by a substantial slowing of contraction rate attributed to the formation of eutectic liquid droplets more than 10× the size of the original nano-particles. These droplets were induced to form well below the B4C-graphite eutectic temperature by the high surface energy of nanoparticles. They were interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded detection of continued solid-state sintering. Starting at 2200 °C, a permanent and interconnected liquid phase formed, which facilitated rapid contraction by liquid phase sintering and/or compact slumping.
Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soa... more Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soak temperature, and holding at that temperature for the minimum time required to reach terminal density. Although the relative densities of pressureless sintered specimens were lower than that of commercially produced hot-pressed B4C, their (Vickers) hardness values were comparable. For 4.45-cm-diameter, 1.35-cm-high disk-shaped specimens, pressureless sintered to at least 93.0% relative density, post-hot isostatic pressing resulted in vast increases in relative densities (e.g., 100.0%) and hardness values significantly greater than that of commercially produced hot-pressed B4C.
Boron carbide, most often used in personnel armor systems, is rather difficult to form and is alm... more Boron carbide, most often used in personnel armor systems, is rather difficult to form and is almost always densified from ceramic powders under heat and pressure. Recent developments by PPG and Georgia Tech are allowing for the full densification of nano-sized powders through a pressureless sintering route. PPG is manufacturing these powders in a plasma processing technique, and they are less expensive than traditionally produced powders. The powders are fonned directly in the plasma, and they can be doped with densification aids. Pressureless sintering teclmiques developed by Georgia Tech offers a low-cost processing route for B4C that was previously unavailable. Together both efforts will lead to boron carbide with shapes and sizes unavailable today and using a fine-grained domestically produced starting powder.
The processing of boron carbide powder including sintering optimization, green body optimization ... more The processing of boron carbide powder including sintering optimization, green body optimization and sintering behavior of nano-sized boron carbide was investigated for the development of complex shaped body armor. Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soak temperature, and holding at that temperature for the minimum time required to reach terminal density. Although the relative densities of pressureless sintered specimens were lower than that of commercially produced hot-pressed B4C, their (Vickers) hardness values were comparable. For 4.45 cm dia. 1.35 cm height disk-shaped specimens, pressureless sintered to at least 93.0% relative density, post-hot isostatic pressing resulted in vast increases in relative densities (e.g. 100.0%) and hardness values significantly greater than that of commercially produced hot-pressed B 4C. The densification behavior of 20-40nm graphite-coated B4C nano-particles was studied using dilatometry, x-ray diffraction and electron microscopy. The higher than expected sintering onset from a nano-scale powder (˜1500°C) was caused by remnant B2O3 not removed by methanol washing, keeping particles separated until volatilization, and the carbon coatings, which imposed particle to particle contact of a substance more refractory than B4C. Solid state sintering (1500-1850°C) was followed by an arrest in contraction attributed to formation of eutectic liquid droplets of size more than 10X the original nano-particles. These droplets, induced to form well below known B4C-graphite eutectic temperatures by the high surface energy of nanoparticles, are interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded continued solid state sintering. Starting at 2200°C, a permanent liquid phase formed which facilitated a rapid measured contraction by liquid phase sintering and/or compact slumping.
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