Density- and hardness-optimized pressureless sintered and post-hot isostatic pressed B 4 C
- PDF / 1,716,846 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 59 Downloads / 228 Views
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.
I. INTRODUCTION
Boron carbide is the third hardest material next to diamond and cubic boron nitride. Combined with its low theoretical density (2.52 g/cm3), it is the premier material for personal armor—typically in the form of front and back inserts into flack jackets in which B4C plates are bonded to a polymer backing. B4C is also used for nuclear shielding applications based on boron’s high neutron absorption cross section. It is used in particulate form as an abrasive and as a nozzle material for slurry pumping and grit blasting because of its excellent abrasion resistance.1 Boron carbide exists as a solid solution in the range of 8.8–20.0 mol% carbon.2 Its crystal structure is complex due to the highly covalent nature of its interatomic cohesion. Twelve atom boron-rich icosahedra reside at the corners of a rhombohedron, and each icosahedron is bonded to six others via direct bonds; three-atom intericosahedral chains reside between the icosahedra.3 Effective ballistic armor materials must have very high hardness4,5 combined with high fracture toughness. When a high-velocity projectile impacts the surface of a armor material such as B4C, a compressive shock wave extends hemispherically from the impact point, generating tensile tangential stresses, in turn forming radial cracks, which emanate from the point of contact. These tangential stresses tear open cracks preferentially at the
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0260 2110
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 8, Aug 2005 Downloaded: 22 Jan 2015
site of pores and fissures. As a result, ballistic performance improves with decreasing porosity, i.e., with increasing fired relative density. Pressureless sintering of B4C to high relative density, however, has proven difficult. Achieving near-theoretical density has required gang-hot pressing (stacked parts under uni-axial pressure), which precludes the formation of complex shapes. Additives such as SiC, Al2O3, TiB2, AlF3, and W2B5 have been used as sintering aids to increase pressureless sintered relative density,6–9 but the second phases formed often have deleterious effects on mechanical behavior.1 The best known additive for B4C is carbon, most successfully added in the form of phenolic resin, which distributes carbon around the B4C particles and
Data Loading...
