TY - GEN
T1 - Root causes of the performance of boron carbide under stress
AU - Fanchini, Giovanni
AU - Niesz, Dale E.
AU - Haber, Richard A.
AU - McCauley, James W.
AU - Chhowalla, Manish
PY - 2006/12/1
Y1 - 2006/12/1
N2 - The absence of a plastic phase in boron carbide and its failure at shock impact velocities just above the Hugoniot elastic limit (HEL) has been the subject of several experimental investigations. Furthermore, the common presence of contaminants, such as disordered graphitic inclusions, oxygen, etc., needs to be addressed. Further, a theoretical picture accounting all these phenomena is still lacking. In the present work, using self-consistent field density functional simulations we are able to account for many experimental observations by noticing that several boron carbide polytypes [e.g. (B 11C)CBC, (B12)CCC, ...] coexist without significant lattice distortions. Our analysis also indicates that above a threshold pressure all such polytypes are less stable than a phase involving segregated boron (812) and amorphous carbon (a-C) but the energy barrier for the transformation into a segregated phase of boron and carbon, is by far lower for the B 12(CCC) polytype. For such a configuration, segregation of carbon occurs in layers orthogonal to the (113) lattice directions, in excellent agreement with recent transmission electron microscopy (TEM) analysis. We will also, in the actual preparation conditions of the material, show that the Gibbs free energy per site in the B12 + a-C segregate phase, in B 12O2 and B4C1-xSix is not significantly lower than in most of the B4C polytypes. Silicon inclusions, however, should strongly reduce the formation of the (B 12)CCC phase.
AB - The absence of a plastic phase in boron carbide and its failure at shock impact velocities just above the Hugoniot elastic limit (HEL) has been the subject of several experimental investigations. Furthermore, the common presence of contaminants, such as disordered graphitic inclusions, oxygen, etc., needs to be addressed. Further, a theoretical picture accounting all these phenomena is still lacking. In the present work, using self-consistent field density functional simulations we are able to account for many experimental observations by noticing that several boron carbide polytypes [e.g. (B 11C)CBC, (B12)CCC, ...] coexist without significant lattice distortions. Our analysis also indicates that above a threshold pressure all such polytypes are less stable than a phase involving segregated boron (812) and amorphous carbon (a-C) but the energy barrier for the transformation into a segregated phase of boron and carbon, is by far lower for the B 12(CCC) polytype. For such a configuration, segregation of carbon occurs in layers orthogonal to the (113) lattice directions, in excellent agreement with recent transmission electron microscopy (TEM) analysis. We will also, in the actual preparation conditions of the material, show that the Gibbs free energy per site in the B12 + a-C segregate phase, in B 12O2 and B4C1-xSix is not significantly lower than in most of the B4C polytypes. Silicon inclusions, however, should strongly reduce the formation of the (B 12)CCC phase.
UR - http://www.scopus.com/inward/record.url?scp=33845924190&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=33845924190&partnerID=8YFLogxK
M3 - Conference contribution
AN - SCOPUS:33845924190
SN - 0470080574
SN - 9780470080573
T3 - Ceramic Engineering and Science Proceedings
SP - 179
EP - 188
BT - Advances in Ceramic Armor II - A Collection of Papers Presented at the 30th International Conference on Advanced Ceramics and Composites
T2 - Advances in Ceramic Armor II - 30th International Conference on Advanced Ceramics and Composites
Y2 - 22 January 2006 through 27 January 2006
ER -