Scientists unlock why boron carbide fails under shock—and find a new form
Researchers have identified a previously unknown crystal structure of boron carbide that forms under extreme pressure, solving a decades-old puzzle about why this aerospace ceramic catastrophically weakens during impacts. The discovery could lead to tougher materials for aircraft, armor, and industrial equipment that better withstand sudden mechanical stress.
Originaltitel: High-pressure formation and characterization of a boron carbide polymorph featuring bent C–B–C chains
Boron carbide is a material of choice for multiple industries, e.g., aerospace, as a lightweight structural ceramic due to its high hardness, high melting temperature, and low density. However, its mechanical properties have been observed to radically degrade under shockwave compression, presumably as a consequence of stress-induced phase transitions resulting in its partial amorphization. So far, the physical mechanism underpinning this behavior remains unclear. Here, we report a pressure-induced phase transition in boron carbide occurring between 78 and 90 GPa during static compression in diamond anvil cells, both at room temperature and after quenching from high temperatures. The crystal structure of the new phase was solved and refined via synchrotron single-crystal x-ray diffraction measurements and further investigated by Raman spectroscopy as well as density functional theory calculations. The discovered high-pressure polymorph, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi mathvariant="italic">mC</a:mi> <a:mn>60</a:mn> </a:mrow> <a:mtext>−</a:mtext> <a:mrow> <a:msub> <a:mi mathvariant="normal">B</a:mi> <a:mn>13</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">C</a:mi> <a:mn>2</a:mn> </a:msub> </a:mrow> </a:math> , has strong resemblance with the known ambient conditions phase, <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mi mathvariant="italic">hR</e:mi> <e:mn>45</e:mn> </e:mrow> <e:mtext>−</e:mtext> <e:mrow> <e:msub> <e:mi mathvariant="normal">B</e:mi> <e:mn>13</e:mn> </e:msub> <e:msub> <e:mi mathvariant="normal">C</e:mi> <e:mn>2</e:mn> </e:msub> </e:mrow> </e:math> , with the important distinction that the linear C–B–C chain linking <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:msub> <i:mi mathvariant="normal">B</i:mi> <i:mn>12</i:mn> </i:msub> </i:math> icosahedra in <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mi mathvariant="italic">hR</k:mi> <k:mn>45</k:mn> </k:mrow> <k:mtext>−</k:mtext> <k:mrow> <k:msub> <k:mi mathvariant="normal">B</k:mi> <k:mn>13</k:mn> </k:msub> <k:msub> <k:mi mathvariant="normal">C</k:mi> <k:mn>2</k:mn> </k:msub> </k:mrow> </k:math> is bent in <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"> <o:mrow> <o:mi mathvariant="italic">mC</o:mi> <o:mn>60</o:mn> </o:mrow> <o:mtext>−</o:mtext> <o:mrow> <o:msub> <o:mi mathvariant="normal">B</o:mi> <o:mn>13</o:mn> </o:msub> <o:msub> <o:mi mathvariant="normal">C</o:mi> <o:mn>2</o:mn> </o:msub> </o:mrow> </o:math> . Such bending of the C–B–C chain has been hypothesized as key to explain boron carbide's drop in strength, phase transitions, and amorphization. The observed reversibility of the phase transition, as well as the formation of covalent bonds between <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"> <s:msub> <s:mi mathvariant="normal">B</s:mi> <s:mn>12</s:mn> </s:msub> </s:math> icosahedra and the bent C–B–C chains, are assessed to decipher the C–B–C chain's bending importance on the amorphization and mechanical properties of boron carbide.