IN-SITU X-RAY DIFFRACTION OF SHOCK LOADED BORON CARBIDE (B₄C)

B₄C is high-strength material of importance for the design of better lightweight impact resistant coatings and tiles for satellites, spacecraft, space stations, vehicles and personal armor. During ballistic impact conditions at pressures below 20 GPa, this ceramic exhibits high strength, which allows it to form a rigid barrier and effectively break impacting projectiles. However, at higher pressures, this material is observed to have a strength collapse and undergo possible phase transitions or amorphization. Currently, the mechanics for the catastrophic loss in strength are unknown as well as the structure of the possible new phases. Previous shock wave experiments have relied on interface wave profiles that lacked the necessary in situ atomic lattice response information needed for constitutive design models.

In this study, we determine the lattice-level structure of B₄C under shock wave compression to 300 GPa. By combining laser compression with pulsed X-ray diffraction, we are able to obtain in situ structural information on this material for the first time. Experiments were performed at the Materials in Extreme Conditions beamline of the Linac Coherent Light Source (LCLS) at Stanford university. At LCLS, shock waves are driven into the sample using a 40-J laser with a ~10-ns pulse. The sample is probed with X-rays form the LCLS free electron laser. Diffraction is recorded using pixel array detectors.

Our diffraction data provide evidence for partial amorphization of B₄C which may explain changes in the material’s continuum-level Hugoniot response. However, there is no evidence for a crystallographic phase transition. Our results also indicate that B₄C is more compressible than indicated by previous low-pressure data.