Crystallographic orientation dependence on nanoscale friction behavior of energetic beta-HMX crystal

Tribology behaviors of energetic crystals play critical roles in the friction-induced hotspot in high-energy explosive, however, the binder and energetic crystals are not distinguished properly in previous investigations. In this study, for the first time, the nanoscale friction of beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX) crystal is studied with nanoscratch tests under the ramping load mode. The results show that the nanoscale friction and wear of beta-HMX crystal, as a typical energetic material, is highly depended on the applied load. The friction coefficient of beta-HMX crystal is initially high when no discernible wear is observed, and then it decreases to a stable value which varies from similar to 0.2 to similar to 0.7, depending on the applied load, scratch direction, and crystal planes. The beta-HMX (011) surfaces show weakly friction and wear anisotropy behavior; in contrast, the beta-HMX (110) surfaces show strongly friction and wear anisotropy behavior where the friction coefficient, critical load for the elastic-plastic deformation transition and plastic-cracking deformation transition, and deformation index at higher normal load are highly depended on the scratch directions. Further analyses indicate the slip system and direction of beta-HMX surfaces play key roles in determining the nanoscale friction and wear of beta-HMX surfaces. The obtained results can provide deeper insight into the friction and wear of energetic crystal materials.