Modifying the surface properties of energetic materials with coating has theoretical and practical significance in the field of their application. Wax, in its role as a functional lubrication layer, has extensive utility in mitigating the sensitivity of explosives to external stimuli. This study aims to investigate the friction behavior and temperature rise of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-cyclotetramethylenetetranitramine, or β-HMX) crystals with and without the wax coating, and the friction coefficient, frictional temperature rise, and frictional work-heat conversion rate were systematically analyzed. The experimental results show that regardless of single and reciprocating scratch conditions, the wax coating significantly reduced the coefficient of friction of the β-HMX surface by up to 63% and effectively suppressed the temperature rise during friction by up to 83%. Further analyses reveal that under single scratch conditions, the frictional work-heat conversion rate of the pristine β-HMX surface is approximately 42%, whereas that of the wax-coated surface increases to about 30% with the frictional power density, which involves the contact pressure, sliding speed, and friction coefficient. In contrast, under reciprocating scratch conditions, the frictional work-heat conversion rate of the uncoated β-HMX surface decreases with increasing number of sliding cycles, which is related to the increase in surface temperature and potential surface damage. The frictional work-heat conversion rate of wax-coated surfaces increases with increasing number of sliding cycles until the contact pressure is high enough that the lubricating effect of the wax layer diminish and the conversion rate begin to decrease. The obtained results not only pave the way for understanding the friction energy regulation and desensitization mechanism of wax on the surface of energetic crystals at the micro and quantitative levels but also provide a necessary basis for establishing friction hotspot models under dry friction and wax lubrication conditions from the aspects of friction characteristic parameters, frictional temperature rise, and the work-heat conversion rate.

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 β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) crystal is studied with nanoscratch tests under the ramping load mode. The results show that the nanoscale friction and wear of β-HMX crystal, as a typical energetic material, is highly depended on the applied load. The friction coefficient of β-HMX crystal is initially high when no discernible wear is observed, and then it decreases to a stable value which varies from ~0.2 to ~0.7, depending on the applied load, scratch direction, and crystal planes. The β-HMX (011) surfaces show weakly friction and wear anisotropy behavior; in contrast, the β-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 β-HMX surfaces play key roles in determining the nanoscale friction and wear of β-HMX surfaces. The obtained results can provide deeper insight into the friction and wear of energetic crystal materials.