Effect of Thermally Aging on Frictional Properties of HMX-Based Polymer Bonded Explosives

In order to solve the problem that the evolution law and mechanism of the friction behaviors in polymer bonded explosives (PBX) after thermally aging is not clear, the current study focused on investigating the evolution and mechanism of friction behaviors, as well as the friction safety properties of octogen (HMX) based PBX explosives. The objective was to gain insights into how these properties change over time and the underlying mechanism involved in the frictional response of PBX explosives subjected to thermally aging. Firstly, the investigation into the variation of the friction sensitivity of PBX after the thermally aging was conducted using the friction sensitivity testing machine. The results revealed a substantial change, as the friction sensitivity of PBX escalated from 16% prior to aging treatment to 46% after undergoing a 50 days thermally aging treatment. This observation underscored the significant impact of thermally aging on the frictional sensitivity of PBX. Using energetic single crystal HMX as the counterpart material, the study delved into the mechanistic influence of thermally aging treatment on the frictional behaviors of HMX-based PBX. This investigation was conducted using a multifunctional friction and wear test machine, aiming to comprehensively understand how thermally aging treatment affected the friction characteristics of PBX. The investigation revealed that, as the durations of thermally aging increased, the steady-state friction coefficient at the PBX interface remained relatively constant, when rubbing with the stainless steel that was used in the friction sensitivity tests. Extending the analysis to include the friction behaviors of both HMX single crystal and the binder in PBX with HMX single crystals, it was observed that the steady-state friction coefficient on the surface of HMX single crystal exhibited almost unchanged with thermally aging treatment. Moreover, the microscale mechanical properties such as nanohardness and elastic modulus, as well as the chemical structure and root mean square (RMS) of surface roughness on the HMX single crystal surface were not significantly impacted by the thermally aging treatment. This suggested that despite the durations of aging, the key characteristics of the HMX single crystal remained stable, contributing to the observed consistency in the steady-state friction coefficient at the PBX interface. The steady-state value of friction coefficient at the interface between the binder and HMX was around 0.3 in the absence of thermally aging treatment. However, after a 50 days thermally aging treatment, the steady-state friction coefficient at the interface rose to approximately 0.6. Subsequent analyses revealed that the binder underwent noticeable crystallization after thermally aging treatment, leading to a substantial increase in both nanohardness and elastic modulus of the binder material. This transformation in the binder's properties provided an explanation for the variations observed in the friction temperature rise at the interface between the binder and HMX, as well as the concurrent increase in the friction sensitivity of the PBX explosives. The observed changes in the binder's crystalline structure and mechanical properties evidently contributed to the variation friction behaviors and sensitivity of the PBX explosive.