Evolution of Frictional Temperature Tise and Its Mechanism of Energetic Single Crystal β-HMX/Copper Interface

In order to understand the evolution law of frictional temperature at the β-octogen (β-HMX) surface when rubbing against copper, where the underlying mechanisms remain poorly understood, the friction characteristics and the evolution law of the interfacial temperature rise of the β-HMX surface when rubbing with a copper ball were studied in the present study, utilizing the multi-functional ball-on-flat tribometer and an in-situ temperature measurement system incorporating ultrafast infrared technology during the tribological experiments. The experimental results showed that the friction coefficient of the β-HMX interface decreased firstly and then increased as the contact pressure increased from 302 MPa to 690 MPa, while it increased firstly and then decreased as the sliding speed increased from 20 mm/s to 200 mm/s. This complex behavior highlighted the intricate interplay of mechanical forces and thermal effects at the interface. Combined with the analysis of the wear scar area at β-HMX surface by optical microscopy and Raman spectroscopy, it was found that the mechanical wear effect such as adhesive friction and brittle failure dominated the friction and wear behavior of the β-HMX surface when rubbing against copper ball. The detailed examination of the wear patterns revealed insights into the material removal mechanisms and the subsequent impact on the structural integrity of β-HMX. Moreover, it was found that with the increase in the contact pressure and sliding speed, the maximum frictional temperature rise on the surface of the β-HMX single crystal gradually increased, which was in contrast to the earlier observed variation of the friction coefficient. Further analysis showed that the maximum frictional temperature rise on the surface of β-HMX single crystal was closely related to the frictional power density, which was the product of contact pressure and sliding speed as well as friction coefficient. The physical significance of frictional power density was defined as the work done by friction per unit area over a unit time. Analyzing this from both spatial and temporal perspectives was critical for understanding the relationship, which was essential for predicting thermal behavior and ensuring the safe operation of β-HMX under various conditions. Furthermore, the frictional work-heat conversion coefficient of the β-HMX single crystal interface increased firstly and then stabilized with the increase of frictional power density, which was closely related to the heat distribution coefficient of the interface and the change of crystal friction damage. This stabilization suggested a limit to the material's heat dissipation capacity, which could lead to critical thermal events under high-load conditions. If the thermal decomposition temperature of β-HMX was used as the critical condition for friction safety evaluation, then the critical frictional power density of β-HMX surface when rubbing with a copper ball at room temperature was determined as 111 W/mm². This indicated that when the frictional power density at the copper/β-HMX interface was below 111 W/mm², the friction conditions were within a safe range. This value could be served as a crucial indicator for assessing the safety of β-HMX in practical applications and holded significant guidance for optimizing the safe production processes and active safety controls of explosive crystals.