Microscopic Mechanisms of Femtosecond Laser Ablation of HMX from Reactive Molecular Dynamics Simulations

With ultrashort duration and ultrahigh energy, femtosecond laser (fs-laser) pulses are very promising for the precision machining of energetic materials. Compared with the mechanical machining methods of energetic materials, fs-laser machining technology has the advantages of high safety, high precision, and absence of pollution. A deep understanding of the mechanisms between fs-lasers and energetic materials is the basis for the development of fs-laser machining technology. In this paper, the method of reactive molecular dynamics (ReaxFF-MD) was adopted to calculate the fs-laser ablation process of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX, a high explosive compound), and the ablation mechanisms of HMX under different fs-laser energies were studied. The results show that the fs-laser ablation mechanisms of HMX are related to the laser power density. When the laser power density is high enough (3.4 × 1014 W/cm2, 1.0 mJ/pulse), HMX undergoes ionization or decomposition reactions at the picosecond level (∼7.65 ps) and produces a high temperature and pressure plasma. Many N, H, and O single atoms and their ionic products occur along with some small molecular fragments of NO2, H2O, CO2, N2, H2, NH, NH2, CO, OH, CNO2 and very few toxic products of NO and HNO2. In this case, the removal process of HMX occurs via a phase explosion mechanism. As the laser energy decreases, the ionization degree of ablation products decreases, in which the number of monatomic and ionic products decreases, while the number of toxic small molecules (such as NO, HNO2, and HNO) increases. When the laser power density is relatively low (0.34 × 1014 W/cm2, 0.1 mJ/pulse), the removal process of HMX occurs via a photomechanical mechanism, and the compound escapes as intact initial HMX molecules. When the laser power density is close to the ablation threshold of the explosive, the HMX molecules only undergo a melting state to some extent without escaping from the surface of the crystal. Therefore, the fs-laser can be used in the precise machining of explosives and preparation of high-purity energetic nanomaterials by a reasonable selection of fs-laser energy.