Multilayer Deposition of Metal–Phenolic Networks for Coating of Energetic Crystals: Modulated Surface Structures and Highly Enhanced Thermal Stability

Phase transition under thermal stimulation remains a long-standing problem for practical applications of polymorphic energetic crystals such as 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). Herein, tannic acid (TA)-based metal–phenolic networks (MPNs), an emerging class of coatings formed through coordination chemistry, were introduced to improve the thermal stability of energetic HMX. Detailed characterizations were systematically conducted and confirmed the formation of a compact core–shell structure upon in situ noncovalent decoration of polyphenols and FeIII under ambient conditions. Additionally, the dynamic nature of MPNs assembly facilitated multilayer deposition with tunable coverage and thickness. As a result, the phase transition (β → δ) temperature of optimized HMX@MPNs was significantly improved by 42.3 °C at low coating loadings (1.8 wt %), as revealed by thermogravimetry–differential scanning calorimetry (TG–DSC) and in situ X-ray diffraction (XRD) techniques. On this basis, an in-depth understanding of the phase-transition mechanism was achieved by employing isothermal kinetics modeling. This work demonstrates a facile and robust coating strategy for modularized surface engineering of energetic crystals, resulting in remarkably enhanced thermal stability.