Micrometer-scale structural alterations and mesoscopic damage mechanisms in coal induced by SC-CO2 impact

Understanding mesoscopic scale damage to coal subjected to SC-CO2 impacts is significant in revealing how transient, high-pressure impact affects micro-nano pore evolution in coal. A custom-designed SC-CO2 impact experimental system was developed to prepare impact-treated bituminous coal samples. The micron-scale surface morphology distribution, mechanical property parameters, and functional group content of coal samples were characterized before and after SC-CO2 impact through atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The mesoscopic failure mechanisms of coal induced by SC-CO2 were systematically investigated by correlating the evolution of surface topography, chemical bonding states, and localized mechanical degradation. It was observed that the impacted coal exhibited an average elastic modulus of 3.021 GPa, which is 10.2 % lower than that for original coal samples. Surface roughness parameters Ra and Rq recorded values of 11.09 nm and 14.60 nm, respectively, which are increases of 24.3 % and 27.4 %. The SC-CO2 impact mechanism comprises three synergistic processes: high-velocity stress waves, through hybrid fluid compression, result in the stress concentration along weak internal planes, producing a gas-wedge effect and subsequent propagating, localized tensile fractures. Oxygen-containing functional groups and aliphatic chains were preferentially removed through selective extraction, promoting the condensation of aromatic structures and reorganization of unsaturated bonds. The strength of intermolecular forces was reduced, and the material porosity was increased due to this molecular rearrangement. On a structural level, this means the transition from pre-damage by stress waves, via gas-wedge effects and extraction, into the continuous, heterogeneous brittle fragmentation of the structure.