Nanoindentation Mechanical Properties of Coal Macerals and Their Response to Evolution of micro-Raman Structure

Abstract

Eighteen coals of various rank with Ro,max = 0.29–3.36% were selected from eight coalfields in North China. Nanoindentation at peak load of 10 mN and 30 mN was used to characterize the nanomechanical properties of vitrinite and inertinite in coals, and the results were combined with micro-Raman analysis to examine their response to evolution of the chemical structure. It indicated that the mechanical properties and the Raman structural responses were both coal-rank-dependent. The Young’s modulus and hardness of inertinite are generally higher than those of vitrinite. The structural parameters of coal macerals derived from Raman first- and second-order analyses have abrupt changes with Ro,max = 0.70%, 1.06%, 1.47%, 1.74%, and 2.06% during the coalification process, which preferably responds to the evolution of Young’s modulus and hardness. The staged evolutionary distributions are consistent with the stages of oil and gaseous generation of organic matters. Thus, the response mechanism of nanoindentation mechanical properties of coal to the evolution of Raman structure was analyzed. As Ro,max = 0.29–1.06%, the nanomechanical parameters decrease due to asphaltification and breakage of alkane branches on aromatic rings. The stage can be divided into two stages by oil-generation peak with Ro,max = 0.70%. As Ro,max = 1.06–1.47%, the degree of aromatic nucleus condensation gradually increases, and the mechanical structure becomes more homogeneous, which results in a dramatic increase of the elastic modulus and hardness. As Ro,max = 1.47–1.74%, oil generation is near completion, the secondary cracking happens, and aromatic clusters form rapidly. The elastic modulus and hardness increase dramatically. As Ro,max = 1.74–2.06%, the gas generation amount decreases slowly. The elastic modulus and hardness decrease sharply due to macromolecular arrangements. As Ro,max = 2.06–3.36%, dry gas generation is continuously ongoing, while the elastic modulus and hardness increase steadly. The findings are helpful to decipher the molecular mechanism of coal nanomechanical evolution, explore the relations between chemical structure and performance, and provide mechanistic insights of coal reservoir stimulation and new technology development.