Mechanical properties and failure mechanisms of different rank coals at the nanoscale

•Hardness, elastic modulus and fracture toughness have a good linear relationship.•Minerals could strengthen the macro-mechanical properties of coal rock.•Six-membered ring structure plays an important role in deformation resistance.•Failure mechanism of coal by the load is revealed. It is of great significance to explore the mechanical properties and failure mechanism of different rank coals at the nanoscale for preventing mine dynamic disasters. X-ray diffraction (XRD), mercury intrusion, and nano-indentation were used to study the relationship between rank, microstructure and mechanical properties of different rank coals, and using Mori-Tanaka method to upgrade the scale. To reveal the failure mechanism of coal, the nanoindentation process was simulated through molecular dynamics (MD). The results indicated that with the increase in coal rank, the peak load, hardness, elastic modulus, resistance of the six-membered ring structure, fracture toughness, the aromatic layer interlayer spacing (d002), positively proportional to lateral sizes (La), stacking height (Lc), and the number of aromatic layers (n) increased, while the plastic deformation, permeability, porosity, mineral content, and thickness of the damaged layer decreased. Minerals could strengthen the macro-mechanical properties of coal rock, while the pores weaken it. The load destroyed the bonds, angles, dihedral angles, and out-of-plane of coal macromolecules, which led to the plastic deformation of coal. And bonds and angles had the greatest influence on the failure of coal, followed by dihedral angles, and finally out-of-plane.