Study on nano-mechanical behavior of coal using nanoindentation tests

The measurement and research on the mechanical properties of coal bodies have important theoretical value for efficient coal mining and disaster prevention. Coal is relatively soft and fragmented, with low strength, making it difficult to prepare standard mechanical test coal samples and impossible to be recycled for repeated testing. There is an urgent need to explore new mechanical testing methods for improving the research on the mechanical properties of coal. Nanoindentation technology can measure the micro mechanical properties of small-scale coal, which has the characteristics of easy sample preparation, fast experiment, and no damage to samples. The physical and micro-mechanical properties of four coals are investigated with the mineral composition testing, morphological scanning and nanoindentation tests. The results show that the minerals of coal samples mainly include amorphous organic matter, clay, quartz and carbonate minerals, and there are significant differences in the mineral compositions of different coals. The 3D morphology of coals indicates that there are huge differences in surface roughness between different coals, and the distribution of minerals in coals exhibits significant heterogeneity. The results of nanoindentation test show that: ① the higher the mass fractions of quartz and carbonate minerals in coal samples, the stronger the surface mechanical properties of samples and the smaller the indentation depth; ② the more complex the components on the surface of samples, the more obvious the heterogeneity of coal, and the higher the degree of dispersion of surface micro mechanical properties distribution; ③ the higher the rank of coal, the higher the degree of metamorphism, and the more developed the external pores, leading to an increase in fracture toughness; ④ fitting the indentation test results reveal a significant linear relationship between elastic modulus and hardness, as well as between elastic modulus and fracture toughness. The linear relationship between elastic modulus and fracture toughness is influenced by peak load. © 2024 China Coal Society. All rights reserved.