## 3 Results and Discussion
### Analysis of Micromechanical Properties of Coal Samples  
#### 3.2.1 Load-Depth Curve Characteristics  
The SEM imaging and EDS elemental analysis can only provide qualitative or semi-quantitative insights into the presence of microstructural surface damage in coal samples under various conditions. Therefore, it is essential to further investigate the mechanical properties and deformation behavior of coal at the microscale. Figure 9 presents the load-displacement curves obtained from nanoindentation tests conducted under different conditions. The curves labeled 1 to 9 in the legend correspond directly to the load-depth responses of indentation Points 1 through 9 within the designated test area. The SEM imaging and EDS elemental analysis are limited to providing qualitative or semi-quantitative assessments of microstructural surface damage in coal samples under various conditions. Therefore, it is necessary to conduct a more in-depth investigation into the mechanical properties and deformation behavior of coal at the microscale. Figure 9 presents the load-depth curves derived from nanoindentation tests performed under varying environmental conditions. The curves labeled 1 to 9 in the legend represent the load-depth responses at indentation Points 1 through 9 within the specified testing region. The test results of the indentation depth (\(h\)) and load (\(F\)) change curves are more consistent with the typical load-depth curve characteristics. As a representative heterogeneous material, coal exhibits variations in its mineral composition even within a single test region, resulting in a noticeable dispersion among the load-depth curves of different indentation points. In the loading stage, before the load reaches the peak, the indentation depth increases rapidly, and then the growth rate slows down. In the load-holding stage, the load remains constant, creep deformation occurs on the surface of the coal sample, and the indentation depth continues to increase. In the unloading stage, the elastic deformation of the coal sample is partially restored, but there is still irreversible plastic deformation. Figure 8: After humidification–heating. (**a**) Surface magnification \(200\times\); (**b**) Surface magnification \(1000\times\). The measured indentation load-depth curves are generally consistent with the typical characteristics of load-depth responses in nanoindentation tests. As a representative heterogeneous material, coal exhibits significant variability in its mineral composition even within a single testing region, leading to the noticeable dispersion among the load-depth curves of different indentation points. During the loading phase, the indentation depth increases rapidly at first, and then the growth rate gradually slows as the load approaches its peak. In the holding phase, the applied load remains constant while creep deformation occurs on the coal surface, resulting in a continued increase in the indentation depth. During unloading, a partial recovery of the elastic deformation is observed, but irreversible plastic deformation remains. As shown in Figure 9, marked variations are evident in the load-depth curves among different indentation points under the same treatment condition. Based on the SEM analysis, Figure 9a illustrates that under the initial condition, the curves at Points 2 and 5 exhibit pronounced deviations from the rest. Point 2 shows a notably higher peak indentation depth, whereas Point 5 presents a significantly lower depth. Figure 9b indicates that after humidification, the coal surface undergoes softening, accompanied by an increase in dissolution pores, which leads to higher peak indentation depths at some locations. Figure 9c reveals that relative to humidification, heating results in the surface hardening of the coal; however, the concurrent development of pores and cracks leads to increased indentation depths at specific locations. Figure 9d demonstrates that following the hygothermal  
Figure 9: Coal sample load–depth curves under different conditions.