## 4 Conclusions  
1. Under initial conditions, the coal surface is relatively dense and intact, exhibiting fewer cracks and pores. After humidification, dissolution pores appear in certain surface regions, resulting in a smoother transition between particles and the matrix. Following heating, the surface roughness of certain areas on the coal sample increased, accompanied by a localized disintegration and collapse. After the hygothermal coupled treatment, the number of fine dissolution pores and microcracks on the coal surface increased markedly.
2. Compared with the initial state, humidification leads to the softening of the coal surface, with average values of \(h_{\text{m}}\) and \(h_{\text{p}}\) increasing by 15.9% and 41.27%, respectively, while values of \(E_{\text{TI}}\) and \(H_{\text{IT}}\) decrease by 17.14% and 29.53%. Heating induces surface  
Figure 14: Characteristic points and corresponding load–depth curves under initial and hygothermal coupled conditions. (**a**) Initial condition. (**b**) Hygrothermal coupling condition. (**c**) Initial state characteristic minerals’ load–depth curve. (**d**) Hygrothermal state characteristic minerals’ load–depth curve.  
hardening, leading to slight decreases in \(h_{\rm m}\) and \(h_{\rm p}\), by 0.4% and 1.15%, and corresponding increases in \(E_{\rm IT}\) and \(H_{\rm IT}\), by 1.56% and 1.32%. Under coupled hygothermal conditions, \(h_{\rm m}\) and \(h_{\rm p}\) increase by 4.45% and 16.01%, respectively, while \(E_{\rm IT}\) and \(H_{\rm IT}\) decrease by 4.94% and 12.45%. The extent of the surface damage under hygothermal coupling falls between that observed under humidification and heating alone.
3. A linear model passing through the origin provides an excellent fit for the relationship between the hardness and elastic modulus under different conditions. The hardness of coal exhibits a positive correlation with its elastic modulus, and this relationship is independent of the coal's condition.
4. The mineral composition on the coal surface has a significant influence on its micromechanical properties. Hard minerals such as quartz tend to result in lower peak indentation depths and higher hardness values, whereas softer minerals like kaolinite lead to higher peak indentation depths and lower hardness values.  
Conceptualization, H.Z.; Formal analysis, J.X. and Y.S.; Methodology, H.Z. and G.L.; Visualization, F.H., H.H., M.H. and B.T.; Writing--original draft, all authors; Writing--review and editing, H.Z. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the National Natural Science Foundation of China (U22A20165); Special Project Support for Key Research and Development Tasks of Xinjiang Uygur Autonomous Region (2023B01010-1); Key R&D Program of Shandong Province (2024 CXGC010322); Graduate Innovation Program of China University of Mining and Technology (2025 WLJCRCZL044); and Guizhou Provincial Program on Commercialization of Scientific and Technological Achivements (LH [2024]ZhongDa024).  
Not applicable.  
The original contributions presented in the study are included in the article material; further inquiries can be directed to the corresponding authors.  
Thanks for the reviewers' comments and editor's work. In addition, [PERSON] acknowledges, in particular, the powerful support received from his girlfriend, [PERSON], over the years.  
The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.