Various classification schemes of TDCs have been discussed in the long term (Li 2013; Yao 2017; Song et al., 2020b; Cheng and Pan 2020). Among them, the classification proposed by Ju et al. has been widely accepted (Song et al., 2013; Zhang 2014; Yu et al., 2017; Liu and Jiang 2019; Song et al., 2020a; Song et al., 2020b), where TDCs are classified into three series (brittle-, transitional-, ductile series) (Ju et al., 2004). Ju et al.’s classification scheme is also adopted here. Coal samples in this study were collected from Suxian mining area and Linhuan mining area, Huaibei coalfield, Anhui Province. The samples are primary coal and brittle TDCs. And the latter includes cataclastic coal, schistose coal, mortar coal, granulitic coal, and flaky coal. Coals’ vitrinite reflectance (R_o) values range from 0.93 to 1.22% (Table 1).

Macrographs and micrographs of coal samples are presented in Figure 1. For primary coal, its original structure is unaltered. The coal band is clear, with few fractures developed (Figures 1A,B). For cataclastic coal, the original structure is relatively intact with several fractures developed (Figures 1C,D). For schistose coal, the coal body is plate-like, and the stratification is stable (Figure 1E). Two sets of parallel fractures can be seen in the micrograph (Figure 1F). For mortar coal, the coal body is broken, and the coal band can hardly be identified (Figure 1G). Fractures are densely developed, and the coal matrix is cut into grains of different sizes (Figure 1H). For granulitic coal, the coal body is pulverized into small grains and powder by intensive fractures, and grains are of similar size compared to mortar coal (Figures 1I,J). For flaky coal, the coal body is broken and sheet-like (Figure 1K). Smooth friction surfaces are widely developed. Several long tortuous fractures can be identified in the micrograph (Figure 1L).

Coal samples need to be polished to a smooth surface because AFM has high requirement for sample roughness. As the TDC can be easily broken in the sample-preparation process, the following procedures were taken: 1) Samples were glued with resin and cut into small pieces of several millimeters; 2) Glued coal pieces were inset into epoxy resin in molds; 3) Samples were polished with sandpapers of different particle size; 4) Argon ion polishing (Leica EM TIC 3X, 4 kV, 30 min) was conducted to produce a smooth surface.

The AFM instrument used in this study is the Multimode 8 AFM from the Bruke Company, Germany. The maximum scan range is 125 × 125 × 5 μm. The resolution is 0.1 nm in the lateral direction and 0.01 nm in the vertical direction. ScanAsyst mode was adopted to obtain microtopography images while PeakForce QNM mode was used to analyze the mechanical properties of samples. A scanning range of 5 × 5 μm is taken in both modes.

Gwyddion is a modular, multiplatform, open-source software for scanning probe microscopy data processing (NeČas and Klapetek 2012), which is used to calculate pore parameters here. The “Grain Analysis” module was used to mark and quantify grains on the surface. Pore parameters calculation can be realized by clicking the “Invert Height” function to invert pores to grains (Jiao et al., 2018; Zhao et al., 2019). There are several grain-related algorithms in Gwyddion, and the watershed algorithm was chosen in this study. The basic theory of the watershed algorithm is as follows: 1) The virtual water drop was placed at each point of the inverted surface; 2) The water drop followed the steepest descent path to minimize its potential energy and reached a local minimum; 3) The first two steps were repeated several times, and a system of lakes of different sizes filling the inverted surface depressions was obtained; 4) Then, the position, area and volume of each of the lakes could be identified (Figure 2). Ten topography images were analyzed to produce average pore parameters for each sample.

PeakForce QNM is an imaging mode that produces topography images and quantitative nanomechanical properties images at the same time. PeakForce QNM is based on force curves analysis. While the probe is scanning the sample surface, the deflection of the cantilever is measured to produce Force-Distance curves (Figure 3) (Li et al., 2017). These curves are then analyzed to obtain the properties of the sample (adhesion, modulus, deformation, and dissipation) (Bruker 2012). To obtain Young’s modulus, the retract curve is fit (the bold green line in Figure 3) using the Derjaguin–Muller–Toporov model (Bruker 2012): F t i p − F a d h = 4 3 E ∗ R ( d − d 0 ) 3

F _tip –F _adh is the force on the cantilever relative to the adhesion force, R is the tip end radius, and d–d ₀ is the deformation of the sample. The result of the fit is the reduced modulus E ^* . If the Poisson’s ratio is known, the software can use that information to calculate the Young’s modulus of the sample (E _s ). This is related to the sample modulus by the equation: E ∗ = [ 1 − ν t 2 E t + 1 − ν s E s ] − 1

ν _t and E _t are Poisson’s ratio and Young’s modulus of the tip, respectively. ν _s and E _s are Poisson’s ratio and Young’s modulus of the sample, respectively. We assume that the tip modulus E _t is infinite. Therefore, the Young’s modulus of the sample can be calculated using the sample’s Poisson’s ratio. Here, we chose a Poisson’s ratio of 0.3 for organic matter (OM) in coals.

In PeakForce QNM mode, choosing a suitable probe is crucial for accurate measurement. As the reported Young’s modulus of OM in coals is under 10 GPa in most literature (Li et al., 2020; Meng et al., 2020; Sun et al., 2020), RTESPA-525 probe was selected in this study because it is suitable for samples with modulus between 1 and 20 GPa (Figure 4A). Probe calibration was conducted using a sapphire standard and a highly-ordered pyrolytic graphite standard (HOPG, 18 GPa). After the calibration, coal samples were scanned at a resolution of 256 × 256 pixels at 0.977 Hz. Data analysis was carried out using the “Roughness” module in Nanoscope Analysis software (Figures 4C,D). Five modulus images were analyzed to obtain the average Young’s modulus for each sample.

Figure 5 shows typical topography images of primary coal and different brittle TDCs. For primary coal, pores are ellipse or circle. Most pores are macropores >200 nm in diameter. Pores are unevenly distributed on the surface (Figure 5A). For cataclastic coal, the pore number increases slightly compared to primary coal (Figure 5B). For schistose coal, the pore number increases to a higher extent and mesopores can be seen (Figure 5C). For mortar coal, pores are densely developed. Macropores and mesopores <100 nm in diameter begin to increase (Figure 5D). For granulitic coal, intensive pores are arranged on the surface. Most pores’ diameter are under 100 nm (Figure 5E). For flaky coal, small mesopores are highly developed with few macropores interspersed (Figure 5F). In summary, the pore number increases gradually, and the mean pore size decreases with enhancing tectonic deformation. From primary coal to schistose coal, this trend is relatively mild. The changes of pore number and pore size become much more obvious since mortar coal.

Nanopore parameters, including mean pore number, mean pore size, mean pore area and plane porosity, are listed in Table 2. It should be noted that the pore size here is equivalent disc diameter—the diameter of the disc with the same projected area as the pore. The mean pore number per image of coal samples ranges from 86 to 685. The mean pore size varies from 68.8 to 131.4 nm. And the plane porosity varies from 8.13 to 17.06%. It is clear that the mean pore number increases and the mean pore size decreases as the tectonic deformation deepens.

The topography images and Young’s modulus images of sample WG-4, RL-2 and QYZ-86 are shown in Figure 6. It can be seen that the Young’s modulus of coal matrix and pores are actually the same, indicating that the mechanical properties of vitrinite is homogeneous. Most of the Young’s modulus histograms obey a normal distribution. As a result, the mean (μ) of the normal distribution can be used to represent the Young’s modulus value per image (Figure 4D). Different samples generally have a Young’s modulus of 1.5–2 GPa. And the sectional analyses show that the modulus mode is not strongly affected by milling ridges and other topographic features, so the overall effect of topography on modulus values is relatively minor (Figures 6C,F,L) (Eliyahu et al., 2015; Liu 2019).

Figure 7 shows the variation in the mean pore number and mean pore size in primary coal and different brittle TDCs. From primary coal to flaky coal, the pore number increases while the pore size decreases. The line graph can be distinctly divided into two stages of different gradients. From primary coal to mortar coal is the first stage, while from mortar coal to flaky coal is the second stage. Here, we name these two stages weak brittle deformation stage (WBDS) and strong brittle deformation stage (SBDS). In WBDS, the mean pore number per image increases from 86 to 245, while the mean pore size decreases from 131.4 to 107.7 nm. However, in SBDS, the mean pore number increases from 245 to 685, while the mean pore size decreases from 107.7 to 68.8 nm. The mean pore number and mean pore size in SBDS change at a higher rate, which indicates that tectonic stress has a more profound impact on pore structure in SBDS than that in WBDS.

To further explore the nanopore structure in primary coal and different brittle TDCs, we plotted the curves of the pore number of four pore-size intervals—10–50 nm, 50–100 nm, 100–200 nm and >200 nm (Figure 8). In WBDS, the pore number increases gradually. 49% of the pore number increment is provided by macropores of 100–200 nm diameter. The pore number of pores with 10–50 nm and 50–100 nm diameter increases slightly, while macropores of >200 nm diameter levels off. In SBDS, the pore number increases quickly. 68% of the pore number increment is provided by macropores of 50–100 nm diameter, and 24% of the pore number increment is provided by mesopores of 10–50 nm diameter. The pore number of 100–200 nm increases slightly, while pores >200 nm in diameter disappear. As shown in Figure 8, tectonic stress generally promotes pore development in brittle TDCs. And the extent of tectonic impact varies greatly among different brittle TDCs. In WBDS, tectonic stress mainly helps to increase macropores of 100–200 nm diameter. In SBDS, tectonic stress mainly helps to increase macropores of 50–100 nm and mesopores of 10–50 nm diameter.

Low-temperature N₂ adsorption (LTN₂A) has been widely used to characterize coal’s pore structure, and its reliable testing range is between 2 and 100 nm (Ju et al., 2005a; Ju et al., 2005b; Ju and Li 2009). The pore size identified from AFM in this study ranges from 10 nm to several hundred nanometers. Therefore, we calculated the total pore area of pores between 10 and 100 nm in diameter and compared the results with that of LTN₂A in previous studies (Figure 9). Figure 9A shows that with increasing tectonic deformation, the total pore area increases accordingly. The graph shows a trend similar to that in Figure 7A. In WBDS, the total pore area increases from 0.0361 to 0.222 μm². In SBDS, the total pore area increases rapidly from 0.222 to 1.66 μm².

LTN₂A results collected from Ju et al. are shown in Figure 9B. Samples in Ju et al.’s studies were collected from Huainan coalfield and Huaibei coalfield, Anhui Province, whose R_{o, max} ranges from 0.76 to 1.51% (Ju et al., 2005a; Ju et al., 2005b; Ju and Li 2009). These samples are in the same coal rank and deformation degree as ours, making the results of AFM and LTN₂A comparable. From primary coal to mortar coal, the total pore area between 2 and 100 nm increases from 0.0383 m²/g to 0.224 m²/g, while the total pore area increases from 0.224 m²/g to 1.377 m²/g. The variation in the total pore area obtained by LTN₂A can also be divided into two stages, indicating that brittle TDCs pore structure does not evolve linearly with tectonic deformation.

Eliyahu et al. (2015) first utilized AFM to map mechanical properties of OM in shales. Since then, AFM has gained increasing attention in characterizing mechanical properties of rocks (Emmanuel et al., 2016; Li et al., 2017; Yang et al., 2017; Khatibi et al., 2018; Liu 2019; Li et al., 2020; Tan et al., 2021). Many of these studies have focused on the relationship between OM’s Young’s modulus and its maturity (Emmanuel et al., 2016; Khatibi et al., 2018; Liu 2019; Tan et al., 2021). However, the relationship between OM’s Young’s modulus and the deformation degree remains poorly understood. Here, we measured the mechanical properties of primary coal and brittle TDCs series, and the results are presented in Figure 10. The Young’s modulus of primary coal and different brittle TDCs varies from 1.5 to 2.0 GPa, which is similar to the Young’s modulus in Li et al.’s study (Li et al., 2020). Overall, the Young’s modulus increases with tectonic deformation degree except for sample QYZ-70. From primary coal to flack coal, the Young’s modulus increases by approximately 20%. In WBDS, the Young’s modulus of different samples levels off. In SBDS, the Young’s modulus increases gradually.

In previous studies concerning OM’s Young’s modulus and its maturity, it has been found that the OM’s Young’s modulus increases with increasing maturity (Emmanuel et al., 2016; Khatibi et al., 2018; Liu, 2019). Moreover, the OM’s Young’s modulus relates well with certain chemical structure indexes. For example, by utilizing atomic force microscopy-based spectroscopy (AFM-IR), Yang et al. (2017) found that macerals enriched in aromatic carbon and lean in aliphatic carbon have relatively high mechanical stiffness. Khatibi et al. (2018) found that OM’s Young’s modulus positively correlates with the D band position and G-D band separation. Liu (2019) found that there is a strong positive correlation between Young’s modulus and the carbon aromaticity (f _a). Tan et al. (2021) also found that the increase in aromatic groups and the decrease in CH₂/CH₃ ration can be regarded as potential reasons for the rise in OM’s modulus. All of these results indicate that the evolution of OM’s chemical structure leads to a modulus change in the maturation process. As OM’s macromolecule arrangements gradually transform from chaotic and mixed layers to an ordered arrangement, the Young’s modulus increases (Khatibi et al., 2018; Tan et al., 2021).

It is generally considered that temperature plays a dominant role in the evolution of OM. However, tectonic stress can also have an impact on the macromolecular structure of coal (Cao et al., 2007). Some researchers have quantified the structural transitions triggered by tectonic stress. Combining Fourier transformation infrared microspectroscopy (FTIR) and X-ray diffraction (XRD), Cao et al. (2007) found that compared to non-deformed coals, deformed coals exhibit weaker aliphatic absorbance peak, stronger aromatic absorbance peak, smaller carbon atomic surface network distance (d ₀₀₂) and larger basic structural unit of coal (BSU) height (L _c) and width (L _a). Li (2013) found that shear stress can greatly increase the carbon aromaticity (f _a) using nuclear magnetic resonance spectroscopy (NMR). Utilizing high-resolution transmission electron microscopy (HRTEM), Pan et al. (2015b) found that with increases in tectonic deformation, the coal fringe length increased and the fringe separation and fringe tortuosity decreased. Pan et al. (2017) also conducted Raman experiments using coals with different deformation degrees. The results showed that strong TDCs exhibit smaller full width at half maximum for the G-peak (fwhm-G) and smaller intensity ratio between the D and G-peaks (I _D/I _G), indicating that tectonic deformation can promote the ordering of coal molecular structure. Song et al. (2020a) found that compared to brittle deformation, ductile deformation can improve poorly ordered BSU to a more ordered BSU.

The above studies indicate that tectonic stress promotes BSU rearrangement and ordering in coal. Since geothermal metamorphism can increase the Young’s modulus of OM by transforming its chemical structure, it is reasonable to consider that tectonic stress has the potential to influence coal mechanical properties. The AFM results here partially confirm this hypothesis. In WBDS, the Young’s modulus level off because tectonic stress has little impact on OM’s chemical structure. Note that the average vitrinite reflectance of sample QYZ-70 is only 1.01. Hence, the lower Young’s modulus of sample QYZ-70 may be attributed to a low coal rank as well as a weak deformation degree. In SBDS, the Young’s modulus increases gradually because strong brittle deformation has a more significant impact on OM’s chemical structure. However, the overall increase in Young’s modulus here is 20%, indicating that coal’s macromolecular ordering improvement contributed by brittle deformation is relatively limited. This finding corresponds well with the results of Li (2013) and Song et al. (2020a).