Effects of coals microscale structural features on their mechanical properties, propensity to crushing and fine dust formation

The work is dedicated to revealing the structural features of coals with different ranks, such as anthracites, metaanthracite and graphite, that determine their ability to crush and form fine dust. For this purpose, a combination of various nanoindentation techniques and Raman spectroscopy was used. The mechanical behavior of the selected coals was investigated by cyclic nanoindentation with increasing peak load and quasi-static loading. The alteration of the mechanical properties was studied by analysis of elastic moduli and damage indices R w . Three groups of coals were identified based on their propensity to crushing during cyclic nanoindentation. Coals assigned to the first and second groups are characterized by local destruction in the contact zone with the indenter and the formation of a core of crushed material. Coals assigned to the third group are characterized by bulk destruction (outside the zone of contact with the indenter). In general, the ability of coals to fracture under mechanical loading decreases in the series of metamorphism due to microscale compaction of vitrinite matter. In the series of anthracite, metaanthracite and graphite, it is established that the coal matter compaction takes place for the anthracite and metaanthracite, whereas graphite reveals rather different behavior due to abrupt change of its structure. The ratios between the amorphous and crystalline phases of carbon ( S ) were determined by deconvolution of coals Raman spectra. The propensity of coals to crushing (a damage index R w ) increases with growth of the proportion of amorphous carbon in the coal matter. For the considered coals and metaanthracite, it is established that the proneness to destruction outside the contact zone with the indenter is determined by the ratio of amorphous and crystalline carbon of 1 and higher. When S parameter is lower than 1, the coals are being crushed only in the zone of contact with the indenter.