ABSTRACT

The current investigation was aimed at characterization of coals micromechanical properties changing with coal type and rank. To this end, depth-sensing nanoindentation experiments were held at four samples of coals. They included: lignite with relatively low carbon contents, low- and medium rank bituminous coals differing by primarily carbon contents and rank, and anthracite. Instrumental indentation tests revealed strong dependence of the evaluated elastic moduli and hardness on coals type. Such dependence is shown by increase of the aforementioned values from lignite to bituminous coals with the maximum reached at anthracite. It also should be mentioned that such dependence is not monotonous and is in good correlation with the previously obtained results on coals microhardness. Moreover, shapes of the typical load-displacement curves obtained at depth-sensing indentation tests also vary with coal rank and type. It was found that, at lignite, such curves have a shape that it typical to strongly plastic material behavior. At the same time, bituminous coals reveal the elastoplastic behavior, whereas for anthracite, they are nonlinear-elastic. Such conclusions were made by characterization of hysteresis between loading and unloading branches of such curves. These studies support the fact that coals micromechanical properties strongly depend on the type and rank.