Development of micro- and nanoindentation methods for assessing the mechanical properties of coals and their propensity to destruction

Abstract

The destruction of coals in the processes of mining, storage, transportation and processing is associated, among other, with their mechanical properties. As a rule, the mechanical properties of coals are evaluated by the resul ts of measurements of such characteristics as tensile and compressive strength, stiffness (elastic modulus) and Poisson’s ratio, etc. For this, experiments are carried out on specially prepared samples in tensile testing machines and mechanical presses. Such integral indicators characterize the mechanical behavior of coals at the macro scale and do not allow assessing the influence of structural features on the initiation of defects leading to destruction. In this regard, the characteristics of the mechanical properties of coals, obtained in areas comparable to the levels of microcracks initiation, are of interest. Micro- and nanoindentation techniques are quite simple to implement for obtaining the reliable information on various mechanical properties of materials at the appropriate scale levels. This paper is dedicated to an overview of the current state of research on the use of these methods to assess the mechanical properties of coals and their individual macerals, as well as their ability to crush with the formation of dust particles. The use of micro- and nanoindentation techniques made it possible to establish the differences in the mechanical properties of individual macerals of coals, as well as to characterize the heterogeneity of the distribution of various indicators characterizing mechanical properties within the individual macerals. Based on experimental data on nanoindentation and Raman spectroscopy, it was shown that the change in the mechanical properties (values of the modulus of elasticity and hardness) of vitrinite in the series of coal metamorphism is associated with an increase in the proportion of graphitized components in organic matter. Nanoindentation methods, combined with an analytical mathematical apparatus, made it possible to obtain data on the ability of coals to crush under mechanical effects with the formation of particles with sizes comparable to those of hazardous aerosol dust (class PM 2.5). The method of cyclic nanoindentation with an increasing maximum load was proposed as a method for determining the degree of coal tendency to fracture with the formation of dust. Its application made it possible to reveal the characteristics of irreversible changes occurring in the structure of coals during destruction. They are expressed in a change in their mechanical properties (stiffness and fracturing ability) with an increase in the applied load. The use of the cyclic nanoindentation method is a promising method for the qualitative and quantitative analysis of the degree of coal propensity to crushing with the formation of dust. This work was carried out with the financial support of the Russian Science Foundation (Grant No. 18-77-10052).