PSI - Issue 13

Igor Bunin et al. / Procedia Structural Integrity 13 (2018) 1971–1976 Author name / Structural Integrity Procedia 00 (2018) 000–000

1975

5

We assessed the structural and chemical variations in surfaces of olivine and serpentine after HPEMP treatment used the data of IR-spectroscopy of diffusive reflection (DRIFTS). Bands relating to valence, deformation and torsional oscillations of silicium-oxygen tetrahedron [SiO 4 ] appeared in IR-spectrum of olivine (forsterite – (Mg 2 , Fe)SiO 4 ). The basic spectrum band ~800–1000 cm –1 is due to splitting of degenerated asymmetric oscillation ν 3 in Si–O bond. Bands of spectrum interval 400–600 cm –1 can be explained by splitting of asymmetric deformation oscillation ν 4 of Si–O bond. Narrow band at 3689 cm –1 refers to ОН–groups adsorbed at surface. HPEMP-treatment of olivine and serpentine particles of caused appreciable alterations in IR-spectra of minerals. Alterations in IR-spectrum of olivine concerned with the increasing intensity of absorption bands 750–1100 cm –1 and dissipation of a weak band at 864 cm –1 as a result of ~ treat t 50 s of nanosecond pulses treatment. Integral characteristics of serpentine IR-spectra indicated that the area of band (850–1150 cm –1 ) pertaining to oscillations of silicium–oxygen bond in tetrahedral mineral structure increased 1.1–1.2 times as a result of a pulsed action. Established variations in spectral (XPS and FTIR) characteristics may be caused by disturbance of microstructure of the surface natural dielectric minerals layers and deal with such processes, as formation, transition and interaction of defects at different structural levels (dislocations, microcracks, etc.), as well as softening, and amorphization of the surface under pulsed high-intensity electric fields. The probable mechanism of the disorders of mineral surface structure and occurrence of micro-damages in crystals under the non-thermal action of high voltage nanosecond pulses is the migration (interlayer) polarization and/or electron-relaxation polarization of the dielectric minerals. According to the scanning electron microscopy (SEM–EDX) data, there were traces of surface electrical break down channels on the polished sections of serpentine as a result of electromagnetic pulse treatment for ~ treat t 10–100 s. These traces of surface breakdowns formed at the sites of the localization of oxide microinclusions (possibly, chromite FeCr 2 O 4 and/or magnesium–chromite MgCr 2 O 4 ,), sulfides (possibly, milleritre NiS), and other metal– containing micro-phases. A gradual process of selective disintegration of the calcite was the main mechanism behind the dissipation of energy in a high-voltage pulse electric field, namely, the opening (softening) of intercrystalline boundaries, the formation and propagation of cracks along the cleavage surfaces, and the formation of microcrystal fragments upon extending the pulse action to  treat t 50 s. Analysis of the AFM images of local zones on the mineral surfaces before and after electropulse treatment revealed an increase in relief height and surface roughness caused by defects. HPEMP treatment of ~ treat t 30–50 s raised arithmetic mean R a and mean square R q deviation of the surface profile of the studied thin sections by 1.8–2 times for calcite; by 3.7–4 times for fluorite; and by 0.8–1.2 times for scheelite. 3.2. Effect of high-voltage nanosecond pulses on microhardness of dielectric minerals Microstructural changes caused by HPEMP in the mineral surface layers effectively softened the mineral surfaces: the greatest relative decrease in microhardness ( HV ) was observed for mineral treatment of ~ treat t 150 s and reached 51.3% for scheelite (from 2087 to 1017 MPa), 53.2% for fluorite (from 876 to 410 MPa), and 66.5% for calcite (from 790 to 265 MPa). The maximum relative drop ( H  ) in calcite microhardness to 66% (from 790 MPa down to 265 MPa) was observed as a result of the HPEMP action for ~ treat t 100–150 s. There was a substantial reduction in the microhardness of the calcite samples (by 45%) for the first 10–30 s of electric-pulse treatment, demonstrating the effectiveness of short pulse energy actions for minerals with relatively low levels of hardness according to Mohs’ scale. The serpentine microhardness fell from 430 MPa ( HV in the initial state) to 260 MPa after HPEMP action for 50– 150 s after increasing the duration of pulse treatment. The maximum relative change in the microhardness H  , % was 42%. The increased duration of pulse treatment  treat t 100–150 s did not substantially alter the mineral HV . The olivine microhardness (Mohs’ hardness, 6.5–7) fell monotonically from 4250 MPa (the average HV of the samples in the initial state) to 1560 MPa after HPEMP treatment as the time of pulse treatment was increased to ~ treat t 150 s; the maximum relative change in HV was approximately 63%. The microhardness test data confirmed the results of spectroscopic investigations: we found using Vickers’ method that the effect of high-intensity pulsed fields leads to reduction in microhardness of rock-forming minerals, in total, by 40–66%. The softening effect of natural dielectric minerals is mainly connected with the damage of microstructure of surface layers, new-formed defects at different structural levels (dislocation, microcracks, incomplete surface break-ups), disordering and amorphisation of the mineral surface. The rate of variations in microhardness of minerals relates to their hardness and reaches the maximum at low doses of electromagnetic pulsed radiation for minerals with relatively low Mohs’ hardness level.