A. V. Ponomarev

Structural changes in the surface of a heterogeneous nanocrystalline body (sandstone) under the friction

The structure of a ~30 nm thick surface layer of a heterogeneous nanocrystalline solid body (sandstone) before and after the friction was investigated using photoluminescence and Raman spectroscopy. Before the friction, this layer contained nanocrystals of quartz, anatase, feldspar, and montmorillonite. The friction caused a sharp decrease in the concentration of nanocrystals of quartz and feldspar.

Relaxation of electric fields induced by mechanical loading in natural dielectrics

An investigation is made of electric fields induced in natural dielectrics by mechanical loading and electrical polarization. It is shown that the relaxation of the polarization is identical in nature for both cases and is basically a thermally activated process. The temperature dependence is obtained for the relaxation time of the electric fields. Also estimated is the activation energy for motion of charge carriers leading to the relaxation of these fields.

Nanosecond dynamics of destruction of the surface layer of a heterogeneous nanocrystalline solid (sandstone) under the friction

It has been found that the friction of a heterogeneous material, namely, sandstone, leads to the appearance of triboluminescence. The phenomenon of triboluminescence corresponds to luminescence of ≡Si–O free radicals and Fe3+ ions. These radicals and ions are formed as a result of the breaking of Si–O–Si bonds in nanocrystals of quartz and feldspar entering into the composition of the sandstone. The time dependence of the triboluminescence intensity represents a set of flashes, each having the duration of a few nanoseconds. It has been assumed that triboluminescence flashes correspond to the appearances of cracks in the material. Сrack opening is found to be approximately 180 nm. The size distribution of the cracks is exponential.

Changes in the surface structure of the heterogeneous body (diorite) under friction

Raman, infrared, and photoluminescent spectroscopy has been used to study changes in the surface structure of quartz diorite caused by friction. Before friction, the diorite surface layer has contained mainly quartz and feldspar crystals. After friction, some quartz and feldspar crystals have been destructed and replaced by a newly formed hydromica mineral with a low friction coefficient such as illite.

Destruction dynamics of a heterogeneous body (diorite) under friction

Triboluminescence bursts are observed in two heterogeneous (diorite) specimens under friction against each other. Triboluminescence appears upon the relaxation of excited free ≡Si–O– radicals and Fe3+ ions and the capture of electrons with acceptor traps formed upon the destruction of the plagioclase crystal lattice. The analysis of the time dependence of these bursts shows that the friction surfaces accumulate clusters, in which the concentration of free ≡Si–O– radicals and electron traps is at least an order of magnitude higher than in their surrounding. The time interval between the appearance of two sequential clusters variates from 0.1 to 1 μs. The linear sizes of clusters are ~0.5 μm.

Variation in the structure of the surface layer of a heterogeneous solid (gneiss) on a shear

The structure of surface layers with a thickness of ~1 μm formed at destruction of gneiss is studied by means of photoluminescent and infrared spectroscopy. It is found that, in this layer, feldspar (plagioclase and microcline) crystals are completely destroyed and replaced by montmorillonite.

The influence of the structure of a nanocrystalline solid (sandstone) on the dynamics of microcrack accumulation on friction

On friction of a heterogeneous material: sandstone, flashes of triboluminescence are observed. Triboluminescence arises on the relaxation of excitation of free radicals ≡Si–O–. These radicals form on disruptions of Si–O–Si bonds and are located at microcrack edges. Microcracks form at the boundaries of feldspar and quartz microcrystals. Their sizes range from ~0.4 to ~7 μm. The microcrack formation on friction leads to the spalling of feldspar and quartz microcrystals from a sample.

Structural analysis of the fracture surface of a heterogeneous body (quartz sandstone)

The structure of surface layers of quartz sandstone with a thickness of ~1 μm before and after destruction by a compressive stress is studied by methods of infrared, photoluminescent, and Raman spectroscopy. Before destruction, this layer contained quartz grains cemented with montmorrillonite and kaolinite. The grains are covered with a thin water layer and have crystallographic defects: Si–O–, self-trapped excitons, AlOH and LiOH compounds, [AlO4]– centers, etc. The destructed surface contains separate quartz grains with sizes of ~2 μm and a reduced defect concentration. It is assumed that the defects reduce the strength of quartz grains, which are destroyed in the first turn.

Structural Changes in the Surface of a Heterogeneous Body (Xenolite) under Friction

The structure of the surface layer of a heterogeneous solid body (xenolite) before and after friction is studied by infrared and Raman spectroscopy. Before friction, the layer contained hornblende and pyroxene crystals. The friction resulted in partial transformation of pyroxenes into hornblende and the latter was transformed into montmorillonite clay. The xenolite surface is covered with a ~60-nm-thick layer of water.

Friction-Induced Changes in the Surface Structure of Basalt and Granite

Friction-induced changes in the structure of the surface layer of basalt and granite samples extracted from a well in the triggered seismicity zone in the Koyna–Warna region, India, have been studied by infrared, Raman, and photoluminescence spectroscopy. It has been found that friction leads to a partial degradation of quartz, albite, and clinopyroxenes crystals. Instead of these crystals, a thin layer of a mineral with a low coefficient of friction—kaolinite—is formed on the surface.