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 $\equiv$ Si–O free radicals and Fe$^{3+}$ 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.