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 $\equiv$ 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 $\sim$0.4 to $\sim$7 $\mu$m. The microcrack formation on friction leads to the spalling of feldspar and quartz microcrystals from a sample.