Formation of microcracks in a heterogeneous solid (sandstone) under the influence of friction

The growth rate and size of microcracks, formed in the surface layer of a heterogeneous natural solid (sandstone) under friction were studied via triboluminescence. It was discovered that destruction of the crystal lattice of plagioclase and quartz, composing sandstones, induces triboluminescence signals. Each $s$-ignal corresponds to a microcrack, whose surface contains electronically excited free radicals $\equiv$ Si–O$^-$ and Fe$^{3+}$ ions. The size and growth rate of microcracks were assessed by means of analysis of the rate and duration of the buildup of these signals. The growth rate of microcracks turned out to be less than the elastic wave velocity ($\sim$ 700 m/s) and varied from $\sim$ 380 to $\sim$ 650 m/s. The size of microcracks varied from $\sim$ 4 to $\sim$ 7 $\mu$m. It was assumed that the variation in the size and growth rate of microcracks were caused by differences in value of the barriers, upon breaking which the microcracks appear.