Initiation of impact fracture in SiO$_2$ ceramics

The results of high-speed recording of fractal luminescence (FL) and acoustic emission (AE) during the impact fracture of the surface of quartz ceramic are presented. FL results from the breakage of covalent bonds, and AE appears during the motion of microscopic and larger cracks. Measurements are performed on samples chosen after each stage of heat treatment, namely, drying at 100$^\circ$C, sintering at 1190$^\circ$C, and flame treatment of the sample surface. After each temperature cycle, the hardness of the material is measured and the connectivity of the silicate network of glass is estimated by IR reflection spectroscopy. The energy released during damage formation is determined from the FL and AE intensities. The energy distribution in the FL signals recorded after an impact in all samples is shown to obey the power law that is characteristic of the cooperative phenomena occurring in nonequilibrium processes. This finding indicates correlated fracture development at a nanostructure level. In AE signal series, a power energy distribution is detected only in sintered not glassy ceramic.