Evaluation of crack initiation load of silica glass surfaces formed during subcritical crack growth

The crack initiation load of freshly fractured surfaces for silica glass was evaluated with ball indentation. The fracture surfaces were formed during subcritical crack growth in the regions I, II, and III of the stress intensity factor (K_I)—crack velocity (V) curve. From the K_I–V curve, we linked the obtained fracture surfaces with the ones in the regions I, II, and III. It was found that the crack-forming probability was the lowest for the fracture surface formed in the region III of the K_I–V curve. In order to understand the controlling factors of the crack formation, some properties which are topography, relative nonbridging oxygens (NBO), hydrogen concentrations, and Si–O three- or four-membered ring structures, of the fracture surfaces were measured by atomic force microscopy, X-ray photoelectron spectroscopy, dynamic secondary ion mass spectroscopy, and Raman spectroscopy, respectively. No distinct difference in NBO and hydrogen concentrations nor the ring structures were found among the fracture surfaces formed in different regions in the K_I–V curve. The peak-to-valley height of the fracture surface, however, decreased with increasing crack velocity. It is concluded that the roughness or topography of the freshly fractured surface is one of the controlling factors which reduce the intrinsic strength of silica glass.