Formation and Evolution of a Confined Liquid Condensate at the Crack Tip in Glasses

Crack propagation in oxide glasses at low crack velocities is controlled by stress corrosion. Proper knowledge of the crack tip chemical environment is thus crucial to understand the slow fracture process of these materials. The formation of a liquid condensate in the confined area of the crack tip is theoretically expected and is classically mentioned to explain specific behavior during the crack propagation. Since this condensate is of the nanometer scale and cannot be traced post mortem, it had yet never been observed. This paper reports the experimental evidences of the presence of a liquid condensate at the tip of a crack propagating by stress corrosion in silica glass. The observation has been performed in real time under carefully controlled atmosphere through in situ phase imaging by atomic force microscopy (AFM). The evolution of the condensate has been followed as a function of the relative humidity. The impact of those evidences of the liquid condensate, added with information on its size or evolution kinetic will be reported. INTRODUCTION The study of crack propagation at slow velocity in oxide glasses is of great interest on both practical and fundamental bases. Long-term behavior of vitreous material is particularly important considering the large proportion of these materials used in architectural structures and their role in the encapsulation of nuclear wastes. On the fundamental aspect, slow advancing crack can be an accurate probe of the molecular structure of glasses. Stress corrosion is known to control slow propagation of crack in oxide glasses. The preferential corrosive role of water molecules at the crack tip sustaining tensile stresses is generally acknowledged. Wiederhorn et al have developed a model that fits well the experimental data and shows how in the stress corrosion regime crack velocity is highly dependent on relative humidity, and can be limited by transport kinetics of the water molecules towards the crack tip. The physical and chemical environment in which stress corrosion takes place, as well as the mechanisms of transport of the water molecules towards the crack tip are less