An Investigation of Crack Initiation and Propagation During Nanoindentation of Silicate Glasses

Abstract

Glass surface damage is a major issue for glass makers, suppliers, and end users. The cracking behavior that governs glass surface damage is not yet completely understood. In many cases, fracture results from a sharp contact loading. Nanoindentation is a promising tool to replicate this sharp contact as it has various advantages: it does not require complicated sample geometries nor preexisting flaws in the material, as plasticity from the indent itself is assumed to create the precursor flaw. Different aspects of the cracking behavior have been studied using nanoindentation with triangular pyramidal indenters with center-line to face angles in the range of cube corner (35.3°) to Berkovich (65.3°). Two different glass structures have been analyzed and compared: anomalous glass (fused silica) and normal glass (soda-lime glass), which deform primarily by densification and shearing processes, respectively. These different deformation processes are anticipated to lead to different stress fields around the contact resulting in different cracking morphologies. Nanoindentation at various loads (1 mN - 20 N) was used to identify a load dependency of the cracking behavior. Subsequently, surface and subsurface cracking was documented using atomic force microscopy, scanning electron microscopy, 3D laser scanning confocal microscopy, and focused ion beam cross-sectioning. Results show that cracking is enhanced by sharper indenters and higher loads. However, there exists a distinct crack initiation threshold, below which no crack formation can be observed, which is dependent on the glass system as well as the indenter geometry. In addition to crack initiation, the crack propagation has been analyzed using relations for the median-radial and Palmqvist-radial crack systems. Results show that the known relations are inadequate to fully describe the fracture toughness of the materials. However, differences in cracking morphology may have to be considered when determining fracture toughness obtained from indentation experiments.