Investigation of the Cracking Threshold of Silicate Glasses Using Nanoindentation

There is a great interest in glass systems for a variety of applications such as electronic devices, displays, and coatings for orthopedic implants. Surface damage is a major issue in each of these applications, and it creates many problems for glass makers, suppliers, and end users. The cracking behavior that governs glass surface damage is not yet completely understood. In many cases, the nucleation of these cracks results from a sharp contact loading. To simulate sharp contact loading conditions, nanoindentation with triangular pyramidal indenters with centerline-to-face angles in the range of cube corner (35.3°) to Berkovich (65.3°) was performed on glass samples, and the resulting crack initiation and propagation was investigated. Two different glass structures have been analyzed - fused silica and soda lime glass - as representative of anomalous and normal glasses, respectively. Fused silica is thought to deform by a process dominated by densification, whereas soda lime glass deforms primarily by shearing processes. Due to these different deformation processes, it is anticipated that the different stress fields for each material around the contact area result in different cracking morphologies. Nanoindentation at various loads (e.g., 3 mN - 20 N) was used to identify a load dependency of the cracking behavior. Surface cracking was documented using atomic force microscopy, scanning electron microscopy, and laser confocal microscopy. Results show that cracking is enhanced by sharper indenters and higher loads. However, there exists a distinct crack initiation threshold, below which no cracking occurs. The threshold depends on the glass system as well as the indenter geometry.