Composition and cooling-rate dependence of plastic deformation, densification, and cracking in sodium borosilicate glasses during pyramidal indentation

• Sodium borosilicate glass mechanical properties through nanoindentation testing
• Examination of the influence of composition and cooling rate on plastic deformation and cracking
• Raman microscopy and post-indentation annealing to examine inelastic densification
• Atomic force microscopy for determining material flow during contact with a Berkovich indenter

Plastic deformation, densification, and cracking of sodium borosilicate (NBS) glasses were examined during indentation with a three-sided pyramidal indenter. Compositions of 74.0SiO2–10.0B2O3–16.0Na2O (NBS1), and 74.0SiO2–20.7B2O3–4.3Na2O–1.0Al2O3 (NBS2) (mol%) were investigated. The effect of thermal history was additionally considered for the NBS2 composition, which lies near the boron anomaly line. Hardness, elastic modulus, and fracture toughness were estimated with instrumented indentation techniques. Atomic force microscopy (AFM), Raman micro-spectroscopy, and post-indent annealing experiments were used to analyze surface topographies, densification, and recovery of deformed material. The results indicate that NBS1 exhibits a greater hardness and elastic modulus, and undergoes less densification than the NBS2 glasses. Different casting conditions influenced the plastic deformation and onset of crack initiation in NBS2. Interpretation of fracture toughness measured by indentation is complicated by residual stresses, densification during contact, and model assumptions. However, distinct differences in elastic modulus, plastic deformation and cracking between the glasses were noticeable. Such results and observations are discussed in terms of structural changes in the glass.