Contribution of interferometry to Vickers indentation toughness determination of glass and ceramic glass

Cracking resistance or fracture toughness of brittle materials, such as optical glasses, can be determined by Vickers indentation, which allows generating cracks along the diagonals of the indent whose length depends of the loading rate. Subsequently, a relationship between the crack length and the corresponding indentation load is applied to calculate the indentation fracture toughness. As a result, the precision of the fracture toughness will depend on the accuracy of the crack length measurement. To minimize the uncertainties of this parameter, we propose its measurement more precisely by a profilometric method. On the other hand, as the glass surface roughness is recognized to have an important influence on the crack growth, different surface preparations are studied to minimize this roughness effect. In this work, four types of glasses are studied, namely Classical Crown K5, Borosilicate Crown BK7, Dense Flint SF2, and Ceramic Zerodur® glasses. The surface polishing is performed by using CeO2 pellets, which allow obtaining a roughness value for Ra to be <10 nm. Vickers instrumented indentation tests, allowing the plot of the applied load as a function of the indenter displacement, are performed with various maximum applied loads of up to 90 N. Different relationships based on assumptions on the crack shape (half-penny/median or Palmqvist cracks) are then used to calculate the indentation fracture toughness. Additionally, to accurately measure the crack length, we present an original approach based on the analysis of the interferometry map obtained with an optical profilometer using a monochromatic wavelength.