Crack opening profiles of indentation cracks in normal and anomalous glasses

A comparative Vickers indentation study is made of two glasses, soda-lime and borosilicate. Indentations in the two glasses reveal substantially shorter radial cracks in the borosilicate, even though toughness values measured by conventional (double-cantilever beam) methods are similar in the two glasses. Here, indentation toughness is measured in two ways: by optical measurement of the indentation crack lengths (ICL) and by atomic force microscopy (AFM) measurement of crack-opening displacements (COD) in the near-crack-tip regions. The ICL measurements indicate artificially high values for the borosilicate relative to the soda-lime, consistent with previously documented indentation results. The COD measurements indicate similar values for the two glasses, in line with expectations from the independent determinations. In the case of soda-lime glass, the COD and ICL values are mutually consistent. In the case of borosilicate, the COD and ICL values differ widely, indicating “anomalous” indentation behavior, typical of glasses with open, network-former structures. It is concluded that the COD route provides more reliable evaluations of intrinsic toughness, albeit at some expense in experimental simplicity. Residual elastic–plastic contact stresses responsible for driving the radial cracks, deconvoluted from COD measurements over the entire radial crack lengths, are shown to be significantly smaller in the borosilicate relative to soda-lime, indicative of a compaction rather than volume-conserving contact deformation mode