Mechanical damage at the surface of glass, including the formation of indents and indentation cracks, still remains a crucial issue for glass makers, designers, and users because it affects the optical and functional properties as well as the durability of glass parts in service conditions. Some new insight into this problem was reached by investigating the crack initiation and the deformation behaviour of oxide glasses from different chemical systems using the Vickers indentation test. It is found that the crack initiation resistance is chiefly governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favoured in glasses with relatively small Poisson's ratio (v) whereas shear is favoured at large v. The residual stresses developing on unloading and which chiefly govern the indentation cracking depend on the extent and on the kinetics of the deformation mechanisms.
