Indentation behaviour of porous materials: Application to the Vickers indentation cracking of ceramics

Classical descriptions of the Vickers indentation technique, used for toughness measurements in dense ceramics, model the indent as an expanding sphere which induces the extension of median radial cracks. They result in the use of a proportionality factor hr relating the toughness of the material to the indentation load and to the radial crack length. Both analysis and experiment yield a value of hr depending on hardness and Young's modulus for dense ceramics. However, experimental data from Vickers indentation toughness measurements on porous YBa 2 Cu 3 O 7 mx ceramics have shown that this proportionality factor diminishes as porosity increases. This is seen as a loss of efficiency of the indent and is interpreted as the sharing of its volume with densification of the porous material. The ratio of a virtual effective indent volume to the actual volume is used to define an indentation efficiency factor, which is then measured as a function of porosity in ceramics. For cross-checking and experimental verification, sintered stainless steel powders, in which median radial cracks do not appear, have been subjected to macroscopic square-shaped indentations. This allowed image analysis measurement of densification beneath the indenter as a function of porosity, and of the actual fraction of the indent volume absorbed by densification. This yields a second data set of efficiency factors. Both sets almost overlap, which suggests that densification behaviours are similar for the investigated ceramic and metallic porous materials.