Influence of porosity on the mechanical properties of cold isostatically pressed and sintered YBa2Cu3O7-x superconductors

Commercial YBa2Cu3O7-x powders have been cold isostatically pressed and pressureless sintered at different temperatures and for various times to obtain porous ceramic samples (5-31 vol.%). The decrease in the mechanical properties with increasing porosity has been studied within the framework of two models, up to now concerned with elastic, that is reversible, behaviours. One model creates porosity from a dense bulk by shrinking randomly its constituting elements and yields a power-law description, whereas the second model describes a densifying microstructure with a given initial stacking of particles and yields a geometrical dependence. Both tit rather well the change in Young's modulus as porosity increases. They were then used to describe the elastic strain energy density of a porous body. After homogenization, a criterion for crack propagation is derived, which describes toughness over the whole domain of porosity. Finally an investigation into the applicability of the indentation technique on porous brittle bodies is started. A median-radial crack system is observed, and toughness measurements are possible. The plastic deformation zone beneath the indenter remains thus a central opening force, but in a less efficient way than in dense materials, owing to densification. However, the way in which indentation plasticity of porous bodies is shared between densification and plastic deformation of the grains is still unknown.