Toughness and thermal shock resistance of YBa2Cu3O7−x composite superconductors containing Y2BaCuO5 or Ag particles

Macroscopic toughness measurements and the Vickers indentation technique have been used to characterise sintered or melt-textured superconducting YBa2Cu3O7−x ceramics. A newly developed thermal shock resistance parameter, based on the Vickers indentation technique, has been used. The crack propagation mechanisms observed in sintered ceramics have been analysed with respect to residual stress fields due to the anisotropy in thermal expansion coefficients, or to the difference with that of the Y2BaCuO5 (‘211’) inclusions. The addition of the ‘211’ brittle phase does not increase the toughness and thermal shock resistance of sintered ceramics, but toughens the textured materials. An enhancement of the thermal shock resistance, through an increased R-curve behaviour, has been achieved by introducing a metallic silver ductile phase into the microstructure, both in sintered and textured ceramics. The evaluation of the latter needed the extension of the thermal shock resistance parameter to the case of Palmqvist indentation cracks. Parallelly, a modelling of the thermal shock behaviour of textured ceramics is proposed through the description of the microstructure as a stacking of YBa2Cu3O7−x layers cracked along the (001) planes, joined by Y2BaCuO5 particles, corresponding to a low-stiffness material.