Mechanical properties of NiO/Ni-YSZ composites depending on temperature, porosity and redox cycling

The Impulse Excitation Technique (IET) was used to determine the elastic modulus and specific damping of different Ni/NiO–YSZ composites suitable for use in solid oxide fuel cells (SOFC). The porosity of the as-sintered samples varied from 9 to 38% and that of the reduced ones from 31 to 52%. For all samples a linear relation between Young's modulus and porosity was found. The temperature dependency of the mechanical properties of both as-sintered and reduced composites was investigated by IET up to 1200 °C. In the as-sintered state, first an increase and peak of stiffness coinciding with the Néel temperature, 250 °C, of NiO was observed. Above this temperature, a linear decrease occurred. Specific damping showed a peak at 170–180 °C and increased above ca. 1000 °C in NiO–YSZ. In the reduced state the elastic modulus decreased linearly with temperature; specific damping increased above ca. 600 °C and was found to be very dependent on microstructure. Damage caused by redox cycling degraded the elastic properties of the composites. Degradation started linearly from 0.5 to 0.6% redox strain leading to macroscopic sample failures at about 2.5% dL/L_o. A simple continuum elastic damage model was fitted to the degradation data.