Electrical and microstructural characterization on nitrogen-stabilized zirconia

When sintered 2Y-TZP (2 m/o yttria doped tetragonal zirconia polycrystals) specimens were embedded in ZrN powder and heat-treated at 1700°C, the cubic phase was stabilized as a result of nitrogen incorporation. The resultant microstructure was similar to a columnar zone in a metal alloy ingot: elongated grains were developed with their length parallel to the nitrogen flux into the specimen. Impedance measurements were made on the nitrogen-stabilized zirconia along the length direction of the columnar grains and perpendicular to the length direction, respectively. The boundary effects of the columnar grains were thus unequivocally distinguished. Different activation energies for the grain boundary and bulk conductivity support the blocking nature of the boundaries in nitrogen-stabilized zirconia as well as in 2Y-TZP. SEM and TEM analyses revealed that the nitrogen-stabilized zirconia consists of a matrix of a cubic phase containing a dispersion of tetragonal precipitates similar to the PSZ (partially stabilized zirconia). No continuous glassy phase at the grain boundaries was identified in the nitrogen-stabilized zirconia as well as in the 2Y-TZP ceramics. The grain boundary resistance was ascribed to a depletion space charge layer at the grain boundary.