Thermal Expansion Coefficient and Residual Stress of Porous Ni/ZrO2-Y2O3 Cermet

Thermal expansion coefficient and residual stress were measured in Ni/ZrO2-8mol% Y2O3 (YSZ) cermet. The microstructure of the cermet was also examined. The average thermal expansion coefficient at 298 to 1273K became larger as the Ni concentration increased. The coefficient versus composition displayed an "S-shaped" curve, and it conformed to the calculated values from Turner's formula at above 44vol% Ni. Less than 32vol% Ni, however, the experimental values were smaller than the calculated ones. The reason was thought to be that most of the Ni particles were isolated and voids existed between the isolated Ni particles and YSZ phase as a consequence of releasing the expansion mismatch between Ni and YSZ. X-ray stress analysis confirmed that a tensile stress of 3 to 34MPa was generated in the Ni phase, and a compressive stress of 55 to 170MPa, in the YSZ phase. The direction of the redidual stresses agreed with that of prediction calculated by Kingery's equation. The experimental values, however, were smaller than the predictions. The reason for these discrepancy may be that the measurement positions were not in the bulk of the samples but on the sample surface region where exist smaller constraints.