Constrained sintering of 8mol% Y 2O 3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y2O3/92 mol% ZrO2 (8YSZ) films approximately 10–15 m thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 ◦C. The results are compared with those reported earlier for 3YSZ films. Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol−1) despite all probably being mainly controlled by grain boundary cation diffusion

Constrained sintering kinetics of 3YSZ films

3YSZ green layers approximately 10 μm thick were screen-printed onto 3YSZ substrates and their constrained sintering kinetics were measured at 1100–1350 °C using an optical dilatometer. The densification rates of the same powder in the form of pellets and free-standing films were also measured. The constrained densification rate was greatly retarded compared with the free densification rate at a given temperature and density. The retardation increased with increasing density and temperature and could not be properly accounted for by existing theories of constrained sintering. As a result the apparent activation energy is much lower for constrained sintering (135 ± 20 kJ mol−1) than for free sintering (660 ± 30 kJ mol−1). It is proposed that this is because the constrained microstructure exhibits larger and more widely separated pores at the higher temperatures

Stress induced by constrained sintering of 3YSZ films measured by substrate creep

3YSZ green layers approximately 10 μm thick were screen printed onto 3YSZ substrates up to 300 μm in thickness. The stress induced by constrained sintering of the film (between 1150° and 1350°C) was measured by monitoring the bending displacement of vertical strips of bilayers using a long-distance microscope. In order to deduce the stress it was first necessary to measure the creep properties of the substrates by monitoring the bending of horizontal beams under gravity. The creep strain rate of the 3YSZ substrates was linearly dependent on applied stress at the low stresses and strains involved in the present work. The creep viscosity appeared to increase with strain (time), which might be due to changes in grain-boundary composition, and had higher activation energy at temperatures above approximately 1250°C. The magnitudes of the creep viscosities are in reasonable agreement with other creep data in the literature for 3YSZ. The in-plane stress induced during constrained sintering of the 3YSZ films had a maximum value of approximately 3 MPa at 1200°C. This behavior is consistent with literature results reported for constrained sintering of bulk alumina. The stress induced by the constraint is of a similar order to the estimated sintering potential