Search

The densification mechanism during the park-plasma-sintering (SPS) processing was examined in high purity MgAl2O4 spinel. As the density ρt increases, that is, as the effective stress σeff decreases, stress exponent n evaluated from σeff dependence of densification rate varies from n ≥ 4 in the low ρt region, n ≈ 2 in the intermediate ρt region to n ≈ 1 in the high ρt region. TEM observation shows that significant stacking faults caused by partial dislocations are observed in the low ρt region, but limited in the high ρt region. The ρt dependent densification behavior and microstructure suggest that the predominant densification mechanism during the SPS processing changes with ρt from plastic flow by partial dislocation motion for the low ρt region (n ≥ 4) to diffusion-related creep for the high ρt region (n ≈ 1).

The fracture strength σf of ZrO2-based ceramics tends to increase with decreasing grain size d. As compared with the data for submicrometer-grain-sized material with d ≈ 350 nm, nano-crystallization of ZrO2 ceramics less than d ≈ 90 nm can improve σf by a factor of 2.0-2.5. The maximum strength reaches σf ≈ 2500 MPa, which is classed as the highest σf of oxide ceramic materials. The high σf can be associated with an increase in the critical t →m transformation stress and a decrease in the flaw size due to nano-crystallization.

Factors limiting the strain rate of superplastic deformation in oxide ceramics are discussed from existing knowledge about the mechanisms of high-temperature plastic deformation and intergranular cavitation. The discussion leads to the following guide: simultaneously controlling the initial grain size, diffusivity, dynamic grain growth, homogeneity of microstructure and the number of residual defects is essential to attain high-strain-rate superplasticity. Along this guide, high-strain-rate superplasticity (HSRS) is attainable in some oxides consisting of tetragonal zirconia, α-alumina and a spinel phase: tensile ductility reached 300-2500% at a strain rate of 0.01-1.0 s-1. Post-deformation microstructure indicates that some secondary phases may suppress cavitation damage and thereby enhance HSRS. The guide is also essential to lower the limit of deformation temperature for a given strain rate. In monolithic tetragonal zirconia, grain-size refinement combined with doping of aliovalnt cations such as Mg2+, Ti4+ and Al3+ led to HSRS at 1350 °C.

The densification mechanism during the park-plasma-sintering (SPS) processing was examined in high purity MgAl2O4 spinel. As the density ρt increases, that is, as the effective stress σeff decreases, stress exponent n evaluated from σeff dependence of densification rate varies from n ≥ 4 in the low ρt region, n ≈ 2 in the intermediate ρt region to n ≈ 1 in the high ρt region. TEM observation shows that significant stacking faults caused by partial dislocations are observed in the low ρt region, but limited in the high ρt region. The ρt dependent densification behavior and microstructure suggest that the predominant densification mechanism during the SPS processing changes with ρt from plastic flow by partial dislocation motion for the low ρt region (n ≥ 4) to diffusion-related creep for the high ρt region (n ≈ 1).

In tetragonal zirconia, possibility is investigated of densification with finer grain sizes under the combination of doping and sintering in air. The materials used are CIP'ed compacts of 3-mol%-yttria-stabilized tetragonal zirconia (3Y-TZP) doped with a small amount of cations. For a given sintering temperature and initial density of the compacts, while the doped cations enhances densification in the latest stage of sintering, the effect is different in grain growth during densification: a doped cation tended to enhance grain growth, whereas the other cations tended to suppress grain growth. As a result, the doping of the latter cations brings about a grain size finer than that of the undoped 3Y-TZP for a given relative density.