R. C. Garvie

Intrinsic size dependence of the phase transformation temperature in zirconia microcrystals

It is argued that because the tetragonal (t) to monoclinic (m) transformation in zirconia is exothermic, this guarantees that the surface free energy of the former is less than that of the latter structure. It is argued further that for pristine, unconstrained, single crystals,As ∼Ms ∼ 1447 K. It follows then, from a thermodynamic analysis that the reciprocal crystallite size is a linear function of the transformation temperature. Quantitative agreement was obtained between calculated and experimental crystallite size-temperature data which ranged over three orders of magnitude.

Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (t→m) occurring in ZrO2 precipitates in a Ca-PSZ alloy and particles in Al2O3-ZrO2 composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t → m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al2O3-ZrO2 alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.

Biocompatibility of magnesia-partially stabilized zirconia (Mg-PSZ) ceramics

The biocompatibility of Mg-PSZ ceramics aged to peak strength at 1100° C was assessed by in vitro and in vivo experiments. The former consisted of immersing the material in saline solution, boiling under reflux, for 1000 hours. A 6% loss in strength was the only discernible change in the ceramic. The in vivo experiments comprised implanting Mg-PSZ samples in the paraspinalis muscles of rabbits for 6 months. There was no significant adverse soft tissue response to the implants. Neither was there any change in the surface phase content, surface roughness or strength of the implants during the course of the experiment.

Thermodynamic analysis of the tetragonal to monoclinic transformation in a constrained zirconia microcrystal

A thermodynamic analysis was made of a simple model comprising a transforming t-ZrO2 microcrystal of sized constrained in a matrix subjected to a hydrostatic tensile stress field. The field generated a critical size range such that a t-particle transformed ifdcl<d <dcu. The lower limitdcl exists because at this point the maximum energy (supplied by the applied stress) which can be taken up by the crystal is insufficient to drive the transformation. The upper limitdcu is a consequence of the microcrystal being so large that it transforms spontaneously when the material is cooled to room temperature. Using the thermodynamic (Griffith) approach and assuming that transformation toughening is due to the dilational strain energy, this mechanism accounted for about one-third of the total observed effective surface energy in a peak-aged Ca-PSZ alloy.

Improved thermal shock resistant refractories from plasma-dissociated zircon

Dissociated zircon (DZ), produced in a plasma furnace recombined and sintered to about 92% of the theoretical density in the range 1300 to 1500° C The work-of-fracture of DZ increased from 20J m−2 to 73 J m−2 with additions of 10 wt% monoclinic zirconia particles which had a mean diameter of about 13μm. Thermal shock data showed that crack propagation in DZ/ZrO2 composites was stable.

Localized impact damage in transformation-toughened zirconia

Abstract Zirconia specimens partially stabilized with calcia and aged for various times to vary KIC were impacted by glass, steel and tungsten carbide spheres. The impact damage was characterized. The zirconia was resistant to damage by the softer spheres, glass and steel, but the tungsten carbide caused visible damage (indentations) at impact velocities as low as 9 m s−1. At higher velocities radial, lateral vent and circumferential cracks were observed. Indentation and stress wave energy loss mechanisms account for most of the observed energy losses. The remaining strengths after impact increased somewhat less than expected with increasing KIC.

Design and Application of a Zircon Advanced Refractory Ceramic

A strong, thermal shock resistant zircon advanced refractory ceramic (ZS10) was designed by dispersing 10 weight percent monoclinic zirconia polycrystalline (MZP) particles in a dense matrix. The dispersion of MZP induced thermal shock resistance via a new toughening mechanism unrelated to transformation toughening. Laboratory tests showed that ZS10 had superior corrosion resistance in molten E-glass compared to conventional zircon refractories and was equivalent to them in thermal shock resistance. Accordingly, a field test was conducted on a ZS10 bushing block. The fabrication and performance of the block are described.