Subcritical crack growth in partially stabilized zirconia(PSZ)

Studies on the subcritical crack growth behaviour of partially stabilized zirconia (ZrO₂-I, 5 to 10 vol% tetragonal phase; ZrO₂-II, 35%) were carried out using the double-torsion technique and data from the dynamic fatigue of unnotched bend specimens. The results of subcritical crack growth support the model of stress induced transformation from the tetragonal to monoclinic modification. Differences in the crack growth parameter n (as-received condition) using the double-torsion technique or bend specimens may be explained by the special nature of subcritical crack extension at stressed surfaces for these different specimen types. The log v-log K _i plot of ZrO₂-I using the double torsion technique shows a plateau of constant velocity, which has to be attributed to a tetragonal-monoclinic transformation. After annealing (1500° C, 5 h) the plateau has vanished and the n value is comparable to bend test in an as-received condition.     

Slip casting of partially stabilized zirconia

Casting behaviour and rheological properties are studied in order to define the appropriate conditions under which to prepare slips for the production of high-temperature ceramics. Various commercial powders have been used, which were characterized with respect to morphology, particle size distribution and specific surface area. Zirconia slips with 75 wt% solid content were prepared with distilled water and ethanol as dispersing agent, with and without deflocculant. Hydrochloric acid and tetramethylammonium hydroxide were used to control the pH. Investigations into rheology, i.e. the dependence of viscosity and shear stress on shear rate, were performed. The slip, green and sedimentation bulk densities were measured.     

Growth morphology of the tetragonal phase in partially stabilized zirconia

The zirconia-rich, metastable tetragonal phase in partially stabilized zirconia—magnesia, zirconia—calcia and zirconia—yttria is examined using electron microscopy and electron and X-ray diffraction. The tetragonal phase precipitate distribution is that normally associated with homogeneously nucleated coherent precipitation. An attempt is made to explain the growth morphology of the tetragonal phase in terms of the cubic—tetragonal lattice parameter mismatch. It is found that the tetragonal phase is retained at room temperature provided coherency with the cubic matrix is retained. Once coherency is lost, due to growth strains or mechanical influences, the precipitate reverts to the room temperature stable monoclinic form.     

A statistical fracture mechanics analysis of time-dependent strength behaviour of partially stabilized zirconia

The time-dependent strength behaviour of a partially stabilized zirconia ceramic (Mg-PSZ) when subjected to constant static and cyclic stresses as well as constant stress rates is analysed in terms of a statistical fracture mechanics model given earlier by the authors. Given the lifetimes for either constant static stresses or constant stress rates it is possible to estimate the lifetimes for constant cyclic stresses. There is good agreement between the predicted and actual lifetimes under cyclic stresses if the data for constant stress rates are used with the theory. The limitations of the conventional single-crack theory for lifetime predictions relative to the statistical fracture mechanics approach are highlighted.     

Plasma spraying of nanostructured partially yttria stabilized zirconia powders

Nanosized partially yttria stabilized zirconia particles, prepared using a co-precipitation method, were reprocessed into agglomerate powders using two methods for plasma spraying. The first method was to make micrometer-sized agglomerates directly following the grinding of the calcined yttria–zirconia agglomerates. The second method was to reconstitute the nanosized particles into micrometer agglomerates using spray drying. The deposition efficiency, porosity, microhardness and average grain size of the deposits made from these two reprocessed powders were studied. Distinct results related to the process parameters were obtained for the two types of powders. The second type of powder was more suitable for plasma spraying than the first one. Using the second type of powder, some unique results distinguished from those of the conventional partially yttria stabilized zirconia powders were observed and an optimized coating with a porosity of 3.8%, Hv_0.3 of 953 and mainly consisting of 1–3 μm columnar grains in the columnar direction and smaller than 100 nm in their cross-sections was achieved.     

Influence of long-term ageing upon the mechanical properties of partially stabilized zirconia (Mg-PSZ) for heat-engine applications

The influence of long-term ageing at 1000° C upon the mechanical strength of a commercial Mg-PSZ ceramic is investigated. The Weibull solution was used to measure the material reliability. The effects of sub-eutectoid ageing were related to the stability of the tetragonal phase. Grain boundaries are shown to often act as fracture origins, and to constitute a fracture-controlling population that is not significantly affected by high-temperature ageing.     

Functionally graded bioactive glass coating on magnesia partially stabilized zirconia (Mg-PSZ) for enhanced biocompatibility

The coating of magnesia partially stabilized zirconia (Mg-PSZ) with a bioactive glass was investigated for enhancing the bioactivity and bone-bonding ability of Mg-PSZ orthopedic implants. Individual coatings of three different bioactive glasses were prepared by depositing a concentrated suspension of the glass particles on Mg-PSZ substrates, followed by sintering at temperatures between 750 °C and 850 °C. Two silicate-based glass compositions (designated 13–93 and 6P68), and a borosilicate glass composition (H12) were investigated. The microstructure and adhesive strength of the coatings were characterized, and the in vitro bioactivity of the glasses was compared by measuring their conversion kinetics to hydroxyapatite in an aqueous phosphate solution at 37 °C. The 6P68 glass provided the highest adhesive strength (40 ± 2 MPa) but showed very limited bioactivity, whereas the H12 glass had lower adhesive strength (18 ± 2 MPa) but the highest bioactivity. A functionally graded coating, consisting of a 6P68 interfacial layer and an H12 surface layer, was developed to provide a coating with high adhesive strength coupled with rapid in vitro bioactivity.     

Phase transitions of partially stabilized zirconia under stepwise shock-wave loading

High-density partially yttria-stabilized zirconia ceramics were manufactured. The primary properties of the manufactured ceramics, such as crack resistance, ultimate bending strength, hardness, elastic moduli, etc., were characterized. The microstructure of the ceramics and the phase transitions under mechanical (including shock-wave) and thermal actions were studied.