Increasing Zirconia Content Research Articles

Effect of microstructure on the wear transition of zirconia-toughened alumina

The mechanical properties of alumina ceramics can be improved by the addition of pure or partially stabilized zirconia particles. In the present study, the wear characteristics of zirconia-toughened alumina (ZTA) composites under a non-reactive fluid (paraffin oil) lubricated condition were investigated. The wear transition load ( i.e. the load at which a rapid increase in wear occurs) increased with increasing zirconia content up to 20 vol.%. The transition from mild to fracture-coritrolled wear of ZTA depends on the material properties ( e.g. hardness, elastic modules), the contact conditions ( e.g. hertzian stress, coefficients of friction) and the microstructure of the material. The effect of the microstructure on wear was demonstrated and a Hall-Petch-type relationship between the microfracture stresses and the grain size was found. The effect of the grain size distribution on the wear transition load was also shown.

Internal friction, crack length of fracture origin and fracture surface energy in alumina-zirconia composites

Two series of alumina-zirconia composites, i.e. alumina-unstabilized zirconia and alumina-partially stabilized zirconia with 3 mol % Y2O3, with different zirconia content were slip casted and fired at 1550°C for 3 h. Elastic constant, bending strength and fracture toughness were measured. Internal friction was determined to follow the formation of cracks, nondestructively, which could be one of the fracture origins. The crack length of the fracture origin and the fracture surface energy were calculated by applying Griffith's fracture theory. Microstructures of the fracture surfaces were observed using a scanning electron microscope. For the unstabilized zirconia system, the increase in the internal friction of the order from 10−4 to 10−3 was a guide to find the formation of cracks which lead to the fracture. The increase in the cracks becoming a fracture origin lead to the increase inKlc and also to the apparent increase in the fracture surface energy. For the partially stabilized zirconia system, the increase in the fracture surface energy with an increase in zirconia content, keeping low internal frictions of the order of 10−4, indicates the intrinsic strengthening of the grain boundaries in comparison to the unstabilized zirconia system. Internal friction is the most suitable nondestructive physical quantity to find the microcracks which leads to the fracture.

Zirconia-Toughened Bioactive Glass-Ceramics

High-strength bioactive glass-ceramics were prepared by hot-pressing the mixtures of glass and tetragonal zirconia polycrystal (TZP) powders at 1150°-1200°C for 2h under the pressure of 30MPa. Mechanical properties of bioactive glass-ceramics were much improved by dispersing TZP powders sintered at 1400°C (TZ) or specially prepared for low temperature sintering (NSZ). The bending strength of the composite containing 40vol% TZ and NSZ showed 600MPa and 900MPa, respectively. The bending strength of the composite toughened by TZ increased with increasing TZ content up to 60vol% showing maximum value of 800MPa, and then decreased over 70vol%. On the other hand, the bending strength of the composite toughened by NSZ increased monotonously with increasing NSZ in whole range. Many defects were found in the specimens containing TZ over 70% by SEM observation and were considered to degrade the bending strength. The number of the points which zirconia grains contact directly with each other may increase with the increase in zirconia content. The defects were formed at these points because TZ powder did not sinter sufficiently at 1200°C. The composite containing 40vol% TZP still maintained high bioactivity though the bioactivity of the composite containing more than 50vol% TZP was poor. The bioactivity was evaluated from apatite formation on the surface in a simulated body fluid at 37°C for 30 days.

Mechanical Properties of Tricalcium Phosphate-Zirconia Composite Ceramics

Mixtures of β-tricalcium phosphate (β-TCP) and zirconia containing 0, 2, 3, 4 and 8mol% Y2O3 were hot-pressed for 10min at 1300°C-1400°C and 20MPa, and the mechanical properties (bending strength, fracture toughness, Vickers hardness and modulus of elasticity) of the hot-pressed bodies were measured. The zirconia content was up to 50vol%. The obtained ceramics consisted of β-TCP and zirconia. The above-mentioned mechanical properties increased with increasing zirconia content. Zirconia containing 2 and 3mol% Y2O3 (TZ-2Y and TZ-3Y) gave better results than other zirconias. Ceramics containing 50vol% zirconia (TZ-2Y or TZ-3Y) fired at 1350° and 1400°C had a bending strength of about 440MPa and a fracture toughness of about 2.7MPa·m1/2. These values were two or three times as large as those of β-TCP monolithic compacts.

Mechanical Properties and Existing Phases of Composite Ceramics Obtained by Sintering of a Mixture of Hydroxyapatite and Zirconia

Mixtures of hydroxyapatite and zirconia containing 3mol% Y2O3 (TZ-3 Y) or 2mol% Y2O3 (TZ-2 Y) were hot-pressed under the conditions of 1300°-1400°C, 10min and 20MPa, and the bending strength and fracture toughness of the hot-pressed bodies were measured. The reactivity between hydroxyapatite and zirconia was also examined. The zirconia content was up to 50vol%. The bending strength and fracture toughness increased with an increase in zirconia content. Properties were improved with zirconia (TZ-2 Y) than with zirconia (TZ-3 Y). Compacts containing 50vol% zirconia (TZ-2 Y) fired at 1400°C showed the highest mechanical properties: bending strength; about 400MPa and fracture toughnees; about 3.0MPa·m1/2. These values were two or three times as large as those of hydroxyapatite single compacts. The following two thermal transformations were observed in the present system: (i) decomposition of hydroxyapatite to α-tricalcium phosphate and (ii) transition of tetragonal zirconia to cubic phase.

Mo-ZrO2系焼結材におけるZrO2のt→m変態および正方晶ZrO2の残留について

In order to investigate the t→m transformation of ZrO2 and the retention of tetragonal-ZrO2 in Mo-ZrO2 composites, the composites containing ∼50 vol%ZrO2 were fabricated by the powder metallurgical process. The t→m transformation temperature (Ms point) was measured by the dilatometric method, and the retention of tetragonal-ZrO2(t-ZrO2) was determined by X-ray diffraction analysis as functions of variables such as powder mixing time, sintered density and zirconia content.The results are as follows:(1) The powder mixing time had a significant influence on Ms point and t-ZrO2. Appropriate mixing time made it possible to retain the t-ZrO2 comparable to that of Al2O3-ZrO2 composite. (2) Although the sintered density did not affect so much, the zirconia content had a great influence, especially in the range from 10 to 15 vol%ZrO2 there was a steep decrease in the t-ZrO2 content with increasing zirconia content. This decrease is due to the growth of zirconia particles. (3) The Ms point and t-ZrO2 content depended on the size of zirconia, and the critical size below which t-ZrO2 was stable lay between 0.4 μm and 0.6 μm. The reason why this size effect occurred could not be explained from the results of this study.