Production and characterization of ZrO<Subscript>2</Subscript> ceramics and composites to be used for hip prosthesis

Tetragonal ZrO₂ polycrystalline (TZP) ceramics with varying yttria and ceria content (2–3 mol%) and distribution (coated or co-precipitated), and varying second phase content Al₂O₃ were prepared and investigated by means of microstructural analysis, mechanical properties, and hydrothermal stability, and ZrO₂-based composites with 35–60 vol% of electrical conductive TiN particles were developed. The effects of stabilizer content and means of addition, powder preparation, sintering conditions, and grain size have been systematically investigated. Fully dense Y-TZP ceramics, stabilized with 2–3 mol% Y₂O₃, 2 wt% Al₂O₃ can be achieved by hot pressing at 1,450 °C for 1 h. The hydrothermal stability increased with increasing overall yttria content. The jet-milled TiN powder was used to investigate the ZrO₂–TiN composites as function of the TiN content. The experimental work revealed that fully dense ZrO₂–TiN composites, stabilized with 1.75 mol% Y₂O₃, 0.75 wt% Al₂O₃, and a jet-milled TiN content ranging from 35 to 60 vol% could be achieved by hot pressing at 1,550 °C for 1 h. Transformation toughening was found as the primary toughening mechanism. The decreasing hardness and strength could be attributed to an increasing TiN grain size with increasing TiN content, whereas the decreasing toughness might be due to the decreasing contribution of transformation toughening from the tetragonal to monoclinic ZrO₂ phase transformation. The E modulus increases linearly with increasing TiN content, whereas the hydrothermal stability increases with addition of TiN content.     

Control of the tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia in hot water

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia caused by annealing in hot water was investigated in the temperature range 80 to 200° C using sintered bodies in zirconia containing 2, 3 and 4 mol % Y₂O₃. Three approaches, alloying ZrO₂(Y₂O₃) with 0 to 20wt% CeO₂, dispersing 0 to 40 wt % Al₂O₃ into ZrO₂(Y₂O₃) ceramics and decreasing the grain size of zirconia, were examined to inhibit the tetragonal-to-monoclinic phase transformation. The amount of monoclinic phase formed decreased with increasing concentrations of CeO₂ alloyed and Al₂O₃ dispersed, and with decreasing grain size of zirconia.     

Thermal shock resistance of ZrO2 based ceramics

Thermal stress fracture behaviour of zirconia based ceramics are described. Although partially stabilized zirconia (PSZ) and tetragonial zirconia polycrystals (TZP) ceramics show superior mechanical properties such as high fracture strength/fracture toughness, the thermal shock resistance of zirconia ceramics is anomalously low. The thermal stress fracture mechanism and improvement of the thermal shock resistance are discussed.     

Mechanical properties of (Y-TZP) — alumina-silicon carbide nanocomposites and the phase stability of Y-TZP particles in it

Al₂O₃-SiC-ZrO₂ composites were investigated to obtain a better understanding of the effect of SiC particles and the stress-induced transformation of Y-TZP on its mechanical properties. The Al₂O₃-SiC-ZrO₂ composites were fabricated by hot pressing using -Al₂O₃, SiC and ZrO₂ mixtures. Fracture toughness and strength of Al₂O₃ were greatly improved by incorporating SiC and ZrO₂ particles which were located mainly inside and between Al₂O₃ grains, respectively. The toughening and strengthening mechanism of these composites and the phase stability of the tetragonal ZrO₂ in the composites before and after high-temperature annealing were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. It was observed that there is a critical volume fraction of zirconia, above which the phase stability of the tetragonal zirconia increases, despite the grain growth of the zirconia. It is considered that another phenomenon, the residual stresses, affect the phase stability of the tetragonal zirconia. To remove the residual stresses the composites were annealed at 1100 °C. After slow cooling, the tetragonal zirconia became very unstable, especially in samples with the highest fabrication temperature and increasing zirconia content. Even quenching from 1100 °C caused an increase in the monoclinic phase of these samples.     

Microstructure and mechanical behaviour of 3Y-TZP/Mo nanocomposites possessing a novel interpenetrated intragranular microstructure

Yttria stabilized tetragonal zirconia polycrystal (Y-TZP)/0-100 vol % molybdenum (Mo) composites were fabricated by hot-pressing a mixture of Y-TZP powder containing 3 mol % yttria (Y₂O₃) and a fine Mo powder in vacuum. This composite system possessed a novel microstructural feature composed of an interpenetrated intragranular nanostructure, in which either nanometer sized Mo particles or equivalent sized zirconia (ZrO₂) particles located within the ZrO₂ grains or Mo grains, respectively. The strength and toughness were both greatly enhanced with increasing Mo content for the 3Y-TZP/Mo composites thus breaking through the strength-toughness tradeoff relation in transformation toughened ZrO₂ and its composite materials. They exhibited a maximum strength of 2100 MPa and a toughness of 11.4 MPa·m^1/2 for the composite containing 70 vol % Mo. These simultaneous improvements in strength and toughness were determined to be the result of a decrease in flaw size associated with the interpenetrated intragranular nanostructure, and a stress shielding effect created in the crack tip by the elongated Mo polycrystals bridging the crack tip in addition to the stress induced phase transformation.     

Critical evaluation of indentation fracture toughness measurements with Vickers indenter on yttria‐stabilized zirconia dental ceramics

The purpose of this study was to investigate and analyze fracture toughness (K_Ic) of yttria stabilized tetragonal zirconia (Y‐TZP) dental ceramics by the Vickers indentation fracture test. In order to determine fracture toughness, the Vickers indenter was used under the load of 294.20 N (HV30). The cracks, which occur from the corners of a Vickers indentation, were measured and used for fracture toughness determination, through five mathematical models according to (I) Anstis, (II) Evans and Charles, (III) Tanaka, (IV) Niihara, Morena and Hasselman and (V) Lankford. Morphology of indentation cracking was determined by scanning electron microscope. The most adequate model for determination of fracture toughness (K_Ic) of yttria stabilized tetragonal zirconia dental ceramics by the Vickers indentation fracture test is Lankford model.     

Thermal Shock Performance of Fine Grained Al2O3 Ceramics With TiO2 and ZrO2 Additions for Refractory Applications

Due to zirconia and titania additions carbon‐free fine grained alumina ceramics are produced with superior thermal shock performance. The decomposition of Al₂TiO₅ in the alumina doped matrix dominates during thermal shock attack and leads to higher strengths in comparison to the as sintered samples after thermal shock. EDX, EBSD, and XRD investigations describe the phase evolution before and after quenching the samples from 950 to 1200 °C in water, respectively.     

Nanocrystalline zirconia-yttria system–a Raman study

Nano sized zirconia (ZrO₂) powders doped with different amount of yttria (Y₂O₃) (3, 5 and 8 mol%) were prepared through coprecipitation method. The crystallite size estimated from the X-ray peak broadening is around 10 nm. Phase identification was carried out using XRD and Raman spectroscopy. Raman spectroscopic study of the synthesized materials show clear evidence of the presence of single phase cubic structure in the case of 8 mol% Y₂O₃ doped fully stabilized zirconia (8Y-FSZ); tetragonal phase in the case of zirconia doped with 3 mol% Y₂O₃ (3Y-TZP-tetragonal zirconia polycrystal) and a mixture of cubic and tetragonal phases for 5 mol% Y₂O₃ doped partially stabilized zirconia (5Y-PSZ). Raman technique is therefore an effective tool to distinguish the phases present in the calcined nano sized powders of zirconia.     

Effect of a small amount of Al2O3 addition on the hydrothermal degradation of 3Y-TZP

The effects of 0–5 wt% Al₂O₃ addition on the aging sensibility and strength degradation of 3-mol-yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) were studied in this study. X-ray diffraction and field emission scanning electron microscopy were used to study the microstructures of aged specimens. The results showed that a small amount of Al₂O₃ addition could effectively improve the aging resistance of 3Y-TZP. The evolution of mechanical properties with aging time is different for 3Y-TZP with and without Al₂O₃ addition. Compared to the flexural strength, the fracture toughness and Vickers hardness of specimen are more susceptible to aging. The grain boundary segregated Al³⁺ plays a key role in retarding the tetragonal-to-monoclinic phase transformation of 3Y-TZP during aging.     

Tough yttria-stabilized zirconia ceramic by low-temperature spark plasma sintering of long-term stored nanopowders

Weakly agglomerated 1.75 and 3 mol% yttria stabilized zirconia nanopowders were used in this study after six years of storage in vacuum-processed plastic containers. The proper storage conditions of the Y-TZP nanopowders avoided the hard agglomeration. Untreated and bead-milled nanopowders were used to obtain dense ceramics by slip casting and subsequent low-temperature sintering. Fully dense nanostructured 1.75Y-TZP and 3Y-YZP ceramics with and without doping of 1 wt% Al2O3 were produced by an optimized spark plasma sintering (SPS) technique at the temperatures of 1050-1150 degrees C at a pressure of 100 MPa. The SPS has revealed the clear advantage of consolidation of the weakly agglomerated nanopowders without preliminary deagglomeration. The Vickers hardness of both the low-temperature and spark plasma sintered samples was found to lie in the range of 10.98-13.71 GPa. A maximum fracture toughness of 15.7 MPa m(1/2) (average 14.23 MPa m(1/2)) was achieved by SPS of the 1.75Y-TZP ceramic doped with 1 wt% Al2O3 whereas the toughness of the 3Y-TZP ceramics with and without alumina doping was found to vary between 3.55 and 5.5 MPa m(1/2).     

Effects of heteroflocculation of powders on mechanical properties of zirconia alumina composites

Zirconia-toughened alumina (ZTA) composites colloidally processed from dense aqueous suspensions (>50 vol% solids) had ZrO₂ content varying from 5 to 30 vol%. Tetragonal zirconia (TZ) was used in the unstabilized, transformable form (0Y-TZ), in the partially transformable form, partially stabilized with 2 mol% yttria (2Y-TZ), and in the non-transformable form stabilized with 3 mol% yttria (3Y-TZ). After sintering in air to 99% theoretical density, the elastic properties, flexure strength and fracture toughness were examined at room temperature. Dynamic moduli of elasticity of fully deagglomerated compositions did not show the effects of microcrack formation during sintering, even for materials with unstabilized zirconia. In all compositions made from submicron powders and with low content of dispersed phase (less than 10 to 20 vol %), the strength increased with increasing ZrO₂ content to a maximum of 1 GPa, irrespective of the degree of stabilization of t-ZrO₂. With increasing content of the dispersed phase (> 20 vol%), heteroflocculation of powder mixtures during wet-processing led to the formation of ZrO₂ grain clusters of increasing size. Residual tensile stresses built within cluster/matrix interfaces upon cooling not only facilitated the t-m ZrO₂ phase transformation in final composites with transformable t-ZrO₂, but also led to lateral microcracking of ZrO₂/Al₂O₃ interfaces. This enhanced fracture toughness, but at larger ZrO₂ contents the flexure strength always decreased due to intensive microcracking, both radial and lateral. The important microstructural aspects of strengthening and toughening mechanisms in ZTA composites are related in discussion to the effects of heteroflocculation of powder mixtures during wet-processing.     

Microstructure and mechanical properties of zirconia-toughened lithium disilicate glass–ceramic composites

The lithium disilicate glass–ceramics composites reinforced and toughened by tetragonal zirconia (3Y-TZP) were prepared by hot-pressing at 800 °C with varying zirconia content from 0 to 30 wt.%. In the case of the composites of small zirconia content (below 10 wt.%), zirconia acted as nucleation agent primarily, and the microstructure was refined continuously. The morphology of Li₂Si₂O₅ crystals transformed from rod-shaped to spherical structure, and the mechanical properties decreased inevitably. For the composites of large zirconia content (from 15 wt.% to 30 wt.%), however, zirconia restrained the phase separation of glass. The morphology of Li₂Si₂O₅ crystals transformed to rod-shaped structure again. The mechanical properties of the composite at zirconia content of 15 wt.% increased up to 340 MPa and 3.5 MPa m^1/2 which were much higher than those of zirconia-free glass–ceramics. The improved properties were attributed mainly to compressive stress reinforcement, phase transformation and bridging toughening mechanisms.     

Toughness tailoring of yttria-doped zirconia ceramics

Despite the impressive development in understanding transformation toughening, tailoring the toughness of yttria-doped zirconia ceramics remained a major challenge. In our research, a simple but innovative route based on the mixing and hot pressing (under identical conditions) of zirconia powders with varying yttria content (3 and 0 mol%) is developed to investigate this critical issue. The experimental results clearly reveal that the fracture toughness of yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics can be tailored by careful mixing of co-precipitated and yttria-free zirconia starting powders.     

Characterization of pure and doped zirconia nanoparticles with infrared transmission spectroscopy

Infrared transmission spectroscopic studies of pure and doped zirconia nanoparticles with different average particle sizes have been made with conventional XRD and TEM techniques. Infrared spectra of these pure or doped zirconia samples are well characterized by their unique spectral features or optical phonons. Blue shifts and broadening of the absorption bands are observed with the decrease of particle size. The seeming “disappearances” of band 628 cm ^{− 1} in pure zirconia nanoparticles and band 610 cm ^{− 1} in the doped zirconia nanoparticles are attributed to the overlap of broad absorption bands. Blue shift of the absorption bands are also caused by the doping of Al₂O₃, which exists metastably in the Y-TZP lattice and decreases the interplanar distance. The contributions of the “interface effect” and “fine size effect” to infrared absorption are also discussed.     

Disintegration process of yttria-stabilized zirconia ceramics using hydrothermal conditions

Capability of the recycling of high strength and high fracture toughness yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) sintered body utilizing “low-temperature annealing degradation” phenomenon was investigated. Hydrothermal treatment was employed to induce the phase transformation from tetragonal to monoclinic zirconia and to disintegrate the Y-TZP sintered body. 3 mol% Y₂O₃–ZrO₂ specimens sintered at 1,550 °C and more were disintegrated without leaving the original appearances when the treatment temperature was between 200 °C and 400 °C. The size of the disintegrated fragments of Y-TZP sintered body was much affected by hydrothermal treatment conditions. Only with hydrothermal treatment and simple ball milling, the sintered body was pulverized into the primary particle level. This technique is expected to apply to a sustainable recycling system for the zirconia ceramics, which restrains an energy consumption compared to crushing zirconia using mechanical procedures.     

Influence of cobalt phase on thermal shock resistance of Al2O3-TiC composites evaluated by indentation technique

Cobalt-coated Al₂O₃ and TiC powders were prepared using an electroless method to improve resistance to thermal shock. The mixture of cobalt-coated Al₂O₃ and TiC powders (about 70 wt.% Al₂O₃-Co + 30 wt.% TiC-Co) was hot-pressed into an Al₂O₃-TiC-Co composite. The thermal shock properties of the composite were evaluated by indentation technique and compared with the traditional Al₂O₃-TiC composite. The composites containing 3.96 vol.% cobalt exhibited better resistance to crack propagation, cyclic thermal shock and higher critical temperature difference (ΔT_c). The calculation of thermal shock resistance parameters (R parameters) shows that the incorporation of cobalt improves the resistance to thermal shock fracture and thermal shock damage. The thermal physic parameters are changed very little but the flexure strength and fracture toughness of the composites are improved greatly by introducing cobalt into Al₂O₃-TiC (AT) composites. The better thermal shock resistance of the composites should be attributed to the higher flexure strength and fracture toughness.     

Thermal stability and mechanical properties of yttria-doped tetragonal zirconia polycrystals with dispersed alumina and silicon carbide particles

Yttria-doped tetragonal zirconia polycrystals in which were dispersed various amounts of Al₂O₃ and SiC particles were sintered at 1500° C for 3 h, and the mechanical properties and the thermal stability of the sintered bodies were evaluated. Dispersion of Al₂O₃ caused no significant effect on sinterability, and increased the hardness and elasticity of the composites. Dispersion of SiC particles decreased the relative density and the grain size of composites. Elasticity and hardness increased by dispersing less than 10 vol% SiC, but decreased above 10 vol% SiC due to the decrease of relative density. Dispersion of both Al₂O₃ and SiC particles slightly increased the fracture toughness of ZrO₂-3 mol% Y₂O₃ ceramics but significantly decreased that of ZrO₂-2 mol% Y₂O₃ ceramics. The rate of the tetragonal-to-monoclinic phase transformation decreased by dispersing both Al₂O₃ and SiC particles. The transformation depth increased rapidly and then slowly with increasing the annealing time. The rate of increase in the transformation depth greatly decreased by dispersing Al₂O₃ particles.     

Texture development in 3 mol% yttria-stabilized tetragonal zirconia

We have developed a method of forming textured tetragonal zirconia. A suspension containing 10 vol% solid loading of monoclinic ZrO₂ mixed with 3 mol% Y₂O₃ was prepared, and then a bead-milling process was performed using 50 μm diameter zirconia beads resulting in a well-dispersed suspension. The mixture suspension of monoclinic zirconia and yttria nanoparticles was slip cast under a magnetic field of 12 T to produce oriented monoclinic zirconia with yttria. The reaction sintering between yttria and the oriented monoclinic zirconia produces a final 3 mol% Y₂O₃ doped tetragonal zirconia that remains oriented.     

Mechanical properties of Al2O3 particle-Y-TZP matrix composite and its toughening mechanism

Dense Al₂O₃ particle-Y-TZP matrix (Al₂O₃<40 vol%) composite was prepared by pressureless sintering at 1550°C. Composites with 10–30 vol% Al₂O₃ particles showed enhanced fracture toughness, bending strength and Vicker's hardness as compared to single-phase Y-TZP. The highest strength (1150 MPa) and highest toughness (12.4 MPa m^1/2) were obtained for the composite containing 10 vol% Al₂O₃. It was found that, in addition to the contribution by the crack-deflection effect, the enhanced phase transformation from tetragonal to monoclinic during fracture was the main toughening mechanism in operation in the composites.     

Microstructure and toughening mechanism of the 3Y-TZP/Al2O3 composite reinforced by SiC whiskers

Microstructures of 10 vol% SiC whisker reinforced 3Y-TZP (3vol% yttria stabilized tetragonal zirconia polycrystals) composite with addition of 5 wt% Al₂O₃ have been investigated using electron microscopy. The investigation focused not only on the morphology of the composite but also on the martensitic transformation of ZrO₂. The results from high resolution electron microscopy (HREM) observations show that the boundaries between whiskers and the matrix are, in general, clean; however, in some areas, little glassy phases exist. The habit plane of t/m ZrO₂ during the transformation was found at c _t//a _m in the present observations. The processes of t m transformation in various conditions have been discussed. The fact that no whisker pull-out was found during material breakage indicates that the toughening of this composite would come mainly from a t/m transformation toughening mechanism, as well as crack deflection and bridging. Finally, a fracture strength enhancing mechanism is discussed.