Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.     

Superplasticity of Hot Isostatically Pressed Hydroxyapatite

Dense and translucent hydroxyapatite polycrystals (Ca₁₀(PO₄)₆(OH)₂ with a grain size of 0.64 μMm) were obtained by hot isostatic pressing at 203 MPa and 1000°C for 2 h in argon. The material exhibited superplastic elongation (>150%) in a tension test at temperatures from 1000° to 1100°C and at strain rates from 7.2×10⁻⁵ to 3.6 × 10⁻⁴ s⁻¹. Extensive strain hardening was observed. The stress exponent of the yield stress was larger than 3.     

Hot Isostatically Pressed Alumina-Silicon Carbide-Whisker Composites

Alumina-silicon carbide-whisker composites were hot isostatically pressed at 1550°C and 200 MPa for 1 h. The silicon carbide whiskers were treated in different acid and gas environments before they were pressed. All samples exhibited linear elastic behavior with no ductility tendency. Improved strength and fracture toughness were obtained compared with unreinforced alumina. Mechanisms for the improved mechanical properties are discussed. These include grain growth control, whisker encapsulation of defects, and related stress relief at the defect.     

Effect of Phase Composition on the Mechanical Properties of Hot Isostatically Pressed SiAION Ceramics

Phase compositions were determined in SiAION ceramics, consisting of only Si, Al, O, and N, which were prepared by high-pressure sintering followed by hot isostatic pressing. The o'-phase contents were correlated with the high-temperature fracture strengths and Vickers hardness. The high-temperature bending strength obtained in this study was over 670 MPa when the o'-phase content was under 18.0 vol% and decreased with increasing o'-phase content. The hardness increase was approximately linear with increasing o'-phase content.     

Grain Growth During Hot Isostatic Pressing of Presintered Alumina

Grain growth during hot isostatic pressing is examined at 1373 to 1673 K at 5 to 200 MPa for 0.5 to 4 h on alumina presintered at 1723 to 1873 K. A linear relation is found between logarithms of grain size and porosity. To account for this result, the development of similar microstructures is suggested regardless of the histories of the starting powder and process. Temperature dependences of grain growth and densification must also be equal during hot isostatic pressing. Both grain growth and densification were controlled by the grain-boundary diffusion of the aluminum ions.     

Mechanical Properties of Hot Isostatically Pressed Zirconia-Toughened Alumina Ceramics Prepared from Coprecipitated Powders

Amorphous Al₂O₃–ZrO₂ composite powders with 5–30 mol% ZrO₂ have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2-propanol. Simultaneous crystallization of γ-Al₂O₃ and t -ZrO₂ occurs at 870°–980°C. The γ-Al₂O₃ transforms to α-Al₂O₃ at 1160°–1220°C. Hot isostatic pressing has been performed for 1 h at 1400°C under 196 MPa using α-Al₂O₃– t -ZrO₂ composite powders. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t / m ratios of ZTA are examined, with emphasis on the relation between strength and fracture toughness.     

Postsintering Hot Isostatic Pressing of Ceria-Doped Tetragonal Zirconia/Alumina Composites in an Argon–Oxygen Gas Atmosphere

Ceria-doped tetragonal zirconia (Ce-TZP)/alumina (Al₂O₃) composites were fabricated by sintering at 1450° to 1600°C in air, followed by hot isostatic pressing (postsintering hot isostatic pressing) at 1450°C and 100 MPa in an 80 vol% Ar–20 vol% O₂ gas atmosphere. Dispersion of Al₂O₃ particles into Ce-TZP was useful in increasing the relative density and suppressing the grain growth of Ce-TZP before hot isostatic pressing, but improvement of the fracture strength and fracture toughness was limited. Postsintering hot isostatic pressing was useful to densify Ce-TZP/Al₂O₃ composites without grain growth and to improve the fracture strength and thermal shock resistance.     

Particle Interactions in Zirconia-Toughened Alumina

In order to determine the effect of the coarse tail of the ZrO₂ size distribution of the monoclinic ZrO₂ content of zirconia-toughened alumina, quantitative hot-stage X-ray measurements were made on (1) A1₂O₃-10% Zr0₂ (submicrometer with a coarse tail >1 μm), (2) A1₂O₃-10% ZrO₂ with the coarse tail of the ZrO₂ carefully removed by centrifugation, and (3) Al₂O₃-10% ZrO₂ in which 10% of Zr0₂(1) and 90% of Zr0₂(2) were used. The monoclinic content of (3) was compared with the weighted average of(1) and (2). A disproportionate amount of monoclinic ZrO₂ was found in (3) and it was concluded that transformation of the coarse particles promotes transformation in finer particles. Microstructural examination supports this conclusion. Results were interpreted as being due both to autocatalytic transformation and to constraint relief from microcracking.     

Effect of Wear Damage on the Strength of Hot Isostatically Pressed Silicon Nitride

Bending tests have been used to characterize the effect of wear damage of hot isostatically pressed silicon nitride in contact with waspaloy at 600°C. The problem arises at the junction between turbine blades and disks where wear may result in premature fatigue cracking of the blade root. The tests show that wear always results in a reduction of strength, whereas the Weibull modulus may decrease or remain unchanged, depending on the amount of surface damage.     

The Dependence of Pore Size Distribution on Porosity in Hot Isostatically Pressed Porous Alumina

Pore size distributions in porous alumina bodies produced by the capsule-free hot isostatic pressing technique have been determined experimentally. The distribution of pore diameter has been found to be dependent on the size of the pre-sintered powders and the amount of open porosity in the sintered body. An empirical model has been developed to predict the modal pore size as a function of median particle size and open porosity. The pore size distributions were found to widen with reduced porosity. They were also shown to be positively skewed. The skew reduced with decreasing porosity. The pore size variation with porosity for specimens produced with a sintering aid could not be described by the same mathematical functions developed for specimens produced by solid-state sintering.     

Hot Isostatic Pressing of Alumina and Examination of the Hot Isostatic Pressing Map

The hot isostatic pressing (HIP) of four alumina powders is studied in the temperature range 1100° to 1400°C, at 5- to 200-MPa applied pressure, and for times ranging from 0.5 to 4 h. Density increases with increasing HIP temperature, pressure, and time; decreasing grain size results in increased density after HIP. An empirical relation is derived for grain growth during HIP, and the HIP map proposed by Helle et al. is found applicable to the present results. Densification is governed by the grain-boundary diffusion of aluminum ions; with the transport coefficient and the grain-growth values found in the present study, the map can be used to express experimental results to within a factor of 4 for all densification stages except near full density.     

Preparation of High-Strength and Translucent Alumina by Hot Isostatic Pressing

A vacuum-pressure slip-casting technique and hot isostatic pressing (HIPing) were used to prepare high-strength and translucent alumina ceramics. A low-viscosity and high-solids-content slurry (46 vol% solids) was prepared, and a dense green compact was formed. The samples were sintered and subjected to capsule-free HIPing. Extremely high-density (99.9%) and fine-grained (0.7 to 15 μm in diameter) alumina ceramics were obtained. The HIPed samples showed high bend strength and translucency with in-line transmittance of 30% to 46% (1 mm thick).     

Analysis of Hot Isotatic Pressing of Presintered Zirconia

Hot isostatic pressing (HIP) of presintered Y-TZP was studied at 1100° to 1400°C, 5 to 200 MPa, for 0.5 to 4 h. The effects of process variables of HIP and the characteristics of the presintered specimens on the densification behavior of HIP were examined. The microstructural development after HIP was also examined. Grain growth occurred during the densification in HIP. Empirically, a linear relation having a rather constant slope was found between the logarithms of porosities and grain sizes, for each starting condition. Provided this relationship was taken into account, the Ashby's model for HIP could express the densification process for this system satisfactorily.     

Rheology of Porous Alumina and Simulation of Hot Isostatic Pressing

Rheology of a porous alumina was studied using sinter-forging, hot-pressing, and sintering tests. The results are analyzed using constitutive equations for porous materials. The deformation and densification rates are found to follow Coble creep behavior with an eventual control by interface reactions. The effect of porosity on shear and densification behavior is studied and compared to results obtained on metal powders showing similar trends for shear and a stronger influence in the case of densification. Large pores are likely to buckle at low densities when external forces are applied. The sintering pressure is also estimated and lies in the range 0 to 3 MPa. Finally, the constitutive equations are used to simulate hot isostatic pressing of test shapes, showing that the proposed model correctly predicts the deformation of the ceramic preforms.     

Phase Analysis and Thermal Stability of Hot Isostatically Pressed Zirconia–Hydroxyapatite Composites

Composites of zirconia and hydroxyapatite (OHAp) have been processed via hot isostatic pressing (HIP) or sintering in air. When the composites were sintered in air at a temperature of 950°C, decomposition of the OHAp to tricalcium phosphate occurred. Using the HIP technique, composites without any detectable degradation of the OHAp phase were produced at 1200°C. The reactivity between zirconia and OHAp was dependent on both the amount of water lost from OHAp and the geometry of the interaction. The phase composition of the materials prepared was evaluated from their powder X-ray diffraction patterns, and their microstructures were studied via electron microscopy.     

Residual-Stress–Defect-Severity Relationships in Ceramics

Equivalent-sized fracture-origin defects in partially stabilized zirconia ceramics show dissimilar behavior in initiating fracture. It is suggested that the difference of relative defect severity is related to local residual stresses which develop during sintering and cooling and to elastic modulus differences in the case of solid inclusions on external loading. The existence of residual stresses associated with various defect types is demonstrated, and their variation with temperature is explored. The relative severity of α-alumina defects in tetragonal zirconia varies with temperature, and it is suggested that this is associated with matrix compressive stress relaxation. The residual stress associated with agglomerate defects is temperature independent, and no residual stress is associated with pores.     

Densification of Precursor-Derived Si-C-N Ceramics by High-Pressure Hot Isostatic Pressing

Densification behavior of precursor-derived Si-C-N ceramics by hot isostatic pressing (HIP) has been investigated to obtain dense ceramics derived from polymer precursor. An as-pyrolyzed ceramic monolith, which had a porosity of about 17%, could be deformed up to a strain of 8% in preliminary uniaxial compression tests. The flow stress of the material was much higher than 200 MPa at 1600°C; thus high stress was necessary for densification by HIP. The density of the monolith increased from 1.9 to 2.4 g/cm³ by HIP at 1600°C and 980 MPa. Although the number of pores decreased, large pores were formed in the hot isostatically pressed monolith. On the other hand, denser ceramics, in which pores were not observed by optical microscopy, were obtained by hot isostatically pressing the pyrolyzed powder compact.     

Mechanical Properties, Thermal Shock Resistance, and Thermal Stability of Zirconia-Toughened Alumina-10 vol% Silicon Carbide Whisker Ceramic Matrix Composite

Zirconia-toughened alumina (Al₂O₃–15 vol% Y-PSZ (3 mol% Y₂O₃)) reinforced with 10 vol% silicon carbide whiskers (ZTA-10SiC _w ) ceramic matrix composite has been characterized with respect to its room-temperature mechanical properties, thermal shock resistance, and thermal stability at temperatures above 1073 K. The current ceramic composite has a flexural strength of ∽550 to 610 MPa and a fracture toughness, K _IC , of ∽5.6 to 5.9 MPa·m^1/2 at room temperature. Increases in surface fracture toughness, ∽30%, of thermally shocked samples were observed because of thermal-stress-induced tetragonal-to-monoclinic phase transformation of tetragonal ZrO₂ grains dispersed in the matrix. The residual flexural strength of ZTA–10 SiC _w ceramic composite, after single thermal shock quenches from 1373–1573 to 373 K, was ∽10% higher than that of the unshocked material. The composite retained ∽80% of its original flexural strength after 10 thermal shock quenches from 1373–1573 to 373K. Surface degradation was observed after thermal shock and isothermal heat treatments as a result of SiC whisker oxidation and surface blistering and swelling due to the release of CO gas bubbles. The oxidation rate of SiC whiskers in ZTA-10SiC _w composite was found to increase with temperature, with calculated rates of ∽8.3×10⁻⁸ and ∽3.3×10⁻⁷ kg/(m²·s) at 1373 and 1573 K, respectively. It is concluded that this ZTA-10SiC _w composite is not suitable for high-temperature applications above 1300 K in oxidizing atmosphere because of severe surface degradation.     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.     

Densification of Lead Selenide and Lead Sulfide by Hot Isostatic Pressing

PbSe and PbS are both volatile and difficult to densify by normal sintering. We tried to sinter these compounds in a closed system, that is, by hot isostatic pressing in a glass capsule. Hot isostatic pressing was carried out for these compounds on the basis of their thermal behaviors in air as studied by TG-DTA. The maximum bulk densities obtained were 8.10 and 7.50 g/cm³, respectively, corresponding to their theoretical densities. The microstructures of the isostatically hot-pressed bodies were also observed by SEM. The present work shows that ceramic compounds which are normally volatile can be densified by encapsulation in glass followed by hot isostatic pressing.