Sinter-Forging of Nanocrystalline Zirconia: I, Experimental

Nanocrystalline (15 nm) yttria (3 mol%)-stabilized zirconia (3Y-TZP) was sinter-forged under conditions of varying temperature (1050–1200°C), plastic strain rate (5 × 10⁻⁵ to 2 × 10⁻³s⁻¹), and green density (33–48%), using constant-crosshead-speed tests, constant-load (i.e., load-and-hold) tests, and constant-loading-rate tests. The densification and pore size evolution results indicate that plastic strain is largely responsible for elimination of large pores, while diffusional mechanisms control the elimination of small pores. Grain growth during sinter-forging is observed to be dependent solely on porosity during intermediate-stage sintering. Once the powder compact enters final-stage sintering, however, both static (time- and temperature-dependent) and dynamic (plastic-strain-dependent) grain growth take place, greatly accelerating the overall rate of grain growth. The use of fast strain rates to impose plastic strain before the onset of dynamic grain growth is proposed as a method of preserving small grain sizes during sinter-forging.     

low-Temperature Sinter Forging of Nanostructured Y-TZP and YCe-TZP

Compacts of ultrafine Y-TZP and YCe-TZP powders have been sinter forged under constant load at low temperatures (1100°–1200°C). The application of a uniaxial load leads to a strong reduction in sintering time, which in combination with the virtual absence of dynamic grain growth makes it possible to effectively limit grain growth. Nanostructured (grain size 100 nm) TZP with high relative densities (90%-93%) could thus be produced by sinter forging under 84 MPa at 1100°C. The contributions of both creep and densification to the dimensional changes of the sinter-forging samples have been analyzed. A strong reduction of the flaw concentration was observed in sinter-forged materials as compared to freely sintered ones.     

Strain Hardening in Superplastic Codoped Yttria-Stabilized Tetragonal-Zirconia Polycrystals

The superplastic characteristics of 3-mol%-yttria-stabilized tetragonal-zirconia polycrystals (3Y-TZP) were drastically improved by the combined addition of germanium oxide and other oxides (titanium, magnesium, and calcium), resulting in up to 1000% elongation under optimum conditions. Extensive work hardening of the codoped compositions for temperatures >1400°C was observed. Dynamic grain growth was not a sufficient mechanism to explain this anomalous high strain hardening. The appearance of a cubic phase that caused abnormal grain growth during testing may have resulted in increased flow stress, which resulted in exaggerated work hardening.     

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.     

Sintering Behavior of 0.8 mol%-CuO-Doped 3Y-TZP Ceramics

In recent years, 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) doped with copper oxide has obtained increasing interest due to its enhanced superplastisity and good potential in tribological applications. In this work, the effect of addition of small amounts (0.8 mol%) of copper oxide on the sintering behavior of 3Y-TZP was studied using a dilatometer and high-temperature X-ray diffraction (XRD). A qualitative sintering model was established based on several reactions during sintering as indicated by thermal analysis and XRD. Some of these reactions remarkably retard densification and consequently result in low final density (86%) of the sample sintered at 1400°C in air. The reaction between molten Cu₂O and yttria as segregated to the Y-TZP grain boundaries at around 1180°C leads to the depletion of yttria from Y-TZP grains, which results in the formation of monoclinic phase during cooling. A relatively higher oxygen partial pressure can inhibit the dissociation of CuO to Cu₂O. This inhibition in dissociation is one of the reasons why a dense (>96%) 0.8 mol% CuO-doped 3Y-TZP ceramic can be obtained after sintering at 1400°C in flowing oxygen.     

High-Temperature Strength of Sinter-Forged Silicon Nitride with Lutetia Additive

A sinter-forging technique was successfully applied to fabricate a silicon nitride with a lutetia (Lu₂O₃) additive. The sinter-forged specimen had a strongly anisotropic microstructure where rodlike silicon nitride grains preferentially aligned perpendicular to the forging direction. The specimen exhibited superior strength of ∼700 MPa at 1500°C. This strength was highest when compared with previous silicon nitrides at temperatures >1400°C. Such superior high-temperature strength was attributed to grain alignment as well as to the refractory grain-boundary glassy phase and the existence of glass-free grain boundaries.     

Submicrometer Transparent Alumina by Sinter Forging Seeded γ-Al2O3 Powders

Seeding boehmite with α-Al₂O₂, followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛ_x=−0.9, and radial flow (ɛ_x= 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha- nism responsible for large pore elimination and compact densification. The sinter forging of sol-gel-derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes.     

Densification of Large Pores: I, Expriments

Three different diameters (0.50, 1, and 2 μm) of monosized spherical pores were introduced, at different volume percents up to 15%, into two submicrometer commercial Zr(Y)O₂ powders which exhibited either sluggish (ZrO₂–3 mol% Y₂O₃) or rapid (ZrO₂–8 mol% Y₂O₃) grain growth kinetics. Densification and grain growth behavior was studied as a function of heat-treatment period at 1400°C and/or 1500°C. Pores in the finer-grain Zr(3Y)O₂ densified more rapidly. The grain growth kinetics of the Zr(3Y)O₂ material were unaffected by porosity, because the pores were always larger than the grains. Consistent with the Zener concept applied to mobile pores on grain junctions, the spherical pores influenced the grain growth kinetics of Zr(8Y)O₂ once the grain size was greater than the pore size. Measurements indicated that the number of pores per unit volume decreased dramatically during densification, whereas their average size remained approximately constant.     

Effect of Pore Distribution on Microstructure Development: III, Model Experiments

Model experiments have been conducted on a series of alumina samples in which the microstructures have been tailored to conform to the classical configuratins depicted in the models of final-stage sintering. Simultaneous measurements of sintered density, grain size, pore number density, and pore size distribution were made as a function of sintering time at constant temperature (1850°C). The data supported a model of grain-boundary-diffusion-controlled densification and surface-diffusion-controlled grain growth. An atom flux equation for grain-boundary diffusion transport was deduced from the data. The kinetics analysis highlights the importance of incorporating the number of pores per grain as an independent variable in mechanistic studies of final-stage sintering. The number of pores per unit volume was identified as a critical factor influencing densification kinetics. The effect of pore distribution on microstructure development was simulated for comparison with the data obtained from the model experiments.     

Grain Growth of Silica-Added Zirconia Annealed in the Cubic/Tetragonal Two-Phase Region

The grain growth in silica-doped 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (SiO₂-doped 3Y-TZP) and undoped 3Y-TZP has been examined in the temperature range of 1400°-1800°C. The presence of a SiO₂ phase inhibits rather than promotes the grain growth in 3Y-TZP, particularly at high temperatures. During the grain growth in 3Y-TZP, yttrium ions are partitioned between grains, and the grain growth mechanism can be understood from Ostwald ripening dominated by lattice diffusion of cations. In SiO₂-doped 3Y-TZP, an amorphous SiO₂-rich phase exists only in the grain-boundary corners or junctions, not in the grain-boundary faces. The grain growth in SiO₂-doped 3Y-TZP is controlled by using different mechanisms below and above the eutectic temperature of the zirconia-silica (ZrO₂-SiO₂) system. The glass phase does not have a major role in grain growth below the eutectic temperature, and the grain growth is dominated by a similar mechanism in undoped 3Y-TZP. The grain growth is more effectively retarded by the presence of a SiO₂ phase above the eutectic temperature and is likely to be controlled by a solution-reprecipitation process in the amorphous phase at the grain-boundary corners or junctions.     

Microstructure and Bending Strength of 3Y-TZP/12Ce-TZP Ceramics Fabricated by Liquid-Phase Sintering at Low Temperature

With the addition of 1 wt% of MgO–Al₂O₃–SiO₂ glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al₂O₃ spinel and an amorphous phase that contains Al₂O₃ and SiO₂ (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al₂O₃ spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.     

Isothermal Sintering Effects on Phase Separation and Grain Growth in Yttria-Stabilized Tetragonal Zirconia Polycrystal

The isothermal sintering behavior in 3 mol% yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) was investigated to clarify phase-separation and grain-growth mechanisms during sintering. In the Y-TZP sintered at 1300°C for 2 h, the Y³⁺ ion distribution of grain interiors in Y-TZP was nearly homogeneous, but Y³⁺ ions segregated along grain boundaries within a width of about 10 nm. When the holding time increased from 2 to 50 h, the cubic-phase regions with high Y³⁺ ion concentrations were clearly formed in the grain interiors adjacent to the grain boundaries, though the average grain size hardly increased. This result shows that the cubic-phase regions were formed without grain growth, which can be explained by the grain-boundary segregation-induced phase transformation mechanism. In the Y-TZP sintered at 1500°C for 2 h, the cubic-phase regions were already formed, and both of the cubic-phase region and average grain size increased with increasing holding time. This grain-growth behavior can be interpreted by the third-power growth low derived based on the solute drag theory, which indicates that the cubic-phase regions do not effectively act as the pinning points.     

Rapid Hot-Pressing of Ultrafine PSZ Powders

The process of compaction and densification of ultrafine (40- to 60-nm grain size) powder of partially stabilized zirconia with 3 mol% of Y₂O₃ (Y3-PSZ) during rapid hot-pressing was investigated. A special apparatus was designed to allow rapid application of 1.6 GPa of quasi-isostatic pressure at temperatures of 1100° to 1300°C to powder compacts encapsulated in glass under vacuum. Pressure was applied for 10 s, then the samples were rapidly cooled to room temperature, removed from the encapsulating glass, and characterized using SEM, TEM, and X-ray diffraction. Density and mechanical properties of the prepared materials were measured and compared with those of similar materials fabricated using conventional hot-pressing. SEM and TEM observations revealed that the ultrafine grains of the starting powder coarsened rapidly during the initial heating, and the compacts developed large (> 10 μm) and small (< 1 μm) pores. The process of densification under pressure consisted of closing of the large pores, whereas the small pores were relatively unaffected by the application of pressure at all investigated temperatures. The major mechanism of densification during the rapid pressing appears to be rearrangement and sliding of grains around the large pores. The material prepared by rapid pressing at 1300°C had higher hardness ( H _v= 1400 versus 1300 kg/mm²) but somewhat lower fracture toughness ( K _{I C} = 5.5 versus 6.0 MPa · m^1/2) compared with the conventionally hot-pressed Y3-PSZ. Density of the material pressed at 1300°C was 97% of theoretical density.     

Synthesis,sintering and microstructure of 3Y-TZP/CuO nano-powder composites

Nanocrystalline 3Y-TZP and copper-oxide powders were prepared by co-precipitation of metal chlorides and copper oxalate precipitation respectively. CuO (0.8 mol%) doped 3Y-TZP powder compacts were prepared from the nanocrystalline powders. Dilatometer measurements on these compacts were performed to investigate the sintering behaviour. Microstructure investigations of the sintered compacts were conducted. It is found that additions of the copper-oxide powders in the nanocrystalline 3Y-TZP leads to an enhancement of densification, formation of monoclinic zirconia phase and significant zirconia grain growth during sintering.     

Spark-Plasma-Sintering Condition Optimization for Producing Transparent MgAl2O4 Spinel Polycrystal

By controlling the heating rate at <10°C/min during spark-plasma-sintering (SPS) processing, transparent polycrystalline spinel with an in-line transmission of 50% and 70% in the visible- and infrared-wavelengths, respectively, can be successfully fabricated for only a 20-min soak at 1300°C. The high transmission can be attained by reducing the residual porosity and pore size, which was achieved by the low-heating rate. At high heating rates, many closed pores are formed due to the high densification rate during the heating process and remain as large pores around grain junctions. At temperatures >1300°C, the coalescence of the residual pores and the precipitation of second phases, which are caused by rapid grain growth, degrade the transparency. The present study demonstrates that although the high heating rates have been regarded as a primary advantage for the SPS processing, the low heating rate is highly effective in attaining a high transparency in the spinel even at low temperatures and for short sintering times.     

两步烧结法制备纳米氧化钇稳定的四方氧化锆陶瓷

采用共沉淀法制备纳米氧化钇稳定的四方氧化锆（yttria stabilized tetragonal zirconia,3Y-TZP）粉体。利用X射线衍射、N2吸附–脱附等温线,透射电子显微镜对3Y-TZP粉体的物理性能和化学性能进行表征。研究了纳米3Y-TZP粉体的烧结曲线,分析了3Y-TZP素坯在烧结过程中的致密化行为和显微结构,探讨了两步烧结工艺对3Y-TZP纳米陶瓷微观结构的影响。结果表明：采用共沉淀法,在600℃煅烧2h后,可获得晶粒尺寸为13nm、晶型发育良好、团聚较少的纳米3Y-TZP粉体;采用两步烧结法,将素坯升温至1200℃保温1min后,再降温到1050℃保温35h,可获得相对密度大于98%,晶粒尺寸约为100nm的3Y-TZP陶瓷。两步烧结法通过控制煅烧温度和保温时间,利用晶界扩散及其迁移动力学之间的差异,使晶粒生长受到抑制,样品烧结致密化得以维持,实现在晶粒无显著生长前提下完成致密化。     

Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP)

Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y-TZP in order to determine their influence on the superplastic behavior between 1200° and 1300°C and on the room-temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y-TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y-TZP with 5 wt% glass, but the room-temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y-TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y₂O₃ and ZrO₂, but the strain rate did not scale inversely with the viscosity of the silicate phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y-TZP with and without glassy grain boundary phases.     

Densification and grain growth of nanocrystalline 3Y-TZP during two-step sintering

Two-step sintering (TSS) was applied on nanocrystalline yttria tetragonal stabilized zirconia (3Y-TZP) to control the grain growth during the final stage of sintering. The process involves firing at a high temperature (T1) followed by rapid cooling to a lower temperature (T2) and soaking for a prolonged time (t). It is shown that for nanocrystalline 3Y-TZP (27 nm) the optimum processing condition is T1 = 1300 °C, T2 = 1150 °C and t = 30 h. Firing at T1 for 1 min yields 0.83 fractional density and renders pores unstable, leading to further densification at the lower temperature (T2) without remarkable grain growth. Consequently, full density zirconia ceramic with an average grain size of 110 nm is obtained. XRD analysis indicated that the ceramic is fully stabilized. Single-step sintering of the ceramic compact yields grain size of 275 nm with approximately 3 wt.% monoclinic phase. This observation indicates that at a critical grain size lower than 275 nm, phase stabilization is induced by the ultrafine grain structure.     

Hot Isostatic Pressing of Transparent Nd:YAG Ceramics

This paper demonstrates that fine-grained (2–3 μm), transparent Nd:YAG can be achieved at SiO₂ doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO₂ levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO₂, especially at higher temperatures, because the SiO₂ solubility limit is exceeded.     

Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering

SPS (spark plasma sintering) process was used to sinter nanocrystalline ZrO₂ (3Y). It was found to be different with the usual rapid sintering method, the density of the samples kept increasing with the rising of the sintering temperature. A higher density could be reached at a lower temperature and shorter dwelling time than that by hot-pressing under the similar pressures. In contrast to the samples with a differential densification from edge to center prepared by a rapid hot-pressing, no obvious densification gradient could be found in the samples sintered by SPS. The grain sizes of the Y-TZP obtained by SPS were smaller than those by the pressureless sintering method, while the grain growth speed was much higher under SPS conditions. All these unique sintering behaviors were explained by the special sintering process of SPS.