Processing-Related Fracture Origins: I, Observations in Sintered and Isostatically Hot-Pressed A12O3/ZrO2 Composites

Strengths of sintered Al₂O₃/ZrO₂ composites are much lower than those of hot-pressed composites due to large flaws associated with the sintering process. Fractography has shown that cracklike internal surfaces are at the fracture origins of the sintered materials. It is hypothesized that the cracklike internal surfaces are developed as a result of differential sintering of agglomerates relative to their surrounding powder compact. Isostatic hot-pressing of the sintered composites quickly eliminated the cracklike internal surface. Flaws responsible for fracture of the isostatically hot-pressed composites were low-density regions that are not as easily eliminated.     

Preparation of High-Density Si3N4 by a Gas-Pressure Sintering Process

Si₃N₄ compacts, containing ≅7 wt% of both BeSiN₂ and SiO₂ as densification aids, can be reproducibly sintered to relative densities >99% by a gas-pressure sintering process. Nearly all densification takes place via liquid-phase sintering of transformed β-Si₃N₄ grains at T =1800° to 2000°C. Compacts with high density are produced by first sintering to the closed-pore stage (≅92% relative density) in 2.1 MPa (20 atm) of N₂ pressure at 2000°C and then increasing the N₂ pressure to 7.1 MPa (70 atm) where rapid densification proceeds at T = 1800° to 2000°C. The experimental density results are interpreted in terms of theoretical arguments concerning the growth (coalescence) of gas-filled pores and gas solubility effects. Complex chemical reactions apparently occur at high temperatures and are probably responsible for incomplete understanding of some of the experimental data.     

Swelling of Hot-Pressed A12O3

Fully dense aluminas, prepared by hot-pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po₂= 10^-7 Pa). The reaction followed the relation p - po = -K log t, where p_o and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain-boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO₂ and SO₂ gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot-pressing. The consequences of internal gas-forming reactions to other processes such as high-temperature creep and sintering are also discussed.     

Effect of Sintering Atmosphere on Densification of MgO-Doped Al2O3

The densification behavior of Al₂O₃-MgO (0.1 wt%) has been studied in O₂ and N₂ atmospheres. Powder compacts have been sintered at 1600°C for 0.5 to 8 h. For some specimens the sintering atmosphere has been changed after 30 min of sintering. Irrespective of sintering atmosphere, sintered densities are approximately the same up to 99% relative density, implying that the capillary pressure effect dominates the atmosphere effect for most of the densification stage. During extended sintering treatment the density of specimens sintered in O₂ becomes higher than that in N₂. When the sintering atmosphere is changed from O₂ to N₂, enhanced densification results, and vice versa. Such an effect of sintering atmosphere is explained by the diffusiveness of gases entrapped in pores, as well as by oxygen potential differences inside and outside of the specimen. Differences in grain growth rate in various atmospheres are discussed on the basis of different densification rates.     

Densification and Microstructural Development of SrTiO3 Sintered with V2O5

Precipitation of TiO₂ occurs during the sintering of SrTiO₃ with V₂O₅ added as a liquid-phase sintering agent. Satisfactory densification can be obtained at 1250°C when using a high content of V₂O₅ during sintering. However, a microstructure of fine grains and large pores results along with the precipitation of TiO₂. The precipitation of TiO₂ can be repressed by the addition of excess SrO. A well-sintered microstructure with superior densification can thus be obtained at 125O°C from specimens sintered with a low content of V₂O₅ and an appropriate amount of excess SrO.     

Low-Temperature Sintering (Ba0.6Sr0.4)TiO3 Thick Film Prepared by Screen Printing

(Ba_0.6Sr_0.4) TiO₃ thick films doped with glass slurry were fabricated by the screen-printing technique. The dielectric properties and the sintering mechanism were investigated. The films can be sintered at 600°C. The dielectric constant is 88 and the dielectric loss is 0.002 with a tunability of 23.86% under 100 kV/cm. Higher dielectric constant and tunability were obtained in the samples sintered at higher temperatures. The highest tunability is 61.12% under 150 kV/cm in the sample sintered at 800°C. The low sintering temperature and dielectric loss of the glass-doped thick films make them potential candidates for LTCC and microwave tunable devices.     

Characterization of Heterogeneous Microstructure Evolution in ZrO2–3 mol%Y2O3 during Isothermal Sintering

Pore boundary tessellation and quantitative stereology were used to characterize microstructure evolution in ZrO₂–3 mol%Y₂O₃ (3YSZ) that had been pressed to a green density of 46% and isothermally sintered at 1275°C for 0.1–10 h. Scanning electron micrographs showed that, relative to classical sintering models, the sintered 3YSZ microstructure was spatially heterogeneous, and that this heterogeneity affected the way in which the microstructure evolved during sintering. Pore boundary tessellation cell maps revealed the presence of dense regions within the microstructure that grew by the preferential elimination of smaller pores and resulted in larger more widely spaced porosity at longer sintering times. In consequence, the average pore separation distance increased much faster than the average grain size. This would call into question the use of the grain size as a measure of microstructural scale for the prediction of densification kinetics for this material.     

Sinterability and Decomposition of Pb0.9175La0.055Zr0.975Ti0.025O3: Influence of Calcination and Sintering Temperature

The sinterability and decomposition of PLZT, (Pb,La)(Zr,Ti)O₃, depend on the temperature and soaking time of both the calcination and sintering temperature. They were determined from the density, linear shrinkage, weight loss, and appearance of extra phases. At moderate calcination temperatures and times, there is no escape of PbO from the PLZT. At calcination temperatures higher than 1050°C and soaking times above 3 h, PbO escapes, and ZrO₂ and La₂Zr₂O₇ can be detected. Even when sintered in a PbO-rich atmosphere, some PbO evaporates during sintering either from free PbO or from the PbO bound in the PLZT in regions in the outer surfaces of the sintered body. An aggressive depletion of PbO during sintering can result in a complete disappearance of the grain boundary phase, giving an intragranular fracture.     

Low-Temperature Sintering of K0.5Na0.5NbO3 Ceramics

The objective of this work was to lower the sintering temperature of K_0.5Na_0.5NbO₃ (KNN) without reducing its piezoelectric properties. The KNN was sintered using 0.5, 1, 2, and 4 mass% of (K, Na)-germanate. The influence of the novel sintering aid, based on alkaline germanate with a melting point near 700°C, on the sintering, density, and piezoelectric properties of KNN is presented. The alkaline-germanate-modified KNN ceramics reach up to 96% of theoretical density at sintering temperatures as low as 1000°C, which is approximately 100°C less than the sintering temperature of pure KNN. The relative dielectric permittivity (ɛ/ɛ₀) and losses (tanδ), measured at 10 kHz, the piezo d ₃₃ coefficient, the electromechanical coupling and mechanical quality factors ( k _p, k _t, Q _m) of KNN modified with 1 mass% of alkaline germanate are 397, 0.02, 120 pC/N, 0.40, 0.44, and 77, respectively. These values are comparable to the best values obtained for KNN ceramics sintered above 1100°C.     

Microstructure and Densification of Pressureless-Sintered Al2O3/Si3N4-Whisker Composites

Composite densification was studied by performing slip casting and sintering experiments on an Al₂O₃ matrix and Si₃N₄ whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si₃N₄ affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.     

Translucent Sintered MgAl2O4

A translucent polycrystalline MgAl₂O₄ ceramic was prepared from finely divided coprecipitated spinel in which a small amount of CaO added as a sintering aid was uniformly distributed. The CaO promotes densification through the formation of a liquid phase at the sintering temperatures. Depending on the sintering treatment, the relative density of the sintered spinel was 99.7 to ∼100% of theoretical. The in-line optical transmission was > 10% from 0.3 to 6.5 μm. Total transmission in the visible region was between 67 and 78%.     

Retrograde Densification in Bi2Sr2CaCu2O8 Superconductors

Bi₂Sr₂CaCu₂O₈ was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850° to 911°C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850° to 890°C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900° to 905°C does the material densify then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion, thus pushing the structure apart. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

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.     

Sintering of Si3N4-Y2O3 Under Nitrogen Pressure

This study shows that the amount ofAl₂O₃ needed to form high density Si₃N₄-15Y₂-O₃ samples can be reduced by using high surface area Si₃N₄ powder and high N₂ overpressure (high sintering temperatures) during the sintering process. The reduction in AI₂O₃ content results in improved oxidation resistance of the sintered samples.     

Microstructure and Short-Term Oxidation of Hot-Pressed Si3N4/ZrO2(+Y2O3) Ceramics

Composite ceramic materials based on Si₃N₄ and ZrO₂ stabilized by 3 mol% Y₂O₃ have been formed using aluminum isopropoxide as a precursor for the Al₂O₃ sintering aid. Densification was carred out by hot-pressing at temperatures in the range 1650° to 1800°C, and the resulting micro-structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si₃N₄ matrix with consequent high room-temperature toughness and strength. Decomposition of the ZrO₂ grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO₂ incorporated some N during densification.     

H2O-Catalyzed Sintering of ∼2-nm-Cross- Section Particles of MgO

MgO produced by vacuum decomposition of Mg(OH) ₂ is formed as aggregates containing both open and closed pores. The surface area and the volume of the open pores, as measured by N₂ adsorption, were found to be 300 m²/g and 0.14 cm³/g, respectively. Through calorimetric and TEM studies, reported elsewhere, the initial surface area and volume of the closed pores are shown to be 550 ± 150 m²/g and 0.19 cm³/g, respectively. The samples were sintered in the presence of water at 823 K. The elimination of the closed pores and coarsening of the open pores dominate early stage sintering under the conditions studied. From the above observations, it is argued that the sintering behavior is governed primarily, not by diffusion, but by a surface step. It is postulated that the nature of the surface step is the formation of new ion layers by random fluctuations in closed pores and in smaller open pores for which edge effects are thermodynamically favorable.     

Effect of Sintering Atmosphere on Isolated Pores During the Liquid-Phase Sintering of MgO-CaMgSiO4

The shrinkage behavior of isolated pores during the liquidphase sintering of MgO-CaMgSiO₄ at 1650°C in O₂ and N₂ atmospheres has been studied. When 90MgO- 10Ca MgSiO₄ specimens containing artificially produced large spherical pores are sintered in O₂, the liquid and grains flow into the pores as oxygen diffuses out. When sintered in N₂ the pores remain intact even after a long time, because the N₂ gas entrapped in them does not diffuse out. The effect of the sintering atmosphere has also been studied in a fine powder mixture of 80MgO 20CaMgSiO₄ composition. Changing the atmosphere from O₂ to N₂ during the sintering treatment reduces the porosity, probably because of the enhanced oxygen diffusion from the pores. The pores grow when the sintering atmosphere is changed from N₂ to O₂, probably because of oxygen diffusion into the pores from the specimen surface. The practical implication of these results is that changing the atmosphere from O₂ to air during the liquid-phase sintering of oxide ceramics can greatly reduce the porosity.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Sintering Behavior and Surface Microstructure of PbO-Rich PbNi1/3Nb2/3O3–PbTiO3–PbZrO3 Ceramics

The sintering behavior and surface microstructure of PbNi_1/3Nb_2/3O₃–PbTiO₃–PbZrO₃ (PNiNb-PT-PZ) ceramics were investigated. The PNiNb-PT-PZ ceramics with the stoichiometric composition and the addition of excess lead oxide (PbO-rich ceramics) were sintered by liquid-phase sintering in accordance with the solution-reprecipitation mechanism at temperatures below the melting point of PbO. The temperature at which the liquid phase forms fell to near the eutectic point of the PbO–Nb₂O₅ and the PbO–TiO₂ system (868°C) with the addition of 5 mol% PbO. As the calcination temperature influenced the sinterability of the stoichiometric PNiNb-PT-PZ ceramic, unreacted PbO was considered to be the source of the liquid phase in the sintering of the stoichiometric powder. The secondary phase was observed at the surface of PbO-rich ceramics and was suggested to be a liquid phase expelled from inside the ceramic. A sintering scheme of PNiNb-PT-PZ ceramics was proposed, and the high sinterability of PNiNb-PT-PZ ceramics was attributed to the low formation temperature of the liquid phase.