Influence of Yttria–Alumina Content on Sintering Behavior and Microstructure of Silicon Nitride Ceramics

The present study investigates the influence of the content of Y₂O₃–Al₂O₃ sintering additive on the sintering behavior and microstructure of Si₃N₄ ceramics. The Y₂O₃:Al₂O₃ ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering-additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β-Si₃N₄ grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction.     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

Densification Behavior in Microwave-Sintered Silicon Nitride at 28 GHz

Si₃N₄ powders were sintered using a 28 GHz gyrotron source, with Y₂O₃, Al₂O₃, and MgO as sintering aids, in an attempt to investigate the effect of microwave radiation on densification behavior. The microwave-sintered samples were compared with identical samples produced by conventional pressureless sintering. The effect of sintering on the microstructural development and grain growth of the samples was assessed using scanning electron microscopy. Phase transformation behavior was assessed using X-ray diffractometry. In the microwave-sintered samples, densification and α→β transformation occurred at temperatures ∼200°C lower than those of the conventionally sintered samples. More importantly, at comparable stages of densification, the microstructures of the microwave-sintered and conventionally sintered samples were significantly different, with the microwave-sintered samples showing the development of elongated β grains at a much earlier stage of the α→β transformation. It was concluded that the effect of microwave radiation on sintering was not simply a decrease in sintering temperatures, but in possibly a different sintering mechanism, clearly related to localized heating within the grain-boundary phase.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Densification and Grain Growth of Al2O3 Nanoceramics During Pressureless Sintering

α - Al₂O₃ nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al₂O₃ seed polyacrylamide gel method were used to sinter bulk Al₂O₃ nanoceramics. The relative density of the Al₂O₃ nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al₂O₃ nanopowders. The densification and fast grain growth of the Al₂O₃ nanoceramics occur in different temperature ranges. The Al₂O₃ nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering.     

Transformation and Densification of Nanocrystalline θ-Alumina during Sinter Forging

The sinter forging behavior of α-Al₂O₃ seeded and unseeded nanocrystalline θ-Al₂O₃ was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI₂O₃ phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al₂O₃ grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al₂O₃ seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al₂O₃ with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min.     

Seeding of the Reaction-Bonded Aluminum Oxide Process

The effect of the initial α-Al₂O₃ particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al₂O₃ to α-Al₂O₃, and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al₂O₃ particles are fine (∼0.2 μm, i.e., 2.9 × 10¹⁴γ-Al₂O₃ particles/cm³), then they seed the phase transformation. The fine α-Al₂O₃ decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics.     

Controlled Gelation and Sintering of Monolithic Gels Prepared from γ-Alumina Fume Powder

The gelation, phase transformation, and densification of a colloidal monolithic gel made from γ-Al₂O₃ fume powder are investigated. Among the six gelation agents that we use, formamide and urea are quick in causing gelation and easy to burn off. The densification rate of this gel decreases rapidly after the γ-to-α phase transformation. TiO₂ is an effective sintering aid to overcome this bottleneck of densification because (1) it enhances the phase transformation rate so that the sintering of α-alumina occurs at a lower temperature, and (2) it promotes sintering rates at the initial and intermediate stages after phase transformation. On the other hand, MgO has an inappreciable effect on gel sintering. The effect of MgO at the final sintering stage is obstructed by this densification barrier after transformation. The titania-doped gel monoliths can be sintered to high density and fine microstructure at 1400°C.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline Alumina

The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of γ-Al₂O₃ nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiC_w/γ-Al₂O₃ nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (∼118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of γ-Al₂O₃ to α-Al₂O₃ of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiC_w/HEBMγ-Al₂O₃ nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m^1/2.     

Pulsed Electric Current Sintering of Silicon Nitride

Pulsed electric current sintering (PECS) has been used to densify α-Si₃N₄ powder doped with oxide additives of Y₂O₃ and Al₂O₃. A full density (>99%) was achieved with virtually no transformation to β-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the α-to-β phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated β-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (∼500 kJ/mol), thereby causing the retention of α-phase during the densification by PECS.     

Novel Two-Step Sintering Process to Obtain a Bimodal Microstructure in Silicon Nitride

A two-step sintering process is described in which the first step suppresses densification while allowing the α-to-β phase transformation to proceed, and the second step, at higher temperatures, promotes densification and grain growth. This process allows one to obtain a bimodal microstructure in Si₃N₄ without using β-Si₃N₄ seed crystals. A carbothermal reduction process was used in the first step to modify the densification and transformation rates of the compacts consisting of Si₃N₄, Y₂O₃, Al₂O₃, and a carbon mixture. The carbothermal reduction process reduces the oxygen:nitrogen ratio of the Y-Si-Al-O-N glass that forms, which leads to the precipitation of crystalline oxynitride phases, in particular, the apatite phase. Precipitation of the apatite phase reduces the amount of liquid phase and retards the densification process up to 1750°C; however, the α-to-β phase transformation is not hindered. This results in the distribution of large β-nuclei in a porous fine-grained β-Si₃N₄ matrix. Above 1750°C, liquid formed by the melting of apatite resulted in a rapid increase in densification rates, and the larger β-nuclei also grew rapidly, which promoted the development of a bimodal microstructure.     

Sintering, Crystallization, and Properties of B2O3/P2O5-Doped Li2O·Al2O3·4SiO2 Glass-Ceramics

Sintering, crystallization, microstructure, and thermal expansion of Li₂O·Al₂O₃·4SiO₂ glass-ceramics doped with B₂O₃, P₂O₅, or (B₂O₃+ P₂O₅) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10^-6 to −0.1 × 10^-6 K^-1. Codoping of B₂O₃ and P₂O₅ resulted in the highest densification and an extremely low coefficient of thermal expansion.     

Mechanism of Alumina-Enhanced Sintering of Fine Zirconia Powder: Influence of Alumina Concentration on the Initial Stage Sintering

The isothermal shrinkage behavior of 2.9 mol% Y₂O₃-doped ZrO₂ powders with 0–1 mass% Al₂O₃ was investigated to clarify the effect of Al₂O₃ concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al₂O₃ addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β₀ ⁿ ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al₂O₃ addition and both nQ and β₀ ⁿ increased with increasing Al₂O₃ concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al₂O₃ addition largely depends on the increase of β₀ ⁿ , that is, the increases of n with GBD→VD change and β₀ with an increase in Al₂O₃ content, although the nQ also increases with Al₂O₃ addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al₂O₃ at ZrO₂ grain boundaries.     

Sintering of Si3N4 with the Addition of Rare-Earth Oxides

The effect of rare-earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N₂ at 1800° to 2000°C was studied. When a single-component oxide, such as CeO₂, Nd₂O₃, La₂O₃, Sm₂O₃, or Y₂O₃, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y₂O₃–Ln₂O₃ (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare-earth oxides is discussed in relation to the additive compositions.     

Densification and Microstructural Development of the Reaction Sintering of Strontium Barium Niobate

SrNb₂O₆ and BaNb₂O₆ were selected as the precursor powders for the reaction sintering of strontium barium niobate of the composition Sr _x Ba_{1− x} Nb₂O₆ ( x = 0.5 and 0.6) (SBN). In comparing the densification behavior of the reaction sintering and the normal sintering of SBN, it was observed that the densification rate of the former was slower during reaction but became faster after reaction. It was also found that the chemical reaction occurred prior to densification, but a high-density and uniform microstructure could be obtained for the reaction-sintered sample. The densification rate of the reaction-sintered sample was pronouncedly enhanced when the reaction was almost completed. We also proposed the reason for the enhancement of the densification rate of the reaction-sintered sample when the reaction was almost completed. A high-density (>98% of the theoretical density), uniform, and fine-grained (3-4 µm) SBN microstructure could be achieved by using reaction sintering.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: I, Effect of Matrix Powder Surface Area

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part I of the study, the effect of the matrix (Al₂O₃) powder surface area on densification behavior and microstructure development is reported. Compacts prepared with higher surface area Al₂O₃ powder showed enhanced densification at lower whisker concentrations (5 and 15 vol%). Samples with 15 vol% whiskers could be pressureless sintered to ∼97% relative density with zero open porosity and ∼1.6-μm matrix average grain intercept size.     

Reaction Sintering of ZnO-AI2O3

The reaction sintering of equimolar mixtures of ZnO and A1₂O₃ powders was investigated as a function of primary processing parameters such as the temperature, heating rate, green density, and particle size. The powder mixtures were prepared by two different methods. In one method, the ZnO and A1₂O₃ powders were ball-milled. In the other method, the ZnO powder was chemically precipitated onto the A1₂O₃ particles dispersed in a solution of zinc chloride. The sintering characteristics of the compacted powders prepared by each method were compared with those for a prereacted, single-phase powder of zinc aluminate, ZnAl₂O₄. The chemical reaction between ZnO and A1₂O₃ occurred prior to densification of the powder compact and was accompanied by fairly large expansion. The mixing procedure had a significant effect on the densification rate during reaction sintering. The densification rate of the compact formed from the ball-milled powder was strongly inhibited compared to that for the single-phase ZnAl₂O₄ powder. However, the densification rate of the compact formed from the chemically precipitated mixture was almost identical to that for the ZnAl₂O₄ powder. The difference in sintering between the ball-milled mixture and the chemically precipitated mixture is interpreted in terms of differences in the microstructural uniformity of the initial powder compacts resulting from the different preparation procedures.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: II, Effects of Sintering Additives and Green Body Infiltration

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part II of the study, the effects of Y₂O₃/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated samples.     

Experimental Design Applied to Silicon Carbide Sintering

Silicon carbide is a promising structural ceramic used as abrasives and applied in metallurgical components, due to its low density, high hardness, and excellent mechanical properties. The composition and content of the additive can control liquid-phase sintering of SiC. Compositions based on the SiO₂–Al₂O₃–RE₂O₃ system (RE = rare earth) have been largely used to promote silicon carbide densification, but most studies are not systematically presented. The aim of this work is to study the effect of several oxide additives in the SiO₂–Al₂O₃–Y₂O₃ system on the densification of silicon carbide using experimental design. This technique seems to be effective in optimizing the values of maximum density with minimum weight loss.