Grain Growth Of Alumina Research Articles

The influence of electric fields on grain boundary mobility in alumina

The influence of an electric field on grain growth of polycrystalline alumina was evaluated. Systematic annealing experiments on fully dense undoped alumina samples were performed in a spark plasma sintering (SPS) furnace at 1600 °C, where heating was either by a current through the graphite die (conventional SPS), or via external heating elements. The grain sizes after annealing in different atmospheres and for different times were measured from polished and thermally etched cross-sections of the samples. A significant increase in the average grain size as a function of time was observed for the samples annealed in SPS with an electric field. SPS with an applied electric field seems to promote grain growth in alumina, although alumina is a non-conductive ceramic (electronic or ionic).

Boron induced abnormal grain growth in alumina

In this work, hot-pressing of alumina in contact with hexagonal boron nitride or doped with boron carbide was conducted at 1500 °C for 30 min. After hot-pressing, abnormal grain growth induced by boron diffusion from these substances into alumina was detected, as clearly demonstrated with SEM, EDS, EBSD, and Raman spectroscopy. Grain boundary complexion transformations, solute drag, or another mechanism relating to interface-controlled grain boundary mobility are presumed to be the fundamental mechanism responsible for abnormal grain growth observed in this work.

Phase field investigation on the alumina nanocrystalline growth controlled by mullite-alumina phase transition in alumina-mullite fibers

The overgrowth of alumina nanocrystalline is the most important cause for the degradation of alumina-mullite fibers at high temperatures. A profound understanding on the kinetics and mechanisms of alumina nanocrystalline growth controlled by the mullite-alumina phase transition is of great significance to the development and service of alumina-mullite fibers. Here, by using the phase field simulations coupling to materials thermodynamic data, we systematically investigated the growth of alumina nanocrystalline and the mullite-alumina phase transition in alumina-mullite fiber at different temperatures and concentrations of the mullite matrix. The growth rates and exponents of alumina grains at different conditions are quantified and agree well with the experimental results of Nextel 720 fibers. We found that the growth of polycrystalline alumina grains undergoes two stages: rapid growth in the very early stage and then grains coarsening. The mullite-alumina phase transition dominates the rapid growth of alumina grains and the grain coarsening proceeds mainly through grain annexation and dissolution-precipitation. Our results clarify the influence of mullite-alumina phase transition on alumina grain growth from mullite matrix, and quantify the mechanisms and kinetics of alumina grain growth from mullite matrix in the alumina-mullite system.

Study of the Effect of a Small Addition of ZrO-=SUB=-2-=/SUB=- on the Density and Grain Growth of Alumina

The effect of an additive of 0.5 wt.% of zirconium oxide on the spark plasma sintering (SPS) kinetics of alumina powders has been studied. Ceramics were obtained by mixing alumina powders in a planetary mill with grinding media from stabilized zirconium oxide. The activation energy of SPS was estimated using the Young-Cutler model. It has been shown that the density and average grain size in ceramics obtained from submicron alumina powders are significantly affected by the nonequilibrium state of the interfaces formed as a result of the transformation of the amorphous phase present on the alumina powder particle surface. The grain size and density of ceramics obtained from fine powders are significantly affected by the coalescence of ZrO2 particles. Keywords: Alumina, zirconium oxide, spark plasma sintering, density, diffusion, activation energy.

Radiation tolerance of alumina scale formed on FeCrAl ODS ferritic alloy

• Fe ion irradiation effect on the alumina scale formed on FeCrAl ODS ferritic alloy was investigated. • Irradiation-induced segregation of reactive elements along alumina grain boundaries was confirmed. • Nanoindentation hardness of alumina scale decreased with the growth of alumina grains during ion irradiation. Alumina-forming FeCrAl oxide dispersion strengthened (ODS) ferritic alloys are considered as promising structure materials for the next-generation nuclear reactors. Current work is aimed at investigating Fe ion irradiation effect on the alumina scale formed on FeCrAl ODS ferritic alloy. A single alpha-alumina layer with a thickness of about 0.7 µm was formed on the surface of the alloy by a pre-oxidation process at 1000 °C with 9 h. The pre-oxidized specimen was subsequently irradiated using 6.4 MeV Fe 3+ ion beam to 2 dpa at 500 °C. Nanoindentation testing demonstrated that the hardness of the alumina scale decreased after the Fe ion irradiation. Corresponding microstructure characterization was carried out by using electron backscatter diffraction (EBSD) and scanning transmission electron microscopy (STEM). Alumina grain growth and softening in accordance with the Hall-Petch relationship are responsible for the hardness decrease by the Fe ion implantation. Irradiation-induced dissolution of Y-Zr oxide particles in alumina grains as well as the irradiation-induced segregation of reactive elements along the alumina grain boundaries were observed.

Test atmospheres affecting voids distribution on MCrAlY-bond coats for TBCs at 1050 °C

The oxidation behaviour of thermal barrier coatings with a NiCoCrAlY bond coat at 1050 °C in different atmospheres was investigated. Water vapour contributed to fewer defects and lesser internal growth of Al2O3 in the bond coat. Additionally, the presence of excess water vapour in the air + water vapour atmosphere suppressed the local inward oxide growth. Electron backscatter diffraction and transmission electron microscopy analyses were performed to investigate the influence of water vapour on the grain growth of alumina. The grain size of Al2O3 depended on the water vapour content. The driving forces for the morphological distribution of voids in various atmospheres were discussed.

アルミナの焼結と粒成長に関するモンテカルロ・シミュレーションと実験の研究

アルミナの焼結と粒成長に関するモンテカルロ・シミュレーションと実験の研究

The influence of carbon on the microstructure and wear resistance of alumina

AbstractThe influence of carbon as a dopant on grain growth and wear resistance of polycrystalline alumina was evaluated. Carbon was introduced into alumina by sintering in a carbon‐rich environment (graphite furnace under flowing He), and/or by residual carbon from organic binders used during the green body consolidation process. Samples were sintered at 1600°C for 2 h. Doping alumina with carbon resulted in a reduced grain size after sintering, correlated to solute‐drag, and graphite particle‐drag for high concentrations of carbon (~3 wt.%). The material response to abrasive wear was quantified by measuring the sample area cut for a defined time using a diamond wafering saw, as a function of grain size and carbon content. Sintering alumina with carbon resulted in a significant increase in wear resistance, as a result of the reduced grain size.

Improved Fracture Toughness and Crack Arrest Ability of Graphene–Alumina Nanocomposite

In this work, high fracture toughness graphene–alumina composite was developed through a novel chemical method using boehmite and graphene, which is followed by extrusion and consolidation. The mixed precursors were consolidated by sintering at 1550 °C in a nitrogen atmosphere. The plate-like structures of boehmite form α-alumina; meanwhile, graphene particles at the grain boundaries hinder the growth of alumina grains. The graphene reinforcement was bonded to α-alumina matrix by van der Waals forces. The XRD pattern reveals the presence of graphene with a plane (002) along with α-alumina. Properties such as fracture toughness (5.6 ± 0.01 MPa m0.5), Vickers hardness (1872 ± 25 kgf/mm2) and true density (3.8 g/cm3) were achieved in 0.5 wt.% graphene–alumina composite when compared to α-alumina with fracture toughness (5.3 ± 0.1 MPa m0.5), Vickers hardness (1984 ± 28 kgf/mm2) and true density (3.91 g/cm3). The bridging and deviation of cracks in 0.5 wt.% graphene–alumina composite are attributed to the anchoring and dissipation of energy during crack growth, which enhances the fracture toughness, whereas α-alumina exhibits failure caused by linear crack propagation. Meanwhile, the slight decrease in Vickers hardness and true density of 0.5 wt.% graphene–alumina composite is due to the tribological and low-density properties of graphene. The obtained properties of composite could be suitable in high-temperature, wear-resistant applications such as crucibles, bearings, etc.

Influences of sintering time on the structures and mechanical properties of zirconia toughened alumina nanocomposites

Abstract The study investigated the influences of sintering time on the phase compositions, microstructural, and mechanical properties of zirconia toughened alumina (ZTA). The specimens containing 15 wt% – 3Y-PSZ were prepared by conventional sintering at 1550 °C for three different holding times (2, 4, 6 h). Densification behaviour, x-ray diffraction analysis, scanning electron microscopy, micro-hardness, fracture toughness, and flexural strength were determined for analyzing the impacts of sintering time on ZTA. The increase in sintering time enhanced the densification, hardness and flexural strength of the nanocomposites gradually. However, the fracture toughness increased up to 4 h and dropped down for 6 h holding time. The percentage of monoclinic zirconia increased slightly with holding time. The grain growth of alumina and zirconia also followed a positive trend with holding time.

Development and evolution of a novel (Zr1-xSnx)O2 toughened alumina-mullite slip cast refractory: Effect of SnO2

Zr1-xSnxO2 reinforced alumina-mullite refractory was manufactured by slip cast methods using SnO2 as a sintering agent. SnO2 had considerable influence in the enhanced reaction kinetics of zircon dissociation and subsequent reaction sintering of alumina and zircon. Presence of pro-eutectic transient Al2O3-SnO2 liquid and SnO2-SiO2-ZrO2 amorphous phases, enhanced densification and mullitisation through liquid phase sintering and, mitigated SnO2 volatilisation by (Zr1-xSnx)O2 solid-solution formation. Based on their morphology and aspect ratios, three types of mullite crystals, MI, MII and MIII, were evolved across the matrix microstructure. Characterization of the evolved microstructure revealed coalescence and grain growth of matrix alumina grains including the presence of an acicular tertiary MIII mullite which acts as both a reinforcement and bridging network of matrix to aggregate grains. SnO2 had the effect of lowering the monoclinic – tetragonal phase transformation temperature. Hot flexural strength values remained nearly unchanged (±2%) compared with the AZS composition without SnO2 doping.

A model of grain boundary complexion transitions and grain growth in Yttria-doped alumina

In this work, we present a physically-parameterized microstructure evolution model for the Yttria-doped alumina system. Yttria-doped alumina is a well-known ceramic system which undergoes first-order phase-like transitions at grain boundaries, which can radically alter interface properties. The change in interfacial properties in turn can radically change microstructure outcomes during processing, including the induction of abnormal grain growth modes. In this work, we develop a simulation that evolves alumina microstructure as a function of yttria concentration and temperature. In the window studied, we achieve strong agreement with reviewed experimental results in identifying the windows for large grains, small grains, abnormal grain growth, and complexion transition kinetics (as measured by JMAK analysis). We then apply the model to study and demonstrate how the possible inclusion of second-phase particles or uneven solute distribution profiles will impact microstructure evolution. It is found that particles do not significantly affect abnormal grain growth in the window studied (but do lead to reduced grain size through pinning effects). It is found that even modest amounts of solute inhomogeneity will result in substantial changes in microstructure outcomes, frequently leading to clusters of abnormal grains. This model largely corroborates the expectations and hypotheses made from recent experimental studies in oxide-doped alumina systems. Further, it is found that there exists a peak transition fraction for the system at which abnormal grain size tends to be maximized.

Preparation and Characterization of An All Oxide Ceramic Matrix Composite by A Novel Method

Due to the characteristics of high temperature oxidation resistance and high reliability, all oxide composites are ideal materials for components in hot end of aeronautics and astronautics. In this paper, all oxide ceramic matrix composite(CMC) was prepared by a novel method which combined warm pressing of prepreg with PIP using polysilialumoxane as the precursor of mullite. The morphology, structure and mechanical properties of the all oxide CMC were characterized. The results show that polysilialumoxane with a ceramic yield of 53wt%can be completely changed to ceramic at 1200°C and form mullite crystals. The composite with alumina and mullite matrix derived from alumina powder slurry indicated the best mechanical properties, the flexural strength and fracture toughness of the composite reach 208.3Mpa and 128.6Mpa respectively, and the pull-out effect of the fibers is obvious in the process of tensile fracture, and the composites show ductile fracture. whereas the mechanical property decreases significantly after heating at 1200°C for100 hours due to the densification of matrix and abnormal growth of alumina grains in fibers.

A Model of Grain Boundary Complexion Transitions and Grain Growth in Yttria-Doped Alumina

In this work, we present a physically-parameterized microstructure evolution model for the Yttria-doped alumina system. Yttria-doped alumina is a well-known ceramic system which undergoes first-order phase-like transitions at grain boundaries, which can radically alter interface properties. The change in interfacial properties in turn can radically change microstructure outcomes during processing, including the induction of abnormal grain growth modes. In this work, we develop a simulation that evolves alumina microstructure as a function of yttria concentration and temperature. In the window studied, we achieve strong agreement with reviewed experimental results in identifying the windows for large grains, small grains, and abnormal grain growth. We then apply the model to study and demonstrate how the possible inclusion of second-phase particles or uneven solute distribution profiles will impact microstructure evolution. It is found that particles do not significantly affect abnormal grain growth in the window studied (but do lead to reduced grain size through pinning effects). It is found that even modest amounts of solute inhomogeneity will result in substantial changes in microstructure outcomes, frequently leading to clusters of abnormal grains. This model largely corroborates the expectations and hypotheses made from recent experimental studies in oxide-doped alumina systems.

Sintering behaviour and properties of manganese-doped alumina

The effect of manganese (0.1, 0.5 and 1.0 wt%) on the sintering and mechanical properties of alumina was studied. Sintering was carried out by the conventional heating method in a box furnace and in a hybrid multimode microwave furnace. XRD analysis revealed the precipitation of a spinel second phase (MnAl2O4) in manganese-doped samples as a result of manganese limited solubility in the corundum lattice. The addition of 0.1 wt% manganese was most beneficial in enhancing the densification of alumina (97.5% relative density when compared to 94.2% for the undoped sample), hindered grain growth, and improved the hardness of the ceramic when sintered at 1500 °C. The study also revealed that microwave sintering was effective in suppressing grain growth of alumina. In addition, the hardness was dependent on the sintered bulk density and that grain coarsening ensued as the density of the sintered alumina exceeded 95% of theoretical.

The grain growth in alumina compacts sintered under high pressure

Abstract In this study, alumina ceramics were produced with high density and nanograin size without additives from nano-size γ-Al2O3. The effects of sintering pressure (3 GPa and 7 GPa) and temperature (600 °C and 1 200 °C) on the grain size of sintered samples were studied. It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase. Pressure-assisted densification decreases grain size, so that nano-scale grains can be achieved in the sintered structure. The effect of various sintering conditions on grain growth in alumina was investigated during high-pressure sintering for better understanding of pressure and temperature effects on microstructural properties.

Faceting, Grain Growth, and Crack Healing in Alumina.

Reactive molecular dynamics simulations are performed to study self-healing of cracks in Al2O3 containing core/shell SiC/SiO2 nanoparticles. These simulations are carried out in a precracked Al2O3 under mode 1 strain at 1426 °C. The nanoparticles are embedded ahead of the precrack in the Al2O3 matrix. When the crack begins to propagate at a strain of 2%, the nanoparticles closest to the advancing crack distort to create nanochannels through which silica flows toward the crack and stops its growth. At this strain, the Al2O3 matrix at the interface of SiC/SiO2 nanoparticles forms facets along the prismatic (A) ⟨2̅110⟩ and prismatic (M) ⟨1̅010⟩ planes. These facets act as nucleation sites for the growth of multiple secondary amorphous grains in the Al2O3 matrix. These grains grow with an increase in the applied strain. Voids and nanocracks form in the grain boundaries but are again healed by diffusion of silica from the nanoparticles.

Microstructure and properties of submicron grained alumina ceramic tool material prepared by two-step microwave sintering

Alumina ceramic with submicron grains was prepared by the two-step microwave sintering. Microstructure evolution and densification kinetics of alumina during microwave sintering were studied. The main densification mechanism of alumina ceramic is grain boundary diffusion at the temperature below 1100 °C, while the main densification mechanism is volume diffusion at the temperature from 1200 to 1300 °C. The mean activation energy of grain growth of alumina is 52.36 kJ/mol, and the minimum activation energy of grain growth (48.32 kJ/mol) appears at the holding time of 10 min. Submicron grained alumina ceramic was prepared by two-step microwave sintering (T1 = 1300 °C, T2 = 1100 °C with the holding time of 5 min). The grain size is 0.64 ± 0.16 µm, and its relative density, Vickers hardness and fracture toughness are 98.23%, 18.70 ± 1.01 GPa and 5.28 ± 0.26 MPa m1/2, respectively.

New materials through a variety of sintering methods

New sintering techniques make it possible to obtain materials with special properties that are impossible to obtain by conventional sintering techniques. This issue is especially important for ceramic materials for application under extreme conditions. Following the tendency to limit critical materials in manufacturing processes, the use of W, Si, B, Co, Cr should be limited, also. One of the cheapest and widely available materials is aluminum oxide, which shows differences in phase composition, grain size, hardness, strain and fracture toughness of the same type of powder, sintered via various methods. In this paper the alumina was sintered using the conventional free sintering process, microwave sintering, Spark Plasma Sintering (SPS), high pressure-high temperature method (HP-HT) and High Pressure Spark Plasma Sintering (HP SPS). Phase composition analysis, by X-ray diffraction of the alumina materials sintered using various methods, was carried out. For the conventional sintering method, compacts are composed of α-Al2O3 and θ-Al2O3. For compacts sintered using SPS, microwave and HP-HT methods, χ-Al2O3 and γ-Al2O3 phases were additionally present. Mechanical and physical properties of the obtained materials were compared between the methods of sintering. On the basis of images from scanning electron microscope quantitative analysis was performed to determine the degree of grain growth of alumina after sintering.

Effect of LZSA Glass-Ceramic Addition on Pressureless Sintered Alumina. Part I: Grain Growth

The aim of this study was to evaluate the effect of an 11.6Li2O-16.8ZrO2-68.2SiO2-3.4Al2O3 (LZSA) glass ceramic on the grain growth of alumina obtained by pressureless sintering. Three contents of the LZSA glass ceramic (7, 15, and 21 vol%; mean particle size of 1.52 µm) and three types of alumina (mean particle sizes of 0.5, 1.7, and 2.8 µm) were used. The powder compositions were formed by cold pressing. The samples (5 mm × 5 mm × 13 mm) were sintered in an optical dilatometer. The grain size and morphology were evaluated by scanning electron microscopy. The results show that elongated alumina particles were obtained for the investigated composites compared with pure alumina because of the LZSA addition. The grain growth of alumina was less pronounced at higher LZSA contents. High relative densities were obtained for higher LZSA contents at lower temperatures and holding times compared with pure alumina.