Effects of Additives on Microstructure and Properties of Alumina-Silicon Carbide Nanocomposites

Al₂O₃/5-vol%-SiC nanocomposites have been fabricated by using pressureless sintering with MgO and/or Y₂O₃ sintering aids and post-hot isostatic pressing (HIPing), which circumvents the limitations of hot pressing. Al₂O₃/SiC nanocomposites that have been doped with 0.1 wt% MgO and 0.1 wt% MgO + 0.1 wt% Y₂O₃ show an increased sintering density and a homogeneous microstructure, as well as a high fracture strength (1 GPa) after HIPing. In contrast, using Y₂O₃ as a dopant has a negative impact on the microstructure and the fracture strength. The results suggest that MgO, as a sintering additive, has a key role in improving the densification and controlling the microstructure of Al₂O₃/SiC nanocomposites.     

Thermochemical Analysis of the Silicon Carbide-Alumina Reaction with Reference to Liquid-Phase Sintering of Silicon Carbide

The stabilities of different phases in the Si-Al-C-O system are calculated from thermodynamic considerations with the objective of identifying the liquid phases formed during sintering of SiC in the presence of Al₂O₃. It is shown that a liquid phase can form at the sintering temperatures by the reaction of SiC with Al₂O₃. Depending on the carbon activity, the liquid can be either of the following: Al₂O₃+ Al₄C₃, SiC + Al₄C₃, or molten aluminum. The stability of the aluminosilicate melts that can form by the reaction of Al₂O₃ with the surface silica layer on SiC powders is also evaluated. Several factors that influence liquid-phase sintering, such as the solubility of SiC in the melts and the generation of gases during sintering, are discussed. The results of the thermodynamic analysis are compared with the observed sintering behavior for SiC.     

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.     

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.     

Al2O3/Ni Laminar Ceramics Shaped by Tape Casting and Electroless Plating

Tape casting and electroless plating were used to fabricate Al₂O₃/Ni laminar ceramic composites with close control of the thickness of the Al₂O₃ and Ni layers. Ninety-seven percent relative density, macrodefect-free composites were obtained by spark plasma sintering. In electroless plating solutions, the stable potential of grain boundary led to the first deposition of nickel on the grain boundary of Al₂O₃. Scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction were used to analyze the structure, elements distribution, and phase composition of the Al₂O₃/Ni laminar composites.     

SiAION Ceramics

The phase relations in and physical properties of SiAlON ceramics prepared by the hot isostatic pressing technique or by pressureless sintering with a sintering aid are reviewed. The sintering aid used is an AIN and Al₂O₃ mixture either pure or in combination with Y₂O₃ and/or various rare-earth oxides. Special attention is paid to the amount and type of phase(s) formed and to how properties such as hardness (HV10), fracture toughness ( K _IC), and oxidation resistance vary with the sintering aid used for different types of SiAlON materials, i.e., O'-, almost monophasic α-, pure β-, and mixed (α+β)-SiAION Ceramics.     

Sintering Kinetics at Constant Rates of Heating: Effect of Al2O3 on the Initial Sintering Stage of Fine Zirconia Powder

The sinterabilities of fine zirconia powders including 5 mass% Y₂O₃ were investigated, with emphasis on the effect of Al₂O₃ at the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The powder compact including a small amount of Al₂O₃ increased the densification rate with elevating temperature. The activation energies at the initial stage of sintering were determined by analyzing the densification curves. The activation energy of powder compact including Al₂O₃ was lower than that of a powder compact without Al₂O₃. The diffusion mechanisms at the initial sintering stage were determined using the new analytical equation applied for CRH techniques. This analysis exhibited that Al₂O₃ included in a powder compact changed the diffusion mechanism from grain boundary to volume diffusions (VD). Therefore, it is concluded that the effect of Al₂O₃ enhanced the densification rate because of decrease in the activation energy of VD at the initial sintering stage.     

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.     

Strengthening of Porous Alumina by Pulse Electric Current Sintering and Nanocomposite Processing

Porous Al₂O₃ and SiC–dispersed-Al₂O₃ (Al₂O₃/SiC) nanocomposites with improved mechanical properties were fabricated using pulse electric current sintering (PECS). Microstructures with fine grains and enhanced neck growth, as well as high fracture strength, could be achieved via PECS of Al₂O₃. The incorporation of fine SiC particles into an Al₂O₃ matrix significantly increased the fracture strength of porous Al₂O₃. Based on microstructural observations, it was revealed that the refinement of Al₂O₃ grains and neck growth occurred by PECS and nanocomposite processing.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide

The mechanical properties of Al₂O₃-based porous ceramics fabricated from pure Al₂O₃ powder and the mixtures with Al(OH)₃ were investigated. The fracture strength of the porous Al₂O₃ specimens sintered from the mixture was substantially higher than that of the pure Al₂O₃ sintered specimens because of strong grain bonding that resulted from the fine Al₂O₃ grains produced by the decomposition of Al(OH)₃. However, the elastic modulus of the porous Al₂O₃ specimens did not increase with the incorporation of Al(OH)₃, so that the strain to failure of the porous Al₂O₃ ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)₃ particles. Fracture toughness also increased with the addition of Al(OH)₃ in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.     

Pore Structure Evolution of Lanthana–Alumina Systems Prepared through Coprecipitation

Pure Al₂O₃ and different compositions of La₂O₃–Al₂O₃ samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La₂O₃·11Al₂O₃ and La₂O₃·13Al₂O₃ had higher surface area compared to the pure Al₂O₃ and the La₂O₃·Al₂O₃ composition. Ethanol washing is an effective way for improving the textural stability of pure Al₂O₃ and La₂O₃–Al₂O₃ samples. The effect of steam on the thermal stability of La₂O₃·11Al₂O₃ has also been studied. La₂O₃·11Al₂O₃ sample is found to be stable in steam.     

Reduction and Sintering of a Nickel–Dispersed-Alumina Composite and Its Properties

High-density nickel–dispersed-alumina (Al₂O₃/nickel) composites with superior mechanical properties were obtained by the hydrogen reduction and the hot pressing of alumina–nickel oxide (Al₂O₃/NiO) mixed powders. The mixtures were prepared by using NiO or nickel nitrate (Ni(NO₃)₂· n H₂O) as a dispersion source of nickel metal. Microstructural investigations of the composite fabricated using nitrate powder revealed that fine nickel particles, } 100 nm in diameter, dispersed homogeneously at the matrix grain boundaries, forming the intergranular nanocomposite. High strength (.1 GPa) and high-temperature hardness were registered for the composite that contained a small amount of nickel dispersion. The ferromagnetic properties of nickel, such as high coercive force, were observed, because of the fine magnetic dispersions, which indicates a functional value of structural composites.     

Effect of Titania and Lithia Doping on the Boundary Migration of Alumina under an Electric Field

Boundary migration under an electric field was investigated for pure, TiO₂-doped, and Li₂O-doped Al₂O₃ specimens. Boundary migration rates in TiO₂-doped and Li₂O-doped Al₂O₃ specimens were much faster compared with that of pure Al₂O₃. In all specimens, the migration rate was observed to depend on the applied bias direction. Compared with pure Al₂O₃, the dependence of boundary migration on bias direction became more pronounced in TiO₂-doped Al₂O₃ but less pronounced in Li₂O-doped Al₂O₃. The results were explained in terms of the variation of grain sizes, mobility, and electrostatic potential of boundaries because of doping.     

Application of a Tough Surface Layer to a Fiber-Bonded Composite

The feasibility of creating "tough surface material" using oxide-fiber-reinforced oxide matrix ceramics was studied. Al₂O₃ fiber/(ZrO₂, Al₂O₃) matrix composite was used as the surface material of a Si–Ti–C–O-fiber-bonded composite. The sintering of the matrix (ZrO₂ and Al₂O₃) of the surface composite layer (SCL) and its bonding to the fiber-bonded composite (FBC) were done simultaneously by vacuum hot pressing. A spherical indentation test demonstrated the advantage of the SCL in reducing the damage of the base FBC from an indenter, because the high fracture resistance of the surface composite layer could reduce the stress concentration by the cumulative microfracture process.     

Microstructure of Alumina Composites Containing Niobium and Niobium Aluminides

The microstructures of niobium-based alumina composites prepared by pressureless sintering of compacts of attrition milled Al₂O₃, Nb, and Al powder mixtures were studied. The addition of a small amount of Al is assumed to assist in rapid sintering. X-ray diffraction analyses show that Al₂O₃, Nb, NbO, and the intermetallics AlNb₂ and AlNb₃ are present in the composites. Electron microscopy studies confirm the existence of these phases and reveal dense, fine-grained (≤500 nm) composites. Al₂O₃ and Nb grains form the matrix. NbO occurs as grains and additionally as small particles within Al₂O₃ grains and at Al₂O₃/Al₂O₃ grain boundaries. The intermetallic AlNb₂ and AlNb₃ phases do not exceed 300 nm in size if they occur at grain boundaries, and possess even smaller dimensions when occluded within Al₂O₃ grains or located at Al₂O₃ triple junctions. While the niobium intermetallics are expected to form during the heating cycle before reaching the sintering temperature, the NbO is assumed to form during the cooling cycle due to precipitation of oxygen dissolved in the niobium.     

Phase Relations and Microstructural Development of Aluminum Nitride–Aluminum Nitride Polytypoid Composites in the Aluminum Nitride–Alumina–Yttria System

AlN–AlN polytypoid composite materials were prepared in situ using pressureless sintering of AlN–Al₂O₃ mixtures (3.7–16.6 mol% Al₂O₃) using Y₂O₃ (1.4–1.5 wt%) as a sintering additive. Materials fired at 1950°C consisted of elongated grains of AlN polytypoids embedded in equiaxed AlN grains. The Al₂O₃ content in the polytypoids varied systematically with the overall Al₂O₃ content, but equilibrium phase composition was not established because of slow nucleation rate and rapid grain growth of the polytypoid grains. The polytypoids, 24 H and 39 R , previously not reported, were identified using HRTEM. Solid solution of Y₂O₃ in the polytypoids was demonstrated, and Y₂O₃ was shown to influence the stability of the AlN polytypoids. The present phase observations were summarized in a phase diagram for a binary section in the ternary system AlN–Al₂O₃–Y₂O₃ parallel to the AlN–Al₂O₃ join. Fracture toughness estimated from indentation measurements gave no evidence for a strengthening mechanism due to the elongated polytypoids.     

Bimodal Sintering of Al2O3/Al2O3 Platelet Ceramic Composites

The sintering behavior of an Al₂O₃ compact containing uniformly dispersed Al₂O₃ platelets has been investigated. The results reveal a significant decrease in the sintering rate as well as the formation of voids and cracklike defects in the presence of nonsinterable platelets. The addition of a small amount (2 vol%) of tetragonal-ZrO₂ particles enhances the sintering rate, increases end-point density (∼99.5% of theoretical density) and prevents formation of sintering defects.     

Aqueous Colloidal Processing of ZTA Composites

Two different zirconia-alumina composites, ZTA-30 (70 wt% Al₂O₃+30 wt% ZrO₂) and ZTA-60 (40 wt% Al₂O₃+60 wt% ZrO₂), with potential for orthopedic applications, were processed in aqueous media and consolidated by slip casting (SC), hydrolysis-assisted solidification (HAS), and gelcasting (GC) from suspensions containing 50 vol% solids loading. For comparison purposes, the same ceramic compositions were also consolidated by die pressing of freeze-dried granules (FG). In the HAS process, 5 wt% of Al₂O₃ in the precursor mixture was replaced by equivalent amounts of AlN to promote the consolidation of the suspensions. Ceramics consolidated via GC exhibited higher green (three-point bend) strengths (∼17 MPa) than those consolidated by other techniques. Further, these ceramics also exhibited superior fracture toughness and flexural strength properties after sintering for 1 h at 1600°C in comparison with those consolidated by other techniques, including conventional die pressing (FG).     

Effect of Titania and Lithia Doping on the Boundary Migration of Alumina under an Electric Field

Boundary migration under an electric field was investigated for pure, TiO₂-doped, and Li₂O-doped Al₂O₃ specimens. Boundary migration rates in TiO₂-doped and Li₂O-doped Al₂O₃ specimens were much faster compared with that of pure Al₂O₃. In all specimens, the migration rate was observed to depend on the applied bias direction. Compared with pure Al₂O₃, the dependence of boundary migration on bias direction became more pronounced in TiO₂-doped Al₂O₃ but less pronounced in Li₂O-doped Al₂O₃. The results were explained in terms of the variation of grain sizes, mobility, and electrostatic potential of boundaries because of doping.