Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

Electroconductive Al2O3–NbN Composites

Electroconductive Al₂O₃–NbN ceramic composites were prepared by hot pressing. Dense sintered bodies of ball-milled Al₂O₃–NbN composite powders were obtained at 1550°C and 30 MPa for 1 h under a nitrogen atmosphere. The bending strength and fracture toughness of the composites were enhanced by incorporating niobium nitride (NbN) particles into the Al₂O₃ matrix. The electrical resistivity of the composites decreased with increasing amount of NbN phase. For a 25 vol% NbN–Al₂O₃ composite, the values of bending strength, fracture toughness, Vickers hardness, and electrical resistivity were 444.2 MPa, 4.59 MPa·m^1/2, 16.62 GPa, and 1.72 × 10⁻²Ω·cm, respectively, making the composite suitable for electrical discharge machining.     

Preparation of Composite Particles by Pulsed Nd-YAG Laser Decomposition of [(CH3)2N]4Ti to TiN-Coat TiO2, Al2O3, or Si3N4 Powders

Irradiation of Ti[N(CH₃)₂]₄ by the 1.064-μm line of a pulsed Nd: YAG laser in the presence of TiO₂, Al₂O₃, or Si₃N₄ particles has been found to form amorphous deposits on the oxide particles. The resulting materials can be processed into TiN/TiO₂, TiN/Al₂O₃, or TiN/Si₃N₄ composites with the TiN component on the surface of the particles. The powders have been characterized by Raman spectroscopy and X-ray powder diffraction studies. The surface analysis of the composites by X-ray photoelectron spectroscopy and high-resolution electron microscopy is presented.     

Uranium Oxide Phase Equilibrium Systems: I, UO2–Al2O3

The UO₂–Al₂O₃ phase equilibrium system was found to contain no new compounds or solid solutions. Uranium dioxide melted at 2878°± 22°C. and Al₂O₃ melted at 2034°± 16°C. The eutectic temperature was approximately 1930°C. There is an indication that two immiscible liquids formed above the eutectic temperature between 53 and 74 mole % Al₂O₃.     

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.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.     

Mechanical Properties and Oxidation Behavior of Al2O3–AlN–TiN Composites

The study examines the effect which the composition of hot-pressed electroconductive ceramics has on their structure, mechanical properties, and oxidation behavior, for ceramics of the type AIN–Al₂O₃–42 wt% TiN, differing in the AIN/Al₂O₃ ratio. The results are physico-mechanical property data, including density, hardness, strength, fracture toughness, and wear resistance. A correlation was found between the wear resistance and fracture toughness. The analysis of oxidation products revealed the formation of α-Al₂O₃ and rutile in the temperature range from 600° to 1100°C and aluminum titanate above 1200°C. The spallation of the oxide layer caused low oxidation resistance of Al₂O₃-rich composites above 1250°C. The oxidation of composites was compared with the oxidation of pure TiN. The relationship is discussed between material properties, composition, phases, and processing parameters.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Activity–Composition Relations in FeCr2O4–FeAl2O4 and MnCr2O4–MnAl2O4 Solid Solutions at 1500° and 1600°C

Activity–composition relations of FeCr₂O₄–FeAl₂O₄ and MnCr₂O₄–MnAl₂O₄ solid solutions were derived from activity–composition relations of Cr₂O₃–Al₂O₃ solid solutions and directions of conjugation lines between coexisting spinel and sesquioxide phases in the systems FeO–Cr₂O₃–Al₂O₃ and MnO–Cr₂O₃–Al₂O₃. Moderate positive deviations from ideality were observed.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Influence of the Characteristics of Spinels on the Slag Resistance of Al2O3–MgO and Al2O3–Spinel Castables

The XRD patterns at ambient temperature and at 1500°C showed that the spinel in the Al₂O₃–MgO castables fired at 1500°C for 3 h has the higher peak intensity, compared to those in Al₂O₃–spinel castables; the interplanar distance in the set (311) is 2.43 Å for the spinel in Al₂O₃–MgO castables as well as the spinels in Al₂O₃–spinel castables using spinels containing 73, 90, and 94 wt% Al₂O₃, respectively. The corresponding alumina contents of the spinels in these castables were estimated to be around 75 wt%. The smaller grain size of the spinel in Al₂O₃–MgO castables compared to that in Al₂O₃–spinel castables is evidenced by the recrystallization of the in situ spinel only occurring in Al₂O₃–MgO castables as revealed by the XRD patterns at ambient temperature and at 1500°C. The larger amount and smaller grain size of the in situ spinel in the matrix mostly account for the better slag resistance of Al₂O₃–MgO castables, compared to Al₂O₃–spinel castables.     

Fabrication and Microstructure of Al2O3–SiC–YAG Hybrid Composites Prepared by Particulate Dispersion

Stable Al₂O₃–SiC–YAG hybrid composites were successfully fabricated by reaction of Al₂O₃ and Y₂O₃ and incorporation of SiC. The hot-pressed bodies consisted of uniformly dispersed grains of microsized YAG particulates and nanosized SiC particulates in an Al₂O₃ matrix. Although the grain size of monolithic A1₂O₃ increases markedly with increased temperature, the grain size of the Al₂O₃–SiC–YAG hybrid composites was effectively restrained due to grain-boundary pinning by the particulates.     

Synthesis, Densification, and Phase Evolution Studies of Al2O3–Al2TiO5–TiO2 Nanocomposites and Measurement of Their Electrical Properties

Alumina–aluminum titanate–titania (Al₂O₃–Al₂TiO₅–TiO₂) nanocomposites were synthesized using alkoxide precursor solutions. Thermal analysis provided information on phase evolution from the as-synthesized gel with an increase in temperature. Calcination at 700°C led to the formation of an Al₂O₃–TiO₂ nanocomposite, while at a higher temperature (1300°C) an Al₂O₃–Al₂TiO₅–TiO₂ nanocomposite was formed. The nanocomposites were uniaxially compacted and sintered in a pressureless environment in air to study the densification behavior, grain growth, and phase evolution. The effects of nanosize particles on the crystal structure and densification of the nanocomposite have been discussed. The sintered nanocomposite structures were also characterized for dielectric properties.     

Experimental Establishment of the CaAl2O4–MgO and CaAl4O7–MgO Isoplethal Sections within the Al2O3–MgO–CaO Ternary System

The isoplethal sections CaAl₂O₄–MgO and CaAl₄O₇–MgO of the Al₂O₃–MgO–CaO ternary system have been experimentally established at 1 bar total pressure and air of normal humidity. The sections obtained provide new data and information that are in disagreement with thermodynamic evaluations and optimizations of the Al₂O₃–MgO–CaO ternary system published to date. These differences arise mainly from the inclusion, or exclusion, of the binary compound Ca₁₂Al₁₄O₃₃, mayenite, as a stable phase in the reported studies of the system. The presence or absence of this compound within the system has an important impact on the solid state and melting relationships of the whole ternary system. The present study confirms the solid-state compatibility CaAl₂O₄–MgO and CaAl₂O₄–MgO–MgAl₂O₄ up to 1372°± 2°C, the peritectic melting point of the later mentioned subsystem.     

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.     

Coherent Precipitation in the System MgO-Al2O3-Cr2O3

An isothermal section of the ternary system MgO–Al₂O₃-Cr₂O₃ was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl₂O₄–Cr₂O₃ and MgCr₂O₄-Al₂O₃, and the binary join MgAl₂O₄-MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al_1-xCr_x)₂O₃, with the corundum structure or through a metastable phase of the probable composition Mg(Al_1-xC_r)₂₆O₄₀. The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Al2O3–Ni Composites with High Strength and Fracture Toughness

Al₂O₃–Ni composites were prepared by the reactive hot pressing of Al and NiO. The composites had a two-phase, interpenetrating microstructure and contained ∼35 vol% Ni. They exhibited an impressively high combination of strength and toughness at room temperature; the four-point bending strength was in excess of 600 MPa with a fracture toughness of more than 12 MPa·m^1/2. Examination of fracture surfaces showed that Ni ligaments underwent ductile deformation during fracture. SEM analysis revealed knife-edged Ni ligaments with a limited amount of debonding around their periphery (i.e., at the Ni–Al₂O₃ interface), indicating a strong Ni–Al₂O₃ bond.     

Phase Relations and Lattice Constants in the MgO-Al2O3-Ga2O3 System at 1550°C

Phase relations and lattice constants in the MgO–Al₂O₃–Ga₂O₃ system at 1550°C have been determined experimentally. In a large part of this system, only a nonstoichiometric spinel is stable. Compositions as extreme as 12.5 mol% MgO–20.5 mol% Ga₂O₃–67 mol% Al₂O₃ for a homogeneous spinel are possible. In the bordering phase diagrams of MgO–Al₂O₃ and MgO–Ga₂O₃, the composition of the spinel is as high as 63 mol% Al₂O₃ or Ga₂O₃, respectively. The contributions of all simple ionic exchange reactions on the lattice constant of the spinel have been deduced from X-ray diffractometry data.     

Major Influences on the Particle-Size–Transformation-Temperature Relation in Al2O3–ZrO2 Composites

The energetics of martensitic transformation in ZrO₂ is studied using a thermodynamic approach, with particular reference to Al₂O₃–ZrO₂ composites. The different characters of three types of transformation-toughened ceramics are analyzed, and several factors affecting the t → m transformation in Al₂O₃–ZrO₂ composites are discussed. The expression of transformation temperature dependence on particle size is derived and has good agreement with experimental results. The energetics concerned with nucleation of martensitic transformation is also discussed.