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.     

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.     

Oxidation Kinetics and Composite Scale Formation in the System Mo(Al,Si)2

The oxidation kinetics of hot-pressed Mo(Al_0.01Si_0.99)₂ and Mo(Al_0.1Si_0.9)₂ were measured at 480°C, and between 1200° and 1600°C. The qualitative oxidation of arc-melted Mo(Al_0.1Si_0.9)₂, Mo(Al_0.3Si_0.7)₂, Mo(Al_0.5Si_0.5)₂, and Mo₃Al₈ was examined after 600°C for 1000 h in air. At all temperatures, the compositional difference between the materials yielded very different oxidation rates and scale microstructures. At 1400° and 1500°C, microstructural evolution of the oxide scales resulted in improved oxidation resistance at long times (>400 h). At these temperatures, a significant reduction in the long-time oxidation kinetics was correlated with the in situ formation of an inner mullite scale. At 480° and 600°C, oxidation resistance improved significantly with increasing aluminum concentration. Contrary to the behavior of MoSi₂, samples of Mo(Al_0.01Si_0.99)₂ did not demonstrate catastrophic oxidation, and samples of Mo(Al_0.1Si_0.9)₂ were very oxidation resistant.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.     

Effects of Additives on the Pressure-Assisted Densification and Properties of Silicon Carbide

The densification of silicon carbide (SiC) was studied using a variety of additives (Al, AlN, Al₂O₃, B₄C, C, Si₃N₄, and Y₂O₃). The onset of densification of SiC with small amounts of additives occurred at temperatures between 1500° and 1900°C with 28 MPa applied pressure. Al, B₄C, and C promoted densification, while N (added as AlN or Si₃N₄) retarded sintering. A 96.75 wt% SiC–2 wt% Al–1 wt% C–0.25 wt% B₄C starting composition yielded the same percent of theoretical density (in the range of 70%–90% theoretical density) 400°C lower than a 95 wt% SiC–5 wt% AlN material. Yttria additions promoted intergranular fracture, which increased the single-edged precracked beam fracture toughness. The appropriate selection and amount of additives allowed for the tailoring of grain size and intergranular fracture, thus controlling the mechanical properties. While oxygen was present in all materials containing aluminum, the incorporation of additional oxygen as alumina resulted in reduced sintering activity compared with Al metal. Corrosion resistance decreased in both HF and NaOH solutions at 80°C for materials containing a grain boundary phase.     

Synthesis and Thermal Stability of Aluminum Titanate Solid Solutions

Aluminum titanate solid solutions with empirical formulas of Al₂Ti_1-xZr_xO₅, Al_6(2-x)(6+x)Si_6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ were synthesized by reaction sintering and annealed at 900° to 1300°C in air to evaluate the thermal stability. Substitution of Al in Al₂TiO₅ by Si and 2Al by Mg and Ti ions to form solid solutions such as AI_{6(2-x)/(6+x)l-Si}6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ was effective in controlling the thermal decomposition, but substitution of Ti by Zr had little effect.     

Reactive Synthesis and Phase Stability Investigations in the Aluminum Nitride–Silicon Carbide System

The effect of AlN on the structure formation of SiC was investigated. SiC was synthesized in the presence of AlN under vacuum at 1500°C, and the result was cubic SiC. The synthesis of AlN–SiC composites through the reaction Si₃N₄+ 4Al + 3C = 3SiC + 4AlN was also investigated and compared with synthesis via field-activated self-propagating combustion (FASHS). Reactants were heated in a vacuum furnace at temperatures ranging from 1130° to 1650°C. Below 1650°C, the reaction is not complete and at this temperature the product phases are AlN and cubic SiC. At 1650°C, the product contained an outer layer which contained β-SiC only and an inner region which contained AlN and cubic SiC. 2H-SiC and AlN composites synthesized via field-activated self-propagating combustion were annealed at 1700°C under vacuum. The AlN dissociated and evaporated and the 2H-SiC transformed to the cubic β phase. Reasons for the differences in products of furnace heating and FASHS are discussed.     

Grain Boundary Evolution in Hot-Pressed ABC-SiC

Silicon carbide with aluminum, boron, and carbon additions (ABC-SiC) was hot-pressed to full density. The samples were examined by transmission electron microscopy (TEM), with an emphasis on high-resolution electron microscopy (HREM). Amorphous grain boundary interlayers, typically less than 2 nm wide, were formed between SiC grains. Heat-treating the ABC-SiC at temperatures as low as 1100°C in Ar crystallized the grain boundary interlayers completely without significantly changing the dominant chemical constituents. Chemical microanalyses demonstrated Al and O enrichment for all examined grain boundaries in both as-prepared and annealed samples. Quantitative EDS analyses revealed Al₂OC- and Al₂O₃-related species (with Si, C, B, or S substitutions) as two of the most likely grain boundary interlayer materials, both before and after heat treatment. Al₂O₃, and (Al_{1− x} Si _x )₂OC with a 2H-type wurtzite structure, were identified as grain boundary films by HREM images. The structural evolution in the grain boundary phases during the hot pressing and postannealing is discussed.     

HRTEM Investigations of New AlN Polytypoids in the High-AlN Region of the AlN–Al2O3–Y2O3 System

The AlN polytypoid phases in selected materials in the AlN–Al₂O₃–Y₂O₃ system has been investigated using TEM methods. Two new AlN polytypoid phases, 24H and 39R, were identified in the pseudobinary AlN–Al₂O₃ system. The 39R phase existed as single grains while 24H was observed only between subblocks of 33R and 39R. The new polytypoids are built on the same structural models that are previously reported for the other polytypoids in the same system, consisting of arrays of planar (oxygen-containing) and corrugated inversion domain boundaries (IDBs). Only one type of interface structure was observed for the planar IDBs of the new polytypoids.     

High-Temperature Neutron Diffraction of the AlN–Al2O3–Y2O3 System

The importance of aluminum nitride (AlN) stems from its application in microelectronics as a substrate material due to high thermal conductivity, high electrical resistance, mechanical strength and hardness, thermal durability, and chemical stability. Yttria (Y₂O₃) is the best additive for AlN sintering. AlN densifies by a liquid-phase mechanism, where the surface oxide, Al₂O₃, reacts with Y₂O₃ to form an Y-Al-O-N liquid that promotes particle rearrangement and densification. Construction of the phase relations in this multicomponent system is essential for optimizing the properties of AlN. The ternary phase diagram of the AlN–Al₂O₃–Y₂O₃ was developed by Gibbs energy minimization using interpolation procedures based on modeling the binary subsystems. This paper aims at testing the resultant understanding experimentally at selected compositions using in situ high-temperature neutron diffractometry. These experimental results agree with the thermodynamic calculations of AlN–Al₂O₃–Y₂O₃. The ternary phase diagram has been constructed for the first time in this work. High-temperature neutron diffractometry has permitted real time measurement of the reactions involved in this ternary system, especially to determine the temperature range for each reaction, which would have been difficult to establish by other means.     

Morphology of Phase Separation in AIN─Al2OC and SiC─AIN Ceramics

Solid solutions in the AIN─Al₂OC and SiC─AlN systems were fabricated by hot-pressing powder mixtures in graphite dies. X-ray diffraction showed the samples to be single phases of 2H structure. The samples were annealed between 1600° and 1900°C for up to 1000 h. In the SiC─AlN system, optical microscopy and X-ray diffraction failed to reveal microstructural or phase changes. However, electron microscopy showed that samples had decomposed. Streaking of diffraction spots occurred along directions orthogonal to {012} planes (∼43° off the c axis), which is approximately the direction along which the elastic energy function is a minimum. The orientation-dependent Young's modulus was also a minimum along this direction. In AIN─Al₂OC, optical microscopy and X-ray diffraction indicated the occurrence of decomposition. The precipitates were disk-shaped with [001] orthogonal to the disks. The occurrence of decomposition along the [001] direction suggests that it is the elastically soft direction.     

Preparation and Properties of Porous Aluminum Nitride–Silicon Carbide Composite Ceramics

Microporous two-phase AlN–SiC composites were prepared using Al₄C₃ and either Si (N₂ atmosphere) or Si₃N₄ (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al₄C₃ under N₂ atmosphere was shown to be a simple processing route for the preparation of porous two-phase AlN–SiC materials. The homogeneous two-phase AlN–SiC composites had a grain size in the range of 1–5 μm, and the porosity varied in the range of 36%–45%. The bending strength was 50–60 MPa, in accordance with the high porosity.     

Effect of Coherency Strains on Phase Separation in the AlN-Al2OC Pseudobinary System

AlN and Al₂OC, both of 2H structure, form a homogeneous solid solution at elevated temperatures. Annealing at lower temperatures leads to the formation of two solid solutions, one AlN-rich and the other Al₂OC-rich. Samples of near equimolar composition form disk-shaped precipitates upon annealing at temperatures below ∼1900°C. In the present work, samples of several compositions were fabricated by hot-pressing mixtures of AlN, Al₄C₃ and Al₂O₃. Lattice parameters of the resulting solid solutions were measured by X-ray diffraction. Samples of an equimolar composition were annealed at 1500°C for various periods of time up to a maximum of 512 h. Disk-shaped precipitates with their axes along the [0001] direction were initially coherent. In the coherent stage, the a lattice parameters of the precipitate and the matrix were identical because of coherent fitting of lattice planes. The shape of the precipitates and their orientation were in accord with predictions based on theoretical calculations of coherency strain energy. Using the experimentally measured values of the aspect ratios, the precipitate/matrix interfacial energy for coherent precipitates was estimated to be on the order of a few mJ/m². Long-term annealing led to the loss of coherency as manifested by the formation of interface dislocations.     

Preparation of Silicon Carbide/Aluminum Nitride Ceramics Using Organometallic Precursors

Solid solutions of 2H -SiC/AlN can be prepared at temperatures less than 1600°C by rapid pyrolysis ("hot drop") of mixtures of [(Me₃Si)_0.80((CH₂=CH)MeSi)_1.0(MeHSi)_0.35] _n (VPS) or [MeHSiCH₂] _n (MPCS) with [R₂AlNH₂]₃, where R=Et, i -Bu or simply by slow pyrolysis of the precursor mixture in the case of [Et₂AlNH₂]₃. In contrast, slow pyrolysis of mixtures of VPS or MPCS with [ i -Bu₂AlNH₂]₃ yields a composite of 2 H -AlN and 3 C -SiC at 1600°C, which transforms into a single 2 H -SiC/AlN solid solution on heating to 2000°C. The influences of the nature of the precursor and processing conditions on the structure, composition, and purity of the SiC/AlN materials are discussed.     

Studies in Lithium Oxide Systems: II, Li2O Al2O3–Al2 O 3

Solid-state reactions between Li₂O and Al₂ O₃ were studied in the region between Li₂O.Al₂ O ₃ and Al₂ O ₃. The compound Li₂ O Al₂ O ₃ melts at 1610°± 15°C. and undergoes a rapid reversible inversion between 1200° and 1300°C. Vaporization of Li₂ O from compositions in the system proceeds at an appreciable rate at 1400°C, as shown by fluorescence. Lithium spinel, Li₂ O -5Al₂O₃, was the only other compound observed. The effect of Li₂ O on the sintering of alumina was investigated.     

Influence of Oxygen on the Formation of Aluminum Silicon Carbide

X-ray diffraction studies have been used to follow the formation of Al₄SiC₄ from Al₄C₃ and SiC and the role played by impurity oxygen. The phase Al₂OC forms in the early stages of reaction and reacts with SiC at ≈ 1700°C to produce Al₄SiC₄ plus a small amount of an aluminosilicate liquid. This liquid dissociates at higher temperatures, the resulting evolution of gases hindering complete densification. Higher densities are obtained on hot-pressing.     

Microstructural Evolution during Sintering of Aluminum Nitride Ceramics Doped with Alumina and Yttria

The microstructural evolution of AlN sintered at >1950°C was studied in a specimen doped with 10 wt% Al₂O₃ and 5 wt% Y₂O₃. The constituent phases of the specimen were AlN, YAG, γ-AlON, and AlON polytypoids (compositional polytypes). Transmission and scanning electron microscopy revealed the microstructural characters: platelike 7AlN·Al₂O₃ first crystallized with concurrent formation of a residual liquid, then spherical AlN crystals formed. The liquid itself changed composition with the progress of the crystallization and reached the eutectic composition in the pseudobinary system AlN–YAG, and crystallized to an aggregate of AlN and YAG during cooling. As a product of the reaction of 7AlN·Al₂O₃, γ-AlON was formed.     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

TEM Investigation of Impurity Phases and the Penetration of Liquid Aluminum in Hot Isostatically Pressed TiB2 Compacts

The microstructure of materials compacted from commercially produced TiB₂ powders was investigated using transmission electron microscopy. A number of impurity phases that are introduced during the various processing stages were identified. After exposure to liquid aluminum, grain boundaries and triple junctions of TiB₂ were found to be penetrated by aluminum. In the penetrated regions pure aluminum, two aluminum oxides, and an (Al₂OC)₁- _x (AlN) _x . phase were identified. A SiO₂ glass phase, introduced during hot isostatic pressing, is believed to be responsible for the formation of alumina. None of the other impurity phases were found to react with aluminum.