Acoustic Emission and Microcracking in Sapphire, Sintered Al2O3, Al/Al2O3 Composite, and Aluminum

A range of materials with brittle to ductile behavior (single-crystal and polycrystalline alumina, aluminum/alumina composite, and metallic aluminum) were investigated by acoustic emission (AE) methods for microcracking during hardness indentations or cooling from elevated temperatures (800°C). During indentation, the extent of crack formation (and the AE counts) decreased in the following order: sapphire, sintered alumina, aluminum/alumina composite with no microcracking in metallic aluminum. During cooling from 800°C, polycrystalline alumina exhibited more extensive microcracking than the aluminum/alumina composite, suggesting that the metallic phase in the aluminum/alumina composite absorbs stresses more than the glassy boundary phase in sintered alumina.     

The Effect of Yttrium on Oxygen Grain-Boundary Transport in Polycrystalline Alumina Measured Using Ni Marker Particles

The grain-boundary transport of oxygen in polycrystalline α-Al₂O₃ (undoped and 500 ppm Y³⁺-doped) was studied in the temperature regime of 1100°–1500°C by monitoring the oxidation of a fine, uniform dispersion of Ni marker particles (0.5 vol%). The annealing treatments were carried out in a high-purity O₂ atmosphere (>99.5%). The Ni particles, which are visibly oxidized to nickel aluminate spinel, were used to determine the depth of oxygen penetration. The thickness of the reaction zone was measured as a function of heat-treatment time and temperature, and a comparison of the oxidation rate constants and activation energies for undoped and Y³⁺-doped alumina was made. The results indicate that the presence of Y³⁺ slows oxygen grain-boundary transport in alumina by a variable factor of from 15 to 3 in the temperature regime of 1100°–1500°C. The values of the activation energy for undoped and Y³⁺-doped alumina were determined to be 430±40 and 497±8 kJ/mol, respectively.     

Controlled wet erosive wear of polycrystalline alumina

Controlled wet erosive wear tests were performed on polycrystalline alumina specimens of mean grain size G = 1.2, 3.8 and 14.1 μm and on sapphire specimens. The tests were performed by using an apparatus consisting of a jet body that rotates immersed in the slurry medium (SiC grits). Impacts are normal to the target surface. The construction and calibration of the apparatus are described. The impacting velocity used was 2.7 m s⁻¹. The weight loss of polycrystalline alumina and sapphire specimens increased linearly with impacting time. The wear rate of polycrystalline alumina specimens increased with grain size. Wear rates of 1.83, 8.36 and 11.3 nm s⁻¹ correspond to G = 1.2, 3.8 and 14.1 μm, respectively. For sapphire specimens the wear rate was 9.56 nm s⁻¹. Worn surfaces of both polycrystalline alumina and sapphire specimens were analysed by scanning electron microscopy.     

The Wulff Shape of Alumina: III, Undoped Alumina

Controlled-geometry cavities were introduced into the m{10     0} plane of undoped sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding the etched sapphire to an undoped high-purity polycrystalline alumina. Pore-boundary separation during growth of the sapphire seed into the polycrystal entrapped the pores within the single crystal. Pores with an equivalent spherical radius of ≈1 μm reached a quasi-equilibrium shape after prolonged anneals at 1600° and 1800°C. The introduction of mechanically induced surface defects accelerated pore shape equilibration. The Wulff shape of undoped alumina was determined by characterizing the shape and facet structure of these equilibrated internal pores using optical microscopy, scanning electron microscopy, and atomic force microscopy. The observed planes in the Wulff shape of undoped alumina, c(0001), r{     012}, s{1     01}, a{11     0}, and p{11     3} planes, were consistent with those reported by Choi et al .; however, a different energy sequence is inferred. The absence of the m-plane in the Wulff shape is consistent with other experimental studies, but inconsistent with those lattice simulations that predict the m-plane to be one of the lowest energy planes in pure alumina. A comparison of Wulff shapes at 1600° and 1800°C suggests that the surface energy of undoped alumina becomes more isotropic as temperature increases.     

Effects of CuO Content on the Wetting Behavior and Mechanical Properties of a Ag–CuO Braze for Ceramic Joining

A silver-based joining technique referred to as reactive air brazing has been recently developed for joining high-temperature structural ceramic components of the type used in gas turbines, combustion engines, heat exchangers, and burners. It was found that additions of copper oxide to silver exhibit a tremendous effect on both the wettability and joint strength characteristics of the subsequent braze relative to polycrystalline alumina substrates. The effect is particularly significant at low copper oxide content, with substantial improvements in wetting observed in the 1–8 mol% range. The corresponding strength of the brazed polycrystalline alumina joints appears to be maximized at a copper oxide content of 8 mol%, with a maximum room temperature flexural strength approaching that of monolithic alumina. While further increases in oxide content lead to improved wetting on polycrystalline alumina, the effect on joint strength is deleterious. It appears that the formation of a continuous brittle copper-based oxide layer along the interface between the braze and alumina faying surface is responsible for the poor mechanical behavior observed in joints fabricated with higher copper oxide content brazes.     

Formation Mechanism of Porous Alumina With Oriented Cylindrical Pores Fabricated by Unidirectional Solidification

Porous alumina whose pores were aligned in one direction was fabricated by the unidirectional solidification method under a pressurized hydrogen atmosphere. The porous structure is formed at the solid–liquid interface during solidification due to a hydrogen solubility gap at the melting point. The hydrogen gas is dissolved into molten alumina according to Sieverts' law and insoluble gas that corresponds to the amount of solubility gap evolves from the solid phase at the solid–liquid interface during the unidirectional solidification to form the pores. The porosity and pore size of the solidified samples decreased with increasing total pressure where the environmental gas consisted of pure hydrogen or hydrogen–argon mixed gases. There is a reverse proportion relation between the pore diameter and the total pressure according to Boyle's law.     

Cracking mechanism in laser directed energy deposition of melt growth alumina/aluminum titanate ceramics

Thanks to its advantages of high efficiency and near-net shaping, laser directed energy deposition (LDED) is rapidly becoming a remarkable preparation technology for high-purity ceramics. However, the cracking problem in shaping process is always a great challenge for LDED to achieve industrial application. For this purpose, alumina/aluminum titanate melt-growth ceramics (A/AT MGCs) were prepared using LDED system, and the corresponding finite element thermal analysis model was developed. The solidification behavior and cracking mechanism of A/AT MGCs were investigated based on the thermal analysis model, and the influence of process parameters on the cracking characteristics was revealed with experiments. Results show that the crack morphology and distribution are controlled by microstructure and temperature gradient together. The scanning speed of 100–150 mm/min, with better microstructure and lower temperature gradient, is a preferred process window. This study provides theoretical guidance and technical support for the cracking suppression during LDED shaping of ceramics.     

Crystallization Atmosphere and Substrate Effects on the Phase and Texture of Chemical Solution Deposited Strontium Niobate Thin Films

Strontium niobate (Sr:Nb  =  1:1) thin films were prepared via chemical solution deposition on (001)‐oriented SrTiO₃, (001)_p‐oriented LaAlO₃, (0001)‐oriented sapphire, and polycrystalline alumina substrates. Crystallization in oxygen at 1000°C yielded Sr₂Nb₂O₇ films on all substrates with strong (010) orientation. Films on LaAlO₃ and SrTiO₃ single‐crystal substrates possessed a small amount of preferred in‐plane orientation, whereas films prepared on sapphire and polycrystalline alumina substrates were fiber textured. Films crystallized at 900°C in a low oxygen atmosphere (~10 ^{? 21} atm pO₂) formed a randomly oriented polycrystalline perovskite, SrNbO_3?δ on all substrates. A similar set of films crystallized at 900°C at a slightly higher oxygen partial pressure (~10^?15 atm pO₂) was comprised of Sr₂Nb₂O₇ and SrNbO_3?δ phases, exposing the dependence of phase formation on oxygen partial pressure. When subjected to a high‐temperature anneal in oxygen, the SrNbO_3?δ phase is shown to transform into Sr₂Nb₂O₇, however, Sr₂Nb₂O₇ did not significantly reverse transform into SrNbO_3?δ after annealing in low oxygen partial pressure atmospheres.     

Synthesis of alumina-bonded polycrystalline diamond by detonation

In this study, nanodiamond, aluminum isopropoxide, and hexogen (RDX) were used as starting materials to synthesize alumina-bonded polycrystalline diamond materials under high-temperature and high-pressure conditions generated by the detonation of the explosive. During detonation, the surface of the nanodiamond is coated with boron, silicon, and chromium through vacuum diffusion. Carbides of boron, silicon, and chromium referred as “bridges” are formed at the diamond/metal interface during the carbonization reaction. The "bridge" formed between diamond and nanoalumina considerably reduced the possibility of oxidation of nanodiamond as well as its graphitization during the detonation reaction. The phase, morphology, microstructure, and elemental composition of the detonation products were characterized by X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The results revealed that the explosion causes alumina and diamond to bond tightly to form alumina-bonded polycrystalline diamond composites. The thermal stabilities of the nanodiamond particles coated with boron, silicon, and chromium were found to be markedly higher, and the diamond phase remained intact even after heating at elevated temperatures. Thus, boron, silicon, and chromium reduced the wetting angle of diamond and alumina and improved the degree of bonding between them. Furthermore, boron facilitated the bonding between nanodiamond and alumina. In contrast, the bond was weaker in the case of silicon. Chromium also aided the bonding of the nanodiamond and alumina but introduced a large amount of oxygen into the composite.     

Modification characteristics of filamentary traces induced by loosely focused picosecond laser in sapphire

The ability of loosely focused picosecond laser to induce filamentation (4.8 times the Rayleigh length) in sapphire was confirmed. The morphology and microstructure of the filamentary traces under typical pulse energy were studied, in which the colorful birefringence phenomenon and damage defects were detected in sapphire. Irreversible ceramic-like polycrystalline microstructures were formed in filamentary traces, and the phase transition from α-Al₂O₃ to γ-Al₂O₃ directly reflected the thermal effect of picosecond laser filamentation. According to the filamentary characteristics, a continuous filamentary channel with a uniform diameter of 33 μm and a length of 7443 μm was obtained by gradually raising the focal position. The research provides first-hand information for the high throughput micro-cutting, micro drilling and thin slicing of sapphire based on ultrafast laser non-linear effects.     

Glass-alumina functionally graded materials: their preparation and compositional profile evaluation

This work was focused on glass-alumina functionally graded materials (FGMs). For the glass phase, a proper composition was chosen belonging to the ternary system CaO–ZrO₂–SiO₂ and the substrate was made up of a sintered, high-purity polycrystalline alumina. Both of the ingredient materials were carefully characterized. The fabricated functionally graded materials were analysed in detail, by observing them under a scanning electron microscope (SEM) coupled with an X-ray energy dispersive spectrometer (X-EDS). The depth of penetration of the glass and the compositional profile were evaluated by means of a SEM-image elaboration. Moreover, this work applied an analytical model to predict the depth of penetration as a function of time and fabricating parameters such as temperature.     

Epitaxial Aluminum-Doped Zinc Oxide Thin Films on Sapphire: II, Defect Equilibria and Electrical Properties

The electrical transport properties of epitaxial ZnO films grown on different orientations of sapphire substrates have been measured as a function of partial pressure of oxygen. After equilibration, the carrier concentration is found to change from a p ^-1/4_O2 to a p ^-3/8_O2 dependence with increasing oxygen partial pressure. The partial pressure dependence is shown to be consistent with zinc vacancies being the rate-controlling diffusive species. In addition, the carrier concentration in ZnO films grown on A-, C-, and M-plane sapphire are the same but that of R-plane sapphire is systematically lower. Electron Hall mobility measurements as a function of carrier concentration for all the substrate orientations exhibit a transition from "single-crystal" behavior at high carrier concentrations to "polycrystalline" behavior at low carrier concentrations. This behavior is attributed to the effective height of potential barriers formed at the low-angle grain boundaries in the epitaxial ZnO films. The trap density at the grain boundaries is deduced to be 7 × 10¹²/cm². The electron mobility, at constant carrier concentration, varies with the substrate orientation on which the ZnO films were grown. The difference is attributed to the difference in dislocation density in the films produced as a result of lattice mismatch with the different sapphire orientations.     

Effect of YSZ-incorporated glass-based composite coating on oxidation behavior of K438G superalloy at 1000 °C

A glass-based composite coating incorporating YSZ particles was prepared by sintering on K438G superalloy substrates. The YSZ additions increased the cyclic oxidation resistance at 1000 °C, while the formation of zircon resulting from interfacial reactions between YSZ and the glass matrix worked reversely. Besides, the YSZ inclusions changed the crystallization behavior of the glass matrix, and only anorthite precipitated during cyclic oxidation. Due to the synergy of sand-blasting and sealing effect of the glass-based coating, the oxidation behavior of K438G was changed and a layer of alumina instead of chromia formed at the substrate/coating interface. Furthermore, a gahnite layer formed at the alumina/gahnite interface because of interfacial reactions between alumina and the glass matrix, leading to the formation of a bi-layered thermally grown oxide. Thus, the alumina layer was protected from the attack of the active glass matrix. Accordingly, the coated K438G superalloy exhibited satisfactory oxidation resistance at 1000 °C.     

Plastic Deformation of Cuprous Oxide

Slip line and transmission electron microscopy observations on the plastic deformation of cuprous oxide were made on large-grained polycrystalline specimens. The specimens were prepared by the complete oxidation of OFHC copper strips in air followed by a high-temperature anneal. Plastic deformation occurred by motion of α(100) dislocations on (100) glide planes. Some dislocation segments of a(110) Burgers vector were present but were probably formed by recombination reactions between α(100) dislocations. The unusual structure of Cu₂0, which can be described as two interpenetrating and identical frameworks of copper and oxygen which are not cross-connected by any primary copper–oxygen bonds, did not result in any unusual behavior of dislocations. The effect of temperature on plasticity was explained in the same way as for other materials of less complex structure, such as MgO.     

Young's Modulus of Various Refractory Materials as a Function of Temperature

Young's modulus as a function of temperature was determined by a dynamic method for single-crystal sapphire and ruby and for polycrystalline aluminum oxide, magnesium oxide, thorium oxide, mullite, spinel, stabilized zirconium oxide, silicon carbide, and nickel-bonded titanium carbide. For the single crystals, Young's modulus was found to decrease linearly with increasing temperature from 100°C. to the highest temperature of measurement. For all the polycrystalline materials, except silicon carbide, stabilized zirconium oxide, and spinel, Young's modulus was found to decrease approximately linearly with increasing temperature until some temperature range characteristic of the material was reached in which Young's modulus decreased very rapidly and in a nonlinear manner with increasing temperature. This rapid decrease at high temperature is attributed to grain-boundary slip. Stabilized zirconium oxide and spinel were found to have the same rapid decrease in Young's modulus at high temperature, but they also had a decidedly nonlinear temperature dependence at low temperature.     

Static and Cyclic Fatigue Behavior of a Polycrystalline Alumina

The fatigue behavior of a polycrystalline alumina was investigated. Stress conditions consisted of a static tensile stress and a static tensile stress with superposed sinusoidal cyclic stress. The alumina exhibited the expected static fatigue behavior; a cyclic fatigue effect characterized by a frequency and amplitude dependence was also observed. Possible mechanisms of cyclic fatigue in brittle materials are discussed.     

Preparation of α-Alumina Platelets by Laser Scanning

A laser scanning with gas jet process was developed to prepare alumina platelets from an alumina powder. When the carbon-dioxide laser scanned the alumina powdery coatings prepared using an electrospraying technique, the alumina particles were heated to a melting state. The coaxial gas ejection force pushed the melting particles to obtain tabular shape grains that recrystallized into alumina platelets in the subsequent rapid-cool solidification. The phase and morphologies of powder bed were characterized by XRD and SEM. Results show that only α-alumina platelets were formed in the scanning process and the average edge length and thickness is 10 μm and 1–2 μm, respectively. Laser processing parameters such as laser energy density, scanning speed, and gas pressure were expected to play a vital role in the melting-crystallization-solidification process for obtaining platelike grains from powder beds. The preliminary experiment showed that the laser-scanning technique could be an effective means of tailoring the morphologies of particles to meet application requirements.     

Fiber Coating Concepts for Brittle-Matrix Composites

The current interest in tough, high-temperature materials has motivated fiber coating development for brittle-matrix composites with brittle reinforcements. Such coatings are needed for controlled interface debonding and frictional sliding. The system investigated in this study was sapphire fiber-reinforced alumina. This system is thermochemically stable for severe use conditions, exhibits little thermal expansion mismatch, and utilizes the excellent strength and creep resistance of sapphire reinforcements. Porous oxide and refractory metal coatings which satisfy requirements for toughness improvement in these composites were identified by employing a variety of newly developed mechanical testing techniques for determining the interfacial fracture energies and sliding resistances.     

Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.     

Evaporation and Energy Transfer for a Partially Molten, Laser-Heated Sapphire Filament

Molten regions were formed on 0.025-cm-diameter sapphire filaments that were heated from one side with a continuouswave CO₂ laser beam in a low-pressure flow reactor. As the laser intensity was increased, the liquid/solid interface moved from the laser-heated edge to the opposite edge of the filament, the apparent temperature measured in the molten region with an optical pyrometer increased from 1470 ± 25 to 2040 ± 30 K, and the filament evaporation rate increased by a factor of 1.6. This change in apparent temperature resulted from an increase in the spectral emittance with the liquid layer thickness. The change in evaporation rate resulted from a 1.09 times larger evaporating area, a 1.24 ± 0.09 times larger evaporation coefficient, and a 10 ± 5 K larger average temperature when the filament cross section was completely liquid than when liquid first formed on the solid filament. Optical and energy-transfer properties of sapphire and liquid Al₂O₃ were calculated from optical pyrometry, energy-balance measurements, and spectral absorption coefficient data for sapphire. At the melting temperature, the total emittance is approximately 0.051 and 0.31 ± 0.03 for the solid sapphire and liquid aluminum oxide filaments, respectively. The thermal accommodation coefficient for Ar atoms is 0.53 ± 0.07 on the solid and approximately unity on the liquid. The spectral absorption coefficient, kλ, at the optical pyrometer wavelength (0.665 μm) is 0.1 ± 0.04 cm⁻¹ for the solid and 12 ± 12 cm⁻¹ for the liquid. This value of kλ for solid sapphire at the melting point is 12 times that of pure, void-free material and reflects the influence of impurities and small voids in the sapphire filaments that were used.