Tribological Behavior of Ti2SC at Ambient and Elevated Temperatures

The tribological properties of Ti₂SC were investigated at ambient temperatures and 550°C against Ni-based superalloys Inconel 718 (Inc718) and alumina (Al₂O₃) counterparts. The tests were performed using a tab-on-disk method at 1 m/s and 3N (≈0.08 MPa). At room temperature, against the superalloy, the coefficient of friction, μ, was ∼0.6, and at ∼8 × 10⁻⁴ mm³·(N·m)⁻¹ the specific wear rate (SWRs), was high. However, against Al₂O₃, at ∼5 × 10⁻⁵ mm³·(N·m)⁻¹ and ∼0.3, the SWRs and μ were significantly lower, which was presumably related to more intensive tribo-oxidation at the contact points. At 550°C, the Ti₂SC/Inc718 and Al₂O₃ tribocouples demonstrated comparable μ's of ∼0.35–0.5 and SWRs of ∼7–8 × 10⁻⁵ mm³·(N·m)⁻¹. At 550°C, all tribosurfaces were covered by X-ray amorphous oxide tribofilms. At present, Ti₂SC is the only member of a family of the layered ternary carbides and nitrides (MAX phases) that can be used as a tribo-partner against Al₂O₃ in the wide temperature range from ambient to 550°C.     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

Some Roles of MgO and TiO2 in Densification of a Sinterable Alumina

Sinterability of undoped, MgO-doped, and TiO₂-doped Al₂O₃ has been examined by applying reported sintering equations. The order of sinterability was MgO-doped ∼ undoped≪ TiO₂-doped Al₂O₃ in the initial and intermediate stages of sintering, but a relative sintered density at 1600°C for 1 h occurred in the order undoped < TiO₂-doped < MgO-doped AI₂O₃. The dispersion of thermal grooving angles increased in the order MgO-doped < undoped < TiO₂-doped Al₂O₃, The change of sinterability by the dopants is explained in terms of mobility of mass transfer estimated from a densification rate in the initial- and intermediate-stage sintering and of dispersed driving forces of densification and grain growth qualitatively evaluated from the width of the dispersion of thermal grooving angles.     

Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High-purity Al₂O₃ compacts were doped with 0–350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700° and 2000°C under hydrogen. The solubility limits of MgO in Al₂O₃ at temperatures of 1720° and 1880°C were very low, ∼75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, ln Mg/Al = 3.80–2.63 × 10⁴/ T . The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain-growth mechanisms in MgO-doped Al₂O₃ were probably not done in single-phase Al₂O₃ solid solutions. However, the conclusions on sintering and grain-growth mechanisms in prior research work in MgO-doped A₂O₃ may be correct. The effects of SiO₂ impurity and grain size on MgO solubility are discussed. Previous grain-growth experiments in MgO-doped Al₂O₃ are described that demonstrate the clearest evidence for grain-boundary mobility controlled by a solid-solution mechanism.     

Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).     

Extended X-ray Absorption Fine Structure (EXAFS) Study on the local Environment around Copper in Low Thermal Expansion Copper Aluminosilicate Glasses

The local environment around copper in the Cu₂O·Al₂O₃·4SiO₃ and CuO·Al₂O₃·4SiO₂ glasses was investigated using the extended X-ray absorption fine structure (EXAFS) technique. It has been found that each Cu(I) in the former glass is coordinated to two oxygens through covalent Cu(I)-O bonds. This is the primary reason for its low thermal expansion coefficient (∼10 × 10⁻⁷ K⁻¹). In the latter glass, which was made by heating the former glass at 600°C in air, each Cu(II) is coordinated to four oxygens and the bonds are ionic in character, contributing to the increase in the thermal expansion coefficient to ∼30 × 10⁻⁷ K⁻¹.     

Dihedral Angles in Magnesia and Alumina: Distributions from Surface Thermal Grooves

Dihedral angles, Ψ_s, from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al₂O₃, and MgO-doped Al₂O₃. The values of Ψ_s span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al₂O₃ at 1870 K, and 90° to 139° for MgO-doped Al₂O₃ at 1870 K. For all three systems, the median Ψ_s values are 105° to 113°, implying that the median γ_gb/γ_s is 1.1 to 1.2, in contrast to metals where γ_gb/γ_s ranges typically from 1/4 to 1/2. The widths in Ψ_s distributions were different for the three materials with the width increasing in the following order: MgO, MgO-doped Al₂O₃, undoped Al₂O₃. For all three materials, the grainboundary grooves and their corresponding Ψ_s were not symmetrical with respect to the surface normal. The asymmetry for MgO was due to the pinning of the grain boundaries by the surface thermal grooves. The range of inclination angles of the grain boundary to the surface was a function of Ψ_s, with the maximum inclination angles of ∼13°, in quantitative agreement with theory.     

Preparation and Characterization of Aluminum Borate

Aluminum borate, 9Al₂O₃·2B₂O₃ or Al₁₈B₄O₃₃, was synthesized by the reaction of stoichiometric amounts of α-Al₂O₃ and B₂O₃. The Al₁₈B₄O₃₃ material was formed into a dense ceramic by pressureless sintering with CaO, MgO, or CaAl₂B₂O₇ additives. The material was characterized by low bulk density, moderate coefficient of thermal expansion (3 × 10⁻⁶/°C to 5 × 10⁻⁶/°C), moderate strength (210 to 324 MPa), and low dielectric constant.     

Formation of ZnAl2O4 in the Presence of Cl2

The formation of ZnAl₂O₄ spinel in diffusion couples of Al₂O₃ and ZnO was investigated between 1000° and 1390°C in air and in air containing 4.8 vol% Cl₂ by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy. The rate of formation of a spinel layer obeyed a parabolic rate law and was accelerated remarkably by the presence of Cl₂. The interdiffusion coefficient, , and the activation energy, E, were calculated to be 10⁻⁸ to 10⁻⁹ cm²/s and 123 kcal/mol (514 kJ/mol) in air and 10⁻⁷ cm²/s and 31 kcal/mol (130 kJ/mol) in air containing 4.8 vol% Cl₂, respectively.     

Conversion of Polycrystalline Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth

In a given batch more than 30%–40% of polycrystalline, MgO-doped Al₂O₃ tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c -axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10⁻¹⁰ m³/(N·s), representing the highest value reported so far in Al₂O₃ and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La₂O₃, or ZrO₂ was added in concentrations of several hundred ppm.     

Interdiffusion in the System MgO-MgAl2O4

Interdiffusion coefficients in single-crystal MgO were determined using an MgO-MgAl₂O₄ diffusion couple. For a concentration of 1 mol% Al₂O₃ in MgO, the interdiffusion coefficient can be expressed as D =2.0±0.2 exp (−76,000±3,000/ RT ) for the MgO-MgAl₂O₄ couple. This relation compares well with previous measurements in the MgO-Al₂O₃ system. The interdiffusion coefficients, which increased with the mol fraction of cation vacancies, were in the range of 10⁻⁸ to 10⁻¹⁰ cm²s⁻¹ for the concentrations and temperatures studied. Diffusion was enhanced below 1640°C if powdered MgAl₂O₄ was used. Self-diffusion coefficients for Al³⁺ ions in MgO were calculated; Al³⁺ diffuses faster than Cr³⁺ in MgO.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Microstructural and Thermoelectric Characteristics of Zinc Oxide-Based Thermoelectric Materials Fabricated Using a Spark Plasma Sintering Process

M-doped zinc oxide (ZnO) (M=Al and/or Ni) thermoelectric materials were fully densified at a temperature lower than 1000°C using a spark plasma sintering technique and their microstructural evolution and thermoelectric characteristics were investigated. The addition of Al₂O₃ reduced the surface evaporation of pure ZnO and suppressed grain growth by the formation of a secondary phase. The addition of NiO promoted the formation of a solid solution with the ZnO crystal structure and caused severe grain growth. The co-addition of Al₂O₃ and NiO produced a homogeneous microstructure with a good grain boundary distribution. The microstructural characteristics induced by the co-addition of Al₂O₃ and NiO have a major role in increasing the electrical conductivity and decreasing the thermal conductivity, resulting from an increase in carrier concentration and the phonon scattering effect, respectively, and therefore improving the thermoelectric properties. The ZnO specimen, which was sintered at 1000°C with the co-addition of Al₂O₃ and NiO, exhibited a ZT value of 0.6 × 10⁻³ K⁻¹, electrical conductivity of 1.7 × 10⁻⁴Ω⁻¹·m⁻¹, the thermal conductivity of 5.16 W·(m·K)⁻¹, and Seebeck coefficient of −425.4 μV/K at 900°C. The ZT value obtained respects the 30% increase compared with the previously reported value, 0.4 × 10⁻³ K⁻¹, in the literature.     

Initial Stages of Growth of Alumina on NiAl(001) at 1025 K

The initial stages of growth of ordered layers of Al₂O₃ on NiAl(001) single-crystal surfaces at 1025 K and 10⁻⁷ mbar (10⁻⁵ Pa) in O₂ have been studied using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The STM results evidence the formation of elongated strips (26 Å wide and 11 Å high) of Al₂O₃ oriented along the [100] and [010] directions of the substrate. With longer oxidation, the substrate is increasingly covered by rectangular and striped islands resulting from the vicinal and parallel growth of the strips. On the ultrathin oxide film formed after 500 L (1 L = 10⁻⁶ torr·s (∼1.33 × 10⁻⁴ Pa·s)) of exposure, STM atomic resolution images have been obtained for the first time. They evidence the [001] orientation of the oxygen sublattice in Bain epitaxy on the substrate. The observation of one-dimensional atomic trenches together with the strips observed on the nanometer scale is consistent with the growth of θ-Al₂O₃. The STS local measurements evidence the insulating behavior of the oxide layer formed with a gap value ranging from 7 and 8 eV for amorphous and ordered domains, respectively.     

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

The low-temperature sintering of lead-based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower-melting frit, B₂O₃–Bi₂O₃—CdO. It exhibited the following dielectric and piezoelectric properties: K_p= 0.52 to 0.58, Q_m= 1000, ε^T₃₃/ε₀= 800 to 1000, tan δ= 50 × 10⁻⁴, ρ= 7.56 to 7.64 g/cm³. Ceramics with the aid of suitable dopants (CdO, SiO₂, and excess PbO) in the Pb-(Ni_1/3Nb_2/3)O₃—PZT family could be sintered at 860° to 900°C. For these materials, K_p= 0.56 to 0.61, Q_m= 1000, ε^T₃₃/ε₀= 1500 to 2000, tan δ≤ 50 × 10⁻⁴, ρ= 7.80 to 8.03 g/cm³. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy.     

Characterization of the Thermally Contracting Tungstates Ta22W4O67, Ta2WO8, and Ta16W18O94

An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta₂₂W₄O₆₇, Ta, WO₈, and Ta₁₆W₁₈O₉₄) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm³, are relatively soft (120 to 655 kg/mm² hardnesses), and are electrical insulators (6× 10³ to 2× 10⁸Ω^.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10⁻⁸/K to −5.1× 10⁻⁶/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity     

Effect of Oxygen Partial Pressure on the Creep of Polycrystalline Al2O3 Doped with Cr, Fe, or Ti

Steady-state creep was studied in hot-forged polycrystalline Al₂O₃ (3 to 42 μm) of nearly theoretical density doped with≤1 cation % of Fe, Ti, or Cr. Tests were conducted at stresses between 10 and 550 kg/cm² at 1375° to 1525°C under O₂ partial pressures of 0.88 to 10⁻¹⁰ atm. Except in the 10-μm Fe-doped material tested at very small stresses, slightly nonviscous creep behavior was generally observed. The effects of P _o2 on the creep rate indicated that increased concentration of a divalent (Fe²⁺) or quadrivalent (Ti⁴⁺) impurity in solid solution enhances the creep rate of polycrystalline Al₂O₃. The activation energies for the creep of Fe- and Ti-doped Al₂O₃ samples (148 and 145 kcal/mol, respectively) were significantly higher than that for Cr-doped material (114 kcal/mol). Taking into account the effects of P_o2, temperature, and grain size, it was concluded that the steady-state creep of transition-metal-doped Al₂O₃ is controlled by cation lattice diffusion.     

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 High-Temperature Corrosion Behavior of a Cr–Al–C Composite

A Cr–Al–C composite was successfully synthesized by a hot-pressing method using Cr, Al, and graphite as starting materials. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses revealed that the composite contained Cr₂AlC, AlCr₂, Al₈Cr₅, and Cr₇C₃. The orientation relationships and atomic-scale interfacial microstructures among Cr₂AlC, AlCr₂, and Al₈Cr₅ are presented. This composite displays both excellent high-temperature oxidation resistance in air and hot-corrosion resistance against molten Na₂SO₄ salt. The parabolic rate constants for the oxidation in air at 1000°, 1100°, and 1200°C are 3.0 × 10⁻¹², 6.2 × 10⁻¹¹, and 6.2 × 10⁻¹⁰ kg² (m⁴·s)⁻¹, respectively, while the linear weight gain rates for the hot corrosion of Na₂SO₄-coated samples at 900° and 1000°C are, respectively, 1.2 × 10⁻³ and 4.4 × 10⁻³ mg (cm²·h)⁻¹. The mechanism of the excellent high-temperature corrosion resistance can be attributed to the formation of a protectively alumina-rich scale.