Essentially pure, ∼1.5 μm thick, polycrystalline diamond films DC–PACVD deposited on polycrystalline α-SiC flats were friction and wear tested against similarly coated α-SiC pins. The oscillatory sliding tests were performed with a Knudsen cell-type, wide temperature range tribometer built into a scanning electron microscope. Experiments were completed in 1.33 × 10−3 Pa (1 × 10−5 Torr) and 13.3 Pa (1 × 10−1 Torr) Pair, at test flat temperatures cycled to 850°C and back to room temperature. The results are compared with similar tests previously completed with diamond versus bare α–SiC and diamond versus Si(100) sliding combinations. In the absence of in situ surface analytical capability in the SEM tribometer, the findings are interpreted based on relevant information collected from the literature. The data indicate that the friction of the respective, tangentially sheared interfaces depends on the generation and annihilation of dangling bonds. Desorption of hydrogen during heating and sliding under the electron beam, in vacuum, create unoccupied orbitals on the rubbing surfaces. These dangling bonds spin-pair with others donated from the counterface, leading to high friction. Resorption of adsorbates such as hydrogen (e.g., by cooling the tribosystem) lowers the interfacial adhesion and friction. At high temperatures, in vacuum, the interaction energy may also be reduced by surface reconstruction and graphitization. At high temperatures, in Pair, the coefficient of friction of diamond versus itself is substantially lower than that in vacuum. This reduction is most probably caused by the generation of oxidation products combined with the temperature-shear-oxygen-induced phase transformation of diamond to graphite. The wide temperature wear rates of pure, polycrystalline diamond, characteristic of our test procedure, varied from ∼4 × 1016 m3/N · m in 1.33 × 10−3 Pa vacuum to ∼1 × 10−15 m3/N · m in 13.3 Pair.

Textured bulk YBa2Cu3Ox superconductor samples were fabricated using directional growth of superconductor grains reacted from Y2BaCuO5, BaCuO2, and CuO powder mixtures. The samples consisted of several mm long grains aligned parallel to the growth direction. The microstructure observation and x-ray diffraction analysis showed that the grains have a preferred orientation to improve critical current density.

Grain size strongly influences the superplastic properties of a fine-grained, 20 wt.% Al2O3-reinforced, Y2O3-stabilized tetragonal ZrO2 (Al2O3/YTZ). The elongation to failure of the material decreases, and the flow strength increases, with increasing grain size. The flow stress at a constant true strain rate and at a given strain is proportional to the grain size raised to a 0.75 power. As a result, the superplastic deformation rate in Al2O3/YTZ is inversely proportional to the grain size raised to a 1.5 power.

Fractal analysis of steady-state erosion surfaces generated by erodents of widely different hardnesses on single crystal sapphire shows surprising similarities in spite of a large difference in erosion rates and single impact morphology. This study quantifies the surfaces in terms of the surface texture measurements using recently developed image analysis techniques. The interpretation for such similarities is that a single mechanism of material removal is operative for all erodents. The differences are explained in terms of the efficiency of crack initiation in the target by the two erodents.

YBa2Cu3O7−x thin films deposited by laser evaporation were studied by transmission electron microscopy. The crystal structure and microstructure of the films depend sensitively on the oxygen partial pressure during deposition. Cooling conditions affect primarily the oxygen sublattice and thus determine electrical properties in the superconducting state. Deposition of technologically relevant thin films requires an oxygen partial pressure of 0.3 mbar during deposition and slow cooling in an oxygen atmosphere. Such films have a Tc of 90 K and a transition width of 0.6 K. Electron diffraction patterns yielded a relative lattice parameter difference (b − a)/b of 1.6%, from which the oxygen content can be determined with considerable accuracy. Extended crystal defects and second phases were analyzed by analytical and high-resolution electron microscopy. Films deposited at lower oxygen partial pressures (≍10−2 mbar) exhibit a strongly strained crystal structure with lattice planes heavily bent on an atomic scale. In these films evidence for a decomposition reaction of YBa2Cu3O7−x was found by the simultaneous presence of BaCu2O2 grains and Y2O3 precipitates. The films deposited at low oxygen partial pressure become superconducting when cooled slowly in an oxygen atmosphere. The onset of the broad transitions lies at 50 K.

Metallic alloy ribbons of Bi–Sr–Ca–Cu with nominal compositions of 2-2–1-2, 2-2–2-3, 2-2–2-4, and 2-2–3-4, with or without Pb, were made by vacuum induction melting followed by melt-spinning. Ag, at the level of 20–80 wt. %, was added to aid in the alloy melting and metallic ribbon formation processes. These ribbons were then subjected to a controlled atmosphere oxidation and annealing to produce superconducting oxide/silver microcomposites. Among the four alloy groups investigated in the present work, 2-2–3-4 alloys with 0.6Bi replaced by Pb possess the best superconducting properties after suitable treatment. These specimens exhibited zero resistance at T = 104–110 K and a critical current density of 600 A/cm2 at 77 K in zero field with excellent reproducibility. The annealing conditions, namely the annealing temperature, time, and atmosphere, are discussed in terms of the formation kinetics of the superconducting phases. While the 85 K Tc “2212” phase formed over a wide temperature range (740 to 845 °C) after an ∼2 h anneal, the 110 K high-Tc transition appeared only after an ∼10 h anneal at 810–840 °C, suggesting that the “2223” phase grew from the 2212 phase controlled by diffusion. The microstructures of these superconducting microcomposites were observed and analyzed by a combination of optical and scanning electron microscopy, x-ray powder diffraction, and electron probe microanalysis. The high Tc “2223” phase exhibited plate-like grains which were distributed randomly in orientation. The silver addition improved the mechanical properties of the metallic precursor ribbons and the superconducting microcomposites, promoted the formation of the 2223 phase, and influenced the oxidation and the annealing conditions.

The low temperature phase (LTP) of MnBi, which is of interest because of its large magnetic anisotropy, has been obtained in almost single-phase form (>95%) by rapid solidification, followed by thermal annealing. X-ray and electron microscope studies indicate that the melt-spun ribbons are amorphous, contrary to the accepted rules for glass formation. The transformation of the amorphous phase is a complex process. The amorphous phase crystallizes first into Bi, Mn3Bi, and ferrimagnetic MnBi. The formation of the LTP begins immediately after the eutectic melting of these phases around 540 K.

Based on the profile of the energy deposition obtained using the binary collision model, we follow the diffusion of energy by solving a simplified version of the heat equation. An estimation of the molten zone compares very well with the molecular dynamics prediction for the same event. We discuss the reasons for this agreement and the relevance of such simplified procedure in terms of present-day computer limitations to simulate high energy cascades using molecular dynamics.

An electron microscope structure image of a σ = 21/[111] tilt grain boundary in Au was obtained and atomic column positions identified to yield a structural unit model of the interface consisting of repeating polyhedron shapes. This result represents the smallest projected spacings at a grain boundary containing defect structures imaged by an electron microscope and interpreted atomistically.

The structure and thermodynamic properties of a Σ5 (001) twist boundary in gold are studied as a function of temperature. This study was performed within the framework of the Local Harmonic (LH) model and employed an Embedded Atom Method (EAM) potential for gold. We find that for the Σ5 (001) twist boundary in gold, a distorted CSL structure is stable at low temperatures, but undergoes a phase transformation to a DSC related structure near room temperature. This transformation is shown to be first order. The temperature dependences of the excess grain boundary free energy, enthalpy, entropy, specific heat, and excess volume are calculated. Discontinuities are observed in the slope of the grain boundary excess free energy (versus temperature), in the value of the grain boundary excess specific heat and excess volume. The stable high temperature grain boundary structure has a smaller excess volume than does the lower temperature structure, and both structures have a coefficient of thermal expansion which is in excess of that for the perfect crystal.

Single-phase aluminum nitride films were deposited onto fused quartz and single-crystal sapphire by current-controlled, reactive, de magnetron sputtering from an aluminum metal target. Optical and structural properties were observed to correlate systematically with the composition of the sputter gas over a wide range of nitrogen partial pressures. A transition in the electrical conductivity of the deposited films occurred as a function of N2 partial pressure. This transition is driven by the condition of the target surface. When the N2 partial pressure was high and the target surface was substantially covered with AlNx, the deposited film was insulating, stoichiometric AlN. When the N2 partial pressure was low and the target surface was substantially Al°, the deposited film was conducting, substoichiometric AlNx.

Inviscid melt spinning yielded the first crystalline alumina fibers directly from the melts. In this experimental process, a liquid jet having a melt viscosity of <101 poise (vs>104 for fiberglass) is extruded into propane and is thus chemically stabilized (vs rapidly quenched) before it forms Rayleigh waves and breaks up into droplets. This letter describes a 65.5% alumina-zirconia fiber, an 81.5% and a 90.6% alumina-calcia fiber, a 98.6% alumina-magnesia fiber, and a 100% alumina fiber. The δ-allomorph was identified as the crystalline phase of the melt spun 100% alumina fibers, compared to the α-allomorph reported for FIBER FP, a slurry spun and sintered 100% alumina fiber.

The effects of self-radiation damage as a function of cumulative alpha-decay events in synthetic zircon doped with 238Pu and natural zircons damaged over geologic time are compared and interpreted in terms of the accumulation of both defects and amorphousness. The radiation-induced unit-cell expansion and amorphization result in macroscopic swelling that increases sigmoidally with cumulative decay events and saturates at a fully amorphous state. The derived amorphous fraction as a function of cumulative dose is consistent with models based on the multiple overlap of displacement cascades, indicating that amorphization in zircon occurs as a result of the local accumulation of high defect concentrations rather than directly within a displacement cascade. Annealing of point defects in the natural zircons suppresses initial swelling and delays the onset of amorphization. Full recrystallization of the zircon structure from the amorphous state occurs in two stages, with kinetics and activation energies consistent with the reported thermal stability of the amorphous state. This study further confirms that actinide doping is a viable accelerated technique to study or simulate radiation effects from alpha decay on geologic time scales.

The isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO2 have been determined at various pH conditions. It is shown that monoclinic ZrO2 precipitates at low pH whereas at high pH tetragonal ZrO2 crystallizes from an amorphous zirconium (hydrous) oxide, Zr(OH)xOy, precursor. At intermediate pH conditions mixtures of the polymorphs are formed suggestive of kinetically competing particle formation mechanisms. The data are explained by the proposed existence of three controlling regimes for the formation of crystalline ZrO2: dissolution/precipitation at low pH, a solubility controlled regime at intermediate pH values, and a gel structure controlled regime at high pH. Apparent activation energies for the nucleation and crystallization of monoclinic and tetragonal ZrO2 formed under hydrothermal conditions are presented.

A CVD process has been developed for coating Textron-Avco SCS-6 SiC fiber with yttria. Both Y(fod)3·H2O and Y(thd)3 (fod = 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato; thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) were examined as potential Y2O3 CVD precursors. Analysis of the deposits by Auger spectroscopy indicated significant F and C'incorporation in the case of Y(fod)3 · H2O whereas, under appropriate conditions, Y(thd)3 gave a deposit which was essentially free of C and other impurities. GCFTIR analysis of the volatile products of the CVD process indicated isobutylene, tetrafluoroethylene, 1,1-difluoroethylene, fluoroform, and fluoroethylene for Y(fod)3 · H2O and mainly isobutylene and propylene for Y(thd)3. The precursor Y(thd)3 was chosen to deposit 1–2 μm of yttria on short lengths of silicon carbide fibers. The coated fibers were then incorporated into a nickel aluminide (Ni3Al) matrix by reactive sintering, with yttria affording protection from the known SiC + 2Ni ⇉ Ni2Si + C degradation process. The SiC/Ni3Al composites, before and after annealing at 1000 °C for up to 100 h, were studied by using SEM and EMPA to determine the extent of reaction. With the exception of certain portions of the fibers that were inadequately coated with yttria, complete protection of the fibers was indicated.

Bi2VO5.5 (Bi4O11), which is the vanadium analog of the first member of the Aurivillius family of oxides of the general formula Bi2An−1BnO3n+3, has been prepared and characterized. The vanadate has the expected layered structure and is ferroelectric with a Curie temperature of 720 K. While we have not been able to synthesize the vanadium analog of the n = 2 member of the Aurivillius family, we have examined the structure and properties of a vanadate of the composition Bi2V3O9.

Cross-links among long chains have been observed in ion bombarded hydrocarbon polymers like polystyrene or polyethylene. Irradiation of fluoropolymers, instead, produces a strong sample erosion with emission of fragments produced along the ion track. Polytetrafluoroethylene foils of thickness ranging from 50 μm up to 2 mm were exposed to MeV helium and proton beams. The ion erosion rate was investigated by changing the target temperature and observing surface topography modifications, using the scanning electron microscopy technique. Etching was measured as removed thickness per irradiation time in the range values of 102−104 μm/h, corresponding to about 104−106 CF2 emitted group/ion. Target temperature (200–400 K), ion mass, and ion energy are the key parameters to follow the evolution of ion erosion of polytetrafluoroethylene.

A serious problem during the fabrication of composite materials by isothermal chemical vapor infiltration is that the matrix forms more rapidly at the external edges of the body and traps a large amount of porosity inside. In theory, this problem can be eliminated by controlling the gas-phase kinetics to obtain densification which is more rapid in the center of a preform than at its outer surfaces. An analysis of a first-order gas-phase reaction followed by a first-order deposition reaction indicates that improved infiltration is possible under a relatively narrow range of conditions.