The excellent biocompatibility of titanium and its alloys used, for example, for medical devices, is associated with the properties of their surface oxide. For a better understanding of the tissue reaction in contact with the oxide layer, knowledge of the chemical and physical properties of this layer is of increasing interest. In this study, titania films were produced on cp-Ti and Ti6Al4V substrates by thermal oxidation, anodic oxidation, and by the sol-gel process. The thickness and structure of the films produced under different conditions were determined by ellipsometry, infrared spectroscopy, and X-ray diffraction measurements. The corrosion properties of these layers were investigated by current density-potential curves under physiological conditions. The oxide layers produced on cp-Ti and Ti6Al4V by thermal oxidation consist of TiO(2) in the rutile structure. For the anodized samples the structure of TiO(2) is a mixture of amorphous phase and anatase. The structure of the coatings produced by the sol-gel process for a constant annealing time depends on the annealing temperature, and with increasing temperature successively amorphous, anatase, and rutile structure is observed. Compared to the uncoated, polished substrate with a natural oxide layer, the corrosion resistance of cp-Ti and Ti6Al4V is increased for the samples with an oxide layer thickness of about 100 nm, independent of the oxidation procedure.

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The slow β relaxation is understood to be a universal feature of glassy dynamics. Its presence in bulk metallic glasses (BMGs) is evidence of a broad relaxation time spectrum that extends to deep within the glassy state. Despite the breadth of research devoted to this phenomenon, its microscopic origin is still not fully understood. The low-temperature aging behavior and atomic structural rearrangements of a Au 49 Cu 26.9 Si 16.3 Ag 5.5 Pd 2.3 BMG are investigated in the regime of the slow β relaxation by employing an ensemble of experimental techniques such as high-intensity synchrotron x-ray scattering, modulated differential scanning calorimetry (MDSC), dynamic mechanical analysis (DMA), impulse excitation, and dilatometry. Evidence of a distinct slow β-relaxation regime is seen in the form of (1) an excess wing of the DMA loss modulus beginning at ∼50• C, (2) a crossover effect of elastic modulus with isothermal aging at 50• C, and (3) a broad, nonreversing and largely irreversible sub-T g endotherm in the MDSC results. Atomic rearrangements occurring at the onset of the measured slow β-relaxation temperature regime were found to be confined mainly to the short-range order length scale while no significant atomic rearrangements occur on the length scale of the medium-range order. Furthermore, evidence is presented that suggests the crossover effect in Young's modulus is due to the evolution of chemical short-range order. These results support the emergent picture of a dynamically heterogeneous glassy structure, in which low-temperature relaxation occurs through atomic rearrangements confined mostly to the short-range order length scale.

ABSTRACT:The thermal aging of an amine-cured epoxy in the glassy state is studied for two network states by using DSC and attenuated total reflection-infrared (IR-ATR). The "low-crosslinked" network possesses a relatively high molecular mobility and a considerable amount of residual reactive groups. In the low crosslinked matrix, the presence of high crosslinked regions is revealed. In contrast, the "highly crosslinked" epoxy system has a reduced molecular mobility and only small reactive groups. The high crosslinked matrix contains low crosslinked regions. Thermal loading for both networks is performed below their glass transition. During thermal aging, an ongoing curing reaction takes place in the low-crosslinked epoxy. Thermooxidative degradation and the disintegration of short-range ordering are observed as well. The highly crosslinked epoxy system undergoes a phase separation of relatively mobile segments in the low mobile matrix, which is a reversible process on heating. Thermooxidative degradation is also detected for this kind of network. In summary, for the "low" and the "highly" crosslinked epoxy, significant chemical and structural changes take place during thermal aging even though the networks are vitrified. It is convincing that these changes in the cured epoxy should exert an influence on the mechanical properties of a bonded structure.

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