High-temperature oxidation of nickel-based alloys and estimation of the adhesion strength of resulting oxide layers

The kinetics of isothermal oxidation (1100°C) of commercial nickel-based alloys with different content of sulfur (0.22–3.2 wt ppm) is studied. The adhesion strength in a metal/oxide system is estimated as a function of sulfur content and duration of high-temperature exposure. The scratch-test technique is proposed to quantitatively estimate the work of adhesion of resulting oxide films. It is found that the film microstructure is composed of an inner α-Al2O3 layer and an outer NiAl2O4 spinel layer, which are separated by discrete inclusions of TiO2. Residual stresses in the oxide film are experimentally determined by X-ray diffraction. spinel layer, which are separated by discrete inclusions of TiO2. Residual stresses in the oxide film are experimentally determined by X-ray diffractio

Effects of near-tip rotation on pre-buckle crack growth of compressed beams bonded to a rigid substrate

The macroscopic pre-cracked line scratch test (MPLST), in which a debonded edge of a film is loaded in in-plane compression, has been modeled as a generic, coupled fracture–buckle problem using simple beam theory. Near crack-tip beam rotation (also called root rotation in literature), which always exists due to the eccentric loading in this type of test, has been incorporated into the governing equations. An analytical solution to the augmented problem has been derived. It is found that the near-tip rotation can introduce pre-buckle bending in the film. One important consequence of this pre-buckle bending is that it leads to the reduction of the critical buckling condition. This agrees well with the results of [Int. J. Fract. 113 (2002) 39] obtained by solving the full elastic field near the crack-tip. Furthermore, the pre-buckle bending moment at crack-tip remains negative (leading to crack closure) as long as the pre-buckle crack length is small, but it becomes positive (leading to crack opening) at larger pre-buckle crack length. The negative bending moment causes the crack-tip energy release rate to decrease as the crack propagates, which results in a stable pre-buckle crack growth. Once it becomes positive, however, the bending moment causes crack-tip energy release rate to increase rapidly as crack length increases and hence leads to an unstable (pre-buckle) crack growth. Further, the nominal phase angle is initially larger than the classic prediction of 52.1° owing to the existence of the negative crack-tip bending moment, but it drops quickly upon approaching the buckle point. All these results are confirmed by a rigorous 2D FEM calculation using cohesive zone modeling (CZM) approach. Finally the derived analytical solution has been used to analyze a set of PLST data reported in the literature. It has been demonstrated that plasticity in the adhesive layer and in the bonded film is responsible for the strong R-curve toughening characteristics in the deduced interface toughness data. It has also been shown that, once the deduced interface toughness is incorporated into a CZM simulation, both the axial loading and buckling point can be accurately predicted

Ion beam-mixed Ge electrodes for high capacity Li rechargeable batteries

Ion beam modification to effect ion beam mixing without changing morphology was investigated as a means to improve the electrochemical performance of Ge thin film electrodes for rechargeable Li batteries. As a result of a minimum tenfold increase in the strength of adhesion of the Ge film to the current collector (substrate), the ion beam-mixed electrodes exhibited stable specific capacities of ∼1500 mAh g -1 (close to the theoretical maximum of 1623 mAh g -1) for galvanostatic cycling rates of 0.2C-1.6C using both single- and multi-rate testing schemes. Electron microscopy investigations showed that the ion beam-mixed electrodes transform from a flat, continuous, nonporous microstructure in the virgin state to a rough, cracked, porous microstructure as a result of electrochemical cycling, but remain in excellent electrical contact with the current collector. The results suggest that ion beam mixing could be used to produce inexpensive, high capacity conversion electrodes for rechargeable Li batteries