J. Grilhé

Atomic force microscopy of in situ deformed nickel thin films

The mechanical behaviour of thin metal films on substrate under stress and particularly the analysis of the first stage of buckling have been characterized. Nickel/polycarbonate samples have been studied using a specific atomic force microscopy which allows the observation in situ of the sample surface during deformation. Straight wrinkle-like shapes are induced in the Ni thin film above a critical stress perpendicular to the compression axis. Undulations of very low amplitude appear also on these debonded regions. The dependence on stress of the shape of the straight wrinkles and of the undulations is discussed. It is shown that theses experiments may be thought of as an alternative method to estimate the localised internal stress s i and the adhesion energy G between the film and substrate. q 1999 Elsevier Science S.A. All rights reserved.

High electronic excitations and ion beam mixing effects in high energy ion irradiated Fe/Si multilayers

Mossbauer spectroscopy (57Fe) shows evidence for mixing effects induced by electronic energy deposition in nanoscale Fe/Si multilayers irradiated with swift heavy ions. A decrease in the mixing efficiency with electronic stopping power is reported; a threshold is found, under which iron environment modifications no longer occur. The kinetics of Fe–Si phase formation after irradiation suggests the existence of three regimes: (i) for high excitation levels, a magnetic amorphous phase is formed directly in the wake of the incoming ion and an almost complete mixing is reached at low fluence (1013 U/cm2); (ii) for low excitation levels, a paramagnetic Si-rich amorphous phase is favored at the interface while crystalline iron subsists at high fluences; (iii) for intermediate excitation levels, saturation effects are observed and the formation rate of both magnetic and paramagnetic phases points to direct mixing in the ion wake but with a reduced track length in comparison to U irradiation. The measured interfac...

Dislocation nucleation from surface steps: Atomistic simulation in aluminium

Abstract The possible role of surface steps in the nucleation of dislocations from a free surface has been studied by means of a static atomistic simulation using a many-body potential for aluminium. The fcc crystal with a {100} free surface containing a monatomic step lying along a (110) dense direction is submitted to an increasing uniaxial stress along a direction belonging to the {100} plane. For a sufficiently high applied stress, well below the theoretical strength. dislocations are nucleated at the step and glide in the {111} planes emerging at the step. The effect of a stress orientation is examined. The type of dislocation formed. that is Shockley partials of 90° and 30° character or perfect dislocations, is rationalized by considering the resolved shear stress in the {111} planes. The plane where glide will occur is favoured well before nucleation; a shear of increasing amplitude and extension is progressively localized on this plane. The role of the stress field due to the step, in the formation of the localized shear, is discussed.

Low-temperature mixing in Cu/W superlattices irradiated with light and heavy ions

Ion irradiation effects, induced at low temperature in Cu/W superlattices prepared by ion sputtering, were studied and compared using in situ resistivity measurements and x‐ray diffraction. The influence of the particle mass was particularly investigated in order to measure the role of spike effects in this immiscible system. It is shown that a more important mixing can be obtained with light‐ion irradiation. Nevertheless, the contribution of ‘‘thermally’’ activated jumps opposing ballistic effects is significant even when small and weakly energetic cascades are formed. A dependence of the microstructural state and of the compositional substructure on the irradiating particle is also evidenced. Light‐ion irradiation favors grain growth, restores grain texture, and preserves the composition modulation.

Atomic force microscopy observations of successive damaging mechanisms of thin films on substrates under tensile stress

Atomic force microscopy has been used to characterize the evolution of cracks in a 100 nm thick nickel film on a polyimide substrate under increasing tensile stresses. Successive damaging mechanisms are described and discussed by means of finite element calculations.

Tribological behaviour of some alloys after surface modification using ion beam techniques

Abstract Two types of ion beam surface modifications have been performed. 1. (i) Superficial amorphization has been induced in an industrial NiTi shape memory alloy by direct ion implantation. A remarkable increase in the wear resistance and exceptional reduction in the friction coefficient has been observed for an implantation of 3 × 10 17 N + cm -2 . 2. (ii) An FeAl coating 1 μm thick on steel has been obtained by iron and aluminium evaporation simultaneously with argon ion irradiation. This coating significantly improves the tribological behaviour of this steel and presents an exceptional adherence to the substrate, which is not so if the coating is performed without ion assistance.

Orientation of H platelets under local stress in Si

Hydrogen is implanted into (001) silicon under the strain field of previously formed overpressurized helium plates. Upon thermal annealing, the hydrogen atoms precipitate into platelet structures oriented within specific {111} or {001} variant determined through the local symmetry of the strain. The behavior is understood in terms of elastic interactions and is described via energy minimization calculations, predicting the formation and distribution of each platelet orientation variant. Our results demonstrate the concept that sublocal organized arrangements of precipitates can be obtained within nanosize domains using local strain fields.

Morphological instabilities of a stressed pore channel

Abstract Under surface or bulk diffusion, a pore channel can lose its cylindrical shape by Rayleigh instabilities, transforming into cavities to minimize surface energy. When the solid is stressed, the surface fluctuations relax partially the stored elastic energy and increase the instability. Calculations show that the new critical wavelength is shifted continuously towards shorter values as the stress is increased. Surface diffusion considerations permit the determination of the most probable wavelength to appear and the increase of instability growing rate with applied stress.

Molecular dynamics simulations of buckling-induced plasticity

Molecular dynamics simulations of thin film buckling demonstrate that high compressive stresses are localized in the straight-sided wrinkle leading to the nucleation of dislocations, which are mobile and plastically release a large amount of the strain. As a consequence, the maximum deflection determined in the framework of elasticity theory is found to be overestimated. Finally, the resulting plastic deformation has been estimated and included in the expression of the deflection.

Quantitative analysis of surface effects of plastic deformation

Single crystals of γ phase nickel based alloy MC2 and of LiF have been studied using an experimental apparatus offering the continuous observation of the slip line development during plastic deformation. Taking advantage of the high resolution of atomic force microscopy, the fine structure of slip lines has been investigated showing drastic differences between the two materials. A statistical analysis of slip line patterns has been carried out to compare deformation mechanisms taking place in the bulk crystals and quantitatively study the surface effects of plastic deformation, with the determination of a few relevant parameters, such as the total number of dislocations emerging at the sample surface, the average dislocation per slip line and the distribution of terrace widths.