Stefano Mischler

Micromechanics of Amorphous Metal/Polymer Hybrid Structures with 3D Cellular Architectures: Size Effects, Buckling Behavior, and Energy Absorption Capability

By designing advantageous cellular geometries and combining the material size effects at the nanometer scale, lightweight hybrid microarchitectured materials with tailored structural properties are achieved. Prior studies reported the mechanical properties of high strength cellular ceramic composites, obtained by atomic layer deposition. However, few studies have examined the properties of similar structures with metal coatings. To determine the mechanical performance of polymer cellular structures reinforced with a metal coating, 3D laser lithography and electroless deposition of an amorphous layer of nickel-boron (NiB) is used for the first time to produce metal/polymer hybrid structures. In this work, the mechanical response of microarchitectured structures is investigated with an emphasis on the effects of the architecture and the amorphous NiB thickness on their deformation mechanisms and energy absorption capability. Microcompression experiments show an enhancement of the mechanical properties with the NiB thickness, suggesting that the deformation mechanism and the buckling behavior are controlled by the brittle-to-ductile transition in the NiB layer. In addition, the energy absorption properties demonstrate the possibility of tuning the energy absorption efficiency with adequate designs. These findings suggest that microarchitectured metal/polymer hybrid structures are effective in producing materials with unique property combinations.

In-situ generated tribomaterial in metal/metal contacts: Current understanding and future implications for implants

Artificial hip joints operate in aqueous biofluids that are highly reactive towards metallic surfaces. The reactivity at the metal interface is enhanced by mechanical interaction due to friction, which can change the near-surface structure of the metal and surface chemistry. There are now several reports in the literature about the in-situ generation of reaction films and tribo-metallurgical transformations on metal-on-metal hip joints. This paper summarizes current knowledge and provides a mechanistic interpretation of the surface chemical and metallurgical phenomena. Basic concepts of corrosion and wear are illustrated and used to interpret available literature on in-vitro and in-vivo studies of metal-on-metal hip joints. Based on this review, three forms of tribomaterial, characterized by different combinations of oxide films and organic layers, can be determined. It is shown that the generation of these tribofilms can be related to specific electrochemical and mechanical phenomena in the metal interface. It is suggested that the generation of this surface reaction layer constitutes a way to minimize (mechanical) wear of MoM hip implants.

Tribocorrosion in Pressurized High-Temperature Water: A Mass Flow Model Based on the Third-Body Approach

AbstractnPressurized water reactors (PWR) used for power generation are operated at elevated temperatures (280–300xa0°C) and under high pressure (120–150xa0bar). In addition to these harsh environmental conditions some components of the PWR assemblies are subject to mechanical loading (sliding, vibration and impacts) leading to undesirable and hardly controllable material degradation phenomena. In such situations wear is determined by the complex interplay (tribocorrosion) between mechanical, material and physical–chemical phenomena. Tribocorrosion in PWR conditions is at present little understood and models need to be developed in order to predict component lifetime over several decades. This paper present an attempt to model PWR tribocorrosion through the combination of a tribological third-body approach with a mechanistic description of the involved flows and the mass balance compartments corresponding to well-defined loci of the contact. The obtained model permits to gain better insight in the phenomenology and in the mechanisms of tribocorrosion of metals in PWR conditions. It also allows assessing the relative role of a variety of materials, mechanical and electrochemical parameters affecting the entire system. Quantitative predictions of the model were found to fit reasonably well experimental observations

Orientation-dependent mechanical behaviour of electrodeposited copper with nanoscale twins

The mechanical properties of electrodeposited copper with highly-oriented nanoscale twins were investigated by micropillar compression. Uniform nanotwinned copper films with preferred twin orientations, either vertical or horizontal, were obtained by controlling the plating conditions. In addition, an ultrafine grained copper film was synthesized to be used as a reference sample. The mechanical properties were assessed by in situ SEM microcompression of micropillars fabricated with a focused ion beam. Results show that the mechanical properties are highly sensitive to the twin orientation. When compared to the ultrafine grained sample, an increase of 44% and 130% in stress at 5% offset strain was observed in quasi-static tests for vertically and horizontally aligned twins, respectively. Inversely strain rate jump microcompression testing reveals higher strain sensitivity for vertical twins. These observations are attributed to a change in deformation mechanism from dislocation pile-ups at the twin boundary for horizontal twins to dislocations threading inside the twin lamella for vertical twins.

Rationalizing the In Vivo Degradation of Metal-on-Metal Artificial Hip Joints Using Tribocorrosion Concepts

In this study, the in vivo degradation of metal-on-metal (MoM) artificial hip joints was assessed based on the present state of the art concepts of tribocorrosion. A recently developed tribocorrosion model, based on the combination of mechanical and corrosion concepts, was used in order to rationalize experimental observations and clinical outcomes. This analysis permitted one to identify and assess the relevance of crucial mechanical (load, velocity), material (hardness, Young’s modulus), chemical (oxidation valence, passivation charge density, electrode potential), and geometrical (head radius, clearance) parameters and synovial fluid viscosity affecting hip joint degradation. Moreover, the tribocorrosion approach taken here shows that in vivo degradation is highly dependent on individual patient features and reveals that in vivo characterization of the corrosion and chemical reactivity of implant materials as well as of synovial fluid properties are needed for predicting degradation of MoM implants.

Prediction of Removal Rates in Chemical–Mechanical Polishing (CMP) Using Tribocorrosion Modeling

The influence of electrochemical parameters, open circuit potential, and passivation kinetics, on the removal rates of tungsten in chemical–mechanical polishing (CMP) was investigated in the presented study. Removal rates were measured on a CMP machine, while electrochemical test were performed separately, both using technical CMP slurries. Electrochemical parameters are shown to have a strong influence on tungsten removal rate in CMP. Tungsten removal rate increases with the open circuit potential (electrode potential) and with passivation charge density. A model of Kaufman mechanism previously developed for sliding tribocorrosion of tungsten was adapted to the CMP conditions. This model describes removal rate in terms of both mechanical and electrochemical parameters and was found to effectively rationalize experimental results obtained in different CMP solutions according to their electrochemical properties.