Bart Blanpain

Surface oxidation of NiTi shape memory alloy

Mechanically polished NiTi alloy (50 at% Ni) was subjected to heat treatment in air in the temperature range 300-800 degrees C and characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Thermogravimetry measurements were carried out to investigate the kinetics of oxidation. The results of thermodynamic calculations were compared to the experimental observations. It was found that NiTi alloy exhibits different oxidation behaviour at temperatures below and above 500 degrees C. A Ni-free zone was found in the oxide layer for oxidation temperatures of 500 degrees C and 600 degrees C. The oxidation at 500 degrees C produces a smooth protective nickel-free oxide layer with a relatively small amount of Ni species at the air/oxide interface, which is in favour of good biocompatibility of NiTi implants. The oxidation mechanism for the NiTi shape memory alloy is discussed.

Calibration procedures for frictional measurements with a lateral force microscope

This paper outlines a procedure for the calibration of the lateral force applied to a triangular cantilever tip by means of a lateral force microscope. The force is directed perpendicular to the cantilever major axis. The lateral force results in a torsion of the cantilever. The torsion signal is monitored as a voltage on a four-sector photodiode. The conversion from a torsion signal to a lateral force depends on the optical geometry of the instrument and the cantilever shape, dimensions and material properties. The calibration procedure used requires a calculation of the elasticity of the cantilever and experimental data. In case tripping forces are ignored, the lateral force can be taken as the friction force.

Lattice Boltzmann modeling of microchannel flow in slip flow regime

We present the lattice Boltzmann equation (LBE) with multiple relaxation times (MRT) to simulate pressure-driven gaseous flow in a long microchannel. We obtain analytic solutions of the MRT-LBE with various boundary conditions for the incompressible Poiseuille flow with its walls aligned with a lattice axis. The analytical solutions are used to realize the Dirichlet boundary conditions in the LBE. We use the first-order slip boundary conditions at the walls and consistent pressure boundary conditions at both ends of the long microchannel. We validate the LBE results using the compressible Navier-Stokes (NS) equations with a first-order slip velocity, the information-preservation direct simulation Monte Carlo (IP-DSMC) and DSMC methods. As expected, the LBE results agree very well with IP-DSMC and DSMC results in the slip velocity regime, but deviate significantly from IP-DSMC and DSMC results in the transition-flow regime in part due to the inadequacy of the slip velocity model, while still agreeing very well with the slip NS results. Possible extensions of the LBE for transition flows are discussed.

Plasma-enhanced chemical vapour deposition growth of Si nanowires with low melting point metal catalysts: an effective alternative to Au-mediated growth

Au nanoparticles are efficient catalysts for the vapour?solid?liquid (VLS) growth of semiconductor nanowires, but Au poses fundamental reliability concerns for applications in Si semiconductor technology. In this work we show that the choice of catalysts for Si nanowire growth can be broadened when the need for catalytic precursor dissociation is eliminated through the use of plasma enhancement. However, in this regime the incubation time for the activation of VLS growth must be minimized to avoid burying the catalyst particles underneath an amorphous Si layer. We show that the combined use of plasma enhancement and the use of a catalyst such as In, already in a liquid form at the growth temperature, is a powerful method for obtaining Si nanowire growth with high yield. Si nanowires grown by this method are monocrystalline and generally oriented in the direction.

Oxidational wear of TiN coatings on tool steel and nitrided tool steel in unlubricated fretting

The fretting wear behaviour of PVD TiN coatings against corundum has been investigated in unlubricated contacts. Analysis of the debris indicated that titanium oxides of different composition are being generated. The volumetric wear on the TiN coatings is shown to be directly proportional to the dissipated friction energy measured on-line during the fretting tests. Furthermore, the effect of oxide debris present in the vibrating contact on the velocity accommodation and, consequently, on the coefficient of friction is discussed. The hardness of some of the ASP 23 tool steel substrates was increased with a plasma nitriding treatment prior to the coating deposition. It was found that the coefficient of friction and the fretting wear rate of the TiN coatings remain unaltered by the plasma nitriding treatment. After perforation of the TiN coating, however, wear proceeds faster on nitrided tool steel substrates.

The influence of humidity on the fretting behaviour of PVD TiN coatings

Abstract The influence of the relative humidity (RH) in ambient air on the friction and wear behaviour of PVD TiN coatings subjected to contact vibrations against corundum and bearing steel (100Cr6) counterbodies has been investigated. The fretting experiments were performed in the gross-slip regime on TiN coatings produced by three different PVD processes. The results indicate two basic friction characteristics. At low relative humidity (RH 80%). A transition from high to low friction was observed during the course of experiments performed in atmospheres of medium relative humidity. The duration of the high friction phase in such transitions was found to depend on fretting parameters such as the normal force and the vibration frequency. This humidity dependence of the friction force was found for both counterbody materials. The size of the damaged surface area as well as the volumetric wear on the TiN coatings were found to be largest at low relative humidity. Fretting damage occurs over a smaller area but extends more into the depth at high relative humidity. The size of the fretted surface area induced on TiN is larger for Cr-steel than for corundum counterbodies.

R.f. plasma-assisted chemical vapour deposition of diamond-like carbon: physical and mechanical properties

Abstract Hydrogenated amorphous diamond-like carbon coatings have been deposited on glass and silicon substrates using an r.f. plasma-assisted chemical vapour deposition process. The mechanical properties of these coatings such as stress (bending beam measurements), hardness and Youngs modulus (nanoindentation measurements) were studied as a function of the r.f. bias voltage in the range 50–500 V. IR absorption spectroscopy and elastic recoil detection analysis were used to correlate these mechanical properties with the total hydrogen content and the hydrogen bonding configuration. Compressive stress, hardness and Youngs modulus are shown to reach their maximum values simultaneously at a bias voltage of about 200 V. At the same time, the hydrogen-bonded sp 3 :sp 2 ratio reaches a local minimum.

Comparative study between macrotribology and nanotribology

The behavior of different materials under macrotribological and nanotribological conditions has been compared. The materials, hydrogenated amorphous diamondlike carbon, highly ordered pyrolitic graphite, and mica, were submitted to a fretting test (macrotribological behavior) and a combined atomic force microscopy/lateral force microscopy (AFM/LFM) (nanotribological behavior). The coefficient of friction in the macroscopic regime under fixed experimental conditions considerably changed with the test duration, whereas the friction coefficient measured from the nanoscopic regime was relatively constant during the test. In the macroscopic regime, evident wear, elastoplastic deformation, and material transfer were noticed, while in the nanoscopic regime under the used testing conditions no wear phenomena were observed. The coefficient of friction showed some dependence on the amplitude and frequency of lateral movement of the sample relative to the counterbody in both the fretting and AFM/LFM tests as well as...

From NdFeB magnets towards the rare-earth oxides: a recycling process consuming only oxalic acid

A chemical process which consumes a minimum amount of chemicals to recover rare-earth metals from NdFeB magnets was developed. The recovery of rare-earth elements from end-of-life consumer products has gained increasing interest during the last few years. Examples of valuable rare earths are neodymium and dysprosium because they are important constituents of strong permanent magnets used in several large or growing application fields (e.g. hard disk drives, wind turbines, electric vehicles, magnetic separators, etc.). In this paper, the rare-earth elements were selectively dissolved from a crushed and roasted NdFeB magnet with a minimum amount of acid, further purified with solvent extraction and precipitated as pure oxalate salts. The whole procedure includes seven steps: (1) crushing and milling of the magnet into coarse powder, (2) roasting to transform the metals into the corresponding oxides, (3) the selective leaching of the rare-earth elements with acids (HCl, HNO3) to leave iron behind in the precipitate, (4) extracting remaining transition metals (Co, Cu, Mn) into the ionic liquid trihexyl(tetradecyl)phosphonium chloride, (5) precipitating the rare earths by the addition of oxalic acid, (6) removing the precipitate by filtration and (7) calcining the rare-earth oxalates to rare-earth oxides which can be used as part of the feedstock for the production process of new magnets. The magnet dissolution process from the oxides utilized four molar equivalents less acid to dissolve all rare earths in comparison with a dissolution process from the non-roasted magnet. Moreover, the less valuable element iron is already removed from the magnet during the dissolution process. The remaining transition metals are extracted into the ionic liquid which can be reused after a stripping process. Hydrochloric acid, the side product of the rare-earth oxalate precipitation process, can be reused in the next selective leaching process. In this way, a recycling process consuming only air, water, oxalic acid and electricity is developed to recover the rare earths from NdFeB magnets in very high purity.

A model of threading dislocation density in strain-relaxed Ge and GaAs epitaxial films on Si (100)

Strain relaxation in large lattice-mismatched epitaxial films, such as Ge and III-V materials on Si, introduces high threading dislocation densities (TDDs). A thermodynamic model of TDD dependence on film thickness is developed. According to this model, the quasiequilibrium TDD of a given strain-relaxed film scales down with the inverse square of its thickness. The quasiequilibrium TDDs in both Ge and GaAs films follow this model consistently. Our model predicts the lowest possible TDD of a large lattice-mismatched film on Si (100), which is determined by the dislocation glide activation energy and the film thickness.