Adhesion forces due to nano-triboelectrification between similar materials

Contact electrification and triboelectrification are well-known in the case of dissimilar materials, however the case of charge exchange during friction between nominally identical insulating materials is less documented. We experimentally investigated the triboelectrification between two smooth monocrystalline α-Al 2O 3 (sapphire) antagonists by surface force measurements with a Surface Force Apparatus (SFA). The force between a sphere and a plane, both in sapphire, was measured as a function of the sphere-plane distance D, before and after nano-friction tests, under dry argon atmosphere. Respective contributions of van der Waals, water meniscus and electrostatic forces were determined. The estimated Hamaker constant was in good agreement with the Lifshitz theory, and the dominant meniscus attraction at low separation could be overcome with small radius sphere. We demonstrated that electrostatic forces were generated by the nano-friction test and we quantified the adhesion that results from this contact-electrification. In the first stage of the unloading process, the short range electrostatic force was found to vary both with time and distance D. Experimental results were correlated with surface densities of mobile charges on the two surfaces, and the time-dependence was related to classical surface transport phenomena on alumina surfaces

Electric charge trapping, residual stresses and properties of ceramics after metal/ceramics bonding

International audienceThe use of ceramic components in electrical engineering and mechanical applications is rapidly increasing. Most of these applications require the use of ceramics bonded with metal. In this paper,we have studied the role of residual stresses occurring after joining between an industrial alumina ceramic (Al2O3) and Ni-based super-alloy, on the dielectric behaviour of ceramics. The electric charging phenomenon i.e. trapping-detrapping or diffusion of electric charges is studied by Scanning Electron Microscope Mirror Effect (SEMME) coupled with the Induced Current Method (ICM). Knowing that localized trapped charges in ceramics is a source of damage, the correlation between residual stress intensity, apparent-toughness of ceramics and ability to trap charges near the interface was demonstrated: the SEMME and ICM measurements of the quantities of trapped charges near the interface, highlighted the changes in the ceramic properties related to residual stresses due to both thermo-mechanical effect and diffusion of metallic species in the ceramics, during the bonding process

Relationships between microstructure, mechanical and dielectric properties of different alumina materials

International audienceDifferent alumina materials were elaborated in order to vary microstructural parameters (grain size, densification, porosity, inter-granular phase). These ceramic materials were then characterized from the mechanical point of view (hardness, toughness, friction and wear) and dielectric breakdown. The comparison of these various results shows that, for all these properties, the grain size and also, the nature of the secondary phases and the microstructural parameters were the most significant. Moreover, from the tribological point of view, the dielectric characteristic of materials (breakdown strength) has a fundamental role in the creation of agglomerated wear debris (“third body”) and its properties: a finely agglomerated third body will be obtained for high breakdown strength. Such third body will be able to protect the substrate and thus to reduce later wear. In the same logic a correspondence between breakdown strength and toughness was established, thus confirming the existence of mechanical–electrical correlation for non-conductive materials

Analysis of electron transfer between electron irradiated metallic ball and insulators in vacuum: A specific alternative to the mirror method

International audienceIn order to improve the knowledge of dielectric properties of insulators, we have imagined an original method of characterization of the charge buildup. Electrons of an electron beam are implanted through a metallic ball directly in contact with the insulator in a scanning electron microscope. By calculating and modeling the capacitance and the electrostatic force between the ball and the insulator plane, it has been possible to determine the relationship between the injected charges in the metallic ball and its surface potential. The major role of the dielectric thickness has been evidenced when the insulator is placed on a grounded metallic plane. At high potential values, a dielectric breakdown of the medium surrounding the sphere occurs and electrical charges are transferred from the ball to the dielectric sample. This transfer has been evidenced and quantified in the case of sapphire and quartz. Analytical calculations and numerical simulations using the finite-element method have been performed for interpreting these experimental results

Effect of thermal residual stresses on the strength for both alumina/Ni/alumina and alumina/Ni/nickel alloy bimaterials

International audienceThis paper describes some technical limitations encountered in joining ceramics-ceramics or ceramics-metals, and how, to some extent, they have been practically overcome. The effect of the residual stresses on the strength of joints fabricated between alumina-alumina or alumina and the nickel base alloy HAYNES 214TM using a solid-state bonding technique with Ni interlayer was studied. Finite element analyses (FEA) for the elastic-plastic and elastic-plastic-creep behavior have also been used to better design the joints and to predict their performance. It was found that the residual stresses caused by the thermal expansion mismatch between alumina (Al2O3) and the Ni-based superalloy (HAYNES 214TM) have severely deteriorated the joints compared to Al2O3-Al2O3 joint fabricated with the same solid-state bonding parameters. The high residual stresses zones obtained through the FEA simulation fitted well with the fractographic observations of the Al2O3/Ni/HAYNES 214TM joints. Also, in order to use the joint material as a structural material, the study about the effect of geometrical parameters has been performed. Optimal geometries have been determined

FEM CALCULATIONS AND EXPERIMENTAL DETERMINATION OF RESIDUAL STRESSES IN ALUMINA/NICKEL ALLOY JOINTS. OPTIMIZATION OF FABRICATION PARAMETERS

International audienceThe study relates to joints fabricated by solid state bonding between alumina and nickel alloy HAYNESTM214®, using an intermediate nickel metallic foil. Experimentally, damages and cracks often are observed close to the metal/ceramics interface. Consequently, the residual stresses distributions in the specimen were characterized experimentally using X-ray diffraction (XRD) and indentation techniques and predicted by Finite Element Analysis (FEA) calculations using an elastic-plastic-creep model. We demonstrate that a good correlation between FEA calculations and experimental results is obtained. Then, the effect of elaboration and geometrical parameters has been studied in order to minimize the residual stresses in alumina close to the metal-ceramics interface. However, the Al2O3/Ni/HAYNESTM214® system always leads to high residual stresses. To solve this problem, we show that the use of a multi-layer Cu/Ni/Cu joint, associated with the Direct Copper Bonding method (DCB), by pre-oxidation of copper, allows reducing significantly the tensile residual stresses in ceramics

Review of in vitro studies on the biocompatibility of NiTi alloys

International audienceNickel-Titanium (NiTi) alloys exhibit special mechanical properties known as shape memory effect and superelasticity. The first effect relates to the fact that after deformation at low temperature, the material regains its original shape after heating. The second effect denotes an unusual flexibility of the material that is far greater than the flexibility of common metallic materials. These properties can be put to excellent use in various biomedical applications, such as orthodontic wires, orthopaedic implants for osteosynthesis, stents for various applications, bone substitution materials and experimental systems for scoliosis correction. The purpose of the present review is to analyze in vitro NiTi biocompatibility compared to other titanium alloys. The problem of Ni's possible toxicity is discussed. Corrosion properties in an oral environment are investigated. The effect of surface chemical composition and surface roughness on cell adhesion, morphology and proliferation is shown as an example. Sterilization is taken into account and its influence on roughness and mechanical properties, chemical composition, wettability and bacterial adhesion inhibition is evaluated. The conclusion is that NiTi is at least as biocompatible as conventional titanium due to the TiO2 oxide layer, and that only in special chemical conditions can Ni diffuse out of the NiTi material

Study of SiC-nickel alloy bonding for high temperature applications

International audienceWe have studied the formation of metal/ceramic joints by solid state bonding technique for applications at temperatures >600 ◦C. The bonding is obtained between silicon carbide (SiC) and Ni-based super-alloy (HAYNES® 214TM) via metallic foils (Ni, Ag). In some cases a thin coating on the ceramic or the alloy by the electroless JetMétalTM process has been used. Often used in brazing, nickel, when added to silicon carbide, usually give silicides. These reactions yield the "Pest Effect" ("pesting") that induces a catastrophic brittleness of this type of assembling. To minimize the reaction of these metals with silicon carbide, addition of elements limiting the "Pest Effect" on the one hand and, diffusion barriers on the other hand, have been performed. Indeed, the choice of the thin Ni0.93 B0.07 coating is based on the ability of boron of improving the mechanical properties of silicides, thus avoiding the "Pest Effect". However, we demonstrate that boron does not allow one to suppress the joint brittleness. Another new joining method employing a thin Ag coating or a Ag foil was tested. This process revealed the absence of chemical reaction at the Ag/SiC interface, thus proving the beneficial role of silver, which acts as an effective diffusion barrier for nickel beyond a certain thickness. This method has led to fabrication of joints presenting high shear resistance (>40MPa)

Adhesion forces due to nano-triboelectrification between similar materials

International audienceContact electrification and triboelectrification are well-known in the case of dissimilar materials, however the case of charge exchange during friction between nominally identical insulating materials is less documented. We experimentally investigated the triboelectrification between two smooth monocrystalline α-Al 2O 3 (sapphire) antagonists by surface force measurements with a Surface Force Apparatus (SFA). The force between a sphere and a plane, both in sapphire, was measured as a function of the sphere-plane distance D, before and after nano-friction tests, under dry argon atmosphere. Respective contributions of van der Waals, water meniscus and electrostatic forces were determined. The estimated Hamaker constant was in good agreement with the Lifshitz theory, and the dominant meniscus attraction at low separation could be overcome with small radius sphere. We demonstrated that electrostatic forces were generated by the nano-friction test and we quantified the adhesion that results from this contact-electrification. In the first stage of the unloading process, the short range electrostatic force was found to vary both with time and distance D. Experimental results were correlated with surface densities of mobile charges on the two surfaces, and the time-dependence was related to classical surface transport phenomena on alumina surfaces