S. Fayeulle

Cracking behaviour of various aluminium alloys during fretting wear

Abstract Fretting wear tests were performed on several aluminium alloys (2024, 2091, 7075) in air. The normal load ranged from 200 to 1000 N and the imposed displacement from ± 15 to ± 100 μm. The tangential force was recorded during each cycle of the fretting test and plotted as a function of the displacement (F-D cycle). Three kinds of F-D cycle were observed depending on the fretting conditions: the closed, elliptic and parallelepipedic cycle. Friction logs (i.e. a three-dimensional representation of the F-D cycles for a given test) are used to describe fretting regimes and fretting transitions. Optical examination of the superficial layers of the samples was performed after each fretting test to characterize the material response. Two degradation modes, cracking and particle detachment, were observed. They were achieved generally simultaneously but, depending on the fretting regime, one or the other was predominant when comparing the behaviour of the three alloys. Results of crack initiation and propagation were found to fit well the classical fatigue properties of the three aluminium alloys. Wear debris originated from the transformed layer, whatever the metal alloy. The results are discussed in term of fretting maps: a running conditions fretting map which describes the fretting regime vs. the fretting conditions (load and displacement), and a material response fretting map which allows one to define the domains of degradation (no degradation, cracking, particle detachment) vs. the fretting conditions.

New insights into adhesion and lubricating properties of graphite-based transfer films

Abstract Thin transfer films were formed on steel surfaces during friction against graphite-based pins. Sodium thiosulfate (Na 2 S 2 O 3 ) was added to graphite to improve the film formation. The lubricating behaviour of these transfer films was tested during friction experiments against a steel block. Microstructural analyses were performed using classical θ/2θ and grazing incidence X-ray diffraction, Raman and micro-Raman spectroscopy, and transmission electron microscopy. Transformations of the graphite structure were observed. In particular, a turbostratic phase was identified in the thinner layers of the transfer films. To study the influence of the gaseous environment, friction tests were performed in ambient air, dry air and dry nitrogen. The friction coefficient was shown to increase dramatically from 0.03 to more than 0.11 in the presence of water vapor as the endurance life decreased. The apparent shear resistance increased with humidity from 10 MPa up to about 18 MPa. An explanation in terms of interfacial thin film shear is given. In ambient air, blisters and fractures were seen, revealing that the entire film has reacted to accommodate velocity.

Tribological behaviour of nitrogen-implanted materials

Abstract Nitrogen implantation is sometimes effective in the improvement of the friction behaviour of materials. Some examples (high speed steel, tungsten carbide and titanium alloy) are presented. They allow us to underline the importance of the different parameters (friction and implantation conditions). The understanding of mechanisms which appear during friction has not yet been achieved and numerous hypotheses have been proposed. One of them, i.e. nitrogen migration during wear, is discussed in relation to parameters such as the ion fluence and the type of material. It is shown that this migration occurs over very short distances but this is not enough to explain entirely the beneficial role of nitrogen implantation.

Role of dielectric properties in the tribological behaviour of insulators

Abstract The friction properties of single-crystal alumina were investigated under dry conditions. The relative humidity was kept to less than 1% and the contact pressure was chosen to be very small to prevent the formation of wear debris at the interface. The tangential force and acoustic emission were recorded continuously during tests. The dielectric properties of samples were characterized before and after tribological tests using scanning electron microscopy. The effect of X-ray irradiation on the dielectric and friction properties was investigated. Sapphire samples annealed at 1500 °C did not charge, but after X-ray irradiation the charging capacity was highly increased, as was the friction coefficient (by a factor of 4). After the friction tests, the charging capacity of sapphire was observed both inside and outside the wear track. X-ray irradiation of the samples outside the wear track also modified the friction behaviour. Based on the results, others from the literature and on the increasingly understood correlation between the mechanical and electrical properties of dielectrics, an energetic explanation of the friction and wear behaviour of insulator materials is proposed. Friction and wear are shown to be related to the mechanisms of storage and dissipation of energy. Because of the dielectric properties of insulators, this energy results from electrostatic interactions: polarization of the material and displacements of electrical charges. Polarization is quickly achieved and increases during friction because of the build-up of a space charge in the material owing to the trapping of charge carriers. This trapping of defects already present in materials before testing or created during friction allows the storage in the lattice of very high amounts of energy (5 eV or more per charge) which, when dissipated, can lead to catastrophic failure.

Influence of the microstructure of plasma deposited MCrAlY coatings on their tribological behaviour

Abstract MCrAlY coatings (CoNiCrAlY) were thermal sprayed under various atmospheres (air, argon, vacuum) on a nickel-based superalloy (Hastelloy X). Their microstructures (porosity, presence of oxides and phase composition) and mechanical properties (hardness and Young modulus) were characterized. The friction behaviour of these coatings sliding against a low-pressure plasma sprayed NiCoCrAlYTa coating was evaluated under fretting conditions. The study focused mainly on the different degradation mechanisms that may occur. Depending on the coating microstructure, two kinds of damage were observed: particle detachment and microstructural transformation (tribologically transformed structure). Adhesion and oxidation wear were also studied.

Effect of X-irradiation and friction on the properties of insulators

The study of ceramics is tackled from the space-charge physics point of view. The role of the polarization and relaxation mechanisms is demonstrated for friction coefficient and wear, as well as for breakdown voltage; therefore these characteristics depend on the permittivity. This result is based on experiments performed with pure or X-irradiated single crystal alumina (sapphire) or polycrystalline alumina. A method called the mirror method is presented, in which the possibility of creating trapped electrical charges in a given material is measured using the electron beam of a scanning electron microscope. This method is based on the fact that X-ray irradiation changes permittivity, Young modulus, and charging properties. Therefore, when a dielectric material is under irradiation, the trap and the bond energies will be modified and consequently changes in electrical and mechanical properties of the material are expected. It is shown that these changes can modify both irradiated and nonirradiated areas. >

STRESS IN DC SPUTTERED TIN/B-C-N MULTILAYERS

Stress in crystalline TiN/amorphous B–C–N multilayered thin films has been determined by the substrate curvature technique. It is established that the total stress is dependent on the number of deposited bilayers and on the bilayer repeat length. The linear relationship between the stress and the inverse of the bilayer repeat length allows calculation of the value of the interface stress. It is found to be compressive with a value between 1.79 and 2.46 J/m2, depending on the calculation method. An apparent dependence between the interface stress and the total thickness of the multilayer film is observed. It is interpreted as an additional relaxation due to an increase of the roughness of the interfaces when the number of deposited bilayers is increased.

Structural modifications of alumina implanted with zirconium, copper, and titanium ions

Microstructural modifications (amorphization, lattice deformation, phase transformations) in alumina induced by implantation of zirconium, copper, or titanium ions and by postimplantation thermal annealings were studied using grazing incidence x-ray diffraction. It was shown that the amount of lattice deformation and the type of damage resulting in the lattice depend on the ion implanted. When zirconium was implanted, the alumina lattice was highly deformed. Amorphization was observed when a high ion dose was implanted. Copper implantation led to the formation of gamma alumina. With titanium ions, very high strain was created and delta alumina was formed. After postimplantation annealings, lattices returned to their equilibrium state through crystallization of alpha alumina and precipitation of oxides of the implanted species (ZrO2, CuO and CuAl2O4, and TiO2).

Molecular dynamics simulation of the lattice: dynamic properties

Molecular dynamics simulation of the crystal was performed. Structural, thermodynamical and vibrational properties were determined by using a modified shell model. In this model, the energy conservation was ensured very accurately by assigning a small mass to each shell and treating them as dynamic variables. Simulation results were compared with experimental data, particularly with inelastic neutron scattering experiments giving the density of phonon states. Calculated structural and thermodynamical properties were in very good agreement with data obtained from the literature. Inelastic neutron scattering experiments were performed; calculated phonon characteristics compared very well with these experimental results.

Characterization of the trapping of charges in polystyrene

Nowadays, many works are done to study phenomena occuring after the charges build up leading to breakdown. But few of them describe the behavior of insulator during electrons injection. In our work, abrupt electron ejections during and after the electron injection in polystyrene have been detected using a SEM by recording simultaneously the secondary electron emission current and the absorbed current. The observed ejection are explained by the reach of a critical field.