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International audience

Aluminium–aluminium nitride (Al–AlN) cermet films have been deposited by RF co-sputtering. The target was an insulating aluminium nitride disk on which small aluminium disks were regularly displayed. The effect of the target configuration, i.e. the influence of the number of aluminium disks and of the sputtering time on the microstructure, has been studied. The Al second phase content of the film can be adjusted by means of the Al disk number placed on the ceramic target. The cermets are made of Al grains (150–200 nm) embedded in a nanocrystalline AlN matrix (5 nm).

Yttria stabilised Zirconia coatings on polycrystalline silicon wafers have been performed using physical vapor deposition with negative bias applied to the substrate. The thicknesses determined by ellipsometry were between 45 and 70 nm and the deposition rate was about 0.25 nm per min. Secondary ion mass spectroscopy analyses revealed no interdiffusion of metallic elements between the zirconia coating and silicon substrate. Grazing incidence X-ray diffraction indicates that the coatings are crystallized, but it can not permit to identify all the crystalline phases. The disparity between the experimental and the k-ratios (Pouchou and Pichoir simulation of electron probe microanalysis analyses) showed the usefulness of others techniques to determine coating and interface compositions. Cross sectional transmission electron microscopy observations demonstrate an amorphous layer between the zirconia coating and the substrate with a thickness of 30–120 nm. If it is considered to be an amorphous silicon layer, the PAP simulation based on the ellipsometric thicknesses and an yttria stabilised zirconia density of 4 g/cm3 fits correctly the experimental values.

Solid state nitriding of titanium under cw C02 laser was investigated. The nitriding kinetics were studied in the caseof C02 laser treatment in isothermal conditions, and were compared to those obtained by using a conventional heatingsystem. Identical kinetics rate constants and activation energy were obtained; therefore C02 irradiation of the titanium surfacedid not influence nitride growth. Studying nitriding treatment in non isothermal conditions showed that high heating ratesinduced modifications of the nitride morphology (compared to isothermal treatments). At last, we investigated the evolutionof the C02-surface coupling during the nitriding treatment. It was found that it depended only on the surface temperature andthe substrate surface roughness since the growth of a few micrometers thick nitride layer did not modify the coupling.Titanium, Nitriding, C02 laser, Kinetics, absorptance, thermal treatment, diffusion. I. INTRODUCTION Nitriding of titanium alloys by C02 laser was extensively studied in the case of liquid phase treatment ,2,3,4Surface melting enables to form thick nitride layers (several hundred micrometers) that are attractive for surface wear resistanceapplications. However, liquid phase nitriding treatment may also reduce the substrate fatigue resistance and in some caseinduce cracks in the nitride layers. An alternative consists in realizing the treatment by maintaining the surface temperaturebelow the melting point (solid phase treatment). This considerabily reduces the nitride thickness (tens of micrometers), but ithas been found to induce also good tribological and mechanical performances on titanium alloys5. We investigated theparamaters that influence the nitride layer growth in solid phase under cw C02 laser radiation, both under isothermal and nonisothermal conditions. For this purpose, we compared, under isothermal conditions, nitriding under C02 laser radiation to theresults obtained when the treatment is carried out using a conventional heating system (thermoresistance heating). We alsocharacterized cw C02 laser nitriding treatment in non isothermal conditions in terms of nitride layers thicknesses andmorphology depending on the process parameters. At last, investigations of the C02

International audience

Colloque avec actes et comité de lecture. Internationale.; International audience; La nitruration gazeuse est une technologie utilisée par les fabricants d'engrenages pour traiter la surface des dentures dans le but d'en améliorer les propriétés mécaniques et d'augmenter la longévité des composants face, entre autre, à la fatigue de contact avec roulement. Certains aciers alliés présentent cependant des réseaux de carbures dans la couche nitrurée. Ces précipités ont la particularité de se retrouver aux joints de grain relativement parallèles à la surface, dont les coupes métallographiques perpendiculaires à la surface leur ont attribué les surnoms de « liserés de carbures » ou de « cheveux d'anges ». De plus, ils constituent des zones d'hétérogénéités à prévenir lors de la nitruration des pièces selon certains auteurs. De par le manque de travaux sur l'impact réel de ces précipités sur la fatigue, la présente étude propose des analyses de résultats d'expériences en fatigue de contact avec roulement. Des observations de réseaux de fissures en 2D et en 3D mettent en évidence l'influence des précipités sur la propagation des fissures. Ces constats permettent de mieux comprendre la divergence des comportements en fatigue de couches nitrurées de propriétés mécaniques équivalentes mais de microstructures différentes en termes de morphologies des liserés de carbures qui dépendent de la taille de grain de l'acier de base. Cela remet en question les modèles de durée de vie en fatigue ne prenant pas en compte les caractéristiques microstructurales du matériau.