Papers by Author: Delphine Retraint

Abstract: Hydroxyapatite (HAP) and 58S Bioactive Glasses (BG) coatings are successfully synthesized by Electrophoretic Deposition (EPD) on Ti6Al4V alloy subjected to Surface Mechanical Attrition Treatment (SMAT). This process uses steel balls impacts on the Ti6Al4V surface to improve its mechanical properties. However when the Ti6Al4V substrate is treated by SMAT the industrial plasma spray technique is not efficient to obtain adherent HAP coatings. This problem is mainly related to the modifications of the Ti6Al4V surface topography due to the SMAT process. Therefore, in this work we demonstrate that EPD offers an efficient solution to solve this technical problem. Indeed we obtain a homogeneous and adherent HAP coating on the SMATed Ti6Al4V surface from a suspension of nanoparticles in ethanol. Moreover EPD is successfully employed to produce a 58S BG coating on the SMATed Ti6Al4V surface. Scanning Electron Microscopy (SEM) associated to Energy Dispersive X-Ray Spectroscopy (EDXS) reveals that the coatings obtained by EPD are adherent and compact without alteration of their chemical composition.

Abstract: The aim of this study is the characterization of hot-dip galvanized S355K2+N steel plates used as components for engineering civil structures. Two thin zinc coatings with a thickness of 145+/-14 μm and 329+/-23 μm, were developed at the surface of the plates. Several experimental techniques were performed to study the microstructure, the chemical composition at the surface of the galvanized plates. The residual stress field was also evaluated in the coatings and the top surface of the plates using the incremental hole drilling method, in the rolling and the transverse directions. The results show the presence of tensile stresses and compressive stresses respectively in the coating and the substrate.

Abstract: In this paper, the effects of Surface Mechanical Attrition Treatment on the high-temperature oxidation of AISI 316L austenitic stainless steel are investigated. Samples treated with different conditions were oxidized at 650°C in order to study the effect of this type of nanocrystallisation on the oxidation resistance of the alloy concerned. X-ray diffraction and in-situ Raman spectroscopy were used to identify the oxides formed at the surface. The results indicate the presence of hematite and chromium oxides. Experimental results obtained by Raman spectroscopy were also used to study the stress evolution in Cr₂O₃ films during isothermal conditions.

Abstract: In this paper the residual stress states induced by conventional shot peening (SP) and surface mechanical attrition treatment (SMAT) are compared. The treated part correspond to plates made of a titanium alloy. Different intensities of these two mechanical treatments are first considered: their influence on the surface characteristics (roughness, hardness...) is studied. These experimental data are then used to develop a model for the residual stress profiles with dimensional analysis, Experimental and analytical approaches are then discussed.

Abstract: In this work, the use of a nanocrystallization surface treatment is considered on a cobalt based alloy. Since the Co28Cr6Mo is widely used as an articular bearing surface for artificial joints like hip and knee prostheses, the improvement of its tribological properties is a matter of concern to extend the lifespan of implants. By means of SMA (Surface Mechanical Attrition) treatment, a nanostructured layer is formed at the surface of a CoCrMo alloy establishing an optimized hardness gradient down to a depth of a few hundred of microns. Different treatment times (5, 15 and 25 minutes) are assessed comparatively and several surface polishing methods are studied: with clothes, brushes and a liquid filled with abrasive particles. The influence of processing parameters is discussed regarding hardness and topography. Moreover, the impact of surface modification is examined in terms of wear strengthening through scratch testing. The use of increasing loads mode gives some evidence of the benefit of SMAT. A significant decrease of penetration depth is noticed, from 30% to 60% on average. A straight correlation is also noticed depending on the surface finish method. This study illustrates both the ability of micro scratch testing to assess comparatively treated surface layer and to highlight the benefit of SMAT for wear strengthening in an abrasive wear mode.

Abstract: The aim of the present study is first to describe an original process, the so called duplex process, whose feature is the coupling between the well-known SMAT (Surface Mechanical Attrition Treatment) and the traditional co-rolling. The first step of this process consists of SMA-Treatment of 316L stainless steel sheets to generate nanocrystalline layers on their top surfaces according to the grain refinement mechanism of austenitic steels which is well described in the literature. During the second step, three treated sheets are co-rolled at 550°C to obtain a semi-massive nanocrystallised multilayer structure with improved mechanical strength alternating nanocrystalline, transition and coarse grain layers. The second part of this work deals with the mechanical and the microstructural characterization of the as-obtained structures. Thus, sharp nanoindentation tests performed over the cross section of the laminates coupled with Transmission Electron Microscopy (TEM) confirm the presence of nanograins after the thermomechanical treatment. In addition, the enhanced yield strength demonstrated by tensile tests correlate well with the theoretical volume fractions of nanoand transition layers. The interface cohesion between the sheets is tested by three-point bending tests and the interface bonding is evaluated by microstructural observations.

Abstract: Surface Mechanical Attrition Treatment (SMAT) is a recent process that enables to nanocrystallise the surface of metallic alloys. It can thus enhance mechanical properties of the treated material by inducing a grain refinement down to the nanometre scale, in the top surface layer. This nanocrystallisation process leads to different effects that were successively studied on several metallic materials. In the present work, investigations are carried out on the modelling of SMAT. A simulation of the shot dynamics is performed using different process parameters, with the aim to obtain the impact velocity field on the treated surface. This field is then used as an input for a finite element model to predict the induced grain refinement. The evolution of the micro and nanostructures are then calculated using a micromechanical approach, which takes into account the dislocations and their interactions. Coupled with a finite element analysis, this approach enables to deduce the influence of the process on the macroscopic material properties, whatever the geometry of the sample.

Abstract: It is possible to produce a nanocrystalline, multilayered composite structure with enhanced mechanical properties by assembling three 316L surface nanostructured stainless steel plates by roll bonding. The Surface Mechanical Attrition Treatment (SMAT) was first used to generate nanocrystalline layers on the elementary plates so that their mechanical properties were improved. They were then assembled through co-rolling. A composite structure of nanocrystalline layers of high strength alternating with more ductile layers was obtained to achieve both high strength and ductility. Microscopy observations and EBSD measurements were carried out and the bonding interfaces were analysed in detail to explore the mechanisms involved during the SMAT/Co-rolling duplex process.

Abstract: . In the present study, a method is presented combining surface nanocrystalline treatment (SMAT) and the co-rolling process. The aim of this duplex treatment is the development of a 316L stainless steel semi-massive multilayered bulk structure with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure by optimizing the volume fraction and distribution of the nano-grains in the laminate. To characterize this composite structure, tensile tests as well as sharp nanoindentation tests were carried out to follow the local hardness evolution through the cross-section of the laminate. Furthermore, transmission electron microscope (TEM) observations were carried out to determine the correlation between the microstructure, the local hardness and the mechanical response of the structure.

Abstract: Severely deformed surface layers have been created by ultrasonic attrition technique on four steel sheets to investigate their influence on fatigue behaviour. A low-carbon (0.05%) ferritic steel and a medium-carbon (0.47%) normalized ferritic-pearlitic steel were selected to study the effect of carbon content on fatigue properties of carbon steels. Two stainless steels, Type 316L and Type 301LN, were also tested to study the influence of stability of the austenitic structure. Microstructural features were characterized by hardness measurements, X-ray diffraction and optical and electron microscopy. Fatigue properties were determined in flexural bending in the range 104 to 107 cycles. Crack nucleation and propagation stages were followed. In the attrition treatment thin severely deformed surface layers were found to form. Highly increased hardness was measured in these layers, especially for stainless steels, where also strain-induced martensite was formed. Drastic improvement in fatigue resistance was observed for all steels due to the surface nanocrystallization treatment.