Description du sujet de thèse

Context and issues

The main interest of the Icarus Fall project concerns the problems of an ageing population and the additional costs associated with post-operative infections during prosthesis fitting. Surgical site infections in trauma surgery are uncommon (1-2% in scheduled surgery) but cause severe complications (bone necrosis, generalised infection, etc.) in an average of 33% of cases, with a mortality rate of almost 5%. These problems represent very significant economic costs, which in the United States will reach almost two billion dollars by 2030. The search for new technologies to overcome these infections is therefore vital. In France, although the Haute Autorité de Santé does not put forward any figures in its report, if the rate of periprosthetic osteoarticular infections were to fall below 0.5%, there would be a considerable financial gain for the State and for insurance companies, amounting to millions of euros a year.

In some prostheses, (I) a more or less thin metal envelope surrounds the outside of the prosthesis and acts as an impassable border between the living tissue and the lattices, so that it can be easily replaced surgically if necessary. In contrast, (II) other prostheses deliberately keep the porous structure in contact with the tissue to encourage osseointegration. In both cases, the lattices allow the apparent moduli of elasticity to be controlled according to density and pattern. The aim is to adapt the modulus of elasticity of the implant to that of the bone in order to reduce stress shielding.

In the first case (lattices with an envelope), the envelopes are the fragile part, as their thinness allows fatigue cracks to propagate rapidly, leading to infection in the bone site. A surface treatment known as SMAT (Severe Mechanical Attrition Treatment) is available to delay cracking, and involves shot peening the surface of a part placed in a closed chamber. This chamber is placed on a sonotrode that sets the balls in motion using ultrasonic vibrations. Repeated shocks to the surface will, on the one hand, cause nano-structuring of the surface microstructure by severe hardening and, on the other, create a gradient of compressive stresses in the subsurface. Coupling these two aspects increases fatigue resistance by preventing crack initiation and propagation. The initial results of Tania Sola Saiz's thesis show that it is possible to severely peened thin layers (0.4 mm) without damaging them, making it possible to lighten prostheses while increasing their strength. Rotating bending tests have also shown a significant increase in fatigue strength (up to 400% in some cases).
The consortium of this project has also shown, via the Resem HypOs project, that SMAT eliminates the need for finish machining on parts with complex geometries by reducing surface roughness and eliminating defects (powder contamination, beads, etc.). However, repeated impacts lead to cross-pollution between the balls and the surface. As a result, after SMAT, the surface may have a modified chemistry that is harmful to biocompatibility and/or conducive to the development of bacterial biofilms. Controlled pollution would increase bio-integration, enhance antibacterial properties and subsequently improve the kinetics and quality of a ZnO nanoparticle deposit.

Goals

The aim of this thesis project is to study the surface modifications induced by ultrasonic shot peening on metallic medical prostheses produced by additive manufacturing. Initially, solid samples cut from TA6V titanium alloy and 316L steel plates will be used to establish an optimum range of parameters (type/size/quantity of beads, treatment time, etc.) before carrying out tests on lattice samples with a shell from additive manufacturing.

Secondly, the SMAT chamber currently available at LEM3 will be improved to enable controlled surface chemistry modification tests to be carried out using filler metals (Cu, Zn, etc.) in various forms (flakes, powder, foil, etc.). Sub-surface metallurgical modifications induced by the new chemistry (phases, microstructures, texture, intermetallics, etc.) and fatigue resistance will be explored. The samples will then be sent to LCPME for ZnO nanoparticle deposition and corrosion tests.

Parameter ranges will be evaluated by studying microstructure gradients, surface modifications and mechanical strength. The study of gradients will be based mainly on hardness filiations as well as more advanced electronic imaging (SEM) and diffraction techniques (EBSD, DRX). Changes in surface chemistry will be studied using energy dispersive spectroscopy (EDS) as well as photoelectron spectroscopy (XPS) to determine oxidation levels, for example. The expected mechanical improvements, particularly in terms of mechanical fatigue, will be verified by rotating bending and/or tension-compression.

As part of the ANR project, the results obtained by the PhD student will be correlated with measurements of potentiodynamic polarisation, electrochemical impedance spectroscopy and ZnO layer growth (LCPME) as well as biological studies (BIOS).

Contexte de travail

This thesis is part of the national ANR Icarus Fall project involving the collaboration of three laboratories: the Laboratoire d'Etudes des Microstructures et de Mécanique des Matériaux (LEM3) in Metz, the Laboratoire de Chimie Physique et Microbiologie pour les Matériaux et l'Environnement (LCPME) in Nancy and the Laboratoire Biomatériaux et Inflammation en site Osseux (BIOS) in Reims.
It will take place within the LEM3, which is a joint research unit (UMR) attached to the University of Lorraine, the CNRS and Arts et Métiers. Its activities are organised into three scientific departments: (1) Mechanics of Materials, Structures and Life (MMSV), (2) Engineering of Microstructures, Processes, Anisotropy and Behaviour (IMPACT), (3) Thermomechanics of Processes and Tool-Material Interactions (TPRIOM). The student will work in the "Development and optimisation of processes related to microstructures" area of the IMPACT department and will be required to travel to partner laboratories during his/her thesis.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.