'Green' Cr(III)-based electrodeposition of stainless steel-like FeCrNi: synthesis, mechanisms, material properties of films and micro-nanocomponents for bio-medical applications

Enrico Bertero
EPFL, 2020
Thèse EPFL

Résumé

The electrodeposition (ED) of stainless steel (SS) -like FeCrNi alloys for miniaturised devices is appealing for bio-medicine as it would allow combining excellent material properties (e.g. corrosion resistance, hardness, bio-compatibility) at low-cost. However, conventional baths often contain hazardous Cr(VI). Alloys ED from environmentally friendly Cr(III) electrolytes is crucial for facilitating the transition towards sustainable and ecological production. Still, this process was not comprehensively studied. ED from Cr(III)-based aqueous electrolytes leads to impurities incorporated in films, hydrogen evolution reaction (HER) and side-effects. The role of both electrolyte composition, containing organic additives (e.g. glycine), and deposition parameters on material properties was not clear. Moreover, passing from films to micro-nanocomponents (M-NEMS) via ED was not properly investigated when dealing with more complex Cr(III)-based alloys. The aim of this thesis has been to study the ED of FeCrNi coatings and M-NEMS using a 'green' Cr(III)-glycine electrolyte, understanding better the relations between ED mechanisms, deposition parameters and material properties, as well as their variation due to miniaturisation. Novel information was attained for all-aqueous electrolytes by investigating: films microstructure evolution (amorphous-nanocrystalline) in correlation to their composition and elemental 3D distribution, influence of Cr(III)-glycine in terms of coatings at%, together with a thorough analysis on metals speciation/complexation in the baths. These results allowed to propose various Cr(III)-based ED mechanisms. The material properties of the as-deposited films were evaluated in dependence of morphology and composition variations, then compared to the standard metallurgical ones. FeCrNi electrodeposits showed good passivation and bio-compatibility comparable to AISI SS, and tuneable soft-magnetism. However, tribological tests revealed that the material was hard but brittle. The main issues were correlated to HER leading to low deposition efficiency, material brittleness and porosity. To overcome these issues, a mixed-solvent electrolyte composed of ethylene glycol (EG) has been investigated and compared to the all-aqueous one, showing similar Cr(III)-glycine complexation, but higher deposition efficiency (decrease in HER). EG-based electrolyte was found to be an effective solution for obtaining FeCrNi M-NEMS via template assisted ED. Nanotubes (NTs) and nanowires (NWs) have been achieved via ED into AAO templates: high currents resulted in NTs, whereas low ones in compact NWs. Micro-pillars have been created via ED into UV-LIGA moulds, combining EG-based electrolyte and a CV-like deposition, avoiding moulds delamination caused by HER and under-deposition. A comparison of the material properties was pursued for FeCrNi electrodeposits as-deposited (amorphous) and annealed (nanocrystalline) for both studied electrolytes. This investigation correlated electrolyte type, material composition/microstructure/morphology, vs. other material properties i.e. corrosion resistance, bio-compatibility, magnetic and mechanical properties. In conclusion, this research studied in depth Cr(III)-based ED, achieving improvements and giving useful guidelines for applying this process for creating SS-like coatings and M-NEMS for advanced bio-medical applications.

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https://infoscience.epfl.ch/record/278616?ln=fr

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Enrico Bertero
EPFL, 2020
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/278616?ln=fr

À propos de ce résultat

Concepts associés (16)

Concepts associés

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Titanium dioxide (TiO2) is a widely investigated material for its biological compatibility, high dielectric constant and refractive index, chemical and mechanical resistance, and catalytic activity. Several different techniques are available to produce TiO2 thin films, which are variously suited to provide the properties required for a specific application. Dense TiO2 coatings with high refractive index and low optical absorption are aimed at for optical applications. High thickness uniformity on large area coatings and a high, precisely controlled, growth rate are further advantages. A reactor for the production of optical coatings at 1% thickness uniformity on 150 mm diameter substrates has been designed according to a mathematical model, which has been extended to evaluate growth rates and precursor efficiency. The reactor design has been made suitable for light assisted TiO2 growth, so as to achieve selective or improved deposition or to locally modify the coating properties. Along this line, a light selective deposition of 3 µm resolution by mask projection is anticipated. The results obtained by operating the reactor in the Low Pressure Chemical Vapour Deposition (LPCVD) and Chemical Beam Deposition (CBD) indicate that the latter mode results in coatings with superior optical properties. Thickness uniformity better than 3% and a precursor impinging efficiency very close to the 5% calculated value have been achieved in the growth rate range from 50 to 1000 nm h-1. The causes of these small discrepancies are discussed and some reactor design improvements are proposed. The estimated resolution limit by mask projection has also been achieved by PET laser ablation, while for TiO2 a selective deposition of 30 µm structures has been obtained. Several characterization methods have been used to investigate the chemical composition (XPS, FTIR, Raman spectroscopy), the crystalline phase (XRD, GAXRD, Raman spectroscopy, TEM) and the morphology (XRR, TEM, SEM, AFM, RBS) of the thermally grown TiO2 films. Pure TiO2 with less than 1% carbon content with anatase crystalline phase is obtained in all the conditions explored (substrate temperature between 330 and 550°C). It is also shown that the anatase content increases with increasing substrate temperature and film thickness. The classical columnar growth related to the anatase crystalline phase is obtained with nanograins of smaller diameter than that of the columns (8-10 nm compared to 50-200 nm). Optical characterisation methods (photospectroscopy, spectral ellipsometry, and in situ Dynamic Optical reflectivity) have also been applied. Dense anatase has been obtained with refractive indexes n and k of 2.5 and 10-2 respectively for films up to 80-100 nm in thickness. For thicker films, an increasing porosity with increasing film thickness is observed resulting in enhanced Rayleigh scattering. A band gap blue shift is also observed when the Tauc model is applied due to increasing nanocrystallinity with thickness. To avoid this problem, SiO2 has been added so as to increase the temperature at which amorphous to anatase phase transition occurs, resulting in TiO2 amorphous phase deposition at 500°C substrate temperature with less than 10% SiO2 content. Alternating layers of TiO2 and TiO2-SiO2 as well as graded index films, both in the growth direction and perpendicular to the growth direction, have also been produced by thermal decomposition and their optical behaviour investigated. Laser assisted deposition has been used to improve optical properties of the film. A densification of the TiO2 anatase phase layer has been achieved resulting in higher refractive index n compared to pure thermal deposition even for films up to 400 nm thickness. A further densification with increasing laser fluence up to rutile phase transition is obtained. The band structure, extrapolated from spectral ellipsometry measurements, shows a progressive shift from the anatase to the rutile phase. Finally, the TiO2 selective deposition and the chemical composition variation in TiO2+SiO2 films as a function of laser repetition rate have been studied. In all cases the results obtained by means of the different characterization techniques are in mutual agreement and consistent with the global picture of the TiO2 growth resulting from the various studies reported in the literature.

EPFL2003

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A unique approach to reveal the nanocomposite nc-MN/SiN-layer architecture of thin films via electrical measurements

Scott Harada, Silviu Cosmin Sandu, Rosendo Sanjines

By addition of Si to a binary transition metal nitride MN (e.g. TiN, ZrN, NbN, CrN), the hardness, thermal stability and chemical inertness of films have been considerably improved. The formation of a ternary M–Si–N ternary phase is possible under specific conditions such as low temperature, high deposition rate and low nitrogen pressure. The formation of nanocomposite materials (e.g. crystallites of MN + a thin layer of SiNx) is also possible under a wide range of deposition conditions. In such nanocomposite thin films the crystallite sizes are on the order of a few nanometers. The grain surfaces and boundaries have an important effect on the physical properties. The arrangement and chemical composition of the so-called “amorphous” minority phase (SiNx) are crucial for electrical and mechanical properties. The location, composition and thickness of the amorphous phase must therefore be known precisely. Their experimental determination is challenging due to the small concentration and in particular the geometry of the “amorphous” phase: approximately one monolayer either completely or partially covering the MN nanocrystallites. TEM investigations on such composites are known to have their limitations. It will be shown that the electrical resistivity, measured as a function of temperature, provides an experimental means for following the thickness evolution of the SiNx coverage layer, in such nanocomposite films.

2010

Chargement

For building integration of solar-powered energy systems, aesthetic aspects play an importantrole. Covering a standard solar collector with a coloured glazing, opaque to the human eye but highly transparent to solar energy, permits a perfect architectural integration of solar thermal panels into glazed building façades. The thermal energy produced can be used for both solar heating and cooling, as well as for domestic hot water. The principle of the coloured appearance is based on interference in the thin-film coating on the reverse side of the cover glass. Different interference filters based on nano-composite materials deposited by the solgel method were presented at CISBAT 2007 [1]. Currently, we are developing new plasma-deposition processes, which are more suitable for industrial large-scale production. A new state-of-the-art ultra-high vacuum (UHV) system for magnetron sputtering deposition of novel nano-composite solar coatings has recently been designed, constructed, and installed at the Solar Energy and Building Physics Laboratory (LESO-PB). Up to five different magnetron sources can be used simultaneously, in reactive and non-reactive mode. The geometric configuration of the chamber has been optimised for best film homogeneity and allows the deposition on substrates up to 100 mm in diameter. The optical and electronic properties of thin films are closely interrelated and highly relevant for solar coatings. Photoelectron spectroscopy provides information on the coating structure, the deposited material and its chemical state inside the coating, as well as the nature of the interface between different layers. A system for ESCA analysis (Electron Spectroscopy for Chemical Analysis) has recently been installed and put into operation at LESO-PB. By ellipsometry and spectrophotometry, we can determine exactly the different optical properties of the coating, such as layer thickness, refractive index, or absorption coefficient. This provides best conditions for highly efficient research and development on new materials for further optimisation of the coloured interference filters.First results have been obtained with our new experimental infrastructure and will be presented in this contribution.

EPFL2011

Chargement

EPFL2003

EPFL2011