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

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.