Rhenium Electrodeposition to Enable Functionally Graded Films

Abstract

Rhenium (Re) has one of the highest melting points of all the metals with a high modulus of elasticity and superior tensile strength when compared to other refractory metals. However, processing challenges from the high temperature stability combined with the relative occurrence require advanced manufacturing approaches to leverage the unique properties of rhenium. The electrodeposition of rhenium has received renewed attention in the past decade as a scalable approach to apply uniform coatings to enable a wide range of applications in aerospace, nuclear, catalysis, biomedical fields. However, Re has traditionally been difficult to deposit by electrolysis from aqueous solutions due to the over potential for hydrogen evolution and complex electrochemical reactions.Beginning with the perrhenate anion (ReO4 -), reduction from the +7 oxidation state must occur to deposit metallic Re. The exact mechanism of this process is still unknown with reductions to ReO2, ReO3, and Re2O5 along with metallic Re. Combined with the hydrogen evolution reaction, Re coatings are generally deposited at low faradaic efficiency, are brittle, and limited in the obtainable thickness (sub-micron).Herein, we present systematic studies progressing towards the development of stable and efficient electrolytes to enable the electrodeposition of thick (100+ microns) Re coatings. Borrowing the water-in-salt electrolyte method from the lithium battery literature enables near room-temperature electrodeposition of Re. Specifically, we correlate nucleation and growth mechanisms of Re with the ability produce thick coatings. Pulse/pulse reverse electroplating was applied to manipulate the obtained grain structure and distribution of rhenium moieties. Correlations between the electrolyte composition and the pulse parameters with the observed morphology will be discussed with an emphasis on enabling industrially relevant electroplating processes.The goal of this project is the electrodeposition of rhenium-cobalt (Re-Co) graded compositional/density samples of millimeter thickness. Thus, understanding how to transition from a coating or pure metallic rhenium to a graded composition of Re-Co as a function of dynamic processing conditions will be presented.