Corrosion behaviour of Ti/Al2O3 interfaces produced by an active metal brazing methodology

Abstract

Metal/ceramic joints are used in a broad range of applications in biomedicine, such as the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or the production of drug delivery systems, biomedical sensors and electrodes. Apart of other characteristics, the corrosion resistance of metal/ceramic interfaces is of prime importance when biomedical applications are considered. Most of metal/ceramic joints are produced by the active metal brazing technique or by diffusion bonding. Both techniques originates a multi-layered interface which should be able of accommodating the abrupt electronic, crystallographic, chemical, mechanical and thermomechanical discontinuity that characterize these metal/ceramic systems. However, galvanic interactions between those chemically distinct layers are likely to occur, affecting the degradation behaviour of the interface. In this work the corrosion behaviour of Ti-Al2O3 interfaces produced by an active metal brazing methodology was studied. SEM analyses evidenced an interface mainly constituted by four different layers. A first layer rich on titanium and copper, located near to the pure titanium, another layer also rich on Ti and Cu, but with a higher Ag content, an intermediate layer rich in silver which contains some little precipitates of Ti-Cu and finally, a reaction layer with a composition profile containing Ti, Cu, Ag, and Al, located near the alumina part. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) measurements, carried out in a simulated physiological solution at ambient temperature, revealed a strong influence of the rich silver layer on the passivating behaviour of the interface. On the other hand, the reaction layer appears to be the main responsible for the degradation process of the interface. This degradation is accompanied by a relatively high release of copper. Through EIS data simulation it was possible to obtain an electrochemical equivalent circuit that describes the corrosion process and allowed an estimation of the polarisation resistance of the constitute layers of the interface. The electrochemical interaction between the different constitutive layers was evaluated, and was correlated with the overall degradation behaviour of the Ti-Al2O3 interface.