Corrosion Mechanisms of Diamond-like Carbon Coated Interlayers & Interfaces

Diamond-like carbon (DLC) coatings can improve the wear resistance of articulating implants. However despite successful in-vitro testing, some DLC coated joint replacements had to be revised prematurely, mainly due to coating delamination induced by corrosion at the substrate/DLC interface. Body fluid will enter defective coating sites and if the reactively formed interface or interlayer materials are instable in the confined fluid, they with dissolve, resulting in delamination of the DLC and ultimate failure of the implant. The aim of this thesis is therefore to investigate the corrosion mechanisms responsible for delamination at a substrate/DLC interface in body-like media. This is achieved by developing and implementing experimental methodologies addressing/simulating aggravating factors such as confinements/crevices, galvanic coupling, stress and fatigue on the interface to investigate corrosion initiation, propagation, and failure.

Firstly, corrosion initiation and galvanic coupling risk are addressed with a methodology for accessing and characterizing the reactivity of buried interlayers. The interlayer is revealed by ion beam polishing, forming a wedge-like profile of the substrate/interlayer/DLC. The composition and chemical state of the interlayer/interface is determined by Auger electron spectroscopy. Reactive interlayers/interfaces, Si, titanium (Ti) and chromium (Cr), formed new phases (carbides and oxides). These new materials are then characterized with a local-electrochemical microcapillary technique. Cr and Si-DLC based interlayers presented good passive behavior, while a Si interlayer corroded in bovine-based wear test fluid (HyClone® WTF). The influence of coating defects on the current response of DLC surfaces was also investigated by varying the exposed working area (cm2 to µm2 range).

To address the failure case of a TiAlV/Si/DLC implant, the influence of confinements/crevices on corrosion propagation is investigated. It is shown that Si is highly vulnerable to corrosion in confined conditions. Pitting corrosion susceptibility is found in a crevice, whereas a Si dissolution rate of ca. 3.6 nm/h at 37 °C occurs within a confined area. The corrosion rates increased at elevated temperatures and yielded linear Arrhenius relations, with activation energies of 106 KJ/mol in phosphate buffered saline (PBS) and 109 KJ/mol in HyClone® WTF. Phosphorous species enhanced Si dissolution, while chlorides were not so influential. Galvanic coupling with Ti and applied stress further accelerated the Si dissolution.

Finally, corrosion related fracture of a CoCrMo/DLC interface is investigated by applying static and cyclic load. A reciprocating sliding rig is utilized to determine the number of sliding cycles required for interface delamination to occur at a critical load, in PBS at 37 °C. The results followed a Wöhler curve relation. Small O2 contaminations (0.5 and 1.0%) at the interface adversely affected the CF behaviour by lowering the critical load and endurance limit. The fitted Wöhler curves could be extrapolated and were found to be in good agreement with long-term experimental results previously obtained on an implant in a spinal disk simulator.