Crevice and stress corrosion cracking of DLC coated implant interlayers

摘要

Diamond-like carbon (DLC) coatings are promising materials for improving the wear resistance of articulating biomedical implants due to their hardness and durability. They can be applied via Plasma Activated Chemical Vapor Deposition (PACVD) either directly onto the substrate or, an adhesion promoting interlayer material can be used to bind the DLC onto the metal. Despite successful wear simulation testing in vitro, some DLC coated hip and knee replacements still failed after only 3-12 years in patients due to coating delamination. Studies into some retrieved failed implants revealed the DLC delamination was induced by corrosion of the adhesion promoting interlayer, silicon. Currently the performance of artificial joints is evaluated through articulating simulators which assess fatigue related problems, however these do not take into account slow corrosion processes which may develop over time. DLC coatings contain inevitable defects, such as miniature cracks and pinholes, which create entries for surrounding joint fluid to contact the binding interface or interlayer and if this material is vulnerable to certain corrosion processes, such as crevice corrosion (CC) and stress corrosion cracking (SCC), the coating will fail. The aim of this work is to gain a better understanding of the corrosive mechanisms responsible for delamination at a coating-substrate interface. An approach is being developed to predict interface/interlayer deterioration in confined electrolytes by utilizing a local micro-electrochemical technique consisting of a glass micro-capillary that acts as a miniaturized electrochemical cell. The coating-substrate interface is revealed via low-angle ion flat milling and electrochemically treated, providing data which can be used to predict the speed of deterioration of the coating in the corresponding electrolyte, and ultimately a more accurate prediction of the implant lifetime. A preliminary procedure was performed in which a sample implant replica was prepared by depositing 1 μm of DLC on a titanium substrate with a 50 nm silicon interlayer. The sample was flat milled with an argon beam at an angle, so that the interlayer is spread across to 1000 times its thickness. A 200 μm capillary containing NaCl was placed on three different areas of the sample; the substrate, interlayer, and DLC coating. The exposed area was polarized to 1 V and the resulting current was monitored. Higher currents were observed at the interlayer compared to the passive substrate and semi-conductive coating, demonstrating this to be a weak point where corrosion occurs.