A methodology for characterizing the electrochemical stability of DLC coated interlayers and interfaces

DLC coatings are often deposited on Si and Cr based adhesion-promoting interlayers to mitigate stress at the DLC/substrate interface. Interlayers reduce the likelihood of mechanical coating delamination. However, their chemical stability at miniature DLC coating defects, such as pores and micro-pinholes, may be adversely affected in a corrosive environment, such as the human body where DLC coated implants are envisaged. An experimental methodology is presented for accessing and characterizing the electrochemical reactivity of buried interlayers and interfaces. The interlayer, with its interfaces, is revealed by ion beam polishing at an angle, forming a wedge-like profile of the substrate/interlayer/DLC system. The chemical binding and composition of the interlayer/interface is determined by Auger electron spectroscopy (AES), and the susceptibility of the different interfaces to carbide formation and oxidation is identified. It is shown that pure interlayer/interface materials can be obtained for the model Co interlayer. On the other hand, for more reactive materials such as Si, Ti and Cr, the formation of new interface phases was observed. These different new materials are characterized for their corrosion susceptibility with a local-electrochemical microcapillary technique both at open circuit potential (OCP) and under potentiodynamic polarization. Cr and Si-DLC based interlayers presented good passive behavior, while a Si interlayer corroded in bovine-based wear test fluid (HyClone (R) WTF). An analogous degradation is found after a long-term immersion experiment and failed implant cases. The influence of coating defects distribution on the current response of DLC surfaces during electrochemical measurements was also investigated by varying the exposed area (from the cm(2) to the mu m(2) range). Microscale characterization allowed for a better representation of the intrinsic reactivity of DLC, while larger areas were highly dependent on the underlying interlayer.

Chemical-mechanical polishing of tungsten

Jelena Stojadinovic

The chemical-mechanical polishing process (CMP) is an essential part of the production of integrated circuits. Metal to be polished in the CMP reacts with the oxidants from the aqueous suspension (slurry) and the passive film is formed on the metal surface. Suspended abrasive nano particles from the slurry cause detachment of the formed friable passive films. The CMP represents the tribocorrosion process where material deterioration results from the interaction of wear and corrosion which takes place in tribological contact exposed to an aggressive environment. Due to increasing number of the materials to be polished in the modern integrated circuits it is necessary to gather fundamental understanding of the removal mechanism of each material. The aim of this work is to elucidate the removal mechanisms of tungsten (W), as a typical microelectronic metal (chosen as a model system), under different oxidising conditions (represented by electrochemically imposed potential). Also, we want to evaluate the effect of the slurry composition (chelating agents lactic and phosphoric acid) on material removal. To achieve this electrochemical techniques (chrono amperometry, cyclic voltametry, passivation kinetics measurements under mass transport control), and tribocorrosion techniques (tests on tribometer with electrochemical cell with reciprocating sliding motion of alumina ball) were used. Surface analysis techniques (SEM, XPS, AES) were used to assess worn and unworn surfaces' morphology and chemistry. In addition, electrochemical tests and polishing tests on the CMP machine were performed with technical CMP slurries. The sliding action of the alumina ball on tungsten in the 0.01 M H2SO4 solution was found to cause three distinct wear responses depending on imposed electrode potential. Below the first threshold potential the wear is low due to negligible oxide film formation. In the following wear region, up to the second threshold potential, materials deterioration in the rubbed area proceeds by cyclic mechanical removal of the WO3 passive film followed by repassivation of tungsten. The amount of anodic oxidation corresponds to the overall removed metal volume. Beyond the second threshold potential a compact tribolayer of WO3 (more than 400 nm thick), probably formed by agglomeration of oxide particles detached from the metal surface, forms and covers the wear track. Similar tribocorrosion behavior of tungsten was also observed in the presence of 0.1 M H3PO4. The presence of chelating agents, lactic acid and phosphoric acid, in the 0.01 M H2SO4 solution does not change the volume of material wastage at potentials below the second threshold potential. Beyond this electrode potential, lactic acid promotes WO3 dissolution and thus impedes the third body formation and enhances wear. Electrochemical factors have a strong influence on the tungsten CMP removal rate. It was found that CMP removal rate increases with the open circuit potential (electrode potential) and with passivation charge density. A good correlation of tribocorrosion results and CMP practice was observed. Tribocorrosion models can be successfully applied to the tribocorrosion of tungsten and as well can be fairly used for modelling the material removal in CMP.

EPFL2009

Corrosion Mechanisms of Diamond-like Carbon Coated Interlayers & Interfaces

Emilija Ilic

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.

EPFL2019

Materials for Methane-fueled SOFC Systems

Stefan Diethelm, Raphaël Ihringer, Joseph Sfeir, Jan Van Herle

Abstract: A short overview of recent work on electroceramic materials relevant to methane-fueled SOFC systems is given. Various fuel feed options are considered, such as pure methane, biogas, and the addition of reforming agents. The principle and implementation into SOFC systems of mixed conducting ceramics for oxygen separation is described, as well as their characterisation by electrochemical methods. Ceramic anodes capable of operating with methane-rich fuel injection are presented. The document concludes with electrochemical results on planar anode supported ceramic fuel cells operating at reduced temperature.