Dirk Drees

Electrochemical techniques for studying tribocorrosion processes

The combined corrosion-wear degradation of materials in sliding contacts immersed in electrically conductive solutions can be investigated in situ by electrochemical techniques. Such techniques are the open circuit potential measurements, the potentiodynamic polarization measurements, and the electrochemical impedance measurements. In this paper, capabilities and present limitations of these techniques are discussed based on a tribocorrosion study of a AISI 316 stainless steel and an iron-nickel alloy immersed in aerated 0.5 M sulfuric acid and sliding against a corundum counterpart. Some novel insights into the tribocorrosion mechanism gathered in this way are discussed.

Corrosion-wear of passivating materials in sliding contacts based on a concept of active wear track area

The concept of active wear track area is proposed as a way to investigate the mechanism of the corrosion-wear behavior of passive materials under sliding conditions. This active wear track area represents that part of the wear track that looses temporarily its passive character due to the mechanical interaction during sliding. It is shown that the active wear track area can be determined from repassivation kinetics measured on electrochemically activated material by a potential pulse method, and from anodic currents measured during sliding ball-on-disk tests. The use of that concept is illustrated for the case of stainless steel AISI 316 immersed in a 0.5 M H2SO4 solution. At loads below 2 N, no breakthrough of the oxide film is noticed. At loads between 2 and 12 N, the corrosion-wear agrees well with Quinn’s mild oxidation wear mechanism. The corrosion-wear of AISI 316 consists then of two processes namely a mechanical delamination of the passive layer in part of the wear track, followed by a progressive electrochemical re-passivation of that active wear track area. The thickness of the passive layer on AISI 316 was derived from the active wear track area and the electrochemical response of passive and active AISI 316 material. A value of 2–3 nm was obtained that agrees well with data obtained by other methods. Above a load of 12 N, the corrosion-wear increases steeply due to a degradation process involving abrasion and/or breakdown of more than just the oxide film.

The electrochemical and wear behaviour of amorphous diamond-like carbon coatings and multilayered coatings in aqueous environments

R.f. plasma CVD deposited diamond-like carbon coatings (DLC) possess interesting wear properties in dry testing conditions or in vacuum applications. Extremely low friction coefficients are displayed in dry test conditions, the friction coefficient being dependent on relative humidity and presence of oxygen. The question arises whether these coatings are equally effective in an aqueous medium. Parameters controlling the through-coating porosity are discussed and their relative importance indicated. A coulometric quantitative porosity measurement has been used. Characterisation by light optical microscopy, profilometric measurements and EPMA suggest the importance of the microstructure of the substrate material. Further, pin-on-disk wear tests in water show a detrimental damage when compared to dry wear tests. De-adhesion of the coating occurs in an early stage with subsequently a rapid destruction of the coating. Important parameters in this failure mechanism are the initial contact stress and the development of this stress by initial wear.

Hybrid processes — a versatile technique to match process requirements and coating needs

Abstract The properties of thin hard coatings and consequently their functionality are influenced by microstructural characteristics such as phase composition and grain size distribution. Recent studies indicate that microlaminated hybrid coatings offer an opportunity to establish unique and controllable microstructural characteristics. A variety of interlayers has been used in hybrid coatings for various purposes: (1) to enhance the adherence between coatings and substrates, (2) to prevent crack initiation, (3) to give good shock resistance, (4) to tailor the thermal expansion characteristics, which leads to good interfacial strength, (5) to prevent diffusion of substrate species into coatings or vice versa during annealing, (6) to enhance corrosion protection of coating and substrate, and (7) to enhance sliding wear resistance or lower sliding friction. In order to obtain the above mentioned properties, various surface modification techniques such as (1) electrodeposition, (2) thermal treatment, (3) ion plating, (4) ion implantation, and (5) sputtering can be used to prepare hybrid coatings. This review includes a discussion on surface modification techniques and properties of different hybrid coatings.

Comparison of wear and corrosion wear of TiN coatings under uni- and bidirectional sliding

Katholieke Univ Leuven, Dept Met & Mat Engn, B-3001 Louvain, Belgium. Limburgs Univ Ctr, Inst Mat Res, Div Mat Phys, B-3590 Diepenbeek, Belgium.Wu, PQ, Katholieke Univ Leuven, Dept Met & Mat Engn, De Croylaan 2, B-3001 Louvain, Belgium.

Quantitative determination of through-coating porosity in thin ceramic physically vapour-deposited coatings

Abstract An experimental method is proposed to determine in a quantitative way the through-coating porosity of thin ceramic coatings deposited on metallic substrates. The method is based on a coulometric measurement performed during potentiodynamic polarization of a coated sample. Since coating and substrate materials may interact electrochemically, a standard addition procedure to derive the porosity index has been worked out. The porosity of different TiN-based physically vapour-deposited coatings has been compared. The porosity values obtained are more realistic in comparison with those obtained by other methods.

Investigation of Friction in the Meso Normal Force Range on DLC and TiN Coatings

In recent years “low load” tribology has received more attention due to the emergence of special devices like MEMS, and new materials such as bioimplants, polymers, and textured surfaces. The tribological characterization of MEMS materials is challenging, because the devices operate at nominal contact pressures of only a few MPa and low wear rates of nm/h. The evaluation of materials with conventional high load tribological tools or with AFM type techniques is not appropriate because the contact pressures or contact geometries are not suitable. Furthermore, even in many conventional applications, the contact pressures and wear rates are more moderate than the ones achieved in conventional tribometers. In this work, the friction and wear behavior of the commonly used industrial coatings titanium nitride (TiN) and diamond-like carbon (DLC) are investigated with normal forces in between Newton (conventional tribology) and nanoNewton (nanotribology). The investigation of friction at such low contact pressures was achieved using a high precision microtribometer MUST® operating in a linearly reciprocating ball-on-flat configuration; a ball-on-disk configuration is also possible. The equipment works on the principle of deflection measurements of a flexible cantilever beam with light optical sensors bearing a force resolution of 0.1 μN. As the applied normal force reduces from mNs to μNs, the effect of surface roughness becomes strikingly apparent in the recorded tangential force. The friction of TiN coating appears to be less sensitive to surface roughness variations and chemical changes of the countermaterial than DLC. The behavior of DLC at low contact pressures is different from the well known low friction behavior of DLC coatings at high contact pressures. The importance of the appropriate tribological tool for evaluating materials at low contact pressures has been illustrated through this study.

Lubricating reaction products on TiN coatings during sliding wear in phosphoric acid

Abstract The debris structure influences the tribological behaviour of TiN coatings sliding against corundum. During sliding wear in environmental air, self-lubricating debris can be generated depending on the conditions. The (micro)structure of the debris varies with the relative humidity, temperature, mechanical attrition and temperature. In this paper, the wear rate and the friction of TiN coatings are compared with uncoated tool steel substrates sliding against corundum in: humid air, demineralised water, and phosphoric acid (0.02 M H 3 PO 4 ). Differences in the tribological behaviour are explained in terms of the interaction with the environment. In phosphoric acid, TiN coatings sliding against corundum show very low friction forces and wear rates due to the formation of a lubricating gel.

Nanocoatings for tribological applications

Abstract: An overview is given of the possible applications of nanostructured coatings for mitigating friction and wear. Different aspects of nanostructured coatings like the available deposition methods, the structure-to-property relationships, and the suitable tribological characterization techniques are reviewed. Finally, the challenges linked to the further implementation of such novel nanostructured coatings into industrial practice are critically discussed.

Tribological behaviour of mineral and rapeseed oils containing iron particles

Purpose – The purpose of this paper is to investigate the tribological behaviour of commercially available SAE 10 mineral and rapeseed oils containing Fe particles synthesized directly in the oil phase. Design/methodology/approach – Sub-micron Fe particles (50-340 nm) were synthesized by wet chemical reduction reaction of FeSO4 by sodium borohydride in the rapeseed and mineral oils in the presence of surfactant: block copolymer (ENB 90R4) or oxyethylated alcohol (OS-20). A four-ball wear tribometer was used to investigate the tribological properties of mineral and rapeseed oil: coefficient of friction (COF), wear scar diameter and wear loss. Viscosity measurements of oil solutions and determination of synthesized Fe particles size were performed as well. Findings – The presence of Fe particles (0.1 weight per cent) in the rapeseed and mineral oils caused the little change in the COF but resulted in marked improvement of anti-wear property. The oils containing Fe particles with slightly higher viscosity ar...