Helena Ronkainen

Coatings tribology—contact mechanisms and surface design

Abstract The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.

Friction and wear properties in dry, water- and oil-lubricated DLC against alumina and DLC against steel contacts

Abstract Diamond-like carbon (DLC) films can be divided into two major categories according to their hydrogen content. These categories have similarities in tribological performance, but the films also behave in a different manner in different tribological conditions. The results of amorphous hydrogenated carbon films (a-C:H) and hydrogen-free hard carbon films (a-C) are reported in this study. The a-C:H films were deposited using the radio frequency (rf) plasma technique, and the hydrogen-free hard carbon films using pulsed vacuum arc. The coatings were characterized and investigated with respect to their tribological performance in dry (50% RH), water-lubricated and oil-lubricated slow sliding conditions (0.004 m s −1 ). The a-C and a-C:H films had a low friction coefficient in dry sliding conditions (0.15 to 0.22), which was further decreased by 10–40% under boundary lubrication. The a-C:H(Ti) films exhibited good self-lubricating properties (0.10) in dry sliding conditions and the a-C films had the lowest friction coefficient in water- (0.03) and oil-lubricated (0.08) conditions. The hydrogen-free hard carbon films showed excellent wear resistance in dry, water- and oil-lubricated conditions, but hydrogenated a-C:H films suffered from severe wear in aqueous conditions. The performance of a-C:H films could be improved by titanium alloying. In dry sliding conditions, the tribolayer formation of DLC films influenced the friction and wear performance, but in oil-lubricated conditions boundary lubrication layers were formed, which governed the tribological mechanisms in the contact.

Tribology of thin coatings

Abstract The fundamentals of coating tribology are presented in a generalised holistic approach to friction and wear mechanisms of coated surfaces in dry sliding contacts. This is based on a classification of the tribological contact process into macromechanical, micromechanical, tribochemical contact mechanisms and material transfer. The tribological contact process is dominated by the macromechanical mechanisms, which have been systematically analysed by using four main parameters: the coating-to-substrate hardness relationship, the film thickness, the surface roughnesses and the debris in the contact. The description covers both soft and hard coatings with thicknesses typically in the range 0.1–50 μm, where the interaction between the coating and the substrate is essential to the tribological behaviour. The concept is supported by experimental observations. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design both regarding friction and wear can be achieved by new multilayer techniques giving reduced stresses, improved adhesion to substrate, more flexible coatings and harder and smoother surfaces. The differences in contact mechanisms in dry, water- and oil lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel ball. The mechanisms of the formation of dry transfer- and tribolayers and lubricated boundary and reaction films are discussed.

Differentiating the tribological performance of hydrogenated and hydrogen-free DLC coatings

Diamond-like carbon (DLC) coatings cover a wide range of different types of carbon-based coatings, which generally have properties such as low friction and high wear resistance. DLC films can be divided into two major groups based on their hydrogen content, namely hydrogenated and hydrogen-free carbon coatings. This presentation describes the research work on amorphous hydrogenated carbon films (a-C:H) and hydrogen-free tetrahedral amorphous carbon films (ta-C). These coating types offer low friction performance and good wear resistance, but they also have some dissimilarities in the friction and wear properties due to their different hydrogen content and microstructure. The a-C:H films deposited for this study were produced by the rf plasma deposition technique and the ta-C films by the pulsed vacuum arc technique. The tribological performance of the a-C:H and ta-C coatings was evaluated by pin-on-disc tests carried out in normal atmosphere. Selected coatings were also evaluated in dry atmosphere. The wear resistance of the ta-C films was higher compared to the a-C:H films, but the ta-C films caused higher wear of the counterpart. The increase in normal load and sliding velocity decreased the friction coefficient of the a-C:H coating against the steel and alumina counter-face, whereas the ta-C coating showed more stable friction performance. The micro-Raman studies showed clear graphite formation for the a-C:H film, whereas the graphite formation on the ta-C film was not so evident. In dry conditions the ta-C had a high friction coefficient, which could be reduced by doping the film with hydrogen. The results show that the hydrogen content together with graphitisation plays an important role in the friction performance of DLC films.

Corrosion performance of some titanium-based hard coatings

Abstract Tools and machine parts which could benefit from wear-resistant titanium-based hard films are often subject to corrosive environments. Physically vapour-deposited coatings frequently exhibit porosity and even small defects, which can cause rapid local corrosion of the substrate material; there is therefore a requirement for dense and chemically inert coatings. This paper presents corrosion data for titanium-based hard coatings such as TiN, (Ti,Al)N, Ti(B, N) and TiB2 and also for multilayered structures where additional aluminium-based insulating surface layers (AlN and Al2O3) were deposited. The corrosion resistance and porosity of the films were analysed by electrochemical techniques. The degree of metallic bonding can play a significant role in influencing the corrosion resistance of refractory transition-metal-based ceramic coatings. Here we demonstrate that, under potentiodynamic corrosion test conditions, resistance to corrosive attack was relatively poor for TiB2, better for (Ti, Al)N and Ti(B, N) and best for TiN. It is also shown that applying the additional protective aluminium-based insulating surface layers on the coating can further improve corrosion resistance.

Effect of tribofilm formation on the tribological performance of hydrogenated carbon coatings

Amorphous hydrogenated carbon (a-C:H) films were deposited by the r.f. plasma technique on stainless steel substrates. Pin-on-disc experiments were carried out over a wide range of normal loads (5–40 N) and sliding velocities (0.1–3.0 m s−1) in order to study the friction and wear performance of the coating against steel and alumina. The friction coefficient of a-C:H films against both steel and alumina pins decreased with increasing load and sliding velocity. On the pin wear surface, tribolayer formation was detected. The wear of the alumina pins increased with increasing load and sliding velocity when they contacted the coating. However, the thick tribolayer formed on the wear surface of the steel pin protected it from excessive wear when high loads and sliding velocities were applied. The wear surfaces were analysed by secondary ion mass spectroscopy and Auger electron spectroscopy. The analyses revealed that the thick tribolayer formed on the pin wear surface mainly consisted of the oxides of the pin material. However, evidence of carbon was found in the sliding deposit formed in front of the contact area of the pin and also in some cases on the pin wear surface. Carbon played an important role in the low friction behaviour although the amount of carbon was low. It is assumed that a thin tribolayer with low shear strength, consisting of carbon species, is formed on the disc wear surface. The coating wear increased when the normal load was increased. Some transfer of pin material was observed on the coating wear surface.

Characterization of wear surfaces in dry sliding of steel and alumina on hydrogenated and hydrogen-free carbon films

Abstract Hydrogenated amorphous carbon coatings were deposited by r.f. plasma and hydrogen-free carbon films in pulsed arc discharge on stainless steel substrates. The coatings were characterized and evaluated in tribological tests. Pin-on-disc tests were used over a wide range of test parameters: normal load, 5–40 N; sliding velocity, 0.1–3.0 m s −1 . The wear of both coatings was of the same order of magnitude (0.7 × 10 −3 −5.1 × 10 −3 mm 3 ). However, the wear of the counterface was one order of magnitude higher for the hydrogenfree carbon coatings. Increasing the normal load generally caused an increase in coating wear and in most cases also an increase in counterface wear. When the steel pin was sliding against the hydrogenated carbon coating with a high sliding velocity and load, a rather thick tribofilm was formed on the pin wear surface, lowering the coefficient of friction and reducing the pin wear. The tribofilm formed on the alumina pin sliding against the hydrogenated carbon film also seemed to reduce the friction coefficient but could not prevent the pin wear. A tribofilm was also formed on the pin wear surface when the hydrogen-free carbon coating was sliding against the steel and alumina pins, but the layer was not able to protect the pins. The tribofilm did, however, lower the coefficient of friction, which was rather insensitive to the different test parameters used. According to secondary ion mass spectroscopy analyses, material transfer of the pin was detected on the disc (coated) wear surfaces. The tribofilms formed on the pin wear surfaces consisted of pin material, hydrogen, oxygen, and carbon.

Tribological properties of plasma nitrided and hard coated AISI 4140 steel

Abstract In the present study, samples made of AISI 4140 steel pre-treated with plasma nitriding and coated with different PVD coatings (TiN, TiAlN and ta-C) were investigated in terms of their microhardness, surface roughness, scratch adhesion and dry sliding wear resistance. Wear tests, in which duplex-treated pins were mated to hardened ball bearing steel discs, were performed with a pin-on-disc machine. To examine the influence of the nitrided zone on the performance of the coating–substrate composite, coatings were deposited on hardened as well as on plasma nitrided samples, prepared under different nitriding conditions. The results of the investigation showed improved mechanical and wear properties of the plasma nitrided hard-coated specimens compared to the uncoated and pre-hardened ones. Furthermore, the compound layer was found to act as an intermediate hard layer leading to superior sliding wear properties of the composite.

Design aspects for advanced tribological surface coatings

Abstract A holistic approach to the study of the important tribological contact mechanisms is described, which provides a basis for effective coating design. The mechanisms include macromechanical effects, defining the stress fields, and these are influenced by the hardness, thickness and surface finish levels of coatings and substrates. Micromechanical mechanisms influence cracking. Tribochemical mechanisms can also determine friction and wear performance. Material transfer is another influencing mechanism, and nanomechanical mechanisms at the atomic level influence friction. Examples are given of coatings and treatments that fulfil the needs of these mechanisms in a range of different contact types.

A comparative study of the corrosion performance of TiN, Ti(B,N) and (Ti,Al)N coatings produced by physical vapour deposition methods

Abstract Thin film coatings produced by physical vapour deposition methods often exhibit porosity. Local defects can cause local and rapid corrosion of the base material. The porosity is difficult to estimate and electrochemical methods are most suitable for evaluating the corrosion resistance of the coated material. This paper compares the corrosion resistance of TiN, Ti(B,N), (Ti,Al)N- and TiB 2 -coated ASP 23 high speed steel. For the materials studied here the corrosion performance of TiB 2 -coated samples was poor. Ti(B,N) coatings obtained by two different methods were quite similar even though the calculated porosity of the coating produced by magnetron sputtering was lower than that of coatings produced by the electron beam technique. These coatings had similar or slightly better corrosion resistance than (Ti,Al)N coatings with a high aluminium-to-titanium ratio. (Ti,Al)N coatings with a low aluminium-to-titanium ratio were better than coatings with a high aluminium-to-titanium ratio. TiN coatings were better than other types excluding (Ti,Al)N + AlN layer coatings, which performed best. (Ti,Al)N + AlN coatings have an insulating layer on top of the coating, which increases the polarization resistance and decreases the corrosion current density.