ICMCTF2015 Session E3-2: Tribology of Coatings for Automotive and Aerospace Applications

Hydrogenated amorphous carbon (a-C:H) thin films can be used in mechanical applications due to its strident properties such as high wear resistance and ultra-low friction. However, a widespread use regarding energy efficient issues in the automobile industry is neglected due to the poor adhesion of a-C:H on steel and/or expensive technologies. a-C:H adhesion on steel can be achieved by nanometric bonding interlayers containing silicon, which are particularly beneficial to mitigate the high compressive stress and the thin film mismatching, promoting stronger chemical bonds between the interfaces. Although the use of such a silicon-containing interlayers is well established in current industrial processes, there is a lack of systematic works reporting the influence of the physicochemical structure of the interlayer on the tribological behavior of a-C:H thin films on steel, mainly from a chemical point of view.

A great effort has been done in the development of carbon based coatings for tribological applications. As a consequence, the slip-rolling resistance of DLC thin films was improved considerably. On the other side, zirconium has been used widely in the decorative industry because of its corrosion resistance and wide range of metallic colors. But the use of Zr in tribological thin films is not very broad. However, recent developments show that zirconium based coatings could perform better than DLC films in tribological contacts under severe contact conditions.

In the present work, zirconium carbonitride multilayered films have been developed on AISI M2 and 52100 steels by cathodic arc evaporation under a wide range of deposition parameters: BIAS voltage (from 0 to 400 V) and processing temperatures (from 100 to 700ºC aprox.), in order to study the possibility of doing the deposition at low temperatures opening the range of substrate steels and energy savings (lower costs).

A wide characterization campaign has been carried out with the resulting ZrCN/ZrN coatings: Calotest, SEM, X-Ray analysis, Raman, XPS, adhesion tests and, finally, a battery of sliding tribological tests. With this we have analyzed the influence of deposition parameters on the films.

We have seen that adhesion improves with higher processing temperatures but bias voltage has to be selected for each tribological application: best adhesion does not lead directly to lowest abrasive wear. Under some sliding conditions it is not necessary to process coatings at higher temperatures as life expectancy of components seem to be unaffected.

Only for specific coatings (and testing conditions) there is a reaction between lubricating additives and coating, forming a tribofilm. This could be related to the contents of elements on the surface and their bonding state. Raman analysis shows more carbon content in the coatings with higher processing temperatures and lower bias voltages.

To meet the future US Corporate Average Fuel Economy (CAFE) standards (54.5 mpg or 4.34 liter/100km in 2025), all efforts are being made to increase the fuel efficiency of vehicles, in which lowering the coefficient of friction (COF) of any moving components including the piston rings is one key area. In addition, efforts are also being made to increase the reliability and sustain the performance including the application of a hard coating on the piston rings. Currently, thick CrN (50-70 µm) is commonly used for diesel engines. The COF of CrN coatings is generally high (0.5-0.7 for dry sliding). Diamond-like carbon (DLC) coating is also being explored due to its low COF, but its current thickness of 2-10 µm is deemed to be insufficient. The main aim of this study is to develop a coating that has both low COF and high wear resistance. Another goal of the development is to reduce the coating thickness (coating time) and hence the coating production cost.

Mo-W doped Carbon coatings were deposited by a combined non-reactive High Power Impulse Magnetron Sputtering (HIPIMS) and Unbalanced Magnetron Sputtering (UBM) technique.

The Mo-W doped C coatings showed nanohardness of 16.5 GPa. In scratch adhesion test critical load values, Lc exceeding 80 N were acieved due to the effective HIPIMS substrate pretreatment. GAXRD patterns indicated that the overal structure was nanocrystalline almost amorphous like.

The coating friction coefficient was measured by room and high temperature pin-on-disc tests under dry and boundary lubrication conditions using 6 mm diameter 100Cr6 steel ball counterpart. Highly viscous non-formulated engine oil (Mobill 10W-60) was used as lubricant.

The friction coefficient in dry sliding conditions was measured to be μ=0.35, which was somewhat higher when compared to state-of-the-art DLC coatings produced by Plasma Assisted Chemical Vapour Deposition or Arc Evaporation techniques. In lubricated conditions however, the Mo-W doped C coatings showed friction coefficient of μ=0.033. which was lower that these reported for a number of state-of-the art DLCs.

Raman spectroscopy of the wear debris was employed to better understand coating wear mechanism in dry and boundary lubrication conditions.

In dry sliding conditions the wear debris consisted of MoO₃ and WO₃ as well as debris of graphitic nature as indicated by the well pronounced D and G bands in the Raman spectra. In these conditions the Me-dopants react with the oxygen from the environment due to the high flash temperatures at the asperity contacts to form oxides, which is typical for the oxidative wear mechanism.

In oil lubrication conditions at elevetaed temperatures, (200^oC) the wear product was a mixture of MoS₂ and WS₂, with Mo and W being the Me-dopants in the coating and sulphur being an element in the oil formulation. Both MoS₂ and WS₂ compounds have a graphite-like layered crystallographic structure, therefore act as solid lubricants. Thus it can be stated that in boundary lubrication condition the tribological behaviour of the Mo-W doped C coatings is goverened by tribochemical reaction wear mechanism.

Nowadays, the control of energy consumption is a priority, especially for the automotive industry due to the problems generated by the global earth warming. The reduction of energy consumption of diesel engines can be reached through the minimization of friction losses in some engine parts, like the piston-ring liner. Indeed, some authors blame this part to be responsible for 40% of friction losses in the diesel engine, particularly under mixed and boundary lubrication regimes. To address this problem, various chemical additives can be added to the engine oil to optimize its friction reduction performance. These additives are supposed to react with steel sliding surfaces to form tribofilms able to reduce friction coefficient. To go further in reducing friction losses and in the protection against mechanical wear, the introduction of Diamond-Like Carbon (DLC) coatings on the mechanical parts is being considered.

A large amount of research has been addressed on the study of the effect of molybdenum dithiocarbamate (MoDTC) additives on the lubricating performances of carbon-based coatings, showing that high wear rate is produced when the MoDTC is blended to the base oil. However, the mechanisms leading to the coating removal are not fully understood yet. On the other hand, the effect of lubricant degradation on the tribological properties of DLC coatings has been poorly investigated although it can provide more information about undue wear of the coatings in the presence of MoDTC-containing base oil.

In this work, the friction and wear performances of different kinds of DLC coatings, in terms of hydrogen content and metallic doping agents, have been analysed for fresh and aged 1% wt. MoDTC-containing oils. The tribological tests have been carried out with DLC/steel and steel/steel contacts under boundary lubrication conditions, using a ball-on-flat tribometer. The wear of the sliding surfaces, steel balls and DLC coated-flat, were evaluated using an interferometer. In order to understand the changes in tribological behaviour as a function of ageing time, tribofilm composition was investigated by X-ray Photoelectron Spectroscopy (XPS). Transmission Electron Microscopy (TEM) technique has been used to observe and analyse the wear particles produced during the friction experiments.

The tribological behavior of technically relevant coatings is closely related to the formation of a third body between two first body surfaces. Usually third bodies form during running-in. A successful running-in results in a long-lived tribo system with low friction and wear, while a system with a third body that evolves towards the “wrong” tribo material results in early failure. Therefore, it is essential to understand the tribo-induced phase transitions that govern third body formation. Despite intense experimental research, many details and most of the underlying mechanisms of the phase transitions that produce third bodies are barely understood. In this contribution, classical molecular dynamics employing realistic bond-order potentials is used as a descriptive and predictive tool to study phase transitions in carbon coatings and simple metallic tribo systems. For carbon films, emphasis is laid on phase transitions that are responsible for the slow wear of these protective coatings. Despite the fact that diamond and diamond-like carbon coatings (DLC) are used in an increasing number of applications, not much is known about the atomic scale processes that cause wear of these films. In our molecular dynamics simulations Diamond and DLC exhibits a mechanically driven phase transformation into a weak sp² phase that can be easily removed from the sliding interfaces. The talk will end with atomic scale insights into third body formation in metals.