ICMCTF2012 Session E3-2/G2-2: Development, Characterization, and Tribology of Coatings for Automotive and Aerospace Applications

During the extreme conditions experienced in automotives and aerospace applications, oil-based lubricants break down at high temperatures. Under such conditions, conventional fluid lubricants either fail early or never are considered as an option. As a result, components of engines that are run at high temperatures to improve their fuel efficiency tend to wear rapidly and require replacement. One solution to extend bearing life is with the implementation of a low friction, high temperature stable, and low wear coatings to the component surface that can perform under extreme conditions. Solid lubricant coatings offer a solution for diverse applications exhibiting extreme and difficult running conditions. Although the most common dry-solid lubricants are graphite, MoS₂, WS₂, TaS₂, and PTFE, they are limited in terms of their high temperature capabilities as well as their wear characteristics. Hence in this paper we propose novel thermal spray lubricious oxide coatings based on a crystal chemical approach. Different combinations of the oxide materials are chosen based on their ionic potential differences and plasma sprayed to a thickness of 150 to 200 microns. Their composition, microstructure and high temperature wear characteristics are reported in this paper.

Increasing the efficiency of a gas turbine engine requires that any leakage of gas from the working gas path is minimized, of particular importance is leakage around the tips of high pressure (HP) turbine blades. This can be approached by using an abradable sealing system, where abrasive particles embedded in an anchor phase are fixed to the end of the turbine blade tip, and an abradable coating. The turbine blades cut a track through the abradable coating on the shroud, the abrasive particles protect the turbine blade from wearing away against the abradable.

However lifetimes of the current sealing system are unacceptably short, with oxidation of the abrasive and creep of the anchor phase being two major factors.

The abrasion behavior of various abrasives against a magnesium aluminate spinel abradable has been studied using a pin-on-disc abrasion rig at temperatures up to 1300 ^oC. It has been shown, even at the velocities of just a few metres per second, that abrasion causes fracture of the particles and frictional heating approximately consistent with predictions in the literature. At elevated temperatures and at these velocities it is shown that correcting for the difference in velocity predicts temperature changes about the melting point of the MCrAlY, and even close to that of the abrasive.

Modern components and systems for automotive and industrial applications have to meet various requirements in multiple technical fields. Apart from properties that affect the part itself – like geometry, stiffness, weight or rigidity – the surface properties must be adjusted to the growing environmental requirements. This includes measures for corrosion and wear protection, for optimum electrical or thermal conductivity and for optical purposes. Beyond those, coatings are increasingly used to reduce the friction losses of car components, improve fuel efficiency and reduce CO₂-emissions.

This article describes how to use surface technology as a modern design element for components and systems to enable the increasing requirements on market leading automotive and industrial products.

Therefore Schaeffler has developed and established a coating tool box for customized surfaces to apply the right solutions for all that needs and requests with the corresponding coating system made by PVD-/ PACVD-, spraying or electrochemical technology. For innovative products it is extremely important that coatings are considered as design elements and integrated in the product development process at a very early stage.

In this article tribological coatings are viewed within a holistic and design-oriented context. The latest developments of amorphous carbon coatings, its characteristics as well as the technical and economical effects of its use in combustion engines and industrial bearing applications are described.

The presented Triondur^® amorphous carbon based coating systems (a-C:H; a-C:H:Me; a-C:H:X and ta-C) are excellent examples for customized tribological systems like bucket tappets, linear guidances and roller bearings.

Triondur^® carbon coatings offer the following advantages:

• Super low friction with highest wear resistance.

• Customized surface energy.

• Optimized wetability and interaction with formulated engine oils.

• Low adhesion to the counterpart.

Close collaboration between designers and surface engineers is required in the future. Schaeffler delivers around 70 million high-quality PVD- and (PA)CVD-coated components (Triondur^®) every year that enable outstanding applications, preserve resources and meet increasing customer requirements.

This study utilized cathodic arc evaporation method to coat Ti-Al-N films on AISI 4340 steel for evaluating the feasibility of prolonging the use-life in the application of landing gears and truck parts. SEM, XRD, and TEM were used to confirm the morphology and structure of the coatings, and some coatings properties, such as adhesion, hardness, Young’s modulus, residual stress, and friction coefficient were all analyzed. The results showed that TiAlN film was indeed a single phase of FCC structure. The TiN/TiAlN multi-layered films had a good adhesion (HF1), high hardness (36.5 GPa), Young’s modulus (461 GPa), and appropriate residual stress (-5.68 GPa). Moreover, the optimum coatings achieved a remarkable reduction in the steel friction coefficient from 0.81 to 0.45.

The fretting wear phenomenon in electrical contacts is a plague in many applications, especially in the automotive industry, but also in other machines exposed to vibration. It induces severe electrical distortions, high electrical contact resistance and micro cuts.

This has led to the development of numerous coating systems consisting of pure metallic materials, noble and non-noble, doped as well as soft coatings.

Five coating systems were studied: a bronze–nickel–with different types of doped silver coating system and a bronze–nickel–with different types of doped gold coating system.

Using this apparatus, most of the physical conditions, such as relative humidity, temperature, frequency, relative displacement and normal force, can be precisely controlled and monitored.

We have observed and understood the kinetics of wear in connection, disconnection and reconnection simulation of electrical connector. This approach is applied to analyze hard coating wear mechanisms focusing on abrasion and oxidation phenomena.

DLC coatings are recognised as a promising way to reduce friction and increase wear performance of automotive parts and are currently being introduced for some engine and transmission components. DLC coatings, especially hydrogenated and doped with W or Si DLC coatings, provide new possibilities in improving tribological performance of automotive components, beyond what normally can be achieved with lubricant design only. However, currently used lubricants have been originally designed for metallic surfaces and there is a lack of knowledge on tribological behaviour of DLC coatings when lubricated with these oils, which limits their use in practical applications.

In this project, a W doped DLC coating is tested in lubricated contact simulating the engine tap/follower interface. The role of lubricant/surface interactions is investigated experimentally against a steel counterface. The tribofilm formation process is characterized chemically and mechanically using EDX mapping, XPS and nanoindentation. A link between the tribofilm structure/composition and tribological performance is studied. Hardness and elastic modulus are mapped within the contact area showing the evolution of the tribofilm mechanical properties. The f riction process is related to the dynamics of tribofilm formation and tribofilm time dependence is investigated. The emphasis of the paper is on how the coating structure and composition can be adapted for optimal interaction with fully formulated lubricant to enhance the tribological “system” performance.