ICMCTF2005 Session TS2-1: Coatings and Automotive Applications

In internal combustion engines, piston rings are required to seal the combustion chamber against the crankcase, this means to prevent gases from passing into the crankcase, and lubrication oil from getting into the combustion chamber. Typically, each cylinder/piston unit is equipped with three piston rings, inserted in grooves at the piston. The top ring, located next to the combustion chamber, is the highest loaded one. Its outer diameter, being in contact with the cylinder liner, belongs to of the most complicated and highest stressed surfaces inside an internal combustion engine. At full load it operates at temperatures up to 300 °C . During the combustion stroke it is pressed to the cylinder wall with pressures up to 200 bar, and oil lubrication is more than poor in modern engines trimmed to ultra-low oil consumption.

In order to prevent wear and scuffing problems at the piston top ring, various coatings are in use, e.g. plasma spray coatings and electroplated hard chromium. The most modern and most powerful coating is a CrN coating deposited by a PVD process.

Typical coating properties are:

Thickness 30 µm. This unusual high thickness is required to survive the 1 million miles operating distance of heavy duty truck engines. Hardness 1200 - 1800 HV. This is enough to achieve very low wear rates, both at the ring side and at the cylinder side.

Engine test results show the superior potential of CrN coatings in terms of wear and scuffing resistance, compared to other types of coatings.

Application of PVD hard coatings on automotive parts in commercial scale was started by Cr-N coatings on the shoes used in the car-air-conditioners in late 80's. For the deposition, AIP (Arc Ion Plating) process was used because of its high production throughput as well as the reliable adhesion of the film. Since then, applications have been expanded by the combination of the successive development on both materials and equipment, and the emerging requirements from the applications, sever tribological condition in automotive engines as an example. Up to now, the PVD hard coatings has been recognized as one of the important surface modification technologies for automotive parts.

This paper intends to give an overview of the successful applications of PVD hard coatings on automotive parts, that have been commercialized by AIP process. Then the discussion will focus on the most popular application, thick Cr-based coatings for piston rings, and the recent development of coating materials and equipment will be discussed.

To avoid wear in roller bearings running under unfavourable conditions, lubricants contain extreme pressure- and antiwear-additives. Because these additives are harmful to the environment it was systematically examined to what extent PVD-coatings are able to take over their function. The effectiveness of the coatings was tested in the FE8 lubricant roller bearing test rig on the basis of cylindrical roller thrust bearings. With the help of this world wide established test rig the roller bearings are tested in mixed friction conditions.

The tests were performed with axial cylindrical roller thrust bearings, which are a quite suitable machine element for tribological investigation. The cylindrical rollers are forced on a circular running track by a cage. Consequently, only in the roller`s centre pure rolling motion occurs, in the other zones positive and negative slippage arise. The coatings have to prove their wear resistance. In dependence on the lubricant and the surface conditions of the friction partners wear is produced on the bearing elements.

Different new developed PVD wear protection coatings like TiAlN, CrAlN, WC/C, W-C:H and ZrCg were systematically investigated on the FE8 test rig with an additive-free mineral oil. The coating systems were quite different in their characteristics like design, hardness, Young`s modulus, critical load and the adhesion strength class. The gravimetric wear for the rollers after fixed time intervals is the criteria for the effectiveness of the PVD coatings in roller bearings. The tests revealed a reliable information for a good coating performance for the parameter critical load and adhesion strength class.

In the automotive industry hard coatings are well established to engine components in order to decrease wear and friction, and to increase lifetime. The performance of the coating depends significantly on the mechanical behavior of the substrate material. Especially low substrate hardness can cause fracture of the coating and a performance loss of the system. This can be overcome by changing the choice of materials or heat treatment strategy, but in many cases, this also means that other performance criteria of the component or the cost targets of the system cannot be met. As a result, the goal is to provide a wear protection by the deposition of a PVD coating without compromising the design or choice of materials.

As an alternative approach, a coating system was developed, combining two different coating types. This approach is already known from some applications. As a bottom layer, a hard layer is deposited as a support and stabilizing layer, offering improved fatigue resistance and dissipating interface stresses. As a top layer there is a wear resistant and friction reducing carbon coating which governs the tribological behavior of the combined coating system. The low friction top layer can be chosen to be a a-C:H or a Me-C:H layer.

The properties and structures of the combined coatings systems are being introduced. The behavior of the coatings under dry and lubricated conditions has been investigated. The respective coating systems have also been tested in various automotive applications. The possible fields of application, from sliding bearings to valve train and power train applications are being discussed.

DLC coatings are now widely used for numerous engineering applications, and are still a favourite topic of academic research. Still, end users know that there is nothing like a universal DLC coating, irrespective of the application. Therefore it often requires specific developments for a new mechanical component to be coated.

In this paper we present a synergistic approach for DLC developments on valvetrain components for high performance engines, by combining basic nano-, micro- and macro-scale mechanical characterizations, pin on disc model tribology, mechanical solicitations simulation through AMSLER test, valvetrain test rig and non fired engine head tests.

It is shown that a parallel processing with continuous interactions and feedbacks in between the various levels of simulation and characterization, offers an efficient and optimised approach for coatings developments. It differs from the classical sequential procedure, based on the successive chronological use of the above tools. In this paper this approach will be illustrated by an example of practical development for a valvetrain application from computer simulation up to the final cylinder-head test.