ICMCTF2010 Session G2: Coatings for Automotive and Aerospace Applications

There is a great motivation to increase the efficiency of automobiles. This motivation mainly arises from the fulfillment of Kyoto protocol, which states the participating countries to reduce their CO₂ emission until 2012. A reasonable amount of CO₂ is produced by transporting vehicles; hence, increasing their efficiency enables reducing CO₂ emission significantly. Enhancing of the efficiency can be achieved by decreasing the friction behavior or increasing the load carrying capacity of the power train elements. These requirements can be fulfilled by means of plasma coating technologies like PACVD (plasma activated chemical vapor deposition) and MSIP (magnetron sputter ion plating). Using PACVD technology, it is possible to deposit DLC (diamond like carbon), which shows excellent tribological performance, i.e. low friction and high wear resistance, under boundary and mixed lubrication regime. The low temperature ternary coating system Cr-Al-N promises to improve the load carrying capacity and wear resistance of the power train elements, with which same performance can be achieved for decreased weight that leads in return to reduction of CO₂ emission. Introduction of new tribological surfaces for the sake of efficiency increase leads to the question, how lubricants are compatible with these new surfaces, which are normally developed for steel surfaces. Therefore the wettability of the new surfaces with conventional lubricants is still a challenging question. DLC and CrAlN show chemical inactive behavior, therefore the physical interactions can be considered for their wettability with lubricants.

In this study, the physical interactions between the plasma coatings and lubricants are investigated by spreading coefficients and adhesion energy, which are determined by contact angle measurement technique. The plasma coatings are a-C:H (DLC-Star^®, Oerlikon Balzers) and CrAlN. The lubricants investigated are mineral oil, polyalphaolefin, synthetic ester, polyether and polyglycol. The tribological behavior of the coatings a-C:H and CrAlN with lubricants are determined in a pin-on-disk tribometer. The tribological results are compared with the physical interactions of the lubricants with the plasma coatings. For an optimum wettability of the plasma coated surfaces with lubricants, it is found that it is important to adapt both the adhesion energy and spreading coefficient, such that increasing the adhesion energy of the lubricants on DLC and CrAlN, whilst decreasing their spreading coefficients leads to a significant decrease of the friction coefficient.

A consistent increase in demand for special tools designed to an efficient honing of engine cylinder bores is observed. Metallic Bonded Diamond Stones (MBD) are more and more used for their wear resistance and their high tool life. However, the hardness of this metallic bond leads to the degradation of the honed surface aspect by the formation of smudgy grooves edges (Blechmantel). This torn and folded metal affects the required functional performances of the cylinder bore. In this study, two new diamond coated stones with Vitrified and Resinoid Bond (respectively VBD and RBD) are tested and compared to the MBD sticks. The influences of bond material on the honing reliability are studied in terms of honed surface aspect, material removal rate and wear properties. Results reveal that the stiffness and the openness structure of the Resinoid bond lead to a better trade off between surface aspect and tool life.

Several case studies that showcase successful applications of coatings and thin films in the automotive industry will be examined in detail. Emphasis is on the diverse performance requirements of thin films and coatings that are unique to vehicle design and performance. These requirements are presenting challenges to materials development by physical vapor deposition (for example) and quantitative materials characterization.

Finally, existing opportunities and some exciting new challenges for coating and thin film materials related to vehicle electrification are reviewed.

Due to the increasing use of advanced high strength steels, die wear prevention has become an important issue in the stamping of automotive parts. Since physical vapor deposition (PVD) coatings usually has a much higher hardness and resistance of wear than electroplated or electroless coatings and nitrided steels, PVD coatings have been considered as necessary top layers on dies surface to battle the wear problems.

Since most lab tests use operating conditions that do not correspond to actual conditions in production stamping dies, the lab tests may provide misleading results. In this work, a ball-on-plate impact fatigue test was proposed as a novel experimental technique to simulate the wear and fatigue behavior of a coating-substrate system. The simulated stamping force consists of an impact load and a pressing load, which can be adjusted by changing the distance between the impact ball and the sample (plate) surfac and by regulating air pressure in the air cylinder of the impact tester, respectively. Various hard coatings (CrN, TiAlN and TiC) were tested at various combinations of impact/pressing loads (i.e., 200N/400N, 400N/400N, 600N/400N and 800N/400N) for 10,000 cycles. The study provides experimental results concerning cohesive and adhesive failure modes. The failure mechanisms of the tested coatings are determined by means of optical microscope, scanning electron microscope and energy dispersive x-ray analysis. It was shown that among all the tested coatings the CrN coating is the best against fatigue.

I will present an overview of our recent work on surface engineering for automotive applications, including coatings and nanostructured materials for electrochemical energy storage.

(1) Is the lotus leaf truly superhydrophobic?

This question was studied by observing water on lotus leaves and lotus-like surfaces at the micro- and macro-meter scales. Our work suggests that these surfaces can be either hydrophobic or hydrophilic. This finding may have significant ramifications on how to make and use “superhydrophobic” surfaces.

(2) Shape memory surfaces

Using micro- and nano-indentation techniques, we demonstrated the existence of shape memory and superelastic effects under complex loading conditions at the micro- and nano-meter scales. These effects form the basis for applying shape memory alloys as “self-healing” surfaces and “metallic-based adhesion” materials. The microscopic shape memory effect can also be exploited to create surfaces with reversible roughness and texture for applications such as friction control and information storage.

(3) Understanding diffusion-induced-stresses in nanostructured materials for durable lithium ion batteries

We examined the effects of surface tension and surface modulus on diffusion-induced stresses in spherical nano-particles. We showed that both the magnitude and distribution of stresses could be significantly affected by surface mechanics. In particular, a tensile state of stress may be significantly reduced in magnitude or even be reverted to a state of compressive stress with decreasing particle radius. This reduction in tensile stress may be responsible for the observed resilience to fracture and decrepitation of nanostructured materials used in electrochemical energy storage.

These examples suggest a wide range of applications of nanostructured materials for the automotive industry and beyond.