ICMCTF2011 Session G2: Coatings for Automotive and Aerospace Applications

There is an increasing interest in developing low friction, high wear resistant coatings to improve the fuel efficiency and durability of automotive engines in which DLC and a few other coatings are already used In this study, we investigated the tribological behavior of MoN-Cu based nanocomposite coatings under oil lubricated sliding conditions at 80-100 ^oC temperature range. The coatings were produced by sputtering molybdenum and molybdenum-copper targets at different power levels. In order to characterize coatings, calotest, scanning electron microscopy time of flight secondary Ion Mass Spectroscopy (ToF-SIMS), and EDS techniques were used.

The tribological behavior of coatings was investigated using ball on disc and reciprocated test machines against steel ball or cylinders at 80-100 ^oC temperature range. The Hertzian contact pressures were kept at around 1GPa. Pure Poly alpha olefin 4 (POA4) , MoDTC and/or ZDDP blended (up to 1% wt.) PAO 4 oils were used as lubricants. Non-hydrogenated and hydrogenated DLC films were also tested under the same conditions for comparison. Wear of tested surfaces was analyzed by using a 3D optical profilometer. MoN-Cu nanocomposite films showed superior friction and excellent wear resistance even under very harsh test conditions. In order to understand superior tribological behaviors of MoN-Cu nanocomposite films, post-test analyses were done on sliding surfaces using ToF-SIMS. Surface analytical findings were correlated with the superior friction and wear behavior of coatings with respect to different lubricant mixtures.

State of the art DLC multilayer coatings designed for fuel injection components were prepared by PVD and PACVD deposition technologies to minimize the wear and friction. Besides the well known tribological properties of DLC coatings also the wetting behavior in motor oil or fuels plays an important role for the performance of the coatings.

The tribological results of different DLC multilayer coatings demonstrated significant differences in various fuels like Gasoline, Diesel, White Spirit, Rasp Methyl Ester (RME) and E85 (Gasoline/Ethanol mixture). The tribological properties of the DLC coatings and the counter body vary over a wide range depending of the used deposition process, coating architecture and the hydrogen content.

Due to the global-level air pollution and global warming, it becomes a pressing need to reduce CO₂ release and to lower emission for automotive internal combustion engines. Improvements on thermal efficiency and mechanical efficiency of the engines are done for fuel economy which is directly connected to these problems. Among them, especially in terms of mechanical efficiency, it is one of the effective methods to the fuel cost reduction by lowering the friction of the piston system that accounts for about 50% of the mechanical loss.

The piston ring is one of the engine components, which is subjected to high gas pressure and reciprocates at high speed in the engine. It mainly has three functions; (1) Gas Sealing Function, (2) Oil Control Function, and (3) Heat Transfer Function from Piston to Cylinder Wall. In order to maintain these functions, base materials for piston rings need to have higher elasticity, better anti-wear property and higher thermal conductivity, and the sliding surface of the piston ring especially requires having better tribological performance. Therefore, based on required performances on applications, various surface modifications have been chosen and applied for piston rings’ surface. On this occasion, we will outline features of the surface modifications.

Diamond-like carbon (DLC) films received considerable interest due to the outstanding mechanical and tribological properties as well as chemical inertness and hydrophobicity, nowadays metallic nanoparticles were used to improve chemical and tribologycal properties.

This paper presents the correlation between surface energy of titanium nanoparticles (Ti-DLC) before and after oxygen/argon plasma treatment. The Ti-DLC coatings with low and high energy before and after plasma treatment were immerged in ethanol, gasoline and diesel oil to study friction coefficient, and wear. The friction and wear tests were run out with AISI 304 sphere as counter face. The goal was to analyze Ti-DLC surface energy after oxygen and argon plasma treatment and correlated trybological behavior of these coatings in regular fuels. Titanium nanoparticles were introduced in DLC films by sputtering and surface coatings were treated by oxygen and argon plasma. DLC films were obtained by PECVD. Thus, surface energy, electrochemical measurements, friction coefficient, wear, optical profiler images, Raman spectra’s and infra-red were use to correlate the most protective coatings to work in contact with AISI 304 and corrosive fuels.

The friction reduction properties of DLC coatings are in competition with other solutions, like new functional designs and technologies, but especially with developments in the field of improved oil additives. For example MoDTC, a friction modifier, is used in many engine oils. Initial tests indicated, that MoDTC in high concentrations is strongly interfering with DLC coatings used for wear protection. Both friction reduction and wear resistance are affected negatively, asking for adjusted solutions.

In this paper the interference of MoDTC with DLC (here a-C:H) is presented and compared with standard oils. As mentioned above, the use of a MoDTC-containing lubricant decreases the wear resistance of a-C:H and therefore limits the friction reduction induced by the coating. Based on this observation it is demonstrated how the analysis of the MoDTC initiated wear mechanism of a-C:H leads to the development of a potential new solution, a modified CrC-coating type. Indeed, a detailed investigation of CrC-type coatings with different compositions has been carried out with respect to their morphology, functionality and tribological properties. Based on these investigations it is demonstrated that by adjusting the composition and morphology of CrC-type coatings the tribochemical interaction with MoDTC additives in engine oils can be strongly influenced. Laboratory tests and first application tests show that the use of CrC-type coatings affect the interference with MoDTC-containing lubricants positively, leading to increased wear resistance and reduced friction compared to a-C:H coatings running with MoDTC oils .