ICMCTF 2022 Session E3-TuM: Coatings for Automotive and Aerospace Applications

DLC films are very resistant to abrasive and adhesive wear making them suitable for applications that experience extreme contact pressure. For this reason, they are one of the most promising solutions for application in piston´s rings of internal combustion engine (ICE). However, recent trends in the automotive industry, such as reduced engine sizes and turbocharging, require higher operating temperatures and loads, which the classical DLC films deposited by magnetron sputtering cannot withstand. High Power Impulse Magnetron Sputtering (HiPIMS) has been actively investigated for hard DLC deposition. In HiPIMS, a large fraction of sputtered atoms is ionized, thanks to 2–3 orders of magnitude higher plasma densities than in classical magnetron sputtering. However, in the standard HiPIMS process based on Ar, the ionized fraction of C is very low (few percent), due to the low carbon ionization cross section by electron impact.

In previous work, the authors have shown that adding Ne to the plasma up to 50 % results in the deposition of denser and smoother DLC films, with improved tribological properties and increased hardness. In this work, the thermal stability of DLC films deposited in Ar-Ne discharge gas by Deep Oscillation Magnetron Sputtering (DOMS), a variant of HiPIMS, was investigated. In a first step, the thermal stability of the films by annealing up to 700 ºC in a protective atmosphere. The structure of the DLC films deposited without Ne in the discharge gas was stable up to 500 ºC, while clear signs of graphitization were detected at higher temperatures by Raman spectroscopy. The stability of the film up to 500 ºC was confirm by pin-on-disk tests and nanoindentation at room temperature after annealing. The structure of the DLC films deposited with Ne in the discharge gas was stable up to annealing at 700 ºC as shown by Raman spectroscopy. However, the hardness of the films decreases from 25 to 20 GPa upon annealing at 700ºC, while the specific wear rate increases from 0.5 to 1.25 x 10^-16 m³/Nm. The thermal stability of the DLC films was also characterized by pin-on-disk tests at high temperature. The coatings were tested up to 400 ºC in ambient atmosphere. Adding Ne to the deposition plasma resulted in an increased thermal stability by 50 ºC for the film deposited with a mixed Ne + Ar discharge gas.

While for ferrous boundary lubricated tribological systems, it is relatively well established that the tribofilms formed from lubricant additives determine friction and wear performance, the effect of lubricant additives on DLC coating wear performance and mechanisms is still not clear. The ability to quantify coating thickness as a function of testing time has the potential to provide new insights on the correlation between tribofilm formation and coating wear, enabling improved lubricant and coating designs.

In this paper, the development of a novel method for measuring coating thickness at the nanoscale level, with the potential to be used for in-situ wear measurement during the test, will be reported.The method is based on using a Raman-active coating underlayer as a sensor of the coating thickness.In this approach, the a-C:H coating is considered a light attenuating layer of the silicon underlayer. The method has been used to study the a-C:H coating wear rate from dry and a boundary lubricated system using molybdenum dialkyldithiocarbamate (MoDTC) additive. A two-stage wear progression mechanism has been proposed for the first time to clarify the detrimental effect of MoDTC-derived tribofilm on a-C:H wear by combining detailed structure and composition analysis. The results have also been supported by post-test Raman spectroscopy, optical profilometer, EELS and TEM analysis.

Keywords: wear measurement, coating, additives, Raman, solid-liquid lubricating