ICMCTF2014 Session E3-1: Tribology of Coatings for Automotive and Aerospace Applications

The increasing demand for more energy efficiency and environmental friendly products leads to more severe conditions to which the surfaces are subjected. Carbon-based coatings with excellent wear resistance are regularly used in lubricated sliding conditions to protect bulk materials in combustion engines. However, the chemical interaction of the coating surface with oil additives is often limited, which results in relatively high friction. To decrease friction, particularly in boundary lubrication, is still an engineering challenge. In this study we summarize our recent work on tribological behaviour of various tungsten doped DLCcoatings in lubricated conditions.

C-based coatings with different W and H content (W-DLC/H) were deposited by DC magnetron sputtering in reactive and non-reactive atmospheres. All deposited coatings have compact morphologies with amorphous or nanocrystalline structures with tungsten carbide nanograins embedded into amorphous carbon matrix. The tungsten content was in range 2-20 at.%, hydrogen content 0-36 at.%. Pure DLC coatings, both hydrogenated and non-hydrogenated, were deposited as reference. The hardness of the coating increased from 10 to 15 GPa with increasing W content. The coatings were tribologically tested in lubricated contact using pin-on-disc with base (PAO), modified (different amount of additives) and fully formulated oils. Selected coating was then applied on valve lifters and tested in real combustion engine. The coated surfaces almost did not show any wear mark, which was in contract with scratched non-coated steel lifters. Raman spectroscopy, HR-TEM and TOF-SIMS was used to identify coating structure, sp2 /sp3 ratio, and particularly tribolayer formed on the coating surface. We can conclude that the surface of W-DLC/H coatings does react with oil additives and form very thin chemisorbed tribolayer with a thickness 2-4 nanometrs.

DLC coatings on piston rings offer a significant potential to reduce mechanical friction losses and therefore fuel consumption and CO₂ emissions of internal combustion engines, as piston rings contribute to 25% of friction loss. The challenge is to still master the thermo mechanical and tribological load conditions that piston rings must endure owing to smoother cylinder bores, reduced lubrication, and the use of alternative fuels. This is why the coating robustness described by wear resistance and scuff resistance plays an increasing role.

Federal-Mogul´s DuroGlide coating meets in an emerging way these demands. It is the first full DLC coating which can fulfill lifetime requirements even in high loaded Diesel engines. Due to the high amount of sp3-hybridized carbon (tetragonal structure) it can be applied with a layer hardness of up to 5000 HV0.2. In contrast to state of the art hydrogen-free DLC coatings, the intrinsic stress of DuroGlide is reduced during the coating process, which makes coating thicknesses of up to 25 microns with an excellent adhesion to cast iron or steel surfaces possible. A coating topography that is already close to the final design as well as optimized manufacturing processes result in very smooth piston ring surfaces.

Performance investigations started with Rig tests which were tailored to determine friction, wear and scuff resistance under overload conditions. Additionally, results out of a so-called floating liner single cylinder gasoline engine are presented with regards to friction losses in comparison to other coatings. Further, robustness studies out of engine tests with more than 500h running time are shown to confirm the lifetime capability of DuroGlide coated piston rings, applied in different engines and different piston grooves.

Carbon-based materials play an important role in science and technology today . Carbon is a very versatile element that can crystallize in the form of diamond and graphite. In recent years , there have been continuous and significant progress in the science of carbon, such as chemical vapor deposition of diamond , the discovery of fullerenes , carbon nanotubes and single-layer graphene . There have also been important developments in the field of disordered carbons . In general, an amorphous carbon system can be any mixture of sp³ (diamond ) and sp² (graphite ) . The management of these fractions allows to enhance the hardness or contain the deleterious effects of friction generating different types of DLC.

The request for the application of DLC on engine components ( pins , valves , camshafts ) and mechanical parts in general is increasingly growing because it has been demonstrated how this coating is able to increase the surface hardness and reduce friction ; for the racing this means an improvement in the overall performance of the engine for the automotive improvement in yield resulting in the possibility to observe the dictates relating to emissions into the atmosphere.

The need to have a layer well bonded and tribological characteristics of hardness and friction coefficient is crucial to the performance of the component.

The deposition process under consideration is a hybrid technology PVD-PaCVD (physical vapor deposition and plasma - assisted chemical vapor deposition ) . The preparatory stages of degassing and etching to enhance the adhesion of the coating to be evaluated with the aid of the scratch test and the test Mercedes . To improve the adhesion will be used several types of interlayers and act on the conditions of preparation dell'etching ( gas flows and potential difference ) .

The change in the conditions of deposition ( reactive flows , applied potential difference , evaporation conditions ) allows the optimization of the tribological properties of hardness and friction coefficient , respectively, with measurable nanoindenter and tribometers to contact. The change of the tribological characteristics will be related with the fraction sp2 - sp3 .

Minimizing friction and wear-related mechanical failures remains as one of the greatest challenges in today’s moving mechanical systems, and the search for new materials, coatings, and lubricants that can potentially avoid such failures continues around the globe. We demonstrate that few layer graphene not only helps in slowing down tribo-corrosion process but also drastically reduces wear (4 orders of magnitude) and friction (4-5 times) in the case of the most commonly used tribo-pairs, in particular, steel against steel sliding under 1 N load regardless of the surrounding environments (i.e., humid air or dry nitrogen) [1-2]. In addition, we show that graphene application as well as re-application does not require any additional processing steps other than just sprinkling a small amount of ethanol solution containing graphene flakes on the surface of interest making this process simple, cost effective, and environmental friendly. Most of all, unlike conventional solid lubricants which are all sensitive to environmental conditions, graphene offers the possibility of being effective regardless of the operating environment.

References:

[1] D. Berman, A. Erdemir, A.V. Sumant: “Few layer graphene to reduce wear and friction on sliding steel surfaces”. Carbon, 54, 454-459 (2013)

[2] D. Berman, A. Erdemir, A.V. Sumant: “Reduced Wear and Friction Enabled by Graphene Layers on Sliding Steel Surfaces in Dry Nitrogen”, Carbon, 59, 167-175 (2013)

More than two centuries after the first account of its use, electroplated gold alloy thin films remain an essential class of materials for engineers and tribologists designing electrical contacts for extreme environments, where low and consistent friction coefficients, wear, and electrical contact resistance are typically all requirements. Modern advances and refinement in physical deposition techniques have enabled practical fabrication of multi-layer and graded thin film topologies better suited to mitigate the common failure modes exhibited by traditional electroplated metal films, such as through-film solid diffusion of underlayer and codeposited species to the contact surface. The use of zinc oxide in a gold matrix as a friction and wear modifier via dispersion strengthening (Au-ZnO nanocomposites), alumina diffusion barrier layers, and gold-alumina multilayer films, and their effect on tribological and electrical contact behavior will be discussed.