Smooth operator - Coating reduces energy loss from engine friction

Materials World magazine
,
8 Jan 2013

The macro-trends driving today’s development of car engines can be summarised as reducing energy consumption, increasing fuel efficiency and minimising CO2 emissions. With consumers expecting unabated performance alongside this, the equation can only be resolved with efforts from all parties involved in the design and manufacturing process.

Since an automobile’s engine and power train are an intricate system of mostly fast-moving components, one of the biggest challenges in this area is friction, which is estimated to be responsible for around 10% of overall fuel consumption. Specialised diamond-like carbon (DLC) coatings for critical under-the-hood components have proven to be some of the most effective means of keeping friction to a minimum throughout the lifespan of an engine. Lubricants are also an important factor, and more recently the interplay between lubricant and coating has been confronting engineers with unexpected challenges.

While state-of-the-art lubricants aim to optimise friction reduction, new additives may also accelerate the degeneration of wear-protective coatings on components. In reaction to this development, comprehensive research has been undertaken into the development of a new, metal-containing DLC coating aimed at further reducing friction with significantly higher resistance to wear.

Keep moving
Due to its friction-reducing properties, the previous material of choice has been a conventional DLC coating. The automotive industry has mainly applied DLC coatings to components subject to high loads and those starved of lubrication – components such as camshafts, piston pins, rocker arm parts, tappets and valve needles.

DLC coatings are amorphous films typically measuring around 2µm thick and exhibiting a hardness of at least 20GPa due to diamond-like (sp3) bonds between the carbon atoms. The hydrogen content of a DLC coating is 14–18%.

Despite the extreme hardness and wear-resistance of conventional DLC coatings, their friction-reducing properties have shown incompatibilities with recent developments in oil additives. Compared to uncoated steel parts, for instance, where DLC-coated valve train components were used with engine oil with added molybdenum dithiocarbamate (MoDTC), only a slight reduction to the coefficient of friction (µ) could be observed above 1,000rpm. In addition, an unexpected degeneration of the DLC coating was observed. Exhaustive testing identified the exact cause of the performance loss and premature wear, the results of which were applied to optimise the DLC coating.

It soon became apparent to the researchers that a fundamental understanding of the entire tribological system was vital. A significant finding was that friction reduction is only possible in the boundary lubrication and mixed lubrication regimes of the Stribeck curve (above), which describes the dependence of the coefficient of friction on speed. In these regimes, the research team determined that the surface roughness of the body and counterbody, as well as running-in behaviour, are important factors in defining friction – particularly for lubrication-starved components with high loads.

Tribological insights enable an understanding of the mechanisms in play between a component’s coating and the MoDTC additive, and in this case proved key to developing the solution of a metalcontaining DLC coating that enabled a significant reduction of the coefficient of friction when using lubricants with MoDTC (see graph below).

On track
Once the new coating had been developed, the next step for the researchers was to apply it on a valve train and test it for friction performance. Over a broad range of revolution speeds, the new coating exhibited friction reduction by a factor of greater than five compared to a conventional DLC coating. The coating was also tested for wear protection using pin-on-disc experiments, which revealed that the wear of the coating’s wear track can be minimised as a function of the newly added metal content (See below)). This not only eliminates the disadvantages of an uncoated system (a high friction coefficient), but effectively overcomes the MoDTC-related issues exhibited by conventional DLC coatings (high wear and only slight friction reduction).

Conventional DLC coatings are applied using plasma-enhanced chemical vapour deposition technology (PECVD) under vacuum conditions (≈3–5*10-3mbar), using hydrocarbon gas (C2H2) as a precursor. In most cases, a chromium (Cr) interface layer is applied to provide sufficient adhesion, followed by a chromium nitride (CrN) support layer. Both the Cr and the CrN layers are deposited using sputter technology in the same process chamber as the DLC deposition.

For the new DLC coating, the metal is introduced in the coating using sputter technology in parallel with PECVD, at a temperature of around 200°C. The new MoDTC-resilient DLC coating is already in production and being applied in engines and valve trains. The new metal-containing DLC coating effectively demonstrates that by analysing behaviour within a tribological system, coatings can be optimised on an industrial scale in a highly targeted manner. Such a coating can make a significant contribution to improved fuel efficiency, reduced fuel consumption and, therefore, lower CO2 emissions.

For further information, email Burkhard Boendel, burkhard.boendel@oerlikon.com