Efficiency improvement in automobile bucket tappet/camshaft contacts by DLC coatings - Influence of engine oil, temperature and camshaft speed

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Dobrenizki, Ladislaus ; Tremmel, Stephan ; Wartzack, Sandro ; Hoffmann, Dennis Christopher ; Brögelmann, Tobias ; Bobzin, Kirsten ; Bagcivan, Nazlim ; Musayev, Yashar ; Hosenfeldt, Tim:
Efficiency improvement in automobile bucket tappet/camshaft contacts by DLC coatings - Influence of engine oil, temperature and camshaft speed.
In: Surface and Coatings Technology. Bd. 308 (25 Dezember 2016) . - S. 360-373.
ISSN 1879-3347
DOI: https://doi.org/10.1016/j.surfcoat.2016.09.041

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Abstract

With the aim to fulfill legal regulations concerning energy efficiency and greenhouse gas emissions, diamond-like carbon (DLC) coatings are increasingly used on highly stressed components of internal combustion engines to reduce friction and prolong component lifetime. The valve train is one of a key system of the combustion engine considerably contributing to frictional losses, especially under boundary and mixed lubrication conditions at lower crankshaft speeds. In this regard, the tribological contact bucket tappet/camshaft offers high potential for friction reduction but imposes high demands on DLC coatings due to highly complex kinematics and repetitive loads depending on the cam contours and the camshaft angle and speed. The aim of this work was to analyze the influence of DLC coatings on the frictional and wear behavior within the contact bucket tappet/camshaft. This tribological contact was analyzed in a one tappet/one cam friction test-rig using series-production tappets, cams and valve springs ensuring high transferability of the results into the real application. Two amorphous hydrogen containing carbon based coatings were deposited on series-production bucket tappets. The zirconium based DLC coating a-C:H/ZrCg was compared to a DLC coating system a-C:H:X deposited under industrial conditions. One mineral motor oil of SAE viscosity grade 0W20 and one synthetic motor oil polyalphaolefin (PAO) of SAE viscosity grade 5W30 formulated with different additive packages containing the anti-wear (AW)/extreme pressure (EP) additive zinc dialkyl dithiophosphate (ZDDP) and the friction modifier (FM) additives molybdenum dialkyl dithiocarbamate (MoDTC) and glycerol monooleate (GMO) were used in the test-rig. Special attention was paid to the interactions of the additive packages in the low viscosity oils with the functional surfaces of the a-C:H:ZrCg coated bucket tappets. The frictional behavior determined by time-resolved measurements of friction forces reveals that higher camshaft speeds led to increased lubrication film formation contributing to reduced frictional losses. Higher motor oil temperature resulted in temperature induced reduced viscosity of the motor oils leading to higher frictional losses. The DLC coatings a-C:H:ZrCg and a-C:H:X caused significant reduction of frictional losses under the operating conditions in the test-rig. The a-C:H:ZrCg coated bucket tappets caused higher frictional losses than uncoated bucket tappets at low camshaft in boundary friction conditions. Friction reduction at all operating points was ensured by a-C:H:X regardless of camshaft speed, motor oil temperature and the choice of motor base oil and additive package proving the great potential for application on components under challenging lubricating and loading conditions. The wear of the uncoated and a-C:H:ZrCg coated functional surfaces was analyzed by means of confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). The a-C:H:ZrCg coating contributed to wear protection in the highly loaded tribological contact despite the different lubricating and loading conditions regardless of the base oil and additive packages. The a-C:H:X coating revealed similar favorable behavior. Comprehensive surface sensitive analysis of the uncoated and a-C:H:ZrCg coated bucket tappets by means of Raman spectroscopy and XPS indicated that no tribo-chemical reaction layer was formed from the motor oil and the additive packages on a-C:H:ZrCg coated bucket tappets under the operating conditions. Moreover, the combination of additives ZDDP and GMO in PAO appears to be disadvantageous in terms of reaction layer formation since no tribo-chemical reaction layer was found on uncoated steel bucket tappets after tribological tests in contrast to the mineral oil containing ZDDP and MoDTC.