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Eingeladener Vortrag
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The slip-rolling resistances of hard and stiff thin films under high Hertzian contact pressures can be improved by optimizing the 'coating/substrate systems'. It is known from former investigations that the so-called 'egg-shell' effect is no general hindrance for high slip-rolling resistance of thin hard coatings. The coating stability depends more on specific deposition process and coating/substrate interface design. In this article it is experimentally shown, that pure amorphous carbon thin films with hardness between 15 and 63 GPa can be slip-rolling resistant several million load cycles under a maximum Hertzian contact pressures of up to 3.0 GPa. Whereas all coatings were stable up to 10 million load cycles in paraffin oil at room temperature, reduced coating lifetime was found in SAE 0W-30 engine oil at 120°C. It was shown how the coating hardness and the initial coating surface roughness influence the running-in process and coating lifetime. No clear correlation between coating hardness and coating lifetime could be observed, but friction coefficients seem to be reduced with higher coating hardness. Very low friction down to ~0.03 in unmodified engine oils was found for the hardest ta-C film.------------------------------------------------------------------------------------------------------------------------------------------------
Die Wälzbeständigkeit von harten und steifen Dünnschichten unter hohen Hertzschen Kontaktpressungen kann durch die Optimierung des 'Beschichtung/Substrat Systems' deutlich verbessert werden. Aus vorangegangenen Untersuchungen ist bekannt, dass der so genannte 'Eierschaleneffekt' kein generelles Hindernis für eine hohe Wälzbeständigkeit für harte Dünnschichten darstellen muss. Die Stabilität der Beschichtungen hängt vielmehr von dem spezifischen Prozessparametern und dem Schicht/Substrat Grenzflächendesign ab. In diesem Artikel wird experimentell nachgewiesen, dass reine amorphe Kohlenstoffdünnschichten mit Härtewerten zwischen 15 und 63 GPa für mehrere Millionen Lastzyklen unter maximalen Hertzschen Kontaktpressungen bis 3.0 GPa überrollbeständig sein können. Während alle Beschichtungen bis 10 Millionen Lastzyklen in Paraffinöl bei Raumtemperatur keine Defekte aufweisen wurde unter Verwendung eines SAE 0W-30 Motorenöls bei 120°C eine Verringerung der Überrollbeständigkeit nachgewiesen. Es wurde nachgewiesen, wie die Schichthärte und ursprüngliche Oberflächenrauheit der Beschichtung den Einlaufprozess und die Lebensdauer beeinflussen. Es konnte keine offensichtliche Korrelation zwischen Schichthärte und Lebensdauer festgestellt werden, allerdings wurde eine scheinbare Reduktion des Reibwertes mit steigender Schichthärte beobachtet werden. Dabei zeigte die härteste ta-C Schicht unter Verwendung des Motorenöls sehr geringe Reibwerte mit ~0.03.
Costs reduction is one of the major objectives in mechanical applications with a parallel increase of the power output. Another driver represents new environmental standards and their increasing restrictions in the automotive industry leading to the question, if materials based concepts may substitute harmful EP/AW additives in lubricants. This paper presents the slip-rolling resistance of different DLC/THC coatings on steel substrates with a definite slip rate of 10%. Industrial DLC coatings of ta-C and a-C:H types from various manufacturers were deposited on steels SAE 52100 and AMS 5898 to compile the slip-rolling resistance under initial average Hertzian contact pressures between 1.5 GPa and 2 GPa. The tests were carried out on Amsler-type twin disc tribometer under the regime of mixed/boundary lubrication in unadditivated paraffinic oil (ISO VG 46) and a factory fill engine oil. The spherical steel sample was uncoated and the cylindrical coated with DLC. Some coatings achieved 10 million cycles without any damage under a maximal Hertzian contact pressure Pmax well over 2.25 GPa, thus exceeding FZG 12.
Nowadays, diamond-like carbon (DLC) coatings are mainly used in many low loaded applications such as in machine tools, computer devices and many more.
Especially the automotive industry anticipates a benefit in applying such coatings in association with the lightweight construction of mechanical parts, for instance in gear components.
The ulterior motive is a global performance increase regarding environmental impact and power efficiency.
In recent years, the slip-rolling resistance of DLC, a-C and ta-C thin film coatings was improved considerably.
In view of the mechanical application of thin film coatings, there is still room for improvements.
It was experimentally shown, that a-C and ta-C coatings can be slip-rolling resistant at room temperature in unaddivated paraffin oil up to ten million cycles under Hertzian contact pressures up to P0max = 2.9 GPa.
Typically, the prime coated steel substrates were made of the hardened and tempered steels 100Cr6H and Cronidur 30.
The aim of the work this to overtake these results, i.e. at higher maximum Hertzian pressures up to P0max = 4.2 GPa.
Under such extreme conditions, it is necessary to choose special steels to avoid the failure of the substrate and to permit a higher load carrying capacity of the coating-substrate-system.
In consideration of the increased substrate properties, two high toughness spring steels and an ultra-high toughness aerospace steel were tested on a twin disc tribometer of the Amsler type as well as on an Optimol TwinDisc test rig under mixed/boundary conditions.
Different factors such as residual stresses of the substrate, influences of the lubrication, and wear behaviour were investigated.
The respective results are presented here.
The slip-rolling resistance of DLC, a-C and ta-C thin film coatings was improved considerably in the last years, but there is still room for improvements in relation to (a) the temperature stability and (b) initial surface roughness after deposition. This paper presents a novel coating-substrate system in comparison to recently developed DLC coatings in a bench-mark test procedure exerting slip-rolling conditions in the presence of liquid lubricants. One of the Zr-based thin film coatings can withstand at least 1 million cycles under initial Hertzian contact pressures of up to P0max = 3500 MPa and oil temperatures of at least 120 °C associated with low coefficients of friction under mixed/boundary conditions or ten million of cycles under P0max = 2940 MPa. In comparison, some of the newly developed DLC coatings are slip-rolling resistant for at least up to 10 million cycles at RT (some of them also at 120 °C oil temperature) under Hertzian contact pressures of Pmax = 2600/2940 MPa. In general, Zr-based thin film coatings do not require special formulated oil formulations and Zr(C,N) bear on a straight coating architecture suited for mass production with nanosized layers.
Zirkonium-basierte Dünnschichten in hochbeanspruchten Wälzkontakten als Alternative zu DLC und ta-C
(2009)
The light-weight approach and fuel economy targets in today's automotive engineering require tribosystems, which can withstand higher contact pressures associated with low coefficients of friction. The application of high-performance coatings represents one approach among others. This paper presents some recently developed DLC coatings (a-C:H and ta-C) as well as a novel coating-substrate system (Zr(C,N)) in a benchmark test procedure under slip-rolling conditions in the presence of liquid lubricants. Various coatings with different thickness, interlayer and substrates were evaluated for their slip-rolling resistance in different lubricants at ambient temperature and at 120 °C. Results indicate that some of these coating systems can withstand at least 10 million cycles under initial Hertzian contact pressures of up to Pmax = 3.500 MPa and oil temperatures of at least 120 °C associated with low coefficients of friction under mixed/boundary conditions. Surface of the coatings and the counter bodies were analysed and compared with untreated substrates.
Wälzbeständigkeit von ta-C Dünnschichten unter hohen Hertzschen Beanspruchungen bis zu 3000 MPa
(2010)
Zirkonium-basierte Dünnschichten in hochbeanspruchten Wälzkontakten als Alternative zu DLC und ta-C
(2009)
Slip-rolling resistant thin film ta-C coatings for up to 3.000 MPa of Hertzian contact pressure
(2009)
Le secteur automobile doit faire face actuellement à de nouveaux challenges au niveau de la construction allégée, des économies de carburant et des coûts. Ces exigences motivent la mise au point de tribosystèmes pouvant résister à des pressions de contact de plus en plus élevées avec de faibles coefficients de frottement. L'optimisation de systèmes existants par l'application de revêtements de surface performants représente une alternative intéressante. Ce texte présente les performances de couches minces dans des conditions d'essais de slip-rolling (roulement à composante de glissement) en présence de lubrifiants liquides. Après une première sélection à température ambiante, les revêtements les plus performants ont été testés à 120 °C. Il s'agit de revêtements DLC en carbone hydrogéné (a-C:H) et en carbone tétraédrique (ta-C) de dernières générations ainsi qu'un nouveau système « revêtement/substrat ». Certains des revêtements DLC développés récemment sont résistants en slip-rolling au moins jusq'à 10 millions de cycles à 120 °C dans l'huile moteur sous des pressions hertziennes de contact de P0max = 2600 / 2940 MPa. De plus, le nouveau système revêtement Zr(C,N)x/substrat peut résister au moins à 1 million de cycles sous des pressions hertziennes initiales de contact allant jusqu'à P0max = 3500 MPa et à des températures de lubrifiant d'au moins 120 °C. -----------------------------------------------------------------------------------------------------------------------------------------------
The light-weight approach and fuel economy targets in today's automotive engineering require tribosystems, which can withstand higher contact pressures associated with low coefficients of friction. The application of surface coatings represents one approach among others. This paper presents thin film coatings performances in a bench mark test procedure exerting slip-rolling conditions in the presence of liquid lubricants. After a first selection at room temperature, the most resistant coatings were evaluated at 120 °C. These are newly developed DLC coatings (a-C:H & ta-C) as well as a novel coating-substrate system. Some of the newly developed DLC-coatings are slip-rolling resistant for at least up to 10 million cycles at 120 °C oil temperature) under Hertzian contact pressures of P0max = 2600 / 2940 MPa. Furthermore, this novel Zr-based thin film coating can withstand at least 1 million cycles under initial Hertzian contact pressures of up to P0max = 3500 MPa and oil temperatures of at least 120 °C associated with low coefficients of friction under mixed/boundary conditions.
Results of rolling contact fatigue (RCF) tests by balls-on-rod (BOR) method and two-roller methods have been compared to those by the balls-on-flat (BOF) method by using three different silicon nitride grades. The results suggest that the two-roller method could differentiate the bearing grade materials in terms of wear behavior.