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- Niobium carbide (16)
- Wear (15)
- NbC (14)
- DLC (11)
- Friction (8)
- Slip-rolling (8)
- Cobalt (6)
- Hardness (5)
- Machining (5)
- Cermet (4)
Organisationseinheit der BAM
OEMs perceive diamond-like carbon (DLC) coatings as part of a strategy for low frictional tribosystems. Especially the automotive industry anticipates a benefit in applying such coatings in association with the lightweight construction of mechanical parts, for instance in gears and engines. In previous investigations it was shown, that DLC, amorphous carbon (a-C) and tetragonal amorphous carbon (ta-C) coatings can be slip-rolling resistant at least up to ten million load cycles at Hertzian contact pressures P0max of 2.9 GPa lubricated by engine oil at 120°C, Further improvements in load carrying capacity up to P0mean of 2.62 GPa (FN= 5,000 N) were achieved with alternative, available Steel metallurgies as Substrates for thin film coatings. Additional factors, such as influences of the lubrication on the surface chemistry (additives and base oils), wear and friction behavior were investigated.
Leichtbaustrategien im Antriebsstrang erhöhen durch Downsizing die Hertz’schen Kontaktpressungen und die mechanischen Lasten. Dadurch müssen neben wälzbeständigen Stählen auch dazu passende reibungsmindernde Konzepte gefunden werden, in der vorliegenden Arbeit wurden alternative, hoch zähe Stähle mit einer Wälzbeständigkeit bis Pomax = 4,2 GPa untersucht, welche mit geeigneten Grundöl- und Additivkombinationen die Reibungszahlen herunter auf 0,025 reduzieren konnten. Diese alternativen Stahllegierungen erlauben nicht nur an der Zahnflanke sehr hohe Kontaktpressungen, sie bieten außerdem entsprechende Zähigkeits- und Festigkeitseigenschaften für den Zahnfuß. In vorangegangenen Untersuchungen wurde eine Vielzahl verschiedener Dünnschichten auf ihre Überrollbeständigkeit unter Extrembeanspruchungen validiert. Allerdings zeigte sich bei den meisten Dünnschichten ein Qualitätsunterschied von Charge zu Charge. Unter Verwendung verschiedener organischer, metallorganischer und anorganischer Additive konnten, sich daran anschließende Versuche, vielversprechende Ergebnisse erzielt werden. Sämtliche Versuche unter extremen Beanspruchungen wurden bei +120°C Öltemperatur auf Zwei-Scheiben-Tribometern vom Typ AMSLER und 2Disk von Optimol Instruments unter Misch-/Grenzreibungsbedingungen durchgeführt und die tribologischen Eigenschaften der neuartigen, unbeschichteten Stähle und in Relation zu leistungsstarken DLC-/ZrCN Dünnschichten gesetzt.
By means of an analysis of future trends in engine oil specifications with the associated consequences for the functional properties performed by base oils and individual additives, the need and potential application of alternative fluids and triboactive materials or lubricious oxides will be displayed. Oxidation resistance and low volatility represent key elements to assure long drains or lifetime lubrication in order to minimize or avoid manual or automatic top-ups. The intrinsic properties of base oils and the additive package define the oxidation resistance and related oxidative volatility. The overall engine oil consumption is also related to design (clearances, thermal stability of geometries, seals,...) and bore surface finishing and are not regarded here. Lubricious oxides or triboactive materials and/or polar base oils substitute the EP and AW properties realized by the additives, thus enabling long drains and responding to "eco-tox" or "bio-no-tox" requirements as well as restrictions from the "chemical box".
Unlubricated tracked chains were realized using soft solid bonded films for the inner wear of the tribosystem of bushing/pin and monolithic ceramic bushings for the outer wear of the tribosystem bushing/sprocket. Both novel approaches were tested on D6 22-to. dozers. Selected solid bonded films and specific monolithic ceramics, offer a potential for simplifying the design, reducing weight and designing a dry-running longlife chain. These results can be further transferred to high-speed running gears or belt drives.
Highly concentrated or loaded contacts in powertrain and gear applications offer significant weight savings in C02 emissions. Today, thin film coatings, alternative base oils and new additives are populär as working fields and strategies of OEMs to realize low friction tribosystems. Especially the automotive industry anticipates environmental and performance benefits in applying light-weight construction of mechanical parts, for instance in gears and engines. Light-weight strategies increase the contact stresses above 2.14 GPa (or FZG load stage 14) due to the reduction of component sizes. These demands directly raise questions about the suitability of the most promising technical solution. Alternative steels with improved mechanical properties, i.e. toughness values, which offer low coefficients of friction, optimal resistance to high contact pressure and potential to apply low friction surface coatings representing a valid alternative to conventional gear steels. Traditional bearing and gear steels, such as 100Cr6 (SAE E52100) or 16MnCr5 (1.7131) have reached their mechanical limits at contact pressures above FZG load stage 14.
Zero wear (Null Verschleiß)
(2012)
Recent developments in contact mechanics and microfribology have shown that it is very important to study surface modification effects for quantification of „Zero wear“ (plastic deformation of upper parts of asperities) during sliding friction. The present work considers experimental and Simulation data on the Formation of elastic and plastic contact areas during sliding friction on rough surfaces. The Simulation of discrete contact was carried out using as a source the AFMimages of real surface topography at micro/nanoscale.
The approach for quantification of „zero wear“, proposed here, is not using common Statistical Parameters of roughness, but it considers contact areas on asperities.
In these areas some regions are undergoing plastic deformation which influences surface Transformation during sliding friction. Also, the workhardening effect was taken into account in the Simulation
of sliding friction. This enabled a rnore successful fit of experimental data. The results obtained of the Computer simulations will provide a way of studying friction force at the initial stage of the running-in process.
The tribological profile of alumina (99.7%) mated against rotating disks in two niobium Carbide (NbC) grades were determined in unidirectional sliding tests (0.1 m/s to 7.5 m/s; 22°C and 400°C) as well as in oscillation tests (f= 20 Hz, Ax= 0.2 mm, 2/50/98% rel. humidity, n= 1062 c* y*c *l es) under unlubricated (dry) conditions. In addition, the microstructure and mechanical properties of NbC were determined as well. The reason for testing hot-pressed NbC was to avoid side effects generated by sintering additives and/or second phases. The tribological data obtained were benchmarked with different ceramics, cermets and thermally sprayed coatings. NbC exhibited low wear rates under dry sliding associated with high load carrying capacity. The tribological profile established revealed a strong Position of NbC under tribological considerations and for closed tribo-systems against traditional references, such as WC, Cr3C2, (Ti,Mo)(C,N), etc..
Environmental legislations prescribe globally more and more environmentally acceptable lubricants (EALs), based on esters and polyglycols. The compatibility of EALs with tribo-materials used in hydraulic System such sliding parts and oil seals needs to be considered. Aluminium alloys are widely used as lightweight material for sliding parts in many applications. It is necessary to provide optimized design on its surface because of the inferior wear resistance under mixed/ boundary lubrication. The AA6061 alloy (uncoated and coated by different conversion coatings) and an alternative AI-Fe-XY alloy (uncoated) were tested by a continuous sliding test (cylinder-on-disk) under mixed/boundary lubrication with commercially available EALs at 80°C against 100Cr6 Steel. Appropriate combinations of aluminium alloys (coated or uncoated) and commercially available EALs have the potential to compete with conventional tribo-system like steel/steel lubricated with mineral hydraulic fluids under mixed/boundary lubrication.
The tribological profile of alumina (99,7%) mated against binderless niobium Carbide (NbC) rotating disks under under unlubricated (dry) friction and the type of motions of unidirectional sliding (0.1 m/s to 7.5 m/s; 22°C and 400°C) and oscillation (f= 20 Hz, Ax= 200 mm, 2/50/98% rel. humidity, n= 105/106 cycles) was determined including the microstructure and mechanical properties. The obtained tribological data were benchmarked with different ceramics, cermets and thermally sprayed coatings. The established tribological profile revealed a strong position of NbC under tribological considerations and for closed tribosystems against traditional references, like WC, Ci‘3C2, (Ti,Mo)(C,N), etc..
Zirkonium-basierte Dünnschichten in hochbeanspruchten Wälzkontakten als Alternative zu DLC und ta-C
(2009)
Das triboiogische Profil von binderlosem NbC und mit 8% bzw. mit 12% Cobalt oder 12% FeaAl gebundenem NbC wird unter unidirektionaler Gleitreibung (v= 0,1-10 m/s, T= 22°C und 400°C) und unter oszillierender Gleitreibung vergleichend gegenüber Ingenieurkeramiken und Hartmetallen dargestellt. Zusätzlich werden die 4-Punkt-Biegebruchfestigkeit, elastische Eigenschaften (E,G) bis 1.000°C die lastabhängige Mikrohärte sowie die Phasenzusammensetzung und Gefügestrukturen vorgestellt.
Die Verschleißkoeffizienten des binderlosen NbC lagen bei T unterhalb von 10'6mm31 / *N* m, während diejenigen der cobaltgebundenen NbCs mit ansteigender Gleitgeschwindigkeit von 2-4 10‘6 mm3/Nm bei 0,1 m/s auf 5-7 10"7 mm3/Nm bei 10 m/s abnahmen. Die Verschleißkoeffizienten bei 400°C lagen für die NbCs generell unterhalb von 10‘6 mm3/Nm. Das triboiogische Lasttragevermögen im Trockenlauf, ausgedrückt als PV-Wert, nimmt mit ansteigender Gleitgeschwindigkeit auf 100 W/mm2 zu. Bedingt durch die geringe Löslichkeit von NbC in Legierungen offenbarten erste Zerspanversuche mit verschiedenen Stählen eine gegenüber WC-basierten Schneidstoffen signifikant angehobene Zerspanungsvolumina. Somit offenbart NbC in offenen und geschlossenen Tribosystemen Anwendungspotentiale.
3 different base-stocks (poly-a-olefin, polypropylene glycol and unsaturated TMP ester) and these blended with 2 different additives (1wt% ZnDTP or 1wt% dibenzyl disulfide(DBDS)) were used as lubricants to evaluate the tribological properties of AA6061-T6 treated by means of plasma electrolytic oxidation in a SRV tribometer (Toi|=80 °C, Fn = 50N, v= 50 Hz, 2mm stroke, 2hrs). The base oils of PPG and the TMP-ester had better lubricities than the PAO with/without the additives. ZnDTP tend to reduce wear, but when added to PAO and PPG increased the friction. The unsaturated TMP-ester +1wt%DBDS showed 45% lower friction than PAO + ZnDTP (reference), whereas PPG + DBDS had 30% lower friction.
This paper presents a dry sliding tribo-couple (DLC/monolithic oxide ceramics), where the tribological quantities are more or less in a wide ränge independent of the sliding speed. The DLC film is composed of a specific mixture of sp3 /sp2 hybridized carbon. Self-mated ceramic/ceramic couples of MgO-Zr02, alpha-AI203 or SSiC are with their coefficients of friction above 0.45 not so lubricious, whereas the coefficients of friction of the tribocouples MgO-Zr02/DLC and alpha-AbOVDLC ranged between 0.075 and 0.15 under dry sliding. The coefficients of friction for these tribo-couples tend to decrease with increase sliding speed from 0.03 m/s to 6 m/s. The dry wear rates of the thin film is low (<10‘7 mm3/Nm) and perfectly independent from sliding velocity.
The tribological profile of rotating disks made in binder-less niobium Carbide (hot-pressed NbC) and cobalt-bonded NbCs (NbC-8Co and NbC-12Co) mated against alumina (99.7%) were determined under unidirectional sliding tests (0.1 m/s to 8.0 m/s; 22°C and 400°C) as well as in oscillation tests (f= 20 Hz, Ax= 0.2 mm, 2/50/98% rel. humidity, n= 105/l 06 cycles) under unlubricated (dry) conditions. In addition, the microstructure and mechanical properties of HPNbCl and NbC bonded with 8 vol.-% and 12 vol.-% cobalt were determined as well. The tribological data obtained were benchmarked with different ceramics, cermets, hard metals and thermally sprayed coatings, where the NbCs bonded with 8% and 12% Co presented above 7 m/s the lowest wear rates so far in such a bench mark. HP-NbCl, NbC-8Co and NbC-12Co exhibited low wear rates under dry sliding associated with PV high load carrying capacities. The tribological profile established revealed a strong position of NbC bearing materials under tribological considerations and for closed tribo-systems against traditional references.
Die Ausbildung von tribologisch günstigen Grenzschichten ist eine häufig für die Phase des Einlaufs angenommene Modellvorstellung. Solche tribologisch günstige, durch Reibungsvorgänge im Kontaktbereich induzierte Triboschichten, werden in der Fachliteratur auch als sog. Beilby- und Cervovit-Schichten oder auch als Tribomutationen bezeichnet. Durch eine Vorkonditionierung mittels modifizierter Schmierstoffe oder durch Kaltverfestigung kann der Verschleiß in Wälzkontakten signifikant erniedrigt werden. Bislang ausgeblieben ist eine deutliche Reibungsminderung, aber es wurde keine Reibungserhöhung gemessen. Es waren die Ziele, mithilfe geeigneter Schmierstoffadditive in Verbindung mit dem Prozess des Kaltverfestigens durch das „Dichtwalzen“ (unter Hertz’schen Kontaktpressungen oberhalb von Pmax = 3,0 GPa), ohne Vorschädigung, eine metallurgische Veränderung zu erzielen und gleichzeitig durch einen induzierten Tribofilm geschützte Stahloberfläche zu erhalten, sodass thermochemische Behandlungen durch diese Konzepte einzeln oder kombiniert substituiert werden können.
Wear tests are often used in engine development in order to characterize the functional profile of surfaces interacting with lubricants. This paper proposes a new test procedure to determine Stribeck-type curves for the tribosystem piston ring/cylinder liner involving test pieces prepared from engine components. The Variation of the friction coefficient with sliding speed depends on the structure and Constitution of the base oil, the viscosity index, the surface topography of the liner and ring, and the friction modifiers used. Novel organic friction modifiers of low molecular mass not only reduce the friction coefficients under mixed / boundary lubrication, but also in the fluid film regime. In addition, the influence of surface roughness on the wettability of surfaces can be significant for both static and dynamic wetting. For this reason, the interaction of the lubricant with the surfaces was also investigated to enable a correlation between the surface properties of the liquid and solid phases and the tribological behavior.
Slip-rolling resistant thin film ta-C coatings for up to 3.000 MPa of Hertzian contact pressure
(2009)
Niobium carbide (NbC) is an alternative cutting material. The Federal Institute for Materials Research and Testing (BAM) and the Institute for Machine Tools and Factory Management (IWF) of the TU Berlin assessed the cutting suitability of NbC with cobalt (Co)-binder compared to WC with Co-binder in the machining of a wide range of materials.
Substitution of tungsten carbide by niobium carbide as cutting materials and for wear protection
(2016)
Several phases, like Nb₄ C₃ , Nb₆C₅ and other short and long range ordered phases occur in the region of homogeneity of NbCₓ (0.75 ≤ x ≤ 1.0) in the binary phase diagram. Properties, like micro-hardness, hot hardness, sliding wear resistance, elastic modulus and toughness can be tailored by the C/Nb ratio, the addition of secondary carbides and the type of binder. Supporting results from different grades with varying C/Nb ratio or binder types were illuminated. Thus, the NbC system offers to producers a wider process window, than WC. The impact of these tailored properties on sliding wear and cutting performance was illuminated by tribological and machining results.
Niobium is today largely available. NbC can be synthesized by carbothermal conversion of Nb₂O₅ or be metallurgically grown and leached out, provides comparably low friction in many relevant tribo-contacts and displays low wear. NbC and Nb₂O₅ have so far no REACH classification related to human toxicology and are not listed as substances of very high concern contrary to WO₃ and Co₃ O₄ .
The tribological behavior under dry sliding of a Fe3Al-NbC composite prepared by pyro-metallurgical synthesis is compared to different SPS sintered NbC-based hardmetals bonded by cobalt or Fe3AI. The dry sliding wear resistance (0. 1-7 /10 m/s) of the present Fe3Al-NbC with -60 % NbC was close to those known ofNbC-based hard metals. No grain pull-outs or fragmentations of the NbC grains were seen in the wear tracks of the Fe3Al-NbC composite, as a metallurgical interphase was formed between matrix and NbC grains. The physical prope1ties of melt grown NbC match those known from powder metallurgically sintered NbC using powders converted from Nb205.
In this present study, the mechanical properties (strength, hardness, moduli) and the dry sliding properties of stoichiometric and sub-stoichiometric NbC were compared. Microhardness and elastic properties of NbC depend from the C/Nb ratio, because the binary phase diagram Nb-C shows a region of homogeneity of NbCx of 0,72≤ x ≤1.0. At RT, hard metals of stoichiometric NbC have an elastic modulus E of ~440 GPa, those of substochiometric NbC0,88 an E of 405 GPa. The hot hardness of sub-stoichiometric NbC is above 600°C higher than of WC. The dry sliding wear resistance (0,1-7/10 m/s) of the present Fe3Al-NbC0,94 with ~61 vol.-% NbC as hard phase was close to those known of NbC-based hard metals. No grain pull-outs or fragmentations of the NbC grains were seen in the wear tracks of the Fe3Al-NbC composite (MMC), as a metallurgical interphase was formed between matrix and NbC grains. Stoichiometric and sub-stoichiometric niobium carbides have at RT and 400°C under dry sliding a prone intrinsic wear resistance more or less independent from sliding speed, either as hardmetal or as hard phase in metal matrix composite, associated with an exceptional high load carrying capacity.
Thc tribological behavior under dry sliding of a Fe3Al-NbC composite prepared by pyrometallurgical synthesis is compared to NbC-based hard metals bonded by cobalt or Fe3Al. The dry sliding wear resistance (0,1-7/10 m/s) of the present Fe3Al-NbC with 60% NbC was close to those known of NbC-based hard metals. No grain pull-outs or fragmentations of the NbC grains were seen in the wear tracks, as a metallurgical interphase was formed between matrix and NbC grains.
The approaching CO2 targets have now enforced automotive OEMs to engage R&D efforts also in powertrain to reduce friction, increase lifetime properties, and load carrying capacities of running systems. Martensitic steels such as 36NiCrMoV1-5-7 have a great potential to be used in automotive powertrain applications due to its favorable mechanical properties. In order to realize lightweight strategies, it is essential that steels with improved fracture toughness values and higher annealing temperatures with regard to higher contact pressures and possible thin film coating application be considered. State-of-the-art steels, like 16MnCr5, 21NiCrMo, 30CrMoV9, and 100Cr6 are not able to sustain a further increase in torque or load during operating time as well as annealing effects under higher oil or deposition temperatures. Therefore, the recent research on the slip-rolling resistance, frictional, and wear behavior of steels such as 36NiCrMoV1-5-7, Cronidur 30 (AMS 5898), 20MnCr5 (SAE 4820 or 5120), and 100Cr6H (SAE E52100) showed that possible lean alloying concept alternatives with promising performance characteristics are already available.
Full densification of Fe, Co and Ni bonded NbC based cermet’s was achieved by pressure less liquid phase sintering in vacuum for one hour at 1420°C. The hardness and toughness of the NbC matrix cermet’s can be mainly tailored by the binder composition and secondary carbide additions. Ni binder based NbC cermet’s allow the combination of high hardness and improved toughness. The addition of lesser amounts of VC/Cr3C2 in a NbC partially substituted WC-Co cemented carbide increased significantly the hardness in combination with a moderate fracture toughness.
The aim of the study presented in this paper is to highlight the potential of niobium carbide cutting tool inserts for machining and to compare their performance to commercially available tungsten carbide inserts. In this work, a comparison between a conventional Wc-Co insert and NbC-Co inserts was carried out under hard machining conditions on specimens made of 100Cr6 bearing steel grade (SAE 52100).
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above-mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2 alumina and antimony impregnated carbon.