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This paper presents the friction and wear behaviour of monolithic titanium-molybdenum carbonitride and tungsten carbide. These materials have been tested sliding against themselves, carbons, fibre reinforced glasses and molybdenum coatings and compared to other ceramics and ceramics composites under unlubricated condition up to 800 °C and 6 m/s.
Self-mated sliding couples of titanium-molybdenum carbonitride display, especially under high-temperature friction, a pronounced wear resistance compared to ceramics and ceramicceramic composites. This very good property is due to the tribo-oxidative formation of so-called lubricious oxides, e.g. Magnéli phases of rutile. They exhibit sliding wear rates under unlubricated friction up to 800 °C as known from mixed/boundary lubrication.
Within the different test conditions, the wear rates of a variety of stationary monolithic materials samples sliding against rotating specimen of (Ti, Mo)(C, N) are always in the group with the lowest wear rates.
The association of the wear resistance, mechanical strength, fracture toughness and low density makes the (Ti, Mo)(C, N) material a complete solution for complex customer demands.
The results presented in this paper have clarified experimentally, that titania-based Magnéli-phases (Ti4O7/Ti5O9 and Ti6O11) with (121)-shear planes exhibit more anti-wear properties than lubricious (low-frictional) properties. The results for dry sliding indicate that the coefficients of friction lie in the range of 0.10.6 depending on sliding speed and ambient temperature. The COF decreased with increasing temperature (T= 22800°C) and increasing sliding speed (?= 1-6 m/s). The dry sliding wear rate was lowest for the Al2O3 at 1 m/s at 800°C with values of 1.7 × 10-8 and 6.4 × 10-8 mm3/N m, comparable to boundary/mixed lubrication, associated with a high dry frictional power loss of 30 W/mm2. The running-in wear length and, more important, the wear rate decreased under oscillating sliding tests with increasing relative humidity. The contact pressure for high-/low-wear transition increased under oscillating sliding tests with increasing relative humidity. At room temperature and a relative humidity of 100% the steady-state wear rate under dry oscillating sliding for the couple Al2O3/Ti4O7Ti5O9 was lower than 2 × 10-7 mm3/N m and therefore inferior to the resolution of the continuous wear measurement sensor. TEM of wear tracks from oscillating sliding revealed at room temperature a work-hardening as mechanism to explain the running-in behavior and the high wear resistance. The hydroxylation of titania surfaces favours the high-/low-wear transition.
Dry friction and wear rates as under liquid lubrication of ceramic/carbon couples up to 450°C
(2004)
In a high temperature tribometer, stationary carbon has been tested against different rotating ceramics (SiC, Si3N4, Al2O3, WC-6Ni, MgO-ZrO2, (Ti, Mo)(C, N)) and stainless steel (DIN 1.4876). The rotating discs were grinded, polished and/or lapped. For most material combinations, the wear morphology is known from available literature. A transfer film with a typical wear pattern was found on the rotating disc. The combination of antimony graphite EK3245 against MgO-ZrO2 did not form carbonaceous transfer layer. Through advanced variation of the roughness up to Rpk=0.011?µm, the wear rate has been reduced to Kv 3.5×10-8?mm3/N?m at a stable coefficient of friction in a millirange of µ~0.008 for a sliding distance of 20.000?m.
Einen großen Beitrag zur Gewichtseinsparung leistet die Substitution von Grauguss-Legierungen durch moderne Aluminiumlegierungen für Pkw Kurbelgehäuse. Zur Erhöhung der tribologischen und thermomechanischen Belastungsgrenzen von Aluminiumoberflächen kommen thermisch gespritzte Schichten in Frage. Zielsetzung nachfolgender Untersuchungen ist es außermotorisch unter Misch-/Grenzreibung das tribologische Potenzial von thermisch gespritzten keramischen Schichtwerkstoffen auf Basis der Stoffsysteme Cr2O3, Cr2O3/TiO2, Cr3C2/NiCr, TiO2, TiO2/Al2O3 im Vergleich zu aktuell serienmäßig eingesetzten Graugusswerkstoffen zu ermitteln. Die Oberflächentopographien und die Schmierstoffe determinieren gegenüber GG26Cr die Verschleißsenkungspotenziale der keramischen Schichten. Eine Reduzierung der Reibungszahlen wird nicht durch keramische Oberflächenbeschichtungen, sondern durch den Einsatz von Polyglykolen als Schmiermedium erzielt.
8 wt% Y2O3-ZrO2 spray powder and thermally sprayed as thermal barrier coatings on outer air seals were characterized by Raman microprobe spectroscopy (RMS) and transmission electron microscopy to investigate the lateral distribution, composition, structure and grain size of ZrO2 phases and to separate ZrO2 and Y2O3. RMS of transfer layers on ZrO2-coated shrouds after rub tests against SiC laser-tipped metallic blades showed the formation of metallic oxides MeIMeII2O4 of spinel type. Wear tracks were partly covered by graphite and spinel-type oxides. In some cases, the latter were better detectable after laser enhanced cleaning by burning off the covering carbon layer.
In situ Acoustic Emission for wear life detection of DLC coatings during slip-rolling friction
(2006)
Different diamond-like carbon (DLC) coatings on a steel substrate (100Cr6) were tested under slip-rolling friction conditions against uncoated counter bodies of the same steel. The initial maximum Hertzian pressure was varied in a range of P0 = 1.52.3 GPa. The friction tests were carried out under dry conditions and with an unadditivated paraffin oil as lubricant. It could be shown that the thickness of the coatings affects the respective wear life. Further, a very important factor for the wear life of a coating under lubricated slip-rolling conditions is the roughness of the surface of the respective counterbody. The wear life tests were monitored by recording in situ the Acoustic Emission (AE) signals. Some causes for a high AE activity could be identified.
Paraffinöl geschmierte Wälztests wurden an extrem harten Diamantschichten auf hartem SSiC Substrat, unterschiedlichen, harten DLC-Schichten auf 100Cr6 Substraten (HRC60) sowie ausgewählten DLC-Schichten auf ungehärtetem Stahl (HRC 20) durchgeführt. Die Wälztests wurden in einem Zweischeibentribometer vom Typ Amsler bei einer anfänglichen, maximalen Flächenpressung nach Hertz von P0=2,3 GPa ausgeführt. Als Abbruchkriterium für die Tests wurde das Erreichen von n=1.000.000 Überrollungen (Langzeittests n=10.000.000 Überrollungen) oder das Auftreten einer Schädigung mit einer zusammenhängenden Fläche von A>1 mm2 festgelegt. Die Wälztests zeigten, dass das harte SSiC Substrat zwar eine stützende Wirkung auf die Diamantschicht hat, diese aber aufgrund von Rissen im Substrat versagte. Ferner gibt es wenigstens zwei DLC-Schichten, die, aufgebracht auf 100Cr6 Scheiben (HRC60), den Wälztests bis n=10.000.000 Überrollungen ohne nennenswerte Schäden widerstanden. Diese Schichten passten sich auch den Verformungen des weichen, stickstofflegierten Stahls (HRC20) an, ohne das es zu größeren Abplatzungen kam (A>1 mm2).
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.
Das ungeschmierte tribologische Verhalten von verschiedenen Stoichiometrien und mit unterschiedlichen Verfahren hergestellten Schichten und Werkstoffen im System Ti-O wird vorgestellt. Der Phasenreichtum geht einher mit dem gleichen Erscheinungsreichtum im tribologischen Verhalten. Obwohl aus den Stoffeigenschaften kaum erkennbar, offenbart das System Ti-O eine phasenweise herausragende Verschleißbeständigkeit, allerdings wurden Reibungszahlen von 0,2 kaum unterschritten.
Diamond like carbon (DLC)-coatings are applied very successfully on computer discs. Sliding friction tests confirm their high performance as dry lubricants with friction coefficients (COFs) below 0.1. In contrast to sliding tests, until very recently most of the DLC-coatings failed in slip-rolling test after fewer cycles than uncoated samples. However, present tests with DLC-coatings of seven different suppliers show a more promising tribological behaviour under slip-rolling conditions. The DLC-coatings were deposited onto steel 100Cr6, HRC 60, with a thickness of 23 ?m. The counter bodies were uncoated, grinded or polished 100Cr6 discs with 30 mm radius of curvature. The tests were performed on a twin disc testing rig (Amsler type) with paraffin oil under boundary/mixed lubrication and rolling with 10% slip. The initial, average Hertzian contact pressure was adjusted to Pm = 1.0, 1.25, and 1.5 GPa. The failure criterion was defined as the occurrence of a single damaged area larger than 1 mm2, which was controlled by optical microscopy (OM). An acoustic emission (AE) measurement system was installed as an additional online control for coating failure. The stressed coatings and the chipping areas were analyzed by OM, SEM, EDX, AFM and Raman spectroscopy.
Until now, the results reveal that a large scatter in lifetime of coatings occurs not only between samples of different coaters but also between the samples of one single batch. The highest lifetime reached by a DLC-coating under these testing conditions is 10 × 106 cycles. That is the highest life time for a coating tested under the testing conditions above in our laboratory so far. The following factors were identified to influence the life time of the coatings: coating thickness, interlayer type, topography of the coatings, counter body roughness and mechanical properties. But the most important factor determining the life time of the coatings is that the coating process does produce homogenous layers free of faults.
Two new substoichiometric titania (TiOx) coatings designated for cylinder liner application were deposited on specimen of grey cast iron GG20HCN with high carbon content by plasma spraying. First, a TinO2n-1 coating was prepared by atmospheric plasma spraying (APS) using a sintered and agglomerated Magnéli-type spray powder. Second a TiO1.95-x coating was deposited with a vacuum plasma spray (VPS) process using a commercial, fused and crushed TiO1.95 powder. The tribological behaviour of these coatings under lubricated conditions was compared with uncoated specimen of this grey cast iron. As counter bodies a widespread used APS-sprayed Mo-NiCrBSi piston ring coating (MKP81A®), an advanced HVOF-sprayed WC/Cr3C2-based (MKJet502®) ring coating as well as non-commercial prototype APS-sprayed TinO2n-1 and APS-sprayed (Ti,Mo)(C,N) + 23NiMo (TM23-1) coatings were tribotested.
The interaction of the pairs with prototype engine oils based on esters and polyglycols were studied under mixed/boundary lubrication using the BAM test method. Lubricants were factory fill engine oils, ester-containing lubricants with low-SAP (sulphurashphosphor) and/or bio-no-tox properties as well as polyglycole-based lubricants. The ester and polyglycole-based engine oils respond both to bio-no-tox criteria and are polymer-free. They follow different strategies to reduce zinc, phosphorus and sulphur to assure a low ash content.
Both TiOx coatings designated for cylinder liners meet or exceed the wear resistance of the grey cast iron with high carbon content when paired with APS-sprayed TinO2n-1 or Mo-NiCrBSi piston ring coatings. Overall, in nearly all pairs the wear rates of the APS TinO2n-1 coating were lower than those of the VPS TiO1.95-x coating.
In order to characterize the tribological behaviour under oil-off, dry-running conditions, additional tests were performed under unlubricated unidirectional sliding conditions at 22 and 400 °C for a sliding speed of 1 m/s against sintered polycrystalline Al2O3 as stationary specimen.
The purpose of this paper is to underline the future need for OEMs to receive lowSAP, polymer- and metal-free engine oils with high-viscosity indices and to illuminate for other OEMs the technical feasibility for application of alternative engine oils based on esters or blends of hydrocarbons with esters or polyglycols.
Design/methodology/approach The strategic goal depends technically on the use of intrinsic properties of alternative base fluids, thus substituing some additives, like anti-wear, extreme pressure and viscosity index improvers. The prone wear resistance of novel triboactive/-reactive materials enables higher portions of mixed/boundary lubrication generated by oils with a lower viscosity.
Findings Overall, the different bionotox and low-ash prototype engine oils with reduced additive contents displayed isoperformance regarding the tribological behaviour against cast iron and triboreactive materials. APS-Tin-2Cr2O2n-1 displayed an overall wear resistance comparable with grey cast iron with high-carbon content and liner wear reduction of one order of magnitude when mated with Mo-based rings. Both tests confirmed the potential for substituing molybdenum-based rings by APS-Tin-2Cr2O2n-1. The most significant reduction in system wear down to zero wear was demonstrated by mating the APS-Tin-2Cr2O2n-1 coated piston rings with smooth machined HVOF-(Ti,Mo)(C,N) liner coatings.
Research limitations/implications As lubricants are today not part of the core business of automotive OEMs, the next steps have to be proposed by the petrochemical suppliers. It is recalled here that some OEMs in their history developed and produced lubricants.
Practical implications The customer will appreciate any increase in longevity resulting in reduced maintenance. The OEM now owns, under increased solicitations, now a future-oriented tool box in order to respond to environmental and CAFÉ demands with reasonable cost management.
Originality/value This OEM report displays the complete methodology in order to adopt alternative engine oils in existing engine architectures.
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.
High-power piezo-electric motors with power densities of 1.4 kW/kg display a potential for substituting hydraulic actuators. For this application, two novel tribometers of the same type have been designed using commercially available components for sliding motion at 40 kHz with amplitudes between 2.5 µm and 5 µm. The tribometers are equipped with means to measure amplitude, frequency, power required to keep the samples in motion and load applied. The effective motion between the two contacting bodies is monitored in each of the tribometers. These data are used to evaluate the coefficient of friction. The wear rate was determined after the tests. The set-ups were tested using well-known 100Cr6H (AISI 52100) samples before investigating novel, non-commercial substrates such as AlFeCrTi-alloys and tungsten carbide-based coatings as well as Magnéli-type coatings (Tin-2Cr2O2n-1 and TinO2n-1). This paper presents the principle of the ultra-high frequency tribometers and first tribological quantities of materials and coatings tested up to and above 1011 cycles. Very low wear rates in the range 10-8 mm³/Nm down to 10-10 mm³/Nm were determined under dry oscillation in air.
The tribological profile of alumina (99.7%) mated against binder-less niobium carbide (NbC) rotating disks were determined unidirectional sliding tests (0.17.5 m/s; 22 °C and 400 °C) as well as in oscillation tests (f=20 Hz, Δx=0.2 mm, 2/50/98% rel. humidity, n=105/106 cycles) under un-lubricated (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.
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.
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.
The historical development on the discoveries of lubrication regimes for lubricated sliding contacts has been reviewed from the very beginning. It was found that the functional relationship between the coefficient of friction and the product of sliding speed and viscosity divided by the normal load well known as the Stribeck curve has been experimentally explored much earlier by Adolf Martens in 1888 long before Richard Stribeck did his pioneering measurements in 1902. However, basic work in this field of experimental hydrodynamics was done and results were published even earlier than that back in 1854 by Gustave A. Hirn, in 1879 by Robert H. Thurston and in 1883 by Beauchamp Tower.
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.
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..
Automotive engine lubricants
(2019)
In 2003, ASTM Manual 37, Fuels and Lubricants Handbook: Technology, Properties, Performance and Testing, featured a chapter discussing automotive lubricants, including engine oils, automatic transmission fluids, manual transmission fluids, gear lubricants, and greases. This chapter, by Schwartz, Tung, and McMillan, surveyed all of these classes of lubricants, up to its publication in 2003.
More recently, the period between 2003 and 2010 has been addressed as part of a book copublished by ASTM and SAE International (coeditors Simon Tung and George Totten), where Fox surveyed the development of engine oil specifications that emerged during those years.
This chapter surveys the evolution of engine oil design and testing since 2002 and concludes with insights into future directions offered by recent tribological research. Our objective is to provide a reader new to the field with an understanding of the following:
- Engine oil composition and formulation
- North American engine oil specification development, and an outline of emergent European specifications
- How emergent specifications and legislative requirements are linked to the introduction of new engine hardware
- How tribological innovation can contribute to future enhancements in engine efficiency
In parallel, we will provide several tables comparing groups of contemporaneous specifications.
Downsizing (power-to-weight ratio) and higher speeds lead to a rise in Hertzian contact pressures in combination with an increase in surface or oil temperatures. Under such conditions, commonly used bearing steels, such as 100Cr6, reach their limits, creating a demand for alternative slip-rolling resistant steel alloys. The present work therefore compares the slip-rolling performance of various steel types with Maraging- and PM-type steel alloys such as e.g. CSS-42L™, ASP2012, BIMAX42+, in the Hertzian contact pressure range up to P0max of 4 GPa. Through-hardened 100Cr6H (AISI 52100), case-hardened 20MnCr5 (AISI 5120H) and nitrogen alloyed Croni-dur30 (AMS 5898) still compete in terms of slip-rolling and wear resistance and load carrying capacity, whereas Maraging- and PM-type steel alloys offer superior strength and toughness properties.
NbC has a relatively high hardness (19.6 GPa) and melting temperature (3600 °C) and is usually applied as a grain growth inhibitor in WC–Co hardmetals. The current study reports on the influence of the sintering technology and overall chemical composition on the microstructure and mechanical properties of NbC-based hardmetals, using Co as a binder. The partial replacement of NbC by 5 wt.% WC, Mo2C, VC or TiC allows one to significantly improve the hardness and toughness of the NbC–Co materials. The influence of different binders, i.e., Co, 316L stainless steel and Fe3Al on the microstructure and mechanical properties of NbC-based hardmetals was also investigated. The powder mixtures were sintered in the solid state by PECS and liquid state by conventional vacuum sintering. Detailed microstructural analysis was conducted by EPMA elemental mapping and WDS point analysis. Mechanical properties, including Vickers hardness and indentation toughness were assessed.
We review contact mechanics with emphasis on the rheological (time dependent) properties of polymers and their relations to surface roughness, material properties, and friction as well as wear behavior of rubbing polymer surfaces. The main concept of polymer mechanics related to tribology consists of three basic elements involved in friction: deformation resulting in the real area of contact of rough surfaces, contact adhesion, and shear and rupture of materials in the contact during the sliding friction. The results of classical work are included, which addresses the real contact area calculation and the description of adhesion interaction between rough surfaces. A brief review of experimental investigations concerning the surface characterization by means of bearing curves, the intermolecular force interaction using the adhesion parameter, the effect of temperature on the real contact area, the formation of transferred polymer film during friction, and tribological behavior of ultrathin polymer layers are presented and their implications discussed.
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.
The tribological and mechanical properties of niobium carbides (NbC) bonded with cobalt of Fe3Al
(2014)
The tribological and mechanical properties of niobium carbide bonded with 8 vol.-% (NbC-8Co), 12 vol.-% of cobalt (NbC-12Co) or 12 vol.-% of Fe3Al (NbC-12Fe3Al) are presented. Rotating discs made of metal-bonded niobium carbide were mated against alumina (99.7%) under unlubricated (dry) unidirectional sliding tests (0.1 m/s to 12.0 m/s; 22 °C and 400 C) as well as in oscillation tests (f=20 Hz, Δx=0.2 mm, 2/50/98% rel. humidity, n=105/106 cycles). Microstructure and phase compositions were determined as well. The tribological data obtained were benchmarked with different ceramics, cermets, hard metals and thermally sprayed coatings, where NbC bonded with 8% and 12% Co presented above 7 m/s the lowest wear rates so far in such a benchmark. Binderless NbC (HP-NbC1) and the metal-bonded NbCs exhibited low wear rates under dry sliding associated with P·V high load carrying capacities. NbC-based hard metal bonded with 12 vol.-% of Fe3Al resulted in a higher hardness level than for 12 vol.-% cobalt. The tribological profile established revealed a strong position of NbC-bearing materials under tribological considerations and for closed tribosystems against established reference tribo-couples.
The research institution TEKNIKER has coordinated the EUROPEAN Project EREBIO, were different biodegradable lubricants have been formulated by FUCHS and BAM for heavy duty engines (GUASCOR), and passenger cars (RENAULT). In the frame of this article, it has been summarised the results obtained when developing biodegradable passenger car lubricants in combination with triboreactive materials.
Replacing hydrocarbon-based oils with biodegradable products is one of the ways to reduce adverse effects on the ecosystem caused by the use of lubricants. The application of low or no sulphur, ash and phosphorous (lowSAP) ester- or polyglycol-based oils, intended for passenger car engine lubricants as substitutes for hydrocarbon-based oils, required the preparation of a composition of lubricants with comparable tribological and functional properties.
The study is focussed on passenger car motor oils (PCMO) with reduced metal-organic additives. This is necessary in order to reduce the ash build-up in the after treatment system and therefore improve its efficiency and lifetime. High fuel efficiency and long drain intervals are requested, as well. To follow a line in a consequent way, these oils have to be biodegradable and non-toxic to the aqueous environment according to the directive EC/1999/45, coherent with other international standard. In a modern diesel or gasoline engine, the engine oils has to fulfil quite a number of different functions, such as lubricating and cooling the system, wear protection, soot and particle handling with less deposit tendency and so on.
In the paper a study of the biodegradability, toxicity and the tribological properties has been carried out for new developed prototype engine bio-oils. Also, some different plasma sprayed triboreactive coatings have been deposited on cast iron piston rings, being studied also their tribological properties. Finally, the behaviour of the new bio-oils selected and plasma sprayed triboreactive coatings on piston rings have been screened in a real engine.
Diamond-like carbon (DLC) coatings are perceived 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 gear and engine components. Therefore, in previous investigations amorphous carbon (a-C) and tetragonal amorphous carbon (ta-C) coatings were inspected. For up to ten million cycles at Hertzian contact pressures P0max of 2.9 GPa it was shown that these coatings are slip-rolling resistant not only at room temperature in the presence of unaddivated paraffin oil, but also up to 120 °C oil temperature. The steel substrates in this investigation were made of the hardened and tempered steels 100Cr6H and Cronidur 30. The aim of the current work is manifold. The Hertzian contact pressures should be increased up to P0max of 4.2 GPa by using new steel metallurgies as substrates for thin film coatings. These steel metallurgies are also assessed, if they can compete as uncoated couples. Two high toughness spring steels and an ultra-high toughness aerospace steel were tested in a twin disc tribometer of the Amsler type as well as in an Optimol 2Disk test rig under mixed/boundary conditions in a factory fill SAE 0W-30 engine oil. Different factors such as influences of the lubrication, surface chemistry and wear behaviour were investigated.
A correlation of adhesion force and electrical conductivity is established for a vanadium oxide and highly oriented pyrolytic graphite (HOPG). Adhesion forces were determined by analysis of pull-off forces obtained from force–distance curves with atomic force microscopy in ultrahigh vacuum, on clean surfaces and with defined contact conditions. The investigated samples include (i) different stoichiometries of Magnéli-type vanadium oxides (V3O5, V4O7, V6O11, and V7O13), which undergo metal-insulator-transitions as a function of temperature, (ii) the (0001) basal plane and the (10–10) prism plane of highly oriented pyrolytic graphite (HOPG), which differ in their respective perpendicular-plane electrical conductivities by several orders of magnitude, and (iii) the (100) surfaces of pure metal single crystals, i.e., silver (Ag), copper (Cu), iron (Fe) and gold (Au). It is shown, that the vanadium oxides as well as the graphite exhibit significantly lower adhesion forces in their electrical conductive state than in their non-electrical conductive state. The values obtained for the electrical conductive states are quantitatively confirmed by the measurements on the single crystal metals.
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.
The tribological profile of alumina (99.7%) mated against rotating disks made in binder-less niobium carbide (NbC) and cobalt-bonded NbC 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, Δx = 0.2 mm, 2/50/98% rel. humidity, n = 105/106 cycles) under unlubricated (dry) conditions. In addition, the microstructure and mechanical properties of binderless NbC and NbC bonded with 8% cobalt 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 and cobalt-bonded NbC exhibited low wear rates under dry sliding associated with high load carrying capacity. The tribological profile established revealed a strong position of NbC bearing materials under tribological considerations and for closed tribo-systems against traditional references, such as WC, Cr3C2 and (Ti,Mo)(C,N).
In order to achieve greater efficiency or to meet light weight requirements, components are downsized. This, however, increases the load, e.g., Hertzian or nominal contact pressures and stresses of tribosystems. This load is expressed as pₐ·v-value, the product of nominal contact pressure and sliding velocity. pₐ·v-values are an effective tool for design engineers for predicting low wear/high wear transitions. Therefore, in the present work, topographical analysis has been combined with the plasticity of micro-asperities and the flash temperatures to estimate the limits of pₐ·v diagrams. The central piece of this set of models presented here is the calculations for flash temperatures and contact mechanics of micro-asperities. This central piece is used to predict the performance of materials in high velocity (turbines, machinery) and low velocity (human joint) applications. It is shown that the model combination suggested here is a useful tool for screening and preselecting a candidate and new materials with respect to tribological requirements before engaging in expensive testing.
Research and development of WC grades is 90 years ahead of niobium carbide-based hard materials, which showed already as light-weight and unexplored material a very high potential for many technical applications, especially for wear protection and machining. NbC evoluted recently from lab scale to pilot scale, especially in areas where established WC-based materials are causing economic, environmental or technical concerns. Benchmark trials of NbC-based hard metals indicated already a favorable performance in machining of steel components.