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- 2015 (22) (entfernen)
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- Niobium carbide (5)
- Cobalt (4)
- Friction (3)
- NbC (3)
- Wear (3)
- Fe3Al (2)
- Hard metal (2)
- High contact pressure (2)
- Molybdenum (2)
- Oscillation (2)
Eingeladener Vortrag
- nein (11)
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 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.
The running-in phase of mechanical systems is critical from a tribological standpoint, though poorly understood. Microcracks accelerate material failure and wear during this phase of heightened friction. With this in mind, the ultimate goal of this current work is to transfer the running-in phase into the final step of the mechanical finishing process through the targeted pre-conditioning of novel, high toughness steel bearings without thermo-chemical treatments and compare these to conventional, case-hardened steels. This pre-conditioning involved the targeted implementation of two specific lubricant packages, the first with CaCO3 as the active ingredient and the second with MoDTC as the active ingredient, to generate chemically reactive tribolayers (tribofilms) on twin disk testing rigs. Pre-conditioning was carried out up to 104 load cycles (approximately 25 min). The films generated in pre-conditioning were analyzed by SEM-EDX with Element-Mapping, Raman spectroscopy, and XPS to elucidate their molecular composition and concentration on the sample surfaces. The combination of these methods of analysis gave a clear indication that 104 cycles were sufficient to generate stable chemical tribofilms. CaO and CaCO3 were the main components of the tribofilm from the first lubricant package, while MoS2, MoO2 and MoO3 were the main components from the second lubricant package. Tribofilm-protected samples were then subjected to slip-rolling endurance testing (T=+120 °C, 10,000,000 cycles, approximately 19 days in a factory fill engine oil) to determine any changes in friction behavior or wear performance. Some significant reductions in coefficients of friction at the end of endurance testing were observed, though in certain cases, no definitive improvement was observed. In contrast, very strong reductions in wear were observed across the entire spectrum of materials and testing loads. In some cases, sample surface wear reduction from pre-conditioning via tribofilms reached over 90%. The observed improvements to friction behavior and wear performance are indicative of a technically simple, cost- and energy-efficient pre-conditioning method that may prove to be competitive with existing thermochemical treatments for steel alloys.
Das tribologische Profil von binderlosem NbC und mit 8% bzw. mit 12% Cobalt oder 12% Fe3Al 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 10-6 mm³/N?m, während diejenigen der cobaltgebundenen NbCs mit ansteigender Gleitgeschwindigkeit von 2-4 10-6 mm³/N?m bei 0,1 m/s auf 5-7 10-7 mm³/N.m bei 10 m/s abnahmen. Die Verschleißkoeffizienten bei 400°C lagen für die NbCs generell unterhalb von 10-6 mm³/N.m. Das tribologische 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.
The inevitable increase of torque or load, because of lightweight approaches, pushes the oil-film temperatures on the teeth flanks of gears above 200°C, and requires alloys with higher strength and toughness properties. Furthermore, the real temperature on surfaces during the physical vapor deposition (PVD) or chemical vapor deposition (CVD) of thin films can exceed the annealing temperature of the state-of-the-art bearing and gear materials, such as AISI 52100 (1.3505), 16MnCr5 (1.7131), 21NiCrMo (1.6523), and 30CrMoV9 (1.7707). Such trends call for slip-rolling-resistant, alternative alloys suited for contact stresses above P0max of >2.2 GPa. The tribological profile under slip-rolling conditions at 120°C in a factory engine oil (SAE 0W-30, ACEA A3/B4, ν120°C = 5.33 mPa, HTHS150°C = 3.0 mPa) until 10 × 106 load cycles between P0max of 2.25 GPa to 3.92 GPa of uncoated 100Cr6 (1.3505, PBQ), 102Cr6 (1.2067), Cronidur 30 (1.4108, DESU), 45SiCrMo6 (1.8062, CVEM), 40SiNiCrMo1 [vacuum arc remelting (VAR)], CSS-42L (AMS 5932, vacuum induction melting (VIM)-VAR, and carburized), 36NiCrMoV1-5-7 (CAB), ASP2012 (ASP), and 20MnCr5 (1.7131, carburized) was established in twin disk machines (Amsler-type and 2Disk). The present benchmark illuminates the tribological impact of uncoated bearing steels in comparison to uncoated alternative steel alloys on friction, wear and slip-rolling resistance, as well as load carrying capacity. Iron-based steel alloyed with silicon or molybdenum reduces alloying costs and avoids thermo-chemical treatments. This benchmark suggests considering an extension to fracture toughness from hardness and strength as a relevant property for slip-rolling alloys. If slip-rolling resistant, the wear rates of the metallurgical different steels were more or less within one order of magnitude, but the influence of increasing contact stresses on the tribological profile differed significantly.
This paper shows how Angle-Resolved Scattering can reveal wear on engineered surfaces. The samples studied, three discs made of steel 100Cr6H used in gear wheels in the automotive industry, were assessed after they had undergone ball-on-disc tests. Scattering maps recorded for spatial frequencies from 0.1 µm-1 to 2.7 µm-1 show a contrast inversion around 0.36 µm-1 revealing the wear trace. Besides measurements of rms roughness, where mean values are 20 nm and 24 nm depending on the locations considered, stationarity and isotropy, we demonstrate the ability of the technique to reveal localized wear on this type of surface in a rapid, robust and convenient way. We show that the tool trace is influenced by the load magnitude rather than by the number of oscillation cycles and highlight the key role played by surface roughness in how the sample responds to wear tests.
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).
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.
The automotive industry places significant importance on downsizing components to achieve greater efficiency. The goal of reducing CO2 emissions has led to the development of lightweight materials that are also able to enhance performance. In light of these aspirations, the aim of this study is to characterize two novel, high-performance steels, as well as a 'classical' gear steel for comparison with each other and currently applied materials. The gear steel underwent carburization and subsequent deep freezing treatments in an attempt to yield discrete sample groups with respect to residual austenite. The high-performance steels were heat treated as recommended by their respective manufacturers, and were not carburized. Elemental analyses were conducted by multiple methods to ensure accurate results. Residual austenite contents of the steels and the depth profiles of residual stresses were determined by X-ray diffraction (XRD). Hardness profiles were taken from the testing surfaces into the material core. The carburization of 20MnCr5 led to higher hardness and the greater concentration of carbon in the carburization zone more representative of a hardened SAE E52100, or 100Cr6/102Cr6, than of a non-carburized 20MnCr5. Residual austenite contents ranging from approximately 6–14 vol% were generated, though effectively providing only two, rather than the desired four discrete sample groups. Residual stresses from machining and carburization were measured directly at the sample surface, and from carburization alone below the surface. The high-performance steels fulfilled manufacturer expectations in terms of elemental content, hardness between 50 and 55 HRC and strongly martensitic microstructure character. Finally, slip-rolling endurance testing (T=+120 °C, 10,000,000 cycles, approximately 19 days in a factory fill engine oil) was carried out on all materials, whereby coefficient of friction distributions during testing and wear coefficients after testing were calculated. Testing was performed up to and including P0Mean=1.94 GPa (P0Max=2.91 GPa, FN=2000 N). Ultimately, the non-carburized high-performance steels showed competitive wear performance and better friction behaviour than the carburized 20MnCr5, which has been attributed to their work hardening capability.