Ingenieurwissenschaften und zugeordnete Tätigkeiten
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The process window and range of functional properties of NbC is wide, because micro-hardness, hot hardness, sliding wear resistance, elastic modulus and toughness can be tailored by the C/Nb ratio, because Nb4C3, 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, and by the type of binder. Further improvements enable the addition of secondary carbides. Supporting results from literature and 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. NbC can be synthesized by carbothermal conversion of Nb2O5 or be metallurgically grown in melts and leached out as coarse NbC cubes. NbC provides comparably lower 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 Co3O4.
The origin of the intrinsic wear resistance of NbC-based materials is investigated through an assessment of the tribological performance of fully dense, crack-free spark plasma sintered Nb2O5 (here as a reduced polymorph: monoclinic Nb12O29 or NbO2.416). The most likely wear mechanism on NbC is the tribo-oxidation to Nb2O5. The unlubricated (dry) friction and wear behavior of alumina (99.7%) mated against rotating disks of crack-free niobium(V)oxide (Nb2O5) under unidirectional sliding (0.03–10m/s; 22°C and 400°C) and oscillation (f=20 Hz, dx=200 mm, 2/50/98% rel. humidity, n=105/106 cycles) will be presented. The microstructure and mechanical properties of the crack-free Nb2O5 are assessed. The tribological data obtained are benchmarked with different NbC grades, ceramics, cermets and thermally sprayed coatings.
Laser-induced periodic surface structures (LIPSS) were generated on titanium nitride (TiN) hardcoating surfaces (deposited on metallic substrates) upon irradiation with multiple linearly polarized femtosecond laser pulses in air (30 fs duration, 790 nm wavelength, 1 kHz pulse repetition rate). The conditions were optimized in a sample-scanning geometry for the processing of large surface areas (5 mm x 5 mm) covered homogeneously by nanostructures with sub-wavelength periods ranging between ~200 nm and 700 nm. For these nanostructures the coefficient of friction was characterized under reciprocating sliding condition against a ball of hardened steel at 1 Hz using different lubricants (regime of mixed friction). After 1000 cycles, the corresponding wear tracks were characterized by optical and scanning electron microscopy. High-resolution energy dispersive X-ray analyzes (EDX) allowed the visualization of chemical alterations within the wear tracks. For specific conditions, the nanostructures endured the tribological treatment. Our experiments provide a qualification of the tribological performance of the fs-LIPSS on TiN surfaces.
Besides carbon, metal sulphides are used in every standard brake pad formulation as friction stabilisers. In order to investigate their impact, various powder mixtures were tribologically stressed in a pin-on-disc device in order to produce and test artificial third bodies. containing the three main components of a real friction film, namely magnetite, graphite and a metal sulphide of interest. Since the sliding behaviour is influenced by the relative humidity the powder mixtures are tested at low, medium and high moisture levels. The tests revealed that the presence of graphite dominates the friction behaviour as well as the humidity sensitivity.
The powders were prepared by either manual mixing or high energy ball-milling. Interestingly, all ball-milled blends provided coefficients of friction within the desired range for braking, which was attributed to the effect of zirconia particles from wear debris of the milling balls. The zirconia particles prevent the formation of solid lubricant films on the surfaces and support a homogeneous mixture of all constituents of the powder. Optimum brake performance is gained by an optimum combination of solid lubricants and abrasives.
To investigate the influence of metal sulphides in brake pads, powder mixtures with up to three components were tribologically stressed in a pin-on-disc device under different levels of relative humidity. The tests revealed that each metal sulphide causes significant differences in resulting coefficients of friction as well in their sensitivity to humidity, so that only one solid lubricant cannot stabilise the friction behaviour during braking in different environments.
New interpretation approaches for seizure load tests on the translatory oscillation tribometer (SRV)
(2016)
When looking in detail at analyses of the tribological load-carrying capacity of high-performance lubricants, it becomes apparent that the stop of the machine of an exclusive evaluation of the development of the coefficient of friction cannot provide any sufficient criteria for determining the occurrence of adhesive failure. This is due on the one hand to the increasing complexity of lubricant formulae and sophisticated additive packages, and on the other hand to the increasing frictional power capacity of modern drive and control concepts in the design of tribometers. For this reason, it is urgently needed to examine the adhesive processes and their detection in more detail with the help of appropriate tribological values and criteria. The evolution of the friction can be coupled with the stroke, contact resistant and/or noise emission. Besides from new criteria for adhesive failure, which were compiled by the work group for the relevant ISO, DIN and ASTM standards, this contribution discusses the inclusion of additional parameters. Among other things, it provides guidance on what evidence the continuous recording of the original coefficient of friction and position signals, the electrical contact resistance, temperature, noise emission, frictional power input of the machine and further derived parameters help to identify adhesive failures or events.
In order to meet the goal of reducing CO2 emissions, automotive industry places significant importance on downsizing components to achieve greater efficiency through lower weight and reduced friction. As friction reductions are associated with energy efficiency and wear protection with resource conservation, ever greater attention has been given to adamantine carbon-
based coatings and high-alloyed steels. Such applications are, however, associated with high production costs and energy expenditures, as well as many technical difficulties. Therefore a key issue in meeting the goals of friction reduction, wear protection and development of comprehensive lightweight strategies is whether or not the functional profiles of state-of-the art alloys can be enhanced by affordable solutions.
The running-in phase of mechanical systems is inevitable and, from a tribological standpoint, critical for the lifetime of such systems, though receives little attention and is poorly understood.
The growth of micro-cracks accelerates premature 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. Two mechanisms were
investigated:
a. Cold work hardening and
b. Chemical tribofilm formation.
Steels that showed a tendency toward work hardening and tribofilm formation in previous testing were chosen for this investigation. Thorough characterization of the chosen Steels was carried out before any pre-conditioning techniques were applied. The widely used 20MnCr5, seen as a reference gear steel, underwent case-hardening and subsequent deep freezing treatments in an attempt to yield discrete sample groups with respect to residual austenite.
This allowed for a more thorough investigation into the effects of residual austenite on the properties of this material. The high-performance alternative steels, 36NiCrMoV1-5-7 (hot working steel) and 45SiCrMo6 (spring steel), were heat treated as recommended by their respective manufacturers, and were not case-hardened. The selection of materials with and materials without case-hardening allows for an investigation into whether or not case-hardening is even necessary to deliver acceptable friction behaviour and wear performance. 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), for 20MnCr5 ranging from approximately 6 – 14 vol.%, and under 2 vol.% for
the alternative alloys. 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-case-hardened 20MnCr5. Residual stresses from machining and case-hardening were measured directly at the sample surface. The high-performance Steels fulfilled manufacturer expectations in terms of elemental content, with hardness values between
50 – 55 HRC and strongly martensitic microstructure character. With characterization of the chosen materials complete, the materials could then be subjected to pre-conditioning.
The first pre-conditioning method involved targeted generation of cold work hardening as induced boundary layers to protect the contact zone against wear. Work hardening was identified both by variations in residual stress profiles, i.e. the introduction of beneficial compressive residual stresses, and hardness increases in the contact zone, providing enhanced wear resistance. Parameters for work hardening were further optimized to reduce damage to the surface substrates of the treated materials. The second pre-conditioning method involved
the targeted generation of chemically reactive tribolayers (tribofilms) on twin disk testing rigs.
The lubrication strategies were based on:
a. CaCO3, which is predominant in engine oils, and
b. MoDTC, which is commonly used in engine and gear oils.
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 and lasting 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.
Finally, slip-rolling endurance testing (T = +120 °C, 107 cycles, approximately 19 days in
a factory fill engine oil) was carried out on all materials. It was shown that both pre-conditioning methods could achieve significant reductions in friction and wear during testing at up to and including P0Mean =1.94 GPa (P0Max = 2.91 GPa, FN = 2,000 N). Ultimately, this research showed that:
1. non-case-hardened high-performance steels offer competitive wear performance and better friction behaviour than the case-hardened 20MnCr5.
2. pre-conditioning led to COF reductions to under 7/10 and wear coefficient reductions to an astonishing 1/10 of the original values for the untreated steels under mixed/boundary lubrication.
3. the observed improvements to friction behaviour and wear performance are indicative of
a technically simple, cost- and energy-efficient pre-conditioning strategy that may prove
Copper is one of the most important components in brake pads and its amount can reach up to 14%. In spite of a number of positive features copper usage in brake pad formulations has recently become the subject of considerable discussions, primarily due to concerns about potential risks related to environmental impacts of copper particles. So, for developing new pad formulations with possible replacements of copper content, it is very important to understand the functionality of copper additions to brake friction materials. In the paper theoretical investigation of the role of copper as a pad ingredient was carried out on the basis of modelling by the method of movable cellular automata (MCA). In the study the concentration of copper particles in a Fe3O4-matrix was varied. The sliding simulations were performed while assuming material properties at 500 degrees C in order to assess the beneficial role of copper during severe braking conditions corresponding to fading cycles during dynamometer testing.
Substituting tungsten carbide (WC) as cutting tools and for wear protection by niobium carbide (NbC)
(2016)
As a refractory carbide, niobium carbide (NbC) is today still a forgotten carbide with hidden properties, such as wear resistance, thus qualifying NbC for the group of tribological materials with enhanced wear resistance. Several phases, like Nb₄C₃, 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, which enable the tailoring of properties and offers a much wider process window than known from WC. The type binder (Co, Ni, NiMo) and its concentration as well as the sintering process (SPS, liquid phase sintering) determine the box “hardness-toughness”, which will be illuminated based on more than 100 grades.
The impact of these tailored properties on sliding wear (T= 22/400°C; v= 0,1-10 m/s) and cutting performance will be illuminated by tribological and machining results. Cutting test results under emulsion and coolant-free operation of cobalt and Fe3Al bonded, straight NbC versus WC based inserts against different alloys (C60, 100Cr6, 42CrMo4, X90CrMoV18, 300WA, GG35) will be highlighted.
Several phases, like Nb₄C₃, 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 Co₃O₄.