Polymer tribology - Plastics in sliding and rolling contact

Lubricated plastics are increasingly being used under sliding and rolling loads (e.g. gear wheels). Their advantages are immense, as they dampen noise and/or bridge gaps and tolerances due to their high elasticity. They have low moments of inertia.

They can also be produced cost-effectively in large quantities.

The tribology of polymers is complex: lubricants can lead to unexpected friction and wear behavior, elastomers show higher wear rates in an aggressive atmosphere. Reliable polymer machine elements require customized tribological solutions.

We clarify the friction and wear behavior of polymer tribological systems and determine the necessary characteristic values. We explain how to select effective start-up polymer systems and how to achieve optimum performance. We carry out standard measurements for you and test prototype components. We help you to design systems with knowledge of load limits. We describe the speed behavior of the friction of lubricated polymer systems from adhesion to sliding. We determine characteristic values concerning the tendency to the formation of noise and can derive possible changes.

Tribological standard tests

• Standard tests in the following geometries:
  • Pin-on-disc and (single direction and oscillating sliding): ASTM G99
  • Ball-on-3-Plates (single direction gliding)
  • Ring-on-ring (single direction and oscillating gliding)
  • Four-ball apparatus VKA (single direction and oscillating sliding): DIN 51350
  • Ball ring and ring-ring (rollers)
• 3 Ball abrasion test or blade abrasion test as a model test for tire treads and seals
  • Sand disk (friction wheel): ASTM G65
  • DIN Abrader (reaming drum): DIN ISO 4649
  • Surface indentation
• Microscopy and measurement of the topography and roughness of the surface by confocal microscopy
• Analysis of materials by spectroscopy: infrared (FTIR), Raman and X-ray photo spectroscopy (XPS)
• Testing of components up to TRL6 or TRL7: O-rings, shaft seals, plain bearings, friction bearings, rollers, bushings, gearing

Evaluation of substitutes and accelerated system design

• Tribological system evaluation based on energy parameters (e.g. for material substitutions and new systems)
• Assessment of the effect of lubricants on tribological properties of thermoplastics and elastomers
• Measurement of accompanying parameters: Interaction energies of the tribological partners (interfacial energy, spreading energy, ...), swelling, surface hardness, surface elasticity, local static friction.

Evaluation of the interaction of polymers with lubricants or atmospheres

• Characterization of friction and wear behaviour under different lubrication conditions
• Contact angle measurement of lubricants
• Calculation of interaction energies from surface energies and cohesion energies
• Determination of the drying behavior
• Measurement of the absorption of lubricant up to saturation, swelling
• Measurement of the change in mechanical and dimensional behavior by aging tests in atmospheres
• Determination of softening or post-curing
• Determination of the change in surface hardness and surface elasticity
• Set-up of tribological and mechanical test environment with atmospheric conditions and comparative tests of friction and wear behavior
• Model development for the influence of changes in characteristic values and system due to lubricants and the atmosphere

Evaluation of polymers for stick-slip tendency

• Characterization of the transition from adhesion to sliding
• Measurement of the speed dependence of friction
• Oscillatory shear load up to the adhesion limit
• Determination of adhesion energy (dry) or spreading energy in lubricated contacts
• Calculation of the elastic length for elastomers
• Numerical simulation of the natural frequencies and excitability of the macroscopic contact

System analysis

• Friction and wear maps
  • Creation of a load strategy for a graduated load ramp
  • Calculation of the contact temperature for each load level
  • Classification of the load windows into thermal parameters of the materials and according to the proportionality of friction and wear rate
  • Impact analysis of the enrichment or depletion of fillers
  • Model adaptation of physically motivated friction models and wear models
• Optimization of the inlet behavior
  • Screening for sensitive load windows for a high break-in effect
  • Testing of an adapted inlet procedure or inlet load
  • Characterization of the broken-in properties: surface hardness, distribution of fillers, polymer transfer, smoothing and roughening
  • Testing the stability of broken-in systems
  • Development of intermittent inlet sequences
• Friction of structured elastomer surfaces
  • Calculation of the elastic length of the contact from surface energy and elasticity
  • Measurement of the periodicity of the roughness intervention
  • Evaluation and modeling of the friction behavior of the structured contact surface under shear

Simulation of component wear

• Creation of a component and contact model abstracted to the bare essentials
• Creation of thermal and mechanical boundary conditions
• Research on thermophysical and parametric data of the tribological system, in particular the friction and wear behavior
• Adjustment of imaging fineness and load focusing
• Simulation campaigns on the stability of the thermal and mechanical stability of the contact
• Creation of an adapted concept for time step mapping of the contact change due to wear
• Simulation of the wear-induced change in the load on the components

Simulation of friction-induced component heating

• Creation of component and assembly models and of thermal boundary conditions for heat sources and heat sinks as well as the transition coefficients between components and materials
• Research on thermophysical data and data on conduction, convection and radiation
• Simulation study to test the stability and dominance of the selected thermal situation
• Simulation of the mapping of transient and steady-state temperature distributions over time
• Simulation of the relevant temperature conditions for the evaluation

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Selected research projects

Wear of tire elastomers

Natural rubber is unrivaled in terms of its mechanical and tribological properties and is, for example, suitable

for highly stressed tires (aircraft, construction vehicles, etc.) and is the "elastomer of choice". Natural rubber crystallizes when stretched - this tendency is more pronounced than in synthetic rubbers and is one of the reasons for its good properties. Biological accompanying substances contained in natural rubber contribute to its elongation crystallization. The Fraunhofer BISYKA project investigated whether these biological accompanying substances can bring about a comparable improvement in material properties when added to highly stereoregular synthetic rubber. The abrasion resistance of these new elastomer samples was investigated at Fraunhofer IWM.

Laser-structured and DLC-coated elastomer coatings

To reduce the frictional forces of dynamic seals, elastomer seals generally utilize the principle of lifting the sealing lip via centrifugal force. This leads to almost friction-free operation at higher speeds, but at lower speeds a correspondingly high contact pressure is required in order to reliably seal. This increased contact pressure leads to high friction and wear and at the same time to delayed lifting or no lifting of the seal at all. Particularly in the case of large seals with large distortions, such as in the wind power sector, some of which have diameters of over 1.5 m, tightness can only be guaranteed throughout the entire operation with a very high contact pressure.

Very hard and low-friction diamond-like carbon layers (DLC) can help to reduce friction on the one hand and significantly increase wear resistance on the other. However, DLC layers are inherently brittle and cannot follow the elastomer's elongation without tearing. The layers then "float" on the materials like a scale armor. The size of the scales is determined by the layer properties. Larger residual compressive stresses can, for example lead to large warping and leaks. In tribocontact, the scales become smaller and smaller and limit the service life of the seals.

Together with the Fraunhofer Institute for Structural Durability and System Reliability LBF and the Fraunhofer Institute for Laser Technology ILT, the Fraunhofer Institute for Mechanics of Materials IWM (the µTC) has developed a combination of adapted elastomers, laser structuring and a corresponding DLC coating in this project, which reduces the friction coefficient of the seals, significantly extends the range of use of the seals (loads, temperatures) and considerably extends their service life.

Thermoplastics in drive technology

In an open access publication in the journal Lubricants MDPI, we describe the effects of the specific pairing of lubricant and thermoplastics at low loads in mixed friction against steel. Lubricants can be absorbed by thermoplastics or change the adhesive friction of the wetted system by their tendency to spread into the gap. Friction, sorption and wear of thermoplastics can be described in their tendency via energetic parameters of spreading and interaction.

In summary, the decrease in hardness and mechanical modulus due to sorption and plasticization were confirmed as the main factors for an increase in the wear rate in mixed friction.

Further information:

Publication: Koplin, C.; Oehler, H.; Praß, O.; Schlüter, B.; Alig, I.; Jaeger, R., Wear and the transition from static to mixed lubricated friction of sorption or spreading dominated metal-thermoplastic contacts, Lubricants 10/5 (2022) Art. 93, 21 pages Link

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Koplin, C.; Oehler, H.; Praß, O.; Schlüter, B.; Alig, I.; Jaeger, R., Wear and the transition from static to mixed lubricated friction of sorption or spreading dominated metal-thermoplastic contacts, Lubricants 10/5 (2022) Art. 93, 21 Seiten Link

Abdel-Wahed, S. A.; Koplin, C.; Jaeger, R.; Scherge, M., On the transition from static to dynamic boundary friction of lubricated PEEK for a spreading adhesive contact by macroscopic oscillatory tribometry, Lubricants 5/3 (2017) Art. 21, 9 Seiten Link

Koplin, C.; Abdel-Wahed, S.; Jaeger, J.; Scherge, M., The transition from static to dynamic boundary friction of a lubricated spreading and a non-spreading adhesive contact by macroscopic oscillatory tribometry, Lubricants 7/1 (2019) Art. 6, 14 Seiten Link

Koplin, C.; Weißer, D:F.; Fromm, A.; Deckert, M.H., Stiction and friction of nano- and microtextured liquid silicon rubber surface formed by injection molding, Applied Mechanics 3/4 (2022) 1270-1287 Link

Vogel, S.; Brenner,  A.; Schlüter, B.; Blug, B.; Kirsch, F.;  Roo, T. van, Laser structuring and DLC coating of elastomers for high performance applications, Materials 15/9 (2022) Art. 3271, 13 Seiten Link

Koplin, Christof  ;Schirmeister, Carl ;Schlüter, Bernadette ;Hohe, Jörg  ;Hees, Timo ;Mülhaupt, Rolf ;Jaeger, Raimund, 3D printing of lightweight "All-Polyethylene Single Component" composite materials designed for circularity (2023) Link

Koplin, Christof  ;Schlüter, Bernadette ;Jaeger, Raimund, Running-in effects of lubricated polyether ether ketone on steel for different spreading and sorption tendencies (2023) Link

Hees, Timo ;Zhong, Fan ;Koplin, Christof  ;Jaeger, Raimund  ;Mühlhaupt, Rolf, Wear resistant all-PE single-component composites via 1D nanostructure formation during melt processing (2018) Link

Stauch, Claudia  ;Ballweg, Thomas  ;Haas, Karl-Heinz ;Jaeger, Raimund  ;Stiller, Stefan ;Shmeliov, Aleksey ;Nicolosi, Valeria ;Malebennur, Sriharish ;Wötzel, Jacqueline ;Beiner, Mario  ;Luxenhofer, Robert ;Mandel, Karl, Silanization of silica nanoparticles and their processing as nanostructured micro-raspberry powders - a route to control the mechanical properties of isoprene rubber composites (2019) Link