Polymer Tribology

The tribology of polymers can be complex: a new lubricant can lead to unexpected friction and wear behavior of a plastic gear wheel, in some hinges of a batch the plastic bearings "creak", or an elastomer shows significantly higher wear under tribological load in an aggressive atmosphere. We develop solutions for these issues.

On this page:

Services and use cases

Influence of lubrication on thermoplastics

Energetic system evaluation and lubricant interaction

Wear of tire elastomers

Structured elastomers and elastomer coatings

"Squeaking" due to sliding movement of plastic-hard pairings

Hydrogels, biological tribosystems

Services and use cases

Services 

• System evaluation based on energy parameters for lubrication (e.g. for material substitutions and new systems)
• Measurement, characterization, understanding of
  • Wear development
  • Degradation development
  • Adhesion tendency of polymer systems (stick-slip, NVH, creaking, etc.)
  • Speed dependence of friction (Stribeck,...)
  • inflow behaviour of structures (softening, polymer transfer,...)

Use cases

• Drive elements: gearing with lubrication, intermittent load between bearing removal, friction and wear
• Seals: wear resistance and sealing effect of surfaces, friction coefficient development with lubrication and structuring
• Noise-vibration-harshness (NVH; jerking, squeaking, jamming, creaking ...): The transition from sticking to sliding determines the tendency to stick-slip excitations and can also be systematically analysed with lubrication
• Abrasion of elastomers
• Lubricant selection for thermoplastics
• Wear behaviour of recyclable systems
• Influence of media (e.g. oxidising or inert gases) on friction and wear
• Substitution of materials in lubricated polymer systems (PTFE replacement, greases, oils, ...)

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Influence of lubrication on thermoplastics

Wear behavior of swelling thermoplastics

How can lubricated thermoplastic systems be selected and evaluated in drive technology? A procedure for assessing the effect of lubricants on the tribological properties of plastics was derived for industrial users and a proposal for a test procedure was developed.

For example, experiments were carried out on the polymers polyoxymethylene (POM), polyketone (PK) and polyamide 4.6 (PA46), which are typical for tribological applications, in combination with polar and non-polar polyalkylene glycol oils (pPG and uPG), an ester-based oil (EST) as well as an oil based on polyalphaolefin (PAO). The polymers in the oil were aged at 100°C, 130°C and 160°C, both in an air atmosphere and in the absence of oxygen for periods of between 4 and around 100 days.

For the systems analyzed, increasing the swelling of the polymers resulted in a decrease in shear modulus, microhardness and an increase in the abrasion rate.

The changes in the tribological application properties due to thermal ageing and oxidative attack were system-specific, but their effect can be classified by the changes in the material properties.

Lubricated thermoplastics already exhibit specific behavior when exposed to air or in the absence of oxygen. Swelling leads to an increased wear rate under mild mixed friction loads.

It makes a difference which lubricants and which thermoplastics are mixed with each other in drive or sliding elements.

What happens with an unfavorable pairing? Does friction increase? Does wear increase?

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 with 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.

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
https://publica.fraunhofer.de/handle/publica/417803

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.

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Energetic system evaluation and lubricant interaction 

Interaction energies

The deposition of lubricant on a surface leads to the formation of a characteristic droplet or film. The size of this drop depends on the interaction of the liquid with the substrate. The surface energy can be calculated from the contact angles of several liquids on a surface. In addition, the lubricant properties can be determined by placing them on several known surfaces. The contact angle results from the balance of cohesive, surface and interfacial energy. If the lubricants are now combined with the polymers, this results in interaction tendencies in the form of spreading or "solving". Spreading means that the triboparticles are separated from each other by the lubricant in the gap, which leads to lower friction as the roughness peaks are further apart. Solving stands for the penetration of the lubricant into the polymer and thus provides important information on softening or hardening effects and therefore on changed friction conditions and wear or material transfer.

Transition from adhesion to sliding in lubricated systems

For a high-resolution analysis of the transition from sticking to sliding, a tribometric cell of a rheometer is used, which forms Hertzian spherical contacts. This increasingly widespread test can also be used to characterise the frictional behavior of the system from boundary friction to elastohydrodynamic friction (Stribeck curve).

In open access publications in the journal Lubricants MDPI, we describe the frictional behavior as a function of the lubricant and for all speed ranges.

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
http://publica.fraunhofer.de/dokumente/N-470474.html

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
http://publica.fraunhofer.de/dokumente/N-531050.html

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Wear of tire elastomers

Natural rubber is unrivalled in terms of its mechanical and tribological properties and is the "elastomer of choice" for highly stressed tires (aircraft, construction vehicles), for example. 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 additives 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 the Fraunhofer IWM. The tribological challenge was to develop an abrasion tester that can make reliable statements about the abrasion of tire elastomers, including on the basis of small sample quantities. This was achieved with the development of a "three-ball tribometer" to analyze fatigue wear, and contributed to the development of a bioidentical synthetic rubber that slightly exceeded the properties of natural rubber.© 

Wear under oxidative or inert atmosphere

Elastomers are exposed to oxidative stress during operation, whether from atmospheric oxygen or from ozone (O3) present in the atmosphere. Ozone in particular, as a strong oxidizing agent, can lead to cracking and failure of the material. Depending on the elastomer and additives, the material is more or less susceptible to oxidative ageing. The description of the damage development is very important for predicting the service life and reliability. Ozone resistance is usually determined under various conditions (e.g. temperatures, humidity, mechanical stress) in an ozone chamber. In most cases, damage due to oxidative ageing only occurs under mechanical stress. The interplay between ozone exposure and tribological stress, i.e. the direct influence of ozone on friction and wear, has been scarcely investigated to date.

For this reason, we have equipped a 3-ball tribometer at the IWM for tests under ozone exposure. The tribometer simulates the fatigue wear of elastomers as it occurs, for example, on rolling tires; the modified set-up is intended to investigate the influence of ozone on fatigue wear and possibly deeper fatigue cracks. In a first approach, it was shown that ozone leads to visibly greater wear for a selection of commercially available elastomers. The degree of wear varied depending on the elastomer.

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Structured elastomers and elastomer coatings

How can the frictional behavior of structured surfaces of elastomers and rubbers be classified schematically? Are there any characteristic values?

Friction of elastomers and rubber has always been something special and still is today. Adolf Schallamach started the scientific description over 50 years ago. Research in this field continues to this day, as the materials exhibit complex non-linear behavior.

As elastomers have a special combination of properties, the highest adaptive deformation with a low tendency to wear, it is still worthwhile researching new ways to change, optimize or adapt the friction behavior.

Their friction is determined, among other things, by the surface elasticity and the adhesive energy of the surface, but can also be changed by structuring the surface.

For industrial molding of surface structures, we at the Microtribology Center of the Fraunhofer IWM (Christof Koplin and Alexander Fromm), together with our colleagues from Esslingen University of Applied Sciences (Dennis Weißer and Matthias Deckert), have demonstrated an increase and anisotropy of friction through embossed nanostructures and the reduction and anisotropy of friction through embossed microstructures. After an exemplary molding of the skin of a chain snake, we were able to determine that the friction of a movement forwards and backwards is already different against a smooth contact. What makes it easier for a snake to move is also technically interesting.

Fo rmore details, please have a look at our publication:

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
https://publica.fraunhofer.de/handle/publica/430952

Laser-structured, DLC-coated elastomer seals

To reduce frictional forces, 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 to ensure reliable sealing. 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 metres, 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 elongation of the elastomers without tearing. The layers then "float" on the material like a scale armour. The size of the scales is determined by the layer properties. Larger residual compressive stresses, for example, can 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 customized elastomers, laser structuring and a corresponding DLC coating that reduces the friction coefficient of the seals, significantly extends the application range of the seals (loads, temperatures) and considerably extends the service life.

The combination of coating and the structuring of the elastomers is particularly helpful here. The laser structures determine how the DLC layers tear under high elongation without the elastomer being damaged or the DLC layer splitting into too small DLC "flakes". Both in tensile tests and in tribological tests, the DLC layers remained completely intact even at 25 % elongation and higher while the effect of smaller and smaller DLC "flakes" forming over time, which otherwise often leads to a shortened service life, does not occur. The laser structures fulfil several purposes. First, the structures determine how and where corresponding relief cracks form under strain in the DLC layer. Second, the structures prevent the DLC layers from bulging due to the high residual stresses on the elastomer. This allows significantly thicker DLC coatings to be applied to elastomers without the elastomer seals leaking during use. At the same time, the structures also serve as a lubricant reservoir, resulting in significantly better lubricant distribution on the sealing surfaces, especially in poorly lubricated systems.

The combination of elastomers adapted to coatings and structuring, a corresponding structuring and a high-performance layer enable significantly more sustainable sealing systems, e.g. significantly more environmentally friendly and sustainable elastomers can be used with the same or even significantly better application properties while at the same time, the operating conditions with regard to application loads, friction and service life can be significantly improved.

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

https://publica.fraunhofer.de/handle/publica/419149

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"Squeaking" due to sliding movement of plastic-hard pairings

Vibratory excitations caused by the sliding of plastics on smooth surfaces are usually elastic vibration excitations of the design, structure and components. For some plastics, however, sliding waves that arise in tribological contact with the friction partner can also cause audible and often disturbing noises. These squeaking noises depend in a complex way on the pressure, the sliding speed, the temperature and the humidity. A tribometer specifically designed for the detection of vibration excitation can be used to investigate acoustic emissions caused by frictional contact and tribologically loaded components.

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Hydrogels, biological tribosystems

Biological tribosystems (e.g. joints, tendons or teeth) are superior to technical tribosystems in many respects: they are generally characterized by low friction coefficients and low wear. The frictional behavior of hydrogels shows similarities in some aspects to the frictional behavior of cartilage, which is why hydrogels are being discussed as potential cartilage replacement materials. We focus on mechanical and tribological structure-property relationships of hydrogels with the aim of evaluating their potential applications. Investigated systems include poly(vinyl alcohol) cryogels and hydrogel-infiltrated networks of bacterial nanocellulose (BNC).

• Poster conference Tampere: Mechanical Characterization of Bacterially Synthesized Nanocellulose Hydrogels [ PDF  0,67 MB ] 

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