Tribology of “soft matter”: Hydrogels, elastomers and thermoplastics

© Photo Fraunhofer IWM

The tribology of “soft matter” is characterized by the complex dependence of the friction value and wear on normal force, speed, temperature, lubricants and surface finish of the friction bodies.  The range of experimental observations can be mapped using appropriate models of friction mechanisms. An understanding of tribological phenomena allows adequate measures to be taken to minimize friction and wear.

Subtopics

Tribology of thermoplastics

In contact with a smooth, hard friction body, the tribology of thermoplastics is determined by the adhesive friction. The speed and pressure dependence of the "coefficient of friction (CoF)” can be influenced by the choice of media or surface structure. Depending on the purpose of application, nano-structured surfaces can have either anti-stick or anti-slip properties. Potential applications of surface structuring include the systematic control of the haptics of polymer surfaces, for example.

”Squeaking” from sliding movement of plastic-hard pairings

Vibrational excitation from plastic sliding on smooth surfaces is usually elastic oscillatory excitation from construction, structure, and components. For some plastics, though, sliding waves resulting from tribological contact with the friction body can cause audible and often disruptive noises. These squeaking noises are dependent on a complex interplay of pressing, sliding speed, temperature and humidity. Acoustic emissions due to frictional contact and tribologically loaded components can be investigated with a tribometer specifically designed for detection of oscillations.

Hydrogels, biological tribosystems

Biological tribosystems (for example, joints, tendons, or teeth) are superior in many respects to technical tribosystems: They are generally characterized by low friction coefficients and low wear. In some respects, the friction behavior of hydrogels shows similarities to the friction behavior of cartilage, which is why hydrogels (although I don't like the z there) are discussed as potential cartilage replacement materials. We are working on mechanical and tribological structure/property relationships of hydrogels with the aim of assessing their possible fields of application. Systems studied include poly(vinylalcohol)-cryogels and hydrogel infiltrated networks of bacterial nanocellulose (BNC).