Dr. Dmitri Kopeliovich
Tribology is the science and engineering of rubbing surfaces.
Polymer is a substance (natural or synthetic), molecules of which consist of numerous small repeated chemical units (monomers) linked to each other in a regular pattern.
Polymers are widely used in tribological applications due the following properties:
The disadvantages of polymers in tribological applications:
Tribological properties of polymers are determined by the parameters and conditions of the polymer molecular structure:
Behavior of polymers under mechanical loads is different from that of Metals, Ceramics and Fluids.
Most of polymers are viscoelastic.
Viscoelasticity is the behavior of the deformed material, which combines the properties of both solid (elasticity) and liquid (viscosity).
The deformation (strain) of elastic materials is proportional to the load (stress) applied to the material. This linear function is time independent: strain forms instantly when the stress is applied. Ceramics and Metals at low temperature demonstrate elastic behavior below a certain level of strain (elasticity limit).
Viscous materials (liquids) resist fast deformation under the load. In contrast to an elastic material, which returns to its original shape immediately after the load is removed, a viscous material flows changing its dimensions for some time after changing the load (time dependent process).
Viscoelasticity of polymers plays an important role in their tribological behavior. The rubbing scratches on the polymer surface may heal due to the viscoelastic “flow” of the material.
Surface energy is the energy required for a creation of a new surface.
Surface energy is summarized from the energies of the intermolecular bonds disrupted as a result of the surface creation.
Surface energy is the fundamental parameter determining the tribological properties of the material: coefficient of friction and wear resistance.
One of the Mechanisms of wear is the adhesive wear, which is a result of micro-junctions formed between the opposing asperities on the rubbing surfaces of the counterbodies. The load applied to the contacting asperities is so high that they deform and adhere to each other creating adhesive bonds. When the bonds break portions of the material are transferred to the counterface. This process includes disappear of some of the interfacial surface and creation of a new surface. The balance of the surface energies determines the adhesive wear process.
The effect of the surface energy on the coefficient of friction is also explained by the formation and disruption of the adhesive bonds between the rubbing surfaces. Lower surface energy results in lower coefficient of friction.
Surface energy of polymers is much lower than that of Ceramics and Metals.
When a polymer material is rubbing against a harder material (e.g. metal) polymer particles are transferred to the metal countersurface and form a transfer film. Formation of the transfer film is determined by the relationship between the interfacial energy polymer-metal (adhesive work) and the polymer surface energy (cohesive work).
Transferred polymer fills the microdefects of the mating surface reducing its roughness, which results in lower coefficient of friction and wear. When a transfer film is formed the polymer part is rubbing against the polymer film on the metal counterpart and not against the metal itself. Due to the lower surface energy of polymers the coefficient of friction with the polymer transfer film is also lower.
Wear is the removal of the material from the surface of a solid body as a result of mechanical action of the counterbody.
There are different mechanisms of wear of polymers:
Tribological properties of polymers may be significantly improved by dispersing filling materials throughout the polymer matrix (Polymer Matrix Composites).
Advantages of polymer composites:
The polymers used as matrices for fabrication of sliding polymer composites:
The fillers for fabrication of sliding polymer composites: