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Wear & Tribology

Tribological effects under the microscope

Wear & Tribology

The solution of wear problems starts with a detailed examination of the tribological system with all influence factors that are involved. From this, it can be deduced what frictional conditions and wear mechanisms are to be reckoned with and when they come into play.

In practice, more than one of these effects is active at any given time or they occur consecutively during the wearing process. However, it is usually one of them that plays the predominant part in failure due to wear.

Experience shows that every tribological system can be optimised by selecting the right coating.

Tribological system

A tribological system consists of the surfaces of two components that are in moving contact with one another and their surroundings. The type, progress and extent of wear are determined by the materials and finishes of the components, any intermediate materials, surrounding influences and operating conditions.

  1. Base object
  2. Opponent body
  3. Surrounding influences: Temperature, relative humidity, pressure
  4. Intermediate material: Oil, grease, water, particles, contaminants
  5. Load
  6. Motion

Typical wear mechanisms

  • Abrasive wear

    Removal of material due to hard and sharp-edged particles that get in between the interacting surfaces. It can also be caused by hard or sharp-edged surfaces and asperity peaks of one or the other partner.

    The consequences are scratches, grooves, micro-chips, dimensional changes, and shiny spots on textured tool surfaces.

  • Adhesive wear

    Under unfavourable lubrication and contact conditions or when running dry, the friction pairs form an intimate, adhesive bond. This is especially likely where the surface materials are of similar composition or have a particular affinity for one another.

    The consequences are cold welding, scuffing, scoring, pits, seizing, built-up edges, tool breakage.

  • Surface fatigue

    Repeated, alternating mechanical stresses lead to the formation and propagation of cracks under the stressed surface, which is thus destroyed.

    The consequences are transverse and crest cracks, pitting and micro-pitting (especially in rolling contact), tool breakage.

  • Tribo-chemical reaction

    Tribo-chemical reaction (tribo-oxidation)

    Tribological sliding contact leads to a chemical reaction. The reaction products influence the tribological processes at the surface; for instance, pairs of components with narrow tolerances can jam.

    Tribo-chemical wear in cutting operations results from diffusion. In general, tribo-chemical wear increases with rising temperature. A frequent cause of tribo-chemical wear is oxidation.

    The consequences of tribo-oxidation include fretting corrosion.

  • Corrosion

    Corrosion arises from the chemical or electrochemical reaction between a metal and substances with which it comes into contact, e.g. electrolyte solutions, humid gases or melts. Mechanical loads can also favour corrosion.

    The consequences are attrition, pitting corrosion, cracks and rust.

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