Microscopic origin of friction and wear

4 problems

• Predicting the friction coefficient from multi-asperity contact mechanics

  Partially resolved

  Two touching surfaces really meet only at many tiny bumps called asperities. Theory can explain why the true contact area grows in proportion to the load, but it cannot yet predict the value of the friction coefficient for a given pair of materials.

• Microscopic basis of Archard's wear law

  Partially resolved

  The amount of material lost by wear grows in proportion to load and sliding distance, but the proportionality constant varies by a factor of a million between materials and cannot be predicted. Its link to what happens at single contacts is unclear.

• Origin of the logarithmic aging of static friction

  Contested

  The longer two surfaces stay in contact at rest, the harder it is to start them sliding, and the increase grows with the logarithm of time. Whether this comes from the contacts slowly squashing and growing or from chemical bonds forming across them is disputed.

• Up to what contact area does structural superlubricity survive

  Open

  Two perfectly flat crystal surfaces whose atomic patterns do not match can slide almost without friction, an effect called structural superlubricity. It is unknown up to what contact area this survives before defects and elastic deformation bring friction back.