Tribology is the science of friction and wear. It is a sub-class of mechanical engineering, and is especially important for machine designing. There has been studies that indicate the influence of friction and wear in design. About 80% of all machine failures are associated with friction and wear.
Tribology usually looks that the reduction of friction and wear. It covers modes of surface failures (e.g. wear, pitting, galling). It talks about material selection, coatings, lubrications, and processes to increase efficiency.
Here is a "Cliff note" what I can remember from my college studies on this subject:
- increasing contact surface area (e.g. line contact of roller bearings instead of point contact of ball bearings) to reduce pitting
- hardening a surface will reduce its wear rate
- reducing friction will reduce wear
- softer materials flow better than harder ones, thus reducing friction
- a great combination of hard and soft would be to coat a hardened material with a softer one. For example, anodizing can put a hard thin shell onto an aluminum part. Dipping it into Teflon afterwards can embed Teflon molecules into the honeycomb matrix of the anodized surface. This leaves a soft Teflon surface (low friction) on a hard material (anodized shell).
- journal bearings only work when the shaft is spinning at a high enough speed to float it in the oil. This is why it is better to leave an engine idling for a few minutes than to stop and restart it. During startup, the shaft grinds against the inside of the journal bearing and create wear.
- some situations require making a profile to keep a part floating a moving fluid (e.g. the read/write head inside a hard drive floating on a very-thin stream of air above the spinning disc). It turns out that a step profile is actually better than a slanted or an airplane-wing one.
- on a macroscopic level, surface area has no significant effect on friction. Only the force pushing 2 surfaces together and their materials (listed as their coefficient of friction against each other) matter. For instance, a 100 lbs block of steel will slide with the same difficulty no matter how you change its surface area in contact with the ground. This is because decreasing the surface area decreases the area that covalent bonding between the block and the ground (decreases friction), but it increases the force-per-area that supports the weight (increases friction), and the two friction methods cancel each other out. This assumes that you don't decrease it so much that the block digs into the ground. This gets more into a mechanical method for stopping the movement instead of surface friction.
- decreasing surface roughness usually reduces "friction" and wear by decreasing the mechanical method for stopping the movement
- however, decreasing surface roughness also increases the contact surface area such that two too-smooth surfaces will experience molecular "sticktion" and can cause them to bind. Hard drive discs are very smooth. On the surface, there is a thin area near their center that have pits to reduce the smoothness. This allows the read/write head to dock there when the disc is not spinning.