Stiction

Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the term "static friction", perhaps also influenced by the verb "stick".

Any solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force.

In situations where two surfaces with areas below the micrometer range come into close proximity (as in an accelerometer), they may adhere together. At this scale, electrostatic and/or Van der Waals and hydrogen bonding forces become significant. The phenomenon of two such surfaces being adhered together in this manner is also called stiction. Stiction may be related to hydrogen bonding or residual contamination.

Stiction is a commonly used term when diagnosing diesel fuel powered engines. The Ford Powerstroke 6.0-L and 7.3-L engines are the most prevalent to experience cold start or injector stiction issues. The 7.3-L was produced between 1994 and 2003, while the 6.0-L was produced between 2003 and 2007. Both engines were manufactured by Navistar International and incorporated the HEUI injector system. These injectors use the engine oil to lubricate themselves and over time the sticky friction build up causes the injector to malfunction and fail. Alternatively to replacing injectors with new units, products are available that rid the injector, turbos and transmission of stiction and prevent build up. Leading engineers in the field have found in their research that 9 out of 10 failing injectors in diesel trucks are cases of stiction, and once removed are still in optimal working condition.

Stiction is also the same threshold at which a rolling object would begin to slide over a surface rather than rolling at the expected rate (and in the case of a wheel, in the expected direction). In this case, it's called "rolling friction" or μ_r.

This is why driver training courses teach that if a car begins to slide sideways, the driver should try to steer in the same direction as the slide with no brakes. It gives the wheels a chance to regain static contact by rolling, which gives the driver some control again. An overenthusiastic driver may "squeal" the driving wheels trying to get a rapid start but this impressive display of noise and smoke is less effective than maintaining static contact with the road. Many stunt-driving techniques are also done by deliberately breaking and/or regaining this rolling friction.

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