How the red oxide(Rust) is formed
This explains how the Rust (Iron Oxide) is formed . Not my words found on Internet.
Fretting wear is surface damage that occurs between two contacting surfaces experiencing cyclic motion (oscillatory tangential displacement) of small amplitude. This type of wear further promotes two-body abrasion, adhesion and/or fretting fatigue (a form of surface fatigue) wear.
When fretting wear occurs in a corrosive environment, both the rubbing-off of oxide films and the increased abrasiveness of the harder oxidized wear debris tend to greatly accelerate wear. When corrosion activity is distinctly evident, as denoted by the color of the debris particles, the process is referred to as fretting corrosion.
Fretting Wear
Fretting wear is also known as vibrational wear, chafing, fatigue, wear oxidation, friction oxidation, false brinelling, molecular attrition, fretting fatigue and corrosion. Because virtually all machines vibrate, fretting occurs in joints that are bolted, pinned, press-fitted, keyed and riveted; between components that are not intended to move; in oscillating splines, couplings, bearings, clutches, spindles and seals; and in base plates, universal joints and shackles. Fretting has initiated fatigue cracks which often result in fatigue failure in shafts and other highly stressed components.
Fretting wear is a surface-to-surface type of wear and is greatly affected by the displacement amplitude, normal loading, material properties, number of cycles, humidity and lubrication.
Fretting Wear Process
Cyclic motion between contacting surfaces is the essential ingredient in all types of fretting wear. It is a combination process that requires surfaces to be in contact and be exposed to small amplitude oscillations. Depending on the material properties of surfaces, adhesive, two-body abrasion and/or solid particles may produce wear debris. Wear particles detach and become comminuted (crushed) and the wear mechanism changes to three-body abrasion when the work-hardened debris starts removing metal from the surfaces.
Fretting wear occurs as a result of the following sequence of events:
- The applied normal load causes asperities to adhere, and the tangential oscillatory motion shears the asperities and generates wear debris that accumulates.
- The surviving (harder) asperities eventually act on the smooth softer surfaces causing them to undergo plastic deformation, create voids, propagate cracks and shear off sheets of particles which also accumulate in depressed portions of the surfaces.
- Once the particles have accumulated sufficiently to span the gap between the surfaces, abrasion wear occurs and the wear zone spreads laterally.
- As adhesion, delamination and abrasion wear continue, wear debris can no longer be contained in the initial zone and it escapes into surrounding valleys.
- Because the maximum stress is at the center, the geometry becomes curved, micropits form and these coalesce into larger and deeper pits. Finally, depending on the displacement of the tangential motion, worm tracks or even large fissures can be generated in one or both surfaces.
As the surfaces become work-hardened, the rate of abrasion wear decreases. Finally, a constant wear rate occurs, which shows that all the relevant wear modes are working in combination.
Fretting Wear Characteristics
The key factor in fretting wear is a mechanically loaded interface subjected to a small oscillatory motion. The relative motion required to produce damage may be quite small, as low as one micrometer, but more often is around a few thousandths of an inch. The wear coefficient depends on the amplitude of oscillation.