Fretting wear is surface damage that occurs between two contacting surfaces experiencing cyclic motion (oscillatory tangential displacement) of small amplitude. At the contact areas, lubricant is squeezed out, resulting in metal-to-metal contact. Because the low amplitude motion does not permit the contact area to be relubricated, serious localized wear can occur. 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 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.
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:
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.
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.
Very little wear occurs at amplitudes below 100 micrometers as shown in Figure 1.
Figure 1. Fretting Wear vs. Slip Amplitude1
At slips below 100 micrometers, nucleation and propagation of cracks that lead to wear debris are too minute to be detected. The wear debris rolling at that degree of oscillation presumably causes this low wear rate. At high amplitudes, direct abrasion of the interface by hard particles (oxide or work-hardened particles) creates the gross wear rate. At large amplitudes of oscillation, the fretting wear coefficient is approximately the same as that of unidirectional wear.
Figure 2. Fretting Wear vs. Running Time2
Changes in the normal load generally affect fretting wear. Although equipment users often presume that high normal loads will dampen vibration sufficiently to reduce fretting, the increase in contact area produces more surface interaction which tends to outweigh this effect. Consequently, increasing load or unit pressures tend to generate higher wear rates as Figure 3 shows.
Figure 3. Fretting Wear vs. Normal Unit Load3
Three separate mechanisms cause fretting wear: adhesion, traction fatigue and delamination (two-body abrasion). Metallic transfer may or may not take place. Plastic deformation geometrically changes surfaces and high load-carrying regions are created that have areas measured in square millimeters.
The material corresponding to these load-carrying areas is highly work-hardened and leads to forming a new structural phase. These work-hardened areas are brittle, prone to fracture and fragmentation, and generate metallic wear debris and particles having initial dimensions of around one micrometer.
Figure 4. Effect of Frequency on Fretting Damage of Mild Steel
Another facet of the fretting process is the influence of humidity on the rate of fretting wear. Fretting wear decreases substantially for most friction couples (metals) as the relative humidity increases from zero to 50 percent. Wear under humid conditions is always less severe because the moisture contained in the air provides a type of lubricating film between the surfaces. In some cases, moisture allows soft iron hydrates to form instead of the harder, more abrasive Fe3O4, magnetite, a magnetic oxide of iron.
Although fretting can occur in an inert environment, this type of environment is not normal. Even under full lubrication conditions, mineral-base oils exposed to the atmosphere contain at least 10 percent air, so oxygen is present at all friction couples or wearing interfaces. Wearing surfaces and wear debris commonly show a large amount of oxide, leading to the name “fretting corrosion.”
In the past, fretting wear was usually called fretting corrosion because oxidation was supposedly the critical factor causing fretting. In fact, the existence of oxidation products has been a ready means of identifying a fretting process.
Today, engineers realize that fretting occurs in materials that do not oxidize, such as cubic oxide, gold and platinum. Although oxidation does not cause fretting in most common materials, removing wear debris leaves virgin metal exposed to the atmosphere and oxidation usually occurs.
Strong visual evidence supports the idea that oxide films form and are subsequently scraped away. The metallic surfaces in the fretted region become slightly discolored. The color of wear debris varies with the type of parent material; the corrosion product of aluminum is white but fretting causes it to become black, the corrosion product of steel is gray but fretting causes it to become a reddish brown.
The second aspect that supports this idea is the increase in wear rate. When fretting occurs in an inert environment, the wear rate is considerably less than when conditions cause an oxide film to form and be scraped off.
Because the effect of frequency on wear is amplitude-dependent, two types of fretting wear need to be defined according to the oscillation amplitude. The first type of fretting is fretting corrosion or wear, as previously discussed. The second type of fretting that occurs, in which less material is removed is called fretting fatigue or traction fatigue.
In fretting fatigue, surface cracks initiate and propagate, thus removing material. The amplitude is small. If the amplitude of slip increases, the fretting fatigue phenomenon can disappear as the wear front begins to advance rapidly enough to remove the initiated cracks before they propagate.
Surface hardness plays a key role in limiting fretting fatigue. If both surfaces are hard, asperities will weld, followed by the shearing of junctions, material transfer and wear particle generation.
If a hard surface is in contact with a soft surface, fretting fatigue wear will likely occur. The harder of the two surfaces creates sufficient traction to cause plastic deformation of the softer surface and particle release through subsurface void nucleation, crack propagation and subsequent loss of surface material. When one surface is much harder and rougher and is driven by less traction force, the asperities will indent into the opposite surface to cause serious abrasion and wire-like wear debris.
Fretting seems to progress more rapidly in friction couples that have smooth surface finishes and close fits. Lubricants do not penetrate wear areas with small clearances (described as close fits). In addition, the smooth finish eliminates lubricant-retaining pockets between the asperities in rougher surfaces.
Under these conditions only boundary lubrication condition, the continuous interaction of oil wetted surfaces, can be achieved. Lubricants are not always successful because the reciprocating action squeezes out the lubricant film and does not allow it to be replenished.
In general, the purpose of the lubricant in most fretting situations is to prevent oxygen from reaching the fretting surface and the wear debris. Liquid lubricants with effective metal deactivator additives can help to reduce the effect of fretting but will not likely stop fretting altogether.
This article originally appeared as a chapter in E.C. Fitch’s book, Proactive Maintenance for Mechanical Systems. 1992.