7/31/2019 Fretting Wear in Lubricated Systems

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Fretting Wear in Lubricated Systems

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E. C. Fitch, Tribolics, Inc.Tags:wear debris analysis,oil analysis

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 tobe 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 theincreased abrasiveness of the harder oxidized wear debris tend to greatly accelerate wear. Whencorrosion activity is distinctly evident, as denoted by the color of the debris particles, the process isreferred to as fretting corrosion.

Fretting WearFretting wear is also known as vibrational wear, chafing, fatigue, wear oxidation, friction oxidation, falsebrinelling, molecular attrition, fretting fatigue and corrosion. Because virtually all machines vibrate, frettingoccurs in joints that are bolted, pinned, press-fitted, keyed and riveted; between components that are notintended to move; in oscillating splines, couplings, bearings, clutches, spindles and seals; and in baseplates, universal joints and shackles. Fretting has initiated fatigue cracks which often result in fatiguefailure 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 ProcessCyclic motion between contacting surfaces is the essential ingredient in all types of fretting wear. It is acombination process that requires surfaces to be in contact and be exposed to small amplitudeoscillations. Depending on the material properties of surfaces, adhesive, two-body abrasion and/or solidparticles may produce wear debris. Wear particles detach and become comminuted (crushed) and thewear mechanism changes to three-body abrasion when the work-hardened debris starts removing metalfrom the surfaces.

Fretting wear occurs as a result of the following sequence of events:

1. The applied normal load causes asperities to adhere, and the tangential oscillatory motion shearsthe asperities and generates wear debris that accumulates.

2. The surviving (harder) asperities eventually act on the smooth softer surfaces causing them toundergo plastic deformation, create voids, propagate cracks and shear off sheets of particleswhich also accumulate in depressed portions of the surfaces.

3. Once the particles have accumulated sufficiently to span the gap between the surfaces, abrasionwear occurs and the wear zone spreads laterally.

4. As adhesion, delamination and abrasion wear continue, wear debris can no longer be containedin the initial zone and it escapes into surrounding valleys.

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5. Because the maximum stress is at the center, the geometry becomes curved, micropits form andthese coalesce into larger and deeper pits. Finally, depending on the displacement of thetangential 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 wearrate occurs, which shows that all the relevant wear modes are working in combination.

Fretting Wear CharacteristicsThe 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 moreoften is around a few thousandths of an inch. The wear coefficient depends on the amplitude ofoscillation.

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 toominute to be detected. The wear debris rolling at that degree of oscillation presumably causes this lowwear rate. At high amplitudes, direct abrasion of the interface by hard particles (oxide or work-hardened

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particles) creates the gross wear rate. At large amplitudes of oscillation, the fretting wear coefficient isapproximately 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 thathigh normal loads will dampen vibration sufficiently to reduce fretting, the increase in contact areaproduces more surface interaction which tends to outweigh this effect. Consequently, increasing load orunit 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-bodyabrasion). Metallic transfer may or may not take place. Plastic deformation geometrically changessurfaces 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 anew structural phase. These work-hardened areas are brittle, prone to fracture and fragmentation, andgenerate metallic wear debris and particles having initial dimensions of around one micrometer.

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Figure 4. Effect of Frequency on Fretting Damage of Mild Steel

Fretting CorrosionAnother facet of the fretting process is the influence of humidity on the rate of fretting wear. Fretting weardecreases substantially for most friction couples (metals) as the relative humidity increases from zero to50 percent. Wear under humid conditions is always less severe because the moisture contained in the airprovides a type of lubricating film between the surfaces. In some cases, moisture allows soft iron hydratesto 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 fulllubrication conditions, mineral-base oils exposed to the atmosphere contain at least 10 percent air, sooxygen is present at all friction couples or wearing interfaces. Wearing surfaces and wear debriscommonly 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 thecritical factor causing fretting. In fact, the existence of oxidation products has been a ready means ofidentifying a fretting process.

Today, engineers realize that fretting occurs in materials that do not oxidize, such as cubic oxide, goldand platinum. Although oxidation does not cause fretting in most common materials, removing weardebris 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. Themetallic surfaces in the fretted region become slightly discolored. The color of wear debris varies with thetype 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 inertenvironment, the wear rate is considerably less than when conditions cause an oxide film to form and bescraped off.

Because the effect of frequency on wear is amplitude-dependent, two types of fretting wear need to bedefined according to the oscillation amplitude. The first type of fretting is fretting corrosion or wear, aspreviously discussed. The second type of fretting that occurs, in which less material is removed is calledfretting fatigue or traction fatigue.

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Fretting FatigueIn fretting fatigue, surface cracks initiate and propagate, thus removing material. The amplitude is small. Ifthe amplitude of slip increases, the fretting fatigue phenomenon can disappear as the wear front begins toadvance 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 thetwo surfaces creates sufficient traction to cause plastic deformation of the softer surface and particlerelease 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 willindent into the opposite surface to cause serious abrasion and wire-like wear debris.

Lubricant Influences on FrettingFretting seems to progress more rapidly in friction couples that have smooth surface finishes and closefits. 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 wettedsurfaces, can be achieved. Lubricants are not always successful because the reciprocating actionsqueezes 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 thefretting surface and the wear debris. Liquid lubricants with effective metal deactivator additives can helpto reduce the effect of fretting but will not likely stop fretting altogether.

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Fretting modes and their driving input

motions

Fretting occurs when two contacting bodies experience small scale reciprocating motion.However, this standard definition does not define the motion as anything more than

reciprocating. The fact that there are many ways in which the contact may be loaded means thatthere is a high degree of variablility in fretting motion. This has led to the definition of several

different fretting modes.

Fretting Modes I-III (Gallego et al., 2010)

Typically three fretting modes are considered (Gallego et al., 2010). The modes are tangential

fretting, radial fretting, and torsional fretting and may be referred to by their mode number (i.e.

Mode I fretting). In reciprocating sliding the moving body has a linear back and forth motion.

In radial fretting the normal force varies with time causing the radius of a Hertzian contact togrow and shrink. In rotational fretting the contact will spin back and forth.

1. Tangential2. Radial3. Torsional

Fretting Modes I-IV (Cai et al., 2010)

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Fretting motion is sometimes categorized into four modes (Cai et al., 2010). The four modes are

tangential, radial, rotational, and torsional. The additional mode, rotational fretting, has a linearmotion similar to tangential fretting. The difference between the similar modes is that in

tangential fretting the moving body translates or slides side to side while in rotational fretting the

ball tips, rocks, or rolls back and forth.

1. Tangential2. Radial3. Rotational4. Torsional

In industrial machinery the fretting motion is typically complex and frequently is not known.

Under most conditions the motion is probably a combination of multiple modes.

In laboratory experimental testing and numerical modeling tangential fretting is nearly always

studied. If you see the word fretting in a text you can assume that it is Mode I unless informed

otherwise.

These fretting modes are closely associated with the Hertzian contact which is by far the most

commonly studied contact geometry. With a more complex contact geometry many more modesof fretting could be defined.