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Fretting wear corrosion

Fretting wear arising from small-amplitude sliding and rolling at bearing contact surfaces manifests as fretting (wear) corrosion and pseudo‑indentations, respectively; these two wear mechanisms are distinct.

Failure mode: fretting wear corrosion

Fretting wear, resulting from small-amplitude sliding and rolling at the bearing contact interface, manifests as fretting (wear) corrosion and pseudo‑indentations; these two wear mechanisms are distinct.

Keywords: fretting (wear) corrosion

Insufficient interference fit Turns dark red Failure mechanism

Principle

1. Fretting (wear) corrosion occurs under unlubricated conditions; the wear mechanism involves severe adhesion, leading to cold welding between the wear debris and the base material across the natural oxide layer. The wear debris consists of α‑Fe₂O₃ and appears dark red.

2. Fretting wear initially manifests as pseudo‑indents; when fretting debris obstructs the lubricant, leading to a lubrication‑free condition on the friction surface, it progressively evolves into fretting (wear) corrosion.
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Fig. Micrographs of the inner ring/outer ring/steel balls, 90× magnification

Phenomenon

Due to vibration, micro‑movements occur between the inner and outer rings of the bearing and their mating components (fixed on the shaft) during operation, leading to the formation of adhesive wear on microscopic asperities. Tangential sliding then causes damage characterized by deeper wear at the edges and shallower wear in the central region of the bearing raceway contact area.

Application Cases

1. Washing Machine Case

20200906 Application Case: A customer in the washing machine industry reported that the end‑user experienced noticeably high noise levels and unusual sounds. The unit was equipped with a 6202‑ZZ bearing. During factory testing, the washing machine showed no abnormal noises; however, once delivered to the end user, such noises emerged.
According to the inspection conducted by our company’s technical personnel, the exterior shows no obvious signs of installation or wear; however, when the bearing is rotated, there is noticeable binding. All dimensional measurements are within normal limits. The detailed analysis results are as follows:
Microscopic image:
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Fig. Micrographs of the inner ring/outer ring/steel balls, 90× magnification

Draw a conclusion Fretting (wear) corrosion caused by vibrations during transportation or movement ultimately leads to… The bearing is damaged, resulting in abnormal noise.

False indentation

◆ Vibration during bearing transportation causes relative motion between the inner and outer rings.
◆Closely spaced corrugated grooves or vibration marks

Failure mechanism

Principle 1. False indentations occur under boundary lubrication; the wear mechanism involves mild adhesion confined to the natural oxide layer, and the wear debris consists of black Fe3O4.
2. Fretting wear initially manifests as pseudo‑indentation features; when fretting debris obstructs the lubricant, creating a dry‑friction condition on the contacting surfaces, it progressively evolves into fretting (wear) corrosion.

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Phenomenon Vibration during rotation generates fine, wave-like grooves or vibration marks on the raceway.

Motor Case

20190926 Application Case: A customer in the motor industry reported that, after being stored for a period of time, their motors began to emit noise and unusual sounds. The motors are equipped with two sets of 608-ZZ bearings.
According to testing by our company’s technical personnel, all dimensional measurements are within specifications; only the noise level exceeds the standard. The detailed analysis is as follows:
1. The bearing returned from the customer’s site is shown in the figure: its exterior exhibits no obvious signs of installation or wear; when rotated, it feels noticeably stiff, and the elevated noise level suggests preliminary evidence of raceway damage.
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3. Internal Conditions

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Fig. Micrographs of the inner ring, outer ring, and steel balls at 90× magnification.

Draw a conclusion Pseudo‑Buchholz indentations are caused by fretting wear. When the installed motor is stationary (not running), minute vibrations between the rotor and stator induce the bearing rolling elements to slide back and forth along the raceways, thereby creating a groove. This type of pseudo‑Buchholz indentation typically occurs during transportation or storage when vibrations are present.

The main causes of bearing false brinelling are:

1. Vibrations and oscillations of the bearing during transportation and when it is stationary;
2. Oscillatory motion with small amplitude;
3. Poor lubrication.

Corresponding measures for bearing pseudo‑indentation damage:

1. During transportation, the shaft and bearing housing must be securely fastened.
2. During transportation, the inner and outer rings must be packaged separately.
3. Add preloading to reduce vibration.
4. Use an appropriate lubricant.

Remedial measures

Reasons for the limited number of improvement measures to address fretting (wear) corrosion in bearings:

1. Differences in design parameters such as the bearing’s contact angle, groove curvature radius coefficient, and clearance result in varying resistance to fretting wear. Under external vibration, the extent of fretting wear in ball bearings is correlated with the product of the tangential force τ and the relative sliding displacement δ at the contact zone; thus, during design, slightly increasing the groove curvature radii of both the inner and outer raceways can significantly reduce the τδ value on the contact surface, thereby mitigating fretting wear. However, this design modification also elevates the contact stress, making it more difficult for lubricant to penetrate the contact region and consequently shortening the bearing’s service life.
2. To mitigate fretting wear in pitch bearings during wind turbine operation, micro‑fretting simulation tests reveal that wear is minimized when the bearing exhibits a certain amount of negative clearance. Furthermore, composite micro‑fretting tests involving radial and tangential components at various inclination angles indicate that setting the initial contact angle of the pitch bearing near 45° can enhance its resistance to fretting wear. However, introducing negative clearance in high‑speed washing machine motors is detrimental to high‑speed operation, as it increases heat generation and torque while reducing service life.

Countermeasure 1 for fretting wear corrosion:

The extent of fretting wear is correlated with the product of the tangential force τ and the relative sliding displacement δ at the contact interface, denoted as τδ.
The tangential force is related to the vector sum of the load and the inertial equivalent at the same resonant frequency (which is difficult to calculate and predict).
The amount of sliding is related to the shaft’s radial displacement angle and axial displacement.
Accordingly, countermeasures such as securing the rotor shaft during transportation, reducing preload, and minimizing transport-induced vibrations can be implemented.

Countermeasure 2 for fretting wear corrosion:

Due to the extremely low velocities associated with fretting, it is difficult to form an elastohydrodynamic lubrication film. Moreover, during fretting, the grease is displaced from the contact zone, making it challenging to establish a stable oil film and leaving the system in a boundary-lubricated state for extended periods. To mitigate fretting‑induced wear and corrosion, one can increase the oil‑film thickness, enhance the grease’s viscosity, and improve the fluidity of its base oil.
Going forward, our company will conduct anti‑fretting wear and corrosion tests using various lubricating oils.

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