Spalling is the flaking or loss of small spalls of material on a bearing rolling element (ball) or Raceway as a
result of contamination or fatigue.
When contaminants in the form of particles get into the bearing, they can get over-rolled by the rolling elements and create indentations in the raceways. The area around these indentation will be subject to cyclic stress and more metal will eventually break away from the raceway. Damage will gradually progress, along with a rise in noise and vibration levels, until the bearing becomes unserviceable.
Solid contaminants enter the bearing and dent the rolling surfaces. skf.com
Over-rolling of particles with sequential Spalling
starting at the back of the dents on a deep-groove ball bearing inner-ring. skf.com
Spalling over the entire surface of an inner-ring of a deep groove ball bearing. skf.com