Which of the following can lead to flaking and pitting of bearing surfaces?

Flaking and pitting of bearing surfaces are primarily the result of hard debris in the lubricant. When hard particles or debris are present in the lubricant, they can create abrasive wear as they move across the bearing surface. These particles can generate localized stresses that exceed the material's fatigue strength, leading to the formation of small pits or flakes as the material degrades.

In the context of bearings, this is particularly problematic as it not only reduces the lifespan of the bearing but can also lead to unexpected failures in machinery, emphasizing the importance of maintaining clean lubrication systems and using high-quality lubricants that minimize the presence of contaminants. While other factors such as water contamination, excessive heat, or using low-quality materials can adversely affect bearings, the specific mechanism of flaking and pitting is most directly attributed to the presence of hard debris in the lubricant.