You might try spring washers. These are typically used in vibration applications to prevent loosening. Whether they will work depends on the magnitude of the vibration. Otherwise you will have to try loctite.
The wear can occur by either adhesive, abrasive, or subsurface fatigue. Since the materials are the same I doubt if abrasive wear is an issue unless foreign particles are be introduced by the environment. This can be check for easily. The number of cycles are pretty low for fatigue unless the...