Will direct metal-to-metal contact inside ball bearings cause wear?

Yes, direct contact between the metals inside ball bearings can indeed cause wear. The design principle of ball bearings is to reduce direct contact between metal surfaces through a lubricating film between the ball and the raceway, in order to achieve smooth rotation and low friction. Lubricants (usually lubricating oil or grease) play a crucial role between the ball and the raceway, forming a thin film to prevent direct contact between metal components.
When the lubricant is insufficient, ineffective, or contaminated, the lubricating film between the ball and the raceway may be damaged, resulting in direct contact between the metal surface. At this point, the friction between metals will significantly increase. Due to the effect of friction, the surface of the ball and raceway will undergo continuous small collisions and sliding, generating significant heat. The accumulation of heat may cause the material surface to soften or degrade, and this abnormal friction process can accelerate wear, damage the smooth surface of the bearing, and form small scratches, dents, or peeling phenomena.
As wear accumulates, the surfaces of the balls and raceways become rough and uneven, further increasing friction and resulting in uneven load distribution, generating more heat and ultimately forming a vicious cycle. This not only leads to a decrease in bearing performance, but may also result in premature failure or jamming of the bearing. In addition, direct contact between metals may also lead to increased vibration and noise of bearings, affecting the stability and accuracy of equipment operation.