What is the lubrication regime when the metal surfaces are touching?

The lubrication regime when the metal surfaces are touching is known as the boundary lubrication regime. In boundary lubrication, the lubricant film is very thin or even non-existent, leading to direct metal-to-metal contact. This contact often results in increased friction and wear, as the protective properties of the lubricant are minimal.

In this regime, the surfaces are highly dependent on the properties of the lubricant, such as its viscosity and the presence of additives that can help form a protective layer on the surfaces. These additives may include anti-wear agents that can chemically bond to the surface, providing some level of protection against wear even when surfaces are in contact.

Other lubrication regimes such as hydrostatic and hydrodynamic involve the presence of a sufficient lubricant film that separates the contacting surfaces, thereby reducing direct contact and minimizing wear and friction. Full film lubrication specifically indicates that the lubricant completely separates the surfaces, preventing any metallic contact, which is not the case in boundary lubrication.