Galling - Clarification and Limitations

Clarification and Limitations

Galling should not be confused with other cases of attraction between surfaces which do not result in plastic deformation. These latter types of attraction involve adhesive surface forces or surface energy theories. Different energy potentials at the surfaces can develop adhesive bonds or cohesive forces that may hold the two surfaces together. Surface energy and the cohesive force phenomenon are not the same as galling, and are only partially correlated. Galling necessarily involves plastic deformation of at least one surface.

However, the present research generally fails to make a clear distinction between energy derived from plastic deformation and the cohesive surface forces with the adjacent counterpart, and chemical attraction between atoms or surface molecules. The latter is likely to be involved in the initial material transfer, as shown in figure 2, where only surface-oxide asperities are in contact. But it is hard to distinguish these adhesive forces from more severe attractions caused by accumulated energy and increased pressure from plastic deformation. Oxides are brittle and it is probable that most of the energy in the fracture mechanism is consumed in brittle fracture, but the created wear debris will instantaneously penetrate the opposing surface. This means that the transferred oxide material will instantly act as a penetrating body and the concentration of energy, pressure and frictional heating is immediate. Without this accumulation of energy, the tendency for material transfer will certainly decrease.

The formation and constitution (physique) of plastic zones around penetrating objects are arguably a prerequisite and the main factor for excessive material transfer, lump growth and galling build-up even in the initial contact process (see figure 2).