Galling - Mechanism

Mechanism

When two metallic surfaces are pressed against each other the initial interaction and the mating points are the asperities or high points found on each surface. An asperity may penetrate the opposing surface if there is a converging contact and relative movement. The contact between the surfaces, initiates friction or plastic deformation and induces pressure and energy in a small area or volume called the contact zone.

The elevation in pressure increases the energy density and heat level within the deformed volume. This leads to greater adhesion between the surfaces which initiate material transfer, galling build-up, lump growth and creation of protrusions above the original surface. An example of accumulated transferred material or “lump growth” on a tool surface can be seen in figure 1. The initial asperity/asperity contact and surface damage on the opposing sheet-metal surface can be seen in figure 2.

If the lump (or protrusion of transferred material to one surface) grows to a height of several microns, it may penetrate the opposing surface oxide layer and cause damage to the underlying bulk material. Damage in the bulk material is a prerequisite for plastic flow that is found in the deformed volume which surrounds the lump. The geometry and the nominal sliding velocity of the lump defines how the flowing material will be transported, accelerated and decelerated around the lump. This torrent or material flow is critical when defining the contact pressure, energy density and developed temperature during sliding. The mathematical function describing acceleration and deceleration of flowing material is thereby defined by the geometrical constraints, deduced or given by the lump's surface contour. The contact damage from deformation of bulk material is seen in figure 3.

If the right conditions are met, such as geometric constraints of the lump that cause less energy transfer away from the contact zone than what is added by movement and plastic deformation, an accumulation of energy can cause a clear change in the sheet materials contact and plastic behaviour; generally this increases adhesion and the friction force needed for further advancement. The sheet damage from this type of high energy contact can be seen in figure 4.

In dynamic contact and sliding friction, increased compressive stress is proportionally equal to a rise in potential energy and temperature within the contact zone or "the system of the mechanics". The reasons for accumulation of energy during sliding can be the lesser loss of energy away from the contact zone due to a small surface area on the system boundary and low heat conductivity. Another reason is the amount of energy that is continuously forced into the system, which is a product of the acceleration of mass and developed pressure. In cooperation these mechanism allows a constant accumulation of energy and increased energy density and temperature in the contact zone during sliding.

The process and contact found in figure 4 can be compared to cold welding or friction welding, because cold welding is not truly cold and the fusing points exhibit an increase in temperature and energy density derived from applied pressure and plastic deformation in the contact zone.