text: In engineering science and other technical aspects, the term galling is widespread. The influence of acceleration in the contact zone between materials has been mathematically described and correlated to the exhibited friction mechanism found in the tracks during empiric observations of the galling phenomenon. Due to problems with previous incompatible definitions and test methods, better means of measurements in coordination with a greater understanding of the involved frictional mechanisms have led to the attempt to standardize or redefine the term galling to enable a more generalized use.
  ASTM International has formulated and established a common definition for the technical aspect of the galling phenomenon in the ASTM G40 standard: "Galling is a form of surface damage arising between sliding solids, distinguished by microscopic, usually localized, roughening and creation of protrusions (e.g., lumps) above the original surface".
  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 called the contact zone.
  The elevation in pressure increases the energy density and heat level within the deformed area. This leads to greater adhesion between the surfaces, which initiates the material transfer, galling build-up, lump growth, and creation of protrusions above the original surface.
  If the lump (or protrusion of transferred material to one surface) grows to a height of several micrometers, it may penetrate the opposing surface oxide-layer and cause damage to the underlying material. Damage in the bulk material is a prerequisite for plastic flow found in the deformed volume surrounding the lump. The geometry and speed of the lump define how the flowing material will be transported, accelerated, and decelerated around the lump. This 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.
  If the right conditions are met, such as geometric constraints of the lump, an accumulation of energy can cause a clear change in the material's contact and plastic behavior, increasing the friction force required for adhesion and further movement.
  In sliding friction, increased compressive stress is proportionally equal to a rise in potential energy and temperature within the contact zone. The energy accumulation during sliding can reduce energy loss from the contact zone due to a small surface area on the surface boundary, thus, low heat conductivity. Another reason is the energy continuously forced into the metals, which is a product of acceleration and pressure. In cooperation, these mechanisms allow constant energy accumulation, causing increased energy density and temperature in the contact zone during sliding.
  The process and contact 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.