Quick
Pitting or marring of finished surface, esp.bearing surface, because of fretting. The localized mutual seizure of two metal surfaces during sliding friction accompanied by the removal of metal particles from one or both surfaces.
Details
Galling can be considered a severe form of adhesive wear. With high loads and poor lubrication, surface damage can occur on sliding metal components. The damage is characterized by localized macroscopic material transfer, that is, large fragments or surface protrusions that are easily visible on either or both surfaces. This gross damage is usually referred to as galling, and it can occur after just a few cycles of movement between the mating surfaces. Severe galling can result in seizure of the metal surfaces.
The terms scuffing and scoring are also used to describe similar surface damage under lubricated conditions. Scuffing is the preferred term when the damage occurs at lubricated surfaces, such as the piston ring-cylinder wall contact. Scoring typically describes damage that takes the form of relatively long grooves.
Materials that have limited ductility are less prone to galling, because under high loads surface asperities will tend to fracture when interlocked. Small fragments of material may be lost, but the resultant damage will be more similar to scoring than to galling. For highly ductile materials, asperities tend to plastically deform, thereby increasing the contact area of mated surfaces; eventually, galling occurs.
Another key material behavior during plastic deformation is the ease with which dislocations cross slip over more than one plane. In face centered cubic (fcc) dislocations easily cross slip. The rate of cross slip for a given alloy or element is usually indicated by its stacking-fault energy. Dislocation cross slip is hindered by the presence of stacking faults, and a high stacking-fault energy indicates a low number of impeding stacking faults and an increased tendency to cross slip and, hence, gall. Nickel and aluminum have poor galling resistance, whereas gold and copper have good galling resistance.
Materials that have a hexagonal close-packed (hcp) structure with a high c/a ratio have a low dislocation cross slip rate and are less prone to galling. This explains why cobalt-base alloys and cadmium-plated alloys resist galling while titanium alloys tend to gall.
Prevention of galling is accomplished through proper design. For example, parts should have sufficient clearance, because tightly fitted parts are more prone to galling. Adequate lubrication and various hard surface coatings also can help prevent galling. Control of surface roughness is another important factor. Highly polished surfaces (<0.25 μm, or 10 μin.) or very rough finishes (>1.5 μm, or 60 μin.) increase the tendency for wear and galling. It is theorized that very smooth surfaces lack the ability to store wear debris because of the absence of valleys between asperities, which means the asperities will have greater interaction. Also, lubricants will tend to wipe off the smoother surface. Too rough a finish results in interlocking asperities, which promote severe tearing and galling.