Editing Galling (section)

==Introduction==
Galling is adhesive [[wear]] that is caused by the microscopic transfer of material between metallic surfaces during transverse motion (sliding). It occurs frequently whenever metal surfaces are in contact, sliding against each other, especially with poor lubrication. It often occurs in high-load, low-speed applications, although it also can occur in high-speed applications with very little load. Galling is a common problem in [[sheet metal forming]], bearings and pistons in [[engine]]s, [[hydraulic cylinder]]s, [[air motor]]s, and many other industrial operations. Galling is distinct from gouging or scratching in that it involves the visible transfer of material as it is adhesively pulled ([[spalling|mechanically spalled]]) from one surface, leaving it stuck to the other in the form of a raised lump (gall). Unlike other forms of wear, galling is usually not a gradual process but occurs quickly and spreads rapidly as the raised lumps induce more galling.
It can often occur in screws and bolts, causing the threads to seize and tear free from the fastener or the hole. In extreme cases, the bolt may seize without stripping the threads, which can lead to breakage of the fastener, the tool, or both. [[Threaded insert]]s of hardened steel are often used in metals like aluminium or [[stainless steel]] that can gall easily.<ref>''Mechanical Fastening Joining Assembly'' By James A. Speck — Marcell Dekker 1997 Page 128</ref>

Galling requires two properties common to most metals, cohesion through [[metallic bonding|metallic-bonding]] attractions and [[plasticity (physics)|plasticity]] (the ability to deform without breaking). The tendency of a material to gall is affected by the [[ductility]] of the material. Typically, [[hardness|hardened]] materials are more resistant to galling, whereas softer materials of the same type will gall more readily. The propensity of a material to gall is also affected by the specific arrangement of the atoms, because crystals arranged in a [[face-centered cubic]] (FCC) lattice will usually allow material-transfer to a greater degree than a [[body-centered cubic]] (BCC). This is because a face-centered cubic has a greater tendency to produce [[dislocation]]s in the crystal lattice, which are defects that allow the lattice to shift, or "cross-slip," making the metal more prone to galling. However, if the metal has a high number of stacking faults (a difference in stacking sequence between atomic planes), it will be less apt to cross-slip at the dislocations. Therefore, a material's resistance to galling is primarily determined by its [[stacking-fault energy]]. A material with high stacking-fault energy, such as aluminium or [[titanium]], will be far more susceptible to galling than materials with low stacking-fault energy, like [[copper]], [[bronze]], or [[gold]]. Conversely, materials with a [[hexagonal close packed]] (HCP) structure and a high ''c/a'' ratio, such as [[cobalt]]-based [[alloy]]s, are extremely resistant to galling.<ref>''[https://books.google.com/books?id=Lt7WBiILHpYC&dq=%22hexagonal+close+packed+%28hcp%29+structure+with+a+high+c%2Fa+ratio%22&pg=PA76 Surface Engineering for Corrosion and Wear Resistance]'' By J. R. Davis -- ASM International 2001 Page 76</ref>

Galling occurs initially with material transfer from individual grains on a microscopic scale, which become stuck or even diffusion welded to the adjacent surface. This transfer can be enhanced if one or both metals form a thin layer of hard oxides with high [[coefficient of friction|coefficients of friction]], such as those found on aluminum or stainless steel. As the lump grows, it pushes against the adjacent material, forcing them apart and concentrating most of the friction heat energy into a very small area. This, in turn, causes more adhesion and material build-up. The localized heat increases the plasticity of the galled surface, deforming the metal until the lump breaks through the surface and begins plowing up large amounts of material from the galled surface. Methods of preventing galling include the use of [[lubricant]]s like [[grease (lubricant)|grease]] and [[oil]], low-friction coatings and thin-film deposits like [[molybdenum disulfide]] or [[titanium nitride]], and increasing the surface hardness of the metals using processes such as [[case hardening]] and [[induction hardening]].