Editing Galling (section)

==Prevention==
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Generally, two major frictional systems affect adhesive wear or galling: solid surface contact and lubricated contact. In terms of prevention, they work in dissimilar ways and set different demands on the surface structure, alloys, and crystal matrix used in the materials.

''In solid surface contact'' or unlubricated conditions, the initial contact is characterized by the interaction between asperities and the exhibition of two different sorts of attraction: [[Cohesion (chemistry)|cohesive]] surface-energy or the molecules connect and adhere the two surfaces together, notably even if a measurable distance separates them. Direct contact and plastic deformation generate another type of attraction through the constitution of a plastic zone with flowing material where induced energy, pressure, and temperature allow bonding between the surfaces on a much larger scale than cohesive surface energy.

In metallic compounds and sheet metal forming, the asperities are usually oxides, and the plastic deformation primarily consists of [[brittle fracture]], which presupposes a very small plastic zone. The accumulation of energy and temperature is low due to the discontinuity in the fracture mechanism.
However, during the initial asperity/asperity contact, wear debris or bits and pieces from the asperities adhere to the opposing surface, creating microscopic, usually localized, roughening and creation of protrusions (in effect lumps) above the original surface. The transferred wear debris and lumps penetrate the opposing oxide surface layer and cause damage to the underlying bulk material, plowing it forward. This allows continuous plastic deformation, plastic flow, and accumulation of energy and temperature.
The prevention of adhesive material transfer is accomplished by the following or similar approaches:

* Low-temperature carburizing treatments such as Kolsterising can eliminate galling in austenitic stainless steels by increasing surface hardness up to 1200 HV0.05 (depending on the base material and surface conditions).<ref>[http://ojs.kmutnb.ac.th/index.php/ijst/article/download/81/81 ''Surface Hardening of Stainless Steels by Kolsterising'' by Gümpel P. -- University of Applied Science, Konstanz Germany AIJSTPME (2012) 5(1): 11-18 (PDF)]</ref>
* Less cohesive or chemical attraction between surface atoms or molecules.
* Avoid continuous plastic deformation and plastic flow, for example, through a thicker oxide layer on the subject material in sheet-metal forming (SMF).
* [[Coating]]s deposited on the SMF work tool, such as [[chemical vapor deposition]] (CVD) or [[physical vapor deposition]] (PVD) and titanium nitride (TiN) or [[diamond-like carbon]] coatings exhibit low chemical reactivity even in high energy frictional contact, where the subject material's protective oxide layer is breached, and the frictional contact is distinguished by continuous plastic deformation and plastic flow.

''Lubricated contact'' places other demands on the surface structure of the materials involved, and the main issue is to retain the protective [[lubrication]] thickness and avoid plastic deformation. This is important because plastic deformation raises the temperature of the oil or lubrication fluid and changes the viscosity. Any eventual material transfer or creation of protrusions above the original surface will also reduce the ability to retain a protective lubrication thickness. A proper protective lubrication thickness can be assisted or retained by:

* Surface cavities or small holes can create a favorable geometric situation for the oil to retain a protective lubrication thickness in the contact zone.
* Cohesive forces on the surface can increase the chemical attraction between the surface and lubricants and enhance the lubrication thickness.
* [[Oil additive]]s may reduce the tendency for galling or adhesive wear.