Galling

For the botanical concept, see bark-galling.

[1] To avoid misinterpretation, the three figures origin and context can be found in the references
The damage,wear mode or characteristic pattern shows no breakthrough of the oxide surface layer which indicates a small amount of adhesive material transfer and a flattening damage of the sheets surface. This is the first stage of material transfer and galling build-up

[2] The characteristic pattern illustrates continuous lines or stripes indicating a breakthrough of the oxide surface layer. This type of contact can, in different proportions, be found simultaneously with the pattern found in Figure [3].
Both characteristic patterns found in figure [2] and [3] are sequential to the pattern in figure [1]

[3] The characteristic pattern illustrates an "uneven surface", a change in the sheet materials plastic behaviour and involve a larger deformed volume compared to flattening of the surface oxides seen in Figure [1]. This type of contact is associated and usually found in different proportions simultaneously with the pattern in Figure [2]

Galling usually refers to adhesive wear and transfer of material between metallic surfaces during sheet metal forming and other industrial applications.
In engineering science and in other technical aspects, the term galling is widely spread and in recent years there have been attempts to standardize or define the word in coordination with greater understanding of the involved frictional mechanisms.

The ASTM G40 standard organization have formulated and established a common definition for the technical aspect of the galling phenomenon, and it reads: "Galling is a form of surface damage arising between sliding solids, distinguished by microscopic, usually localized, roughening and creation of protrusions (i.e., lumps) above the original surface".

Mechanism

The initial interaction and the mating points between the two metallic surfaces are the asperities (i.e., high points found on the surfaces).
If movement are applied, will the asperities penetrate the opposing surface, causing friction or plastic deformation and induced pressure which increase heat and adhesion whom in it´s term initiate material transfer, galling build-up and lump growth. The surface damage from initial asperity/asperity contact can bee seen Figure [1].

If the lump, (growth of transferred material), grows to a certain height it will penetrate the surface oxide-layer and even the underlying bulk material, reaching deep down into the bulk, (several microns), and create a plastically flowing deformed volume around it.
The geometry and the nominal sliding velocity of the lump defines how the flowing material will be transported, (accelerated), around the lump and is critical for defining the lumps contact pressure and developed temperature during sliding. The mathematical function or curve for acceleration of flowing material is thereby defined by the lumps surface contour.
The damage from this type of contact can bee seen in Figure [2].

If the right conditions are achieved and the energy transfer away from the contact zone is less than the added energy from movement and plastic deformation, will the accumulated energy cause a clear change in the sheet materials contact and plastic behaviour which increase adhesion between the surfaces and the friction. The damage from this type of high energy tempered contact can be seen Figure [3].

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Note: In dynamic contact and sliding friction, increased compressive pressure is equal to a raise in potential energy and temperature into the system and acceleration of mass contributes to the force needed for further progression. And initially there are only a limited loss of energy and thermal conductivity away from the contact zone due to a small surface area on the system boundary, also new energy is continuously forced into the system as the sliding progresses allowing a constant increase in energy content and temperature in the contact zone.

To enlighten the matter further, the process and contact found in Figure [3] can be compared to cold welding or friction welding, because cold welding is not cold and the fusing points exhibits an increase in temperature and energy content derived from applied pressure and plastic deformation in the contact zone.

Galling can be found in

In metalworking that involves cutting (primarily turning and milling), galling is used to describe a phenomenon which often occurs when cutting soft metal. The work material is transferred to the cutter and develops a "lump", the developed lump changes the contact behaviour between the two surfaces which usually increase the friction and resistance for further advancement, see Figure [3].

Galling often occurs with aluminium compounds and is a common cause of tool breakdown. Aluminium is a ductile metallic compound and possesses the ability to plastically flow which is needed to develop a plastic zone around the cutter. The ability to deform plastically can be considered as a general prescription for excessive material transfer and galling build-up because frictional heating is closely linked to the constitution (physique) of plastic zones around penetrating objects. In comparison, brittle fracture seldom generates a great amount of heat.

Galling can occur even at relatively low loads and velocities because it's the real pressure or energy content in the system that inflict phase transition which often leads to an increase in material transfer and higher friction.

Clarification and limitations about the galling definitions

Galling should not be confused with attraction between surfaces without involving plastic deformation, this type of attraction should only be compared with adhesive surface energy theories. Different energy potentials at the surfaces can develop adhesive bonds or cohesive forces that holds the two surfaces together even though they are separated by a distance. But surface energy and the cohesive force phenomenon is not the same as galling, because galling involve plastic deformation of at least one surface.

However, the present research generally lacks a clear distinction between energy derived from plastic deformation and the adjacent counterpart cohesive surface energy between atoms or surface molecules. The last are likely involved in the initial material transfer, see Figure [1], where only surface-oxide asperities are in contact. But it´s hard to distinguish these adhesive forces from more severe attraction caused by accumulated energy and increased pressure from plastic deformation. Oxides are brittle and it is probable that most energy in the fracture mechanism is consumed in brittle fracture, but the created wear debris will instantaneously penetrate the opposing surface. This means that the transferred oxide material will instantly act as a penetrating body and the concentration of energy, pressure and frictional heating is immediate which without would certainly reduce the tendency for material transfer.

The formation and constitution (physique) of plastic zones around penetrating objects are arguably a prerequisite and the main factor for excessive material transfer, lump growth and galling build-up even in the initial contact process, see Figure [1].

The difference between welding, solders and material degradation are not always obvious, but generally they can be separated by the following:

• Friction welding is an industrial manufacturing method and demands phase transition in both the mating materials which creates a weld after cooling.
  
• A solder is an industrial manufacturing method and demands phase transition in at least one of the mating materials and generally involve chemical or metallic bonding in a much smaller volume between the mating surfaces compared to friction welding.
  
• Cold welding counts as a wear phenomenon and demands phase transition in at least one of the mating materials which creates a minimum amount of chemical or metallic bonding between the surfaces to increase the friction.
  
• "Materials degradation" is material failure but may include wear phenomenons and the same mechanisms found in both friction welding and solders which in this case means concentration of kinetic energy, pressure and/or heat.
  

Prevention

Adhesive wear and material transfer from one surface to another during sliding, so called galling, occur for a number of different materials and frictional systems.
Generally there are two major frictional systems which have affects on adhesive wear or galling and 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.
The two frictional systems are:

1. Solid surface contact, (unlubricated conditions)
2. Lubricated contact

In solid surface contact or unlubricated conditions, the initial contact is characterised by interaction between asperities and the exhibition of two different sorts of attraction.
Cohesive surface energy or chemical attraction between atoms or molecules, connect and adhere the two surfaces together notably even if they are separated by a measurable distance, (within the micro scale).
Direct contact and Plastic deformation generates another type of attraction through the constitution of a plastic zone whit flowing material where induced energy, pressure and temperature allow bonding between the surfaces on a much larger scale than the previous mentioned cohesive surface energy.

In metallic compounds and SMF sheet metal forming the asperities are usually oxides and the plastic deformation mostly consists of brittle fracture which presuppose a very small plastic zone and the accumulation of energy and temperature is low due to the discontinuity in the fracture mechanism.

However, during the initial asperity/asperity contact, wear derbies or bits and pieces from the asperities adhere to the opposing surface creating microscopic, usually localized, roughening and creation of protrusions (i.e., lumps) above the original surface.
The transferred wear debris and creation of lumps, penetrate the opposing oxide surface layer and and cause damage to the underlying bulk material, allowing continuous plastic deformation, plastic flow, accumulation of energy and temperature.

Whit regards to the previously defined difference between the initial two types of attraction in "solid surface contact" or unlubricated conditions is the prevention of adhesive material transfer accomplished by the following:

1. Less cohesive or chemical attraction between surface atoms or molecules.
2. Avoid continuous plastic deformation and plastic flow trough a thicker oxide layer on the subject material in SMF sheet metal forming.
3. Coatings such as titanium nitride(TiN) or carbon diamond like coatings exhibit low chemical reactivity even in high energy frictional contact where the protective oxide layer is breeched and the frictional contact is distinguished by continuous plastic deformation and plastic flow.
   

Lubricated contact set other demands on the materials surface structure and the main issue is to withhold the protective lubrication thickness and avoid plastic deformation because it heightens the temperature of the oil or used lubrication fluid and change the viscosity and deteriorate the ability to withhold a full-film lubrication thickness.
A full-film lubrication thickness can be withhold by the following:

1. Surface irregularities can create a favourable geometric situation for the oil to withhold a full-film lubrication thickness in the contact zone.
2. Numbered list item

Galling is prevented by the presence of grease or surface coatings, even if the surface coatings increase friction. It usually does not occur when joining dissimilar materials (e.g., threading 18-8 stainless steel into 17-4 stainless steel) even though both of those materials are susceptible to galling.

See also

• Tribology
• wear
• Rheology
• Surface engineering
• Pin on disc tribometer

References

• "urn:nbn:se:kau:diva-2790: An investigation of friction graphs ranking ability regarding the galling phenomenon in dry SOFS contact : (Adhesive material transfere and friction)". Diva-portal.org. Retrieved 2009-11-03.