What Is Galling and How Does Is It Caused?

Galling is a wear mechanism that occurs between two bodies in contact during sliding and is caused due to the disruption of oxide layers on metals. Galling is more severe than scuffing, which results in macroscopic surface texture changes due to inadequate lubrication. Galling can also occur when a nominal lubrication film is present.

During galling, two contacting bodies slide over each other and a microscopic transfer of material occurs. The material transfer occurs adhesively, giving a distinct appearance. As the material has been removed from one body adhesively, the other body gains a lump of material at the corresponding location.

As galling occurs, opposing asperities come into contact, which tends to be oxide layers for metals. The wear debris from the contact adheres to the asperity, resulting in growth. The bulk material underneath will be damaged if the lumps grow too large and penetrate the oxide layer.

Galling occurs rapidly, unlike many other wear processes, and as a result, local welding occurs via friction welding or cold welding. Cold welding occurs due to the metallic and intermetallic forces within the structure.

When two similar materials undergo large pressures, the boundary of the two is difficult to identify and hence the material, then behaves like one. If an oxide layer or protective layer separates the two similar materials, the atoms know they are not one. The disruption of the oxide layer or protective layer is the initiation of galling.

The mechanism can occur under a variety of conditions, including low speed and high loads or high speeds and low loads.

How Is Galling Caused?

An example of galling, which many people have come across, is with threads of a bolt/nut. The threads on a bolt and nut can seize due to galling. Normally, this results in the threads becoming damaged, however, in extreme cases, the bolt can shear during attempts to remove it. High pressure and friction are present during installation, which sometimes ruptures the oxide layers and causes cold welding.

Factors Affecting Galling

As discussed above, galling occurs at both high pressures at low speeds and low pressures at high speeds. However, there are factors we can alter to help reduce the probability of galling occurring.

Lubrication

A sufficient lubrication film will minimise the probability of galling, however, galling is commonly caused due to inadequate lubrication. A lubrication film separating the two contacting bodies prevents contact or reduces the direct contact pressure if a mixed lubrication regime is present. Anti-galling/anti-seizing compounds have been specially developed to use on bolts and nuts which may undergo galling.

Material

A materials atomic structure will influence its susceptibility to galling. Therefore, we can reduce galling by choosing a metal that has a favourable structure. Materials with an FCC (Face Centre Cubic) structure are susceptible to galling because the FCC structure has a greater tendency to produce dislocations and favours lattice slip resulting in it being more prone to galling. Examples of metals with an FCC structure are Aluminium, Silver and Lead. It is well known that Aluminium is susceptible to galling.

A BCC (body centre cubic) structured metal does not tend to produce dislocations and hence is less likely to gall. BCC structured metals include Lithium, Potassium and Chromium.

The structure that offers the best resistance to galling is HCP (Hexagonal Close Packed), and matched with a high c/a ratio, offers good resistance to galling.

Ductility/Hardness

Ductility is an influential mechanical property of metals that’s influences galling. Materials with a higher ductility offer less resistance to galling than their hard and brittle counterparts. Therefore, if we case-harden or induction harden a metal, it is less likely to experience galling.

A harder material is more resistant to galling due to its larger resistance to plastic deformation – a key mechanism in galling.

Surface Roughness

Surface roughness is also an influencing factor because it influences the actual contact area between contacting bodies and therefore affects junction growth. If a material has a high surface roughness, it is less likely to experience galling due to a reduced contact of asperities.

Temperature

At higher temperatures, a metals mechanical properties worsen, meaning they are susceptible to more damage. As a result, the oxide coating becomes more prone to deformation and modification, resulting in an increased probability of galling. A higher temperature generally occurs due to high levels of friction.

How To Prevent Galling

Lubrication

The prime prevention method for galling is the use of a lubricant. Using a lubricant will prevent/limit contact between the two surfaces, depending on the film thickness. An acceptable lubrication film will prevent high-pressure contact and hence will not disrupt the oxide layers on metals, exposing the reactive bulk material.

As discussed above, one of the ways lubricants are used to prevent galling is through the use of purpose-built lubricants for preventing galling. These are anti-seize/anti-galling compounds.

Alternative Materials

Using alternative materials will decrease the chance of galling. When two very similar metals are used, which come into contact, a larger risk of galling is apparent because of the adhesive forces present in the bulk material. The very similar metals will not be able to determine a boundary between themselves if the protective coating is ruptured, resulting in them becoming one.

Selecting different materials means they will have different hardness and ductility properties resulting in varying levels of damage to their oxide layers – reducing the chances of galling.

However, this can introduce another issue, such as galvanic corrosion.

Speed

Reducing the relative speed of motion between the two surfaces reduces the likely hood of galling. Tightening bolts at a controlled pace will help reduce the probability of them seizing.

Stainless Steel Galling

Stainless steel is an extremely popular metal in many industries and systems due to its corrosion-resistant properties. However, Stainless Steel is susceptible to galling.

Stainless steel is not as strong as many of its steel counterparts (it is considered a soft metal) and is more ductile. As discussed above, higher levels of ductility increase the probability of galling.

From experimental data gathered in labs to measure galling stresses of paired materials, it was witnessed that when stainless steel was coupled it exhibited inferior galling resistance by a factor of two or more compared to other types of steel (except a handful of Nitronic steels). One of the most notorious types of stainless steel known to undergo galling is Austenitic Stainless Steel.

Summary

Galling is a wear process prevalent in the engineering industry, and you are more than likely to experience it during your career. It is important we understand the essentials of Galling, how it is caused, and understand the consequences of excessive wear. We have discussed the definition of Galling, how it is caused, covered the influential factors of Galling. In addition to this, we looked at the Galling of stainless steel.

• Related Posts:
• What is Abrasive Wear?
• Design and Selection of Engineering Materials for Tribological Applications

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