Galling is a common mechanical issue that occurs when metal surfaces are repeatedly pressed together under load. Most metals are susceptible to galling if they do not slide easily against one another, but some types of metals are more prone than others. The primary cause of galling is poor lubrication, but metal type and surface hardness can also play a role. Generally, soft metals such as aluminum and stainless steel are more prone to galling than harder metals like tool steels and martensitic stainless steels.
The most common form of galling involves two different metallic surfaces that are not lubricated and come into contact with each other at high speeds. The friction between the two metals creates a small amount of heat which is transferred from one surface to the other and causes the surfaces to adhere together, or “weld”. If this process continues for long enough, it can lead to severe damage to both components.
Various mechanisms have been proposed to explain galling, including mechanical deformation and chemical adhesion. Chemical adhesion has been described as a “snowball effect” in which the accumulation of hard oxide particles on one surface causes frictional heating and eventually leads to welding of the two adjacent metals. This is a form of adhesive wear that can be difficult to overcome because the oxide layers are relatively stable, and removing them requires very strong mechanical force.
Another form of galling involves the transfer of ductile material from the contacted surface to the other. This can occur even if the surface is not lubricated, but is more likely to occur with soft metals such as aluminum and austenitic stainless steels. The ductility of these metals allows them to weld to the opposing material, and this is especially true if the surface is contaminated by oxidation.
A final mechanism is based on elastic interaction. This is most commonly seen when a fastener is tightened into a flange and, as the bolt load increases, the elastic interaction between the fastener and flange forces it against the flange and creates more friction. The resulting thermal energy is transferred through the fastener and into the flange, creating an area of localized heat that can melt or burn the flange.
To reduce the risk of galling, metal surfaces should be thoroughly lubricated and cleaned before assembly. This will prevent abrasion and increase the smoothness of the mating surfaces. Surface hardness can be increased by using a harder metal such as martensitic stainless steel or duplex stainless steel, but this will decrease corrosion resistance. Surface finish can also be improved by using a hot rolled or cold rolled surface, or by electropolishing. The surface roughness of the mating surfaces can also be reduced by using a deburring tool or inspection with a low power lens.
Research is needed to better understand how galling herbivore-induced responses affect neighboring ecological interactions, particularly in beech forests. This will involve analyzing volatile cues emitted from galls during different stages of the herbivore’s life cycle to see how these can influence neighboring herbivory patterns.
