Galling is a severe form of adhesive wear that occurs when two metal surfaces slide against one another under pressure. This phenomenon is a significant concern in mechanical and industrial applications because it can rapidly lead to equipment failure, component seizure, and the locking of threaded fasteners. Understanding the underlying physics of this surface damage is the first step toward mitigating its effects. This analysis explores what galling is, the mechanism driving this material failure, which metals are most affected, and strategies for prevention.
Defining Material Galling
Material galling is a rapid and destructive type of surface damage characterized by the gross transfer of material from one sliding surface to the other. The visible result is a rough, torn, and lumpy surface, often appearing gouged or heavily scored. This damage is distinct from simple abrasion because it involves significant physical adhesion between the two parts.
The process begins at microscopic high points, or asperities, on the metal surfaces where localized pressure is extremely high. This intense pressure and friction cause the microscopic peaks to bond together momentarily. As sliding continues, these fused points are torn apart, pulling material from the weaker surface and depositing it onto the opposing surface. The resulting protrusions, known as galls, further increase friction and pressure, creating a self-accelerating cycle of destruction that quickly leads to component seizure.
The Mechanism of Cold Welding
The underlying physical process that causes galling is a form of solid-state fusion known as cold welding. Under normal conditions, metal surfaces are protected by a thin oxide layer that acts as a barrier, preventing the atoms from bonding. However, when two metal surfaces are placed under high load and subjected to sliding, intense localized pressure causes the microscopic asperities to plastically deform.
This plastic deformation and friction generate enough heat and pressure to fracture and shear away the protective oxide layer at the contact points. Once this barrier is removed, the ultra-clean, reactive metal surfaces are exposed and forced into intimate contact. The atoms are then close enough to recognize each other’s strong metallic bonds, causing them to fuse together momentarily.
As sliding continues, the fused junction must break, but the fracture often does not occur exactly at the original interface. Instead, the break happens within the crystal structure of the weaker material. This results in a permanent transfer of material to the opposing surface, creating the characteristic raised lumps or galls. This cycle of micro-weld formation and subsequent tearing drives the progression of galling damage.
Materials Most Susceptible to Galling
Certain material properties significantly increase a metal’s susceptibility to galling, primarily high ductility and the ability to work-harden easily. Highly ductile metals, such as aluminum, easily undergo the plastic deformation under load required for intimate contact and cold welding. Austenitic stainless steels, like the 300-series, are particularly prone to galling because they are ductile and have a high rate of work hardening, meaning they become harder as they are deformed.
Titanium and its alloys also exhibit a high tendency to gall, primarily due to their low thermal conductivity and the reactive nature of their surface when the oxide layer is disrupted. The phenomenon is often worse when identical or very similar materials slide against each other. This is because the uniform atomic structure and hardness across the contact area make the formation and tearing of cold welds more consistent and destructive.
Strategies for Prevention
The most effective approach to mitigating galling involves a combination of material selection, operational modification, and surface treatment. A primary strategy is selecting dissimilar materials for the mating surfaces, such as pairing a bronze component with a steel component. This difference in atomic structure and chemical composition reduces the likelihood of the clean metals forming a strong metallic bond.
Introducing a significant hardness differential between the two sliding components is also highly effective, often requiring a difference of at least 50 Brinell units. This ensures that if cold welding occurs, the resulting material transfer is confined almost entirely to the softer surface, preventing the seizure of both parts. Operational changes include applying specialized high-pressure lubricants, often called anti-seize compounds, which contain solid particles like graphite or copper to physically separate the surfaces under high load.
Surface modification techniques can dramatically improve galling resistance by altering the near-surface properties of the metal. Processes such as nitriding or carburizing introduce elements like nitrogen or carbon into the surface layer, significantly increasing its hardness and wear resistance. Additionally, specialized coatings, such as physical vapor deposition (PVD) coatings, provide a hard, low-friction layer that prevents direct metal-to-metal contact, disrupting the cycle of cold welding and material transfer.