Glossary

An anti-galling coating is a protective surface treatment applied to fasteners and other metal components to prevent a specific type of wear known as galling. Galling occurs when sliding or threaded metal surfaces, particularly those made from materials like stainless steel, titanium, or aluminum, experience high friction under pressure. This friction can cause the surfaces to seize and weld together at contact points, which then tear apart and lead to severe damage or the complete seizure of the fastener.

The primary purpose of these coatings is to reduce friction and prevent metal-to-metal adhesion, allowing fasteners to be easily tightened, loosened, or reused without the risk of seizing or thread damage. They achieve this through several mechanisms: forming a thin, low-friction lubrication layer, creating a barrier to prevent direct metal-to-metal contact, and in some cases, improving wear resistance and surface toughness.

A variety of materials are used for anti-galling coatings, each with its own properties. Common types include PTFE (Teflon®) and other fluoropolymer coatings known for their low friction, and dry film lubricants such as molybdenum disulfide or graphite. Other options include nickel or silver plating for added lubricity and corrosion resistance, zinc flake coatings that offer both anti-galling and corrosion protection, and hard, smooth finishes like nitride and DLC (Diamond-Like Carbon) coatings.

These coatings are used extensively on fasteners and threaded inserts, especially in assemblies involving stainless steel. They are critical in industries like aerospace and automotive, where seizing in joints could lead to failure, as well as in oil and gas for high-pressure connections, and in medical implants where titanium orthopedic screws could be compromised by galling. The main benefits of these coatings are that they extend the service life of fasteners, ensure a consistent torque-tension relationship, prevent costly part damage, and allow for the reusability of components.

Galling is a type of wear and surface damage that occurs when two metal surfaces slide against each other under pressure, causing material to transfer from one surface to the other. It is sometimes referred to as a form of “cold welding” because the high friction and localized adhesion make the surfaces stick together and tear apart unevenly. This often results in rough, damaged threads, seized fasteners, or scratched and scored surfaces.

In fasteners, galling is most common with stainless steel, aluminum, and titanium bolts and nuts, because these metals have a tendency to form adhesive bonds when rubbed together. As torque is applied, microscopic high points (asperities) on the threads weld together under pressure. When the fastener continues to turn, these welded spots tear, pulling material from one surface to the other. This not only damages the fastener but can also cause it to seize, making removal extremely difficult.

Contributing factors to galling include high loads, lack of lubrication, soft or ductile metals, high-speed installation, and repeated tightening/loosening of the same fastener. The risk is especially high in applications with stainless steel fasteners used without lubrication.

Prevention methods include:

- Using anti-seize lubricants or specialized thread lubricants.

- Choosing fasteners with surface treatments or coatings (such as PTFE, zinc, or nitriding).

- Reducing installation speed and avoiding power tools that generate heat and friction.

- Mixing dissimilar metals (e.g., using a stainless steel bolt with a different nut alloy) to reduce adhesion.

- Using fasteners specifically designed with anti-galling properties.