Galling is a form of severe adhesive wear that occurs when two metal surfaces slide against each other under pressure, causing material transfer and surface damage. This phenomenon, sometimes called “cold welding,” begins when microscopic high points (asperities) on the surfaces contact one another. Localized heat and pressure cause them to fuse momentarily. As motion continues, these welded junctions tear apart, pulling material from one surface and depositing it onto the other, which rapidly escalates the damage. Galling is a common and destructive problem in machinery, particularly in threaded fasteners made from materials like stainless steel, aluminum, and titanium, and its prevention is a primary concern for component longevity.
Selecting Materials and Surface Finish
The initial defense against galling begins with the proper selection of mating materials and their surface characteristics. A highly effective preventative measure involves using dissimilar metal pairings for components that slide or rub against each other. For instance, combining an austenitic stainless steel bolt with a bronze nut significantly reduces the tendency for the surfaces to cold-weld. This works because the different atomic structures and inherent properties of the two metals make it more difficult for them to fuse together under pressure.
A significant difference in material hardness between the two components is also beneficial for galling resistance. The harder material is better able to resist the localized deformation that initiates cold welding. A difference of at least 50 Brinell units is often cited as a target for improved performance. Designers frequently specify a harder bolt to be paired with a softer nut to manage this hardness differential.
The surface finish of the components plays a role in managing galling risk, though the ideal specification is not simply the smoothest possible finish. Surfaces that are either too rough (typically over 1.5 micrometers Ra) or extremely smooth (below 0.25 micrometers Ra) have a higher tendency to gall. Slightly rougher surfaces can sometimes be better because they can retain a thin film of lubricant, whereas a mirror-like finish may increase the actual contact area and struggle to hold a lubrication film in place.
Applying Lubricants and Anti-Seize
Applying a protective layer of lubricant or anti-seize compound is one of the most effective ways to prevent galling, especially in threaded applications. These compounds create a physical barrier that prevents direct metal-to-metal contact and minimizes friction. Anti-seize compounds are typically formulated as a grease or paste containing a high percentage of solid lubricating particles.
These solid lubricants can include metals like copper, nickel, or aluminum, or non-metallic solids such as graphite, molybdenum disulfide (MoS2), or PTFE. The choice of compound should be governed by the operating conditions, particularly the temperature and load. Nickel-based anti-seize is often preferred for extremely high-temperature applications, as it can be rated for service up to 2500°F (1370°C). The grease base will burn off, leaving the solid metal particles to continue their protective function.
Proper application involves ensuring adequate coverage of the threads, without applying so much material that it creates excessive buildup. The lubricant should be applied to the load-bearing surfaces so that when the fastener is tightened, the compound spreads evenly across the contact area. Using a lubricant also alters the torque-tension relationship of a fastener, meaning the specified torque value may need adjustment to achieve the correct clamping force.
Permanent Surface Modification Techniques
Permanent surface modification techniques chemically or physically alter the metal’s outer layer to inherently improve galling resistance.
Coatings and Platings
Coatings like chrome plating and nickel plating increase the surface hardness and provide a low-friction layer, which is highly effective at reducing adhesive wear. The thickness of these platings can range from 0.003 to 0.5 millimeters, offering a durable protective shell.
Surface Hardening
Surface hardening processes, such as nitriding or carburizing, diffuse elements into the metal’s surface to create a hardened case layer. Plasma nitriding, for example, can significantly increase the surface hardness to over 1000 Vickers (HV) on stainless steel, improving the metal’s ability to withstand high contact stresses. These diffusion processes are advantageous because they do not change the component’s dimensions, making them suitable for parts with tight tolerances.
Conversion Coatings and Dry Films
Conversion coatings, like phosphating or black oxide, create a softer, non-metallic crystalline layer on the surface that acts as a sacrificial barrier. This layer shears easily under pressure, preventing the underlying parent metal from contacting the mating surface and initiating galling. Dry film lubricants, such as tungsten disulfide or PTFE, can also be applied as a coating that cures to a thin, permanent film, offering a clean, low-friction solution that will not attract dirt or debris.
Assembly and Operational Controls
Even with the correct materials and surface treatments, the way a component is assembled and operated can still increase or decrease the risk of galling.
Control Assembly Speed
Controlling the assembly speed is a simple yet powerful measure. Rapid rotation from power tools generates excessive friction and heat between the mating surfaces. Slow, steady tightening is recommended, or using hand tools for high-risk applications, to minimize the heat-induced adhesion that causes galling.
Maintain Cleanliness
Maintaining component cleanliness is a practical control measure that prevents galling. Debris, metal chips, or contaminants on the threads act as abrasive particles that increase friction and disrupt the protective oxide layer, accelerating the onset of cold welding. Components should be inspected for burrs or damage before assembly to ensure a smooth, uniform contact area.
Apply Proper Torque and Alignment
Proper torque application is essential, as over-tightening a fastener increases the surface pressure and stress, significantly raising the risk of galling. Using a calibrated torque wrench and adhering to the manufacturer’s recommended specifications ensures the correct clamping force is achieved. Ensuring correct alignment during assembly also prevents uneven pressure distribution, which can lead to localized high-stress points where galling is most likely to begin.