Galling

This article is about adhesion between metal surfaces. For the botanical concept, see bark-galling.

Galling on the threads not protected by PTFE tape on a NPT fitting (zoom in on first few threads for better view).

An electron microscope image shows transferred sheet-material accumulated on a tool surface during sliding contact under controlled laboratory conditions. The outgrowth of material or localized, roughening and creation of protrusions on the tool surface is commonly referred to as a lump.

The damage on the metal sheet, wear mode, or characteristic pattern shows no breakthrough of the oxide surface layer, which indicates a small amount of adhesive material transfer and flattening damage of the sheet's surface. This is the first stage of material transfer and galling build-up.

The damage on the metal sheet illustrates continuous lines or stripes, indicating a breakthrough of the oxide surface-layer.

The damage on the metal sheet or characteristic pattern illustrates an "uneven surface," a change in the sheet material's plastic behavior and involves a larger deformed volume compared to mere flattening of the surface oxides.

Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface. This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material will appear worn, chipped, or even gouged and may have balled-up or torn lumps of material stuck to its surface.

Galling is most commonly found in metal surfaces that are in sliding contact with each other. It is especially common where there is inadequate lubrication between the surfaces. However, certain metals will generally be more prone to galling, due to the atomic structure of their crystals. For example, aluminium will gall very easily, whereas annealed (softened) steel is slightly more resistant to galling. Steel that is fully hardened is very resistant to galling.

Galling is a common problem in most applications where metals slide in contact with other metals. This can happen regardless of whether the metals are the same or different. Alloys such as brass and bronze are often chosen for bearings, bushings, and other sliding applications because of their resistance to galling as well as other forms of mechanical abrasion.

Introduction

Galling is adhesive wear that is caused by the microscopic transfer of material between metallic surfaces during transverse motion (sliding). It occurs frequently whenever metal surfaces are in contact, sliding against each other, especially with poor lubrication. It often occurs in high-load, low-speed applications, although it also can occur in high-speed applications with very little load. Galling is a common problem in sheet metal forming, bearings and pistons in engines, hydraulic cylinders, air motors, and many other industrial operations.

Galling is distinct from gouging and scratching, in that it involves the visible transfer of material as it is adhesively pulled (mechanically spalled) from one surface and deposited upon the other, in the form of a raised gall (lump). Unlike other forms of wear, galling is not usually a gradual process; it both occurs quickly and spreads rapidly, as the galled material becomes more effective at inducing further galling.

Galling can often occur in screws and bolts, causing the threads to seize and tear free from the fastener or the hole. In extreme cases, the bolt may seize without stripping the threads, which can lead to breakage of the fastener, the tool, or both. Threaded inserts of hardened steel are often used in metals like aluminium or stainless steel that can gall easily.

The process requires two properties common to most metals: cohesion through metallic-bonding attractions, and plasticity (the ability to deform without breaking). The tendency of a material to gall is affected by the ductility of the material. Typically, hardened materials are more resistant to galling, whereas softer materials of the same type will gall more readily. The propensity of a material to gall is also affected by the specific arrangement of the atoms, because crystals arranged in a face-centered cubic (FCC) lattice will usually allow material transfer to a greater degree than a body-centered cubic (BCC). This is because a face-centered cubic has a greater tendency to produce dislocations in the crystal lattice, which are defects that allow the lattice to shift, or "cross-slip," thereby making the metal more prone to galling. However, if the metal has a high number of stacking faults (a difference in stacking sequence between atomic planes), it will be less apt to cross-slip at the dislocations.

Thus, a material's resistance to galling is primarily determined by its stacking-fault energy: a material with high stacking-fault energy, such as aluminium or titanium, will be far more susceptible to galling than will a material with low stacking-fault energy, such as copper, bronze, or gold. Conversely, materials with a hexagonal close-packed (HCP) structure and a high c/a ratio—e.g., cobalt-based alloys—are extremely resistant to galling.

Galling occurs initially with material transfer of individual grains on a microscopic scale, which become stuck or even diffusion-welded to the adjacent surface. This transfer can be enhanced if one or both metals form a thin layer of hard oxides with high coefficients of friction, such as those found on aluminum or stainless steel. As the lump of transferred material grows, it pushes against the adjacent surface, forcing them apart and concentrating most of the frictional heat into a very small area; this, in turn, causes yet more adhesion and material build-up. The localized heat increases the plasticity of the galled surface, deforming the metal until the lump breaks through the surface and begins plowing up still larger amounts of material from the galled surface.

Methods of preventing galling include the use of lubricants like grease and oil, low-friction coatings and thin-film deposits like molybdenum disulfide or titanium nitride, and increasing the surface hardness of the metals using processes such as case hardening and induction hardening.