What is Galling
Galling is a common yet often misunderstood type of metal wear that can significantly impact the performance and longevity of mechanical components. It occurs when two metal surfaces under pressure adhere and tear material from one another, leading to surface damage, friction, and eventual part failure. Found frequently in ductile metals such as stainless steel and aluminum, galling poses challenges in industries ranging from manufacturing to aerospace.
Understanding the causes and prevention of galling is essential for engineers, designers, and manufacturers who work with high-performance machinery. In this article, we explore what galling is, how it occurs, its key causes, and practical methods to prevent it—including the role of advanced surface treatments like BOROCOAT®.
How Galling Occurs
Galling is a severe form of adhesive wear that happens when two metallic surfaces come into high-pressure contact. The process starts with surface asperities (tiny high points) penetrating the oxide layers of both surfaces, causing microscopic welding. As the surfaces slide against each other under stress, this adhesion intensifies, leading to tearing and the transfer of material between the surfaces.
The resulting wear compromises the mechanical integrity of components, often leading to equipment failure. While galling can occur in many metals, it is particularly common in ductile metals such as stainless steel, where surface characteristics and high ductility can lead to adhesion under stress.
Real-World Examples of Galling
- Threaded fasteners like stainless steel bolts and nuts often seize due to galling when tightened without proper lubrication.
- Sliding components such as valves and bearings are prone to galling in high-friction environments.
- Metal forming processes like stamping and extrusion can cause galling if the surfaces are not adequately treated or lubricated.
Key Causes of Galling
Several factors contribute to galling, making it a common yet preventable issue.
Material Properties
Metals with high ductility and certain surface characteristics are more likely to experience galling. For example, stainless steel is prone to this type of wear due to its tendency for surface adhesion under pressure.
Surface Conditions
Uneven or rough surfaces create friction points that accelerate galling. Smooth surfaces, in contrast, reduce friction and improve galling resistance.
Poor Lubrication
Insufficient or inadequate lubrication exacerbates friction and heat generation, which are primary drivers of galling.
High Contact Pressure
Components subjected to high compressive forces, such as bolts or sliding mechanisms, are particularly vulnerable. These forces intensify the interaction between surfaces, increasing the risk of galling.
Differences Between Galling and Other Wear Types
- Cold welding occurs without sliding motion and results in two metal surfaces permanently bonding under pressure.
- Abrasive wear involves material removal caused by a harder surface grinding against a softer material.
- Corrosive wear results from chemical reactions that degrade the material over time.
How to Prevent Galling
Preventing galling requires a combination of strategies tailored to specific applications and operating environments.
- Material selection: Choosing metals with higher hardness or lower ductility can reduce the risk of galling. Pairing metals with contrasting properties, such as stainless steel and a harder alloy, minimizes adhesion.
- Surface finishing: Polished or coated surfaces reduce friction points and improve galling resistance.
- Proper lubrication: Applying suitable lubricants, such as oils or greases, forms a protective layer between surfaces. Advanced lubricants for extreme conditions may be required in high-stress applications.
- Advanced surface treatments: Treatments like BOROCOAT® significantly enhance surface hardness and wear resistance, providing a smooth and durable barrier against adhesion.
- Design optimization: Optimizing component geometry, such as using larger contact surfaces or rounded edges in fasteners, helps distribute forces more evenly.
Applications and Industry Impact
Galling is a critical concern in industries where high-performance components operate under extreme conditions.
- In the automotive sector, galling can lead to failure in drivetrain components, fasteners, and engine parts.
- In aerospace, sliding mechanisms and high-pressure systems are at risk, where galling can compromise safety and performance.
- In manufacturing, processes like stamping and extrusion require smooth surfaces to prevent galling and ensure consistent quality.
