{"id":22470,"date":"2023-11-29T14:03:20","date_gmt":"2023-11-29T12:03:20","guid":{"rendered":"https:\/\/fractory.com\/?p=22470"},"modified":"2024-01-30T14:51:30","modified_gmt":"2024-01-30T12:51:30","slug":"fretting-explained","status":"publish","type":"post","link":"https:\/\/fractory.com\/fretting-explained\/","title":{"rendered":"Fretting Explained – Definition, How It Works & Prevention"},"content":{"rendered":"

Fretting is a phenomenon that causes damage when metal surfaces come into contact through repeated rubbing or cyclic sliding motion. The friction-induced damage from fretting results in material transfer and corrosion, leading to mechanical wear<\/a>.<\/p>\n

At first, this description may sound similar to galling but these two processes have fundamental differences.\u00a0Galling<\/a> generally refers to the wear damage associated with gross relative motion on a much larger scale than the small amplitude relative motion associated with fretting contacts.<\/p>\n

Let’s discover the reasons behind the fretting process, and how to protect mechanical systems from this type of wear.<\/p>\n

What Is Fretting Wear and When Does It Occur?<\/h2>\n

Fretting is a type of wear<\/a> that stems from low-amplitude oscillating motion or vibration under high contact pressure. These small cyclic movements and micro-sliding generate stresses on the contact surface, degrading the metals over time.<\/p>\n

Fretting is a combination of adhesive and abrasive wear. The oscillatory motion causes fatigue wear which is enhanced by the adhesion of contact surface asperities. This bond is short-lived though, as it will be broken shortly after by the small movements, resulting in wear debris.<\/p>\n

If the debris and\/or surface subsequently undergo a chemical reaction, mainly oxidation, the mechanism is termed fretting corrosion. Fretting leads to increased surface roughness<\/a> and micropits, which further reduces the fatigue strength of the components.<\/p>\n

Many machine components<\/a> and systems are exposed to small movements causing fretting. These small oscillations can be from parts intended to move by design (e.g. camshafts and leaf springs) or not intended to move (e.g. bolted flanges, riveted lap joints and keyways) and caused by various external factors.<\/p>\n

The relative sliding motion has a small amplitude, generally ranging from micrometeres to millimetres but in some cases, it can also be measured in nanometres. Fretting can cause serious damage even with a small friction path.<\/p>\n

Smooth surfaces are more prone to fretting wear since the stress at the contact points between the mating surfaces is amplified.<\/p>\n

The term fretting wear was considered to be interchangeable with fretting corrosion. Interestingly, fretting wear can also occur in a vacuum (space mechanisms for example) and in materials that do not oxidise, such as gold and platinum. This is why it’s important to cover the terms fretting corrosion and fretting fatigue separately, although these phenomena most often appear in tandem.<\/p>\n

Fretting Corrosion<\/h3>\n

\"Fretting<\/p>\n

Fretting corrosion develops due to the removal of protective oxide layers on the metal surfaces, exposing fresh metal to corrosive elements. Wear debris created by the constant sliding motion accelerates the corrosion process<\/a> since the particles cannot escape contact, causing abrasive wear and subsequent oxidation of the freshly exposed metal, continuing the process and leading to elevated wear volumes.<\/p>\n

Corrosion depends on the material’s inertness. In the case of steel, iron oxide is harder than the steel itself and thus it acts as an abrasive and causes a lot of damage to the surfaces.<\/p>\n

Fretting corrosion may occur in metals such as steel, aluminium, cast iron, and other non-ferrous metals<\/a> but also in polymers and ceramics. The color of debris particles for fretting corrosion appears different from normal corrosion. Aluminium corrodes<\/a> to a white color under normal conditions and turns black with fretting corrosion, whereas steel becomes gray and reddish brown respectively.<\/p>\n

Examples of fretting corrosion involve wind turbine pitch bearings (false brinelling<\/a>) wherein the wear mechanism oscillates and orthopedic implants when the two materials in contact are in relative motion with each other.<\/p>\n

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