Mysterious Micro-movements (corrosion) Wear

Continuing from last week, today I would like to introduce micro-dynamic corrosion wear, the English name: Fretting.

foreword

The introduction of micro-motion (corrosion) wear is primarily based on my previous work experience. For example, even though there is no obvious relative motion between two components, wear still occurs at the contact surface. Even though the two components are interference-fitted and should not have any relative motion, rust and corrosion similar to those seen in the bearing outer ring and bearing housing, as well as the spline fit, occur.

In fact, these are micro-movements (corrosion) wear.

What is micro-damage (corrosion)?

Micro-motion (corrosion) wear refers to the slight oscillatory displacement (100 micrometers or less) that occurs between contact surfaces that are nominally stationary. Due to the minimal amplitude of movement, lubricating oil cannot redistribute itself on the contact surfaces, leading to direct metal surface contact and interaction with oxygen, which results in corrosion of the metal surfaces and the production of oxide particles. These particles interact with the friction surfaces, causing wear. Micro-motion (corrosion) wear has several characteristics: it typically forms indentations on the contact surfaces, similar to those seen in Brinell hardness tests, hence it is also known as pseudo-Brinell indentations. When these pseudo-pressures expand to a certain extent, they can trigger the initiation and propagation of micro-cracks, potentially leading to fracture failure. This type of damage is referred to as micro-fatigue. In everyday life, many typical cases of micro-motion (corrosion) wear include interference fits and press fits, bearing housings on rotating shafts, elastic couplings and splines, vane connections between turbine blades and shafts, steel cables and wires in cranes and elevators, tube supports in heat exchangers, car leaf springs, ball bearings, ball and raceway, aircraft riveting and fastener connections, etc. Micro-motion wear primarily occurs due to free clearance in different directions, as illustrated in the figure below, which lists some micro-motion positions of connecting parts.

Mechanism of micro-dynamic (corrosion) wear

The micro-motion (corrosion) wear process in the contact area involves complex physical and chemical interactions. Therefore, no single unified theory has been developed to describe the mechanism of micro-motion (corrosion) wear. Different operating conditions may correspond to different micro-motion mechanisms. Typically, micro-motion (corrosion) wear is considered a combination of adhesive wear, corrosion wear, abrasive wear, and fatigue wear. However, the primary cause is the shear stress resulting from slight relative vibration at the adhesion points between the contact surfaces, followed by oxidation. Based on the relationship between wear and oxidation, two distinct mechanisms have been proposed: wear-oxidation and oxidation-wear.

Under load-bearing conditions, the protruding parts of the two metal surfaces in actual contact are in a state of adhesion and welding. During relative motion, these contact points are damaged, causing metal particles to fall off. Due to abrasive wear, these particles are oxidized, and the harder oxide particles act as abrasives during subsequent micro-corrosion, enhancing the mechanical wear process.In the oxidation-wear mechanism, in relative motion, the oxide film on the protruding parts of the actual contact surface of the metal is worn down and turned into oxide particles, and the fresh metal exposed is re-oxidized. This process is repeated, resulting in micro-movement (corrosion) wear.

Factors affecting micro-dynamic (corrosion) wear

Tests on the experimental setup revealed that, in general, vibration frequency, amplitude, and acceleration significantly affect micro-motion (corrosion) wear. The amount of micro-motion wear increases with the rise in vibration frequency and acceleration. However, regarding amplitude, the wear initially increases as the amplitude grows. When the amplitude reaches a certain level, typically 2 to 3 times the clearance, the wear reaches its peak and then decreases as the amplitude continues to increase. Under specific test conditions, the wear rate increases rapidly during the early stages of micro-motion but slows down as the frequency of micro-motions increases. This phenomenon is known as the three-body theory of micro-motion.