Basic Wear Modes in Lubricated Systems

This article provides a basic definition and understanding of the major wear modes or mechanisms based around the ISO 15243.2004 rolling bearing failure mode classification. Several other modes of wear that occur in gears, journal bearings, hydraulic pumps and pistons - but don't occur in rolling bearings - will be discussed.

The ISO system discusses wear in six major categories with 15 subcategories.

Not contained in the ISO classification is Erosion from particles and Cavitation.

Wear mechanisms can also be thought of as occurring in two separate categories: contact and noncontact modes. Contact wear requires the components to have direct metal-to-metal contact for wear to occur. Noncontact modes do not require the surfaces to come into direct contact for them to wear; in other words, a full fluid lubricant film may exist.

Subsurface Fatigue

Subsurface fatigue is a form of wear that occurs after many cycles of high-stress flexing of the metal. This causes cracks in the subsurface of the metal, which then propagate to the surface, resulting in a piece of surface metal being removed.

It begins with inclusions or faults in the bearing metal below the surface. Subsurface microcracks form due to long-term repeated load cycles and stress (500,000 psi), causing elastic deformation (flexing) of the metal. This is typical in all rolling bearing elements and races and gear teeth, all of which operate in the elastohydrodynamic (EHD) lubrication regime. The contact stress is concentrated at a point below the metal surface.

These microcracks normally propagate to the surface, which eventually results in a piece of the surface material being removed or delaminated. They appear as surface damage or wear (large pits) referred to as spalling. Other terms for subsurface fatigue include flaking, peeling and mechanical pitting. A full oil film exists and no metal-to-metal contact or surface damage is needed. Subsurface fatigue is not a common issue if better quality metals are used in bearing manufacture. Most bearings will fail by another mechanism first.

Subsurface fatigue failure is the result of a bearing living out its normal life span based on the load, speed and lubricant film thickness that it is exposed to. The L10 fatigue life of a bearing is the average time (in hours or cycles) to fail 10 percent of a set of identical bearings under certain conditions. An estimate of the L10 life can be calculated, providing a rating life of a bearing.

Surface-initiated Fatigue

This begins with reduced lubrication regime and a loss of the normal lubricant film. The oil film is reduced to boundary or a mixed regime. Some metal-to-metal contact and sliding motion occurs. Surface damage occurs. The high points of the metal surface asperities are removed, which initially appear as a matted or frosted surface. This is not smearing, as in adhesion (discussed below). This type of surface damage is usually visible with a magnification of three to five times.

The surface damage is coupled with the cyclic loading of the rollers rolling over the race. This creates asperity microcracks and microspalling. The cracks start at the surface and migrate down into the metal. An edge of metal is created at the surface which flexes at the edge of the surface crack. This creates a cold worked edge which is lighter in color. The cracks propagate and may intersect within the metal, and a piece of surface material is then removed. Flaking, mechanical pitting and micropitting are other names used to describe spalling.

Surface fatigue can also occur as a result of plastic deformation (described below). Contaminant particles in the oil enter the high-load rolling contact area between rollers and the race, or between gear teeth, and cause some form of surface damage - a dent. Improper handling of bearings can cause similar surface damage.

These round-bottomed dents often have a raised berm around their edges. The raised berm of metal acts as a point of increased load or stress, or creates a reduced lubrication regime (mixed or boundary), and leads to a lower surface fatigue life. Improved filtration reduces plastic deformation, and therefore indirectly reduces the occurrence of surface fatigue.

Notice that the term "contact fatigue" is not used by ISO. This is a vague term sometimes used to describe both forms of fatigue. It does not specify whether metal flexing damage started in the subsurface or from some initial surface damage. It encompasses any change in the metal structure caused by repeated stresses concentrated at a microscopic scale in the contact zone between the rolling elements and raceways, and between gear teeth.

Abrasive Wear

Abrasive wear is estimated to be the most common form of wear in lubricated machinery. Particle contamination and roughened surfaces cause cutting and damage to a mating surface that is in relative motion to the first.

Three-body abrasion occurs when a relatively hard contaminant (particle of dirt or wear debris) of roughly the same size as the dynamic clearances (oil film thickness) becomes imbedded in one metal surface and is squeezed between the two surfaces, which are in relative motion. When the particle size is greater than the fluid film thickness, scratching, ploughing or gouging can occur. This creates parallel furrows in the direction of motion, like rough sanding. Mild abrasion by fine particles may cause polishing with a satiny, matte or lapped-in appearance. This can be prevented with improved filtration, flushing and sealing out small particles.

Two-body abrasion occurs when metal asperities (surface roughness, peaks) on one surface cut directly into a second metal surface. A contaminant particle is not directly involved. The contact occurs in the boundary lubrication regime due to inadequate lubrication or excessive surface roughness which could have been caused by some other form of wear. Higher oil viscosity, increased metal hardness and even demagnetizing bearings after induction heating during installation may help to reduce two-body abrasion.

Adhesive Wear

Adhesive wear is the transfer of material from one contacting surface to another. It occurs when high loads, temperatures or pressures cause the asperities on two contacting metal surfaces, in relative motion, to spot-weld together then immediately tear apart, shearing the metal in small, discrete areas.

The surface may be left rough and jagged or relatively smooth due to smearing/deformation of the metal. Metal is transferred from one surface to the other. Adhesion occurs in equipment operating in the mixed and boundary lubrication regimes due to insufficient lube supply, inadequate viscosity, incorrect internal clearances, incorrect installation or misalignment. This can occur in rings and cylinders, bearings and gears.

Normal break-in is a form of mild adhesive wear, as is frosting. Scuffing usually refers to moderate adhesive wear, while galling, smearing and seizing result from severe adhesion. Adhesion can be prevented by lower loads, avoiding shock loading and ensuring that the correct oil viscosity grade is being used. If necessary, extreme pressure (EP) and antiwear (AW) additives are used to reduce the damage.

Corrosion

Moisture corrosion involves material removal or loss by oxidative chemical reaction of the metal surface in the presence of moisture (water). It is the dissolution of a metal in an electrically conductive liquid by low amperage and may involve hydrogen embrittlement. It is accelerated, like all chemical reactions, by increased temperatures. No metal-to-metal contact is needed. It will occur with a full oil fluid film.

Corrosion is often caused by the contamination or degradation of lubricants in service. Most lubricants contain corrosion inhibitors that protect against this type of attack. When the lubricant additives become depleted due to extended service or excessive contamination by moisture, combustion or other gases or process fluids, the corrosion inhibitors are no longer capable of protecting against the acidic (or caustic) corrosive fluid and corrosion-induced pitting can occur. The pits will appear on the metal surface that was exposed to the corrosive environment.

This may be the entire metal surface or just the lower portion of the metal that may have been submerged in water not drained from the oil sump or at the roller/race contact points. Generally, an even and uniform pattern of pits will result from this form of attack. Mild forms of moisture corrosion result in surface staining or etching. More severe forms are referred to as corrosive pitting, electro-corrosion, corrosive spalling or rust.

Frictional corrosion is a general form of wear caused by loaded micromovements or vibration between contacting parts without any water contaminant being present, although humidity may be necessary. It may also be referred to as fretting wear. It includes both fretting corrosion and false brinelling, which in the past were often considered to be the same mechanism.

Fretting corrosion is the mechanical fretting wear damage of surface asperities accompanied and escalated by corrosion, mostly oxidation in air with some humidity present. It occurs due to many oscillating micromovements at contacting interfaces between loaded and mating parts in which the lubricant has not been replenished (an unlubricated contact). Adhesion is occurring and it is generally considered more severe than false brinelling.

It usually appears as a reddish-brown oxide color (rust without water being present) on steel and black on aluminum. Metal wear debris flakes are created or shed off.

Fretting corrosion occurs on many mechanical devices such as gear teeth and splines, not just rolling element bearings, and can occur on surfaces other than the rolling contact. In bearings, it is also associated with bearing fit on the shaft and in the housing. It occurs where there is not any large relative motion between the mating parts such as between the shaft and the inner race and between the housing and the outer race. Fretting corrosion can occur on materials that do not oxidize.

False brinelling occurs due to micromovements under cyclic vibrations in either static or rotating boundary lubrication contacts. Mild adhesion of the metal asperities is occurring. Shallow depressions or dents are created in which the original machining marks are worn off and no longer visible due to the wearing damage of the metal. False brinelling occurs on the rolling elements and raceway, similar to small-scale plastic deformation or brinelling (see below) and hence the name "false brinelling".

False brinelling is usually associated with static nonrotating equipment and, thus, the wear appears at the roller contacts with the exact same spacing as the rollers. The depressions in the metal can appear shiny with black wear debris around the edges. If the equipment is rotating, the wear appears as a gray, wavy washboard pattern on the raceway. Reduced bearing life or failure ultimately occurs, sometimes in a catastrophic fashion, through surface fatigue initiating in these damaged surface layers.

An example of false brinelling occurs in standby electric motors and pumps (and others) which sit idle for periods of time, but are subjected to vibration from the plant floor up through the load-bearing rolling elements of the bearings. Antiwear additives may be beneficial in reducing the wear damage.

 

Electrical Erosion

This type of wear occurs when electric current passes between two metal surfaces (for example, bearing roller and race) through the oil or grease film. It is subdivided based on the severity of the damage. Electrical erosion should not be confused with erosion caused by particles (discussed below).

 

Excessive voltage (electrical pitting) is caused by a high electrical current or amperage passing through only a few asperities on the metal. Voltage builds up and then arcs, causing localized heating/melting and vaporization of the metal surface. This causes deep, large craters or pits in the metal surfaces, which may correspond to the spacing between the rolling elements of the bearing. It is possibly due to welding in the area and inadequate grounding or insulation. It may also be referred to as electrical pitting, arcing or sparking.

Current leakage (electrical fluting) is a less severe form of damage caused by a lower continuous electrical current. The damage may be shallow craters that are closely positioned and appear dark gray in color. If the electrical discharge occurs while the bearing is in motion, with a full fluid film, a washboard effect or grooves appear on the entire bearing raceway and is called fluting or corduroying.

 

Plastic Deformation

This is the denting, indentations or depressions in the race or rollers caused by impact or overloading. The surface metal flows, causing irreversible deformation (not wear). The machining marks are still visible in the bottom of the dent. The dents often have a raised lip which increases stresses and leads to surface-initiated fatigue (surface cracks) and eventual pit formation or adhesive wear. Plastic deformation consists of three subcategories.

 

Overload or true brinelling is characterized by static or shock loading, or impact from operational abuse, causing a permanent dent in the metal without cutting or welding of the metal. An example occurs in roller bearings when impact causes the rollers to create a series of dents in the bearing race surface at intervals that match the roller spacing exactly. Some people consider denting from the impact of hammering on a bearing as overload; others may consider it as an indentation from handling.

Indentation from debris is a form of plastic deformation but it is caused by a particle trapped within the dynamic clearances between two machine elements and being over-rolled. The force causes a round-bottom dent to form in the race or rolling element. Cracks may propagate down into the metal.

Indentation from handling is similar to that from debris, but results from a bearing being dropped or hammered, causing localized overloading. It can also be due to nicks from hard or sharp objects.

It is common to encounter erosion from particles in the oil and cavitation, although this is not included in the ISO standard for rolling bearings.

Erosion

Erosion could be considered a form of abrasive wear. It occurs principally in high-velocity, fluid streams where solid particle debris, entrained in the fluid (oil), impinges on a surface and erodes it away. Hydraulic systems are an example where this type of wear may occur. Flow rates have a significant influence on these wear rates, which are proportional to at least the square of the fluid velocity. Erosion typically occurs in pumps, valves and nozzles. Metal-to-metal contact does not occur. The mechanism of erosion is used to an advantage in water-jet cutting.

Cavitation

This is a special form of erosion in which vapor bubbles in the fluid form in low-pressure regions and are then collapsed (imploded) in the higher-pressure regions of the oil system. The implosion can be powerful enough to create holes or pits, even in hardened metal if the implosion occurs at the metal surface. This type of wear is most common in hydraulic pumps, especially those which have restricted suction inlets or are operating at high elevations.

Restricting the oil from entering the pump suction reduces the pressure on the oil and, thus, tends to create more vapor bubbles. Cavitation can also occur in journal bearings where the fluid pressure increases in the load zone of the bearing. No metal-to-metal contact is needed to create cavitation.

Just to be clear, pitting is a general term used in failure analysis to describe almost any small, rough-bottomed, circular potholes in the metal surface. Pits can be caused by mechanical pitting (fatigue or cavitation), chemical pitting (corrosion) or by electrical pitting (stray arcing), all of which are described above.

Failure analysis is used to assign a wear mechanism to a specific failure. If the wear mechanism can be determined, then some corrective action can be applied to prevent the failure from recurring. Often, it can be useful to use the process of elimination to determine which wear mechanisms could not have produced the observed wear pattern, thus reducing the number of possible mechanisms. Unfortunately, combinations of wear mechanisms exist in most situations, thus complicating the selection of the optimum wear-resistant system.

Acknowledgment
Several portions of this article may contain residual wording from an article that was originally written by Rees Llewellyn of the National Research Council of Canada for the Alberta section of the Society of Tribologists and Lubrication Engineers (STLE).