Basic Wear Modes

7/30/2019 Basic Wear Modes

1/6

Basic Wear Modes in LubricatedSystems

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.


2/6

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 FatigueThis 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.


3/6

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 WearAbrasive 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 WearAdhesive 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


4/6

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.

CorrosionMoisture 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 f luid 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


5/6

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 ErosionThis 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 positionedand 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 DeformationThis 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.


6/6

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.

ErosionErosion 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.

CavitationThis 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.

AcknowledgmentSeveral 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).

Documents

Basic Wear Modes