STLE - Basic of Wear

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Basics of Wear

Mechanisms of Wear

Mild Adhesion

Severe Adhesion

Abrasion

Erosion

Contact Fatigue

Corrosion

Fretting

False Brinnelling

Cavitation

Polishing

Electro Corrosion

Electrical Discharge

Summary

Special Thanks

Mechanisms of Wear

Early investigators could not do much to investigate wear because it is usually a very gradual process, and they did have accurate means to quantify it. Although friction and wear influence each other, the relationship is complex and involves many other factors including the chemical environment and temperature, each of which influences the othe simple correlation between friction and wear data is therefore hardly ever found. Modern research has shown that there are 12 main types of wear.

These are:

Mild Adhesion

Severe Adhesion

Abrasion

Erosion

Polishing

Contact Fatigue

Corrosion

Fretting Corrosion

Brinelling

Electro-Corrosion


Cavitation Damage

1. Wear can be prevented by:

2. Recognizing the type of wear.

3. Making changes in the lubricant, design or operation.

Most common regimes:

Adhesive Wear

Fatigue

Abrasive Wear

Corrosive Wear

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Mild Adhesion

Generally, removal of surface film material due to adhesion and subsequent loosening during relative motion. Mild Adhesion transfer and loosening of surface films only.

Other names Susceptible Machine Parts

Normal/Common All

Mild Adhesion - How to detect it

Unaided Eye Microscopically

Low rates of wear Smooth micro plateaus among original grinding marks

No damage Slight coloration due to films

Deeper original grinding marks still visible Slight coloration due to films

Mild Adhesions - Solutions Prevention Mechanical - None LUBRICANT - None

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Severe Adhesion

Cold welding of metal surfaces due to intimate metal to metal contact.

Earlier, we stated that when two surfaces are brought together under load, asperities of the two surfaces adhere to other. The conditions at the interface of these junctions are similar to those of a cold weld. A strong bond is formed without much interdiffusion of atoms and recrystallization as would occur in a hot weld. During sliding, these junctions are sheared. Shearing may occur at the interface or within one of the two asperities. junctions shear at the interface, but occasionally shearing will occur in one of the two materials. This will result in a fragment being transferred from one surface to the other. It would seem that shearing should always occur at the interface since it should have the greatest weakness. However, the junction plane, for example, may be parallel to t sliding direction so that it may not have the smallest cross-sectional area. Then the break may occur in the softer material occasionally in the harder material should it contain a local weak spot. Some junctions may be stronger tha base metal itself because of plastic yielding and work hardening. In the normal process of adhesive wear, there will some transfer of particles from one surface to the other. Some particles may be transferred back to the original surf or break off as loose wear particles. The adhesive wear theory expresses the total volume of wear particles generated V, per unit sliding length L, as: V/L=K P/H

With:

K=wear coefficient

P=load

H=hardness of the softer materials

Other Names Susceptible Machine Parts

Scuffing Piston rings and cylinder

Galling Rolling and sliding bearings

Smearing Gears

Cutting Tools

Chains

Valve Trains

Metal Seals

Severe Adhesion - How to detect it


Rough, torn, melted or plastically deformed metal, bands or streaks Rough, irregular surface

High temperature oxidation Metal from other surface adhering to other surface by spot te or microprobe analysis

High friction, high rates of wear

Possible seizure

Conditions Promoting Wear

High loads, speeds and/or temperatures

Use of stainless steels or aluminum

Insufficient lubricant

Lack of anti-scuff additives

No break in

Abrasive wear interrupting film allowing adhesion

Oil Analysis

Large metallic wear fragments of irregular shape

Severe Adhesion - Solutions Prevention

MECHANICAL

Reduce load, speed and temperature

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Improve oil cooling

Use compatible metals

Apply surface coatings such as phosphating

Modify surface, such as ion implantation

LUBRICANT

Use more viscous oil to separate surfaces

Use "extreme pressure" (anti-scuff) additives such as a sulfur-phosphorous or borate compounds

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Abrasion

Cutting and deformation of material by hard particles (3-body) or hard protuberances (2-body).

Abrasive wear occurs when either a rough, hard surface or a soft surface with hard particles embedded in its surface slides over a softer material. A plowing action takes place. When abrasive wear is the result of loose wear particles a contaminants, it is called three-body abrasive wear. Intentional abrasive wear is produced by grinding wheels, files sandpaper. The abrasive wear theory expresses the total volume of wear particles generated V, per unit of sliding length L as: V/L=K P/H Wear coefficients for abrasive wear are generally larger than for adhesive wear.


Cutting All surfaces in relative motion

Scratching

"Wire wool" damage The most common industrial wear problem

Gouging

Scoring

Abrasion - How to detect it


Scratches or parallel furrows in the direction of motion, similar to "sanding" Clean furrows, burrs, chips

High rates of wear Embedded abrasive particles

In sliding bearings with soft overlay embedde particles


Hard particles contaminating oil

Insufficient metal hardness

Hard metal with rough surface against soft metal

Oil Analysis

High metal content in oil and high silicone (10ppm) by emission spectroscopy

Burrs by via microscopic evaluation

Abrasion - Solutions Prevention

MECHANICAL

Remove abrasive by improved air and oil filtering, clean oil handling practices, improved seals, flushing and frequent oil changes

Minimize shot peening, beading, or sand blasting of surfaces because abrasives cannot be completely remov

Increase hardness of metal surfaces

LUBRICANT

Use oil free of abrasive particles

Use more viscous oil

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Erosion

Cutting of materials by hard particles in a high velocity fluid impinging on a surface.

This type of wear results from sharp particles impinging on a surface such as the cutting of materials by hard partic a high velocity fluid impinging on a surface. This action is very much like that of sandblasting.


Solid particles impact erosion Journal bearings near oil holes

Valves

Nozzles

Erosion - How to detect it


Smooth, broad grooves in direction of fluid flow Short V-shaped furrows by scanning electron microscopy

Matte texture, clean metal Embedded hard particles

Similar to sandblasting


High velocity gas or liquid containing solids impinging on a surface

Oil Analysis

High wear rates by emission spectrograph

Chips and burrs via microscopic evaluation

Contamination in oil

Erosion - Solutions Prevention

MECHANICAL

Remove abrasive by improved air and oil filtering, clean oil handling practices, improved seals, flushing and frequent oil changes

Increase hardness of metal surfaces

Reduce impact angle to less than 15 degrees

LUBRICANT

Use oil free of abrasive particles


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Contact Fatigue

Metal removed by cracking and pitting, due to cyclic elastic stress during rolling and sliding.

This type of wear is produced when repeated sliding or rolling occurs over a track. The most common example is the

action of a ball or roller bearing race. As the rolling element passes over a given spot on the raceway, it is stressed load is applied and released. Even though these stress reversals may be within the elastic limits of the material, cra will eventually occur. Depending on the stress pattern and material properties, these cracks may be initiated at or b the surface in the bodies in contact. With time, a relatively large piece of material will be released, leaving an uneve hole or pit. This type of wear will also occur on gear teeth.


Fatigue wear Rolling and sliding bearings

Frosting Valve train parts

Surface fatigue Gears

Spalling

Contact Fatigue - How to detect it


Cracks, pits and spalls Combination of cracks and pits with sharp edges

Subsurface cracks by metallographic cross-section. Numerous metal inclusions


Cyclic stress over long periods

Water, dirt, in oil

Inclusions in steel

Particles of metal with sharp edges

Metal spheres by microscopy

Contact Fatigue - Solutions Prevention

MECHANICAL

Reduce contact pressures and frequency of cyclic stress

Use high quality vacuum melted steels

Use less abusive surface finish

LUBRICANT

Use clean, dry oil


Use oil with higher-pressure viscosity coefficient

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Corrosion

Rubbing off of corrosion products on a surface.

Corrosive wear occurs when the sliding of surfaces are in a corrosive environment. This wear action generally takes in two stages. First, there is corrosive attack on the surfaces. Then the sliding action wears off the corroded surface film. If the compounds formed are harder than the original material and if loose particles are formed. The wear rate is accelera abrasive sliding action. Some corrosive reaction products are softer than the base metal and act to reduce the wear rate especially when th sliding action does not remove the entire layer. Hence, some corrosion products can be used beneficially for their go lubricating characteristics. An example of a highly corrosive situation occurs in the engines of ocean-going vessels. Many ships are powered by engines that use inexpensive, high-sulfur fuel. However, their combustion products contain oxidized sulfur and wate vapor. These combine to form sulfuric acid, which is highly corrosive to the cast iron cylinders and piston rings. This corrosion is effectively controlled by using highly alkaline-type cylinder lubricants that neutralize the acids.


Chemical wear All bearings

Oxidative wear Cylinder walls

Corrosive film wear Valve train

Seals and chains

Corrosion - How to detect it


Corroded metal surface rust, FeOHO (hydrated iron oxide) is a common iron corrosion product

Scale, films, pits containing corrosi products

Dissolution of one phase in a 2-pha alloy


Corrosive environment

Corrodible metals

Rust promoting conditions

High temperatures

Oil Analysis

Detection of corrosion products in oil and wear debris

Detection of anion, such as chlorine by X-ray fluorescence

Corrosion - Solutions Prevention

MECHANICAL

Use more corrosion resistant metal (not stainless)

Reduce operating temperature

Eliminate corrosive material

LUBRICANT

Remove corrosive material such as too chemically active additive and contaminates

Use improved corrosion inhibitor

Use fresh oil

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Fretting

Wear between two solid surfaces experiencing oscillatory relative motion of low amplitude.

Fretting wear occurs where there is oscillatory motion with a small displacement ( ~1 micron) of the contacting surf under load. Small wear particles are formed through the mechanism of adhesive wear. Because of the small amplitu motion the wear particles are not carried out of the contact area and removed from the system. The particles produ contribute to the wear through abrasive action. Where the material is steel, which is usually the case, corrosive wea enter the picture. The wear particles oxidize and generally, being harder than the base metal, expose clean metal to further oxidation.


False brinelling Vibrating machines

Fretting Bearing housing contacts

Friction oxidation Splines, keys, couplings

Fasteners

Fretting - How to detect it


Corroded stained surfaces where damage on one surface is mirror image of mating surface

Thick films of oxide and metal. Red and for steel

Loose colored debris around real contact areas

Rouge (Fe?O?) colored films, debris, grease or oil for steel


Vibration causing relative motion

Oil Analysis

Identify metal oxide (alpha Fe?O? for steel) by X-ray diffraction

Fretting - Solutions Prevention

MECHANICAL

Reduce or stop vibration by tighter fit or higher load

Improve lubrication between surfaces by rougher (then honed) surface finish

LUBRICANT

Use oil of lower viscosity

Relubricate frequently

Use oxidation inhibitors in oil

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False Brinneling

Localized wear spots made by rolling elements on raceways due to limited rolling/repeated impact.

Localized wear spots formed by rolling elements on raceways due to limited rolling/repeated impact. False Brinelling typically characterized by indentations on the inner or outer raceway of a rolling element bearing. The indentation

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corresponded to the position of the rolling elements.


False brinelling Rolling element bearing raceways

Fretting

False Brinelling - How to detect it


Indentations on raceway Indentations on raceway


Protective film is broken continually by repeated impacts

Oil Analysis

None

False Brinelling - Solutions Prevention

MECHANICAL

Reduce or eliminate impact

Rotate bearings occasionally

LUBRICANT

Change viscosity

Consider additives

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Cavitation

Removal of metal by vapor cavity implosion in a cavitating liquid.

Cavitation happens, for example, in high-pressure pumps adjacent to the intake ports and on the blades of a ship's propellers. When a sudden local reduction in fluid pressure occurs, a vapor cavity is formed. When the vapor cavity collapses near a solid surface it produces a mechanical shock. Surface damage similar to surface fatigue takes place particles are removed. The particle sizes are smaller, however, than those generated by surface fatigue.


Cavitation erosion Hydraulic parts, pumps, valves, gear teeth

Fluid erosion, NOT pump cavitation same name, different thing Cylinder liners

Piston rings

Sliding bearings

Cavitation - How to detect it


Clean frosted or rough appearing metal Clean, metallic bright rough metal, pits

Deep, rough pits or grooves Removal of softer phase from 2-phase metal (graphite phase in cast iron is susceptible)


Sudden changes in liquid pressure due to changes in liquid velocity or to shape or motion of parts

Cavitation damage is increased by a corrosive environment, and by abrasives in the oil

Oil Analysis

Observation of large chunks or spheres of metals in oils

Cavitation - Solutions Prevention

MECHANICAL

Use hard, tough metals, such as tool steel

Reduce vibration, flow velocities and pressures

Avoid restriction and obstructions to liquid flow

LUBRICANT

Avoid low vapor pressure, aerated, wet oils

Use noncorrosive oils

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Polishing

Continuous removal of surface films by very fine abrasives.

Polishing wear is characterized by very shiny, very smooth, mirror like metal surfaces. Fine abrasives wear away the

surface films as they form and reform.


Bore polishing Cylinder bores (diesel engines)

Gear teeth

Valve lifters

Polishing - How to detect it


High wear but a bright mirror finish Featureless surface except scratches at high magnification by electron micros

Wavy profile

Polishing - Solutions Prevention

MECHANICAL

None

LUBRICANT

Choose less chemically active additive

Remove corrosive contaminant

Remove abrasive

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Electro-Corrosion

Dissolution of a metal in an electrically conducive liquid by low amperage currents.


"Erosion" Aircraft hydraulic valves

Electrical erosion Hydraulic pumps and motors

Electro-chemical wear

Electrical attack

Electro-corrosion - How to detect it


High wear but a bright mirror finish Featureless surface except scratches at high magnification by electro microscopy

Local corroded areas Corrosion pits, films, dissolution of metals

Black spots such as made by a small drop of acid

Corroded, worn metering edges


High velocity liquid flow causing streaming potentials

Stray currents

Galvanic metal combinations

Oil Analysis

Detection of corrosion products

Electrically conducive liquids

Electro-corrosion - Solutions Prevention

MECHANICAL

Decrease liquid velocity and velocity gradients

Use corrosion-resistant metals

Eliminate stray currents

Use nongalvanic couples

LUBRICANT

Decrease or increase electrical conductivity of lubricants or hydraulic fluids

Highly compounded oils can act as electrolytes, and be conductive. Phosphate ester hydraulic fluids are cond

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Removal of metal by high amperage electrical discharge or spark between two surfaces.


Electrical pitting Bearings in high speed rotating machinery such as compressors, atomizers

Sparking Static charge producers

Electrical Discharge - How to detect it


Metal surface appears etched. In thrust bearings, sparks make tracks like an electrical engraver

Pits, near edge of damage, showing once molten state, suc smooth bottoms, rounded particles, gas holes

Rounded particles welded to surface near pits


High-speed rotation

High velocity two-phase fluid mixtures

High potential contacts

Sparks

Oil Analysis

Detection of large rounded particles by microscopic examination of filtrate or on microscope slide

Electrical Discharge - Solutions Prevention

MECHANICAL

Improve electrical insulation of bearings

Degauss magnetic rotating parts

Install brushes on shaft

Improve machine grounding

LUBRICANT

Use of oil of higher electrical conductivity

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Summmary

Congratulations! You have completed the materials for Fundamentals of Lubrication and Tribology II - Wear. We hope that you have achieved the learning objectives listed on the Overview page of this module such that you a able to:

Describe the different wear processes;

Recognize the manner in which certain wear processes can lead to other wear processes;

Discuss the factors which are primarily responsible for accelerating each type of wear process;

DUnderstand the Archard equation for wear rate and the conditions under which it is valid; and,

Know how to minimize wear through proper selection of lubricant, material, and operating conditions.

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Special Thanks

Technical Contributors We would like to thank the following people for their valuable contributions to this course:

Principal Contributor: Dr. James Ziemer, Chevron

William R. Herguth, Herguth Laboratories, Inc.,

Robert W. Bruce, GE Aircraft Engines,

Douglas Godfrey, Wear Analysis,

Ray Ryason, Tamalpais Tribology,

E.R. Booser, Independent Consulting Engineer,

Andrew Flaherty, Flowserve Corp.,

Dr. Simon Tung, General Motors,

Philip J. Guichelaar, Western Michigan University

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