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Wear of MetalsWear of Metals

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What is Metallic Wear?What is Metallic Wear?The deterioration of metal surfaces isThe deterioration of metal surfaces is known as the Metallic Wear.

Wear is the result of erosion, abrasion,Wear is the result of erosion, abrasion, impact, metal-to-metal contact, oxidation and corrosion or aoxidation, and corrosion, or a combination of these.

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Introduction to WearPlastic deformation at the interface often leads toPlastic deformation at the interface often leads to wear, i.e., deformation induced wear. Wear can also be caused by chemical processes. y pThere are many different kinds of wear mechanismsWe have to analyze these wear mechanisms using mechanics, thermodynamics, etc. Tribology is a multi-disciplinary subject

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Types of Wear:Types of Wear:There are generally 12 Types of MetallicThere are generally 12 Types of Metallic Wear:

1 Mild Adhesion1. Mild Adhesion 2. Severe Adhesion 3. Abrasion 4 E i4. Erosion 5. Polishing g

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Types of Wear:Types of Wear:6 Contact Fatigue6. Contact Fatigue 7. Corrosion

F tti C i8. Fretting Corrosion 9. Brinelling 10.Electro-Corrosion 11 Electrical Discharge11.Electrical Discharge 12.Cavitation Damage

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

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Example:Example:

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Other names Susceptible Machine PartsOther names Susceptible Machine Parts

Normal/Common All

Mild Adhesion - How to detect it :

U id d E Mi i llUnaided Eye Microscopically

Low rates of wearSmooth micro plateaus among original grinding marksoriginal grinding marks

No damage Slight coloration due to films

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Deeper original grinding marks still visible

Slight coloration due to films

Mild Adhesions Solutions:Mild Adhesions Solutions:PreventionPrevention

Mechanical None

LUBRICANT None

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Severe Adhesion:Severe Adhesion:Severe Adhesion:Severe Adhesion:Cold welding of metal surfaces due to intimate metal to metal contact.

Mechanism:when two surfaces are brought together under loadwhen two surfaces are brought together under load, asperities of the two surfaces adhere to each other. The conditions at the interface of these junctions are jsimilar to those of a cold weld. A strong bond is formed but without much interdiffusion of atoms and

t lli ti ld i h t ldrecrystallization as would occur in a hot weld.11

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Detection:Detection:Severe Adhesion - How to detect it :Unaided Eye Microscopically

Rough torn melted or plasticallyRough, torn, melted or plastically deformed metal, bands or streaks

Rough, irregular surface

High temperature oxidationMetal from other surface adhering to other surface by spot tests or microprobe analysismicroprobe analysis

High friction, high rates of wear

Possible seizure 13

Conditions Promoting Wear:Conditions Promoting Wear:

Conditions:Conditions:1. High loads, speeds and/or temperatures 2. Use of stainless steels or aluminum 3. Insufficient lubricant 4. Lack of anti-scuff additives5. No break in 6. Abrasive wear interrupting film allowing

adhesion

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Severe Adhesion Solutions:Severe Adhesion Solutions:1 Reduce load, speed and temperature1. Reduce load, speed and temperature 2. Improve oil cooling 3 Use compatible metals3. Use compatible metals4. Apply surface coatings such as phosphating5 Modify surface such as ion implantation5. Modify surface, such as ion implantation

LUBRICANT:Use more viscous oil to separate surfacesUse more viscous oil to separate surfaces Use "extreme pressure" (anti-scuff) additives such as a sulfur-phosphorous or borate compoundsa sulfur phosphorous or borate compounds

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Abrasion:Abrasion:Abrasion is the wearing of surfaces by rubbing,Abrasion is the wearing of surfaces by rubbing, grinding, or other types of friction.It usually occurs due to metal-to metal contact. It yis a scraping, grinding wear that rubs away metal surfaces and can be caused by the scouring action of sand, gravel, slag, earth, and other gritty material.

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Example:Example:

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Detection:Detection:Abrasion - How to detect it :Abrasion How to detect it :Unaided Eye Microscopically

Scratches or parallel furrows in the direction of motion, similar to Clean furrows, burrs, chipsd ec o o o o , s o"sanding"

C e u ows, bu s, c ps

High rates of wear Embedded abrasive particlesHigh rates of wear Embedded abrasive particles

In sliding bearings with soft overlay b dd d i lembedded particles

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Conditions Promoting Wear:Conditions Promoting Wear:

Conditions:Conditions:1. Hard particles contaminating oil 2. Insufficient metal hardness 3. Hard metal with rough surface against soft metal

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Prevention:Prevention:MECHANICALRemove abrasive by improved air and oil filtering, clean oil handling practices, improved seals, flushing and frequent il hoil changes

Minimize shot peening, beading, or sand blasting of surfaces because abrasives cannot be completelysurfaces because abrasives cannot be completely removed Increase hardness of metal surfaces LUBRICANTUse oil free of abrasive particles U i ilUse more viscous oil

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Erosion:Erosion:Definition:Definition:Cutting of materials by hard particles in a high velocity fluid impinging on a surfacevelocity fluid impinging on a surfaceOccurrence:

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

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Mechanism:Mechanism:

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Erosion - How to detect it:Erosion How to detect it:

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Unaided Eye Microscopically1

Smooth, broad grooves in direction of fluid flow

Short V-shaped furrows by scanning electron microscopy

Matte texture, clean metal Embedded hard particles, p

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Similar to sandblasting

Example:Example:

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Erosion – Solutions:Erosion Solutions:Prevention:Prevention:

MECHANICAL:R b i b i d i d il filt i l ilRemove abrasive by improved air and oil filtering, clean oil handling practices, improved seals, flushing and frequent oil changesgIncrease hardness of metal surfaces Reduce impact angle to less than 15 degrees LUBRICANT:Use oil free of abrasive particles U i ilUse more viscous oil

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Contact Fatigue:Contact Fatigue:Definition:Definition:Metal removed by cracking and pitting, due to cyclic elastic stress during rolling and slidingcyclic elastic stress during rolling and sliding.

Occurrence: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 rollercommon 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 as thea given spot on the raceway, it is stressed as the load is applied and released. 26

Contact Fatigue - How to detect it:Contact Fatigue How to detect it:

Detection:Detection:Unaided Eye Microscopically

Cracks, pits and spallsCombination of cracks and pits with sharp edgesp g

Subsurface cracks by ymetallographic cross-section. Numerous metal inclusions

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Example:Example:

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Prevention:Prevention:MECHANICAL:MECHANICAL:Reduce contact pressures and frequency of cyclic stressUse high quality vacuum melted steels Use less abusive surface finishUse less abusive surface finish LUBRICANT:Use clean dry oilUse clean, dry oil Use more viscous oil Use oil with higher pressure viscosity coefficientUse oil with higher-pressure viscosity coefficient

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Corrosion Wear:Corrosion Wear:Corrosion wear is the gradual deterioration ofCorrosion wear is the gradual deterioration of unprotected metal surfaces, caused by the effects of the atmosphere, acids, gases, alkalies, etc. This type of wear creates pits and perforations and may eventually dissolve metal parts.

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Mechanism:Mechanism:

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Corrosion - How to detect it:Corrosion How to detect it:Detection:Detection:

Unaided Eye Microscopically

Corroded metal surface rust, l fil i i i

Co oded e a su ace us ,Fe²O³H²O (hydrated iron oxide) is a common iron corrosion product

Scale, films, pits containing corrosion products

Dissolution of one phase in a 2-

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phase alloy

Example:Example:

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Corrosion – Solutions:Corrosion – Solutions:PreventionPrevention MECHANICAL

1 Use more corrosion resistant metal (not stainless)1. Use more corrosion resistant metal (not stainless)2. Reduce operating temperature3. Eliminate corrosive material3. Eliminate corrosive material

LUBRICANT1. Remove corrosive material such as too chemically active

additive and contaminates 2. Use improved corrosion inhibitor3 U f h il3. Use fresh oil

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Fretting:Fretting:Definition:Definition:Wear between two solid surfaces experiencing

ill t l ti ti f l lit doscillatory relative motion of low amplitude.

Occurrence: Fretting wear occurs where there is oscillatory motion with a small displacement ( 1 micron) ofmotion with a small displacement ( ~1 micron) of the contacting surfaces under load. Small wear particles are formed through the mechanism ofparticles are formed through the mechanism of adhesive wear 35

Mechanism:Mechanism:

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Fretting - How to detect itFretting How to detect itUnaided Eye MicroscopicallyUnaided Eye MicroscopicallyCorroded stained surfaces where damage on one

Thick films of oxide and where damage on one surface is mirror image of mating surface

metal. Red and black for steel

mating surfaceLoose colored debris around

l t treal contact areasRouge (Fe?O?) colored films, debris, grease or oil for steel

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Example:Example:

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Fretting - SolutionsFretting - Solutions PreventionPreventionMECHANICAL

1 Reduce or stop vibration by tighter fit or higher load1. Reduce or stop vibration by tighter fit or higher load2. Improve lubrication between surfaces by rougher

(then honed) surface finish(then honed) surface finish LUBRICANT

1 Use oil of lower viscosity1. Use oil of lower viscosity 2. Relubricate frequently 3 Use oxidation inhibitors in oil3. Use oxidation inhibitors in oil

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False Brinneling:False Brinneling:Definition:Definition:Localized wear spots made by rolling elements on raceways due to limited rolling/repeated impactraceways due to limited rolling/repeated impact.

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

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Example:Example:

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False Brinelling - SolutionsFalse Brinelling SolutionsDetection:Detection:

Unaided Eye MicroscopicallyIndentations on raceway Indentations on raceway

PreventionMECHANICAL

Indentations on raceway Indentations on raceway

MECHANICALReduce or eliminate impactRotate bearings occasionallyg yLUBRICANTChange viscosity Consider additives

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Cavitation:Cavitation:Definition:Definition:Removal of metal by vapor cavity implosion in a cavitating liquidcavitating liquid.

Occurrence:Cavitation happens, for example, in high-pressure pumps adjacent to the intake ports and on the blades of a ship's propellers When a suddenblades of a ship's propellers. When a sudden local reduction in fluid pressure occurs, a vapor cavity is formed When the vapor cavity latercavity is formed. When the vapor cavity later collapses near a solid surface it produces a mechanical shock.

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Cavitation - How to detect itCavitation How to detect it

Detection:Detection:Unaided Eye MicroscopicallyClean frosted or rough appearing metal

Clean, metallic bright rough metal, pitspp g , pRemoval of softer phase from 2-phase metal

Deep, rough pits or groovesfrom 2 phase metal (graphite phase in cast iron is susceptible)

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is susceptible)

Cavitation - SolutionsCavitation - SolutionsPrevention:Prevention:

MECHANICALUse hard, tough metals, such as tool steelReduce vibration, flow velocities and pressuresAvoid restriction and obstructions to liquid flow LUBRICANTAvoid low vapor pressure, aerated, wet oils Use noncorrosive oils

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Polishing:Polishing:Definition:Definition:Continuous removal of surface films by yvery fine abrasives.Polishing wear is characterized by veryPolishing wear is characterized by very shiny, very smooth, mirror like metal surfaces. Fine abrasives wear away thesurfaces. Fine abrasives wear away the surface films as they form and reform

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Mechanism:Mechanism:

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Polishing - How to detect itPolishing How to detect it

Detection:Detection:Unaided Eye MicroscopicallyUnaided Eye Microscopically

Featureless surface except High wear but a bright mirror finish

scratches at high magnification by electron microscopy

Wavy profile48

Wavy profile

Example:Example:

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Polishing - SolutionsPolishing - Solutions PreventionPreventionMECHANICALNoneLUBRICANTChoose less chemically active additiveRemove corrosive contaminant Remove abrasive

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Electro-CorrosionElectro-CorrosionDefinition:Definition:Dissolution of a metal in an electrically

d i li id b l tconducive liquid by low amperage currents.

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Mechanism:Mechanism:

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Electro-corrosion - How to detect itElectro corrosion How to detect it

Detection:Detection:Unaided Eye Microscopically

F l fHigh wear but a bright mirror finish

Featureless surface except scratches at high magnification by electron microscopyby electron microscopy

Local corroded areasCorrosion pits, films, dissolution of metalsof metals

Black spots such as made by a small drop of acid

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p

Corroded, worn metering edges

Example:Example:

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Electro-corrosion – Solutions:Electro corrosion Solutions:

PreventionPrevention MECHANICALDecrease liquid velocity and velocity gradientsDecrease liquid velocity and velocity gradientsUse corrosion-resistant metalsEliminate stray currentsyUse nongalvanic couples LUBRICANTDecrease or increase electrical conductivity of lubricants or hydraulic fluidsHighly compounded oils can act as electrolytes and beHighly compounded oils can act as electrolytes, and be conductive. Phosphate ester hydraulic fluids are conducive

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Electrical DischargeElectrical DischargeElectrical Discharge:Electrical Discharge:Removal of metal by high amperage electrical discharge or spark between two surfacesg p

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Electrical Discharge - How to detect itElectrical Discharge How to detect it

Unaided Eye MicroscopicallyUnaided Eye Microscopically

Metal surface appears Pits, near edge of damage,Metal surface appears etched. In thrust bearings, sparks make tracks like an

Pits, near edge of damage, showing once molten state, such as smooth bottoms,sparks make tracks like an

electrical engraversuch as smooth bottoms, rounded particles, gas holes

Rounded particles welded to surface near pits

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Example:Example:

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Electrical Discharge - SolutionsElectrical Discharge Solutions

PreventionPrevention MECHANICALImprove electrical insulation of bearingsDegauss magnetic rotating parts Install brushes on shaft Improve machine groundingLUBRICANTUse of oil of higher electrical

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