In-Person Wear Assessment Practice

Wear (definition)

Progressive damage involving material loss on a surface due to relative motion with adjacent parts or interaction with the environment

What the A3 information card gives you

The second body / the wear situation (motion and forces) / the wear environment / enough history to build a narrative

Answer style in the wear assessment

Bullet points and single sentences - back every answer with a visible feature or a line on the card

MCQ trap: one answer vs all that apply

Read the instruction - shoe example wanted one answer but turbocharger had two correct (2-body and 3-body abrasion)

Five steps of wear analysis

1 Examine the tribosystem 2 Predict the wear 3 Examine the wear 4 Find the root cause 5 Wear prevention

Examine the tribosystem - what to check

Wear situation (sliding / rolling / impact) and environment: steady or cyclic stress - metal on metal - contaminants - heat changing material properties

Three wear scar features to examine

Location / appearance / magnitude of the wear scar

Even wear scar across the whole contact suggests

Normal wear and NOT misalignment

Wear scar heavier on one edge suggests

Misalignment or edge stress concentration

Patchy uneven wear scar suggests

Load variation or lubricant film breakdown (scuffing)

Running-in

Early wear period that improves conformity and load capacity before steady normal wear then wear-out

Mild wear characteristics

Very smooth surfaces (often smoother than new) / tiny debris about 100 nm / high contact resistance / little metallic contact

Severe wear characteristics

Rough deeply torn surfaces / large metallic debris up to about 0.01 mm / low contact resistance / metallic junctions

More than one wear mechanism present means

Worse wear - mechanisms coexist / interact and can be stages of one process

Wear mechanisms linked to sliding

Oxidative (mild) / adhesive (severe) / abrasive (if particles present) / fretting (small reciprocating motion) / seizure (very severe)

Wear mechanisms linked to rolling

Fatigue - pitting / spalling / crack propagation

Impact - single high energy

Elastic and plastic deformation / ductile extrusion / brittle fracture

Impact - repeated low energy

Fatigue - plastically deformed layers with cracks parallel to surface leading to delamination

Impact - many small particles

Erosive wear

Sliding - large vs small displacement

Large = adhesive and abrasive wear / small (10-150 µm) = fretting

Abrasive wear - appearance

Parallel grooves / scratches / lines in the direction of motion with material cut (removed) or ploughed (displaced)

Two-body abrasion

Harder surface asperities (or embedded particles) cut the softer surface - continuous scratches along the motion

Three-body abrasion

Loose hard particles (debris / dirt / soot / oxide) roll and slide between surfaces - about 10x slower than 2-body

How much harder must an abrasive particle be

Only about 20% harder than the surface it wears

Scoring

Severe abrasive wear - large areas of deep parallel grooves in the sliding direction from little or no lubrication or embedded hard particles

Share of industrial wear due to abrasion

About 50%

Abrasive wear - causes

Dirt or debris in contact / contaminated or inadequate lubricant / much harder counterface / rough surfaces / high load speed or temperature

Abrasive wear - operational or design fixes

Lubrication / filtration / seals to keep dirt out / reduce load speed and temperature

Abrasive wear - material fixes

Hard coatings (low stress) / weld deposits or thermal or thermo-chemical hardening (high stress) / keep hardness of mating surfaces within about 20%

Low-stress abrasion - surface treatment

All hard coatings - low stress so unlikely to delaminate

High-stress abrasion - surface treatment

Weld deposits / thermal and thermo-chemical treatments - thin coatings would delaminate

Adhesive wear - mechanism

Asperity welding (solid-phase welding at junctions) - softer asperity plucked off and stuck to harder surface or becomes debris

Adhesive wear - appearance

Material transfer / smeared metal / torn patches / built-up edge on tools / seizure

Adhesive wear progression

Mild local adhesion -> scuffing -> galling -> seizure

Scuffing

Onset of adhesive wear when the lubricant film breaks down - local welding at high load and speed - common in running-in or with degraded oil

Galling

Severe adhesive wear - macroscopic chunks torn out under high load with poor lubrication and metal-to-metal contact - worse in ductile or similar metals

Adhesive wear - conditions that make it worse

Sliding with boundary or poor lubrication / like materials / high temperature / high load / protective oxide removed

Adhesive wear - fixes

Improve lubrication / keep COF low / oxide layer / surface coating / reduce load and temperature / dissimilar materials

Galling - specific fixes

Shot peening / increase hardness / reduce load / appropriate surface finish / gall-resistant steel

Adhesive wear - surface treatments

Sprayed copper / thermo-chemical treatment / phosphating / sprayed cermets / electroless nickel / TiC (CVD) / TiN (PVD)

Why surface treatments work for adhesion

They change the surface chemistry so the two surfaces are dissimilar and less likely to bond

Adhesion vs abrasion - how to tell

Adhesion = material transferred or torn / abrasion = clean grooves with material removed

Fatigue wear - mechanism

Material removal due to cyclic stress - cracks form and propagate

Rolling contact fatigue - where cracks start

Below the surface in pure rolling (Hertzian stress) / from the surface with traction (rolling + sliding)

Pitting

Small rough-bottomed circular pits - linked to fatigue / fretting / corrosion / cavitation / oxidation

Spalling

Cracks propagate and pits join forming large craters - starts at surface or subsurface - degradation then accelerates

Delamination

Large regions removed from the surface - horizontal cracks (impact) / weakness inside a coating (porosity) / corrosion within the material

Fatigue wear - fixes

Material with high fracture toughness / surface treatments / keep stress below fatigue limit / thicker lubricant film / planned maintenance with crack detection

Contact fatigue - surface treatments

Thermal and thermo-chemical treatment / weld deposits / spray and spray-fused coatings

Fatigue pitting vs corrosion pitting

Fatigue = pits in the rolling or contact track / corrosion = pits also outside contact with rust or discolouration

Fretting wear

Small amplitude oscillation (10-150 µm) between parts meant to be fixed - occluded contact with stick and slip zones

Fretting wear - appearance

Combined adhesive / abrasive / oxidative marks in a fixed joint with trapped oxide debris

Fretting fatigue

Fretting combined with cyclic loading leading to crack initiation

Why fretting gets worse over time

Wear opens clearances so oscillations grow / oxide debris has larger volume than base metal and can cause seizure

Fretting wear - fixes

Reduce load / prevent relative motion (tighter fit or clamping) / adequate lubrication

Fretting - surface treatments

Sprayed copper alloys / anodising / nitrocarburising / plasma sprayed and electrodeposited cermets - thin hard distinct layers

Erosive wear

Material removal by a fluid carrying solid particles (or liquid droplets) - depends on particle size / shape / hardness / velocity / angle

Erosion - ductile vs brittle materials

Ductile wear most at LOW angles (ploughing) / brittle wear most at HIGH angles (cracking)

Erosive wear - appearance

Matt sandblasted surface / worn leading edges / pitting or cracking depending on ductility

Erosive wear - fixes

Screens or filters to remove particles / reduce fluid velocity / change impact angle / hard coatings

Erosion high angle - surface treatments

Weld deposits / plasma sprayed cermets and metals / PVD hard coatings

Erosion low angle - surface treatments

All hard coatings - adhesion of coating important as particles skip across surface

Corrosive wear

Surface deterioration from reaction with a second substance (water / acid) - rubbing removes protective layer exposing fresh metal

Galvanic corrosion

Electrochemical reaction between dissimilar metals in the presence of an electrolyte

Corrosive wear - appearance

Flaky surface / pitting / delamination / grooves / uniform pattern / rust or discolouration

Corrosive wear - fixes

Change materials / surface coating / reduce loading or cycles / environmental changes (dry / clean / seal / ventilate)

Corrosion + cyclic load

Corrosion pits act as stress concentrations - fatigue cracks join pits leading to spalling

Oxidative wear

Formation and wear of oxide films - high speed / low load - lower wear rate than mechanical wear

Oxide layers - good or bad

Protective with lower friction if formed faster than worn / hard oxide debris can cause 3-body abrasion

Oxidative wear - fixes

Oxidation inhibitors / lubrication / surface treatments

Glaze layer (Inconel blade root)

Trapped debris compacted and sintered at high temperature into a protective nickel oxide glaze - less wear than predicted

Impact wear - appearance

Dents / flattening / mushroomed or lipped edges / chipping / flaking

What makes impact wear worse

Superimposed sliding

Impact wear - fixes

Reduce impact speed / remove the sliding aspect / hard surface treatment (not a thin coating)

Impact wear - surface treatments

Weld deposits / thermal and thermo-chemical treatment / sprayed cobalt alloys / sprayed cermets

Surface treatment better than coating for which mechanism

Impact wear - thin very hard coatings crack and delaminate

Lack of lubrication - evidence

Scoring / scuffing / galling / seizure / heat discolouration / card mentions low oil or high temperature or sudden stops

Contaminated lubricant - evidence

Deep scoring / 3-body marks / embedded debris / soot or wear debris in oil / missed oil or filter changes

Misalignment - evidence and rule-out

Wear concentrated on one side or edge - ruled out when wear is even across the whole contact

Corrosion as root cause - rule-out

Rejected if materials were selected for the environment and no corrosion product is visible (turbocharger example)

Important parameters affecting wear

Load / speed / temperature / material / surface roughness / lubrication / working environment / operator

First step in preventing wear

Remove the wear situation (contact / relative motion / contamination / force) - if not possible reduce it

Why fix the root cause before surface engineering

Treating one part alone can move the wear to the counterface and surface engineering adds cost

Operational vs design change

Operational changes (how it is used or maintained) are cheaper than design changes

Material change options (order)

Change material / modify properties (heat treatment) / modify composition (carburising or nitriding) / apply a coating

Simplest effective treatment for well-lubricated steel

Surface hardening or carburising - weld facing for extreme wear - advanced coatings for poorly lubricated cases

Coating drawbacks

Sharp interface with lower bond strength -> delamination / CTE mismatch stresses / thicker coatings delaminate more / cost

Where peak sub-surface stress must NOT sit in a coated part

At the coating-substrate interface (interlayer) - keep it within the coating

Three families of surface engineering

Modify existing material (heat or mechanical treatment) / compositional change (diffusion) / coatings

Flame hardening

Flame heat then quench - 0.25-0.6 mm - widely available but hard to control depth and can warp

Induction hardening

Induction coil heat then quench - 0.3-10 mm - depth set by frequency - good control - part size limited by coil

Laser hardening

Laser heats track and conduction self-quenches - 0.1-1 mm - small regions on large parts - expensive and high skill

Cryogenic treatment

After quench hold at about 80 K in liquid nitrogen then temper - retained austenite to martensite plus fine carbides - harder without losing toughness

Shot peening

Deforms surface causing work hardening and compressive residual stress which resists cracks - not for thin parts

Carburising (case hardening)

Heat in carbon-rich atmosphere so carbon diffuses in then quench - hard case and tough core for low carbon steel

Carbonitriding

Carburising with added nitrogen - easier martensite transformation so oil quench with lower thermal stress

Nitriding

Nitrogen diffused into alloy steels with Al Cr Mo Ti or V forming hard nitrides - hardness plus fatigue and corrosion resistance

Gas vs plasma nitriding

Gas: over 500 C in ammonia / about 4 days for 500 µm / Plasma: about 3x faster at about 350 C

Thermo-chemical treatment - advantage and limitation

No delamination as change is gradual / high temperature can distort and mostly limited to ferrous metals

Hydrogen embrittlement

Risk in gas-based diffusion processes - hydrogen reacts with iron causing micro-cracking