TRIBOLOGY AND WEARWEAR AND ITS TYPES

TYPES OF WEAR

• The removal of material from solid surfaces as a result of mechanical action. Wear may

occur in different modes with different failure mechanisms.

• A material may exhibit high resistance to wear in a specific mode but perform poorly in

another mode.

• Major modes of wear:

• Abrasive wear (About half of all wear failures)

• Adhesive wear (~15% of failures)

• Erosion (~ 8% of failures by erosion and fretting)

• Fretting

• Corrosive wear

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ABRASIVE WEAR

a) Phenomena

• Occurs when a rough hard surface, or a soft surface containing hard particles, slides on a

softer surface and plows a series of grooves in it.

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ABRASIVE WEAR

a) Phenomena

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ABRASIVE WEAR

a) Phenomena

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ABRASIVE WEAR

b) Mechanism

• Ductile Materials: abrasion occurs, resulting from plowing and cutting

• Brittle Materials: abrasion occurs, resulting from fracture and delamination

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ABRASIVE WEAR

b) Mechanism

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ABRASIVE WEAR

b) Mechanism

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ABRASIVE WEAR

b) Mechanism

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ABRASIVE WEAR

b) Mechanism

• Quantitative expression for abrasive wear in brittle materials:

• No plastic deformation occurs

• Fracture toughness (Kc) is a major player

• Kc V

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ABRASIVE WEAR

c) Influential factors in abrasive wear

1. Hardness

• H Wear

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ABRASIVE WEAR

c) Influential factors in abrasive wear

2. Size of abrasive particle

• D Wear

• Clogging of small particles

• Higher flow stress in smaller volume

• Higher stress concentration for larger particles

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ABRASIVE WEAR

c) Influential factors in abrasive wear

3. Shape of abrasive particle or asperity

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ABRASIVE WEAR

c) Influential factors in abrasive wear

4. Microstructure

• D >> d: Significant role of microstructure in wear behavior

• D<< d: Wear behavior is mainly affected by the properties of the single crystal.

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ABRASIVE WEAR

c) Influential factors in abrasive wear

5. Lubricant, Moisture content and Environment

• Reducing frictional force

• Penetration of the asperities into the lubricating film

• Cracking by lubricant in high speed wear

• Corrosive environment

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ABRASIVE WEAR

d) Testing

• Measuring weight loss and volume loss

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ADHESIVE WEAR

a) Phenomena

• Adhesive wear occurs during sliding, accompanied with metal transfer From softer one to the

hard one.

• When the shear strength of the softer metal is smaller than the interfacial bonding strength,

adhesive wear occurs.

• Examples:

• Adhesive wear of satellite components (outside space, no air strong adhesion)

• Wear of machine parts and tools due to frictional heating

• Adhesion between head and magnetic medium (tape, diskette, rigid disk)

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ADHESIVE WEAR

a) Phenomena

• Happens due to strong interfacial bonding

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ADHESIVE WEAR

b) Mechanism

• When distance between two surfaces <1nm, short-range force becomes effective, e.g.,

metallic bonds for metal-metal contact and covalent bonds for ceramic-ceramic contact.

• Adhesive wear often obeys Archard’s equation

• Wear coefficient: A function of geometry, material compatibility, E, ….

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ADHESIVE WEAR

c) Influential factors in adhesive wear

1. Compatibility of material pair

• More compatibility, more adhesive wear

• Note: when perform tests, use smooth surfaces and vacuum

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ADHESIVE WEAR

c) Influential factors in adhesive wear

1. Compatibility of material pair

• Elements which are closer in the periodic

table usually have larger adhesive force.

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ADHESIVE WEAR

c) Influential factors in adhesive wear

2. Electron behavior

• Electron work function (EWF): The minimum

energy required to move electrons from inside

a metal to its surface.

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ADHESIVE WEAR

c) Influential factors in adhesive wear

3. Crystal structure

• Greater number of slip systems More adhesion

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ADHESIVE WEAR

c) Influential factors in adhesive wear

3. Crystal structure

• Greater number of slip systems More adhesion

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ADHESIVE WEAR

c) Influential factors in adhesive wear

4. Microstructure

• Larger grain size Less wear

• Morphology of second phases: Second

phases can strengthen materials and

thus decrease adhesive wear. The

morphology and orientation of second

phase affect adhesive wear

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ADHESIVE WEAR

c) Influential factors in adhesive wear

4. Microstructure

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ADHESIVE WEAR

c) Influential factors in adhesive wear

5. Lubricants

• Lubricants reduce adhesion

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ADHESIVE WEAR

d) Testing

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ADHESIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

a) Phenomena

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EROSIVE WEAR

a) Phenomena

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EROSIVE WEAR

b) Mechanism

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EROSIVE WEAR

b) Mechanism

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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EROSIVE WEAR

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CORROSIVE WEAR

a) Phenomena

• Degradation of materials, in which both corrosion and wear mechanisms are involved.

• In situations where the environment surrounding a sliding surface interacts chemically

with it. If the products of reaction are worn off the surface, corrosive wear has occurred.

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CORROSIVE WEAR

b) Mechanism

• Corrosive wear may cause material loss through following processes:

I. Accelerated corrosion + Stress Concentration

• Porous and brittle oxide scale can be easily destroyed causing accelerated

corrosion and stress concentration

II. Preferable interfacial corrosion and cracking

• Preferable interfacial corrosion due to galvanic effect causing removal or

reinforcing phases and stress concentration at the interfaces

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CORROSIVE WEAR

b) Mechanism

III. Grain boundary corrosion and cracking

• Corrosion happens at GB due to defects and impurity and second phase

segregation. This makes the alloys weaker against applied force.

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CORROSIVE WEAR

b) Mechanism

III. Accelerated corrosion under wearing force

• Heavy plastic deformation makes the material more anodic

V. Corrosive wear of corrosion –resistant materials (passive alloys)

• Under corrosion wear attack, stainless steel could be as ineffective as

carbon steel.

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CORROSIVE WEAR

b) Mechanism

• Synergism of wear and corrosion

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CORROSIVE WEAR

b) Mechanism

• Large percentage of total volume loss results from the

synergy of corrosion and wear.

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CORROSIVE WEAR

c) Influential factors in corrosive wear

I. pH level of the corrosive medium

II. Solution

• Wear of steel in sulfuric acid and hydrochloric acid is 14-16 times higher than that in NaOH

and 2-4 times higher than those in water and salt water.

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CORROSIVE WEAR

c) Influential factors in corrosive wear

II. Solution

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CORROSIVE WEAR

c) Influential factors in corrosive wear

III. Corrosion inhibitor

• Inhibitors can promote the formation of protective passive films to reduce the synergy of

corrosion and wear, thus reducing material loss.

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CORROSIVE WEAR

c) Influential factors in corrosive wear

IV. Properties of passive film

• Inhibitors can promote the formation of protective passive films to reduce the synergy of

corrosion and wear, thus reducing material loss.

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CORROSIVE WEAR

c) Influential factors in corrosive wear

V. Microstructure

• Inhibitors can promote the formation of protective passive films to reduce the synergy of

corrosion and wear, thus reducing material loss.

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CORROSIVE WEAR

d) Testing

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CORROSIVE WEAR

d) Testing

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FATIGUE WEAR

a) Phenomena

• Fatigue wear occurs during repeated sliding or rolling over a track, caused by cyclic loads,

involving nucleation and propagation of cracks.

• It is similar to fatigue of bulk materials but much complicated, affected by surface

geometry, adhesion, …

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FATIGUE WEAR

a) Phenomena

• Fretting or Fretting Wear or Fretting Damage:

• Occurs between two closely contacting surfaces having oscillatory relative motion of

extremely small magnitude.

• Arises between surfaces which are intended to be fixed in relation to each other, but which

nevertheless are experiencing a small oscillatory relative movement. (Different from sliding

wear)

• Examples:

• Bolts, rivets, and pins under the heads and in their holes

• Wire rope between the strands

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FATIGUE WEAR

a) Phenomena

• Surface Fatigue Wear or Rolling-Contact Fatigue:

• Surface damage that results by fatigue from repeated rolling, or rolling and sliding contact

between curved metal surfaces.

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FATIGUE WEAR

a) Phenomena

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FATIGUE WEAR

b) Mechanism

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FATIGUE WEAR

b) Mechanism

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FATIGUE WEAR

b) Mechanism

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FATIGUE WEAR

b) Mechanism

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FATIGUE WEAR

b) Mechanism

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FATIGUE WEAR

b) Mechanism

• Propagation of cracks

• For many materials, there is a fatigue limit or σa below which no failure occurs.

• However, such limit is not observed in fatigue wear.

• S-N curves still is useful in evaluating fatigue behavior of materials.

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FATIGUE WEAR

c) Testing

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FATIGUE WEAR

c) Testing

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FATIGUE WEAR

d) Worn surface analysis

• Worn surface, cross-section of worn surface layer and wear debris may be examined to

understand the mechanisms responsible for fatigue wear (of course, you also need to

know the loading condition.)

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