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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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