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MEC446 > Topic 1 Wear Mechanisms > Flashcards

Topic 1 Wear Mechanisms Flashcards

1
Q

Where do cracks start in rails

A

start at peak shear stress which is below surface and then grow towards the surface

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2
Q

What impact can chemical effects have on wear

A 
High ph (7+) causes significantly increased wear at the same load, lower ph (3) minimises  wear rate but very low ph (1.2) and significant wear with little load
More acidic decreases hardness
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3
Q

What is wear analysis process

A

Examine tribosystem -> characterise wear situation > select analytical wear relationship and databases > evaluate and model wear situation using analytical relationship and databases > Use models to > develop recommendations or select material evaluations and preform evaluations > verify > finish

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4
Q

Where does the wear mechanisms, wear features, contact mechanics come into the wear analysis process

A

When characterising the wear situation

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5
Q

What is wear

A

Wear is progressive damage, involving material loss, which occurs on the surface of a component as a result of its motion relative to the adjacent working parts

Surface damage and material removed from object

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6
Q

Do you need relative motion for wear

A

No, corrosion oxidation require no movement

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7
Q

What is factors influence wear

A

design, chemistry, manufacturing, materials, contact mechanics

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8
Q

What are the wear features that need to be examined

A

location of wear scar
appearence of the wear scar
magnitude of the wear scar

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9
Q

What are the important aspect of wear scar shape

A

shape - does it tie with contact geomtry

uniformity and continiuity - misalignment or load variation

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10
Q

What would a normal wear scar look like for two discs in contact

A

rectangular uniform shape

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11
Q

What would the wear scar look like if there was load variation

A

would be more triangular, thicker at the bottom, thinner at the top

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12
Q

What would the wear scar look like if there was separation and what would cause this

A

two separate ovals

caused by lubrication issues and vibration

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13
Q

What would cause scuffing in a wear scar

A

load variation
lubrication film breaking down
(lubricant film need sufficient speed in contact to support shear force if you dont have will break down

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14
Q

What are the significant wear scar features

A

location, size and shape of the scar
state of lubrication (are there bits in it)
wear situation (rollnig/slide/impact)
sliding marks in rolling and impact situations
scatches and other indications of abrasive wear
corrosion
adhesion and material transfer
type of deformation, brittle or ductile (fracture and cracks; plastic flow)
thermal damage (paricularly with polymers)
flaking or delamination of coatings
gross plastic deformation

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15
Q

What are the three types of wear motion and give examples of where you ight see it

A

sliding - in a slider (piston crankshaft)
rolling - in a rail wheel
Impact - hammer hitting block (big impact, little impact = erosion)
ARE THESE SINGLE OR REPEATED CYCLE MECHANISMS

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16
Q

What happens in sliding

A

everything that roles does also slide a little bit
causes number of wear mechanisms
- oxidative wear in mild contact conditions (low load and sliding velocity)
- adhesive or galling wear in more severe conditions
- fretting wear may occur with reciporacting motion
- if paricles are present in the contact abrasive wear may also occur
- very severe sliding conditions can lead to seizure and high heat generation in contact
- likely to occur in combination with other wear situations such as rolling and impact

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17
Q

Wear features of sliding

A

line in direction of motion
copper surface looks all even - sponge across a window
- ceramic surface very rough lot of variation in surface

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18
Q

What happens in rolling

A
  • FATIGUE mechanisms associated with rolling
  • subsurface cracks form and grow - material lost from surface
  • with traction (rolling/sliding) cracks grow from surface
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19
Q

What happens in impact

A

Deformation - single or repeated cycle
percussive and erosive
normal or compound impact

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20
Q

Whats likely to happen with interaction of wear mechanisms

A
  • Different wear mechanisms often coexist, interact and compete (same time or different time in process)
  • When examining wear scars it is typical to find evidence of different mechanisms
  • could be different wear mechanisms acting independently or they could be features indicating different stages of complex wear process
  • result from variations in contact conditions
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21
Q

Describe some features of abrasive, delaminated wear scar

A

Abrasive got grooves, material scraped out by sharp particles, evidence of being one direction
Delaminated areas, looks really rough, lot of variation in high (clouds almost)

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22
Q

What is running in

A

Process of preconditioning to improve conformity and increase ultimate load carrying capacity
(end goal higher load carrying capacity)
Old cars had to drive first 1000 miles slow
Get some polishing of componenets

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23
Q

What is a typical component history in terms of wear volume vs time or sliding distance

A

initially very high wear volume (during running in phase)
then more gradual wear volume
then at end of life very quick again
not all components bit of everything is possible

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24
Q

What are the qualities of mild wear

A
  • results in extremely smooth surfaces - often smoother than the original
  • debris is extremely small, typically only 100nm diameter
  • high elctical contact resistance, little true metallic contact (due to good contact)
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25
Q

What are the qualities of severe wear

A
  • results in rough, deeply torn surfaces, much rough than the original
  • large metallic wear debris, typically up to 0.01 nm
  • low contact resistance, true metallic junctions formed (as only contact at asperities)
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26
Q

What is adhesion

A
  • Contact between surfaces occurs at discrete points
  • bonding occurs between surface asperities
  • on sliding junctions are broken and new ones formed
  • usually the tip is plucked off the softer asperity leaving them adhering to the harder surface
  • may become loose and give rise to wear debris
    (two faces sit together, get brittle or ductile fracture)
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27
Q

Which way is material transfer likely to take place in adhesion

A

From softer to harder materials, but not always retained

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28
Q

Draw diagram of adhesive wear

A

see powerpoint

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29
Q

What is galling and how might you reduce it

A

Type of adhesion, may ccur in severe contacts (tearing away of macroscopic chunks of materials)
improving lubrication, keep COF low, oxide layer, surface coating reduce speed/load/temperature

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30
Q

What is not always conveniant but useful to do i adhesion to reduce wear

A

reduce speed/load/temperature

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31
Q

What does lubrication do in the case of adhesion

A

Prevents chemical bonds forming

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32
Q

What does adhesion look like under the microscope

A

areas torn away, very rough, variation in height, sometimes fresh oxidised layer, material transfer to another body

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33
Q

what does severe adhesion look like on the major scale

A

smearing of material

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34
Q

What is scuffing

A
  • Onset of adhesive wear, linked to inadequate lubrication and local temperature increases
  • local welding
  • high loads and speeds
  • high friction in sliding contacts due to breakdown in lubrication film
  • can lead to seizure
  • occurs in the “runningin” period and when lubricatn degrades
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35
Q

What does scuffing look like

A

Rough dirt on the surface almost (see powerpoint)

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36
Q

What is galling

A
  • Severe adhesive wear
  • tearing of microscopic chunks
  • occurs in high loading, poor lubrication with metal to metal contacts
  • protective oxide layer removed
  • worse in ductile or similar material
37
Q

How do you reduce galling

A

shot peening ( compressive stresses), increase hardness, reduce load, appropiate surface finish (smoother leads to increased area for bonding, but rougher more sites that interact)

38
Q

Whats the difference between galling and scuffing

A

bigger chunks of material removed in galling, more material transferred

39
Q

What are the types of abrasion

A
  • two surfaces are in relative motion (two body abrasion)

- or particles (three body abrasion) remove material from softer materials

40
Q

How hard do the particles need to be in abrasion

A

20% harder than containing surface

41
Q

Which is faster 3 body to 2 body

A

3 body 10 times slower than 2 body, layers gradually removed, particles roll so less wear

42
Q

Draw a diagram of two body abrasive wear

A

see powerpoint

43
Q

Draw a diagram of three body abrasive wear

A

see powerpoint

44
Q

Draw a diagram of two body abrasive wear, with embedded particles

A

see powerpoint

45
Q

How does hardness effect abrasion

A

harder something is less wear

46
Q

How does roughness impact abrasion

A

Rougher surfaces, exhibit higher abrasive wear

47
Q

How is abrasion characterised

A

Parallel grooves in the direction of motion, material displaced or removed

48
Q

50% of wear in industry is due to

A

abrasion

49
Q

What does severe abrasion lead to

A

gouging

50
Q

How do you reduce abrasion

A

lubrication, surface coating, reduce speed, load, temperature, filtration system

51
Q

Why does temperature impact adhesion

A

high temperature, high chemical reaction rate, higher bonding, higher wear

52
Q

What does the mechanism of impact wear involve

A
  • elastic and plastic deformation (hgih impact energy)
  • fatigue accompanied by wear debris release due to crack formation
  • oxidative mechanism may also take place (low impact energy)
53
Q

Draw a diagram of impact wear mechanism

A

see powerpoint

54
Q

Impact wear is dependent on

A

formation of deformed layers, crack extend parallel to surface causing delamination

55
Q

What can happen to impact wear if you impose sliding

A

accelerate wear

56
Q

How do you reduce impact wear

A

reduce speed of impact, surface engineering (thermo/thermos chemical, weld deposits or thick spray coating), remove sliding aspect

57
Q

What is fatigue wear

A
  • caused by cyclic loading, subsurface plastic strain build up over repeated loading and contact
  • cracks form and propagate to form pitting
  • if cracks propagate and pits meet, larger sections of material may be removed, this is known as spalling
  • once particles spall degradation progagates rapidly
58
Q

What is fatigue linked to

A

delamination, fretting fatigue and impact wear

59
Q

Reduce fatigue by

A

Selecting material with high fracture toughness, use surface treatments, ensure stresses below fatigue limit, planned maintenance with crack detection.

60
Q

Draw a diagram of fatigue

A

see powerpoint

61
Q

What is erosion

A
  • material removal by the impingement of particles

- exploited as a cleaning process (using iron shot or sand)

62
Q

When do problems occur due to erosion

A
  • transport of powders and slurries
  • the impact of dust particles on the blades of turbo machinery
  • operation of fluid bed combusters
63
Q

Reduce erosion by

A

relatively thick surface coatings, reduce risk of contact of erosive particles with screens or filters, reduce velocity, change angle of contacting fluids and or particles

64
Q

What type of particles cause erosion

A

solid or liquid

65
Q

How does erosive wear vary for brittle bodies with impact angle

A

At low impact angle, particles are skimming surface, at 90 degrees have high impact energy which leads to cracking so increased wear

66
Q

How does erosive wear vary for ductile bodies with impact angle

A

At low impact angle very high - high plastic deformation (imagine meteor), at higher impact angle the high toughness can withstand impact

67
Q

Draw diagram of hardness/brittle material erosion vartiaon with impact angle

A

see diagram

68
Q

What is pitting

A
  • small rough bottom, circular potholes pits on surfaces
  • linked to fatigue, fretting, corrosion, spalling, caviation, oxidation
  • common in roller bearings
69
Q

How to reduce pitting wear

A
  • surface coating

- reduce risk of contact of erosive particles with screens or filters

70
Q

Where is pitting common

A

roller bearings

71
Q

How is pitting different to erosion

A

its more localised, concentrated wear

72
Q

What is spalling

A
  • Cracks that propagage and cause large pits on surface
  • Can start at the surface or subsurface
  • can get spalling in severe corrosion
  • Also linked to fatigue, pitting
73
Q

How to reduce spalling

A

selecting materials with high fracture toughness, use surface treatments, ensure stresses below fatigue limit, planned maintenance with crack detection

74
Q

Whats the difference between spalling and pitting

A

spalling is a more extreme version of pitting

75
Q

What is oxidation

A
  • Formation of oxides on the surface of the material
  • wear rates are lower than those seen with mechanical wear processes
  • linked to spalling and most other forms of wear that expose surfaces to oxygen
76
Q

How to reduce oxidation

A

oxidation inhibitors, lubrication, surface treatments

77
Q

Draw a diagram of oxidation

A

see powerpoint

78
Q

What is corrosion

A
  • deterioration of a surface due to a reaction with a second substance such as acidic depost or water
  • corrosion of metallic surfaces is dependent on electro (electron exchange) and phyical (mechanisms) and chemistry, the type of metal, and themodynamics
79
Q

What type of environments can be corrosive

A

All environments! Air and moisutre and chemical

80
Q

What is corrosion linked to

A

pitting, cavitation, fretting, seizing, galling

81
Q

What are the characteristics of corrosion

A

pits, grooves, flaking, uniform patterns

82
Q

How to reduce corrosion

A

changing materials (noble material, inert), surface coating, reduce loading/cycles, remove moisture

83
Q

What are the most common wear mechanisms

A
  • adhesion (scuffing and galling)
  • abrasion
  • impact
  • fatigue
  • erosion
  • pitting
  • spalling
  • oxidation
  • corrosion
84
Q

What does adhesion look like under a microscope

A

Depending on which surface looking at, smooth, chunks removed/added on

85
Q

What does impact wear look like

A

lots of cracks (think like hard cracked sand)

86
Q

Why is it important to know counter suface

A

if similar surfaces likely adhesion

87
Q

If there is a hard surface moving against a soft surface whats the wear mechanism likely to be

A

abrasion

88
Q

What wear mechanisms are you likely to get with a show

A

Wear situation is sliding and impact

Wear mechanism adhesion, abrasion, impact

MEC446 (8 decks)

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