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MEC446 > Topic 2 Surface Engineering > Flashcards

Topic 2 Surface Engineering Flashcards

1
Q

Why do we do surface engineering

A

95% of parts fail due to surface related effects, 70% of equipment failure blamed on lubrication breakdown and wear loss, frictional losses in automtove engine = 28% of fuel consumption
By altering surface using treament/coating we can reduce change of failure
Historically have to optimise bulk of material + lubricants could only take you so far
Advanced coatings and surface texture will reduce losses by upto 61% in the next 25 years

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2
Q

What links into surface effects

A

wear/corrosion/fatigue/friction forces

surface treatments/surface coatings/solid lubricant films

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3
Q

Examples of surface coatings

A

Pan low friction teflon
Tap - appearence and corrosion resistance (plated)
Gears - hardened, wear, contact fatigue resistant
Bearing - Solid lubricant(where can’t use lubricant use solid)

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4
Q

What are surface treatments

A

surface modified physically or chemically

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5
Q

What are thermal surface treatments (simple)

A

change temperature usually hardening

induction, flame, laser hardening

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6
Q

What are mechanical treatments (simple)

A

Hit things itll get harder

Shot peening, cold working

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7
Q

What are thermo chemical diffusion surface treaments

A

Implant chemical into surface of metallic component

carburizing, nitriding, carbon-nitriding

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8
Q

What are the 4 main types of surface treatments

A

thermal treatment
mechanical treatment
thermo chemical diffusion
ion implanation

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9
Q

What are surface coatings

A

could be wear resistant

could be there to provide easy shear - solid lubricant

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10
Q

How might you apply a surface coating

A 
electro plating
weld cladding
thermal spraying
chemical vapour deposition
phyiscal vapour deposition
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11
Q

Whats the advantage of surface coatings over treatments

A

can put on any material, not all parts can heat treat due to geom/material

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12
Q

How might you optimise a surface coating

A

Look at surface tensions, look to optimise surface tension to reduce sticking or liquids spreading over them, for use with lubrication might want opposite to allow lubricatn to spread

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13
Q

What points must you consider when selecting a coating

A
  • composition of base material (some work better than others, same for surface treatment)
  • Heat treatment history of base material (will impact how coating adheres)
  • whats possble coating or treatment
  • Which areas of the part are to be treated/masked off
  • Required thickness or case depth (stress distribution in contact)
  • Required hardness or other mechnical property
  • Tolerance on the final dimensions (thickness varies massively may impact tolerances)
  • Any pre or post coat treatment or finishing
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14
Q

What surface treatments might you select for fretting

A

Spraed copper alloys, anodising, nitrcarburising, plasma sprayed and electrodeposited cermets

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15
Q

What surface treatments might you select for contact fatigue

A

thermal and themro-chemical treatment, weld deposits, spray and spray fused coatings

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16
Q

What surface treatments might you select for adhesive wear

A

Sprayed copper, thermo chemical treatment, phosphating, sprayed cermets, electroless nickel, tic (CVD) or tin (PVD)

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17
Q

What surface treatments might you select for impact wear

A

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

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18
Q

What surface treatments might you select for low stress abrasion

A

All hard coatings

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19
Q

What surface treatments might you select for high stress abrasion

A

Weld deposits, thermal and thermo-chemical treatments

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20
Q

What surface treatments might you select for machining wear

A

All hard coatings

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21
Q

What surface treatments might you select for erosion - high angle impact

A

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

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22
Q

What surface treatments might you select for erosion - low angle impact

A

all hard coatings

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23
Q

Summary for selecting coatings

A
  • simpler ther better
  • well lubricated steel component?
  • surface hardening or carburising is sufficient (well understood)
  • fatigue life will be extended
  • in the case of extreme wear - weld facing
  • Advanced coatings - best suited to low lubricated cases
  • cost is always a factor
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24
Q

Why do we need surface treatments/coatings

A

low speeds lubricant film isnt well formed - surface engineering can solve this problem

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25
Q

How do thermal treatments work

A

temperature transition transformation
heat and cool changes microstrcuture - hardness/ductility
maternsitic transformation through heating and cooling

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26
Q

What does thermal treatments result in

A 
inreased hardness
compressive stresses induced
improve fatigue life
improved impact resistance
improve wear resistance
due to increased hardness
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27
Q

What are the pros and cons of thermal treatments

A 
good mircostructural control
localised treatments possible
small dimensional changes
roughness changes negligble
induced compressive stresses
improved fatigue life
simple and well understood
BUT
difficult with complex geometries
oxide layers
distortion possible
Need to remachine after treatment
whole component treated so difficult to tailor
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28
Q

What are localised thermal treatments often used with

A

gear teeth, cam/crank shafts, bearings.

Gears to 700Hv depth of 0.8mm (tip) to 0.6mm (root) with laser hardening due to accuracy

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29
Q

Depth for flame hardening/induction hardening/laser hardening

A

flame - 0.25 - 0.6mm deep (rough not as easily controlled microstructure)
Induction hardening 0.3-10mm deep (good surface finish)
Laser hardening 0.01-1mm deep (difficult to get right)

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30
Q

What properties can be altered by thermal treatments

A

increase hardness, reduce ductility, increase toughness (resistance to impact wear), link to fatigue wear as well

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31
Q

What are the main benefits of thermal treatments

A

cheap as well understood, small dimension changes and importantly increases the hardness

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32
Q

What are the main limitations of thermal treatments

A

difficult with thin geom as might get distortion

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33
Q

What can happen if you apply a thermal treatment twice in the same place

A

can make the part very brittle - have to be careful

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34
Q

What are the advantages of laser hardending

A

better surface finish
good for complex geom if you can get the laser in
importantly its very precise and have a lot of control
Gives smaller micro structure so less prone to cracking

35
Q

What issues arise with laser hardening

A

go over twice gets very brittle (start finish point of laser path bit that isnt treated or bit thats double treated)

36
Q

Is all thermal treatments heating

A

No cyrotreatments - steadily cooling material to 80K and soak for 72 hours
control return to room temp and temper to relieve stresses
increased hardness without the brittleness due to increased carbides in surface

37
Q

What is cyrotreatment great for

A

impact wear - less plastic defomration and brittle fracture, little wear debris (as less brittle fracture)
As you get the hardness without the brittleness

38
Q

What is shot peening

A

mechanical treatment
relies on plastic deformation
induces compressive residual stress
fatigue mechanisms
surface effects have a knock on effect sub surface
used in crankshafts con rods
might not be suitable for thin componenets

39
Q

What do you require in shot peening

A

line of sight

40
Q

What are thermo chemical treatments

A

Thermal treatments limited by base material, by altering chemical composition can improve

41
Q

How are thermo chemical treatments done

A

Heat in chamber with gas, gas diffuses into surface
diffusing small atoms into the surface (carburizing, carbonnitriding)
Chemical reaction at surface to form next compounds (nitriding, nitrocarburizing, chromizing)
Gradual change from substrate structure to surface

42
Q

What are you limited by in thermo chemical treatments

A

what material will allow to diffuse onto it
Expensive high energy
high temps can cause distortion
might require post machining
generally limited to ferros materials, but are some nonferros can be done with

43
Q

How do thermos chemical treatments differ from thermal treatments

A

Diffusing chemicals onto surface, thermal is just heat

44
Q

What are the advantages of thermo chemical treatments

A

Can diffuse different chemical, different surface properties

45
Q

What are surface coatings bad for

A

thin hard coatings terrible for impact wear, break off and damage component
imagine ice on lake, might not see any surface damage but sub suface big impact

46
Q

What are zinc surface coatings good for

A

corrosion resistance

47
Q

Why do you have a gradual transition from your base material to coating with coatings like zing

A

To help adhere and prevent oxidation

48
Q

What is it important to have a good base surface before applying a surface coating

A

imperfection in substrate due to crack transfer to coating

49
Q

What are diamond like carbon surface (DLC) coatings good for

A

really low friction

50
Q

Why should you aim for similar hardness values between coating and substrate

A

as it offers better protection

51
Q

What are the 3 main functions of surface coatings

A

reduce friction (easy shear layer), reduce wear, prevent corrosion

52
Q

What are the 2 main benefits of surface coatings over surface treatments

A

surface treatment limited by base material

surface coating can use any material as long as not very different hardness

53
Q

In which wear mechanism would a surface treatment be better than a surface coating

A

impact

54
Q

What are of a coating would you generally not want to design the sub surfaces stresses to act

A

at the boundary

55
Q

What is weld cladding

A

used a remedial surface coating treatment
component cladded in weld material
used on worn rail section and in marine

56
Q

What is weld cladding good for

A

erosion

57
Q

What is thermal spray

A

Moltern droplets of liquid fired onto the substrate
Particles rapidly cool on impact - little component heating
Metals or ceramics
Porosity is an issue (difficult to get consistent density)

58
Q

What does porosity on thermal spray help on

A

abradability - can get abraded away easily, ie on gas turbines/jet engines lining can get damaged not the blades

59
Q

What type of wear mechanism is thermal spray good for

A

erosion or abrasion

60
Q

What is the main issue with thermal spraying

A

cant control the porosity

61
Q

What does PVD and CVD stand for

A

Physical and chemical vapour deposition

62
Q

How do PVD and CVD work

A

Rectants supplied as component as a gas
Condenses and reacts on component surface
PVD - evaporation
CVD - Gas reactant (higher temperatures)

63
Q

Whats the issue with PVD and CVD process

A

can be slow - CVD is plasma assisted and PVD is ion assisted

64
Q

What are the adv/disadv of PVD and CVD

A 
thin, hard and corrosion resistant
low surface enrgy
low friction
BUT 
high cost
and difficult to adhere to substrate
65
Q

How do solid lubricants work

A

Low shear layers
certain types of materials strong in plane bonding, weak inbetween layers
allows planes to slide over each other very easily

66
Q

Draw a diagram of solid lubricant structure

A

see powerpoint

67
Q

When would you use solid lubricants

A

when liquid lubricants arent suitable (inhospitable environment/inaccessable)

  • space
  • unreactive sufaces
  • high temperatures
  • extreme pressure
  • Reciprocating or soliding motion to reduce wear
68
Q

What are the common properties of fretting surface treatment/coatings

A

All form very thin distinct layers on original surface that are hard

69
Q

What are the common properties of contact fatigue surface treatment/coatings

A

Surface treatments that harden surface of entire component or applying a thick coating, relatively thick coatings for weld and spray type, low accuracy compared to other types

70
Q

What are the common properties of adhesive surface treatment/coatings

A

Alter the chemical composition of the surface, reduce similarity of the contact surfaces to reduce risk of chemical bonding

71
Q

What are the common properties of impact surface treatment/coatings

A

Harden the surface, relatively thick, or change chemical composition to make harder, really thin coatings are terrible for this

72
Q

What are the common properties of low stress abrasion surface treatment/coatings

A

hard coatings that may be harder than counterface, coatings suitable as unlikely to delaminate under low stress situations

73
Q

What are the common properties of high stress abrasion surface treatment/coatings

A

thicker coatings or surface transformation to make surfaces harder, less liekly to use coating as high stress could cause delamination

74
Q

What are the common properties of machining wear treatment/coatings

A

Hard coatings

75
Q

What are the common properties of erosion (high angle impact) surface treatment/coatings

A

hard coatings, relatively thick, absorb some particles without damaging main part, PVD can be very hard and tailored to meet the need of application, high angle therefore less risk of delamination

76
Q

What are the common properties of erosion (low angle impact) surface treatment/coatings

A

Hard coatings!! Adhesion of coating will be important as particles may skip across surface, coating should resist abrasion cause by the low angle

77
Q

What is induction hardening

A

components are heated inside a copper coil through providing an alternating current in the coil, causing alternating magnetic field on the steel components and therefore heats it

78
Q

Name advantages of induction hardening over other methods

A

You can control the depth of hardening, induction hardening better if you can put components inside the coil because of the way the electro field round round the component, can do flat surface though
good surface finish
good understanding of the stress field so you can design the stress in the substrate

79
Q

What are the limiations of induction hardening

A

The deeper the less quality, higher risk of cracks forming and distortion due to rapid heating and cooling

80
Q

What are some common applications of induction hardening

A

axels, screws, gears

81
Q

What are the advantages of thermo chemical diffusion treatments over surface coatings

A

Do not suffer adhesion issues as coatings would as the change in gradual through the depth of the component

82
Q

Whats the main difference between CVD and PVD

A

CVD is done a higher temperatures than PVD, CVD bonds are chemical, PVD bonds are physical

83
Q

What are the most common applications and material for PVD

A

TiN and TiAIN are commonly used on cutting tools, punches and dies

MEC446 (8 decks)

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