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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
How do thermal treatments work
temperature transition transformation heat and cool changes microstrcuture - hardness/ductility maternsitic transformation through heating and cooling
26
What does thermal treatments result in
``` inreased hardness compressive stresses induced improve fatigue life improved impact resistance improve wear resistance due to increased hardness ```
27
What are the pros and cons of thermal treatments
``` 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 ```
28
What are localised thermal treatments often used with
gear teeth, cam/crank shafts, bearings. | Gears to 700Hv depth of 0.8mm (tip) to 0.6mm (root) with laser hardening due to accuracy
29
Depth for flame hardening/induction hardening/laser hardening
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)
30
What properties can be altered by thermal treatments
increase hardness, reduce ductility, increase toughness (resistance to impact wear), link to fatigue wear as well
31
What are the main benefits of thermal treatments
cheap as well understood, small dimension changes and importantly increases the hardness
32
What are the main limitations of thermal treatments
difficult with thin geom as might get distortion
33
What can happen if you apply a thermal treatment twice in the same place
can make the part very brittle - have to be careful
34
What are the advantages of laser hardending
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
What issues arise with laser hardening
go over twice gets very brittle (start finish point of laser path bit that isnt treated or bit thats double treated)
36
Is all thermal treatments heating
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
What is cyrotreatment great for
impact wear - less plastic defomration and brittle fracture, little wear debris (as less brittle fracture) As you get the hardness without the brittleness
38
What is shot peening
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
What do you require in shot peening
line of sight
40
What are thermo chemical treatments
Thermal treatments limited by base material, by altering chemical composition can improve
41
How are thermo chemical treatments done
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
What are you limited by in thermo chemical treatments
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
How do thermos chemical treatments differ from thermal treatments
Diffusing chemicals onto surface, thermal is just heat
44
What are the advantages of thermo chemical treatments
Can diffuse different chemical, different surface properties
45
What are surface coatings bad for
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
What are zinc surface coatings good for
corrosion resistance
47
Why do you have a gradual transition from your base material to coating with coatings like zing
To help adhere and prevent oxidation
48
What is it important to have a good base surface before applying a surface coating
imperfection in substrate due to crack transfer to coating
49
What are diamond like carbon surface (DLC) coatings good for
really low friction
50
Why should you aim for similar hardness values between coating and substrate
as it offers better protection
51
What are the 3 main functions of surface coatings
reduce friction (easy shear layer), reduce wear, prevent corrosion
52
What are the 2 main benefits of surface coatings over surface treatments
surface treatment limited by base material | surface coating can use any material as long as not very different hardness
53
In which wear mechanism would a surface treatment be better than a surface coating
impact
54
What are of a coating would you generally not want to design the sub surfaces stresses to act
at the boundary
55
What is weld cladding
used a remedial surface coating treatment component cladded in weld material used on worn rail section and in marine
56
What is weld cladding good for
erosion
57
What is thermal spray
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
What does porosity on thermal spray help on
abradability - can get abraded away easily, ie on gas turbines/jet engines lining can get damaged not the blades
59
What type of wear mechanism is thermal spray good for
erosion or abrasion
60
What is the main issue with thermal spraying
cant control the porosity
61
What does PVD and CVD stand for
Physical and chemical vapour deposition
62
How do PVD and CVD work
Rectants supplied as component as a gas Condenses and reacts on component surface PVD - evaporation CVD - Gas reactant (higher temperatures)
63
Whats the issue with PVD and CVD process
can be slow - CVD is plasma assisted and PVD is ion assisted
64
What are the adv/disadv of PVD and CVD
``` thin, hard and corrosion resistant low surface enrgy low friction BUT high cost and difficult to adhere to substrate ```
65
How do solid lubricants work
Low shear layers certain types of materials strong in plane bonding, weak inbetween layers allows planes to slide over each other very easily
66
Draw a diagram of solid lubricant structure
see powerpoint
67
When would you use solid lubricants
when liquid lubricants arent suitable (inhospitable environment/inaccessable) - space - unreactive sufaces - high temperatures - extreme pressure - Reciprocating or soliding motion to reduce wear
68
What are the common properties of fretting surface treatment/coatings
All form very thin distinct layers on original surface that are hard
69
What are the common properties of contact fatigue surface treatment/coatings
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
What are the common properties of adhesive surface treatment/coatings
Alter the chemical composition of the surface, reduce similarity of the contact surfaces to reduce risk of chemical bonding
71
What are the common properties of impact surface treatment/coatings
Harden the surface, relatively thick, or change chemical composition to make harder, really thin coatings are terrible for this
72
What are the common properties of low stress abrasion surface treatment/coatings
hard coatings that may be harder than counterface, coatings suitable as unlikely to delaminate under low stress situations
73
What are the common properties of high stress abrasion surface treatment/coatings
thicker coatings or surface transformation to make surfaces harder, less liekly to use coating as high stress could cause delamination
74
What are the common properties of machining wear treatment/coatings
Hard coatings
75
What are the common properties of erosion (high angle impact) surface treatment/coatings
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
What are the common properties of erosion (low angle impact) surface treatment/coatings
Hard coatings!! Adhesion of coating will be important as particles may skip across surface, coating should resist abrasion cause by the low angle
77
What is induction hardening
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
Name advantages of induction hardening over other methods
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
What are the limiations of induction hardening
The deeper the less quality, higher risk of cracks forming and distortion due to rapid heating and cooling
80
What are some common applications of induction hardening
axels, screws, gears
81
What are the advantages of thermo chemical diffusion treatments over surface coatings
Do not suffer adhesion issues as coatings would as the change in gradual through the depth of the component
82
Whats the main difference between CVD and PVD
CVD is done a higher temperatures than PVD, CVD bonds are chemical, PVD bonds are physical
83
What are the most common applications and material for PVD
TiN and TiAIN are commonly used on cutting tools, punches and dies

MEC446 (8 decks)

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