Topic 2 Surface Engineering Flashcards
Why do we do surface engineering
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
What links into surface effects
wear/corrosion/fatigue/friction forces
surface treatments/surface coatings/solid lubricant films
Examples of surface coatings
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)
What are surface treatments
surface modified physically or chemically
What are thermal surface treatments (simple)
change temperature usually hardening
induction, flame, laser hardening
What are mechanical treatments (simple)
Hit things itll get harder
Shot peening, cold working
What are thermo chemical diffusion surface treaments
Implant chemical into surface of metallic component
carburizing, nitriding, carbon-nitriding
What are the 4 main types of surface treatments
thermal treatment
mechanical treatment
thermo chemical diffusion
ion implanation
What are surface coatings
could be wear resistant
could be there to provide easy shear - solid lubricant
How might you apply a surface coating
electro plating weld cladding thermal spraying chemical vapour deposition phyiscal vapour deposition
Whats the advantage of surface coatings over treatments
can put on any material, not all parts can heat treat due to geom/material
How might you optimise a surface coating
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
What points must you consider when selecting a coating
- 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
What surface treatments might you select for fretting
Spraed copper alloys, anodising, nitrcarburising, plasma sprayed and electrodeposited cermets
What surface treatments might you select for contact fatigue
thermal and themro-chemical treatment, weld deposits, spray and spray fused coatings
What surface treatments might you select for adhesive wear
Sprayed copper, thermo chemical treatment, phosphating, sprayed cermets, electroless nickel, tic (CVD) or tin (PVD)
What surface treatments might you select for impact wear
Weld deposits, thermal and thermo-chemical treatment, sprayed cobalt alloys, sprayed cermets
What surface treatments might you select for low stress abrasion
All hard coatings
What surface treatments might you select for high stress abrasion
Weld deposits, thermal and thermo-chemical treatments
What surface treatments might you select for machining wear
All hard coatings
What surface treatments might you select for erosion - high angle impact
Weld deposits, plasma sprayed cermets and metals, PVD, hard coatings
What surface treatments might you select for erosion - low angle impact
all hard coatings
Summary for selecting coatings
- 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
Why do we need surface treatments/coatings
low speeds lubricant film isnt well formed - surface engineering can solve this problem
How do thermal treatments work
temperature transition transformation
heat and cool changes microstrcuture - hardness/ductility
maternsitic transformation through heating and cooling
What does thermal treatments result in
inreased hardness compressive stresses induced improve fatigue life improved impact resistance improve wear resistance due to increased hardness
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
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
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)
What properties can be altered by thermal treatments
increase hardness, reduce ductility, increase toughness (resistance to impact wear), link to fatigue wear as well
What are the main benefits of thermal treatments
cheap as well understood, small dimension changes and importantly increases the hardness
What are the main limitations of thermal treatments
difficult with thin geom as might get distortion
What can happen if you apply a thermal treatment twice in the same place
can make the part very brittle - have to be careful
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
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)
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
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
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
What do you require in shot peening
line of sight
What are thermo chemical treatments
Thermal treatments limited by base material, by altering chemical composition can improve
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
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
How do thermos chemical treatments differ from thermal treatments
Diffusing chemicals onto surface, thermal is just heat
What are the advantages of thermo chemical treatments
Can diffuse different chemical, different surface properties
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
What are zinc surface coatings good for
corrosion resistance
Why do you have a gradual transition from your base material to coating with coatings like zing
To help adhere and prevent oxidation
What is it important to have a good base surface before applying a surface coating
imperfection in substrate due to crack transfer to coating
What are diamond like carbon surface (DLC) coatings good for
really low friction
Why should you aim for similar hardness values between coating and substrate
as it offers better protection
What are the 3 main functions of surface coatings
reduce friction (easy shear layer), reduce wear, prevent corrosion
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
In which wear mechanism would a surface treatment be better than a surface coating
impact
What are of a coating would you generally not want to design the sub surfaces stresses to act
at the boundary
What is weld cladding
used a remedial surface coating treatment
component cladded in weld material
used on worn rail section and in marine
What is weld cladding good for
erosion
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)
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
What type of wear mechanism is thermal spray good for
erosion or abrasion
What is the main issue with thermal spraying
cant control the porosity
What does PVD and CVD stand for
Physical and chemical vapour deposition
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)
Whats the issue with PVD and CVD process
can be slow - CVD is plasma assisted and PVD is ion assisted
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
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
Draw a diagram of solid lubricant structure
see powerpoint
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
What are the common properties of fretting surface treatment/coatings
All form very thin distinct layers on original surface that are hard
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
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
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
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
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
What are the common properties of machining wear treatment/coatings
Hard coatings
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
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
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
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
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
What are some common applications of induction hardening
axels, screws, gears
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
Whats the main difference between CVD and PVD
CVD is done a higher temperatures than PVD, CVD bonds are chemical, PVD bonds are physical
What are the most common applications and material for PVD
TiN and TiAIN are commonly used on cutting tools, punches and dies
