Topic 1 Wear Mechanisms Flashcards
Where do cracks start in rails
start at peak shear stress which is below surface and then grow towards the surface
What impact can chemical effects have on wear
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
What is wear analysis process
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
Where does the wear mechanisms, wear features, contact mechanics come into the wear analysis process
When characterising the wear situation
What is wear
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
Do you need relative motion for wear
No, corrosion oxidation require no movement
What is factors influence wear
design, chemistry, manufacturing, materials, contact mechanics
What are the wear features that need to be examined
location of wear scar
appearence of the wear scar
magnitude of the wear scar
What are the important aspect of wear scar shape
shape - does it tie with contact geomtry
uniformity and continiuity - misalignment or load variation
What would a normal wear scar look like for two discs in contact
rectangular uniform shape
What would the wear scar look like if there was load variation
would be more triangular, thicker at the bottom, thinner at the top
What would the wear scar look like if there was separation and what would cause this
two separate ovals
caused by lubrication issues and vibration
What would cause scuffing in a wear scar
load variation
lubrication film breaking down
(lubricant film need sufficient speed in contact to support shear force if you dont have will break down
What are the significant wear scar features
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
What are the three types of wear motion and give examples of where you ight see it
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
What happens in sliding
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
Wear features of sliding
line in direction of motion
copper surface looks all even - sponge across a window
- ceramic surface very rough lot of variation in surface
What happens in rolling
- FATIGUE mechanisms associated with rolling
- subsurface cracks form and grow - material lost from surface
- with traction (rolling/sliding) cracks grow from surface
What happens in impact
Deformation - single or repeated cycle
percussive and erosive
normal or compound impact
Whats likely to happen with interaction of wear mechanisms
- 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
Describe some features of abrasive, delaminated wear scar
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)
What is running in
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
What is a typical component history in terms of wear volume vs time or sliding distance
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
What are the qualities of mild wear
- 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)
What are the qualities of severe wear
- 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)
What is adhesion
- 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)
Which way is material transfer likely to take place in adhesion
From softer to harder materials, but not always retained
Draw diagram of adhesive wear
see powerpoint
What is galling and how might you reduce it
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
What is not always conveniant but useful to do i adhesion to reduce wear
reduce speed/load/temperature
What does lubrication do in the case of adhesion
Prevents chemical bonds forming
What does adhesion look like under the microscope
areas torn away, very rough, variation in height, sometimes fresh oxidised layer, material transfer to another body
what does severe adhesion look like on the major scale
smearing of material
What is scuffing
- 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
What does scuffing look like
Rough dirt on the surface almost (see powerpoint)
What is galling
- 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
How do you reduce galling
shot peening ( compressive stresses), increase hardness, reduce load, appropiate surface finish (smoother leads to increased area for bonding, but rougher more sites that interact)
Whats the difference between galling and scuffing
bigger chunks of material removed in galling, more material transferred
What are the types of abrasion
- two surfaces are in relative motion (two body abrasion)
- or particles (three body abrasion) remove material from softer materials
How hard do the particles need to be in abrasion
20% harder than containing surface
Which is faster 3 body to 2 body
3 body 10 times slower than 2 body, layers gradually removed, particles roll so less wear
Draw a diagram of two body abrasive wear
see powerpoint
Draw a diagram of three body abrasive wear
see powerpoint
Draw a diagram of two body abrasive wear, with embedded particles
see powerpoint
How does hardness effect abrasion
harder something is less wear
How does roughness impact abrasion
Rougher surfaces, exhibit higher abrasive wear
How is abrasion characterised
Parallel grooves in the direction of motion, material displaced or removed
50% of wear in industry is due to
abrasion
What does severe abrasion lead to
gouging
How do you reduce abrasion
lubrication, surface coating, reduce speed, load, temperature, filtration system
Why does temperature impact adhesion
high temperature, high chemical reaction rate, higher bonding, higher wear
What does the mechanism of impact wear involve
- 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)
Draw a diagram of impact wear mechanism
see powerpoint
Impact wear is dependent on
formation of deformed layers, crack extend parallel to surface causing delamination
What can happen to impact wear if you impose sliding
accelerate wear
How do you reduce impact wear
reduce speed of impact, surface engineering (thermo/thermos chemical, weld deposits or thick spray coating), remove sliding aspect
What is fatigue wear
- 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
What is fatigue linked to
delamination, fretting fatigue and impact wear
Reduce fatigue by
Selecting material with high fracture toughness, use surface treatments, ensure stresses below fatigue limit, planned maintenance with crack detection.
Draw a diagram of fatigue
see powerpoint
What is erosion
- material removal by the impingement of particles
- exploited as a cleaning process (using iron shot or sand)
When do problems occur due to erosion
- transport of powders and slurries
- the impact of dust particles on the blades of turbo machinery
- operation of fluid bed combusters
Reduce erosion by
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
What type of particles cause erosion
solid or liquid
How does erosive wear vary for brittle bodies with impact angle
At low impact angle, particles are skimming surface, at 90 degrees have high impact energy which leads to cracking so increased wear
How does erosive wear vary for ductile bodies with impact angle
At low impact angle very high - high plastic deformation (imagine meteor), at higher impact angle the high toughness can withstand impact
Draw diagram of hardness/brittle material erosion vartiaon with impact angle
see diagram
What is pitting
- small rough bottom, circular potholes pits on surfaces
- linked to fatigue, fretting, corrosion, spalling, caviation, oxidation
- common in roller bearings
How to reduce pitting wear
- surface coating
- reduce risk of contact of erosive particles with screens or filters
Where is pitting common
roller bearings
How is pitting different to erosion
its more localised, concentrated wear
What is spalling
- 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
How to reduce spalling
selecting materials with high fracture toughness, use surface treatments, ensure stresses below fatigue limit, planned maintenance with crack detection
Whats the difference between spalling and pitting
spalling is a more extreme version of pitting
What is oxidation
- 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
How to reduce oxidation
oxidation inhibitors, lubrication, surface treatments
Draw a diagram of oxidation
see powerpoint
What is corrosion
- 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
What type of environments can be corrosive
All environments! Air and moisutre and chemical
What is corrosion linked to
pitting, cavitation, fretting, seizing, galling
What are the characteristics of corrosion
pits, grooves, flaking, uniform patterns
How to reduce corrosion
changing materials (noble material, inert), surface coating, reduce loading/cycles, remove moisture
What are the most common wear mechanisms
- adhesion (scuffing and galling)
- abrasion
- impact
- fatigue
- erosion
- pitting
- spalling
- oxidation
- corrosion
What does adhesion look like under a microscope
Depending on which surface looking at, smooth, chunks removed/added on
What does impact wear look like
lots of cracks (think like hard cracked sand)
Why is it important to know counter suface
if similar surfaces likely adhesion
If there is a hard surface moving against a soft surface whats the wear mechanism likely to be
abrasion
What wear mechanisms are you likely to get with a show
Wear situation is sliding and impact
Wear mechanism adhesion, abrasion, impact
