Lecture 14 Flashcards
friction–even the most polished surfaces are made up of hills and valleys on the atomic level; asperities (aspect ratios height to rel. width ratio); smaller aspect ratio = less friciton
ye
height of asperities of optically polished metal?
10e-7 to 10e-8 m
height of asperities of finely turned metal
5 e-6 m
height of asperities of surface polished with 600 grit carborandum
10e-7 m
height of asperities of articular cartilage
2 to 20 x 10e-8 m
in general, friction between like and unlike materials is LESS than that betweeon similar materials
ye
what is one way to lower friction?
lubrication–separates the asperities
what are the types of lubrication?
- hydrodynamic
- boundary
- elastrohydrodynamic
- squeeze film
- mixed
- hydrostatic
- weeping
describe hydrodynamic lubrication
thickness of lubricant prevents the solid surfaces from touching;
layer thickness = η v/p
where η = viscosity
v = relative velocity of motion
p = applied pressure
describe boundary lubrication
film thickness is too low
η v/p is too low
surface to surface contact occurs–high friction
what is elastohdrodynamic lubrication
between hydrodynamic and boundary lubrication
what is squeeze film lubrication
lubricant which is sufficiently viscous to respond almost elastically to a sudden increase in pressure for a short period of time
–e.g. heel strike
describe mixed lubrication
occurs when lubricant is normally hydronamic in nature
can also coat the surface and provide some boundary lubrication at high pressure
describe hydrostatic lubrication
needed external pressure source to maintain the fluid lubricant film
describe weeping lubrication
- lubricant is taken up in porous, deformable surfaces
- pressure builds up in liquid in the pores by load applied to the surface
what kind of lubrication do we find in normal joints?
boundary lubrication during initiation of motion
elastohydrodynamic during motion
squeeze film during short high load transients
how are artifical prosthetic joints lubricated?
-boundary lubrication
but wear does occur; transfer film from a worn polymer may act as a lubrication, and synovial fluid contains hyalauronic acid which binds to metal
what is synovial fluid?
ultrafiltrate of serum with long chain polysacch. called hyaluronic acid as a major component
synovial fluid is thixotropic
ye
trauma to a synovial joint decreases the percentage of hyaluronic acid and produces low viscosity but still thixotropic fluid
ye
study slide 21
e
what are the types of wear joints can undergo/
adhesive wear–atomic bonding takes place at the interface –>lift softer material and transfers it to a harder one
abrasive wear – asperities of hard surface gouge and plow softer surface
corrosive wear: protective oxide is softer than metal–>wears easily, subject to underlying metal corrosion
fatigue wear: surface undergoes cyclic loading and unloading–>cracks–>flaking
study aslide 9
ye
