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Boundary Lubrication

Boundary lubrication occurs when solid surfaces are very close together, allowing contact between surface asperities. Friction and wear are determined by interactions between the solids and between the solids and lubricant. Lubricant films are very thin, less than 100nm. Load is supported partly by asperity contacts and partly by pressure from thin lubricant films. Examples include piston rings and cylinders at top and bottom dead centers, and starting and stopping of machinery.

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389 views8 pages

Boundary Lubrication

Boundary lubrication occurs when solid surfaces are very close together, allowing contact between surface asperities. Friction and wear are determined by interactions between the solids and between the solids and lubricant. Lubricant films are very thin, less than 100nm. Load is supported partly by asperity contacts and partly by pressure from thin lubricant films. Examples include piston rings and cylinders at top and bottom dead centers, and starting and stopping of machinery.

Uploaded by

adityanarang147
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© Attribution Non-Commercial (BY-NC)
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Boundary Lubrication
Definition: Campbell [1] "Boundary lubrication is lubrication by a liquid under conditions where the solid surfaces are so close together that appreciable contact between opposing asperities is possible. The friction and wear in boundary lubrication are determined predominantly by interaction between the solids and between the solids and the liquid. The bulk flow properties of the liquid play little or no part in the friction and wear behavior. "

Stribeck Curve

Characteristics
Liquid-solid interactions Contact between asperities ! Increased friction ! Wear Friction & wear determined by properties of solid surfaces as well as properties of lubricant Shear of thin surface films ! Lubricant films < 100 nm thick ! Several to hundreds of molecules thick ! Solid or liquid ! Lubricant starved contact Load sharing ! Asperity-asperity contacts ! Lubricant pressures generated by ! Squeeze films ! Sliding "Oiliness" of surfaces determines friction ! Lubricant film thickness < 100 nm thick

Examples
! Engines: Piston rings & liner @ TDC & BDC ! Biological o Synovial joints, e.g., knee with synovial fluid o Teeth/saliva during chewing ! Oiled gun barrel & bullet ! Start up & shut down of equipment: o Engines o Bearings o Cams ! Lubricated metal cutting or machining

Protective Films
Chemical interactions form (protective) films ! Surface absorption ! Surface reactions Diamond like (protective) films ! Very low (dry) friction ! 0.001 ! Chemically inert, very hard ! Ali Erdemir: "Near-Frictionless Carbon.ultrahard coating many times slicker than Teflon." ! Hydrogenated carbon bonded to surface, unreactive. Design of protective boundary films (hydrocarbons) ! Film absorbs to surface ! Bodies don't touch ! Class of oil additives for bonding to surfaces (surfactants)

GREASES MOLECULAR DYNAMICS OF FILMS FLOW THROUGH ROUGH SURFACES ADHESION OF LUBRICANT GOOD BOUNDARY LUBRICANTS

BOUNDARY LUBRICATED CONTACT MODELS


Rough surfaces contact:

Asperity forces Pi support load @ discrete points Hydrodynamic fluid pressures pfluid support load elsewhere

Total load support

P = "asperities Pi +"#area p fluid dA !


Notes: Real contact area Areal << A, apparent contact area. Thus
$
#""Pi(z)"F(z)"dz N$

+"#A p fluid dA

#(A - Areal) p fluid dA ! #A p fluid dA


Many asperities of many heights, thus

"asperities Pi !

$
#""Pi(z)"F(z)"dz N$

References
1. W.E. Campbell, Boundary Lubrication, Boundary Lubrication, an Appraisal of World Literature, ASME, 1969, pp. 87-117.. 2. N.K Myshkin, Chung Kyun Kim, Mark I Petrokovets, Introduction to Tribology, Cheong Moon Gak, 1997.

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