Tribology is the science of interacting surfaces in relative motion, including friction, lubrication, and wear. It studies how surfaces in contact interact and move over each other. Key aspects include the different types of friction (solid, fluid, mixed), lubrication processes like hydrodynamic and boundary lubrication, and wear modes like abrasive, adhesive, and fatigue wear. Synovial fluid in the joints provides lubrication through processes like boundary lubrication, where lubricants like lubricin form protective monolayers on cartilage surfaces to prevent direct contact and reduce friction and wear.

1. TRIBOLOGY
Dr Sharad shirol

2. What is Tribology?
• Tribos means “to rub” or “rubbing”
• Coined by Dr. H. Peter Jost in his Jost Report which noted a potential savings of
over £515 million per year ($800 million) for industry by better application of
tribological principles and practices, published in 1966

3. Dr. H Peter Jost

4. Definition
• Tribology is the science and engineering of interacting surfaces in relative motion
• It includes the study and application of the principles of friction, lubrication and
wear
• Tribology is a branch of mechanical engineering and materials science

5. • Tribology Down the Centuries
• Early civilisations developed quite sophisticated tribological devices such as potter’s wheels,
door hinges and wheeled carriages. The carvings on the tomb at Saqqara shows an Eygptian
tribologist bending down to lubricate the sled that carries a statue of Ti (c. 2400 BC).

6. • Military engineers rose to prominence in the days of the Roman empire by devising
both war machinery and methods of fortification, using tribological principles. War
ships (c. 50 AD) recovered from Lake Nemi near Rome, contain broze balls and rollers
used to support rotating platforms

7. •It was the renaissance engineer-artist, Leonardo da Vinci (1452-
1519), celebrated engineer, painter, and sculpter, who discovered
that the tangential force of friction between moving solid bodies is
proportional to the normal force. His notebooks show many designs
for
moving parts and
machines that
show a remarkable
similarity to those
in use today

8. Friction
• Friction is the force that hinders or resists the relative motion of the two contacting bodies.
• Friction originates from complex molecular and mechanical interactions between the
contacting surfaces.
• Friction causes wear and generates heat which can lead to premature failure of the
functioning implants.

9. Types of Friction
• Solid Friction
• Fluid Friction
• Mixed Friction
• Internal Friction
Friction between two solids is independent of the materials and dependent upon:
• The size of the contact zone
• Surface roughness, asperities
• Load or pressure on surfaces

10. Cases of Friction
• Friction is also related to the type of motion of the two contacting bodies.
• Rolling friction and sliding friction are two general cases of friction.

11. Friction Coefficient
•Frictional force is proportional to the load, therefore
F(Force) = µ P(Load)
•Friction is commonly represented by the friction coefficient µ . The coefficient of
friction is a unit-less ratio, where “F” represents the frictional force experienced
by the two contacting bodies in motion, and
“P” represents the normal force pressing the same two bodies together.
•The value of the coefficient of friction typically ranges from 0 to 1; the higher the
value, the higher the frictional force or the resistance of the contacting bodies
towards motion. Under boundary lubrication conditions, usually approaches 1.

12. Metal Surfaces
• All metal surfaces, irrespective of their finish, contain ridges, peaks, and valleys,
They stick out of the surface forming peaks and valleys at a microscopic level.
These peaks are called asperities.
• When two metal surfaces come in contact, solid friction, sometimes called static
or adhesive friction, ensues and the surfaces undergo adhesion and cold
welding.
• When surfaces start to move, kinetic friction comes into play. Kinetic friction
results from plowing of the asperities of the one surface across the other surface,

13. Wear
Wear is a process of removal of material from one or both of two solid surfaces in
solid state contact, occurring when these two solid surfaces are in sliding or rolling
motion together. Bhushan and Gupta (1991)
Wear is the progressive damage, involving material loss, which occurs on the surface
of a component as result of its motion relative to the adjacent working parts.
John Williams

14. Types of wear process

15. Abrasive wear
Abrasive wear occurs when a harder materialharder material is rubbing against a softer materialsofter material
Two body wearTwo body wear
Three bodyThree body
wearwear

16. Types of abrasive wear
Gouging abrasion
• LargeLarge particles
• HighHigh compression loads
High stress oror grinding abrasion
• SmallerSmaller particles
• HighHigh compression load
Low stress oror scratching abrasion
• NoNo compression loadload
• Scratching abrasion while material is slidingmaterial is sliding

17. Erosive wear
The impingementimpingement of solid particles, or small drops of liquid or gas on the solid
surface cause wear what is known as erosion of materials and components.

18. Types of erosion
•Solid particle erosion
Surface wear by impingement of solid particles.
• Liquid drop erosion
Surface wear by impingement of liquid drops.liquid drops.
• Cavitation erosion
Surface wear in a flowing liquid by the generationgeneration and implosive collapseimplosive collapse of
gas bubblesgas bubbles.

19. Frictional wear / adhesive wear
Two bodies slidingbodies sliding over or pressedpressed into each other which promote the materialmaterial
transfer from one to another.transfer from one to another.

20. Surface fatigue
•Two surfaces contacting to each other under
pure rolling, or rolling with a small amount of
sliding in contact
Contact fatigue
•As one element rolls many times
over the other element

21. Delamination wear
A wear process where a material loss from the surface by forces of another
surface acting on it in a sliding motionsliding motion in the form of thin sheets.thin sheets.
Mechanisms of delamination wear
•Plastic deformation of the surfacePlastic deformation of the surface
•Cracks are nucleated belownucleated below
the surface
•Crack propagationpropagation from these
nucleated cracks and joiningjoining with
neighbouring one
• After separation from the surface,
laminates form wear debris

22. Wear modes
• Conditions under which the prosthesis was functioning when the wear occurred
• Mode 1 constitutes normal functioning prosthesis
• Mode 2, 3, and 4 constitutes malfunctioning prosthesis
• Mode 1 : Motion of 2 primary bearing surfaces against each other
• Mode 2: Primary bearing surface moving against a secondary surface that was not
intended to come into contact with the first

23. • Mode 3 : Contaminant particles directly abrade one or both of the primary bearing
surfaces
•Third body abrasion or wear
• Mode 4 : 2 secondary surfaces rubbing together

24. Synovial Fluid
• Synovial
• syn(like) + ovia (egg)
• “Joint Fluid”
• Viscous fluid found in the cavities of
movable joints
• Non Newtonian fluid
• Synovial membrane
• Inner membrane of synovial joints
• Secretes synovial fluid into the joint cavity
• Contain specialized cells (synoviocytes)

25. • Hyaluronic acid
• synthesized by the synovial membrane
• increase the viscosity and elasticity of articular cartilages
• lubricate the surface between synovium and cartilage.
• Lubricin secreted by synovial cells.
• It is chiefly responsible for so called boundary layer lubrication, which reduces friction
between opposing surfaces of cartilage.

26. Major Functions
• Reducing friction
• Lubrication
• Lessen shock
• Supplying oxygen and nutrients

27. Lubrication Processes for Articular Cartilage
Boundary LubricationFluid-film Lubrication
Hydrodynamic Lubrication Squeeze-film Lubrication

28. Fluid-film Lubrication
• Thin film of lubricant separates bearing surfaces
• Load on bearing surfaces supported by pressure developed in fluid-film
• Lubrication characteristics determined by lubricant’s properties
• Rheological properties
• Viscosity and elasticity
• Film geometry
• Shape of gap between surfaces
• Speed of relative motion of two surfaces

29. • The relationship of the coefficient of friction and the oil film thickness to
lubricant viscosity Z, equipment speed N, and equipment load, or pressure P,
are graphically presented by the Stribeck curve.
• The ratio of ZN/P is related directly to the oil film thickness but inversely to
the coefficient of friction .
• This implies that high lubricant viscosity Z, high equipment speed N, and low
equipment load P will allow the formation of a thick lubricant film, and hence
the equipment will encounter little or no friction.

30. • Conversely, low lubricant viscosity, low equipment speed, and high equipment
load will create a situation where the film thickness will be inappropriate and
the equipment will encounter high friction,

31. Hydrodynamic Lubrication
• Occurs when 2 nonparallel rigid bearing surfaces lubricated by a fluid-film that
moves tangentially with respect to each other
• Wedge of converging fluid formed
• Lifting pressure generated in wedge by fluid viscosity as the bearing motion drags
fluid into gap

32. Schematic of Hydrodynamic Lubrication

33. Squeeze-film Lubrication
• Occurs when weight bearing surfaces move perpendicularly toward each
other
• Wedge of converging fluid formed
• Pressure in fluid-film result of viscous resistance of fluid that acts to
impede its escape from the gap
• Sufficient to carry high loads for short durations (eventually contact
between asperities in bearing surfaces)

34. Schematic of Squeeze-film Lubrication

35. Weeping Lubrication
• Compression of articular cartilage surfaces result in weeping of fluid
from the cartilage into load supporting regions within joint cavity.
• It is form of hydrostatic lubrication
Boosted lubrication
•Water and smaller electrolytes are forced into the cartilage as a result of pressure
generated from joint surfaces
•As pores of articular surfaces are smaller compared to hyaluronate particles,
they are left behind forming highly viscous synovial fluid concentrate
with enhanced lubrication.

36. Schematic of Squeeze-film Lubrication

37. Boundary Lubrication
• Surfaces of cartilage protected by an adsorbed layer of boundary lubricant
• Direct surface-to-surface contact is prevented
• Most surface wear eliminated
• Lubricin (glycoprotein) synovial fluid constituent responsible for boundary lubricant
• Absorbed as monolayer to each articular surface
• Able to carry loads (normal forces) and reduce friction
• Independent of physical properties of lubricant (e.g., viscosity) and bearing
material (e.g., stiffness)
• Primarily depends on chemical properties of lubricant
• Functions under high loads at low relative velocities, preventing direct
contact between surfaces

38. Schematic of Boundary Lubricant

39. Modes of Mixed Lubrication
1. Combination of fluid-film and boundary lubrication
• Temporal coexistence of fluid-film and boundary lubrication at spatially distinct
locations
• Joint surface load sustained by fluid-film and boundary lubrication
• Most friction in boundary lubricated areas; most load supported by fluid-film

40. Modes of Mixed Lubrication
2. Boosted lubrication
• Shift of fluid-film to boundary lubrication with
time over the same location

41. Elastohydrodynamic Lubrication
• Beneficial increase in surface areas
• Lubricant escapes less rapidly from between the bearing surfaces
• Longer lasting lubricant film generated
• Stress of articulation lower and more sustainable
• Elastohydrodynamic lubrication greatly increases load bearing capacity

42. • The Boundary lubrication appears to be the most important when the joint is
stationary and under conditions of severe loading.
• As movement commences and loading is reduced, there is a transition to a mixture
of Boundary and fluid film lubrication.
• Under these conditions Boundary lubrication occurs between asperities while fluid
film lubrication occurs at other regions.
In this ‘mixed’ lubrication, it is probable that most of the friction is generated in the
boundary lubricated areas while most of the load is carried by the fluid film

43. • As speed increases, a conversion to elastohydrodynamic lubrication occurs.
• During slowing, squeeze film lubrication begins to operate and this
continues until the limb is at rest.
• After a period of immobility, boundary lubrication again takes over.

44. CHARNLEYS LOW FRICTION ARTHROPLASTY
• Small head to reduce wear
• Bone cement for fixation
• Stainless steel head on UHMWPE cup articulation

45. MATERIALS USED IN HIP ARTHROPLASTY
METALS:
• Cobalt, chromium and molybdenum alloys
• Alloys are biphasic materials with primary cobalt alloy matrix phase and
secondary metal carbide phase
• Chromium- improves mechanical properties and promote formation of a
passive oxide layer
• Molybdenum- corrosion resistance
• Carbide- increases material hardness
• Lost wax process: wax mould – silica slurry

46. POLYMERS
• Commonly used are polytetrafluoroethylene(PTFE), polyacetal, high density
polyethylene(HDPE), polysters, ultra high molecular weight
polyethylene(UHMWPE) and carbon fibre reinforced polyethylene(CFRP)

47. ultra high molecular weight
polyethylene(UHMWPE)
• Good toughness, durability and biologically inert
• [C2H4] n, where n is number of monomers
• Usually molecular weight is 2- 6 million units with n- 100-200 thousands
• Crystalline and amorphous phase
• Osteolysis – wear- immune response- osteoclasts-periprosthetic bone-
loosening
• Oxidation- gamma irradiation- free radical generation- oxygen-
hydroperoxides-free radicals- post irradiation aging.

48. Cross – linked polyethylene
• Gamma irradiation – hydrogen atoms removed from polyethylene chain- free
radicals
• Free radicals combine with neighbouring chains- cross link.
• Increases wear resistance and stiffness of material
• post irradiation aging can occur
• Prevented by removing excess free radicals by annealing or remelting
• Vit E incorporation.

49. Benefits of cross linked polyethylene
• -High wear resistance
• -No toxicity
• -Relatively low cost
• -Multiple liner options (elevated rim etc)

50. Risks of cross-linked polyethylene
• -Reduction in other material properties -gross material failure
• -Increased bioactivity of wear particles

51. CERAMICS
• Smaller grains:
• Low surface roughness
• Reduced friction
• High hardness
• Low wear rate
• High wettabilty
• Fluid film lubrication
• Alumina and oxidized zirconium

52. Sterilization
• Ethylene oxide
• Gas plasma
• Gamma radiation in air (2.5 to 4 mrad)
• Gamma radiation in inert atmosphere (nitrogen, argon or vacuum)

53. Benefits of Metal on metal
• Very high wear resistance
• Favors larger diameters (lowers wear)
• Long in vivo experience

54. Risks of Metal on metal
• Increased ion levels
• Delayed type hypersensitivity
• Metal sensitivity
• Organ toxicity
• Acute lymphocytic vasculitis
associated lesions (ALVAL)
• Pseudo tumour
• Carcinogenesis

55. Benefits of ceramic on ceramic
• Highest wear resistance
• No toxicity
• Long in vivo experience

56. Risks of ceramic on ceramic
• Position sensitivity
• Stripe wear
• Liner chipping
• Liner canting
• Edge loading
• Fracture risk
• Clicking and squeaking

57. Material
properties
CoC MoM MoUHMWPE
HardnessMPa 2300 350 Low
# reported no +remelted

58. CoC MoM MoUHMWPE
Wear 1 25 100
Particle size <0.02and >0.2 0.05 0.4
Metal ion Not increased increased Not increased

59. CoC MoM MoHCLUHWPE
Cell toxicity No Yes No
Local tissue
reaction
Low Low Low
Hypersensitivity NR R NR
Carcin. NR * NR

60. Coc MoM MoHCLUHMWPE
Squeaking + + -
Clicking + + -
Seizing - + -

62. THANK YOU