Introduction to Tribology
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Introduction to Tribology
Tribology is the study of how surfaces in relative motion interact, and how
friction, lubrication, and wear affect them.
•Friction: The resistance to relative motion
•Wear: The loss of material due to relative motion
•Lubrication: The use of a fluid or solid to reduce friction and wear.
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material
elements sliding against each other.
There are several types of friction:
a) Dry friction is a force that opposes the relative lateral motion of two solid surfaces in
contact. Dry friction is subdivided into static friction between non-moving surfaces,
and kinetic friction between moving surfaces.
b) Fluid friction describes the friction between layers of a viscous fluid that are moving
relative to each other.
c) Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid
surfaces.
d) Skin friction is a component of drag, the force resisting the motion of a fluid across the
surface of a body.
e) Internal friction is the force resisting motion between the elements making up a solid
material while it undergoes deformation.
Wear
The process leading to loss of material is known as
"wear”.
Types of wear
a) Adhesive wear
b) Abrasive wear
c) Erosive wear
d) Surface fatigue
e) Fretting wear
Adhesive wear
Adhesive wear are caused by relative motion, "direct
contact" and plastic deformation which create wear
debris and material transfer from one surface to
another.
Example of Adhesive Wear:
• A Shaft rotating in a bushing
• Chalk on board-while writing
Abrasive Wear
Abrasive wear occurs when a hard rough surface
slides across a softer surface. ASTM International
(formerly American Society for Testing and Materials)
defines it as the loss of material due to hard particles or
hard protuberances that are forced against and move
along a solid surface.
Types of Abrasive wear
• Abrasive wear is commonly classified according to the type of contact and
the contact environment
• The two modes of abrasive wear are known as two-body and three-body
abrasive wear
• Two-body wear occurs when the grits or hard particles remove material
from the opposite surface.
• Three-body wear occurs when the particles are not constrained, and are free
to roll and slide down a surface.
Two-body wear Three-body wear
Erosive Wear
• Erosive Wear
Erosive wear can be described as an
extremely short sliding motion and is executed
within a short time interval. Erosive wear is
caused by the impact of particles of solid or
liquid against the surface of an object.
Surface Fatigue
Surface fatigue is a process in which the surface of a material is
weakened by cyclic loading, which is one type of general material
fatigue. Fatigue wear is produced when the wear particles are detached
by cyclic crack growth of micro-cracks on the surface. These
micro-cracks are either superficial cracks or subsurface cracks
Fretting wear
Fretting is the repeated cyclical rubbing between
two surfaces, which is known as fretting, over a
period of time which will remove material from one
or both surfaces in contact
Lubrication
Lubrication is the process or technique employed to
reduce wear of one or both surfaces in close proximity,
and moving relative to each another, by interposing a
substance called lubricant between the surfaces to carry
or to help carry the load (pressure generated) between
the opposing surfaces.
Regimes of Lubrication
As the load increases on the contacting
surfaces three distinct situations can be
observed with respect to the mode of
lubrication, which are called regimes of
lubrication:
• Fluid film lubrication
• Hydrostatic lubrication
• Hydrodynamic lubrication
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