ME6623D Industrial Tribology
Course Faculty: Dr. P.K. Rajendrakumar
Professor
Department of Mechanical Engineering
Tribology
The word tribology was first reported in a landmark report by Peter Jost
(1966) in England
Derived from Greek words - tribos (rubbing) and logy (knowledge)
Defined as, “the science and technology of interacting surfaces in
relative motion and of associated subjects and practices”
Study of friction, wear and lubrication
Friction is the principal cause of energy dissipation
Wear is the major cause of material wastage and loss of
mechanical performance
Lubrication is an effective means of controlling wear and
reducing friction
Tribology
The practices of tribology dates back from 1800 B.C. or earlier
Egyptians used a lubricant to aid movement of Colossus,
El-Bersheh, Circa 1880 B.C.
Leonardo da Vinci wrote in 1519 :
“All things and everything whatsoever, however thin it be, which interposed
in the middle between objects that rub together, lighten the difficulty of
friction”
Concept of a lubrication
Tribology
Interactions of tribo-couple elements are highly sophisticated and
their understanding entails knowledge of multiple disciplines
Industrial Significance of Tribology
Jost Report (1966) – United Kingdom made a wastage of about 500 million
pounds per year, due to the ignorance of mechanical surface interaction
phenomena in machinery
A coherent program of education and research was launched to remedy this
situation, and the term tribology was coined to describe this program
At present it is known that the report greatly under-estimated the real financial
importance of tribology (Report neglected loss due to wear)
United States (1976) – estimated a possible savings of at least 16 billion
dollars per annum by better tribological practices
Economic importance of tribology in USA
Loss of usefulness of material objects with their economic importance
Surface Interactions
Surface interactions take
place at regions where atom-
to-atom contact occurs
These regions are called
junctions
Real Area of Contact, Ar
Sum of the areas of all the
junctions
Nature of interaction between two
surfaces is determined by Ar
Apparent Area of Contact, Aa
Total interfacial area (as
appeared from outside)
Ar ( Load, L) /(Penetration Hardness, p)
Surface Topography
All solid surfaces are found to be uneven (except carefully cleaved mica)
Surface texture is the repetitive or random deviation from the nominal
surface that forms the three-dimensional topography of the surface.
Surface texture includes -
Waviness (macro-roughness)
Roughness (nano- and micro-roughness)
Lay
Flaws
Surface Topography (Contd)
Waviness (macro-roughness) is the surface irregularity of
longer wavelengths.
Results from such factors as machine or workpiece deflections,
vibration, chatter, heat treatment, or warping strains.
Includes all irregularities whose spacing is greater than the roughness
sampling length and less than the waviness sampling length
Surface Topography (Contd)
Nano- and micro-roughness are formed by fluctuations in the
surface of short wavelengths, characterized by hills (or asperities)
and valleys of varying amplitudes and spacings
These are large compared to molecular dimensions.
Asperities are referred to as peaks in a profile (2-D) and summits in a
surface map (3-D)
Includes traverse feed marks and other irregularities within the limits of the
roughness sampling length
Surface Topography (Contd)
Lay is the principal direction of the predominant surface pattern,
ordinarily determined by the production method
Flaws are unintentional, unexpected, and unwanted interruptions
in the texture
In addition, the surface may contain gross deviations from nominal
shape of very long wavelength, which is known as errors of form
Three-dimensional Profile Map
(a) A ground steel surface
(b) A shot blasted steel surface
(c) A diamond-turned surface
Measurement of Surface Topography
Contact and non-contact methods are used to study the topography
of surfaces
Stylus measurements
Most common, less sensitive
Electron or light microscopy and other optical methods
Scanning Tunneling Microscopy / Atomic Force Microscopy
Very high resolution
Stylus Profilometer
A fine stylus is dragged
smoothly and steadily
across the surface under
examination
Vertical displacement of
the stylus is picked up by a
transducer and plotted
against its travel along the
surface
This method gives
roughness in one-
dimension
Transducer Stylus tip
Stylus Profilometer (Contd…)
Since the vertical
and horizontal
magnifications are
generally
different, the
generated profile
appears to be
different from the
actual one
Stylus Profilometer (Contd…)
An unavoidable
limitation of this
method results from
the shape of the
stylus
The stylus tip cannot
fully penetrate into
deep narrow features
of the surface
Digital Optical Interferometer
Determines surface profile without mechanical contact
Atomic Force Microscope
Developed by Gerd Binnig et al. in 1985.
AFM measures ultra small forces (less than 1 nN) present between the AFM tip surface
mounted on a flexible cantilever beam, and a sample surface.
These small forces are measured by measuring the motion of a very flexible nano-sized
cantilever beam having an ultra small mass, by a variety of measurement techniques
including optical deflection, optical interference, capacitance, and tunneling current.
The deflection can be measured to within 0.02 nm, so for a typical cantilever force
constant of 10 N/m, a force as low as 0.2 nN can be detected.
Advantages of the AFM for tribological studies
AFM can be routinely used
on all types of materials
Three-dimensional atomic force
ceramics, metals, polymers, microscope (AFM) image of a
polish mark on a piece of steel.
semiconductors, magnetic, The scan range in X and Y is 3μm
optical, and biomaterials and the entire Z range is 40 nm
AFM investigations are
usually made in ambient air
environment Metrological study of a
scratch mark in the surface of
a polished material (stainless
AFM studies in a vacuum or liquid steel). Because the AFM
environment is also possible directly measures three
dimensional data, the depth of
Direct 3-dimensional the scratch mark is easily
quantified.
visualization of wear tracks
and scars is possible
images may be displayed in 2-D
projection and 3-D projection
Quantification of Surface Roughness
Roughness : Small scale irregularities of a surface
Form error : Measure of the deviation of the shape of the surface
from its intended ideal shape (plane, cylindrical etc.)
Waviness : Periodic surface undulation intermediate in scale
between roughness and form error
Roughness is extracted from the surface profile recorded by a
profilometer, by subtracting form error and waviness
Mechanical filtering is achieved by supporting the measuring head on a
small skid which rides on the surface just behind or in front of the stylus
The profilometer records the displacement of the stylus relative to the skid
Electrical or electronic filtering can also be done
Quantification of Surface Roughness (contd…)
Graph of the surface profile contains most of the information needed
to describe the topography along one direction
Several typical quantities can be computed from the surface profile
Average Roughness, Ra (Centre Line Average)
Arithmetic mean deviation of the surface height from the mean line
through the profile
Mean line is defined so that equal areas of the profile lie above and below it
1 L
Ra y ( x) dx
L 0
Quantification of Surface Roughness (contd…)
R.m.s. roughness, Rq
Root mean square deviation of the profile from the mean line
1 L 2
Rq 2
y ( x) dx
L 0
Rq and Ra are similar
For a Gaussian distribution of surface heights, Rq = 1.25 Ra
Quantification of Surface Roughness (contd…)
Amplitude density function, p(y)
Proportional to the probability of finding a point on the surface at height y
above the mean line
Describes the distribution of surface heights
Bearing Ratio : Ratio of the contact length to the total length of the profile
Bearing ratio curve is a plot of bearing ratio against surface height
Differentiation of bearing ratio curve yields amplitude density function
Quantification of Surface Roughness (contd…)
Skewness : Measure of the
asymmetry of the amplitude density
curve
1
Sk
Rq3
y 3 p ( y ) dy
Kurtosis : Measure of the sharpness
of the peak of the distribution curve
1
K
Rq4
y 4 p( y ) dy
Gaussian (normal) probability
distribution has a skewness value
of zero and kurtosis of 3.0
Quantification of Surface Roughness (contd…)
Autocorrelation function, C() : Describes the distribution of hills and valleys
1 L
C ( ) y ( x) y ( x ) dx
L 0
Power spectral density function, P(ω) : Conveys information about the
spatial frequencies (or wavelengths) present in the surface profile
Fourier Transform of the autocorrelation function
2
0
P( ) C ( ) cos( ) d
