Industrial Significance of Tribology
Tribology is crucial to modern machinery which uses sliding and rolling surfaces. Examples
of productive friction are brakes, clutches, driving wheels on trains and automobiles, bolts,
and nuts. Examples of productive wear are writing with a pencil, machining, polishing, and
shaving. Examples of unproductive friction and wear are internal combustion and aircraft
engines, gears, cams, bearings, and seals.
According to some estimates, losses resulting from ignorance of tribology amount in the
United States to about 4% of its gross national product (or about $200 billion dollars per year
in 1966), and approximately one-third of the world’s energy resources in present use appear
4 Introduction to Tribology
as friction in one form or another. Thus, the importance of friction reduction and wear control
cannot be overemphasized for economic reasons and long-term reliability. According to Jost
(1966, 1976), savings of about 1% of gross national product of an industrial nation can be
realized by better tribological practices. According to recent studies, expected savings are
expected to be of the order of 50 times the research costs. The savings are both substantial
and significant, and these savings can be obtained without the deployment of large capital
The purpose of research in tribology is understandably the minimization and elimination of
losses resulting from friction and wear at all levels of technology where the rubbing of surfaces
is involved. Research in tribology leads to greater plant efficiency, better performance, fewer
breakdowns, and significant savings.
Since the 1800s, tribology has been important in numerous industrial applications requiring
relative motion, for example, railroads, automobiles, aircraft, and the manufacturing process
of machine components. Some of the tribological machine components used in these applications
include bearings, seals, gears, and metal cutting (Bhushan, 2001a). Since the 1980s,
other applications have included magnetic storage devices, and micro/nanoelectromechanical
systems (MEMS/NEMS) as well as biomedical and beauty care products (Bhushan, 1996,
1998, 1999, 2000, 2001a, 2001b, 2010a, 2010b, 2011, 2012b). Since the 2000s, bioinspired
structures and materials, some of which are eco-friendly, have been developed and exploited
for various applications (Nosonovsky and Bhushan, 2008, 2012; Bhushan, 2012a).
Tribology is not only important to heavy industry, it also affects our day-to-day life. For
example, writing is a tribological process. Writing is accomplished by the controlled transfer
of lead (pencil) or ink (pen) to the paper. During writing with a pencil there should be good
adhesion between the lead and the paper so that a small quantity of lead transfers to the paper
and the lead should have adequate toughness/hardness so that it does not fracture/break. The
objective when shaving is to remove hair from the body as efficiently as possible with minimum
discomfort to the skin. Shaving cream is used as a lubricant to minimize friction between the
razor and the skin. Friction is helpful during walking and driving. Without adequate friction,
we would slip and a car would skid! Tribology is also important in sports. For example, a low
friction between the skis and the ice is desirable during skiing. Fabric fibers should have low
friction when touching human skin.
Body joints need to be lubricated for low friction and low wear to avoid osteoarthritis and
joint replacement. The surface layer of cartilage present in the joint provides the bearing
surface and is lubricated with a joint fluid consisting of lubricin, hyaluronic acid (HA) and
lipid. Hair conditioner coats hair in order to repair hair damage and lubricate it. It contains
silicone and fatty alcohols. Low friction and adhesion provide a smooth feel in wet and dry
environments, reduce friction between hair fibers during shaking and bouncing, and provide
easy combing and styling. Skin creams and lotions are used to reduce friction between the
fingers and body skin. Saliva and other mucous biofluids lubricate and facilitate the transport
of food and soft liquids through the body. The saliva in the mouth interacts with food and
influences the taste–mouth feel.
1.3 Origins and Significance of Micro/Nanotribology
At most interfaces of technological relevance, contact occurs at numerous levels of asperity.
Consequently, the importance of investigating a single asperity contact in studies of the
Figure 1.3.1 Comparisons between macrotribology and micro/nanotribology.
fundamental tribological and mechanical properties of surfaces has long been recognized. The
recent emergence and proliferation of proximal probes, in particular tip-based microscopies
(the scanning tunneling microscope and the atomic force microscope) and of computational
techniques for simulating tip-surface interactions and interfacial properties, have allowed
systematic investigations of interfacial problems with high resolution as well as ways and
means of modifying and manipulating nanoscale structures. These advances have led to the
development of the new field of microtribology, nanotribology, molecular tribology, or atomicscale
tribology (Bhushan et al., 1995; Bhushan, 1997, 1998, 2001b, 2010a, 2011). This field is
concerned with experimental and theoretical investigations of processes ranging from atomic
and molecular scales to microscales, occurring during adhesion, friction, wear, and thin-film
lubrication at sliding surfaces.
The differences between the conventional or macrotribology and micro/nanotribology are
contrasted in Figure 1.3.1. In macrotribology, tests are conducted on components with relatively
large mass under heavily loaded conditions. In these tests, wear is inevitable and the bulk properties
of mating components dominate the tribological performance. In micro/nanotribology,
measurements are made on components, at least one of the mating components, with relatively
small mass under lightly loaded conditions. In this situation, negligible wear occurs and the
surface properties dominate the tribological performance.
The micro/nanotribological studies are needed to develop a fundamental understanding
of interfacial phenomena on a small scale and to study interfacial phenomena in microand
nano structures used in magnetic storage systems, micro/nanoelectromechanical systems
(MEMS/NEMS), and other industrial applications. The components used in micro- and nano
structures are very light (of the order of few micrograms) and operate under very light loads
(of the order of a few micrograms to a few milligrams). As a result, friction and wear (on
a nanoscale) of lightly-loaded micro/nano components are highly dependent on the surface
interactions (few atomic layers). These structures are generally lubricated with molecularlythin
films. Micro- and nanotribological techniques are ideal ways to study the friction and
wear processes of micro- and nanostructures. Although micro/nanotribological studies are
critical to study micro- and nanostructures, these studies are also valuable in the fundamental
understanding of interfacial phenomena in macrostructures to provide a bridge between science
The scanning tunneling microscope, the atomic force and friction force microscopes, and
the surface force apparatus are widely used for micro/nanotribological studies (Bhushan
et al., 1995; Bhushan, 1997, 1999). To give a historical perspective of the field, the scanning
tunneling microscope (STM) developed by Doctors Gerd Binnig and Heinrich Rohrer and their
6 Introduction to Tribology
colleagues in 1981 at the IBM Zurich Research Laboratory, the Forschungslabor, is the first
instrument capable of directly obtaining three-dimensional (3D) images of solid surfaces with
atomic resolution (Binnig et al., 1982). STMs can only be used to study surfaces which are
electrically conductive to some degree. Based on their design of the STM, in 1985, Binnig et al.
(1986, 1987) developed an atomic force microscope (AFM) to measure ultrasmall forces (less
than 1 µN) present between the AFM tip surface and the sample surface. AFMs can be used
in the measurement of all engineering surfaces which may be either electrically conducting
or insulating. AFM has become a popular surface profiler for topographic measurements
on the micro- to nanoscale. AFMs modified to measure both normal and friction forces,
generally called friction force microscopes (FFMs) or lateral force microscopes (LFMs), are
used to measure friction on the micro- and nanoscales. AFMs are also used for studies of
adhesion, scratching, wear, lubrication, surface temperatures, and for the measurement of
elastic/plastic mechanical properties (such as indentation hardness and modulus of elasticity).
Surface force apparatuses (SFAs), first developed in 1969, are used to study both static and
dynamic properties of the molecularly thin liquid films sandwiched between two molecularlysmooth
surfaces (Tabor and Winterton, 1969; Bhushan, 1999).
Meanwhile, significant progress in understanding the fundamental nature of bonding and
interactions in materials, combined with advances in computer-based modeling and simulation
methods, have allowed theoretical studies of complex interfacial phenomena with high resolution
in space and time (Bhushan, 1999, 2001b, 2011). Such simulations provide insights into
the atomic-scale energetics, structure, dynamics, thermodynamics, transport and rheological
aspects of tribological processes. Furthermore, these theoretical approaches guide the interpretation
of experimental data and the design of new experiments, and enable the prediction
of new phenomena based on atomistic principles.
Tribology exam questions:
(a) Composites are tailor made materials-comment.
(b) Name different types of ceramic matrix composites.
(c) Differentiate thermoplastic and thermo setting matrix materials with examples.
(d) How vacuum assisted RTM is different from RTM?
(e) What is the difference between orthotropic and transversely isotropic?
(f) Explain monoclinic materials.
(g) Explain rule of mixtures to evaluate modulus in longitudinal direction.
(h) Explain the major difference in strength of materials approach and elasticity approach in predicting
(i) Define the flexural modulus of a laminate.
(j) What is meant by warping of laminate?