. Introduction
As it is well known different physical processes contribute to
rubber friction. Dry friction of rubbers is controlled, among others,
by adhesion, macro hysteresis, micro- or surface roughness generated
hysteresis, and cohesion (friction contribution from rubber
wear) [1,2]. In presence of lubricant, the viscous component of
friction also appears and can be interpreted as energy loss due to
the viscous nature of lubricant. Contribution of different components
to sliding friction depends on the macro geometry, detailed
surface micro-topography, relative tangential velocity, cleanliness
of surfaces, temperature, material and surface properties of rubbing
bodies, applied normal load, filler material and content,
lubricant and its additives, and surface treatment/coating of the
harder counterpart or the rubber surface. Interaction of friction
mechanisms and complexity of physical processes involved in
rubber friction, however, make the friction prediction very
challenging.
From engineering point of view reciprocating rubber seals are
of primary importance because they are frequently used machine
elements. During operation most of them slide on (apparently)
smooth hard (compared to rubber) countersurface in presence of
lubricant. Seals have smooth surface but the surface of harder
counterpart is usually even smoother (see [3]) in order to reduce
friction, rubber wear and wear induced leakage. It is well known
that reciprocating rubber seals operate frequently in boundary and
mixed lubrication regime. However different explanations exist in
the literature for the friction contribution arisen in the boundary
lubrication regime. On the one hand [4] states that the friction is
determined predominantly by interaction between the solids and
between the solids and the liquid. Bulk flow properties of the
liquid play little or no part in friction. In other words, the friction
contribution arisen in the boundary lubrication regime is considered
to be due to the shearing of a thin boundary lubricant layer
separating contacting surfaces or shearing of the interface
between the boundary layer and the solid surfaces. Shear strength
of the boundary lubricant layer is influenced by both properties of
the contacting surfaces and those of the lubricant. Its magnitude
can only be determined by experiments conducted at sufficiently
low sliding velocity where the hydrodynamic effect is negligible.
In [5], it was also pointed out that the applied normal pressure and
the adhesion may induce solid/solid contact between the adhered
boundary lubricant islands (discontinuous boundary lubricant
layer) causing relatively high coefficient of friction. In the boundary
lubrication regime, applied normal load is carried by asperity
contacts and closed lubricant pools formed in the roughness valleys
of harder surface. On the other hand, in [6,7], the importance
of micro-hysteretic friction component is emphasized in case of
boundary lubrication. The authors hypothesized that the friction
contribution arisen in boundary lubrication regime is mainly due
to the surface roughness generated hysteresis (micro-hysteresis).
The reason why this was hypothesized is that the very thin
boundary layer formed typically from few layers of lubricant
molecules makes solid type asperity contacts possible. In contrast,
Smith’s theory (see [1]) states that if the adhesion propensity is
very low (this is the case when adhesion eliminating boundary
layer separates contacting surfaces) the micro-hysteretic friction
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/triboint
Tribology International
http://dx.doi.org/10.1016/j.triboint.2015.09.018
0301-679X/& 2015 Published by Elsevier Ltd.
E-mail address: goda.tibor@gt3.bme.hu
Tribology International 93 (2016) 142–150
will disappear. Consequently Smith’s theory is based on “adhesion-related
surface deformation hysteresis” where the decreasing
adhesion propensity results in decreasing micro-hysteretic friction.
Additionally Smith assumes that the micro-hysteretic friction
contribution is practically independent of the nominal contact
pressure.
Main objective of the current study is to investigate the contribution
of micro-hysteresis to rubber friction in case of apparently
smooth surfaces. In order to attain this objective it is needed
to reanalyze measurement results of [6], and compare them
additional test results. Like in [6], friction test results reported for
nitrile butadiene rubber (NBR) are in the focus of this work
because, contrary to its great practical importance, surprisingly
little attention is paid in the literature to oil lubricated sliding
friction of NBR squeezing against (apparently) smooth steel
. IntroductionAs it is well known different physical processes contribute torubber friction. Dry friction of rubbers is controlled, among others,by adhesion, macro hysteresis, micro- or surface roughness generatedhysteresis, and cohesion (friction contribution from rubberwear) [1,2]. In presence of lubricant, the viscous component offriction also appears and can be interpreted as energy loss due tothe viscous nature of lubricant. Contribution of different componentsto sliding friction depends on the macro geometry, detailedsurface micro-topography, relative tangential velocity, cleanlinessof surfaces, temperature, material and surface properties of rubbingbodies, applied normal load, filler material and content,lubricant and its additives, and surface treatment/coating of theharder counterpart or the rubber surface. Interaction of frictionmechanisms and complexity of physical processes involved inrubber friction, however, make the friction prediction verychallenging.From engineering point of view reciprocating rubber seals areof primary importance because they are frequently used machineelements. During operation most of them slide on (apparently)smooth hard (compared to rubber) countersurface in presence oflubricant. Seals have smooth surface but the surface of hardercounterpart is usually even smoother (see [3]) in order to reducefriction, rubber wear and wear induced leakage. It is well knownthat reciprocating rubber seals operate frequently in boundary andmixed lubrication regime. However different explanations exist inthe literature for the friction contribution arisen in the boundarylubrication regime. On the one hand [4] states that the friction isdetermined predominantly by interaction between the solids andbetween the solids and the liquid. Bulk flow properties of theliquid play little or no part in friction. In other words, the frictioncontribution arisen in the boundary lubrication regime is consideredto be due to the shearing of a thin boundary lubricant layerseparating contacting surfaces or shearing of the interfacebetween the boundary layer and the solid surfaces. Shear strengthof the boundary lubricant layer is influenced by both properties ofthe contacting surfaces and those of the lubricant. Its magnitudecan only be determined by experiments conducted at sufficientlylow sliding velocity where the hydrodynamic effect is negligible.In [5], it was also pointed out that the applied normal pressure andthe adhesion may induce solid/solid contact between the adheredboundary lubricant islands (discontinuous boundary lubricantlayer) causing relatively high coefficient of friction. In the boundarylubrication regime, applied normal load is carried by asperitycontacts and closed lubricant pools formed in the roughness valleysof harder surface. On the other hand, in [6,7], the importanceof micro-hysteretic friction component is emphasized in case ofboundary lubrication. The authors hypothesized that the frictioncontribution arisen in boundary lubrication regime is mainly dueto the surface roughness generated hysteresis (micro-hysteresis).The reason why this was hypothesized is that the very thinboundary layer formed typically from few layers of lubricantmolecules makes solid type asperity contacts possible. In contrast,Smith’s theory (see [1]) states that if the adhesion propensity isvery low (this is the case when adhesion eliminating boundarylayer separates contacting surfaces) the micro-hysteretic frictionContents lists available at ScienceDirectjournal homepage: www.elsevier.com/locate/tribointTribology Internationalhttp://dx.doi.org/10.1016/j.triboint.2015.09.0180301-679X/& 2015 Published by Elsevier Ltd.E-mail address: goda.tibor@gt3.bme.huTribology International 93 (2016) 142–150will disappear. Consequently Smith’s theory is based on “adhesion-relatedsurface deformation hysteresis” where the decreasingadhesion propensity results in decreasing micro-hysteretic friction.Additionally Smith assumes that the micro-hysteretic frictioncontribution is practically independent of the nominal contactpressure.Main objective of the current study is to investigate the contributionof micro-hysteresis to rubber friction in case of apparentlysmooth surfaces. In order to attain this objective it is neededto reanalyze measurement results of [6], and compare them
additional test results. Like in [6], friction test results reported for
nitrile butadiene rubber (NBR) are in the focus of this work
because, contrary to its great practical importance, surprisingly
little attention is paid in the literature to oil lubricated sliding
friction of NBR squeezing against (apparently) smooth steel
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