Wear 241 Ž2000. 151–157
www.elsevier.comrlocaterwear
Wear in relation to friction — a review
Koji Kato )
Laboratory of Tribology, School of Mechanical Engineering, Tohoku UniÕersity, Sendai 980-8579, Japan
Abstract
Soft or hard film coating, multi-phase alloying and composite structuring have been developed to control wear and friction by
improving materials and surfaces with some aspects for better properties of friction and wear. On the other hand, it is well recognized
recently that the coefficients of friction and wear are not material properties but two kinds of responses of a tribo-system. They are always
reasonably related with each other when the necessary functions of the tribo-system are well considered. Typical wear behaviors of
representative materials of coatings, composites, metallic alloys and ceramics are reviewed in relation to their friction behaviors, and
fundamental mechanisms of wear are confirmed for the technical development of wear control. q 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Wear; Friction; Roughness; Hardness; Ductility; Oxide film; Transfer
1. Introduction
Friction and wear are responses of a tribo-system. Coefficients of friction and wear are parameters describing the
state of contact of bodies in a tribo-system, and they are
not material constants of the bodies in contact w1,2x. They
may be treated as material properties for technical conveniences with an engineering sense only in some special
states of contact.
Friction and wear, as two kinds of responses from one
tribo-system, must be exactly related with each other in
each state of contact in the system, although a comprehensive simple relationship should not be expected. Technical
senses of past tribologists, on the other hand, have already
introduced successful methods of controlling wear without
asking details of wear mechanisms. They are soft or hard
film coating, multi-phase alloying and composite structuring in addition to traditional method of lubrication.
It would be helpful for the understanding of wear
mechanisms to confirm the tribo-characters of materials by
those methods, in the viewpoints of wear and friction, by
describing the tribo-phenomena with the representative
terms of ‘‘roughness, hardness, ductility, oxide film, reaction layer and adhesive transfer’’.
)
Corresponding author. Tel.: q81-80-22-217-6954; fax: q81-80-22217-6955.
E-mail address: [email protected] ŽK. Kato..
The purpose of this paper is to come to the general
understanding of wear mechanisms by reviewing the characteristics of wear and friction of very different materials.
2. Wear of soft coatings: roughness effect
When either one of two surfaces of tribo-elements in
sliding or rolling contact has thin soft surface layer that
can partly transfer to the counter surface by adhesion,
relative displacement takes place at the interface between
the surfaces of coating and transfer layer with smaller
shear strength of the soft material than that of the underlying element material. Low friction is obtained as a result,
and wear of the tribo-elements is much reduced. Soft metal
coating is introduced for this purpose, and Au, Ag, Pb and
In are representative ones.
For any selected coating metal, roughness of the substrate has the strong effect on wear of the coating under
given frictional conditions. As Fig. 1 shows for the deposited coating of about 70 nm thickness of In on stainless
steel ŽSUS440C. disk sliding against a silicon nitride pin
in high vacuum, friction coefficient becomes smaller at
smaller roughness and life of the coating becomes longer
w3x. The critical number of rolling cycles, where the friction increases quickly as a result of coating wear, is shown
in Fig. 2 as the function of the substrate surface roughness
R max together with the corresponding friction coefficient
w3x.
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 4 3 - 1 6 4 8 Ž 0 0 . 0 0 3 8 2 - 3
152
K. Kato r Wear 241 (2000) 151–157
Fig. 1. The effect of substrate roughness on friction coefficient of In coated stainless steel sliding against Si 3 N4 ball in high vacuum w1x.
It is clear from these two figures that wear of very thin
soft coatings is much reduced by reducing the surface
roughness of the substrate through the change in friction
coefficient. Similar roughness effect on friction was observed for Pb film thicker than 0.2 mm w4x.
It must be noted, on the other hand, that the substrate
surface roughness effect on friction and wear can be
shown differently from the results of Figs. 1 and 2, when
the coating thickness and the materials of coating and
substrate are quite different from those in the figures and
when adherence between coating and substrate plays the
major role in wear of the coating w5–7x.
Fig. 2. The effect of substrate roughness on the friction coefficient sliding
against Si 3 N4 ball in high vacuum w1x.
Fig. 3. Wear volume as a function of time for different coatings and
uncoated substrate w9x.
3. Wear of hard coatings: hardness effect
When the surface of a tribo-element has a coating that
is much harder than that of the element, wear of the
element is much reduced by the coating as long as it exists
on the surface, because hard materials generally wear less
than soft materials under the same frictional conditions w8x.
Fig. 3 shows the wear curves of five different coatings
and uncoated substrate of high speed steel in the abrasive
wear tests with Al 2 O 3 abrasives w9x. The numbers in the
K. Kato r Wear 241 (2000) 151–157
153
Fig. 4. Wear modes Ža. and wear rate Žb. of Al 2 O 3 coating in abrasive sliding against a diamond pin w10x.
Fig. 5. The distribution of sx x along the friction surface under the friction coefficient, the maximum contact pressure Pmax and the contact width 2 a. Ža.
m s 0.25, Žb. m s 0.70. The arrow shows the maximum tensile stress at the trailing edge of contact Žwhere 1 F xra F 1; contact interface, tra; Žcoating
thickness.rŽhalf contact width., EfrEs ; ŽCoating elastic modulus.rŽsubstrate elastic modulus. w13x.
154
K. Kato r Wear 241 (2000) 151–157
coefficient tends to be high since asperities on the coating
surface work as abrasive ones. It varies from 0.4 to 0.05,
in the cases of diamond and DLC coatings depending on
their roughness w15x.
Therefore, the initial surface roughness of a coating
should be minimized when its hardness is high in order to
avoid its delamination.
Fig. 6. Local yield map for coating and substrate which shows the
possible local plane of yield initiation Ža. for m s 0.25 and Žb. m s 0.70
in sliding contact; t, coating thickness; a, half of the contact width; m ,
friction coefficient; Yc , Ys : yield strength of the coating and substrate, Ec ,
Es : elastic modulus of the coating and substrate, Hc , Hs : hardness of the
coating and substrate w14x.
parentheses show the micro-hardness values. It is clear
from those curves that the wear rate of Ti–B–C coating is
about one tenth of that of uncoated substrate where the
hardness of Ti–B–C is about 10 times larger than that of
the substrate.
Even when the wear rate of coating in steady state of
wear is smaller due to its high hardness, delamination of
coating may also take place because of the same reason. It
can happen under inappropriate contact pressure and shear
stress at the early stage of contact cycles, as shown in Fig.
4 w10x or it can be introduced at a critical coating thickness
after a certain amount of wear as shown in Refs. w11,12x.
High friction coefficient is especially influential for the
maximum tensile stress at the trailing edge of sliding
contact as calculated in Fig. 5 w13x. It also increases the
possibility of crack propagation from the surface defects of
the coating as shown in the local yield maps of Fig. 6Ža.
and Žb. w14x. High tensile stress and local yield on the
surface of coating are decisive functions for the delamination of coating.
In sliding friction of hard coating such as diamond or
DLC against softer material of metal or ceramic, friction
Fig. 7. Variations of wear volume Ža. and friction coefficient Žb. with
sliding distance for PEEK and PEEK CuS composites sliding against a
hardened tool disk. ŽTest condition: contact pressures 0.654 MPa, velocity s1.0 mrs, ambient condition. w16x.
K. Kato r Wear 241 (2000) 151–157
4. Wear of soft composites: transfer effect
When a tribo-element made of soft material such as
plastics is rubbed and effectively worn by a harder counterpart, its wear can be reduced by forming a transfer layer
of the soft material on the counter surface of harder
material. Wear rate of the soft material in sliding against
itself must be small in this case.
Fig. 7Ža. and Žb. shows the effect of a filler of CuS in
polyetheretherketone ŽPEEK. on wear and friction of PEEK
sliding against steel w16x. As shown in Fig. 7Ža., wear of
PEEK is much reduced by having a filler of CuS through
the effect of enhanced adhesive transfer of PEEK on to the
counter surface of steel disk, although the friction coefficient is increased by having contact between PEEK of the
element and transferred PEEK layer. Similar effects of a
filler of CuS on wear was observed with PTFE and high
density polyethylene w17,18x.
5. Wear of hard composites: ductility effect
When a tribo-element is made of a ductile material of
moderate hardness such as Al, Cu, Ni, Fe or an alloy with
a combination of them, material in the contact region
155
plastically deforms severely under the combined stresses
of compression and shear. Large plastic deformation generally introduces large wear rate since wear surface tends
to become rough and protective surface layers are easily
destroyed. The introduction of a harder reinforcing phase
in the ductile matrix by a certain volume fraction can
reduce ductility of the matrix material in the contact region
without having brittleness, and wear of matrix can be
reduced as a result.
If a tribo-element is made of a brittle material such as
Al 2 O 3 , Si 3 N4 or SiC, the material in the contact region
tends to have brittle fracture of microscopic scale under
concentrated large contact pressure at each contact asperity. It generally gives large wear rate together with rough
wear surface. The introduction of a soft reinforcing phase
in the brittle matrix by a certain volume fraction can give a
certain amount of ducitility to the brittle matrix material in
the contact region, and wear of matrix can be reduced as a
result.
Fig. 8Ža. shows the effects of size and volume fraction
of SiC particles on wear of Ni matrix, which is the case of
a hard reinforcing phase in a ductile matrix w19x. It is clear
in the figure that the volume fraction of about 10% of SiC
particles reduces the matrix wear by a half. Fig. 8Žb.
shows the effect of volume fraction of a soft cellular
eutectic HfO 2 –Al 2 O 3 phase Ža half hardness value of that
of Al 2 O 3 . and the effect of hardness of the HfO 2 alloyed
alumina on wear w19x. It is clear in the figure that the
volume fraction of about 10% of soft phase reduces the
matrix wear by more than a half.
6. Wear of metallic alloys of multi-phases: hard oxide
film effect
There are many metallic alloys that have been well used
for tribo-elements. They are alloys based on iron, copper,
aluminum, tin, lead or other metals . These alloys generally have more than two phases, and one is harder and the
other is softer. When a tribo-element is made of one of
these alloys and its surface is rubbed in air by other solid
surface, the rubbed surface shows the following responses
of multi-phases as the result of repeated stress cycle and
frictional heat cycle:
Fig. 8. The effect of a reinforcing phase in a matrix on its abrasive wear
w19x. Ža. SiC dispersion-hardened galvanic nickel, Žb. HfO 2 alloyed
alumina.
1. Growth of oxide film on the surface and its fracture,
2. Severe plastic deformation in the surface layer and
restructuring of its micro-structure,
3. Adhesive transfer of materials to the counter surface
and its retransfer between two mating surfaces,
4. Generation of wear particles,
5. Detachment, agglomeration, compaction andror bedding of wear particles,
6. Conforming of surface morphology between two mating surfaces after roughening andror smoothing of
wear surfaces.
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K. Kato r Wear 241 (2000) 151–157
These responses take place at the same time in different
phases at the contact interface with different stages depending on the differences in physical and chemical properties of each phase and of each oxide film.
Fig. 9 shows the effect of different iron oxide structure
on friction observed with steelrsteel contact w20x. Reddish
a-Fe 2 O 3 covers the wear surface in air, and it gives the
friction coefficient of about 0.6 and the wear rate of about
2.2 = 10y3 mm3rN m. Dark gray Fe 3 O4 covers the wear
surface at the medium vacuum of 1.3 Pa, and it gives the
friction coefficient of about 0.3 and the wear rate of about
1.0 = 10y4 mm3rN m. Like hard coatings shown in Figs.
3 and 4, oxide films formed in air and medium vacuum
cover the wear surface, and the observed results of friction
and wear are those of the oxide film.
In high vacuum of 5.0 = 10y3 Pa, metallic contact
becomes predominant and high friction coefficient of about
0.8 and the wear rate of about 3.0 = 10y4 mm3rN m are
observed. Like soft coatings shown in Fig. 7, ductile iron
well transfers to the counterface by adhesion in high
vacuum, and low wear rate is generated in spite of high
friction coefficient.
Fig. 10 shows that 10% of Sn, Bi or Pb in aluminum
matrix has negligible effect on friction coefficient in both
air and vacuum of 5.0 = 10y3 Pa. Al–10Sn gives the
smallest wear amount and Al–10Pb the largest in air and
vacuum w21x. The effects of oxygen on friction and wear
shown by their differences in air and vacuum are much
more drastic than those by Sn, Bi and Pb. Adhesive
transfer of aluminum takes place in both air and vacuum,
but the amount of transfer is larger in air than in vacuum.
Large values of friction coefficient and wear volumes in
air can be explained by the large hardness values of
transfer layers Ž659.4 for Al–10Pb, 107.5 for Al–10Bi,
108.1 for Al–10Pb by Hv in kgfrmm2 .. against the softer
wear tracks Ž75.4 for Al–10Sn, 56.9 for Al–10Bi, 83.4 for
Al–10Pb by Hv in kgfrmm2 .. These differences in hardness values of the transfer layer and the wear track tell that
Fig. 9. Friction coefficient observed with sliding test of steel on steel in
high vacuum of 5.0=10y3 Pa, medium vacuum of 1.3 Pa and air w20x.
Fig. 10. The effects of Sn, Bi and Pb in Al-alloy and the atmosphere of
air and vacuum on friction coefficient and wear in repeated sliding of 10 4
cycles against 52100 steel w21x.
the contact is ideally abrasive. By comparing the initial
hardness values of the alloys Ž31.8 for Al–10Sn, 24.7 for
Al–10Bi, 26.0 for Al–10Pb by Hv in kgfrmm2 . with
those of wear tracks and transfer layers, the extreme
hardening in the wear track and the transfer layer can be
noticed. It is the effect of oxygen that forms oxide films on
the Al-alloys and the steel of counter surface.
7. Wear of ceramics: soft reaction film effect
Because of the hardness and brittleness of ceramics
such as Al 2 O 3 , Si 3 N4 and SiC, they are recommended to
be used in the mild wear state to avoid large scale fracture.
Humid air or water is active enough to form silicon oxide
or silicon hydro-oxide under repeated rolling contact
w22,23x or sliding w24x. Alumina also forms aluminum
Fig. 11. The smoothening of initial rough surface of Si 3 N4 by the
repeated sliding in water against itself Ža. and the resultant low friction in
water Žb. w27x.
K. Kato r Wear 241 (2000) 151–157
hydro-oxide by sliding in water w25x. These oxides or
hydro-oxides in humid air or water are soft or soluble, and
resultant wear surfaces become very smooth and available
for hydro-dynamic lubrication with water w26x. Fig. 11
shows the smoothening process of initial rough surface of
Si 3 N4 by wear in repeated sliding against Si 3 N4 and the
corresponding reduction in friction coefficient w27x.
The wear state in Fig. 11 is mild, and the wear mode is
tribo-chemical wear where generated chemical products as
wear debris are soft andror soluble in water. Therefore, a
reaction layer on a hard ceramic surface works as a very
soft coating and its wear mechanism must be similar to
that of an artificial soft coating such as MoS 2 , Pb, or In.
8. Conclusion
For the technical development of wear control in the
near future, the characteristics of wear of coatings, composites, metallic alloys and ceramics were reviewed in
relation to their frictional characteristics.
It is confirmed in this review that the introduced observations on wear and friction of representative materials are
well explained in terms of roughness effect, hardness
effect, ductility effect, oxide film effect, reaction layer
effect and transfer effect. New technologies for better
control of wear will become possible by combining these
effects better.
Acknowledgements
The author would like to express his special thanks to
Dr. M.W. Bai Žat Tohoku University since 1997, formerly
at Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences. for his help in preparing materials
for this paper.
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