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Tribology - Materials, Surfaces & Interfaces
ISSN: 1751-5831 (Print) 1751-584X (Online) Journal homepage: https://www.tandfonline.com/loi/ytrb20
Embeddability behaviour of some Pb-free engine
bearing materials in the presence of abrasive
particles in engine oil
Daniel W. Gebretsadik, Jens Hardell & Braham Prakash
To cite this article: Daniel W. Gebretsadik, Jens Hardell & Braham Prakash (2019): Embeddability
behaviour of some Pb-free engine bearing materials in the presence of abrasive particles in engine
oil, Tribology - Materials, Surfaces & Interfaces, DOI: 10.1080/17515831.2019.1574452
To link to this article: https://doi.org/10.1080/17515831.2019.1574452
© 2019 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group
Published online: 01 Feb 2019.
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Embeddability behaviour of some Pb-free engine bearing materials in the
presence of abrasive particles in engine oil
Daniel W. Gebretsadik, Jens Hardell and Braham Prakash
Division of Machine Elements, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden
ABSTRACT
One of the tribological requirements on engine bearing material is its ability to safely embed
contaminant particles onto its surface and minimise damage to both the bearing and
crankshaft surfaces. In this work, a journal bearing test rig that operates under constant load
has been employed to investigate the embeddability behaviour of selected multi-layered Pb-
free engine bearing materials at three different rotational speeds using engine oil
contaminated with SiC particles. Experimental results have shown that third-body abrasive
wear is influenced by the lubricant film thickness. There was also difference in embeddability
of the different materials. Bismuth-based overlay and MoS
2
containing polyamide-imide-
based overlay-coated materials show higher wear compared to tin-based overlay and a
polyamide-imide-based composite overlay-coated material. Steel counter surfaces sliding
against bismuth-based overlay and MoS
2
containing polyamide-imide-based overlay
exhibited higher wear than those sliding against tin-based overlay and polyamide-imide-
based composite overlay.
ARTICLE HISTORY
Received 12 June 2017
Accepted 22 January 2019
KEYWORDS
Embeddability; third-body
abrasive wear; hydrodynamic
lubrication; engine bearings;
Pb-free
1. Introduction
The extent of abrasive wear on journal bearings is sig-
nificant compared with the other wear mechanisms.
For instance, Vencl and Rac reported that abrasive
wear is the most dominant type of wear (∼60%) in
engine bearings [1]. It is caused either by asperities lar-
ger than the lubricant film thickness or by contaminant
particles. Third-body abrasive wear due to contami-
nant particles can occur while the engine is operating
either in hydrodynamic, mixed or boundary lubrica-
tion condition [2,3]. It also depends on the nature of
the abrasive particles including their size, shape, hard-
ness and fracture toughness. The hardness of the bear-
ing and the shaft surfaces as well as operating
conditions such as surface speeds will also affect the
occurrence of three-body abrasion. Depending on
these factors, various damages such as indentation
and abrasion (micro-cutting and ploughing) can
occur [4].
Sources of third-body abrasive particles include deb-
ris from the machining process in recently assembled
components, internally generated wear particles and
ingested particles from external sources. These third-
body abrasive particles in engines include a wide
range of materials such as Al, Fe, Cu, Sn, Ag, SiC,
SiN, sand, and silicates. Their hardness can range
from soft materials of 40 HV to very hard materials
up to 1300 HV [4–6].
For dynamically loaded bearings, it has been
reported that abrasive wear increases significantly at
locations where the film thickness is smaller [7]. In
general, if the film thickness is larger than the size of
the abrasive particles circulating with the oil, the lubri-
cant film may not break and hence there might not be
any wear. However, if the film thickness is smaller than
the size of the abrasive particles, abrasive wear can
occur on both the bearing and the rotating shaft surface
since the abrasive particles are dragged between the two
surfaces and act as a third-body abrasive particles [8].
The ability of engine bearing surface to safely embed
contaminant particles without causing severe damage
to the expensive crankshaft is referred to as embedd-
ability property [9,10]. To improve embeddability of
bi-metal bearings a soft phase is usually incorporated
in the bearing alloys (lining) such as Pb in Cu-Pb alloys
and Sn in Al-Sn alloys. According to Ronen et al. for
steady loaded hydrodynamic bearings, the shaft and
bearing liner wear due to contaminant particles depend
mainly upon the shaft to liner hardness ratio [11]. The
most widely used approach to improve embeddability
is to apply a soft overlay as the outermost surface.
Most overlays are soft materials that can easily be
deformed and embed abrasive particles. According to
Spikes et al., embeddability indices of tri-metallic bear-
ings (overlay plated) are about four times better than
bimetallic bearing [12]. However, the use of an overlay
will not completely avoid abrasive wear. The abrasive
© 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-
nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or
built upon in any way.
CONTACT Daniel W. Gebretsadik daniel.gebretsadik@ltu.se
TRIBOLOGY - MATERIALS, SURFACES & INTERFACES
https://doi.org/10.1080/17515831.2019.1574452
particles in the engine oil can cause wear of the entire
overlay in the minimum film thickness region exposing
the intermediate layer and lining and consequently
resulting in severe shaft damage [13].
The most conventional engine bearing material is
Cu-Pb based alloys coated with Pb based overlay. How-
ever, the use of Pb-containing materials for vehicle
components is prohibited due to environmental con-
cern. Thus new Pb-free bearing materials are becoming
more common. The tribological performance of these
Pb-free bearing materials in the presence of contami-
nant particles in the engine oil is not available in the
open literature and hence investigation into their
embeddability behaviour is important in terms of
developing Pb-free engine bearings. This work thus
focuses on experimental studies pertaining to the
embeddability behaviour of some multi-layered Pb-
free engine bearing materials under lubricated con-
dition in the presence of SiC abrasive particles.
2. Experimental
2.1. Experimental materials
Elemental composition of the bearing materials is given
in Table 1. Bearing samples are designated as D1,D2,
D3, D4 and D5.D1,D3,D4 and D5 have copper-tin-
based lining with some amount of bismuth (Bi) and
D2 has Al-Sn-based lining. All bearing materials are
overlay coated. D1 has Sn-based overlay. D2 has a com-
posite overlay composed of polyamide-imide (PAI), Al,
PTFE and silane. D3 has an overlay composed of PAI
and MoS
2
.D4 and D5 have Bi-based overlay. D4 has
a silver (Ag) interlayer. The cross-sections of the bear-
ing materials are shown in Figure 1(a–e). As received
bearing specimens are used and hence regardless of
the differences in the thickness of the different layers
more focus is given to the overlays since these will
have a more pronounced effect on the embeddability
behaviour. For these tests, a steel shaft (EN 10297-1)
is used as a counter surface. SiC particles (#1000 grit
size, corresponding to the larger particle size of
∼18 µm) were used as abrasive contaminant particles.
In practice, as shown in Figure 1(f), these abrasive par-
ticles span a wide range of sizes and shapes and they are
mainly smaller than 18 µm. Engine oil (Scania refer-
ence oil 10W-30) was used as a lubricant.
Hardness values for the overlays and linings of the
different materials are shown in Figure 2. In general,
hardness of the overlay is lower than the lining
materials. The metallic overlays show lower hardness
compared to the composite overlays. The nanoindenta-
tion measurements were carried out on the polished
cross-sections of the bearing materials. Indentations
were carried out in a row along the horizontal direction
as shown in Figure 1(a). At least nine measurements
were carried out for each layer and an average hardness
value was calculated based on these. The hardness
Table 1. Nominal composition of bearing materials and thickness of the different layers.
Sample
Composition Thickness (µm)
Lining Interlayer overlay Lining Interlayer Overlay
D1 CuSn4Bi(3-5)Ni(0,7-1,3) Ni Sn ∼325 ∼2∼12
D2 AlSn(5-7)Cu1Ni1Si(1,5-3)Mn0,3V0,15 –PAI; Al (10-15) PTFE (5-7) Silane5 ∼384 –∼10
D3 CuSn10Bi4 –PAI 45; MoS
2
55 ∼328 –̴∼10
D4 CuSn10Bi4 Ag Bi ∼328 ∼4.2 ∼5.7
D5 CuSn10Bi4 –Bi ∼333 –∼5.7
Figure 1. SEM images of cross-sections of bearing materials (a) D1, (b) D2, (c) D3, (d) D4, (e) D5 and (f) SiC abrasive particles.
2D. W. GEBRETSADIK ET AL.
values are not absolute hardness values especially for
the overlays since there is some influence from the
adjacent layers. In addition, for the composite overlays,
the scatter in the hardness is related to the fact that the
indentation could be on the metallic particles or the
polymer matrix. Moreover, the nanoindentation
measurement values in polymeric materials can also
be affected by the direction and length of the polymer
back chain. For the linings, nanoindentation was car-
ried mainly on the copper-tin matrix to avoid the Bi
soft phase in D1,D3,D4 and D5. For the lining of
D2 nanoindentation was carried out mainly on the alu-
minium matrix. But there could be some influence
from the soft phase Sn and hard Si particles in the lin-
ing. The nanoindentation was carried out using a
Micro Materials NanoTest Vantage system in a load-
controlled manner with a maximum load of 5mN.
2.2. Experimental techniques
A journal bearing test rig that operates under constant
load condition has been employed to investigate the
embeddability behaviour of the bearing samples. A
simple schematic of the test rig is given in Figure 3.
A half bearing shell (Ø108 and 34 mm width) is
mounted in the lower section of the bearing housing.
The steel counter surface, which is a sleeve with outer
diameter Ø107.5 mm and hardness value of ∼610
HV and Ra value of ∼0.2 µm, is mounted onto the
shaft that is coupled to the drive motor. The load is
applied by placing a dead weight at the end of a lever
arm. The bearing housing has freedom of movement
to compensate for misalignment. The rotational
speed of the shaft has a range from 0 to 1500 rpm.
There is an oil tank equipped with a heater and stirrer
to continuously mix the SiC abrasive particles in the oil.
The oil is supplied into the bearing housing from the oil
tank through a pipe. Once the oil lubricates the system
it is collected in an oil reservoir located beneath the
bearing housing. The oil is then recirculated using a
hose pump.
Weight loss due to abrasive wear was measured on
both the bearing and shaft samples using a weighing
balance that reads two decimal places to the right of
the decimal point. Two measurements were carried
out for each bearing material and the average weight
loss values are reported. The weight loss results are
average of two tests using new bearing specimens as
well as shaft specimens for each test. Surface analysis
was carried out to investigate damages on both bearing
and shaft surfaces. Scanning electron microscopy
(SEM) with integrated energy dispersive spectroscopy
(EDS) was used for studying the abrasive wear on bear-
ing materials and counter surfaces. White light inter-
ferometry (Veeco Wyko 1100NT) was also used to
study topography changes on the counter surface.
2.3. Test conditions
The test conditions employed for the embeddability
tests are shown in Table 2. Three different speeds are
Figure 2. Hardness values of linings and overlays of bearing
materials.
Figure 3. Schematic of the embeddability test rig and a front
view of the bearing housing.
Table 2. Test conditions and parameters used for
embeddability test.
Test Parameter Value
Test load 1800 N
Speed 30 rpm (h
0
= 2.1 µm)
200 rpm (h
0
= 6.5 µm)
800 rpm (h
0
= 15.6 µm)
Viscosity at 90°C 0.0129 Pa s
Oil temperature 90°C
Flow rate 0.012 L/s
Concentration of contaminants 2.0 g/L
Test duration 60 min
TRIBOLOGY –MATERIALS, SURFACES & INTERFACES 3
employed to generate different lubricant film thickness
to see if there is any correlation between the abrasive
wear and the minimum film thickness. It should be
noticed that the sliding distances for the three test con-
ditions are not similar. For each speed the correspond-
ing minimum film thickness (h
o
) is calculated using 1-
D Reynolds equation and finite difference method is
used for discretization. The surface roughness is not
taken into consideration for the calculation. The con-
centration of the abrasive particles used in this study
is higher than the amount of abrasive particles experi-
enced in engine oil in order to increase the severity of
tests.
3. Results and discussion
Embeddability behaviour of five different bearing
materials was investigated at different minimum film
thickness conditions. In this section effect of the mini-
mum film thickness on abrasive wear, weight loss of the
bearing and counter surfaces due to abrasive wear and
abrasive wear mechanisms are presented and
discussed.
3.1. Effect of minimum film thickness
Damage due to abrasive wear on the bearing surfaces
was found to be high at 200 rpm (h
0
= 6.5 µm) com-
pared to those at 30 rpm (h
0
= 2.1 µm) and 800 rpm
(h
0
= 15.6 µm) for the same test duration of 60 min.
Figure 4 shows photographs of surfaces of bearing
material D4 after tests carried out at 30, 200 and
800 rpm. The damage on bearing materials tested at
800 rpm is considerably lower compared to those
tested at 200 rpm. This can be explained by the fact
that at 800 rpm the minimum film thickness is large
enough to allow free circulation of most of the abrasive
particles without causing severe abrasion on the sur-
face. There are scratches on the overlay caused by
some particles larger than the minimum film thickness.
However, at 200 rpm the overlay is worn out around
the middle of the bearing shell which corresponds to
the location where the minimum film thickness occurs.
At 200 rpm, most of the abrasive particles cannot pass
through the interface and hence when the abrasive par-
ticles are dragged between the two surfaces the overlay
material is gradually worn out and subsequently
exposes the intermediate layers. At lower rotational
speed of 30 rpm, the number of particles that are fed
into the interface is fewer and their velocity is lower
since the rotational speed, as well as the total sliding
distance, is lower compared with that at 200 rpm for
the same test duration of 60 min.
In general, the severity of the abrasive wear varies
along the circumferential direction of the bearing sur-
faces. This is a direct implication of the lubricant film
thickness distribution. Away from the minimum film
thickness area in the entrainment direction, the film
thickness is larger and therefore the numbers of abra-
sive particles that pass through without causing severe
damage are higher. However, as the abrasive particles
continue moving forward with the oil in the sliding
direction, the gap between the bearing and shaft sur-
faces becomes smaller to the point where the gap
becomes equal with the particle size and hence they
cause more scratches and removal of material. Once,
the abrasive particles left the minimum film thickness
area, the film thickness become larger and hence the
number of abrasive particles that can scratch the bear-
ing surface becomes less and less as they move out of
the interface.
Figure 5 shows white light interferometry images of
part of the counter surfaces sliding against bearing
material D4 tested at 30, 200 and 800 rpm. More
material was removed from the counter surface sliding
at 200 rpm than those sliding at 30 and 800 rpm. At
800 rpm the minimum film thickness is large enough
to allow most particles to circulate without causing
severe damage on the shaft surface. At 200 rpm, how-
ever, the film thickness becomes smaller and the abra-
sive particles that circulate with the oil can cause
abrasion as they are dragged between the two surfaces.
Figure 4. Photograph of worn surfaces of bearing material D4
after tests at three different test conditions.
Figure 5. White light interferometry images of counter surfaces used against bearing material D4.
4D. W. GEBRETSADIK ET AL.
In addition, abrasive particles that are partially
embedded on the bearing surface abrade the counter
surface. At the lower rotational speed of 30 rpm, as
shown later in Figure 12, large numbers of these par-
ticles are trapped by the overlay before entering the
minimum film thickness zone. The trapped abrasive
particles cause damage to the shaft.
3.2. Weight loss of bearing materials and shaft
specimen
The weight loss due to abrasive wear of the bearing
materials tested at three different speeds is shown in
Figure 6(a). Although the discussion is based on weight
loss of bearing materials, it should be noted that as
rotational speed decreases wear rate (gram/meter)
increases. The difference in weight loss due to abrasive
wear among bearing materials is more visible and easy
to compare at 200 rpm than the other two test con-
ditions. D1 which is Sn-based overlay-coated material
and D2 which is composite PAI-based overlay contain-
ing PTFE and metallic (Al) particles shows lower
weight loss than D3 with PAI-based overlay containing
MoS
2
, and D4 and D5 which are Bi-based overlay-
coated materials. The Sn-based overlay has slightly
lower hardness compared to the Bi-based overlay but
both of them can be considered as soft metals. In soft
metals, abrasive particles tend to embed without
causing severe abrasive wear of the overlay. However,
the Bi-based overlay shows significantly higher wear
compared to the Sn-based overlay. The main reason
for this difference is that the Bi overlay of D4 and D5
are significantly thinner than the Sn-based overlay of
D1. The thickness of the Sn-based overlay is about
12 µm but the Bi-based overlay is about 5.7 µm. Con-
sidering the size of the abrasive particles which spans
a wide range (up to 18 µm) the soft thin Bi overlay
can easily be abraded exposing the intermediate layer.
However, the thick Sn-based overlay can safely
embed the particles and reduce the abrasive wear. On
the other hand, the PAI-based overlay-coated materials
D2 and D3 show a significant difference in their weight
loss. The weight loss is found to be higher for D3 com-
pared to D2. Regardless of the PAI polymeric matrix,
there is a difference in their structure. For example,
overlay of D2 contains PTFE and metallic particles in
a very small amount in the PAI polymer matrix. In
contrast, overlay of D3 has a large amount of MoS
2
par-
ticles in the polymer matrix. In such composite
materials in addition to the wear resistance of the poly-
mer matrix, other factors such as the interfacial bond-
ing between polymer matrix and the reinforcement can
influence abrasive wear. The volume fraction of the
reinforcement in the polymer matrix can also affect
abrasive wear [14]. Abrasive wear due to particle
removal by the sharp abrasive particles is also possible.
Figure 6. Weight loss (grams) of (a) bearing materials, (b) shaft specimens and wear rate (grams/meter) of (c) bearing materials and
(d) shaft specimens.
TRIBOLOGY –MATERIALS, SURFACES & INTERFACES 5
The observed high abrasive wear of the PAI-based
overlay containing MoS
2
particles could be because of
these factors. In D3,D4 and D5, once the overlay is
entirely worn out the intermediate layers which are
harder materials with less tendency of embedding abra-
sive particles are exposed. These exposed harder layers
are then abraded by the SiC abrasive particles. At 30
and 800 rpm, the differences in weight loss among
the tested bearing materials are not very significant
considering the error associated with the weight
measurement. However, the Bi-based overlay plated
materials still show slightly higher weight loss com-
pared with the other tested bearing materials.
The weight loss of the bearing materials may not be
enough to describe the embeddability behaviour since,
by definition, embeddability is the ability of bearing
materials to minimise damage on the crankshaft by
the abrasive particles circulating with the oil. Hence,
the severity of abrasive wear on the counter surface is
also important in characterising the embeddability
behaviour of the bearing materials. Figure 6(b) shows
weight loss of the shaft specimens at the three test con-
ditions. At 200 rpm, it is easy to compare the weight
loss caused by abrasive wear on the counter surfaces
sliding against the different bearing materials. The
shaft specimens sliding against D3,D4 and D5 shows
higher wear than those sliding against D1 and D2.
This is mainly related to the ability of the bearing
materials to embed abrasive particles. Those materials
that tend to safely (fully) embed and reduce the num-
ber of abrasive particles circulating with the oil reduce
the abrasive wear on the counter surface. In this regard,
D1 and D2 which show the lowest wear also reduce
wear on the counter surface.
Figure 6(c,d) shows the normalised wear rate at
different rotational speeds for the bearing and shaft
specimen, respectively. As rotational speed increases,
the minimum film thickness increases and hence the
abrasive particles circulate with the oil without causing
significant wear.
Mainly two types of damages were observed on the
steel counter surfaces. There is a polishing of the shaft
surface accompanied by microgrooves as shown in
Figure 7(a). This occurs mainly in the counter surfaces
sliding against the bearings run at 200 and 30 rpm
where the polishing and microgrooves were located
on the edge of the rotating shaft. It is most likely
more pronounced around these locations because of
small misalignment that causes variation of the oil
film thickness in the axial direction. According to Wil-
liams [8], such damages occur when the dimension of
particles to film thickness ratio is above a certain criti-
cal value which results in the abrasive particles getting
embedded on the softer material (bearing surface) and
will spend some time sliding against the harder surface
(shaft). In addition, abrasive wear due to indentation
and micro-scratches is observed on the counter surface
as shown in Figure 7(b). These scratches and the
indent marks result in a dented surface. This kind of
wear was mainly observed in the shaft surfaces used
at 800 rpm where the minimum film thickness is larger
than those used at 30 and 200 rpm. Such kind of abra-
sive wear occurs when the abrasive particles roll or
tumble through the lubricant film that separates the
Figure 7. SEM images of steel counter surface: (a) shiny surface due to polishing accompanied with microgrooves, (b) micro-
scratches and indents, (c) high magnification images of micro-scratches and (d) indents and removed material.
6D. W. GEBRETSADIK ET AL.
bearing and the shaft surface. Detailed image of
the micro-scratches and indent marks and
materials removed due to micro-cutting are shown in
Figure 7(c,d).
3.3. Damages on bearing surfaces
Surface analysis carried out on the tested bearing
materials shows different features. In general, abrasive
wear increases in severity from the beginning of the
entrainment direction towards the location where the
minimum film thickness occurs. Figure 8(a–d) shows
the differences in the severity of indents and micro-
scratches at different locations on bearing material
D1 tested at 200 rpm. Figure 8(a) shows the original
surface of D1 which is Sn-based overlay. Figure 8(b)
shows a smaller number of indents marks and micro-
scratches at location far away from the minimum
film thickness area. The abrasion is caused by the
sharp edges of the abrasive particles. As shown in
Figure 8(c) the number of indents and abrasion
increases as the gap between the bearing and the
shaft surface decreases. The scratches become more
severe at the location very close to the minimum film
thickness area as shown in Figure 8(d). In this region,
the surface is severely abraded and the features
observed in the original surface disappear since the
space between the two surfaces is smaller and the abra-
sive particles remove more material from the overlay.
This trend is observed in all tested bearing materials
in all the three test conditions, however, the extent of
severity was highest at 200 rpm where in most cases
the lining, which was originally protected by the over-
lay, is exposed as shown in Figure 4.
Various forms of damage are observed on the bear-
ing surfaces. Some of the features observed on the bear-
ing surfaces indirectly show the causes of damages on
the counter surfaces. The typical damages caused by
abrasive particles on bearing materials are discussed
in the following section.
In most of the bearing materials, grooves caused by
abrasive particles are common on the overlays. For
example, as shown in Figure 9(a), in D1 these grooves
are deeper and wider than the micro-scratches
observed on most of the overlays and exposed linings.
They are also relatively long and extend in the sliding
direction. This indicates that the abrasive particles
that cause these grooves are dragged along the circum-
ferential direction when they are forced to circulate
with the oil. Based on the width of these grooves, it is
possible to say that they are caused by larger abrasive
particles and can also cause abrasive wear on the coun-
ter surface. Deep and long grooves caused by abrasive
particles are also observed on the composite overlay
of D2 as shown in Figure 9(b). The grooves are charac-
terised by displaced material on both sides of the
groove. In addition, there are also other patterns
associated with the way abrasive particles remove
material from the bearing surface while they circulate
with the oil. Figure 9(c) shows abrasive wear track on
the overlay of D5 left by abrasive particle rolling
between the interfaces. Although most of the grooves
are observed on the overlays of most materials, there
Figure 8. Severity of abrasion on bearing surface along the circumferential direction in Sn overlay-coated bearing material D1: (a)
original surface, (b) far away, (c) closer and (d) very close to the minimum film thickness area.
TRIBOLOGY –MATERIALS, SURFACES & INTERFACES 7
are also grooves caused by abrasive particles on the
exposed linings. For example, the SEM analysis of the
grooves on the exposed lining region cut out from
bearing specimen of D3 is shown in Figure 9(d).
There are also micro-scratches and indents on both
the overlay and the exposed linings of most of the
bearing materials. Even though the size of the micro-
scratches and indents looks small, there is a significant
amount of material removed from the surface. In
addition to the micro-scratches, there is wear of
material as metallic chips by micro-cutting on the over-
lay of D1 on a location away from the minimum film
Figure 9. SEM micrographs showing (a) grooves along the circumferential direction on Sn-based overlay of D1, (b) deep groove on
the composite overlay of D2, (c) wear track left by rolling abrasive particle on Bi overlay of D5 and (d) micro-scratches and grooves
on the exposed lining of D3.
Figure 10. SEM micrographs showing (a) micro-scratches and chip formation on the overlay of D1, (b) abraded overlay of D2 and (c)
random oriented scratches on the exposed lining of D4.
8D. W. GEBRETSADIK ET AL.
Figure 11. SEM micrographs showing (a) abrasive particle embedded after scratching composite overlay of D2, (b) abrasive particle
embedded after scratching Bi overlay of D4 (c) (c) embedded abrasive particle without scratching Bi overlay of D4 and (d) crushed
embedded particle.
Figure 12. Embedded particles at 30 rpm and corresponding elemental mapping (a) on Sn overlay of D1 (b) on composite overlay
of D2 and (c) on Bi overlay of D5.
TRIBOLOGY –MATERIALS, SURFACES & INTERFACES 9
thickness area at 200 rpm as shown in Figure 10(a). In
general, the micro-scratches and indents are less pro-
nounced in locations away from the minimum film
thickness area. However, along the circumferential
direction more micro-scratches and indents become
visible at the minimum film thickness area. The sur-
faces are also more abraded as shown in Figure 10(b)
for the overlay of D2 on locations close to the mini-
mum film thickness area at 30 rpm. In most of the
bearing materials, especially at 200 rpm, the overlay
is entirely worn out and the lining is exposed. The
exposed linings that are located around the minimum
film thickness area also suffer damages due to micro-
scratches and indents as shown in Figure 10(c).
Embedded particles are observed mainly on the
overlays of most of the bearing materials. Abrasive par-
ticles are embedded on the overlay in two different
ways. As shown in Figure 11(a,b), some abrasive par-
ticles first scratch the surface and then get embedded.
This is confirmed by the grooves they leave behind
them. Other abrasive particles get embedded without
scratching the surface as shown in Figure 11(c).
These abrasive particles are pressed deep into the over-
lay. This might happen as soon as the abrasive particles
enter the area with a gap comparable to their size they
are pressed against the overlay by the counter surface.
In the metallic overlays, there is pile up of material
around the embedded particles due to displace
material. Both types of embedded abrasive particles
are seen in all bearing materials except in D3.One
possible reason for the observed behaviour of the over-
lay of D3 could be its hardness. The overlay of D3 has a
higher hardness than the other metallic overlays. Fur-
thermore, it has a large amount of metallic MoS
2
par-
ticles in the PAI matrix which may also prevent
embedding of particles. Embedded particles are also
crushed as shown in Figure 11(d) and are observed
mainly in D4 and D5. This happens when a ceramic
material such as SiC, which has lower fracture tough-
ness than metallic alloys such as steel, is loaded
between the shaft surface and the intermediate layer
beneath the overlay.
Safely embedded abrasive particles on the Sn-based
overlay at 30 rpm and corresponding elemental map-
ping of the embedded SiC particles are shown in Figure
12(a). Such embedded particles are also observed on
the composite overlay of D2 (Figure 12(b)) and Bi over-
lay of D4 and D5. SiC Particles embedded on the Bi
overlay are shown in Figure 12(c). This is an indication
that as expected from its low hardness, the Bi overlay
can embed abrasive particles; however, its effectiveness
depends on both the size of the abrasive particles and
the thickness of the overlay since if the abrasive par-
ticles are bigger, part of the abrasive particles protrudes
from the surface and causes more damage on the shaft
specimen.
There were partially embedded particles on the
exposed lining of D3 as shown in Figure 13(a)
even though embedded particles were not observed
onitsoverlay.Thisisquitedifferent from the
embedded particles seen on most of the overlays.
The abrasive particles are not fully embedded since
the lining is relatively harder than the overlays.
Mostpartoftheabrasiveparticleisprotrudingout
of the surface and can act as hard asperity and
causes more damage on the counter surface. Similar
features are seen on the exposed lining of D4 and
D5.Figure 13(b) shows an abrasive particle that is
not fully embedded on the exposed lining of D5.
These particles can be big enough to rupture the
lubricant film and cause more damage on the rotat-
ing shaft. This may contribute to the high wear
observed on the steel counter surface sliding against
D3,D4 and D5, whoseliningsareexposedaftertests
at 200 rpm, and thereby the ability to safely embed
hard particles was limited.
4. Conclusions
Embeddability characteristics of some Pb-free engine
bearing materials have been investigated in the pres-
ence of SiC abrasive particles in engine oil and abrasive
wear mechanisms were studied.
Figure 13. SEM micrographs showing partially embedded abrasive particle (a) on the exposed lining of D3 (b) on exposed lining of
D5.
10 D. W. GEBRETSADIK ET AL.
.Third-body abrasive wear of the bearing surfaces
and steel counter surfaces is influenced by the mini-
mum film thickness.
.Abrasive wear was found to be higher on Bi-based
overlay and MoS
2
containing PAI-based overlay-
coated materials compared to the Sn-based overlay
and a PAI, Al and PTFE containing composite over-
lay-coated material.
.There is a direct relationship between the wear of
bearing materials and the wear of shaft specimens
tested at 200 rpm. Shaft specimens sliding against
bearing specimens that exhibited lower wear shows
lower wear and vice versa.
.Most of the damages on both the bearing and the
counter surface involve micro-scratches, ploughing,
micro cutting and indents.
.Abrasive particles are embedded on most of the
overlay surfaces except the PAI-based overlay con-
taining MoS
2
particles. There are no embedded
abrasive particles on the counter surface.
Acknowledgments
The authors would like to thank Scania CV AB for providing
test samples and financial support. The authors would also
like to thank Dr Mattias Berger from Scania CV AB for his
active interest in this work and his helpful feedback.
Disclosure statement
No potential conflict of interest was reported by the authors.
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