Materials Transactions, Vol. 44, No. 5 (2003) pp. 1024 to 1028
#2003 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Sliding Properties of Composite Sprayed Coating between Bronze Powder
and Solid Lubricant*1
Takeshi Kobayashi, Toru Maruyama and Tsutomu Yasuda*2
Department of Material Science and Engineering, Faculty of Engineering, Kansai University, Suita 564-8680, Japan
Intake of lead is harmful to the human body. Therefore, it is necessary to substitute other alloying elements for lead in Cu alloys. Using a
Cu–9.5 mass%Sn alloy powder and CaF2 , which has sliding properties equal to lead, a composite sprayed coating (by flame spraying) was
developed. The results were as follows. A composite sprayed coating with the desired characteristics was successfully produced. The area
fraction of the CaF2 layer in the composite coating increased with the increase in the blend ratio of CaF2 in the blended powder. The wear
resistance of the composite coating containing CaF2 was excellent. As a result it was determined that CaF2 in the composite coating was
effective as a sliding material substitute for lead.
(Received February 27, 2003; Accepted March 27, 2003)
Keywords: calcium fluoride, composite coating, composite spraying, copper base alloy, lead free material, powder flame spraying, sliding
property, solid lubricant, wear test
1.
Introduction
Copper base alloys such as bronze, lead bronze and
phosphor bronze have excellent sliding properties. These
materials contain small amounts of lead because of the
excellent self-lubricating properties of the lead. However, the
standard amount of lead in drinking water was limited to less
than 0.01 mg/L by the WHO in 1992, because the intake of
lead is harmful to the human body.1) Furthermore, in order to
obtain the certification of ISO14000 series, enterprise must
consider environmental safety manufacturing. Due to these
factors, a lead-free material with the same sliding properties
as lead was produced. This material consisted of a Cu alloy
spray coating in which the solid lubricant was dispersed in
the composite material. Generally, graphite is commonly
used as a solid lubricant. However, the composite thermal
spraying of graphite powder and Cu alloy powder is difficult
because the density of the graphite is much smaller than that
of the Cu alloy. In this study, calcium fluoride (CaF2 ) was
chosen as the substitute solid lubricant. A composite coating
that dispersed CaF2 in a lead free Cu–Sn alloy was created by
a composite spraying of both powders. This paper details the
microstructure and wear characteristics of the lead-free
composite sprayed coating.
2.
Experimental Method
2.1 CaF2 Characteristics
The characteristic of CaF2 used as solid lubricant are as
follow.2,3) (1) The lubrication properties of CaF2 at high
Table 1
2.2 Thermal sprayed materials
The Cu–9.5 mass%Sn alloy powder used in this experiment was produced using an argon gas atomization method.
The size of the powder particles were 45–150 mm, with most
of the particles falling in the 75–106 mm range. CaF2 powder,
which is used as a reagent, has a wide particle size
distribution (16 to 192 mm) even though 80% of the particles
are less than 75 mm. A disk of S45C (JIS standard) was used
as the thermal spraying substrate. Two disk sizes, 50 mm 50 mm 6 mm and 30 mm 3 mm, were used for the
structural analysis of the sprayed coating and wear testing,
respectively. Alumina grit was used for blasting (average
particle size, 328 mm; blasting was carried out for 20 seconds
from a distance of 100 mm).
2.3 Composite thermal spraying
Composite spraying conditions are shown in Table 1. The
blend ratio of CaF2 powder to Cu–Sn was varied (0, 10, 20
and 30 vol%) and thermal spraying was carried out from a
constant distance of 250 mm. The thermal spraying time was
Composite spraying conditions during powder flame spraying.
Gas flow rate, FR/m3 h1
Combustion gas pressure, PCOM /kPa
*1This
temperature, high pressure, and high speed are excellent. (2)
The melting point (1633 K) and boiling point (2724 K) of
CaF2 are high. (3) The density of CaF2 (3.18 gcm3 ) is
considerably higher than that of graphite (2.24 gcm3 ). (4)
CaF2 has good oxidation resistance and no hygroscopic
tendencies. (5) The crystal structure of CaF2 is cubic and has
a clear cleavage plane (111). In addition, CaF2 is easy to
obtain and inexpensive. These characteristics are essential for
blended powder flame spraying.
Carrier gas pressure,
Acetylene
Oxygen
Acetylene
Oxygen
PCAR /kPa
88
343
2.4
1.3
343
Paper was Presented at the Spring Meeting of the Japan Thermal
Spraying Society, held in Osaka, on June 20, 2002.
*2Graduate Student, Kansai University.
Sliding Properties of Composite Sprayed Coating between Bronze Powder and Solid Lubricant
20 seconds for on the structural analysis substrate, and 10
seconds for the wear testing substrate. The thermal spraying
distance for structural analysis was varied, 200, 250 and
300 mm, at a constant 30% CaF2 powder blend ratio.
2.4 Wear testing
The disk wear test ball is shown in Fig. 1. The ball
(6 mm, JIS, SUJ2) was set at 5.0 or 10.0 mm from the center
of the test disk (the wear test piece) and a load of 100 g was
applied to the ball. The frictional speed of the ball was
maintained at 0.05 ms1 and the wear test time was 20
Fig. 1 Schematic illustration of ball-on-disk wear test.
Fig. 2
1025
minutes for each position (5.0 mm and 10.0 mm). The wear
loss of the disk was measured after wear testing by examining
the wear streak on the disk and the ball trace with a
stereoscopic microscope. The depth of the wear streak on the
disk was also measured with a traceable surface roughness
meter.
3.
Experimental Results and Discussion
3.1 Structure of composite sprayed coating
The microstructure of a cross section of the composite
coating, at various blend ratios, is shown in Fig. 2. As the
CaF2 powder blend ratio in the mixed powder increased, the
thickness of the black colored layer and black spots
increased. The black layer and spots are believed to be
melted CaF2 , the oxide of the Cu based alloy, and pores.
However, it was difficult to distinguish these differences only
from a microstructure observation.
Next, the treatment of the microstructure image analysis
was carried out in order to measure the melted CaF2 area
fraction. The microstructure was magnified 200 times and
converted into a digital image using a CCD camera mounted
on a optical microscope. Then, the image was converted into
a digital black and white image and analysed on a computer.
Using special software, the black area as a fraction of the
entire area was calculated. The schematic illustration of the
relationship between the CaF2 blend ratio and net fraction of
the black area is shown in Fig. 3. The fraction of the melted
CaF2 area in the microstructure, fCa , is expressed by:
fCa ¼ fI f0 , (where fI and f0 are the total area fraction
of black part in the sprayed coating with and without CaF2
powder, respectively). The quantity of the oxide in the Cu
based alloy and the composite sprayed coating porosity were
assumed to be almost constant. Figure 4 shows the effects of
blend ratio on the area fraction of melted CaF2 and the yield
percentage of CaF2 in the composite sprayed coating. The
yield percentage of CaF2 , YCa , is expressed as a ratio of the
area fraction of the melted CaF2 to the CaF2 powder blend
ratio. Both the CaF2 area fraction and the yield percentage
Influence of CaF2 blend ratio on microstructure of composite coating.
T. Kobayashi, T. Maruyama and T. Yasuda
70
Fraction of dark
layer on coating
( f I)
Fraction of dark layer on
non-blended coating ( f0)
0
CaF2 Area Fraction, fCa (%)
f I - f 0 = f Ca
Area fraction of CaF2
( fCa)
: CaF2 fraction
: CaF2 yield
60
Spray distance : 250mm
15 Spray time : 20s
50
10
40
20
5
30
n:10
0
10
20
CaF2 Blend Ratio, BR(vol%)
CaF2 Blend Ratio, BR(vol%)
Fig. 3 Schematic diagram for calculating CaF2 area fraction.
20
Fig. 4 Effects of CaF2 blend ratio on CaF2 area fraction and CaF2 yield of
composite coating.
Influence of spray distance on microstructure of composite coating.
were directly proportional to the CaF2 blend ratio. The size
and density of the CaF2 powder were smaller than those of
the Cu base alloy powder. As mentioned above, the CaF2 fine
particle ratio was large. When the blend ratio of CaF2 is lower
than 10 vol%, the CaF2 powder is easily scattered. Thus, the
yield percentage is low. However, the yield percentage
increases when the blend ratio is increased from 20 to
30 vol%, since the CaF2 powder is easily melted by the latent
heat4) of the CaF2 despite the fact that it is lower than that of
Cu. This is probably due to the increase in the distribution
density of CaF2 powder. Melted CaF2 adheres to substrate
surface as the powder melts and welds itself to the surface.
The CaF2 yield percentage increases inversely to the
scattering powder ratio.
The microstructure of the cross section of the sprayed
coating when the spray distance changed is shown in Fig. 5.
The black area increases as the thermal spraying distance
increases. However, large differences are not recognizable
from the microstructure. Figure 6 shows the effects of the
22
CaF2 Area Fraction, fCa (%)
Fig. 5
30
20
80
: CaF2 area fraction
: CaF2 yield
n:10
18
60
16
14
12
40
CaF2 blended ratio: 30vol%
CaF2 Yield, YCa (%)
Fraction of Dark Layer (%)
25
CaF2 Yield, YCa (%)
1026
Spray time: 10s
10
200
250
Spray Distance, D/mm
300
20
Fig. 6 Effects of spray distance on CaF2 area fraction and CaF2 yield in
composite coating.
Sliding Properties of Composite Sprayed Coating between Bronze Powder and Solid Lubricant
spray distance on the CaF2 area fraction and yield percentage
in composite coatings, calculated using the image analysis
method shown in Fig. 3. The CaF2 area fraction decreases as
the spray distance increases up to 300 mm. The CaF2 area
fraction remains almost constant at 17–18% when the spray
distance is increased from 200 to 250 mm. The CaF2 yield
percentage also decreased at a spray distance of 300 mm.
This might be caused by flight stalls or the dispersion of the
CaF2 powder (because of its lower density and the high
proportion of fine particles).
Wear characteristics of the composite sprayed coating
Figure 7 shows the effects of CaF2 area fraction on the disk
surface streak width and steel ball damage after disk wear
testing. The width of the streak and steel ball damaged
decrease as the CaF2 area fraction in the sprayed coating
increased. The profile curve of the streak passing through the
disk surface is shown in Fig. 8. The maximum irregularity
height, Rmax , also decreased as the CaF2 area fraction
increased. In Fig. 8, the maximum irregularity height
indicates the depth of the streak. In addition, it is possible
to visualize the width of these streaks from the profile curve.
Figure 9(A) though (C) show the effects of CaF2 area fraction
on wear loss, streak depth, and streak width on the composite
coating, respectively. As shown in Fig. 9(A), wear loss
decreases as the CaF2 area fraction increases. Clearly, the
addition of CaF2 powder is very effective in reducing wear
loss. Moreover, from an environmental point of view, CaF2 is
an excellent solid lubricant substitute for lead. CaF2 has selflubricating5) characteristics comparable to lead. The sintered
ferrous alloy which contained 6 mass% CaF2 (about
15 vol%)6) displayed excellent wear resistance and has been
used for vehicle intake-valve seats. As shown in Fig. 9(B)
and (C), streak depth and width decreases as the CaF2 area
Rrotation Radius: 5mm
Level, L/ µm
3.2
1027
10
0
-10
-20
-30
10
0
-10
-20
-30
10
0
-10
-20
-30
10
0
-10
-20
-30
(A)
Area fraction of
CaF2, fCa: 0%
Rmax: 42.50 µm
(B)
fCa: 3.65%
Rmax: 34.90 µm
(C)
fCa: 8.47%
Rmax: 20.25 µm
fCa: 18.07%
(D)
Rmax: 16.25 µm
0
1000
Distance, D/ µm
2000
Fig. 8 Profile curve on disk surface of composite coating.
fraction increases, as was true for wear loss, as shown in Fig.
9(A). In addition, wear loss, streak depth and streak width
increased as the location of the ball from the center of disk
decreased (from 10 to 5 mm; at a constant rotation time and
friction speed). Calculations showed that the number of
rotations at 5 mm exceeded the number of rotations at 10 mm.
The CaF2 fraction varied inversely with steel ball damage, as
shown in Fig. 9(D), which means that CaF2 might also reduce
Fig. 7 Effects of CaF2 area fraction on streak width of disk surface and steel ball damage after wear test.
1028
T. Kobayashi, T. Maruyama and T. Yasuda
66
6
Radius of rotation: 5mm
Radius of rotation: 10mm
5
n=4
4
3
Streak Depth, Ds /µm
900
Ball Damage Area, fd/mm
2
40
Streak Width, Ws /µm
2
(B) Streak dipth of disk
62
0.8
58
0.6
54
0.4
50
n=4
30
Rotation radius: 5mm
Rotation radius: 10mm
Hardness
1.0
Hardness, HRB
(A) Wear loss of disk
Relative Wear Ratio, WR
Wear Loss, Wl/mg
7
0.2
0
20
5
10
15
CaF2 Area Fraction, fCa (%)
20
Fig. 10 Effects of CaF2 area fraction on relative wear ratio and hardness of
composite coating.
10
(C) Streak width of disk
800
while the tested hardness of the copper alloy matrix in
Fig. 10 is 96 of Hv (or 52 of HRB). This indicates that CaF2
is much harder than the copper base alloy.7)
n=4
700
600
500
4.
Conclusion
400
(D) Damage area of steel ball
2.5
n=1
2.0
1.5
Test time: 20min
0
5
10
15
CaF2 Area Fraction, fCa (%)
20
Fig. 9 Effects of CaF2 fraction on wear loss (A), streak depth (B), streak
width (C) of disk and damage area (D) of steel ball.
the wear loss of the steel ball. Steel ball damage increases as
the ball’s position from the center decreases for the same
reasons that disk wear loss, streak depth and streak width
increase.
Figure 10 shows the effects of CaF2 area fraction on the
relative wear ratio and the hardness of the composite sprayed
coating. The relative wear ratio, WR , is expressed as a ratio of
wear loss with CaF2 to wear loss without CaF2 . As the figure
indicates, the relative wear ratio decreases as the CaF2 area
fraction increases, while the hardness of sprayed coating
increases as the CaF2 area fraction increases. From these
results it was concluded that the decreasing relative wear
ratio resulted from the increase in hardness. Theoretically,
steel ball damage would be expected to increase if wear loss
decreased with increasing hardness. However experimental
results showed that steel ball damage decreased as hardness
increased. This proves that CaF2 has excellent self-lubricating properties. The actual hardness of CaF2 is 189 of Hv,
The feasibility of a composite sprayed coating between a
Cu–9.5 mass%Sn alloy and CaF2 by flame spraying was
examined, and the coating’s wear characteristics were
examined.
The results can be summarized as follows:
(1) A composite sprayed coating between a Cu–Sn alloy
and CaF2 by flame spraying is possible.
(2) The wear loss of composite sprayed coating decreases
as the CaF2 fraction increases. The composite sprayed
coating was also shown to have excellent self-lubricating properties.
(3) Composite sprayed coatings made with CaF2 instead of
lead were proven to have sliding properties equivalent
to lead without the environmentally damaging effects of
lead.
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