rticle history:eceived 3 August 2011eceived in revised form 14 August 2011ccepted 25 September 2011vailable online 1 October 2011
a b s t r a c t
The mechanical behavior of Sn–Pb–Sb Babbitt bearing alloys has been modified with 5 wt% copper andprocessed by melt spinning technique. Results on the rapid solidification structure and the ribbon inden-tation creep tests are discussed. The stress exponent values in the range 2.11–2.75 indicate that grainboundary sliding is the possible mechanism during room-temperature creep deformation of melt-spunbearing alloys.
The development of new advanced material is an importantctivity for continues progress in science and technology. Consid-rable research and development efforts are underway towards theevelopment of tin-based bearing alloys [1–10]. In 1839, Isaac Bab-itt received the first report for a white metal alloy that showedxcellent journal bearing properties. Since this, the name Babbittas been used for other alloys involves similar ingredients. Tin haslow coefficient of friction, which is the first consideration in itsse as a bearing material. Tin is structurally a weak metal and whensed in bearing applications, it is alloyed with antimony and copperor increasing hardness, tensile strength and fatigue resistance. Theery good frictional properties, reasonably good corrosion resis-ance and low cost of the tin–lead–antimony with small amountsf copper make them ideal for a wide range of applications.
The purpose of this paper is to discuss the copper effects inhe mechanical properties of rapidly solidified Sn–Pb–Sb bearinglloy from melt as engineering materials and often employed inpplications which involve loading-carrying characteristics, low
ear, good run-in properties and good emergency behavior in the
The experimental techniques utilized have been described indetails [11–14] and will be explained here in briefly. Two alloys ofcompositions Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu wereproduced by a single copper rollers melt-spinning technique. Melt-spun ribbons were between 80 �m and 150 �m thick and 4 mmwidth and generally have a smooth appearance. The solidificationfront velocity in the ribbon was estimated to be 30.4 m/s. If weassumed that the ribbon becomes completely solid at a distanced from the point of impingement of the molten stream, then thesolidification rate R is given by:
R = t × vd
(1)
where t is the ribbon’s thickness, v is the surface velocity ofthe surface wheel. Thus the solidification rate R is in the range10−5–10−4 m/s. Fully solid ribbons can be seen to leave the sur-face wheel after approximately a tenth of one revolution so thatd ≈ 50 mm [15]. Due to the small thickness of the samples, thecooling rates can be assumed to be very high. Techniques usedfor the subsequent characterization were X-ray diffraction analy-
sis, dynamic resonance method and creep indentation behavior ofused samples which was studied by Vickers micro hardness testerusing loads 10, 25 and 50 gf (gram-force) for dwell times up to 90 s.The analysis of the indentation creep was done by using Juhasz et al.
Fig. 1. X-ray diffraction pattern for Sn–30%Pb–10%Sb.
16] method to obtain the stress exponent (n) in the steady-statereep by the following expression:
=[
∂ ln d
∂ ln HV
]d
(2)
here HV is the Vickers hardness number, d is the indentationiagonal length, and d is the rate of variation in indentation diag-nal length. This implies that if d is plotted against HV on doubleogarithmic scale, a straight line would be obtained whose slopeives the stress exponent (n) [17] which can be used to identify theechanisms controlling the deformation process in this test.
. Results and discussion
.1. X-ray results
The diffraction patterns of rapidly solidified from meltn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu alloys, as shown inigs. 1 and 2, have lines corresponding to �-Sn, �-Pb, SbSn andu6Sn5 only. The d-spacing of tin-phase lines and lead-phase lines
n these alloys indicated that some small solubility of tin in leadr lead in tin has occurred. For rapidly quenched alloy containing
wt% Cu, extra lines of Cu6Sn5 and SbSn of intermetallic com-ounds were found in addition to the lines corresponding to �-Snnd �-Pb phases.
90807060504030200
200
400
600
800
1000
1200
Sn-25% Pb-10% Sb-5% Cu
Cu 6S
n 5 (1
21)
Cu 6S
n 5 (1
02)
sb s
n (2
08)
sb s
n (2
26)
sb s
n (2
26)
sb s
n (4
04)
sb s
n (2
20)
α−pb
(420
)
α−pb
(331
)
α−pb
(400
)
α−pb
(222
)
α−pb
(220
)α−pb
(200
)
β−sn
(431
)
β−sn
(312
)
β−sn
(411
)β−
sn (4
20)
β−sn
(321
)β−
sn (4
00)β−
sn (1
12)
β−sn
(301
)
β−sn
(211
)
β−sn
(101
)α−
pb (1
11)
sb s
n (2
02)
β−sn
(200
)
Inte
nsity
Cu 6S
n 5 (1
10)
2θ (degree)
Fig. 2. X-ray diffraction pattern for Sn–25%Pb–10%Sb–5%Cu.
ngineering A 530 (2011) 327–332
Tables 1 and 2 summarize the possible corresponding phases. Itseems quite clear that by the addition of 5 wt% Cu, only the stablephase remains in addition to the appearance of the equilibriumCu6Sn5 phase in reinforced.
Table 3 gives lattice parameters, axial ratio (c/a) and cell vol-ume for �-Sn and �-Pb phases spun alloys. We found that thecell volume decreased by the addition of 5 wt% Cu, because of theatomic diameter of copper (0.2560 nm) which is smaller than thatof lead (0.3499 nm) around room temperature. It is known that ifthe cell is expanded or contracted uniformly but still remains cubicas �-Pb phases; the diffraction lines merely shift their position butdo not increase in number, since no change in cell symmetry isinvolved. However, if the cubic cell is distorted along one axis, thenit becomes tetragonal, its symmetry decreases and more diffractionlines are formed. Kane et al. [18] reported that rapidly quenchingfrom the melt leads to the formation of metastable intermediatephase if certain physical factors such as valance electron concen-trations (VEC) favor their formation although others as bad atomicsize fit and presence of strongly bonded adjacent phases oppose itand tend to establish the equilibrium structure.
3.2. Elastic constants
Young’s modulus is one of the important characteristics thatreflect strongly the interaction and the bonding nature among con-stituent atoms [19]. The elastic constants of the metallic alloyswhich were fundamental physical properties especially for themechanical properties such as strength, plastic deformation andfracture were reported previously using single crystals [20]. A num-ber of researchers have given the empirical relationships to relateshear modulus (G), bulk modulus and the Young’s modulus (E) forvarious elements [21,22]. Poisson’s ratio, defined as the lateral con-traction per unit breadth divided by the longitudinal extension perunit length in simple tension, is reported to provide more infor-mation about the character of the bonding forces than any otherelastic coefficients [23,24]. Poisson’s ratio (�) is related to Young’smodulus (E), shear modulus (G) and bulk modulus by the followingequations:
G = E
2(� + 1)(3)
B = E
3(1 − 2�)(4)
The dynamic Young’s modulus (E) was calculated according to thefollowing equation:
E = 38.32dL4f0
2
t2(5)
where the density of the sample is d, L is the length of the sample,f0 is the resonance frequency of the sample and t is the sample’sthickness. Fig. 3 shows the resonance curves for melt spun alloysSn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu. The Poisson’s ratiosof Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu melt-spun alloyswere about 0.379 and 0.374, respectively. Table 4 shows Young’smodulus, shear modulus, bulk modulus and Poisson’s ratio for meltspun alloys Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu. Theaddition of 5 wt% Cu on the ternary alloy shows slightly decrease inthe internal friction while a great increase in the thermal diffusivityhas been measured as shown in Table 5.
The magnitude of elastic constants defect-free metal alloys isonly a function of the magnitude of the stiffness of the atomic bonds.
In real polycrystalline melt-spun alloys, other factors such as poros-ity, concentration of impurities, intergranular phases and alloyingelements may influence the magnitude of the elastic constants. Thenominal elastic modulus value for Sn and its alloys commonly is
M. Kamal et al. / Materials Science and Engineering A 530 (2011) 327–332 329
Table 1Phases observed and their details for Sn–30%Pb–10%Sb rapidly solidified from melt.
2� d-Spacing FWHM (h k l) Phase I/I0 Particle size
Table 3Lattice parameters, axial ratio (c/a) and cell volume for �-Sn and �-Pb phases in Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu rapidly solidified from melt.
Composition �-Sn �-Pb
a (A) c (A) c/a Volume, a2c (A3) a (A) Volume, a3 (A3)
330 M. Kamal et al. / Materials Science and Engineering A 530 (2011) 327–332
0
5
10
15
20
25
4035302520151050Frequency (HZ)
Am
plitu
de (c
m)
Sn 25%Pb 10%Sb 5%CuSn 30%Pb 10%Sb
Fig. 3. The resonance curves for melt spun alloys Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu.
Table 5Internal friction (Q−1) and thermal diffusivity (Dth) for melt spun alloysSn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu.
Composition Q−1 Dth (m2/s)
tfTm
mtspneifd[
ibtwsimcmcu5
3
etc[m
Sn-30%Pb-10%Sb
70
80
90
100
110
120
130
140
150
1009080706050403020100t (sec)
HV
(Mpa
)
Load 10 gfLoad 25 gfLoad 50 gf
Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu rapidly solidifiedfrom melt, respectively. It is found that the indentation diagonallength increases with the loading time and the applied load.
aken as 50 GPa but experimental values collected from literatureor pure tin and its alloys can vary between 16 and 55 GPa [25].he reasons for this wide variation in experimental value of elasticodulus have been investigated in this study.It should be pointed out that the measurements of the elastic
odulus is quite complicated for highly rate sensitive micro struc-urally evolving and highly sensitive to the effect of solute in theolution, porosity, the cross-section thickness, volume fraction ofrecipitates and may be sensitive to electron concentration. It isoted in this study that, by the addition of 5 wt% Cu, the increase inlastic modulus is about 73%. It is well known that Young’s moduluss determined by the bonding force among atoms, so this bondingorce is not only related to the crystal structure, but also to theistance among atoms and it can be affected by alloying additions26].
The unit-cell volume of both phases (Sn-phase and Pb-phase)s indicated in Table 3. It can be seen that the unit-cell volume ofoth phases (Sn-phase and Pb-phase) are expanded with the addi-ion of Cu content. This means that the increased unit-cell volumeith Cu content normally leads to decrease elastic modulus of the
tudied alloy in this study. But, it is found that Cu atoms tend tonteract much more strongly with Sn than Sb and Pb to form high
elting point intermetallic compound Cu6Sn5 than that of SnSbompound [6,27]. Therefore a small amount of copper is added toelt in this study to facilitate formation of yet another metallic
ompound Cu6Sn5. Hence, it is reported to increase the elastic mod-lus of the Sn–Pb–Sb melt-spun bearing alloy with the addition ofwt% Cu.
.3. Indentation creep results
It has been reported from Mahmudi et al. [28] and Rouminat al. [17] that there are three methods to analyze and to calculatehe stress exponent of indentation creep (n) in the steady-state
reep. It is found that the most suitable method is Juhasz et al.16] to investigate the power-law indentation creep behavior of
elt-spun Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu alloys
Fig. 4. The variation of hardness with time for Sn–30%Pb–10%Sb at loads 10, 25 and50 gf.
at room temperature by measuring the stress exponents of thesealloys through Juhasz et al. method of analysis.
Micro hardness indentation creep measurement is a form ofhigh-stress creep testing in which indentation deformation is afunction of dwell time indentation of a loaded indenter [17]. Thevariation of micro hardness (HV) with time was studied at roomtemperature for the same materials. Tests were carried out withloads of 10, 25 and 50 gf. Fig. 4 shows the variation of hardnesswith time for Sn–30%Pb–10%Sb at loads 10, 25 and 50 gf. It is seenthat HV decreases to an apparently equilibrium value. Results forSn–25%Pb–10%Sb–5%Cu at loads 10, 25 and 50 gf are shown in Fig. 5.It should be noted that HV decreased to equilibrium value. Tests onSn–25%Pb–10%Sb–5%Cu showed higher hardness than that with-out copper. The difference in hardness for tests at different loadsis within the used statistical error involved in measuring a meanhardness by making several indentation tests.
Juhasz et al. [16] carried out testes on lead-tin solder alloysusing Vickers testes to investigate the power-law indentation creepbehavior by measuring the stress exponent (n) in steady-state creepby using Eq. (2). This implies that if d is plotted against HV on dou-ble logarithmic scale, a straight line would be obtained whose slopegives the stress exponent (n).
The variation of indentation length with loading time under con-stant loads 10, 25 and 50 gf has been plotted in Figs. 6 and 7 for
Fig. 5. The variation of hardness with time for Sn–25%Pb–10%Sb–5%Cu at loads 10,25 and 50 gf.
M. Kamal et al. / Materials Science and Engineering A 530 (2011) 327–332 331
Sn-30%Pb-10%Sb
36.47
46.47
56.47
66.47
76.47
86.47
96.47
106.47
116.47
9585756555453525155t (sec)
d (μ
m)
Load 10 gfLoad 25 gfLoad 50 gf
Fig. 6. The variation of indentation length with loading time under constant loads10, 25 and 50 gf for Sn–30%Pb–10%Sb rapidly solidified from melt.
Sn-25%Pb-10%Sb-5%Cu
26.74
36.74
46.74
56.74
66.74
76.74
86.74
9585756555453525155t (sec)
d (μ
m)
Load 10 gfLoad 25 gfLoad 50 gf
F1
hfsaofstte
Fn
Sn-25%Pb-10%Sb-5%Cu
-2.9
-2.7
-2.5
-2.3
-2.1
-1.9
-1.7
-1.5
-1.3
-1.1
-0.9
5.435.385.335.285.235.185.135.085.034.98Ln (HV)
Ln (d
. )
Load 10 gfLoad 25 gfLoad 50 gf
Fig. 9. Log–log plot of the rate of diagonal variation against the Vickers hardnessnumbers for Sn–25%Pb–10%Sb–5%Cu.
Table 6The averages stress exponent value (n) for Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu.
Composition Stress exponent (n)
ig. 7. The variation of indentation length with loading time under constant loads0, 25 and 50 gf for Sn–25%Pb–10%Sb–5%Cu rapidly solidified from melt.
The rate of diagonal variation is plotted against the Vickersardness number on a double logarithmic scale as shown in Fig. 8
or Sn–30%Pb–10%Sb and in Fig. 9 for Sn–25%Pb–10%Sb–5%Cu. Atraight line of slope equal (n) is obtained for each melt-spunlloy and load. The average stress exponent values obtained fromur experimental method were found to be about 2.11 and 2.75or Sn–30%Pb–10%Sb and Sn–25%Pb–10%Sb–5%Cu, respectively as
hown in Table 6. It is indicated that grain boundary sliding ishe possible mechanism during room temperature creep deforma-ion of the melt-spun Sn–30%Pb–10%Sb. However values of stressxponent equal to 2.75 or approximately 3 may suggest that it
Sn-30%Pb-10%Sb
-2.5
-2
-1.5
-1
-0.5
0
4.754.74.654.64.554.54.454.44.354.3Ln (HV)
ln (d
. )
Load 10 gf
Load 25 gf
Load 50 gf
ig. 8. Log–log plot of the rate of diagonal variation against the Vickers hardnessumbers for Sn–30%Pb–10%Sb.
Sn–30%Pb–10%Sb 2.11Sn–25%Pb–10%Sb–5%Cu 2.75
is associated with the viscous glide deformation mechanism dur-ing room temperature in the melt-spun Sn–25%Pb–10%Sb–5%Cu.According to the power law creep, an increase in stress exponentfor Sn–25%Pb–10%Sb–5%Cu would result in a decrease in creep ratedue to an increase in yield strength [29]. Therefore the melt-spunSn–25%Pb–10%Sb–5%Cu with higher (n) values is more resistant toindentation creep compared to the melt-spun Sn–30%Pb–10%Sb. Itis also been demonstrated that grains slide along one another attheir boundaries. This results when dislocations produced duringcreep accumulate at grain boundaries.
4. Conclusions
Based on observations described in the present paper, the fol-lowing conclusions may be formulated.
1. For rapidly quenched alloy containing 5 wt% Cu, extra lines ofCu6Sn5 and SbSn of intermetallic compounds were found inaddition to the lines corresponding to �-Sn and �-Pb phases.
2. Rapidly quenching from the melt leads to the formation ofmetastable intermediate phase if certain physical factors suchas valance electron concentrations (VEC) favor their formationalthough others as bad atomic size fit and presence of stronglybonded adjacent phases oppose it and tend to establish the equi-librium structure.
3. It can be seen that the unit-cell volume of both phases (Sn-phaseand Pb-phase) are expanded with the addition of Cu content. Thismeans that the increased unit-cell volume with Cu content nor-mally leads to decrease elastic modulus of the studied alloy inthis study. But, it is found that Cu atoms tend to interact muchmore strongly with Sn than Sb and Pb to form high melting pointintermetallic compound Cu6Sn5 than that of SnSb compound.Therefore a small amount of copper is added to melt in thisstudy to facilitate formation of yet another metallic compoundCu6Sn5. Hence, it is reported to increase the elastic modulus ofthe Sn–Pb–Sb melt-spun bearing alloy with the addition of 5 wt%Cu.
4. According to the power law creep, an increase in stress expo-nent for Sn–25%Pb–10%Sb–5%Cu would result in a decreasein creep rate due to an increase in yield strength. Thereforethe melt-spun Sn–25%Pb–10%Sb–5%Cu with higher (n) values is
3 and E
R
[
[[[
[[[[[[
[[[[[[[
32 M. Kamal et al. / Materials Science
more resistant to indentation creep compared to the melt-spunSn–30%Pb–10%Sb. It is also been demonstrated that grains slidealong one another at their boundaries. This results when dislo-cations produced during creep accumulate at grain boundaries.
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