Research over NiTi wires.

1. INFLUENCE OF NITI WIRES SURFACE FINISHING IN THE FATIGUE LIFE
Tiberio Cesar Uchoa Matheus a
, William Marcos Muniz Menezesb
, Odair Dona Rigob
,
Leonardo Kyo Kabayamab
, Carlos Sergio da Costa Vianac
, Jorge Otubob
a
Universidade Federal de Pernambuco – UFPE
Departamento de Fisica, Av. Prof. Luiz Freire s/n, Cidade Universitaria, CEP: 50670-901,
Recife, Pernambuco, Brazil. e-mail: tcmatheus@yahoo.com.br
b
Instituto Tecnologico de Aeronautica – ITA, S. J. Campos, S. Paulo, Brazil.
c
Universidade Federal Fluminense – UFF, V. Redonda, R. Janeiro, Brazil.
Abstract:
The aim of this study was to evaluate the surface finishing of NiTi wires and its influence on
fatigue life. The wires were produced from two superelastics NiTi SMA with different
chemical compositions and final diameter of 1 mm thick. The wires were subjected to rotary
bending fatigue tests varying strain amplitude (1, 0.8 and 0.67%) and rotational speed (250
and 455 rpm). The wires surfaces were examined after fatigue tests with scanning electron
microscope. It was observed that the quality of wire surface finishing play an important role
on the rotary bending fatigue resistance justifying the scattering of some results.
Key words: NiTi wires, SMA, fatigue tests, surface finishing.
INTRODUCTION
The NiTi Shape Memory Alloy (SMA) wires can be used for aplication in several areas
such as medical and dentistry. At dentistry emphasize the orthodontics wires to correct dental
arc and the endodontics files for root canal treatment. Superelasticity property is used for
these applications(1,2)
.
For endodontics, the NiTi wires are machined to reach the final product that is the files.
The NiTi instruments have been introduced to facilitate the instrumentation of curved canals
and have the combination of good bioinercety, good mechanical strength, and specific
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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properties, such as the shape memory effect (SME) and superelasticity (SE). NiTi instrument
can be bend far more than stainless-steel instruments before exceeding their yield point(3)
.
This flexibility is an important property that allows the preparation of curved canals while
minimizing deviation(4)
. The major concern in the use of NiTi rotary instrument is the
possibility of the occurrence of unexpected failure. Usually, the file does not exhibit visible
indication that the failure will occur especially when misused(5,6)
. Therefore, the rotary fatigue
loading is an important aspect of mechanical properties study to the final product and also to
the wire from which the file is produced.
The fatigue life of a rotary endodontic instrument could be related to the degree to
which it is flexed when placed in a curved root canal, with greater flexures leading to a
shorter expected life(7,8)
. For NiTi SMA wires and endodontics files the fatigue loading can be
influenced by some factors such as surface defects and precipitates that act as stress
concentration points. The defects present as cracks, scratches and fold like surface
irregularities are from the machine processing and are associated with crack initiation
process(9,10)
. Through chemical and microstructural characterization of superelastics NiTi
alloys it is possible to detect the presence of TiC particles coming from carbon contamination
during melting stage. Usually the NiTi SMA alloys are produced by Vacuum Induction
Melting – VIM using graphite crucible that contaminates the bath with carbon and the degree
of contamination depend upon graphite crucible quality. Consequently the wire produced by
those materials will also contain different amount of carbon content and therefore different
amount of TiC particles influencing the resistance of the wire to rotary bending fatigue tests.
Aiming a future application as raw material to produce endodontics files, NiTi wires
produced at Instituto Tecnologico de Aeronautica were tested using RBF test. In this study it
was evaluated the surface finishing of NiTi wires and its influence on fatigue life. The
surfaces were examined after fatigue tests with scanning electron microscope.
EXPERIMENTAL PROCEDURE
In this study two NiTi alloys were used produced by VIM having atomic compositions
of Ti-49.81at%Ni and Ti-50.33at%Ni, respectively, alloy 01 and alloy 02. The ingots were
then hot forged and cold drawn down to 1 mm in diameter wire. The cold drawing was done
with 15% area reduction per pass with intermediate annealing at 773 K per 10 minutes. The
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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cold drawn wire was heat treated (straightened) by passing the wire through a tube furnace at
673 K under a longitudinal tension of 125 MPa for 2 minutes.
A Rotary Bending Fatigue (RBF) device was proposed and the details can be seen in
references(9,11)
. The Fig. 1 indicates the bent wire forced into rotation by the loading grip
connected to a driving motor. A low-friction nylon®
bearing was used to keep torsional load
of the wire at a minimum. The MoS2 (Molybdenium Bisulfate) lubricant was chosen for
minimizing the friction. The number of cycles, rotational speed and the time to failure of the
test is automatically controlled by computer.
They were used three curvature radius of 50, 62.5 and 75 mm resulting in three strain
amplitudes εa, of 1, 0.8 and 0.67%. The selected rotational speeds were 250 and 455 rpm.
Three samples with 1 mm in diameter wire were tested for each condition of curvature radius
and rotational speed. The tests were carried out at room temperature.
Fig. 1: NiTi wire mounted in RBF device
The samples were evaluated by MEV after RBF tests aiming to find at wires surface
possible irregularities that can justify the scattering results of the fatigue tests.
RESULTS AND DISCUSSION
The wires chemical compositions are shown in Table 1. The wire 2 wire is nickel rich
alloy with its oxygen and carbon contents given according to ASTM 2063-00 (carbon,
maximum 0.070 and oxygen, maximum 0.050 in wt%). The nickel content of wire 1 is about
0.52% lower than that of wire 2. According to nickel content, wire 1 should present higher
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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martensitic transformation temperature compared to wire 2 but as shown in Table 2, their
martensitic transformation temperatures are almost the same. This result comes from the very
high carbon content of wire 1 which is 0.188%, around 3.6 times higher than that of wire 2.
As shown in an earlier work, the presence of carbon, beside interfering on the mechanical
performance (as shown below), modifies the martensitic transformation temperature. Carbon
reacts with titanium forming TiC modifying the relation Ni/Ti and transforming the matrix
richer in nickel, therefore, lowering the martensitic transformation temperature (13)
. As shown
in Table 2, both wires are superelastic at room temperature.
Table 1: Nickel, Carbon and oxygen content in the wire 1 and wire 2 alloys.
Ni (at.%) C (wt.%) O (wt.%)
wire 1 49.81 0.188 0.036
wire 2 50.33 0.052 0.057
Table 2: Transformations temperatures, in K.
MS MP MF AS AP AF
wire 1 263.5 255.0 245.5 267.4 277.1 285.9
wire 2 275.8 268.1 256.8 279.0 290.0 330.2
The RBF tests varying strain amplitude (ε) and angular speed (ω) are shown in Table 3
and 4. Starting with ω= 255 rpm and wire 1, one can see that the number of cycles, Nf,
increased as strain amplitude decreased. The maximum individual value was 108,323 cycles.
Except the last data, the average performance of wire 2 was slightly better than that of wire 1
showing the influence of lower carbon content for the first. The influence of the increase in
rotational speed can be seen in Table 4 lowering the average Nf values for both wires. Again,
excepting last data for εa= 0.67%, the best performance of wire 2 is also demonstrated.
18º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais, 24 a 28 de Novembro de 2008, Porto de Galinhas, PE, Brasil.
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Table 3: RBF tests results for the rotational speed of 250 rpm.
250 rpm
wire 1 wire 2
Curvature
radius/strain
amplitude
Nf Average Nf Average
21,014 43,835
34,903 48,285
50 mm
1.00%
33,529
29,815
22,785
38,301
17,779 28,281
36,385 41,714
62.5 mm
0.80%
36,351
30,171
24,147
31,380
61,441 52,571
36,500 40,856
75 mm
0.67%
108,323
68,754
25,336
39,497
Table 4: RBF tests results for the rotational speed of 455 rpm.
455 rpm
wire 01 wire 02
Curvature
radius/strain
amplitude
Nf Average Nf Average
10,339 24,855
14,028 29,694
50 mm
1.00%
14,320
12,895
33,775
29,441
7,229 35,323
11,580 25,665
62.5 mm
0.80%
8,362
9,057
26,395
29,127
50,203 39,363
37,911 34,955
75 mm
0.67%
37,087
41,733
38,394
37,570
Figures 2 and 3 summarize the Nf values for both wires as a function of strain amplitude
and rotational speed. The influences of those parameters are small for wire 2 but one can see a
clear difference for wire 1. The Nf values decrease with increasing the strain amplitude and
rotational speed. The large scattering of the data seen in Table 3 and 4 is attributed to surface
quality of the wires as shown later. It was expected a much better performance of wire 2
compared to wire 1 what did not happen.
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0
10000
20000
30000
40000
50000
60000
70000
80000
90000
0.50 0.80 1.10
Nf
wire 1 (250 rpm)
wire 2 (250 rpm)
Fig 2: Average Nf values as a function of strain amplitude and rotational speed, ω= 250 rpm.
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
0.50 0.80 1.10
Nf
wire 1 (455 rpm)
wire 2 (455 rpm)
Fig. 3: Average Nf values as a function of strain amplitude and rotational speed, ω= 455 rpm.
Following, the irregularities found at the wires surface from machining process were
carried out to analyze by MEV. The worst result to fatigue tests was presented by wire 1 with
Nf= 7,229 cycles and the reason for that was a very large longitudinal crack nucleated at the
fracture as shown in Fig. 4 (a). For the wire 2, the worst result was 24,147 cycles with
intermediate strain amplitude and lower rotational speed, and fracture was nucleated at
folding like defect as can be clearly seen in Fig 4 (b). The lightest condition (lower strain
amplitude and lower rotational speed) with worst result for wire 2 wire is shown in Fig 5
showing scratch like defect (promoted by the wire drawing stage) presenting breakage with
25,336 cycles. According to the literature(10, 14)
the crack initiation processes is very
εa
εa
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dependent on the presence of high stress raisers like cracks, scratches and fold-like surface
irregularities which can be associated to large scattering of Nf .
In relation to the NiTi SMA endodontics files, the presence of groove patterns at the
cutting edge which comes from machining stage also can act as stress raisers during
loading(15)
. Therefore, the surface finishing of the raw NiTi wires and the endodontics files as
final product should be improved with no defects.
Fig. 4: Wire and fractures surfaces of: (a) wire 1 εa= 0.8%, ω= 455 rpm, Nf= 7,229 with
longitudinal cracks and (b) wire 2, εa= 0.8%, ω= 250 rpm, Nf= 24,147 with folding like
irregularities, SEM.
Fig. 5: wire 2 wire and fracture surfaces, εa= 0.67%, ω= 250 rpm, Nf= 25,336 cycles with
scratches like defects, SEM.
As can be seen in Fig. 6, the expected results for some strain amplitude and rotational
speed were presented by the wires with no surface defects withstanding more than 100,000
cycles for wire 1 at lowest strain amplitude and lowest rotational speed which is coherent.
Now, to the highest strain amplitude of 1% but with same rotational speed, the expected result
(a) (b)
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presented by wire 2 was 48,285 cycles at failure. The wires surfaces for both are smooth with
no defects.
Fig. 6: Wires surfaces with no defects: (a) VIM 1, εa= 0.67%, ω= 250 rpm, Nf= 108,323 and
(b) wire 2, εa= 1%, ω= 250 rpm, Nf= 48,285 (b), SEM.
Taking into account the carbon content, the wire 1 should have presented lower
performance when compared to wire 2, but this aspect is sometimes clouded by the surface
quality as shown in the results of this work. Therefore, to enhance the effect of carbon content
on the performance at RBF test, the quality of the wire surface should be improved. This is
the aim for the next step for producing wire with better quality.
CONCLUSIONS
In average the numbers of cycles to failure were higher for wire 2 wires with lower
carbon content as expected.
The presence of wire surface damages associated to mechanical processing stages play
a very important role for RBF resistance hiding the effect of carbon content.
The quality of the wire should be improved to better understand the real effect of
carbon on RBF performance.
(a) (b)
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ACKNOWLEDGMENTS
We would like to aknowledge FAPERJ (grant #E-26/100.130/06-DSC 10) for
scholarship to one of the authors and to FAPESP (grant #00/09730-1), FINEP (grant #
01.04.0255.00 – CT INFRA 03/2003; PROINFRA 01/05, Protocolo 153) for financial
support.
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