The mechanical properties of Nitinol 60, 60 w/o Ni, 40 w/oTi (55 a/o Ni, 45 a/o Ti) are sufficiently attractive to warrant its consideration as a lubricated spacecraft triboelement. The ability to lubricate Nitinol 60 by the oils usually used on spacecraft mechanisms--Pennzane 2001A, Krytox 143AC and Castrol 815Z - was experimentally determined. These oils were run in the boundary lubrication regime for Nitinol 60 balls running against a 440C steel counterface in the vacuum spiral orbit tribometer. Test results consisting of the coefficient of friction versus time (friction traces) and relative degradation rates are presented. Contrary to the inability to successfully lubricate other metal alloys with high titanium content, it was found that Nitinol 60 is able to be lubricated by these oils. Overall, the results presented here indicate that Nitinol 60 is a credible candidate material for spacecraft bearing applications

DellaCorte, Christopher

Glennon, Glenn

Hall, David R.

Noebe, Ronald D.

Pepper, Stephen V.

NASA Technical Reports Server

NITINOL 60 AS A MATERIAL FOR SPACECRAFT TRIBOELEMENTS
Stephen V. Pepper (1), Christopher DellaCorte(2), Ronald D. Noebe(3), David R. Hull(4), Glenn Glennon(5)
(1)NASA-Glenn Research Center, Cleveland, OH 44135, (United States) Email: stephen.v.pepper@nasa.gov
(2)NASA-Glenn Research Center, Cleveland, OH 44135, (United States) Email: christopher.dellacorte@nasa.gov
(3)NASA-Glenn Research Center, Cleveland, OH 44135, (United States) Email: ronald.d.noebe@nasa.gov
(4)NASA-Glenn Research Center, Cleveland, OH 44135, (United States) Email: david.r.hull@nasa.gov
(5)Abbott Ball Company, Inc., West Hartford, Connecticut 06133-0100, (United States) Email: gglennon@abbottball.com
ABSTRACT
The mechanical properties of Nitinol 60, 60 w/o Ni, 40
w/oTi (55 a/o Ni, 45 a/o Ti) are sufficiently attractive to
warrant its consideration as a lubricated spacecraft
triboelement. The ability to lubricate Nitinol 60 by the
oils usually used on spacecraft mechanisms—Pennzane
2001A, Krytox 143AC and Castrol 815Z - was
experimentally determined. These oils were run in the
boundary lubrication regime for Nitinol 60 balls running
against a 440C steel counterface in the vacuum spiral
orbit tribometer. Test results consisting of the coefficient
of friction versus time (friction traces) and relative
degradation rates are presented. Contrary to the inability
to successfully lubricate other metal alloys with high
titanium content, it was found that Nitinol 60 is able to
be lubricated by these oils. Overall, the results presented
here indicate that Nitinol 60 is a credible candidate
material for spacecraft bearing applications.
1. INTRODUCTION
Binary nickel-titanium (NiTi) alloys are in widespread
use in the medical and dental industries in applications
where their biocompatibility and unique superelastic or
Shape Memory Effect (SME) characteristics are readily
exploited [1,2]. In the present paper, we present evidence
that NiTi alloys can be tailored to avoid shape memory
or superelastic behavior and that such alloys display
excellent tribological properties under oil-lubricated
contact conditions.
Materials for high performance bearings, gears and other
mechanical components require a number of specific
properties and characteristics. Among these key
attributes are high strength and hardness, high thermal
conductivity, and the ability to be manufactured to very
high levels of precision with regards to final dimensions
and surface finish. In addition, excellent corrosion
resistance and good tribological properties are often of
importance especially for applications in extreme
environments.
Traditional tool steel based bearing materials, such as
M50 and 52100 enjoy widespread application due to
their high hardness, ease of manufacture and good
tribological properties. However, these alloys suffer
from corrosion attack if not protected and though
electrically conductive they are also highly magnetic. In
addition, when used as bearing rolling elements, their
high density leads to high centrifugal forces and limited
fatigue life. These considerations have driven the search
and development of alternate bearing and mechanical
component alloys, namely stainless steels and ceramics.
Stainless steels such as 440C are widely used in the
bearing and gear industry where corrosion resistance and
high hardness are required. These martensitic stainless
steels are reasonably low cost, easy to machine prior to
heat treatment and are dimensionally stable. When
prepared through vacuum melting processes, they
achieve very uniform, fully dense microstructures which
lead to fine surface finishes and good fatigue behavior.
Despite being referred to as stainless, however, the 400
series martensitic steels are prone to corrosion and are
more accurately referred to as corrosion-resistant alloys
rather than stainless. They are also highly magnetic
which can be problematic in certain applications.
Silicon nitride ceramics, on the other hand, are
essentially corrosion proof. They can be polished to very
fine surface roughness and are quite wear resistant.
Silicon nitride’s low density compared to steels also
makes it ideal for ultra high-speed applications because
lower centrifugal stresses result. These attributes make
silicon nitride the material of choice for high stiffness,
high load, high-speed bearings and for applications that
include corrosive conditions and aggressive sliding
environments. Such applications include bearings for gas
turbine hot sections, cryogenic oxidizer turbopumps and
components for diesel engine fuel injection systems.
Though nonmagnetic, silicon nitride is an electrical
insulator. It is also more expensive to manufacture than
steels owing to the complexity and cost of the high
temperature, high pressure powder metallurgy
processing required. Silicon nitride’s low thermal
expansion coefficient can present challenges in
applications involving wide temperature variations.
This paper assesses the feasibility of using NITINOL 60
(60 w/o Ni, 40 w/o Ti (55 a/o Ni, 45 a/o Ti)) for bearings
and mechanical components. NITINOL 60, when
appropriately heat-treated, does not exhibit SME
https://ntrs.nasa.gov/search.jsp?R=20090034488 2019-08-30T07:57:02+00:00Z
properties at normal ambient and anticipated use
temperatures and is dimensionally stable. It has high
hardness when properly heat-treated and yet can be
readily machined prior to final heat treatment. Like
silicon nitride, NITINOL 60 is nonmagnetic and is
intrinsically highly resistant to corrosion. Unlike
ceramics, NITINOL 60 is electrically conductive.
Historically, metallic alloys with high concentrations of
titanium are poor tribological materials in that they do
not respond well to lubrication by organic fluids [3-5].
For example, alloys such as Ti-6Al-4V exhibit galling
behavior in dynamic contacts even under conditions well
lubricated by oils and greases. There is presently no
understanding at the fundamental level of why metallic
titanium alloys perform so poorly under lubricated
tribological conditions. A consideration of NITINOL 60
as a tribological material thus requires an experimental
study of its performance in a lubricated configuration.
This paper reports the results of tests with oils
commonly used on spacecraft tribological systems -
Pennzane 2001A, Krytox 143AC and Castrol 815Z -
running in the boundary lubrication regime for a
NITINOL 60 ball against 440C steel counterfaces in the
vacuum spiral orbit tribometer. Test results consisting of
the coefficient of friction versus time (friction traces)
and relative degradation rates will be presented. Initial
results have been reported [6].
2. MATERIALS
The NITINOL 60 balls evaluated in this work were
manufactured via a high temperature proprietary powder
metallurgy process roughly similar to that described in
the literature [7]. The finished 12.5 mm diameter balls
were bright and shiny in appearance and resembled
conventional polished steel balls. Density was measured
at 6.71 g/cc, about 13% percent lower than steel, and
microhardness measurements indicated values in the
range of 58 to 62 on the Rockwell C scale in the
hardened condition. The elastic modulus is estimated to
be 114 GPa, comparable to that of the Ti-6Al-4V alloy.
Differential scanning calorimetry (DSC) indicated that
the final hardened balls are microstructurally stable
down to at least –100 °C. The 440C steel counterfaces
on which the NITINOL 60 balls ran in the spiral orbit
tribometer had Rockwell C hardness values about 60.
3. TRIBOMETER
For the tribological evaluations, NITINOL 60 balls were
lubricated with a thin film of the different oils and
subjected to a rolling-sliding contact lubricant wear life
test in a Spiral-Orbit-Tribometer (SOT), described
before [8,9]. The SOT, depicted in Fig. 1, is basically a
thrust bearing with one ball and flat races (plates). It may
be regarded as a simplified version of the usual angular
Figure 1. Components of the SOT. The top plate and
guide plate are stationary, while the bottom plate rotates
to drive the ball.
contact ball bearing. One of the plates is stationary and
the other rotates to drive the ball into an orbit that is an
opening spiral. The ball contacts a guide plate at the end
of each orbit, which forces the ball back into its initial
orbital radius. The ball then exhibits, for a given
coefficient of friction (CoF), a stable orbit, repeatedly
over-rolling the track on both flat race plates and guide
plate. The spiral’s pitch and the length of the contact on
the guide plate increases with the increase in the CoF. A
piezoelectric force transducer supporting the guide plate
senses the frictional force developed on the ball as it
slides on the rotating plate during the contact of the ball
with the guide plate. During this contact, the coefficient
of friction is obtained from this force and the load
imposed on the system. The tribometer is housed in a
stainless steel chamber that can be evacuated by a
turbomolecular pump to <2× 10 –8 torr.
The 440C plate specimens were initially clean and only
the ball was lubricated with about 25 µg of oil. Tests
were run at a load of 30 lb, corresponding to a Hertz
pressure of 1.06 GPa. Under these conditions, the system
is obviously operating in the boundary lubrication
regime. The characteristic of a test in which boundary
lubrication is operative is a low and constant coefficient
of friction for a number of orbits and then an eventual
transition to a much higher value of the CoF. This
eventual increase has been attributed to the consumption
of the organic lubricant by tribochemical attack on the
lubricant molecules in the ball/plate contact by the
bearing materials between which the lubricant is
captured. Each member of the ball/plate contact can
exhibit tribochemical activity that degrades molecular
structure, consuming the lubricant and leading to high
CoF in the absence of lubricant and the end of the test. In
a symmetric system, 440C steel/440C steel for example,
each member in the ball/plate contact contributes equally
to the tribochemical attack rate and a normalized lifetime
(orbits to failure per µg of lubricant initially applied) is
obtained. In the asymmetric system considered here
where members of the contact may have different
tribochemical activities, smaller or greater lifetimes may
occur. An extreme case is one in which one of the
partners exhibits such great tribochemical aggressiveness
that the lubricant does not survive thecontact at all and
failure of the lubrication is immediate, with no
observable lifetime. The other extreme case is that in
which one of the partners exhibits no tribochemical
activity at all and the test’s (longer) lifetime is
determined only by the activity of the other partner. The
goal in this report is to provide an assessment of the
ability to lubricate the NITINOL 60 via vacuum SOT
tests with the NITINOL 60/440C steel/oil system.
4. TRIBOLOGY RESULTS
Determination of the ability of NITINOL 60 to be
lubricated is best illustrated by first referring to a test of
Pennzane 2001A on specimens that are all 440C steel.
This is an oil/metal combination that is well established
as a system that can be successfully lubricated. The
friction trace of a typical test shown in Fig. 2 illustrates
the basic characteristic—a low constant CoF illustrated
for the first 250 orbits. The sensitivity of the SOT test to
the surface chemical constitution of the ball is also
illustrated in Fig. 2, which shows a test with a 440C ball
coated with a thin film of titanium. It is evident from the
high erratic CoF of the test with the titanium-coated ball
that the system is not operating in a lubricated manner.
This is attributed to the destruction of the Pennzane’s
molecular structure and attendant loss of lubrication
capability by the tribochemically aggressive titanium
film with which it is in contact. This test demonstrates
that only one partner of a tribological pair needs to be
tribochemically aggressive to prevent effective
lubrication. Such highly aggressive tribochemical
behavior was also observed with a NITINOL 60 ball
coated with titanium, so that the effect is not dependent
on the particular mechanical properties of the ball, but is
really of chemical origin. Fig. 2 thus illustrates the
extreme cases within which a test of the capability of
lubricating a particular material falls—successful
lubrication, indicated by low and constant CoF and lack
of lubrication, indicated by high and erratic CoF.
The friction trace of a test with a NITINOL 60 ball
lubricated with Pennzane 2001A and running on 440C
plates is also shown in Fig. 2 and is seen to exhibit the
same low and constant CoF typical of a successfully
lubricated contact as does the all-440C steel system.
Tests of all three oils with the NITINOL 60 balls
running on 440C steel plates exhibited this behavior in
the first few hundred orbits, indicating that NITINOL 60
is definitely not so tribochemically aggressive so as to
preclude its lubrication by these oils.
Figure 2. Friction traces for a 440C steel ball, a 440C
ball coated with a thin film of titanium and a NITINOL
60 ball, all lubricated with Pennzane 2001A and running
on 440C steel plates.
Figure 3. The complete friction traces of the NITINOL
60 ball and 440C steel ball lubricated with Pennzane
2001A in Figure 2.
The complete friction traces to failure of both the
NITINOL 60 ball and 440C steel ball lubricated with
Pennzane 2001A is shown in Fig. 3. The progressive
increase in CoF exhibited by NITINOL 60 is typical of
Pennzane in all tests with 440C steel. In fact the
similarity of friction traces with NITINOL 60 and 440C
steel balls was found for all three lubricants used here.
The normalized lifetime of a lubricant is an inverse
measure of its consumption rate and thus of the
tribochemical aggressiveness of a bearing material
combination. These lifetimes for tests for NITINOL 60
balls on 440C steel and also for tests with an all-52100
steel system already published [9] are given in Tab. 1. It
has been found that the lifetimes of the lubricants on
52100 steel are similar to those on 440C steel, thus
justifying the comparison to the results of NITINOL 60
on 440C steel.
Table 1. Normalized lifetime(orbits/ug) of 3 lubricants
for tests with NITINOL 60 on 440C steel (average of 3
tests with each lubricant) and for tests with 52100 steel
(average of 4 tests with each lubricant).
Lubricant NITINOL 60 on440C Steel
52100 Steel on
52100 Steel
Castrol 815Z 100 45
Krytox 143AC 68 73
Pennzane 2001A 1100 4100
The normalized lifetimes with the PFPE oils—Krytox
and Castrol—on the NITINOL 60 ball are rather similar
to those of the 52100 steel system, indicating that
NITINOL 60 is, generally, no more tribochemically
aggressive for those oils than either 440C or 52100 steel.
The lifetime of Pennzane on NITINOL 60 is an order of
magnitude greater than that of Castrol or Krytox, similar
to the ordering of lifetimes of these lubricants for the all-
steel system. However, the average lifetime of Pennzane
on the NITINOL 60 ball is noticably shorter than the
lifetime with steel, indicating a possibly more aggressive
tribochemistry of NITINOL 60 for this oil. The
differences observed between tests with and without
NITINOL 60 may be due to (a) differences in
tribochemistry (b) differences in the Hertz pressure at
which the tests were run or (c) simply due to statistical
scatter in the test values.
5. CONCLUSION
It is shown here by vacuum spiral orbit tribometry that
the bearing system consisting of a NITINOL 60 ball and
440C steel plates can be successfully lubricated by
Castrol, Krytox and Pennzane oils that are used on
spacecraft bearing systems. Such a hybrid system,
however, does not take full advantage of the unique
combination of properties of NITINOL 60, such as being
highly resistant to corrosion, nonmagnetic, electrically
conductive and less dense than steel. NITINOL 60 plates
for the SOT will soon be available. Tests with these oils
are planned for an all-NITINOL 60 system and the
results will be reported in the near future.
6. REFERENCES
1. Hodgson, D.E., Wu, M.H. & R.J. Biermann. (1990).
Shape Memory Alloys. Metals Handbook 2, 10th
edition, 897–902.
2. Shabalovskaya, S.A. (2002). Surface, Corrosion and
Biocompatibility Aspects of Nitinol as an
ImplantMaterial. Bio-Medical Materials and
Engineering, 12(1), 69–109.
3. Rabinowicz, E. (1954). Frictional Properties of
Titanium and Its Alloys. Met. Prog., 65(2), 107–
110.
4. Rabinowicz, E. & Kingsbury, E.P., (1955) Lubricants
for Titanium. Met. Prog. 67(5), 112–114.
5. Roberts, R.W. & and R.S. Owens, R.S. (1963)
Titanium Lubrication, Nature, 200, 357–358.
6. DellaCorte, C, Pepper, S.V., Noebe, R., Hull, D.R. &
G. Glennon, (2009) Intermetallic Nickel-Titanium
Alloys for Oil-Lubricated Bearing Applications.
NASA/TM—2009-215646.
7. McNeese, M.D., Lagoudas, D.C., & Pollock, T.C.
(2000). Processing of TiNi from ElementalPowders
by Hot Isostatic Pressing, Materials Science and
Engineering A280(2), 334–348.
8. Pepper, S.V. & Kingbury, E.P. (2003). Spiral Orbit
Tribometry —Part I: Description of the Tribometer.
Trib. Trans. 46(1), 57–64.
9. Pepper, S.V. & Kingbury, E.P. (2003). SpiralOrbit
Tribometry—Part II: Evaluation of Three Liquid
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Nitinol 60 as a Material For Spacecraft Triboelements

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