Wear, 151 (1991) 203-217
Tribological
Kenneth
Eastman
(Received
203
properties
of titanium
alloys*
G. Budinski
Kodak
Company,
Building
23, 5th Floor, Kodak
Park,
Rochester,
NY I4650
(U.S.A.)
April 30, 1991)
Abstract
Titanium is often the most cost-effective
corrosion-resistant
material for applications
in
the chemical process industry that involve resistance
to halides. Unfortunately,
there are
usually some components
in these systems that involve relative sliding-titanium
tribosystems
- e.g. bolts and nuts, valves, piping connections,
etc.
Titanium and its alloys have had a reputation for poor tribocharacteristics,
but detailed
information
on suitable counterfaces
and wear specifics is scarce. This paper summarizes
a study conducted
on the two most widely used titanium alloys, Grade 2 commercially
pure titanium and the age-hardenable
Ti6Al4V. Dry sand-rubber
wheel tests were conducted
to assess abrasion resistance;
fretting, galling and reciprocating
pin-on-plane
tests were
conducted
to determine
if there is a “best” counterface
for these two titanium alloys.
The test results are distilled into recommendations
for use of titanium alloys in
tribosystems
in the chemical process industry. Both alloys have poor abrasion resistance.
Grade 2 pure titanium should be avoided in all titanium
tribosystems
and there are
preferred
counterfaces
for the Ti6Al4V alloy, but the best metal-to-metal
wear resistance
is obtained when the alloy is anodized and coated with a dry film lubricant.
1. Introduction
In the past decade,
titanium
alloys have been adopted
as standard
materials
of
construction
for pipes, fittings, valves and similar equipment
in the chemical
process
industry. Titanium
alloys are often the best choices for handling halides and bleaches.
They have far better resistance
to these environments
than the best 300 series stainless
steels. The most popular alloys for this type of service are pure titanium (ASTM B348
Grade 2) and the Ti6Al4V alloy (ASTM B348 Grade 5) [l]. The Grade 2 pure titanium
is one of four grades of “pure” titanium
which differ in impurity level and strength;
the 6A14V alloy can be solution treated
and age hardened.
The Rockwell C hardness
is normally
30-36 HRC in the annealed
condition
and may increase
to 39 HRC on
age hardening.
The hardness
is about 36 HRC in the solution-treated
and over-aged
condition.
Pieces of equipment
made from these alloys do their job from the corrosion
standpoint
but occasionally
the alloys must be used in parts of equipment
that involve
sliding of titanium
on other counterfaces
- titanium
tribosystems.
Titanium
is an
extremely
reactive metal and has a reputation
for poor tribological
properties
[2-51.
For this reason, machine
designers
have tried to avoid its use in sliding systems, but
*Paper presented
at the International
U.S.A., April 7-11, 1991.
0043-1648/91/$3.50
Conference
on Wear of Materials,
0 1991 -
Elsevier
Orlando,
FL,
Sequoia, Lausanne
204
there are times when this is not possible. For example, it is often necessary to use
titanium fasteners to assemble titanium components. Periodic disassembly and reassembly
of fasteners causes tapped holes in extremely expensive equipment to wear out. This
eventually causes components to become useless. There are other unavoidable titanium
tribosystems where similar wear problems can cause loss of extremely expensive (and
critical) equipment.
It is the purpose of this paper to present the results of a comprehensive laboratory
study on the tribological properties of two titanium alloys (Grade 2 and Grade 5).
The objective of this study was fourfold: (I) to determine the severity of the problem,
(2) to determine
if there are any counterfaces
that mitigate wear of the titanium
member, (3) to determine if surface treatments alter wear effects and (4) to determine
the abrasion resistance of titanium. Using this information, we make recommendations
to machine designers on how to prevent potential wear problems when titanium
tribosystems
are unavoidably
present in chemical-handling
machines. The overall
objective of this study is to prevent loss of equipment or degradation of product quality
due to wear failures.
Titanium may be subject to different forms of wear in chemical process systems;
the plan of this study was to assess the tribocharacteristics
of titanium in the three
wear modes that are likely to occur in these systems: abrasion, metal-to-metal
wear
and fretting wear. We will present the results that we obtained in this study in galling,
abrasion, metal-to-metal
and fretting tests. We will conclude with design recommendations for the use of titanium in tribosystems.
2. Galling
studies
In a 1989 unpublished
study we conducted tests to find an appropriate material
to use for nuts on titanium bolts. There was a concern that galling would occur,
making disassembly difficult or impossible. In test on actual bolts and nuts it was
determined
that titanium bolts did not gall with a titanium nut or with nuts made
from hard and soft stainless steels (type 316, 90 HRE3; 17-4, 43 HRC, 440 C, 58 HRC).
However, after about 20 tightenings of a nut on a bolt 12.5 mm in diameter the
threads on the titanium bolts were almost completely removed. It appeared that
titanium does not gall like soft stainless steels. When soft stainless steel slides on
itself under high normal forces, it forms macroscopic excrescences that in turn can
lead to seizure. The titanium alloy that we tested, 6Al4V alloy, wore by adhesive
transfer of Aat platelets to the mating material (Fig. l(a)). This type of transfer has
been noted by other investigators [63 and is probably due to the reactivity of titanium
when surface fihns have been removed by mechanical action. The material removal
rate was higher than can be tolerated on bolts that are reused. As a result of this
study we recommend a PH stainless steel counterface (43 HRC) coupled with surface
lubrication (polytetrafluoroethylene
(PTFE) or MO& coatings) to mitigate thread wear.
In this study we also investigated the effect of heat treatment
on the galling
characteristics
of the Ti6Al4V alloy. This alloy is received in the annealed condition
(30-36 HRC). Most parts are made from this alloy in this condition; some parts receive
a stress relief at 760 “C (as part of the fabrication process) and occasionally, where
maximum strength is required, the Ti6A14V alloy is solution treated and aged at 538
“C (35-39 HRC). One technique that has been used to mitigate wear problems on
heat-treated alloys is to perform the heat treatment in air and to leave the oxide that
is formed on the titanium in place to act as a wear adjuvant. Galling tests were
Fig. 1. (a) Typical adhesive transfer of titanium: Ti6Al4V vs. itself (original magnification, X40).
(b) Large excrescence formed in tantalum rider after rotation on Ti6Al4V at 14 MPa apparent
pressure.
on the following
couples:
(1) annealed
Ti6A14V stress relieved at 760 “C
vs. itself (oxide removed); (2) annealed pure titanium stress relieved at 538 “C (oxide
on contact surfaces) vs. itself; (3) annealed Ti6A14V stress relieved at 538 “C (oxide
on contact surfaces) vs. itself; (4) solution-treated
and aged Ti6A14V vs. itself (oxide
removed); (5) annealed pure titanium vs. Ti6A14V stress relieved at 760 “C (oxide
removed).
The ASTM G98-89 galling test [7] was used with modified sample geometry. In
this test the end of a 12.5 mm pin is rotated through 360” on a flat counterface under
increasing normal forces and both surfaces are visually examined for galling. In our
studies the upper sample was an annular ring with a contact area of 0.29 cm’. Test
surfaces were freshly abraded to produce a random scratch pattern and a roughness
R, of 0.1-0.5 pm. The criterion for galling was the formation of macroscopic excrescences.
Samples were tested with increasing normal forces until they galled or until we reached
the capacity of the machine, 3.5 X lo4 N (276 MPa). Typical load increments were 4
kN. The criterion for galling was the ASTM G98 definition of galling: “in Tribology,
a severe form of wear characterized by localized, macroscopic material transfer, removal
or formation of surface protrusions when two solid surfaces experience relative sliding
under load”. Surfaces were examined after rotation of the annular ring; if galling did
not occur, the samples were refinished and retested. The surfaces were used as abraded
and pure dry air was used to remove fine grinding detritus. Three replicates of each
couple were tested. The results were essentially the same for all these couples; the
threshold galling stress was over 276 MPa and there was no diminution of the tendency
for wear and transfer. The coefficient of friction was higher on the pure titanium
couples (Fig. 2) and definitely shows significant deformation
during testing.
In an effort to determine if there is some material that will run against titanium
without the wear and transfer problem, we tested a variety of materials against solutiontreated and aged 6A14V in the same ASTM G98 galling test. The results shown in
Table 1 indicate that titanium only galled against one material, tantalum, out of the
performed
206
COEFFICIENT
OF FRICTION
1
SO
-
PURE
+
TieAl4V
160
100
APPARENT
Ti VS PURE
(HT)
Ti
VS Ti6Al4V
200
260
CONTACT STRESS (MPa)
ItiT)
f
TiBAI4V
-4
PURE TI VS TIEAl4V
VS TI6Al4V
Fig. 2. Coefficient of friction of various titanium couples measured in the ASTM G98 test for
threshold galling stress (galling did not occur up to the maximum stress obtainable in the test
rig, 276 MPa).
TABLE
1
Friction coefficients and threshold galling stresses for various titanium
ASTM G98 modified with an annular upper specimen
Couple
Stress
(MPa)
Friction”
couples
CL,
/&
Ti6A14V vs.
> 276
> 276
> 276
> 276
> 276
> 276
> 276
> 14
> 276
> 276
> 276
0.36
0.35
0.38
0.38
0.38
0.37
0.36
0.53
0.44
0.36
0.45
0.31
0.29
0.29
0.30
0.31
0.31
0.31
0.53
0.37
0.27
0.36
Ti transfer
Ti transfer
Ti transfer
Ti transfer
Ti transfer
Ti Transfer
Ti transfer
Galled
Ti transfer
Ti transfer
Ti transfer
Pure Ti vs.
Itself
Ti6Al4V
17-4
> 276
> 276
> 276
0.47
0.43
0.48
0.40
0.36
0.48
Transfer
Transfer
Transfer
apparent
stress relieved
as per
Comments
17-4
Stellite 12
Waukesha 88
Ferralium 25.5
Nitronic 60
Gall Tough
Itself
Tantalum
440 “C
Al bronze
Stellite 6
aAt maximum
tested
and deformation
and deformation
and deformation
at 760 “C.
15 counterfaces
tested. It galled at a threshold
stress of only 14 MPa. The threshold
galling stress for all the other couples was above the 276 MPa capacity of our test
machine.
All the couples
showed
significant
titanium
wear and the pure titanium
showed severe deformation
in addition to high wear. We tested a variety of lubricants
to determine
if any were effective in preventing
the titanium wear and transfer.
Heat
207
treating oxides by themselves was not effective but several proprietary anodizing-lubricant
treatments did prevent transfer in the galling tests.
In summary, Grade 2 pure titanium and the 6Al4V alloy do not gall (formation
of macroscopic excrescences) when mated against each other in any condition of heat
treatment, but severe wear and transfer do occur. In the case of the pure alloy the
plastic deformation
that occurs makes this alloy totally inappropriate
for use in
tribosystems. The Ti6Al4V alloy does not gall against a wide variety of other corrosionresistant materials, but the titanium wear and transfer are such that these systems
should not be used without a wear adjuvant. Suitable palliative treatments are: anodizing
plus resin-bonded
MoS, or PTPE; fluorocarbon
enamels; MO&--Pb-Cu or similar
antiseize compounds. Grade 2 pure titanium is totally unsatisfactory even for unlubricated
metal-to-metal
sliding and its use should be avoided.
3. Abrasion
Tests
A widely used test for assessing abrasion resistance of metals is the ASTM G65
dry sand-rubber
wheel abrasion test [8]. This test produces abrasion on the large face
of a 12 X 25 X 75 mm3 sample cut from the test material. The sample is pressed against
a rubber-tired
wheel and 50-70-mesh silica sand is metered between the specimen
and the rotating wheel. After a prescribed number of wheel revolutions the wear on
the sample is assessed by gravimetric techniques and a volume loss is calculated. In
this study we performed three replicate tests on Grade 2 pure titanium and on the
6Al4V alloy in the age-hardened
condition. The test results are compared with a
variety of other materials in Fig. 3. These results indicate that the Grade 2 pure
titanium has better abrasion resistance than the harder Ti6Al4V alloy. However, neither
grade had abrasion resistance comparable to soft stainless steel. The results obtained
on pure titanium appear to be incongruous
with the Archard equation [9], which
predicts abrasion resistance to be inversely proportional
to material hardness:
MATERIAL IDENTIFICATION
CEMENTED
CARBIDE
STELLITE
I, ,d HRC,
l
(66 HRC,
1016 ’
D2 TOOL STEEL ’
1060 STEEL*
17-4 STAINLESS STEEL’
316 STAINLESS STEEL’
TI GRADE 2
TIBAUV
ALUMINUM 6061 T6
_I
0
* FROM ASTM
REFERENCE
200
400 600 600 100012001400
VOLUME LOSS (mm)
066
Fig. 3. Abrasion rates of various materials compared with Grade 2 pure titanium and Ti6Al4V
(ASTM G65 test, procedure A).
208
where W is the wear rate, K is the wear coefficient, D is the sliding distance, P is
the load (normal force), h is the penetration
hardness and LYis the included angle
of abrasive particles.
We did not perform additional experiments to explain these results, but metallographic studies on the wear scars suggest that material removal occurs by different
mechanisms in the two materials. As shown in Fig. 4, the Grade 2 pure titanium wear
scars had a lapped appearance and the Ti6Al4V scars showed a pattern suggesting
hard and soft regions in the material. Neither material showed normal abrasion patterns.
This type of wear scar was also obtained in abrasion tests vs. SC and A1203 by
Hutchings and Mercer [lo]. Ferrous materials usually have a scar consisting of fine
scratches - similar to a ground surface. It is possible to hypothesize that the mottled
pattern in the Ti6Al4V alloy is from microstructural
heterogeneities.
It is a two-phase
alloy, a mixture of stable a: phase and metastable p phase. The pure alloy is singlephase a. The fact that the pure titanium had better abrasion resistance than the
harder (36 HRC) 6A14V alloy could be attributed to differences in the behavior of
the sand in going through the rubber tire-sample
interface. We have seen similar
results in copper alloys - pure copper (60 HRB) had better abrasion resistance in
this test than 1% Be-Cu at 42 HRC. There was strong evidence that the round and
subangular sand used in this test rolls through the sample-tire nip rather than becoming
fixed in the rubber. The way that this test produces abrasion is that grains of sand
become embedded in the rubber and are momentarily fixed to the rubber wheel so
that they are capable of plowing a furrow or scratch in the specimen. Rolling grains
of sand do not produce abrasion. This is probably the explanation
of the lack of
correlation with the Archard equation. The adhesion of the sand to the pure titanium
is greater than to the alloy and this produces rolling grits rather than fixed grits, hence
reduced abrasion.
Irrespective of the differences between the pure and 6Al4V titanium alloy, the
important finding of this test is that both grades of titanium have poor abrasion
resistance. The abrasion rates of both grades were at least 15 times the abrasion rates
that would be obtained on a hardened tool steel such as Type D2. These data suggest
that titanium should be avoided for applications involving low stress abrasion from
(4
0)
Fig. 4. Wear scars on Ti6A14V and Grade 2 pure titanium after ASTM G65 sand abrasion test,
procedure A (original magnification, x 100): (a) Grade 2 - note lapped appearance; (b) Ti6Al4V
- note discontinuous removal of material.
209
hard particles. Ayers [ll] verified the poor abrasion characteristics
of titanium and
proposed surface modification by injection of carbide particles by laser cladding. This
technique is not in wide use and is not ready for use in everyday machine design.
4. Fretting damage
Titanium components used in chemical process systems are often intended to be
non-sliding systems, but many non-sliding parts can experience fretting motion in bolted
assemblies. Temperature
fluctuations or machine vibrations produce oscillatory relative
motion in the amplitude range 10-300 pm where fretting damage occurs.
In this study we tested titanium (Ti6Al4V) under fretting conditions against a
variety of couples. We used the test rig shown schematically in Fig. 5 and the following
test conditions: oscillatory motion, amplitude 50 pm, frequency 3.3 Hz, normal force
30 N, test duration 3 x 10’ cycles.
Hemispherical
riders with an end radius of 6.25 mm were made from Ti6Al4V
and oscillated against flat samples of the following materials: (1) itself (36 HRC); (2)
cemented carbide (WC-Co);
(3) 17-4 stainless steel (43 HRC); (4) 440C stainless
steel (57 HRC); (5) Stellite 6B @* (43 HRC); (6) chromium plate (on 1020 steel); (7)
316 stainless steel (90 HRB); (8) itself plus anodize plus MO&
The test is described in detail in a previous publication [12], but essentially damage
to the ball and counterface was assessed by profilometry. The scar depths were measured
in both members and a volume loss was calculated from these data.
IIILOCKJ
Fig. 5. Schematic diagram of fretting test rig. Test conditions: amplitude 50 pm, frequency 3.3
Hz, normal force 30 N, test duration 25 h.
*Registered trademark of Stoody Deloro Stellite Co.
210
The test results presented in Fig. 6 suggest that this alloy is quite susceptible to
damage self-mated and mated against stainless steels. Photomicrographs
of the couples
with the most damage (Ti6Al4V vs. itself) and the least damage (Ti6A14V vs. Stellite
6B) are presented in Figs. 7 and 8 respectively.
The damage is substantial even against a hard stainless steel counterface (440C
stainless steel). Of the eight couples tested, the lowest system damage was obtained
with a couple of Ti6Al4V vs. SteIlite 6B. The anodized and lubricated couple did not
wear as well as the bare Steliite-titanium
couple. However, most of the system wear
was in the anodize coating; the coating was 5 pm thick and did not penetrate. Thus
the wear on the titanium was low. Essentially, titanium vs. anodized and lubricated
36
30
26
20
VOLUME LO88
IS
IO
* 10 EXP-a
m
RIDER
6
0
6
10
VOLUME
hn?)
m
16
LOW
20
26
. 10 EXP-8
30
316
htl
COUNTERFACE
Fig. 6. Results of fretting corrosion tests on Ti6A14V vs. various counterfaces. Titanium hemisphere
rider is oscillated on a flat counterface. Data are averages for three tests.
Fig. 7. Wear scars on (a} Ti6Ai4V rider and (b) counterface
magnification, X 50).
after 25 h fretting
test (original
(4
(b)
Fig. 8. Wear scars on (a) titanium
(original magnification,
X 50).
VOLUME LOSS * EXP-2
rider and (b) Stellite 6B counterface
mm3
4
0
after 25 h fretting test
I
I
0
160
HO
COUNTERFACE
-
COUNTERFACE
Fig. 9. Effect of hardness
660
HARDNESS
on fretting
-
660
(kg/mm’)
RIDER
damage: Ti6Al4V rider vs. various stainless
steels.
is a reasonable
couple if the lubricant can be tolerated. In a similar study
Ball [12] concluded that a polyimide dry film lubricant was somewhat better than
MO& types of lubricants.
These results suggest that wherever titanium may be subject to fretting damage,
a suitable counter-face would be Stellite 6B. We tested wrought Stellite but we anticipate
that the same or similar Stellite alloys applied as a hardfacing deposit would be
satisfactory. The test data also suggest that the use of a hardened counter-face helps
(Fig. 9). If the use of Stellite is not possible, the anodized and lubricated titanium
would be the second choice. Additional
counterface
options are cemented carbide,
chromium plating or hardened 440C stainless steel (58 HRC). Komanduri and Read
[13] determined that the low binder WC-Co is a more suitable counterface for titanium
than the high cobalt grades. The grade that we tested had 7% cobalt. A lower cobalt
titanium
212
grade may have produced better results. An easy-to-use palliative would be to chromium
plate the titanium counterface (titanium cannot be chromium plated by conventional
means). This is probably the minimum protection that should be taken to prevent
fretting damage. In summary, unlubricated
self-mated titanium couples should be
avoided in systems where fretting damage could affect serviceability. If they must be
used, a suitable counterface would be Stellite 6. If both members must be titanium,
they must be separated by a lubricious film.
5. Metal-to-metal
wear
To assess the wear of self-mated titanium in simple sliding applications, we decided
to test the metal-to-metal
wear characteristics
of the pure titanium and the 6Al4V
alloy against a variety of counterfaces
in a reciprocating ball-on-plane
test. This test
uses a ball rider (12.5 mm diameter) oscillating on a flat counterface under a given
normal force for a given sliding distance at a controlled velocity (1 kgf normal force,
2.5 cm amplitude, 0.08 m s-r sliding velocity, 516 m of sliding). Wear of both members
is assessed by profilometry of the ball scar and counterface scar (three traces each)
and wear volumes are calculated from geometry changes. In this study we tested the
couples shown in Table 2.
There are not a significant number of options available when looking for a suitable
counterface
for titanium. Acceptable
counterfaces
should have corrosion resistance
comparable to titanium since in the chemical process industry it is common to only
use titanium where conventional
stainless steels have inadequate corrosion resistance
(titanium costs five to ten times as much as stainless steel). The uncoated metals that
we selected as candidates
all have significant corrosion resistance: Nitronic 60 similar to 304 stainless steel; Ferralium
255@* - exceptional pitting resistance in
TABLE 2
Test couples for ball-on-plane reciprocating wear test. Three tests of each ball were conducted
on each counterface, with fresh surfaces each time
Ball riders
Flat counterfaces
(1)
(2)
(3)
(1)
(2)
(3)
Grade 2 Ti + anodize + MoSr dry film lube
Nitronic 60 stainless steel (Ammo steel)
Gall Tough stainless steel (crucible steel)
(4)
Tantalum
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
Zirconium
Grade 2 Ti + anodize +PTFE dry film lube
Ti6Al4V + anodize + PTFE dry film lube
Ti6Al4V plated with Ni-P+PTFE
Ti6Al4V chromium plated
316 stainless steel plated with Ni-P+PTFE
Grade 2 Ti plated with Ni-P+PTFE
Ferralium 255
Ti6Al4V PVD coated with TiN 2 pm thick
(4)
Pure Ti Grade 2
Ti6Al4V
Chromium-plated
440C stainless
steel
Grade 2 Ti + anodize + MoSr dry
film lube
*Registered
trademark
of Cabot Corp.
213
chlorides; Gall Tougha - similar to 304 stainless steel; tantalum - the “most corrosionresistant” non-noble
metal; zirconium
- resistant to a wide variety of corrodents.
Thus, if one of these metals worked as a suitable counterface for titanium, it would
probably have adequate corrosion resistance to be used in hostile environments.
A similar approach was taken in selecting coatings. We chose candidate corrosionresistant coatings that can be applied to titanium. Coatability is a concern because
titanium has a very passive oxide on its surface; it cannot be plated by conventional
techniques. It can be anodized but this is not a standard coating and only proprietary
treatments are available to the average machine designer. The proprietary processes
are combined with fluorocarbon
(PTFE) treatments
or other resin-bonded
dry film
lubricants to provide lubricity. Some electroless nickel platings have shown promise
for adhering titanium and we tested three proprietary Ni-P-PTFE
platings. They all
contained from 10% to 20% PTFE which was codeposited with the nickel. Finally
we tested a TiN coating since this is much harder than titanium and there are no
adhesion problems. In summary, the types of coatings that we evaluated were essentially:
(1) anodizing impregnated
with PTFE or MoSz (4 pm thick); (2) Ni-P electroless
plating with PTFE (25 pm thick); (3) TiN physical vapour deposition (PVD) coating
(2 pm thick); (4) chromium plating (1 pm thick).
The other part of the test matrix was a counterface
of chromium-plated
440C
stainless steel. The rationale for selecting this as a candidate is that it is the hardest
corrosion-resistant
counterface that is readily available. It can be chromium plated to
make a composite surface which will have respectable corrosion resistance (but not
as good as titanium or the other metals tested). It is essentially the easiest-to-implement
counterface.
The reciprocating
wear test results are presented in Fig. 10. Two counterfaces
produced low system wear against Ti6A14V, Grade 2 pure titanium and chromiumplated pure titanium: (1) pure titanium plus anodize plus MO& dry film lube plus
460 lube; (2) Ni-P-PTFE
plating on 316 stainless steel. Previous investigations
[14]
found that dilIi.rsed Ni-P platings improved the wear properties of Ti6Al4V. We believe
the PTFE addition produced a lubrication effect that allows the use of these coatings
as deposited.
The most suitable counterface for chromium-plated
440C stainless steel was the
anodized titanium alloys with MO& dry film lubricant. The couples that produced low
system wear are: (1) Ti6Al4V vs. anodized Grade 2 titanium plus anodize plus MO&;
(2) Ti6A14V vs. Ni-P-PTFE
plating; (3) Grade 2 titanium vs. anodized Grade 2
titanium plus anodize plus MO&; (4) Grade 2 titanium vs. Ni-P-PTFE
plating; (5)
chromium-plated
Grade 2 titanium r~. Grade 2 titanium; (6) chromium-plated
pure
titanium vs. Ni-P-PTFE
plating; (7) chromium-plated
440C stainless steel vs. Grade
2 titanium plus anodize plus MO&.
These results suggest that whenever titanium (pure or alloy) is used in a sliding
system, the counterface should have a lubricious coating. None of the pure metal or
specialty alloy counterfaces compared with the PTFE or MO&-lubricated systems. The
results are in agreement with previous investigations
[15]. The friction results (Fig.
11) show that the anodized plus MO&-treated
counterfaces
had the lowest kinetic
coefficient of friction against pure and alloy titanium. This result is in line with what
one would expect. However, these data also show that the wear characteristics of the
various lubricous coatings differed significantly. Some were more effective than others
in reducing system wear. These friction data suggest that designers could effectively
prevent metal-to-metal
wear in titanium tribosystems by having one of the titanium
members treated with an anodize that is subsequently
coated with a PTFE or MO!&
214
0
OLUME
LOSS (mm
!
COUNTERFACE
Fig. 10. Results of metal-to-metal
in a reciprocating ball-on-plane
1.6 Hz, 1.1 kgf normal force, 2
were at their maximum working
wear tests on titanium and other corrosion-resistant
materials
rig. Data are average values for three tests. Test conditions:
h test duration, 2.5 cm stroke amplitude. All metal samples
hardness.
c
. FTFE
,1-S
REFFER
TC DIFFERENT
GOUtNO
VENDORS
0.2
0.4
COEFFICIENT
Fig. 11. Kinetic coefficient of friction of couples in bail-on-plane
derived from average force readings during the test.
0.6
0.8
OF FRICTION
1
wear test. Coefkients
were
216
dry film lube. All the dry film coatings tested were applied by spray and bake techniques.
The MO& coating had an epoxy binder.
6. Summary
and conclusions
The various wear tests conducted in this study have shed considerable
light on
the idiosyncracies of titanium alloys when they are used in sliding systems. The following
are the conclusions from these tests and recommendations
for use of titanium in
design situations.
(1) Titanium does not gall against most metal counterfaces, but it does adhesively
transfer and wears at a high rate (it can be used for threaded fasteners without
concern about seizure, but the threads wear). It can be used mated with 300 series
stainless steels to form a galling-resistant
(but not wear resistant) tribosystem.
(2) The wear of titanium alloys in sliding systems can be significantly reduced by
the use of an anodizing and subsequent treatment with dry film lubricant coating on
one member of the couple (anodizing plus MO& or PTFE).
(3) Both the pure and alloy grade of titanium have poor abrasion resistance
(poorer than soft 300 series stainless) and their use should be avoided in systems
involving low stress abrasion.
(4) Titanium is quite prone to fretting damage (coupled to itself and to soft and
hard stainless steels). A counterface that reduces damage to both members is Stellite
6B. This counterface
should be used whenever possible. Other counterfaces
can be
used if they have a lubricous coating (anodize plus PTFE or MO& dry film lubricant).
(5) Pure titanium transfers and deforms badly in unlubricated
tribosystems; its
use in any tribosystem should be avoided.
We have not learned everything that there is to know about the wear characteristics
of titanium, but adoption of the above guidelines by design engineers will produce
major reduction in wear problems with titanium alloys.
References
1 Standard specification for titanium and titanium alloy bars and billets, ASTM 8348-83.
2 S. Fayeulle, Tribological behavior of nitrogen implanted materials, Wear, 107 (1986) 61-70.
3 T. S. Eyre and H. Asalin, Effect of boronizing on adhesive wear of titanium alloys, Tribologv,
10 (1977) 281-285.
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Wear, 3 (1960) 332.
5 I. J. Polmear, Light Alloys, Arnold, London, 1981, p. 198.
6 S. L. Rice, Materials transport phenomenon in the impact wear of titanium alloys, Wear,
65 (1980) 215-226.
7 Standard test method for galling resistance of materials, ASTM G98-89, 1989.
8 Practice for conducting dry sand/rubber wheel abrasion test, ASZ’M G65-87, 1987.
9 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965, p. 168.
10 I. M. Hutchings and A. P. Mercer, The influence of atmosphere composition on the abrasive
wear of titanium and Ti6Al4V, Wear, I24 (1988) 165-176.
11 J. D. Ayers, Wear behaviour of carbide-injected
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12 R. C. Bill, Selected fretting-wear-resistant
coatings for Ti-d%AU%V
alloy, Wear, 102 (1985)
283-301.
13 R. Komanduri and W. R. Reed Jr., Evaluation of carbide grades and a new cutting geometry
for machining titanium alloys, Wear, 92 (1983) 113-123.
14 Guang-Xi Lu and Jung-Roug Liu, Unlubricated
sliding wear behavior of nickel diffusion
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15 F. deLaat and T. Adams, Inhibiting the wear and galling characteristics of titanium, Met.
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pp. 118-133.
17 K. G. Budinski, Evaluation of fretting corrosion by means of a new device for control of
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