Force-transmitting surfaces of titanium protected from pretting fatigue by a coating of Co-Ni-Fe

A method is disclosed for protecting a force-transmitting or force-receiving surface of titanium from fretting fatigue. The method constitutes coating the surface with a coating of an alloy of nickel, cobalt, and iron.

2. Field of the Invention 
The invention relates to a method for protecting a force-transmitting 
surface of titanium from fretting, and to a method for reducing fretting 
fatigue resulting from high velocity contact of mating surfaces. 
3. Description of the Prior Art 
Fretting wear occurs on force-transmitting surfaces of parts which contact 
each other with a high frequency, low amplitude motion. Specifically, this 
phenomenon occurs in assemblies wherein a force-transmitting surface is in 
rotating, sliding or oscillating contact with a second surface. Since each 
surface contains many microscopic asperities in contact, the fretting 
motion tends to cause local adhesion at these contact points, which may 
fracture, producing material transfer, wear debris, or both. If the 
materials in contact are similar, or their constituent elements have high 
mutual solubilities, the majority of the fretting wear will be adhesive in 
nature. Elevated temperatures will greatly accelerate the process, as may 
large loads and high frequencies. If the mating surfaces become extremely 
abraded and roughened, stress concentrations will result in wear scarring, 
and a reduction in fatigue strength (known specifically as fretting 
fatigue) will result, causing failure of the part or parts. Thus, fretting 
fatigue stress comprises a surface pressure or compression and an 
alternating shear stress or strain between force-transmitting and 
force-receiving surfaces. Specific examples of materials subject to 
fretting fatigue include compressor blade roots of jet aircraft engines, 
friction dampers, bearings on shafts with loose fits, and drive-coupling 
components. In these applications, fretting fatigue failures may occur at 
loads far under the fatigue strength limit of the alloy. It is also to be 
noted that fretting fatigue is aggravated by conditions of high 
temperature, and by higher loadings, or pressure, at the interface of the 
contacting surfaces. 
Prior art efforts to overcome the onset of fretting have involved providing 
anti-fretting coatings on the surfaces in contact, which coatings 
constitute soft metallic films, keeping the substrate surfaces from coming 
in direct contact, and dissipating vibrational energy by intracoating 
shear mechanisms. Such prior art anti-fretting coatings have generally 
constituted copper based alloys, such as Cu-Ni or Cu-Ni-In, such as set 
forth by Bamberger et al, U.S. Pat. No. 3,143,383. Such coatings have, in 
general, been limited to use where temperatures did not exceed 
approximately 750.degree. F. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to minimize fretting fatigue of at 
least one of a pair of contacting titanium or titanium alloy surfaces by 
providing an anti-fretting coating on one or both surfaces. 
SUMMARY OF THE INVENTION 
In accordance with the method of the invention, force-transmitting and 
receiving surfaces of titanium may be protected from fretting fatigue by 
coating either one or both of the surfaces with a coating of an alloy of 
cobalt, nickel, and iron. The thickness of the coating may be within the 
range of 1 to 1000 microns, preferably 10 to 20 microns. Such a coating 
may be applied by any technique which provides uniform coatings of such 
thicknesses, exemplary of which are physical vapor deposition techniques, 
such as cathodic arc deposition; chemical vapor deposition; sputtering; 
thermal spray techniques, such as plasma spray; and electrodeposition 
techniques, such as electroplating. 
The term "titanium" as used herein and in the claims includes both 
commercially pure titanium and titanium-base alloys, such as those 
suitable for aerospace application, and titanium intermetallics. Exemplary 
titanium alloys include Ti-5Al-2.5Sn, Ti-3Al-2.5V, Ti-6Al-4V, 
Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1V-1Mo, and Ti-6Al-2Sn-4Zr-6Mo.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
It has been determined that providing a surface coating in accordance with 
the method of the invention yields a significant improvement in fretting 
fatigue life of titanium-base alloys as compared to uncoated surfaces at 
temperatures up to at least 900.degree. F. The invention is also effective 
for use with commercially pure titanium, as well as titanium-base alloys. 
The benefit is greater, however, with titanium-base alloys, which are used 
in higher temperature applications than commercially pure titanium. The 
coating of the invention finds particular utility with titanium-base alloy 
surfaces of compressor blade roots, but can be applied with beneficial 
results to any titanium-base alloy surface configuration where fretting 
fatigue is a problem. 
Since the coating of the invention has been found to be beneficial at 
temperatures of from 900.degree. F. to 1200.degree. F., it has been found 
to provide notable advantages over conventional methods of minimizing 
fretting fatigue, which include the use of dry film lubricants and 
coatings of alloys, such as aluminum-bronze, copper-nickel, and 
copper-nickel-indium. These conventional coatings are deficient in being 
effective only at temperatures up to about 750.degree. F. This maximum 
useful temperature is below the temperatures encountered in many 
applications where fretting fatigue is a problem, including compressor 
blade roots. 
With the coating of the present invention, it has been determined that the 
presence of a solid solution of cobalt and nickel minimizes oxidation of 
the contact surfaces in the high-temperature environments encountered 
during use of the coated titanium-base alloy surfaces. This results in 
retention of the lubricating properties of the coating at temperatures 
exceeding 750.degree. F., at which temperatures conventional coatings are 
rendered ineffective. The iron content of the present coating serves to 
further improve the lubricant properties thereof at elevated temperature 
by oxidizing to form hematite (Fe.sub.2 O.sub.3). By retaining lubricating 
properties at these elevated temperatures, resistance against fretting 
fatigue is maintained in applications of titanium-base alloys for which 
conventional coating materials are not effective. 
In addition, it has been found that titanium surfaces coated with the 
anti-fretting coating of the present invention are capable of withstanding 
higher loadings than surfaces coated with conventional anti-fretting 
coatings of the prior art. That is, less fretting is observed for high 
loadings applied to the coatings of the present invention than for the 
same loadings upon a surface having a Cu-Ni anti-fretting coating. 
As previously indicated, surfaces of titanium may be protected from 
fretting fatigue by coating either one or both of the surfaces with a 
coating of an alloy of cobalt, nickel, and iron. The alloy composition may 
be within the range of 30 to 70% cobalt, 25 to 55% nickel, and 5 to 25% 
iron, by weight. Preferably, the coating comprises from 40 to 60% cobalt, 
30 to 50% nickel, and 5 to 15% iron. Most preferably, the coating is of an 
alloy of about 45 to 55% cobalt, about 35 to 45% nickel, and about 10% 
iron. 
Preferably, the coating is deposited to a thickness of from 10 to 20 
microns, with a surface finish not exceeding 16 microinches A.A., or 
R.sub.a, roughness average. Roughness average is the arithmetic average 
(A.A.) of the absolute values of the measured profile height deviation 
taken within the sampling length and measured from the graphical 
centerline, as defined by the American National Standards Institute, 
ANSI/ASME B46.1-1985, which is incorporated herein by reference. 
EXAMPLE 1 
In accordance with the present invention, test specimens of a titanium-base 
alloy of the composition Ti-6Al-2Sn-4Zr-2Mo were coated by ion vapor 
deposition coating. Coating was performed in a conventional low pressure 
inert gas ion vapor deposition chamber to deposit a 10 micron coating of 
the alloy composition, in weight percent, 45 cobalt, 45 nickel, and 10 
iron. 
Evaluation of the coating for fretting fatigue resistance was accomplished 
by fully reversed bending (R=-1) high cycle fretting fatigue testing. All 
specimens were vibrated to failure or 10 million cycles, which was 
considered the endurance limit. Each specimen was individually loaded and 
calibrated to determine test deflection limits. The test involved varying 
the bending load at approximately 100 Hz and determining cycles to failure 
(or endurance limit). An electromagnet provided a vibrational stimulation 
to the specimen. A magnetic pickup was attached to the titanium specimens. 
During the specimen vibration the fretting bars, loaded normal to the 
specimen surface, produced a small oscillatory displacement at the 
specimen surface such that fretting wear and fatigue cycling of the 
specimen occurred simultaneously. Testing was performed at ambient 
temperature and 900.degree. F. A complete uncoated baseline at both 
temperatures was generated and the results of the test with coated 
specimens was plotted against the baseline regression curve. 
The Figure shows the results of the evaluation. A substantial increase in 
fretting fatigue life was realized at all vibratory stress levels with the 
application of the nickel-cobalt-iron coating in accordance with the 
invention at both ambient temperature and at 900.degree. F. The bearing 
stress was 552 MPa. Surprisingly, it was also found that when the bearing 
load was increased from 172 to 552 MPa, the number of cycles withstood by 
the sample prior to failure increased, rather than decreased, at 
900.degree. F. Although copper-nickel coatings demonstrate some slight 
degree of improvement under higher loadings at elevated temperature, the 
coating of the present invention provided a substantially increased degree 
of improvement under higher loadings at 900.degree. F. 
EXAMPLE 2 
A sample of gamma titanium aluminide was coated to a nominal thickness of 
10 microns with an alloy of 45% cobalt, 45% nickel, and 10% iron, as 
above. Upon testing at both ambient temperature and at 1200.degree. F., it 
was found that the effect of the coating was to reduce fretting and 
subsequent fretting fatigue of the intermetallic, in that the coating 
permitted dramatically increased loading on the part, or alternatively, 
permitted a substantial increase in the number of cycles to which the part 
could be subjected, prior to failure. 
It is to be understood that the above description of the invention is 
subject to considerable modification, change, and adaptation by those 
skilled in the art, and that such modifications, changes, and adaptations 
are intended to be considered within the scope of the present invention, 
which is set forth by the claims which follow.