Six 440-C hardened stainless steel roller bearing test rods were ion plated with various chromium films of thicknesses from .2 microns to 7 microns. The thinner (approximately .2 microns) coating sample had 3 times the fatigue life of the unplated (standard) specimens. Contrastingly, the samples having thicker coatings (several microns) had short fatigue lives (about 3% of the unplated standard)

Davis, J. H.

English

NASA Technical Reports Server

1982
NASA/ASEE SUMMER FACULTY RESEARCH
FELLOWSHIP PROGRAM
MARSHALL SPACE FLIGHT CENTER
THE UNIVERSITY OF ALABAMA
CHROMIUM ION PLATING STUDIES FOR
ENHANCEMENT OF BEARING LIFETIME
Prepared by:
Academic Rank:
University and Department:
NASA/HSFC :
(Laboratory)
(Division)
MSFC Counterpart:
Date :
Contract No.:
Jack H. Davis, Ph.D.
Associate Professor
The University of Alabama
in Huntsville
Department of Physics
Materials and Processes
Engineering Physics
Dr. Raymond L. Gause
August 18, 1982
NGT-01-008-021
University of Alabama
XIV-i
https://ntrs.nasa.gov/search.jsp?R=19830009102 2020-03-22T00:15:10+00:00Z
Abstract
Last summer six 440-C hardened (_ R.C. 60) stainless steel
roller bearing test rods were ion plated with various chromium
films of thicknesses from .2_ to 7_. During the past six months
Dr. B. N. Bhat (Weekly Notes EH23, February 2, 1982) reported
that the thinner (_ .2_) coating sample had 3X the fatigue life
of the unplated (standard) specimens. Contrastingly the samples
having thicker coatings (several microns) had short fatigue
lives (about 3% of the unplated standard).
This year initially only one specimen has been chromium ion
plated (_ I0_ thick) as the VTA system has required almost
continuous refurbishment. However the VTA system was gradually
returned to as good as new condition and during the final week
six more specimen rods were ion plated. Thus, the summer objective
was met, but little time was available for film characterization
or report writing during the final week.
Acknowl edgement s
I wish to express thanks to the National Aeronautics and
Space Administration, the American Society of Engineering Education
and the University of Alabama for operating and supporting this
program. Special thanks to many who interrupted their higher
priority Space Shuttle work to make on-the-spot repairs of the
VTA system. Mr. Webster and Mr. Clark performed expert welding
on leaks as they were found in delicate parts. Mr. Jack Reed
contributed precision machining and general advice. Mr. Long
and Willmore provided emergency grinding and lapping. Mr. Bobby
Cothren and Charlie Torstenson provided quick replacement parts
and help as needed. Alan Biddle, Raymond Gause, B. N. Bhat,
and Ann Whitaker provided both suggestions and encouragement.
Last year's
on Ion Plating.
Introduc t ion
report I provided introductory background material
Objective
This year's objective is to ion plate about six roller bearing
rods with about 0.3_ of chromium.
XlV-i
Equipment
The ion plating apparatus VTA Model 7375 was custom built
by Vacuum Technology Associates and consisted of three main
components: (I) a Varian (NRC) vacuum system Model 3117.
(2) an electron beam gun system, a Sloan Multihe_==h 27v6 gun
powered by a Sloan Model Five/Ten power supply; (3) the high
voltage power supply for bias voltage (I0 KV, .5a), also made
by VTA.
General Comments
Testing separate parts of the VTA system is difficult due
to vacuum interlocks which shut off the electronics, e,g.,
without a vacuum the e-beam voltage will not activate. The VTA
bias supply is extremely powerful 10KV at 500 ma giving a 5 KW
power output which is equal to that of the electron beam itself.
Thus insulation and metal may be easily vaporized or melted if
the system arcs, producing plating shorts on electrical insulators.
Samples
Eight smooth (% 8_") hardened (R.C. 60) 440-C stainless
steel roller bearing test rods, 3-1/4" long by 3/8 ° diameter,
supplied by Dr. B. N. Bhat were similar to those used last year I
except for some carbon black (from hardening) on the end disks
and in the centering cavities,
Exp er imenta I Me thod s
Patience seems to be a chief ingredient in successful electron
beam ion plating, e.g., allowing the vacuum system to stabilize
at steady state. The bias voltage Vc was slowly increased during
discharge cleaning so as not to: (I) damage the feedthrough
insulation, (2) evaporate Teflon hydrocarbons into the system,
nor (3) cause pressure current runaway. A very low rate of
increase of e-beam power minimized the outgassing partial pressure.
Samples I0, Ii, 12, 13, 14 and 15 were bead blasted on the
ends with a large orifice (_ 1/8") while the holes were blasted
with a pen type small orifice (_ i/2mm)jet. The excess grit
was removed by air blown jets and by an acetone rinse, Surface
oxygen was removed by a 2 to 2.7 KV d.c. glow discharge at _ 8 ma
for about one hour. Since the substrate is hot water cooled, over-
heating and arcing to the ground shield occurred at 3 KV after
about 30 minutes.
XIV-2
The specimen rod axis was mounted coaxially with the jar
axis to produce a symmetric uniform coating. For further
uniformity NASA personnel had replaced during the past six months
the top center stationary high voltage feedthrough with an NRC
rotary feedthrough so the sample could rotate about its own axis.
This should provide less destructive vibrations during subsequent
bearing testing of samples 9, 12, and 13, but it was unused on
#'s 8, i0, 12, and 14. The rotator was switched on only a few
seconds at a time because of an increase in arcing with continuous
rotation.
Results
The Cr plating thickness in sample #3 varies from +10% to
-10% (II_ to 9_) from the front to the back of the rod although
the system had circular symmetry about the rod axis. The 150 mg
mass gain predicts an 8.2 average film thickness using the bulk
density of Cr of 7.19 gm/cm3.
Note that the three thinnest films had the least visibility
as expected. Also note that the darkest sample had the highest
plating pressure (35_) in keeping with last year's trend.
Optical rechecks of the above estimated film thicknesses should
be made by others before any destructive testing.
Refurbishment of VTA 7375 lon Plating System
Initially the VTA Model 7375 system ion plating system was
in only fair condition with a rather high ultimate jar pressure
in the 10-6 torr range. During the first two weeks a lightning
damaged fore pump motor was replaced, a magnetic stress free,
quick mounting sample holder was machined, shorts and misadjustments
in the e-beam gun were corrected and 440C roller specimen #8
was successfully ion plated with a I0_ chromium film.
Specimen #9 was mounted in the jar but not coate_ because
the ultimate bell jar pressure was too high (_5 x i0 "= torr).
Based on the time rate of increase of pressure in the closed system
jar, the apparent air leak rate was judged to be a large 5% of
the Ar leak rate so that considerable oxygen contamination could
be expected. During the third week no leak could be detected with
acetone, thermal protection on the diffusion pump failed, thus
requiring the system disassembly and methodical decontamination.
During the fourth week the jar pressure level was still poor
so a helium leak detector (Veeco MS-9) was acquired on loan from
UAH. A large leak was pinpointed in the foreline valve which was
eventually corrected by tightening the valve's bellows slot nut.
XlV-3
The blanked-off, i.e., high vacuum v_ive shut, diffusion pumppressure was then checked at 3 x I0" torr (without the cold trap) -
a good indication that all was now well at least below the bell
jar high vacuum valve. This NRC valve should be watched as the
manual shows an O-ring seal but the present seal apparently is
formed by a soft elastic flat washer.
Grounded insulator shields were made and installed over the
high voltage feedthrough. Filament shields were enlarged and
longer e-beam current #i0 leads were installed to zig-zag around
the above new shields.
During the fifth week the NRC foreline valve leak reappeared
so an "extension spreader wrench tool" was fashioned from 5/8"
copper pipe which helped to correct the problem. Hand polishing
of the top AI plate was begun around the seal areas. Etch marks
near the jar seals were found in the stainless steel base plate
on the AI top plate which could have contributed to the high
pressure. Thus a decision was made to send the top AI plate,
the Varian ring and the bottom plate to Building 4705 for resur-
facing by lapping. The glass cylinder which had a chipped sealing
surface, but not all the way through, was also sent for lapping.
During the sixth week helium leak testing of the e-beam 8
water cooling section was begun; two leaks of greater than i0"
std. cc/sec were found. About eight welds, rewelds and rechecks
with the helium detector were required to produce the leak proof
welds which are now undetectable on the Veeco MS-9 Leak Detector
of sensitivity 10-8 std. cc/sec.
During the eighth week helium leak checking indicated a torque
of about 50 ft.lbs, is required to stop the leak in the metal to
metal Swagelock flange. The carousel hearth showed a slight He
leak during rotation but the leak stopped when the rotation stopped.
The 18.75" AI top plate was received, the lap (_800 grit) looked
basically good but several heavy scratch marks had to be hand
polished in the O-ring contact area. A set screw induced bulge
in the O-ring groove of the center rotary feedthrough was found
and resurfaced. This tap hole had been recently added as a
terminal for the high bias voltage wire. The center (3/8" shaft)
rotary NRC feedthrough was helium tested and found leak free(<i0 -° std. cc/sec).
The base plate acid pits were lapped but presumably some
foreign grit has introduced severe (_.2 mil. deep) scratches into
the base plate and into the Varian ring.
An 18" x 3/4" AI "proof" disk (too big for the Bldg. 4711
lathes) has been machined at UAH to blank off either the top of
the Varian ring or the top of the glass jar to isolate troublesome
leaky parts of the VTA system without a He leak detector.
XIV-4
Several 58 X Leitz photomicrographs of specimen #8 show
that the Cr film peeled away with a knife blade rather easily
from the interface with the 440C stainless steel.
During the ninth week the lapping problems were overcome
by Mr. Richard Long who ground out the scratches in both surfaces
of the Varian ring and both surfaces of the base plate to an
estimated i0 microinch finish.
The e-beam gun was left out (the two waterline Con Flat
Flanges being blanked off) during initial leak testing of the
ring, the new replacement (recleaned in building 4706) glass
cylinder jar and the lapped top plate. The suspected combination
high voltage (modified for rotation) feedthrough proved leak free.
However a makeshift leaky weld of a stainless (_i/4") tube to
a Swagelock fitting at the argon leak inlet was found and rewelded(but not leak tested). An NRC 1/8" pipe thread fitting which did
not require a weld was used as a replacement. The stainless steel
flexible Ar leak line was sprayed with helium with no response
on the leak detector which in the above system test had less
sensitivity since it was connected by a 6' x .5" I.D. hose to the
small fore line (_!/8" I.D.) tap-in valve of the NRC-3117 Vacuum
System.
During the tenth week a second ion plating attempt on sample
#9 was made. However, the e-beam voltage would not initially
come on due to high voltage arcs in the -I0 KV feedthrough. Dis-
assembly and subsequent bead blasting of the inside of the feed-
through cleared the short. However the e-beam high voltage still
tripped each time it was activated. The 2.5 minute delay timer
for the high voltage take was found to be closing but not holding.
Mr. C. A. Torstenson found a close substitute timer and wired it
into the Sloan Five/Ten power supply.
During the tenth week Mr. Torstenson procured a hot water
heater thermostat and wired it into the diffusion pump current
line for protection against the previous overheating. This
provided an overnight pumping capability which (wi_h LN cold
trap) yielded an estimated jar pressure of 6 x 10 -I torr before
the first plating with the refurbished system.
During the last week samples 9, i0, II, 12, 13, and 14 were
each ion plated with roughly less than iv of Cr. The film
thicknesses are not known since most effort was toward coating
all samples.
Conclus ions
The VTA lon Plating System operated rather well (once refurbished)
plating 0.3_ films compared to a few years ago when 40_ films were
attempted. This is not surprising since most of the VTA downtime
has been related to film and powder buildup.
XIV- 5
REFERENCES
I. Davis, J. H., "AI to Cr Ion Plating Studies for Surface
Enhancement," NASA Contract Report, Jan. 1982, NASA CR-161855,
p. XXVIII
2. Davis, J. H., "Ion Plating for High Temperature Applications,"
NASA Contractor Report CR-161511, p. V, Oct. 1980
XIV- 6
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Chromium ion plating studies for enhancement of bearing lifetime

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