Rolling contact fatigue (RCF) test specimens of heat treated 440C bearing steel were chromium ion plated in thicknesses from 0.1 to 8.0 micron and tested in RCF tester using 700 ksi maximum Hertzian stress. Heavy coatings, greater than about 5 micron in thickness, peeled off or spalled readily, whereas thin coatings, less than 3 micron thick, were tenacious and did not come off. Furthermore, significant improvement in RCF life was obtained with thin chromium ion plated test specimens. The average increase in B10 life was 75% compared with unplated 440C. These preliminary results indicate that ion plating is a promising way to improve bearing life

Bhat, B. N.

Davis, J. H.

English

NASA Technical Reports Server

ROLLING CONTACT FATIGUE LIFE OF 
CmOMIUM I O N  PLATED 440C BEARING STEEL 
B i l i y a r  N.  Bhat 
Ma te r i a l s  and Processes  Laboratory 
NASA/MSFC 
Marshal l  Space F l i g h t  Center ,  Alabama 35823 
U.S.A. 
Jack  B. Davis 
Department o f  Physics  
The Univers i ty  of Alabama i n  Hun t sv i l l e  
Huntsv i l le ,  Alabama 35899 
U.S .A .  
MSTPZACT 
Rol l ing  contac t  f a t i g u e  (RCF) t e s t  specimens 
of hea t  t r e a t e d  44QC bear ing  s t e e l  were chromium 
ion p l a t e d  i n  th icknesses  from 0.1 pan t o  8 - 0  pxn and 
tested i n  RCF t e s t e r  us ing  780 k s i  maximum Nfzsrtzian 
s t r e s s .  Heavy coa t ings ,  g r e a t e r  t han  about 5 p m  
P a  th ickness ,  peeled o f f  o r  s p a l l e d  r e a d i l y ,  where- 
as t h i n  coa t ings ,  l e s s  than  3 p m  t h i c k ,  were tena- 
c ious  and d id  not  come o f f ,  Furthermore, s i g n i f i -  
cant improvement i n  RCF l i f e  g a s  obtained wi th  
t h i n  chromium i o n  p l a t e d  t e s t  specimens. The 
average inc rease  i n  B ~ Q  l i f e  was 75% c o q a r e d  w i t h  
urnplated 4406. These pre l iminary  r e s u l t s  i n d i c a t e  
that i o n  p l a t i n g  i s  a promising way t o  inprove 
bearing l i f e .  
https://ntrs.nasa.gov/search.jsp?R=19850018571 2020-03-20T19:07:18+00:00Z
ROLLING CONTACT FATIGUE LIFE OF CmOMIUM! 
ION PLATED 440C BEARING STEEL 
Introduction 
The high efficiency operation of the Space 
Shuttle Main Engine (SSME) appears to limit the 
service life of 440C steel bearings in the engine, 
especially those of the high pressure oxidizer 
turbopump (MgSOTP). Bearing life enhancement is 
expected to contribute significantly to increased 
life and reliability of space transportation systems, 
To achieve this goal, efforts have been made to 
thoroughly understand the complexities of turbopump 
bearing service life! Improved bearing materials 
with greater resistance to fatigue and wear are 2 being developed through powder metallurgy techniques, 
Furthermore, surface modification techniques such as 
ion plating and ion implantation appeared to offer 
potential for improving the fatigue life of bearing 
materials? Ion plating, in particular, seemed to 
have several positive qualities, the most important 
of which are outstanding film adhesion and deposi- 
tion on all sides of the substrate (including some 
coverage into cavities), whereas normal vacuum 
deposition and ion implantation give the usual 
line-of-sight coverage. Other qualities are fine 
grain structure, low coefficient of friction and 
improved mechanical properties. The latter two 
qualities of ion plated coatings make it attractive 
for application in turbopump bearings. Therefore, 
a study of ion plating 440C with chromium was con- 
ducted. This paper presents some preliminary 
results obtained in that study. 
Ion Plating Procedure 
Chromium was selected for ion plating 440C 
steel because of its common use in electroplating 
steel for corrosion resistance. Furthermore, 440C 
steel contains about 16-18 wt. % chromium, thus 
providing a continuity in chemistry which is an 
important consideration in obtaining a good bond. 
The test specimen used in this study and the heat 
treatment are shown in Figure 1. The apparatus used 
for ion plating is shown schematically in Figure 2. 
Chromium is melted by an electron beam and vaporized. 
Some chromium atoms are ionized in the plasma glow 
discharge formed by the ionized argon gas, acceler- 
ated by the applied electric potential and deposited 
on the 440C rolling contact fatigue (RCF) test 
specimen. The specimen rod axis was mounted co- 
axially with the jar axis to produce a symmetric 
uniform coating. The specimen was initially sputter- 
cleaned in situ to remove the oxide layer before the 
electron beam was turned on to vaporize chromium. 
Plating thickness was varied so that the effect of 
plating thickness on RCF life could be determined. 
The plating thickness was calculated based on weight 
gain during plating. Argon pressure during plating 
was typically 15-30 vm. Substrate temperatures 
using an infrared pyrometer varied between 20O0C- 
300°C. Table 1 gives a summary of the test specimens 
ion plated with chromium in this program. Estimated 
plating thickness varied from 0.1 ym to 8.0 vm. 
Coating appearance was variable, fr m nearly invisible 8 
and bright to bluish, grey and dark . 
Metallurgical Analysis 
All RCF test specimens were examined under a 
stereo viewer. Where the coatings were very thin 
(< 1 pm) it was difficult to tell that the specimens 
were coated. Dark coatings were generally porous 
and were readily scratched by knife. Good, thin 
coatings were not scratched-by knife. One such 
specimen (No. 5) was sectioned, electroless nickel 
plated, mounted in epoxy, polished, etched and 
examined under o p t i c a l  rcicroscope. A t h i n  i o n  p l a -  
t e d  chroniurc l a y e r  was observed (Figure 3 ) .  The 
p l a t i n g  w a s  uniform and followed t h e  s u r f a c e  contours .  
There w a s  a good bond between t h e  chrome p l a t i n g  and 
t h e  su r f ace ,  e s p e c i a l l y  wi th  t h e  carb ides .  
Ro l l i ng  Contact Fat igue (RCF) Tes t ing  
RCF t e s t i n g  was done us ing  t h e  Polymet m ~ d e l  
RCF-1 machine which used two, 1.5 in. diameter 
loading d i s c s  (Figure 4) .  The specimen was loaded 
t o  a maximum Her tz ian  s t r e s s  of 7 00,000 p s i .  The 
t e s t i n g  speed was 10,000 rpm. The number of revo- 
l u t i o n s  were counted au tomat ica l ly  by a r evo lu t ion  
counter .  The t e s t i n g  machine w a s  shu t  down auto- 
ma t i ca l ly  by a f a i l u r e  sensor  which picked up 
v i b r a t i o n s  t h a t  increased  sha rp ly  when t h e  specimen 
f a i l e d  by s p a l l i n g .  The number of r evo lu t ions  t o  
f a i l u r e  was noted. About e i g h t  tests were run  wi th  
each specimen, f o u r  on each end. The d a t a  were 
p l o t t e d  on a Weibltll p r o b a b i l i t y  c h a r t ,  which 
p l o t t e d  cumulative percent  f a i l u r e  ve r sus  number of 
cyc l e s  t o  f a i l u r e  (Figure 5). The B10 l i f e  was read  
o f f  t h e  c h a r t .  The r e s u l t s  a r e  summarized i n  Table 
1. Some specimens had coa t ings  t h a t  were too  t h i c k  
and r e a d i l y  s p a l l e d  o f f .  These samples were no t  
t e s t e d  any f u r t h e r .  Specimen No. 1 ( i n  Table 1)  
was not  i o n  p l a t e d ,  bu t  was simplY s p u t t e r  cleaned 
by argon. Auger a n a l y s i s  of t h i s  sample showed a 
chromium r i c h  (40 w t ,  % Cr) s u r f a c e  l a y e r ,  approxi- 
mately 15 nm th i ck .  Such a s u r f a c e  l a y e r  obviously 
provides a good t r a n s i t i o n  l a y e r  f o r  i o n  p l a t i n g  
chromium. RCF t e s t i n g  produced t y p i c a l  wear t r a c k s  
and s p a l l s  i n  t h e  specimen. Figure 6 g ives  t h r e e  
examples. The s p a l l s  of  both i o n  p l a t e d  and s tandard 
specimens were examined i n  t h e  scanning e l e c t r o n  
microscope (Figure 7 ) .  F a i l u r e  mode appears  t o  be 
t h e  same i n  both cases .  
On examination of RCF t e s t  r e s u l t s  (Table 1) 
chromium ion  p l a t i n g  obviously helped t o  improve 
t h e  RCF l i f e  of 440C s t e e l  t e s t  specimens. The 
r a t i o  B10 (specimen)/B (s tandard)  i s  l i s t e d  i n  
t he  l a s t  column of Table 1.  This  r a t i o  va r i ed  from 
1.07 t o  2.86. The average was 1.75, i n d i c a t i n g  a 
75% improvement i n  RCF l i f e .  The f a i l u r e  mode i n  
RCF t e s t i n g  was by s p a l l i n g  (Figure 7 ) .  I n  t h i s  
mechanism subsurface crack i n i t i a t i o n  occurs  under 
high Her tz ian  s t r e s s e s  and t h e  crack propagates t o  
t h e  su r f ace  r e s u l t i n g  i n  s p a l l i n g .  The reason f o r  
t h e  improvement i n  RCF l i f e  may be due t o  t h e  f a c t  
t h a t  a t h i n  chromium l a y e r  i s  l i k e l y  t o  produce a 
more even d i s t r i b u t i o n  of contac t  s t r e s s e s  which 
r e s u l t s  i n  a somewhat reduced maximum shear  s t r e s s ,  
thus  extending t h e  f a t i g u e  l i f e .  It i s  not  c l e a r  how 
w e l l  t h e  coa t ing  would perform under a c t u a l  s e r v i c e  
condi t ions  of t h e  turbopump where some s l i d i n g  
motion i s  present  and where l u b r i c a t i o n  i s  marginal.  
To answer t h i s  ques t ion  i t  would be necessary t o  
run cryogenic t e s t s  wi th  i o n  p l a t ed  bear ings .  
The prel iminary r e s u l t s  obtained i n  t h i s  s tudy a r e  
encouraging. However, t h e r e  i s  a cons iderable  
s c a t t e r  i n  t h e  B10 l i v e s  of i on  p l a t ed  specimens, 
taken a s  a whole. The reason f o r  t h i s  s c a t t e r  i s  
not  very c l e a r .  One p o s s i b i l i t y  i s  t h a t  t h e  ion  
p l a t i n g  process  i s  no t  f u l l y  optimized y e t .  For 
i n s t ance ,  f u r t h e r  op t imiza t ion  i n  e l e c t r o n  beam 
cu r ren t  (which determines t h e  r a t e  of chromium 
evaporat ion)  may be necessary.  However, i t  does 
appear t h a t  t h e  h ighes t  B10 l i v e s  a r e  a s soc i a t ed  
wi th  t h i n  coa t ings  (0.1 ym and 0.2 vm,  Table 1 ) .  
This  avenue should be pursued f u r t h e r  t o  gain con- 
f idence  i n  t h e  imporved RCF l i f e  of i on  p l a t ed  
440C bear ing  s t e e l .  
Summary and Conclusions 
RCF tests were conducted with chromium ion 
plated 440C bearing steel specimens. The results 
showed significant improvements in B10 life. The 
average improvement was 75% for all samples. Thin 
coatings, 0.3 to 0.2pm thin, gave the best results, 
with B10 life improvements greater than 150%. Fur- 
ther process optimization and tests in cryogenic 
environments are necessary to qualify these coatings 
for use in the SSME turbopump bearings. 
Acknowledgements 
The technical support of F. J. Dolan, R. R. Rowe, 
and other personnel of Materials and Processes Labora- 
tory, NASA-MSFC, and the advice and assistance of 
A. P. Biddle of Space Science Laboratory, NASA-MSFC, 
in setting up and operating the ion plating unit are 
gratefully acknowledged. 
References 
1. B. N. Bhat, "Fracture Analysis  of HPOTP Bearing 
Bal ls , '$  NASA-TM-82428, May 1981; B.N. Bhat and 
P. J. Dolan, "Past Performance Analysis of 
WPOTP ~ e a r i n g s " ' ,  NASA-TM-92470, March 1982; 
B.N. Bhat and F.J. Dolan, "Analysis of Cryogenic 
Turbopump Bearing Serv ice  Li fe , "  unpublished 
r e p o r t ,  May 1983. 
2, "Application of Powder Metallurgy Techniques 
t o  Produce Improved Bearing Elements f o r  Liquid 
Rocket Engines," by TRW, Inc . ,  Ma te r i a l s  and 
Manufacturing Technology Center,  Cleveland, 
Ohio. (NASA Contract  NAS8-34763) Work i n  
progress .  
3 ,  "lon P la t ing"  by R.F. Hochman and D.U. Mattox; 
"Ion Implantationl"y R.F. Hochman, Metals 
Handbook, Volume 5 ,  American Socie ty  f o r  
Metals (1982). 
4. Jack  H. Davis, "Aluminum and Chromium Ion  
P l a t i n g  S tud ie s  f o r  Enhancement of Surface  
Properties,'"NASA CR-162051 and NASA CR- 
161851, Univers i ty  of Alabama i n  Huntsv i l le ,  
AL, August 11, 1981 and August 18s 1982. 
A. RCF TEST SPECIMEN 
I 
END HOLES 
NOT TO EXCEED 
118 IN DEPTH 
SURFACE FINISH - 6 TO 8 RMS 
ON CURVED SURFACE. 
SURFACES GROUND BOTH 
PARALLEL AND PERPENDICULAR 
B. HEAT TREATMENT OF RCF TEST SPECIMEN 
AFTER ROUGH MACHINING, HEAT TREATMENT IS CARRIED OUT AS FOLLOWS: 
A. AUSTENITIZE AT 1930° * 30°F FOR ONE HOUR AT TEMPERATURE. 
0. HARDEN (OIL QUENCH 120° - 150°F OIL TEMP.). 
C. TEMPER ONE HOUR MIN. AT 3250+ 25°F 
- OOF 
D. COOL IN  AIR TO 70° 2 10°F (ROOM TEMP.) 
E. COLD SOAK IN LIQUID NITROGEN FOR 30 MINUTES. 
F. TEMPER ONE HOUR MINIMUM AT 325O + 25OF 
- OOF 
FINAL HARDNESS -TYPICALLY R t  59-61 
FIGURE 1 . RCF TEST SPECIMEN CONFIGURATION 
AND HEAT TREATMENT 
FIGURE 2. ION PLATING APPARATUS WITH AN ELECTRON BEAM GUN (SCHEMATIC) 


FIGURE 5. WEIBULL PLOT OF RCF TEST RESULTS FOR STANDARD 
AND CHROMIUM ION PLATED 440C 


Sample 
Number 
-
0.1 Bright RCF tested 8 . 0 ~ 1 0 ~  2.86 
2.0 Bluish RCF tested 3 . 0 ~ 1 0 ~  1.07 
0.9 Bright RCF tested 4 . 2 ~ 1 0 ~  1.5 
0 . 4  Bright RCF tested 3 . 4 ~ 1 0 ~  1.21 
0.0 Bright Standard 440C specimen 2 . 8 ~ 1 0 ~  ---- 
AVC. 1.75 


Rolling contact fatigue life of chromium ion plated 440C bearing steel

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