An evaluation of a duplex silicon-slurry/aluminide coating is presented. The coating is cyclically tested in Mach 1 combustion gases for oxidation and thermal fatigue resistance at 1093 C and in Mach 0.3 gases for hot-corrosion resistance at 900 C. The base metal superalloys are ViA and B-1900. The coated B-1900 specimens performed much better in oxidation than similar specimens coated with aluminides and almost as well as the more expensive Pt-Al and MCrAlY (where M is Ni and/or Co) coatings deposited by the physical vapor deposition process. The coating also provided good hot corrosion protection. Metallographic, X-ray, and electron microprobe studies are used to characterize the coating, determine failure mechanisms, and study some of the changes due to exposure

Deadmore, D. L.

Young, S. G.

English

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NASA Technical Memorandum 8145 5 
AN EX PER IMENTAL, LOW-COST- 
- - I  
S ILICON-A LUMINIDE H IGH-TEMPERATURE 
COATING FOR SU PERALLOY S 
(rrs~-~a-~t~ss) AH E X P ~ I M E E T A L ,  LOU-COST, 
SILICOI-ALOlIl9IDB HIGH-TBIIPIPATURl C0AZIPG 
FOR SUPERALLOYS (NASA) 16 p HC A02/11F A01 
CSCL 11P Unclas 
6 3 / 2 6  47616 
Stanley G. Young and Daniel L. Deadmore 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
International Conference on Metal Coatings 
sponsored by the American Vacuum Society 
San Diego, California, April 21-25, 1980 
I 
, 
AN EXPERIMENTAL, LOW-COST, SILICON-ALUMINIDE 
H I GH TEMPERATURE COAT I NC FOR SUPERALLOYS* 
by 
Stanley G. Young and Daniel  L. Deadmore 
Nat iona l  Aeronautics and Space Adm in i s t r a t i on  
Lewis Research Center 
Cleveland, Ohio 
INTRODUCTION 
I L n  QD The e f f i c i ency  ~f gas- turb ine engines increases as opera t ing  tempera- rn I u tu res  increase. However, the  temperature i s  r e s t r i c t e d  by design l i m i t a t i o n s ,  
ma te r i a l  s t rength,  and environmental c a p a b i l i t i e s .  Fcr  metal temperatures 
above 900' C,  n ickel-and-cobal  t-base superal l o y  components usual l y  must be 
coated t o  r e s i s t  ox i da t i on  and hot  cor ros ion.  A t  t he  Lewis Research Center 
there i s  a con t inu ing  e f f o r t  t o  improve convent ional  a lumin ide coat ings and 
t o  develop new, a1 t e r n a t i v e  coat ings ( r e f s .  1 t o  5 ) .  These systems form an 
adherent A1 0 f i l m  which p ro tec t s  the subs t ra te  from f u r t h e r  ox ida t ion .  2 3 
Refractory  metals and a l l o y s  developed f o r  much h igher  temperatures than 
superal loys can be p ro tec ted  by fused-si  1 i c i d e  coat ings ( r e f s .  6 and 7) .  
These a l l o y s  a re  genera l l y  p ro tec ted  by M S i  and HSlp compounds, which p r i -  5 3 
m a r i l y  form S i O  upon o x i d a t i o n  ( r e f .  8).  L i t t l e  i s  known, however, about the 2 
ox ida t i on  o f  N i s i  a l l o y s  as coat ings compared w i t h  the MA1 and M C r A l Y  coa t ing  
a l l o y s .  Even less  i s  known about the complex combinations o f  oxides, i n t e r -  
metal 1 i c  compounds, and e u t e c t i c s  t h a t  could r e s u l t  from s i l  i c ide /a lumin ide  
* 
Based on NASA Techni c a l  Memorandum 79178, An Experimental , Low-Cost , S i  1 I con 
SLURRY/Alurninide High-Temperature Coating f o r  Superal loys by Stanley H. Young 
and Daniel  L. Deadmore. 
9 
and S i O  /A1 0  combined w i t h  the n i ne  o r  more o the r  elements contained i n  2 2 3  
.y superal loys.  
Most a v a i l a b l e  in fo rmat ion  ind ica tes  t h a t  smal l  amounts o f  s i l i c o n  have 
a favorab le  e f f e c t  on the o x i d a t i o n  res is tance o f  i r o n  and n icke l -base a l l o y s  
( r e f s .  9 t o  12). But d i f f e r e n t  r e s u l t s  are obtained w i t h  d i f f e r e n t  ma te r i a l s  
( r e f s .  8, 13). I t  appears t h a t  s i  1 icon i n  a  coa t ing  must be t a i l o r e d  f o r  use 
w i t h  the p a r t i c u l a r  subs t ra te  t o  be bene f i c i a l .  Unpublished c y c l i c  furnace 
o x i d a t i o n  data (by D. Deadmore o f  Lewis) showed t h a t  a  h i g h - p u r i t y  s i l i c o n -  
s l u r r y  spray fo l lowed by a  pack ~ l u m i n i z i n g  t reatment g r e a t l y  improved the  
h i gh  temperature o x i d a t i o n  res is tance  o f  NASA V I A  a l l o y .  
The purpose of t h i s  i n v e s t i g a t i o n  was t o  eva luate the S i - A 1  coat ing sys- 
tem i n  severe environments t h a t  more c l o s e l y  s imulate  gas- turb ine engines 
and t o  compare i t  w i t h  p rev ious ly  tes ted  o r  i n - se r v i ce  coa t i ng  systems. The 
Si-A1 coa t ing  system, i f  successful ,  prov ides a  la rge  p o t e n t i a l  f o r  cost  re-  
duct ion. 
EXPERIMENTAL PROCEDURES 
The nominal composit ions o f  the subs t ra te  a1 loys ( i n  wt%) a re  f o r  V I A ,  
6 Cr, 7 Co, 5 A l ,  2 Mo, 1 T i ,  6 W, 9 Ta, 0.5 H f ,  0.5 Nb, 0.4 Re, balance Ni ; 
and f o r  8-1900, 8 Cr, 10 Co, 6 A l ,  6 Mo, 1 T i ,  0.1 W, 4 Ta, 0.1 C ,  0.1 Z r ,  
0 .0 \5  0 ,  balance N i .  
The two specimen con f i gu ra t i ons  used i n  t h i s  i n v e s t i g a t i o n  a re  shown i n  
f i g u r e  1 .  Specimens were g r i t - b l a s t e d  and cleaned before coa t ing .  S i l i c o n  
powder was sprayed ontc  the supera l loy  spzcimens, they were a i r  d r ied ,  and 
3 
then pack a lumin ized a t  1 10oO C f o r  16 hours under argon. A more complete 
desc r i p t i on  o f  the process i s  g iven i n  reference 15. 
Ox ida t ion  and thermal f a t i g u e  specimens were run i n  a na tu ra l -sss  burner 
w i t h  a gas v e l o c i t y  of Mach 1 .  A d e t a i l e d  d e s c r i p t i o n  o f  the burner f a c i l -  
i t y  i s  g iven  i n  references 16 and 17. I n  each t e s t  c yc l e  the specimens were 
ro ta ted  i n  the  ho t  gas f o r  1 hour a t  a metal temperature o f  1093' C ,  and 
cooled t o  room temperature by lowering i n t o  a h i gh -ve loc i  t y  a i r  b l a s t  f o r  3 
minutes. 
The ho t -co r ros ion  specimens were run ' n  a sma l le r  f a c i l i t y  opera t ing  on 
a Je t  A fue l  w i t h  a gas v e l o c i t y  o f  Mach 0.3. F ive  ppm s a l t  was added t o  
the combustion gases. The specimens were ro ta ted  i n  the gas flame for. 1 hour 
a t  900' C and then cooled t o  near room teloperature i n  3 minutes. F.t  i n t e r v a l s  
o f  20 o r  50 hours o f  t e s t i n g  the specimens were weighed, photographed, and 
inspected f o r  cracks. Each t e s t  was run w i t h  d u p l i c a t e  specimens. 
A f t e r  burner t e s t s  were complete, as determined from the ap~aarance  o f  
the specimen o r  weight loss r a t e ,  the specimens were inspected metal lograph- 
i c a l l y .  Samples o f  surface ox ide  were scraped from se lected specimens f o r  
X-ray d i f f r a c t i o n  analyses. Microprobe a~ ia l yses  were conducted on the coated 
V I A  specimens defore and a f t e r  exposure to determine the ex ten t  o f  elemental 
d i f f u s i o n  and t o  i n d i c a t e  elements presenr i n  the  m e t a l l i c  and ox ide  forms. 
O X  l DAT l ON AND THERMAL FAT l CUE RESULTS 
The r e s u l t s  o f  the Mach 1 burner t e s t .  on coated and uncoated specimens 
are shown i n  f i g u r e  2. The curves drawn hri,uglr dctual  po in t s  a re  f o r  the 
4 
Si-A1 c o a t i n g  da ta  o f  t h i s  paper. The o t h e r  curves  a r e  d a t a  f rom p r e v i o u s  
t e s t s  i n  t h e  same f a c i l i t y ,  under t h e  same c o n d i t i o n s .  Specimens were re- 
moved from t e s t i n g  a f t e r  a  we igh t  change o f  -50 mg o r  i f  a  t h e r m a l - f a t i g u e  
c rack  had propagated app rox ima te l y  1 / 3  o f  t h e  way th rough t h e  specimen. 
From f i c u r e  2 (a )  t h e  Si-A1 coated specimens have 1 i v e s  (de termined as 
t h e  ze ro  c rossove r  p o i n t  on  t h e  x - a x i s )  o f  f rom 675 t o  960 hours,  n e a r l y  4 
t imes the  l i f e  o f  commercial pack-alurn in ide specimens t e s t e d  under t h e  same 
c o n d i t i o n s  The uncoated specimen shows r , lp id  we ight  l oss .  The Pt-A1 spec- 
imen had an e x t r a p o l a t e d  l i f e  app rox ima te l y  t w i c e  t h a t  o f  t h e  Si-A1 coated 
V I A  specimen. 
When the  S i - A l  c o a t i n g  was a p p l i e d  t o  J-1900 a  bu rne r  l i f e  o f  one o f  
t h e  specimens surpassed t h e  e x t r a p o l a t e d  1 i f e  o f  t h e  Pt-A1 on VIA system by 
100 hours.  The S i - A I  coated 8-1900 specimen ran  >2100 hours  b e f o r e  removal.  
No thermal  f a t i g u e  c racks  were observed u n t  i 1 a f t e r  800 hours  (200 hours  
longer  than f o r  t h e  Pt-A1 specimen). These t e s t s  c l e a r l y  demonstrated t h e  
p o t e n t i a l  o f  t h e  S i - A I  c o a t i n g  app l  i e d  t o  < 1 1  l o y s  t h a t  fo rm f a v o r a b l e  combina- 
t i o n s  w i t h  t h i s  p r o t e c t i v e  c o a t i n g .  
For the  0-  1900 s u b s t r a t e  t h e  specimen ~ h i c h  r a n  t h e  l onges t  (2129 h r )  
looked remarkably good even thouqh i~ was : racked and had l o s t  >200 mg. A  
f i n e  f i ssur -e  was observed on t h e  t r a i l i n g  :dye a f t e r  800 hours wh ich  lengthened 
th roughout  the  t e s t .  Other c racks  appeare l  l a t e r ,  i n  b o t h  t h e  l e a d i n g  and 
t r a i  1 i n g  edges. For the  o t h e r  ti-1500 spec .men the  c o a t i n g  appeared t o  erode 
over  a  l a r g e r  a rea o f  t he  l e a d i n g  edge b e f < ) r e  c racks  appeared. 
5 
Photomicrographs o f  the as-coated specimens and cross sect ions o f  the 
as- tested specimens o f  the S i - A 1  on V I A  a l l o y  a re  shown I n  f i g u r e  3, and 
e l e c t r o n  m i  croprobe ana 1 yses were made on these specimens. 
Two d i s t i n c t  l ayers  were observed above the as-coated V I A  subst ra te .  
The inner  layer  i s  what remains o f  the o r i g i n a l  s i l i c o n  l aye r  a f t e r  the 
16-hour pack treatment.  Th is  iayer  i s  a  nu l t i phase  reg ion  o f  small e lon-  
gated g ra ins ,  h i gh  i n  N i ,  and S i ,  w i t h  va ry ing  amounts o f  the o ther  elements. 
S i l i c o n  d i f f u s i o n  i n t o  the base metal  was less  than expected, bu t  S i  d i f f u -  
s ion  outward was s i g n i f i c a n t .  The top layer  was most ly N i A l ,  w i t h  nodules 
o f  the  S i - r i c h  phase. The surface t ex tu re  cons is ted o f  rounded g ra i ns  w i t h  
an est imated average diameter of 1QO Urn. 
A f t e r  exposure, the surface became smoother and the m ic ros t r uc tu re  o f  
the coa t i ng  appeared e n t i r e l y  d i f f e r e n t .  Layers were no t  as d i s t i n c t ,  and 
l a rge r  o ra ins  o f  d iscont inuous phases were present. S i  1 icon d i f f u s e d  about 
200 Lm i n t o  the subst ra te .  Two d i s t i n c t  rhases were observed i n  the o r i g i -  
n a l l y  S i - r i c h  zones c lose  t o  t+e  subst ra te .  The continuous phase i s  r i c h  
i n  S i ,  T i ,  W, Cr, Mo, and Ta. Above t h i s  reg ion a large-gra ined phase, r i c h  
i n  C r ,  S i ,  N i ,  A l ,  and Co was observed. :his phase may be a form o f  low-A1 
martensi  t i c  B - N i A l  as descr ibed i n  rc.ferecce 8. A t  the sur face a metal 1 i c  
phase r i c h  i n  N i ,  A l ,  Ta, W ,  and T i  ( l i g h  reg ion)  was present.  Th is  i s  be- 
l i e v e d  t o  be e i t h e r  Ni s o l i d  s o l u t i o n  o r  ' . i  A l .  I n  the ox ides,  Mo, Ta, S i ,  3 
and A l  were detected. X-ray ana l ys i s  o f  he surface ind ica ted  A 1  0 Ni 
2 3 '  
s o l i d  so l u t i on ,  sp i ne i ,  and o ther  un ident  f i e d  phases. 
6 
I t  appears tha t  S I ,  combined w i t +  Ta and Mo from the  subs t ra te ,  d i f f used  
i n t o  the ou te r  N i  and A l - r i c h  phases and poss ib l !  formed p r o t e c t i v e  oxides 
i n  a d d i t i o n  t o  A t  0 The p r o t e c t i v e  a b i l i t y  o f  t h i s  coa t i ng  seems t o  be 2 3 '  
h i g h l y  s e n s i t i v e  t o  the elements i n  the subs t ra te .  Small amounts o f  S i  i n  
the a1 l o y  have been shown t o  increase the ox idat  ion res is tance  o f  B-1900 
( r e f .  18) and N i  A1 ( r e f .  19). Fur ther  analys i :  i s  needed t o  more c l e a r l y  3 
i d e n t i f y  s p e c i f i c  phases, but  the complex i ty  anc h i gh  number o f  elements 
makes t h i s  very d i f f i c u l t .  
HOT-CORROS I ON RESLLI S 
The r e s u l t s  o f  the ho t -co r ros ion  t e s t s  on bare and S i - A 1  coated 8-1900 
are shown i n  f i g u r e  4. Specimens were subjected t o  exposure t o  Mach 0.3 
0 
combustion gases con ta in ing  5 ppm se3 s a l t  and bare cyc led between 900 C 
and room temperature. The coa t ing  p ro tec ted  8-1900 f o r  more than 500 hours 
compared t o  less than 50 hours f o r  the uncoated ipecimens. 
I n  the coated specimen b l  i s t e r s  formed a f te r  345 hours. Hot co r ros ion  
occurred a t  these spots and made deep ho les i n  t le specimens by 779 hours. 
SUMMARY OF RESULTS 
(1) I n  Mach 1 .  1093 C cyr. l ; c  o x i d a t i o n  the i i - A 1  coat ings p ro tec ted  
the V I A  a l l o y  a t  l eas t  four  tinies lcriger than ccf ivent ior ,a l  a lumin ide coat ings 
and prov ided a tou t  the same thermal f a t i gue  p r o t x t i o n .  
( 2 )  One S i - A 1  coa t ing  on 8-1900 exce(:ded t t  o x i d a t i o n  and thermal 
f a t i g u e  performance o f  more expensive P t - . \ I  coat lngs on V I A  and s i g n i f i c a n t l y  
7 
surpassed PVD CoCrAlY nn  V I A  i n  o x i d a t i o n  and thermal fa t igue .  
(3 )  The Si-A1 g r e a t l y  improved B-1900's res fs tance  t o  ho t  co r ros ion .  
(4)  The Si-A1 coa t i ng  f a i l u r e  i n  the  o x i d a t i o n  t e s t s  s t a r t e d  a t  a  
small p i t  i n  the coa t ing  fo l lowed by a  thermal f a t i gue  crack a t  t h a t  spot,  
which propagated t o  a l l ow  an expanded area of base metal t o  be exposed t o  
ox i da t i on .  
( 5 )  The S i - A l  f a i l u r e  i n  ho t -co r ros ion  s t a r t e d  wl t h  a  b l i s t e r  which 
V\ 
aD 
rn formed i n  the coa t ing  exposing the  base metal. Rapid co r ros ion  and ox ida-  
I 
W 
t i o n  o f  the base metal masked any o ther  e f f e c t s .  
( 6 )  Pre l  iminary m i c ros t r uc tu ra l  , X-ray, and e l e c t r o n  microprobe s tud ies  
a f t e r  o x i d a t i o n  t e s t s  i nd i ca ted  t h a t  the h i g ; ~  S i  phase, which a l s o  contained 
h i gh  amounts o f  Ta and Mo from the subs t ra te ,  supp l ied  a combination o f  these 
elements t o  t he  o r i g i n a l  N i A l  and/or Ni s o l i d  s o l u t i o n  a t  the sur face t o  a l -  
low combinations o f  ox ides o f  S i ,  Ta, and Mo, t o  form i n  a d d i t i o n  t o  A1 0 2 3' 
The p r o t e c t i v e  a b i l i t y  o f  t h i s  coa t ing  appears t o  be h i g h l y  s e n s i t i v e  t o  the 
e len~ents  contained i n  the  subs t ra te .  
REFERENCES 
1.  Deadmore, Daniel  L.: High Ve loc i t y -Ox ida t ion  Performance o f  Metal-  
Chromium-Aluminum ( M C ~ A I ) ,  Cermet, and Modi f ied Aluminide Coatings 
on IN-100 and Type V I A  A l l oys  a t  1093' C .  NASA TN D-7530, 1974. 
2. Talboom, F. P . ;  Elam, R. C. ;  and Wilson, L. W . :  Eva lua t ion  o f  Advanced 
Superal l oy  P ro tec t i on  Systems. (Pw/ -4055, P r a t t  & Whi tney A i r c r a f t ,  
NASA Contract  NAS~-1241 5.) NASA CR. 72813, 1970. 
3. Levlne, Stanley R.: Cycl ic  Furnace and High-Veloci ty Oxidation o f  an 
Aluminide-Coated High Strength Nickel A l l o y  (8-1900). NASA TM X-2370, 
4. Gedwil l ,  M. A.; and Gr isaf fc ,  S. J.: Oxldat lon Resistant Claddings f o r  
Superalloys. Met. Eng. Q., vo l .  12, May 1972, pp. 55-61. 
5 .  Freche, John C. ; and Aul t, G. Marvin: Progress i n  Advanced High Tempera- 
tu re  Mater ia ls  and Coating Technology. NASA TM X-73628, 1977. 
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ings f o r  Columbium A l l o y  Gas Turbine Blades. F ina l  lech.  Rep. 1 Ju ly  
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(AFML-TR-71 - 172, AD-748837. ) 
7. Packer, C. M.; and Perkins, R. A.: Modif ied Fused S i l i c i d e  Coatings f o r  
Tantalum (Ta- IOW) Reentry Heat Sh i t 1  ds. (0316824, Lockheed Miss i les  and 
Space Co.; NASA Contract NAS~-14316.) NASA CR-120877, 1973. 
8. Smialek, James L. : Fused S i  1 icon-Rich Coatings fo r  Superal loys. NASA 
TH X-3001, 1974. 
9. Ztmskov, G. V . ;  e t  a l . :  Ef fect  o f  S i l i c o n  Addit ions on the Oxidation 
Resistance of  Aluminided Layers on a Nickel-Chromium Al loy.  Zashchitnye 
Pokryt i  l a  na Metal lakh, No. 4, 1971, pp. 151-158. Pro tec t ive  Coatings 
on Metals. Vol. 4, G. V .  Samsonov, ed. ( 2 .  S. Michalcwicz, t rans l . )  
Consultant Bureau, 1972, pp. 110-112. 
10. Moore, V ,  S.; Bretrall, W. D.; and Stetson, A. R.: Evaluat ion of  Coatings 
fo r  Cobalt- and Nickel-Base Superalloys, Vol. 2, Aluminide Pro tec t ive  
Coatings fo r  Nickel and Cobalt A1 loys. (RDR-1474-3, Solar; NASA Con- 
t r a c t  NAS3-9401.) NASA CR-72714, 1"O. 
9 
Crouch, A. G.; and Dooley, R. 9. :  The Hechanlcal I n t e g r i t y  and Pro- 
t c c t l v e  Performance of S i l l c d  Coatings. Corros. Sci., vo l .  16, no. 
6, June 1976, pp. 341-347. 
Lavendel, H. W. :  Corrosion-Resistant Alumlnide Coatl fgs fo r  Iron- and 
Nickel-Base Al loys. J. Metals, vo l .  27, Ju l y  1965, pp. 4-10. 
F ledler ,  H. C.; and Sieraski,  R. J.: Alumlnide Coatings for Nickel -  
Base A l  loys. (SRO-72-028, General E l c c t r i c  Co. ; NASA Contract NAS3- 
14300.) NASA CR-120871, 1971. 
Johnston, James R.; and Young, Stanley G.: Oxidation and thermal 
Fat lgue o f  Coated and Uncoated NX-188 Nickel-Base A l  loy  i n  a Hlgh- 
Ve loc i t y  Gas S t  ream. NASA TN 0-6795, 1972. 
Young, Stanley G.; and Deadmore, Daniel L.: An Experimental, Low-Cost, 
S i  l icon-Slurry/Aluminide Hi gh-Temperature Coating for  Superal loys. 
NASA TH- 79 1 78, 1979. 
Deadmore, Daniel L.: Cycl ic  Oxidat ion of Cobalt-Chromium-Aluminum- 
Yt t r ium and Alumlnide Coatings on IN-100 and V I A  A l loys  i n  High Veloc- 
i t y  Gases. NASA TN D-6842, 1972. 
Johnston, James R.; and Ashbrook, Richard L.: Oxidat lon and Thermal 
Fatigue Cracking o f  Nlckel -  and Cobalt-Base Al loys I n  a High-Veloci ty 
Gas Stream. N,4SA TN 0-5376, 1969. 
Miner, Robert V . ,  J r . ;  and Lowell, Carl E.: E f fec ts  o f  S i l i c o n  Addf- 
t ions  on Oxidat ion and Mechanical Behavior o f  the Nickel-Base Super- 
a l l o y  8-1900. NASA TN 0-7989, 1975. 
10 
19. Lowell, Carl E . ;  and Santoro, Gilbert J. :  Tho 1 2 0 0 ~  C Cycllc Oxidr- 
t ion Behavior o f  Two Nickel-Aluminum Alloys (Nl A1 and ~ i A 1 )  with 3 
Additions of  Chromium, Sil icon, and Titrnlum. NASATN 0-6836, 1972. 

f lqurr j. . ? \ l ~ r ~ \ l r u c t l ~ ~ ~  111 5 1  A1 lorlrcl v !A r l l o k  twforr ,and r f l r r  
cr,llnq 10 U c t t i  I Irurnrr 11 l l ~ P  ( 
'blAn[R 1m. 1 hour C Y C L E S  IN MACII 1 a U R h l f R  A T  10930C. 
Fiqurp 3. - C o n c l ~ ~ d ~ d  
0 100 200 MO 4O(r HX: 6a) 700 800 900 10M) 
EXPOSURE, h r  
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Figure 4. - Bare and coated 0-1900 after Mach 0.3 
hat-corrosion tests at C for 1 h r  in 5 ppm 
sea salt. 


An experimental, low-cost, silicon-aluminide high-temperature coating for superalloys

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