A low cost coating for protecting metallic base system substrates from high temperatures, high gas velocity oxidation, thermal fatigue and hot corrosion is described. The coating is particularly useful for protecting vanes and blades in aircraft and land based gas turbine engines. A lacquer slurry comprising cellulose nitrate containing high purity silicon powder is sprayed onto the superalloy substrates. The silicon layer is then aluminized to complete the coating. The Si-Al coating is less costly to produce than advanced aluminides and protects the substrate from oxidation and thermal fatigue for a much longer period of time than the conventional aluminide coatings. While more expensive Pt-Al coatings and physical vapor deposited MCrAlY coatings may last longer or provide equal protection on certain substrates, the Si-Al coating exceeded the performance of both types of coatings on certain superalloys in high gas velocity oxidation and thermal fatigue. Also, the Si-Al coating increased the resistance of certain superalloys to hot corrosion

Deadmore, D. L.

Young, S. G.

English

NASA Technical Reports Server

United States Patent 4,310,5444 
Deadmore et al. 1451 Jan. 12, I982 
FOREIGN PATENT DOCUMENTS [54] METHOD OF PROTECTING A SURFACE WITH A SILICON-SLURRY/ALUMINIDE 
COATING 
[75] Inventors: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed: 
[51] Int. Cl.3 ... 
[52] U.S. C1. .... 
Daniel L. Deadmore, Middleburg 
Heights; Stanley G. Young, North 
Olmsted, both of Ohio 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
161,254 
Jun. 20, 1980 
.......................................... B05D 7/26 
............................... 427/405; 427/205; 
427/253;428/938;428/941 
[58] Field of Search ................ 428/938, 941; 427/405, 
427/252, 253, 38, 202, 204, 205, 255 
1561 References Cited 
U.S. PATENT DOCUMENTS 
2,059,770 11/1936 Bopp ................................ 106/14.05 
2,109,485 3/1938 Ihrig .................................... 428/938 
3,073,015 1/1963 Wachtell et 
3,251,704 VI966 Nellessen .. 
3,372,038 3/1968 Weldes et a 
3,708,318 1/1973 Reinhart et al. ............... 106/193 M 
3,754,968 8/1973 Reznik 
3,779,719 12/1973 Clark e 
4,155,769 5/1979 Almagro .......................... 106/193 J 
3,698,943 10/1972 Colantuono ........................... 117/53 
7675 2/1980 European Patent Off. ..... 427/255.7 
2756437 6/1978 Fed. Rep. of Germany ...... 427/405 
2745812 4/1979 Fed. Rep. of Germany ...... 428/678 
OTHER PUBLICATIONS 
Grant, J.; Hackh’s Chemical Dictionary 4th Edition, 
McGraw Hill, pp. 141, 458 (1969). 
Primary Examiner-Michael L. Lewis 
Attorney, Agent, or Firm-Norman T. Musial; John R. 
Manning; Gene E. Shook 
[571 ABSTRACT 
A low cost coating protects metallic base system sub- 
strates from high temperatures, high gas velocity oxida- 
tion, thermal fatigue and hot corrosion. The coating is 
particularly useful for protecting vanes and blades in 
aircraft and land based gas turbine engines. 
A lacquer slurry comprising cellulose nitrate containing 
high purity silicon powder is sprayed onto the superal- 
loy substrates. The silicon layer is then aluminized to 
complete the coating. 
The Si-A1 coating is less costly to produce than ad- 
vanced aluminides and protects the substrate from oxi- 
dation and thermal fatigue for a much longer period of 
time than the conventional aluminide coatings. While 
more expensive Pt-A1 coatings and physical vapor de- 
posited MCrAlY coatings may last longer o r  provide 
equal protection on certain substrates, the Si-A1 coating 
exceeded the performance of both types of coatings on 
certain superalloys in high gas velocity oxidation and 
thermal fatigue. Also, the Si-A1 coating increased the 
resistance of certain superalloys to hot corrosion. 
7 Claims, 3 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19820020565 2020-03-21T17:12:07+00:00Z
U.S. Patent Jan. 12, 1982 Sheet 1 of 2 4,3 10,574 
I- 
S 
(3 
w 
- 
3 
22 
lbo 2bo 360 4bo 5bo 6bo 7 b  8 b  lobo ll]00 
TIME IN MACH I BURNER AT 1 0 9 3 O  C, hr 
FIG. I 
-120 
I- 
S 0 -160 
W 
- 
/ 46 
- - 
3-200- 
-240 - 
200 400 600 800 loo0 1200 1400 1600 1800 2Mx) 2200 
TIME IN MACH I BURNER AT 1 0 9 3 O  C, hr 
FIG. 2 
U.S. Patent Jan. 12, 1982 Sheet 2 of 2 4,3 10,574 
~ U J  '39NtlH3 lH913M 
4.3 10,574 I -  
1 
METHOD OF PROTECTING A SURFACE WITH A 
SILICON-SLURRY/ALUMIIDE COATING 
Origin of the Invention 
The invention described herein was made by employ- 
ees of the U.S. Government and may be manufactured 
and used by or for the Government for governmental 
purposes without the payment of any royalties thereon 
or therefor. 
Technical Field 
This invention is directed to the protection of super- 
alloys, a metallic-base system substrate. The invention is 
particularly concerned with low cost coatings for pro- 
tecting turbine blades from high temperatures, high gas 
velocity oxidation: thermal fatigue, and hot corrosion. 
Nickel- and cobalt-base superalloys and dispersion- 
strengthened alloys are used in vanes and blades in 
aircraft and land-based gas turbine engines. The effi- 
ciency of gas turbine engines increases as the operating 
temperatures increase. However, these operating tem- 
peratures are now restricted by design limitations, by 
material strength deterioration, and by material capabil- 
ity to resist oxidation and hot corrosion attack. For 
metal temperatures above 900" C., nickel- and cobalt- 
base superalloy components usually must be coated to 
resist such attack. 
These superalloys may be protected by conventional 
2 
BRIEF DESCRIPTION O F  THE DRAWING 
The details of the invention will be described in con- 
nection with the accompanying drawings illustrating 
FIG. 1 is a comparison of bare and coated VI-A alloy 
substrate response to high gas velocity cyclic oxidation, 
FIG. 2 is a comparison of bare and coated B-1900 
alloy substrate response to high gas velocity cyclic 
FIG. 3 is a comparison of bare and coated B-1900 
5 weight change curves wherein 
10 oxidation, and 
alloy substrates after hot corrosion testing. 
BEST MODE FOR CARRYING OUT THE 
15 INVENTION 
Various test specimens were prepared in accordance 
with the present invention to illustrate the beneficial 
technical effect of the improved coating. The superal- 
loys used for the test specimen substrates are known as 
*O NASA VI-A and B-1900. It is contemplated that other 
nickel- and cobalt-base superalloys, as well as compos- 
ites and directional eutectics, can be used as the metallic 
base system for the substrate. 
The nominal composition of the VI-A superalloy, in 
25 weight percent, is 6% chromium, 7% cobalt, 5% alumi- 
num, 2% molybdenum, 1% titanium, 6% tungsten, 9% 
tantalum, 0.5% hafnium, 0.5% niobium, 0.4% rhenium 
and the remainder nickel. The nominal composition of 
3o the B-1900 superalloy, in weight percent, is 8% chro- 
mium, 10% cobalt, 6% aluminum, 6% molybdenum, - -  
aluminide coatings, or by alternate coatings such as 
claddings, physical vapor deposited MCrAlYs, alumi- 
nized NiCrAlSi, and Pt-A1 systems. The primary ad- 
vantage of such systems is that they form an adherent 35 
A1203 film which protects the substrate from further 
oxidation. However, many of these systems not only are 
expensive to produce, but also lose their protective 
ability with the passage of time. 
Background Art 
protective coating for high-speed tool steel billets. The 
coating protects against oxidation and includes sodium 
3,372,038 discloses a corrosion-resistant metal coating 
having aluminum powder dispersed in organic ammo- 
nium silicate. 
discloses coatings in which a ductile oxidation resistant 
aluminide coating is then applied to the metallic alloy. 
Stecura et a1 U.S. Pat. No. 4,055,705 is directed to a ne specimens were subjected to oxidation and ther- 
bond coating and a reflective oxide thermal barrier dation and thermal fatigue specimens were run in a 
coating. The reflective oxide is preferably zirconia that natural gas fuel burner rig operating with a gas velocity 
is stabilized with another oxide. at the burner nozzle of Mach 1. In each test cycle the 
specimens were rotated in the air-rich natural gas com- Disclosure of Invention 
60 bustion products for one hour at a metal temperature of 
In accordance with the present invention a low Cost 1093" c. and then cooled to near room temperature in 
coating is formed by applying a slurry spray of high three minutes. 
purity silicon to the superalloy substrate. The silicon Hot corrosion specimens were run in a facility oper- 
layer is pack aluminized to complete the coating which ating on Jet A fuel with a gas velocity of Mach 0.3. Five 
is characterized by a sublayer of high purity silicon 65 ppm of synthetic sea salt was added to the combustion 
which combines with aluminum and other elements gases. The specimens were rotated in the gas flame for 
from particular substrates to create oxides which pro- one hour at 900" C. and then cooled to near room tem- 
vide exceptional surface protection. perature in three minutes. 
1% titanium, 0.1% tungsten, 4% tantalum, 0.1% car- 
bon, 0.1 % zirconium, 0.015% boron and the remainder 
nickel. 
The test specimens were initially cleaned in a wet 
slurry grit-blast facility. They were subsequently ultra- 
sonically cleaned in alcohol and then acetone. 
A lacquer slurry Containing 4 grams of high Purity 
silicon powder per 10 ml vehicle was sprayed by air 
4o brushing onto the superalloy specimens to a specific 
weight of 6 mg Si/cm2. The silicon was 99.9% pure and 
quer was a cellulose nitrate solution that served as both 
the transport and binder. 
After the 'pecimens were they were placed in a 
pack comprising 98% A1203, 1% NaCI, and 1% A1 
powder. 
The pack was heated to 1100" C. for 16 hours under 
mg/cm. The pack was then removed from the furnace 
before the specimens were removed from the pack. 
U.S. Pat. No. 3,698,943 to Colantuono discloses a was of a nominal powder size of -325 mesh. The lac- 
borate, silica, and alumina. Weldes et a] U.S. Pat. No. 45 The 'prayed 'pecimens were air dryed for 24 hours* 
Gedwill et a1 u*s. pat. Nos- 3,849,865 and 3,869,779 5o argon. ~h~ weight pickup in the pack averaged 17 
is to a An and allowed to cool four hours to room temperature 
therma1 barrier coating system comprising a NiCrAIY 55 mal fatigue testing as well hot corrosion testing. Oxi- 
4.3 10,574 
3 
Before testing, the specimens were degreased in tri- 
chloroethylene vapor and weighed to the nearest 0.1 
milligram. At intervals of either 20 or 50 hours of test- 
ing, the specimens were removed from the apparatus, 
reweighed, photographed and inspected for cracks. 
Each test was run with two specimens coated in accor- 
dance with the invention. 
After the burner tests were complete, as determined 
from the appearance of the specimen or weight loss as 
well as by observation of severe cracking or erosion, the 
specimens were cut into sections and mounted for me- 
tallographic inspection. Some samples of surface oxides 
were scraped from the edges of the oxidation and ther- 
mal fatigue specimens for X-ray diffraction analyses. 
Microprobe analysis was conducted on the coated VI-A 
specimens before and after exposure to determine the 
extent of diffusion of silicon into the alloy and to indi- 
cate elements present in the metallic and oxide forms. It 
appears that silicon in the coating contributed to the 
formation of silicon-oxide compounds in the protective 
A1203. The presence of Ta and Mo, also detected in the 
complex protective oxide scale, indicated that they had 
diffused from the substrate to the protective scale. 
The results of the 1093" C .  cyclic, Mach 1 burner-rig 
5 
10 
15 
20 
tests on coated and uncoated alioy specimens are shown 25 
in FIGS. 1 and 2. In these figures curves 10,12,30 and 32 
which are drawn through actual points are for the Si-AI 
coating data. The other curves are data from tests per- 
formed previously in the same burner facilities under 
the same conditions. They are included for comparison. 30 
Weight change curve 10 in FIG. 1 is drawn through 
actual test data points obtained for a first specimen of 
VI-A coated in accordance with the inventibn. A simi- 
lar curve 12 is drawn through data points obtained in 
the test of a second VI-A specimen. 
Studies of the failure processes indicated that for 
oxidation a small pit, fissure, or blister of the coating at 
either the test specimen leading edge or trailing edge 
appeared to lead to a thermal fatigue crack, which prop- 
agated and provided an expanded area for oxidation. 
For hot corrosion, the failure process was started by a 
blister that 
formed in the coating and allowed very rapid corrosion 
of the substrate from that site. 
A leading edge crack was observed at point 14 on the 
curve 10 while a trailing edge crack occurred at point 
16. Similarly, a leading edge crack appeared at point 8 
on the curve 12 while the trailing edge crack appeared 
at point 20. 
A weight change curve for bare VI-A superalloy is 
shown at 22 while a similar curve 24 is for a VI-A speci- 
men protected by a commercial pack aluminide. The 
protective ability of a more expensive Pt-AI system on 
a VI-A specimen is illustrated by the curve 26. 
Referring to FIG. 2, a weight change curve 30 is 
drawn through actual test data points obtained for one 
specimen of B-1900 superalloy coated in accordance 
with the present invention. Curve 32 is drawn through 
data points obtained in the test of another B-1900 speci- 
men. 
A leading edge crack was observed at point 34 on the 
curve 30 while a trailing edge crack occurred at point 
36. Two leading edge cracks appeared at points 38 and 
40 on curve 32 while trailing edge cracks appeared at 
points 42 and 44. 
A weight change curve for bare B-1900 superalloy is 
shown at 46 while curve 48 is for an aluminide coated 
B-1900 specimen. Although there is no test information 
35 
40 
45 
50  
55 
60 
65 
4 
available regarding the protective ability of Pt-AI sys- 
tems on B-1900 substrates, weight change curve 26 of 
FIG. 1 is extrapolated past IO00 hrs. by the dotted line 
50 in FIG. 2. 
In FIG. 1 large spreads are noted between the weight 
change curves 10 and 12 of supposedly identical speci- 
mens. Likewise, in FIG. 2 similar spreads are noted 
between the curves 30 and 32. Such a scatter is very 
common, however, in this type of test because of many 
uncontrollable variables in the coating application pro- 
cess. It is contemplated that automated spray or electro- 
phorectic deposition systems might provide improved 
reproducibility. 
FIG. 1 shows that the Si-A1 coated specimens of lines 
10 and 12 have lives from 675 to 960 hrs. Such a life is 
nearly four times that of commercial pack-aluminide 
specimens identified by curve 24 which were tested 
under the same conditions. The uncoated specimen of 
curve 22 shows rapid weight loss. The Pt-A1 specimen 
had an extrapolated life of approximately twice that of 
the Si-AI coated VI-A specimen. The curve for the 
specimen is shown in FIGS. 1 and 2. 
A comparison of curves 24 in FIG. 1 and 48 in FIG. 
2 shows that the B-1900 specimen of FIG. 2 had a 
shorter life with an aluminide coating than the alumi- 
nide coated VI-A specimen of FIG. 1. Yet when the 
Si-AI coating was applied to a B-1900 superalloy, a 
burner rig life of one of the specimens shown in line 30 
of FIG. 2 even surpassed the reported and extrapolated 
data for the Pt-AI on VI-A systems by 100 hrs. The 
Si-AI coated B-1900 specimen was actually run to 2100 
hrs. in the burner rig before removal. No thermal fa- 
tigue cracks were observed until after 800 hrs., and this 
is 200 hrs. longer than that reported for the Pt-AI speci- 
men. 
The results of the hot corrosion tests on bare and 
Si-AI coated B-1900 superalloy specimens are shown in 
FIG. 3. Weight change curves 60 and 62 are for two 
specimens coated in accordance with the invention. 
Weight change curves 64 and 66 are for two bare B- 
1900 specimens. FIG. 3 shows that the coating protects 
the B-1900 in that the weight change did not become 
negative until after 500 hrs., as compared to less than 50 
hrs., for the uncoated specimens. 
While the invention has been described and illus- 
trated with reference to certain preferred embodiments 
thereof, those skilled in the art will appreciate that vari- 
ous modifications, changes, omissions and substitutions 
may be made without departing from the spirit of the 
invention. It is intended, therefore, that the invention 
will be limited only by the scope of the subjoined 
claims. 
1 
We claim: 
1. A method of protecting the surface of a substrate of 
a metallic base system selected from the group consist- 
ing of nickel-base and cobalt-base superalloys, compos- 
ites and directional eutectics comprising the steps of: 
covering said surface with a layer of substantially 
pure silicon, and 
aluminizing the outermost surface portion of said 
layer by pack cementation in argon thereby form- 
ing an outer aluminide coating thereon. 
2. A method of protecting the surface of a substrate as 
claimed in claim 1 wherein the surface is covered by 
spraying a slurry containing substantially pure silicon 
powder onto said substrate and said surface is air dried. 
3. A method of protecting the surface of a substrate as 
claimed in claim 2 wherein the slurry comprises a lac- 
4,3 10,574 
5 6 
quer which serves as both the transfer vehicle and the 
binder. substrate. 
claimed in claim 3 wherein the lacquer comprises a 
cellulose nitrate solution. 
5. A method of protecting the surface of a substrate as 
claimed in claim wherein the slurry contains about 
four grams of high purity silicon powder per ten ml of 
cellulose nitrate. 
square centimeter of surface area is sprayed onto the 
4. A method of protecting the surface of a substrate as 7. A method of protecting the surface o f a  substrate as 
claimed in claim 
prises 
wherein the aluminizing step com- 
covering the coated substrate with a pack comprising 
98% AI203 1% NaCl and 1% AI powder, and 
heating the covered substrate to 1100" C. for about 16 
hours. 6. A method of protecting the surface of a substrate as 10 
claimed in claim 5 wherein about six mg of silicon per * + * * $  
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 


Method of protecting a surface with a silicon-slurry/aluminide coating

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