Coating composition having good corrosion and oxidation resistance

A coating composition comprising an alloy having the formula RCrAlR'R" wherein R is nickel, cobalt or the like; R' is yttrium or hafnium and R" is tantalum, rhenium and/or platinum, preferably mixed with an oxide dispersion such as alumina, to provide an improved class of coatings suitable for operating in high temperature oxidizing environments.

FIELD OF THE INVENTION 
The invention relates to an improved class of coatings suitable for 
operating in high temperature oxidizing environments in which said 
coatings comprise an alloy, such as NiCrAlYTa, or NiCrAlYPt preferably 
mixed with an oxide dispersion such as alumina or any combination thereof. 
BACKGROUND OF THE INVENTION 
There are many good oxidation and corrosion resistant coatings used in 
industry for various applications and for use in various environments. 
Articles composed of iron-, cobalt-, or nickel-based superalloys have been 
developed for use in applications, such as aerospace applications, and for 
use as blades, vanes, seals and other components utilized in gas turbine 
engines. In these applications, it is important that the articles have 
sufficient protection against undue oxidation and sulfidation since such 
corrosion can affect the useful life of the article resulting in reduced 
performance and possible safety problems. Although various superalloys 
have a high degree of corrosion resistance, such resistance decreases when 
the superalloys are operated in or exposed to high temperature 
environments. 
To increase the useful life of components made of alloys and superalloys, 
various coatings have been developed. Aluminide coatings were initially 
used to provide a corrosion resistant outer layer but such layer was 
observed to crack when subjected to mechanically or thermally induced 
strain. Another class of coatings developed was the MCrAlY overlay 
coatings where M represents a transition metal element such as iron, 
cobalt or nickel. The coatings have been found to be more effective than 
the aluminide coatings in extending the useful life of alloy components in 
high temperature environments. 
Modern gas turbine engines operate in a high temperature environment in 
excess of 2000.degree. F. in which hot gases are expanded across rows of 
turbine blades. These turbine blades are typically nickel base alloys 
chosen for excellent high temperature creep and thermal fatigue 
resistance. In general, the design of the blade alloy sacrifices 
resistance to oxidation and hot corrosion in order to achieve the 
optimized mechanical properties. Therefore, the blade is coated with a 
thin layer of material designed to provide only high resistance to 
oxidation or hot corrosion, with little regard to mechanical properties of 
the coating. This thin coating, typically 3 to 8 mils thick, is generally 
applied by argon shrouded plasma spraying, plasma spraying in a vacuum 
chamber, or by physical vapor deposition methods. 
In the field of gas turbine engines, designers continually look toward 
raising the operating temperature of the engine to increase efficiency. In 
turn, higher temperatures act to reduce the life of the current coatings 
on the turbine blades and vanes. Components of a gas turbine engine can 
also be subjected to hot corrosion. This can occur when there is salt 
ingested into the engine via the intake air, or when the fuel has even low 
levels of sulfur concentration, or both. The attack of bare blades or even 
coatings on blades can be very rapid in hot corrosion, where the sulfur 
and salt can form liquid compounds on the surface that are able to 
dissolve the otherwise protective oxide scale on the substrate. This hot 
corrosion mechanism is most aggressive when the blade temperature falls 
between the temperatures where the complex salt-sulfate compounds melt and 
the temperature where the compounds evaporate. In the intermediate 
temperature range a liquid film of the corrodant can exist on the surface 
of the substrate and be very deleterious. Even in engines that generally 
run at high temperatures, above the evaporation temperature of such liquid 
corrodants, there may be conditions where the components pass through the 
lower temperature regime, such as during reduced power operation or at 
idle waiting for take-off in an aero engine. If the corrodants are present 
in the air or fuel, they can enhance the rate of attack during these 
periods. 
In operation, the turbine blade experiences a range of temperatures as the 
power demand is raised or lowered. The blade also experiences a range of 
axial stresses as the rotation speed of the blade is increased or 
decreased. Of course, both the temperature change and the stress change 
happen concurrently to the rotating blade. One mode is when temperature 
and tensile stress both increase together as the demand for power is 
increased, and they both decrease together as power is reduced. If the 
blade temperature were plotted on the abcissa and the stress on the 
ordinate of an x-y graph, the above mode would look like a single upward 
sloping line in the positive stress and temperature quadrant. It is 
possible when temperature changes quickly or the surface of the blade is 
heated or cooled faster than the core of the blade, that the graph of the 
total power cycle is not the same simple curve for heating and cooling. 
Rather the heating and cooling legs of the stress-temperature graph may be 
different, and the total cycle looks like an open loop. This is an 
indication of hysteresis in the system between stress and temperature. 
If now a thin coating is applied to the blade surface, and the coating has 
a different thermal expansion rate than the blade alloy, the situation 
becomes more complicated. One can envision separate stress-temperature 
graphs for the coating and the blade alloy for the same power cycle. In 
many cases, the thermal expansion rate of MCrAlY coatings is greater than 
that of typical nickel base blade alloys. Considering the 
stress-temperature graph of the coating, there would be two contributions 
to its stress state. One would be the radial tensile stress due to 
increasing the rotational speed of the blade. The stress in the coating 
would be the same as in the underlying blade due to this effect. In 
addition, since the coating is assumed to expand faster than the blade 
alloy, it wants to become longer than the blade but is well bonded to the 
substrate so it is constrained and a compressive stress develops in the 
coating. The total coating stress is the sum of the two contributions. The 
heating leg of the coating graph will thus have less tensile stress than 
the blade because of the compression component, so its curve would 
increasingly fall below the simple line assumed for the bare blade. If all 
the high temperature stress was able to be stored in the coating, when it 
experienced the cooling leg of the cycle it would trace back along the 
heating leg for the coating. However, most MCrAlY coatings are weak at 
high temperatures in comparison to blade alloys, and some of the stress in 
the coating would be reduced due to annealing or creep. In that case, when 
the cooling leg of the cycle occurs, the coating stress will end up at a 
lower value at the final low temperature than what it was at the start. 
This is due to the stress relaxation effect of the weak coating at the 
high temperature. Depending on the relative contributions of the stress 
due to blade spinning compared to the differential thermal expansion 
stress effects, and the number of cycles of heating and cooling, the 
coating could become increasingly more in compression. A mechanism such as 
described here could be responsible for the observation that some coatings 
become buckled and cracked after many cycles. 
A further current problem with conventional MCrAlY coatings on superalloy 
substrates is interdiffusion of coating elements into the substrate and 
substrate elements into the coating after long times of high temperature 
exposure. The loss of coating aluminum to the substrate is noticed by an 
aluminide depletion layer in the coating. Certain substrate elements like 
titanium have been found to diffuse through the MCrAlY coating to the 
external surface oxide scale and to make said oxide scale less protective. 
It would be desirable to modify current MCrAlY coatings to reduce this 
interdiffusion effect. 
Although MCrAlY has overall been a successful class of coatings having good 
oxidation and corrosion resistance for superalloys, improvements have been 
made to the MCrAlY coatings. 
U.S. Pat. No. 3,676,085 discloses that the oxidation-erosion and 
sulfidation resistance of the nickel- and cobalt-based superalloys is 
markedly improved through the use of a coating consisting of cobalt, 
chromium, aluminum and an active metal such as yttrium, particularly at 
the composition, by weight, of 15-40 percent chromium, 10-25 percent 
aluminum, 0.01-5 percent yttrium, balance cobalt. 
U.S. Pat. No. 3,754,903 discloses a coating alloy for the gas turbine 
engine super-alloys which consists primarily of nickel, aluminum and a 
reactive metal such as yttrium, particularly at the composition, by 
weight, 14-30 percent aluminum, 0.01-0.5 percent reactive metal balance 
nickel. A preferred embodiment also includes 15-45 weight percent 
chromium. 
U.S. Pat. No. 3,928,026 discloses a highly ductile coating for the nickel- 
and cobalt-base superalloys having long term elevated temperature 
oxidation-erosion and sulfidation resistance and diffusional stability 
which coating consists essentially of, by weight, 11-48% Co, 10-40% Cr, 
9-15% Al, 0.1-1.0% reactive metal selected from the group consisting of 
yttrium, scandium, thorium, lanthanum and the other rare elements, balance 
essentially Ni, the nickel content being at least about 15%. 
U.S. Pat. No. 3,993,454 discloses coatings which are particularly suited 
for the protection of nickel and cobalt superalloy articles at elevated 
temperatures. The protective nature of the coatings is due to the 
formation of an alumina layer on the surface of the coating which serves 
to reduce oxidation/corrosion. The coatings contain aluminum, chromium, 
and one metal chosen from the group consisting of nickel and cobalt or 
mixtures thereof. The coatings further contain a small controlled 
percentage of hafnium which serves to greatly improve the adherence and 
durability of the protective alumina film on the surface of the coating. 
U.S. Pat. No. 4,585,481 discloses a similar coating except that yttrium 
and hafnium are used together along with silicon. 
U.S. Pat. No. 3,918,139 discloses a nickel, cobalt and nickel-cobalt alloy 
coating composition having improved hot corrosion resistance. In 
particular, an improved MCrAlY type alloy coating composition consists 
essentially of, by weight, approximately 8-30 percent chromium, 5-15 
percent aluminum, up to 1 percent reactive metal selected from the group 
consisting of yttrium, scandium, thorium and the other rare earth elements 
and 3-12 percent of a noble metal selected from the group consisting of 
platinum or rhodium, the balance being selected from the group consisting 
of nickel, cobalt and nickelcobalt. 
U.S. Pat. No. 4,677,034 discloses an MCrAlY coating in which silicon is 
added. U.S. Pat. No. 4,943,487 disclosed a NiCrAlY or NiCoCrAlY coating to 
which tantalum is added. U.S. Pat. No. 4,743,514 discloses a coating for 
protecting the surfaces of gas turbine components such as single crystal 
turbine blades and vanes, wherein the coating has a composition (in weight 
percent) consisting essentially of chromium, 15-35; aluminum, 8-20; 
tantalum, 0-10; tantalum plus niobium, 0-10; silicon, 0.1-1.5; hafnium, 
0.1-1.5; yttrium, 0-1; cobalt, 0-10; and nickel, balance totalling 100 
percent. A preferred coating, which is particularly desirable for use with 
single-crystal turbine blades and vanes, has a composition consisting 
essentially of chromium, 17-23; aluminum, 10-13; tantalum plus niobium, 
3-8; silicon, 0.1-1.5; hafnium, 0.1-1.5; yttrium, 0-0.8; cobalt, 0-trace; 
and nickel, balance totalling 100 percent. A process for preparing the 
coated component is also described. 
U.S. Pat. No. 4,615,864 disclosed coatings for iron-, nickel- and 
cobalt-base superalloys. The coatings are applied in order to provide good 
oxidation and/or sulfidation and thermal fatigue resistance for the 
substrates to which the coatings are applied. The coatings consist 
essentially of, by weight, 10 to 50% chromium, 3 to 15% aluminum, 0.1 to 
10% manganese, up to 8% tantalum, up to 5% tungsten, up to 5% reactive 
metal from the group consisting of lanthanum, yttrium and other rare earth 
elements, up to 5 percent of rare earth and/or refractory metal oxide 
particles, up to 12% silicon, up to 10% hafnium, and the balance selected 
from the group consisting of nickel, cobalt and iron, and combinations 
thereof. Additions of titanium up to 5% and noble metals such as platinum 
up to 15% are also contemplated. 
U.S. Pat. No. 4,101,713 discloses a coating made from mechanically alloyed 
MCrAl with a dispersoid of Al.sub.2 O.sub.3, ThO.sub.2 or Y.sub.2 O.sub.3. 
It is an object of the present invention to provide an improved coating 
having good high temperature oxidation resistance characteristics. 
It is another object of the present invention to provide a coating for 
substrates that are intended to operate in high temperature oxidizing and 
sulfidizing environments. 
It is another object of this invention to provide a coating for superalloy 
substrates that will have a thermal expansion rate that is similar to that 
of the substrates and will have a greater high temperature strength so 
that it will resist stress relaxation. 
It is another object of the present invention to improve the diffusional 
stability of the coating toward nickel and cobalt base substrates. 
SUMMARY OF THE INVENTION 
The invention relates to a coating composition comprising an alloy of 
RCrAlR'R" wherein R is at least one element selected from the group 
consisting of iron, cobalt and nickel, R' is at least one element selected 
from the group consisting of yttrium and hafnium and R" is at least one 
element selected from the group consisting of tantalum, platinum or 
rhenium and said alloy mixed with an oxide dispersion such as alumina, 
thoria, yttria and the rare earth oxides, hafnia and zirconia. 
The amount of R, R' and R" in the coating alloy will depend on the specific 
composition of the coating and the environment that the coating will be 
used in. For most applications, the following amounts of the components 
would be suitable. 
TABLE 1 
__________________________________________________________________________ 
Elements-weight percent of Composition* 
Composition 
Co Ni Cr Al Y** Ta Pt 
__________________________________________________________________________ 
NiCrAlYPt -- Bal. 
15-25 
7-14 
0.1-1 
-- 3-6 
NiCoCrAlYPt 
10-40 
Bal. 
15-25 
7-14 
0.1-1 
-- 3-6 
CoCrAlYPt Bal -- 10-50 
4-12 
0.1-1 
-- 3-6 
NiCrAlYTa -- 53-75 
15-25 
7-14 
0.1-1 
3-8 -- 
NiCoCrAlYTa 
10-40 
Bal 15-25 
7-14 
0.1-1 
3-8 -- 
CoCrAlYTa 30-83 
-- 10-50 
4-12 
0.1-1 
3-8 -- 
NiCrAlYTaPt 
-- 47-72 
15-25 
7-14 
0.1-1 
3-8 3-6 
NiCoCrAlYPtTa 
10-40 
Bal 15-25 
7-14 
0.1-1 
3-8 3-6 
CoCrAlYPtTa 
24-80 
-- 10-50 
4-12 
0.1-1 
3-8 3-6 
__________________________________________________________________________ 
Bal = balance of weight 
**When hafnium is used in place of yttrium or along with yttrium, it can 
be added in an amount between about 0.1 and 2.0 weight percent. 
Generally, the coating composition of this invention would have R between 
19 and 83 weight percent of the alloy; Cr between 10 and 50 weight percent 
of the alloy; Al between 4 and 14 weight percent of the alloy; R' between 
0.1 and 3 weight percent of the alloy; and R" between 3 and 14 weight 
percent of the alloy. 
The oxide dispersion in the coating mixture can be added in an amount from 
5 to 20 volume percent based on the volume of the coating mixture, 
preferably between 8 and 12 volume percent. The preferred oxide dispersion 
would be alumina. To prepare the coating mixture, the alloy should be 
prepared with the elements in an amount to provide an alloy composition as 
shown, for example, in Table 1. Preferably, the alloy could be made by the 
vacuum melt process in which the powder particles are formed by inert gas 
atomization. The oxide component could then be added to the alloy in an 
appropriate amount and blended to produce a composite powder by 
ball-milling, attritor milling or any other technique. The preferred 
powder size would be about 5 to 100 microns, and more preferably between 
10 to 44 microns. The composite powder produced can then be deposited on a 
substrate using any thermal spray device. Preferred thermal spray methods 
for depositing the coating are inert gas shrouded plasma spraying, low 
pressure or vacuum plasma spraying in chambers, high velocity oxygen-fuel 
torch spraying, detonation gun coating or the like. The most preferred 
method is inert gas shrouded plasma spraying. It could also be 
advantageous to heat treat the coating using appropriate times and 
temperatures to achieve a good bond for the coating to the substrate and a 
high sintered density of the coating, and then to peen the coating. Some 
suitable substrates are nickel base superalloys, nickel base superalloys 
containing titanium, cobalt base superalloys, and cobalt base superalloys 
containing titanium. Preferably, the nickel base superalloys would contain 
more than 50% by weight nickel and the cobalt base superalloys would 
contain more than 50% by weight cobalt. A sample of specific substrates 
are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
(All elements in weight percent*) 
Alloy Ni Co Cr W Mo Ta Ti Al Hf Zr C B Cb Re V 
__________________________________________________________________________ 
Ma, M-002 
Bal 
10 9 10 -- 2.5 
1.5 
5.5 
1.5 
.05 
.15 
.015 
-- -- -- 
Rene 80 Bal 
9.5 
14 4 4 -- 5 3 -- .06 
.17 
.015 
-- -- -- 
Mar-M-200 + Hf 
Bal 
10 9 12.5 
-- -- 2 5 2 -- .14 
.015 
1 -- -- 
CMSX-4 Bal 
9.5 
6.5 
6.4 
0.6 
6.5 
1 5.6 
0.1 
-- .006 
-- -- 3 -- 
IN-100 Bal 
15 9.5 
-- 3 -- 4.75 
5.6 
-- .06 
.17 
.015 
-- -- 1 
B-1900 Bal 
10 8 -- 6 4.25 
1 6 1.1 5 
.08 
.11 
.015 
-- -- -- 
Mar M-509 
10 Bal 
22.5 
7 -- 3.5 
0.2 
-- -- 0.5 
0.6 
.01 
-- -- -- 
Max 
__________________________________________________________________________ 
*Balances of compositions were minor traces of other elements 
Mar M is a trademark of Martin Metals Co. 
Rene is a trademark of General Electric Co. 
CMSX is a trademark of CannonMuskagon Co. 
IN is a trademark of International Nickel Co.

EXAMPLE 1 
Several different alloys were made in which single element additions of 
tantalum or platinum were added to a melt containing Ni and/or Co plus 
CrAlY prior to argon atomization to powder. The addition of the tantalum 
or platinum was at the expense of cobalt in the composition of the alloy. 
Additional coatings were made in which an oxide dispersion was added. The 
alloy powder was mixed with 0.3 micron diameter alumina and attriton 
milled to produce a powder mixture passed through a -325 tyler mesh (44 
microns). The various powder compositions were plasma sprayed to various 
thicknesses on various substrates using an argon shrouded plasma torch 
operating at 150 amperes. The various powder compositions are shown in 
Table 3. 
TABLE 3 
______________________________________ 
Volume 
percent of 
oxide in 
Composition 
Weight percent of element in alloy 
mixture 
Sample Co Ni Cr AI Y Ta Pt Al.sub.2 O.sub.3 
______________________________________ 
Sample A 38 32 21 8 0.5 
CoNiCrAlY 
Sample B 35 32 21 8 0.5 3 
CoNiCrAlYTa 
Sample C 30 32 21 8 0.5 8 
CoNiCrAlYTa 
Sample D 35 32 21 8 0.5 3 
CoNiCrAlYPt 
Sample E 32 32 21 8 0.5 6 
CoNiCrAlYPt 
Sample F 74 18 8 0.7 
CoCrAlY 
Sample G 74 18 8 0.7 10 
CoCrAlY 
Sample H 74 18 8 0.7 20 
CoCrAlY 
Sample I 15 53 20 11 0.5 
NiCoCrAlY 
Sample J 15 53 20 11 0.5 10 
NiCoCrAlY 
Sample K 15 53 20 11 0.5 20 
NiCoCrAlY 
______________________________________ 
*Balances of compositions were minor traces of other elements 
The various coating compositions were coated on a substrate of Mar M-002 to 
a thickness of about 6 mils. The coatings were tested in a burner rig with 
the following characteristics: 
______________________________________ 
Mass air flow 60 lbs./min 
Gas velocity 650 ft/sec. 
Fuel Standard aviation kerosene 
Sulfur content 
0.2% in fuel 
Synthetic salt 
0.5 ppm into pre-combustor line 
Specimen Temp. 
1050 C. 
Hot time 13 minutes per cycle with 1 minute 
cool-down then 1 minute heating for 
the next cycle 
______________________________________ 
The data obtained are shown in FIG. 1. As can be seen for Table 1, Samples 
B, C D and E increased the life of the coating by 40 percent or more over 
Sample A. Also Samples G and H increased the life of the coating over 40 
percent or more over Sample F, and Samples J and K increased the life of 
the coating over Sample I. In each case, an additive of platinum, tantalum 
or oxide to the base Samples A, F and I made significant improvements to 
the burner rig life test of the coating. 
The thermal expansion of the samples of Table 3 were measured in a vertical 
dilatometer constructed of a tripod support legs and a central pushrod, 
all made from the same single crystal of sapphire. The heating rate was 
held constant at 5.degree. C./min., while a computer recorded specimen 
temperature and length signal from a linear variable differential 
transformer (LVDT) attached to the pushrod. The mean coefficients between 
25.degree. and 1050.degree. C. are shown in FIG. 2. The Samples were 
tested in the as-coated condition and as a second set having been first 
vacuum heat treated 4 hours at 1080.degree. C. In comparison, typical 
nickel base superalloys for turbine blades have CTE's to 1050.degree. C. 
of 16.6, 17.0, and 16.1 uin./in./C. for Mar-M-002, Rene' 80, and 
Mar-M-200+Hf, respectively. The as-coated FIG. 2 and heat treated FIG. 3 
results are only slightly different for most of the coatings, possibly due 
to relaxation of residual stress. The Samples A, F and I coatings have 
CTE's of about 18 uin./in./C. and one of the object was to reduce this to 
be closer to the blade alloys. Adding tantalum does not change the CTE. 
However, the platinum addition makes a substantial reduction in coating 
CTE after heat treatment. This would be the normal state of the coating on 
the turbine blade. It may be that the heat treatment allows the formation 
of a platinum aluminide or some other phase that has low CTE, that reduces 
the overall CTE of the coating. This finding was an unexpected benefit of 
the platinum addition since up to now its main attribute has been to 
improve hot corrosion resistance. Finally the oxide additions were found 
to be very effective in reducing CTE. The results show that 10 vol. 
percent alumina reduces the CTE from about 18 to 17 uin./in./C., and 20 
vol. percent reduces the CTE to about 15.7 uin./in./C. 
Thick coatings of each of the candidate materials were heat treated, 4 
hours at 1080.degree. C., ground flat and machined to a tensile test 
profile, with reduced strip width in the gage section. The tensile tests 
were done at 800.degree. and 1000.degree. C., using a strain rate of 0.005 
to 0.006 in./in./min. up to the 0.2% offset yield stress. These data are 
shown in FIG. 4. Coating Samples A and F were also included. We found that 
by adding tantalum to the CoNiCrAlY (Sample B) the yield strength at 
800.degree. C. more than doubled for a 3 wt. percent addition, and more 
than tripled for an 8% Ta addition (Sample C). The platinum additions to 
Sample D increased the yield strength by a more modest 40 percent. When we 
added an aluminum oxide dispersion to CoCrAlY, the yield strength tripled 
for a 10 vol. percent addition (Sample G), and increased by a factor of 5 
for a 20 vol. percent addition (Sample H). As evident from FIG. 5, at 
1000.degree. C., we found that the tantalum or platinum additions were no 
longer contributing to higher yield strength, but that the oxide continued 
its strengthening role. For both the CoNiCrAlY (Sample A) and the CoCrAlY 
(Sample F) base alloys, the increased yield strength was about twice for a 
10 vol. percent addition and about 4 times for a 20 vol. percent addition. 
EXAMPLE 2 
Sample coating powders were produced as above except that the compositions 
were different. The compositions of the various powders are as shown in 
Table 4. The object here was to explore multiple additions and 
combinations of the Example 1 single component additions to look for even 
greater synergestic improvements in coating properties. 
TABLE 4 
______________________________________ 
Oxide addi- 
tive Vol- 
Elements by Weight ume percent 
Composition 
of Composition of mixture 
Sample Co Ni Cr Al Y Ta Pt Al.sub.2 O.sub.3 
______________________________________ 
I 40.5 30.7 21.3 7.8 0.3 
II 32.1 31.0 21.0 7.7 0.6 2.8 4.8 
III 38.7 32.4 21.2 6.8 0.6 11.2 
IV 38.2 31.1 21.7 6.3 0.6 3.2 11.4 
V 35.6 32.1 21.2 6.6 0.4 4.9 11.3 
VI 33.0 31.1 21.5 7.2 0.6 2.8 5.0 10.6 
VII 74.7 18.6 5.9 0.5 11.5 
VIII 64.7 20.2 6.4 0.4 3.1 4.9 11.1 
______________________________________ 
The coating powders shown in Table 4 were coated on smooth substrates, such 
that samples could be easily removed for free-standing density 
measurement. The coating samples were heat treated in vacuum for 4 hours 
at 1080.degree. C., then tested for density by the water immersion method 
(ASTM B-328). The theoretical density is the density of the material in a 
porosity-free condition. This can be calculated from the composition by 
the Hull method (F. C. Hull, "Estimating Alloy Densities", Metal Progress, 
November 1969, p. 139) and with correction for oxide addition for certain 
coatings of Table 4. The density results are shown in Table 5. The fully 
metallic coatings (I and II) were found to reach a high percentage of 
their theoretical density, about 95 to 96 percent. The oxide dispersed 
coatings were found to only reach lower densities after heat treatment, 88 
to 90 percent of theoretical. 
TABLE 5 
__________________________________________________________________________ 
1050.degree. C. EXTERNAL 
*DENSITY gm/cm3 HOT HARDNESS, Hv 0.6 Kg 
ALUMINIDE 
Coating TEMPERATURE, .degree.C. 
DEPLETION 
Sample 
Apparent 
Theoretical 
% Theor 
22 200 
400 
600 
800 
900 
100 Hrs 
300 Hrs 
__________________________________________________________________________ 
I 7.27 7.57 96.1 604 
441 
277 
21 61 33 0.57 0.59 
II 7.58 7.99 94.9 459 
302 
281 
221 
74 17 0.44 0.83 
III 6.58 7.28 90.4 467 
371 
291 
216 
77 39 0.71 1.07 
IV 6.62 7.39 89.5 242 
189 
163 
143 
59 25 0.53 1.36 
V 6.83 7.71 88.6 260 
201 
143 
111 
51 25 0.61 1.12 
VI 6.88 7.82 87.9 326 
243 
118 
61 
31 24 0.42 0.80 
VII 6.65 7,45 89.2 511 
359 
294 
219 
52 23 0.76 3.36 
VIII 6.96 7.80 89.3 537 
413 
374 
306 
114 
51 0.29 0.93 
__________________________________________________________________________ 
*average density after 4 hour 1080.degree. C. vac. heat treatment 
The hot hardness of the coatings was measured for temperatures up to 
900.degree. C. using a Vickers type indentor under a load of 0.6 Kg. The 
results are shown in Table 5. It was found that the coatings with the 
oxide and platinum and/or tantalum additive were generally softer than the 
Sample I coating and this effect is believed to be strongly related to the 
lower coating densities. Even so, the three-way addition coating Sample 
VIII was found to have significantly higher hardness at high temperature 
than Sample I. It was found that Sample VII had about the same hot 
hardness as Sample I and that Sample II was softer than Sample I even 
though the densities of Sample I and Sample II were reasonably close. The 
Ta plus Pt addition softened the coating, while the Oxide addition made 
little change, and yet, Ta+Pt+Oxide had a substantial hardening effect. 
Thus an unexpected interaction occurred in the three-way addition that 
gave a significantly improved hot hardness result. 
The coatings of Table 4 were deposited on Mar M-002 pin substrates at 
nominally 6 mils thickness, heat treated 4 hours at 1080.degree. C., 
finished smooth and peened at 12N Almen intensity. They were given a 
cyclic oxidation test exposure at 1050.degree. C. in air, with a 50 minute 
period in the furnace and a 10 minute cooling period. Separate samples 
were tested to 100 hours total, and to 300 hours total. The tested pins 
were gold and nickel plated, then mounted in cross section to measure the 
width of the aluminide depletion layer beneath the external surface. We 
consider the thickness of this depletion to be a measure of the 
consumption of coating life, and thus smaller depletion layers for the 
same time and temperature exposure is a sign of longer coating life. The 
data observed is shown in Table 5 and demonstrates that after 100 hours, 
several coatings have less depletion than Sample I, and these all contain 
Ta as one of the additives. We also found that when the additive was the 
oxide alone, the depletion thickness increased over Sample I, an effect 
which may be due to the lower density. However, when Ta+Pt is additionally 
added to the oxide bearing coating the depletion is significantly reduced. 
These particular data indicate that Ta is the most effective addition for 
reduced aluminide depletion in oxidation, and that it or in combination 
with Pt, allow the oxidation resistance of oxide addition coatings to be 
restored. This combination will allow oxide additions to be made for other 
purposes, such as creep resistance, without sacrificing oxidation 
resistance. Considering the overall performance at 100 and 300 hours, the 
three-way addition of Ta+Pt+Oxide produced the best results. 
The coatings of Table 4 were deposited on 7 mm diameter by 85 mm long bar 
for burner rig testing. The coatings were heat treated, finished and 
peened. In this test, the rig was operated as in Example 1, except the 
temperature was increased to 1100.degree. C. The results for cycles to 
failure are shown in FIG. 6. The Sample I coating was not in this test, 
but from Example 1 we see that the oxide-only addition coatings are better 
than the Sample I coatings. The data indicate that the oxide+platinum and 
the oxide+tantalum+platinum multiple additions to be significantly better. 
They had about two to three times greater life in the burner rig test than 
the oxide-only additions, which would make them significantly better than 
the no-addition simple Sample I coating. 
Some of the compositions of Tables 3 and 4 were deposited on bars and 
subjected to a burner rig test. In this case the substrate was single 
crystal CMSX-4, an advanced nickel base alloy for high performance turbine 
blades. The burner rig test was as described above, except the injected 
salt concentration was reduced to 0.25 ppm. The test was at 1100.degree. 
C., with 13 minutes in the burner and 1 minute out of flame cooling, as 
before. It took about 1 minute to come back to 1100.degree. C. when cycled 
back into the burner. The results are presented in FIG. 7, as the cycles 
to failure divided by the coating thickness in mils. This essentially 
normalizes out any life difference due to different coating thickness. The 
coatings were still made to nominally 6 mils thickness. It was found that 
all coatings containing platinum had about twice the life as coatings with 
only tantalum or oxide single additions. Furthermore, tantalum plus 
platinum was slightly better than platinum alone, and tantalum plus 
platinum plus oxide was the best of all. 
To assess the ability of the new coating formulations to resist diffusion 
into the substrate, we ran long exposure isothermal tests at 1100.degree. 
C. in argon, then measured the interdiffusion zone width in cross section. 
We used the same Sample I coating to which we added the additives. FIG. 8 
shows the results up to 60 hours. We found that the coatings with the 
greater diffusion zone widths contained platinum. The presence of tantalum 
alone or the combination of tantalum and platinum additions reduced 
interdiffusion. The further addition of oxide to a platinum bearing 
coating also reduced interdiffusion. The best coating for reducing 
interdiffusion from our studies was the three-way addition of platinum 
plus tantalum plus oxide. The overall observation is that the tantalum and 
oxide additions are the most effective components for reducing 
interdiffusion. 
The data show that tantalum, particularly in combination with oxide are 
effective additions for our purpose of reducing interdiffusion with the 
substrate. We want to limit the loss of Al and Cr from the coating to 
diffusion into the substrate, but we also want to keep deleterious 
elements from the substrate from entering the coating and reducing its 
protective nature. To help understand the particular effect of tantalum, 
we had coatings with and without the tantalum addition analyzed by the 
electron microprobe after an oxidation exposure of up to 400 hours at 
1050.degree. C. The substrates were In-100 and Mar M-002. It was 
discovered that the tantalum in the coating was reacting with titanium 
which was trying to diffuse from the substrate into the coating. Ta-Ti 
particles would form, at first in the coating near the substrate, and 
later higher up in the coating as time progressed and the tantalum was 
consumed in this reaction. Thus one role of Ta in improving the oxidation 
resistance was thus the trapping of Ti and reducing its migration to the 
outer oxide scale on the coating, where it had been adversely affecting 
the protective scale. This finding makes the value of tantalum additions 
to coatings particularly useful on titanium bearing superalloy substrates 
such as IN 100, Mar-M-002, Rene' 80, and on variants of Mar-M 200 and 
B-1900. 
The coatings containing particularly the oxide additions were found to have 
reduced densification in a typical heat treatment (2 to 4 hours at 
1080.degree. C. to 1100.degree. C.). The lower density obtained in Coating 
Samples III through VIII as shown in Table 5 are expected to reduce all 
mechanical properties compared to what they would be at a higher density, 
and reduce the oxidation and corrosion resistance somewhat. The thermal 
expansion and Poisson's ratio values would not be expected to be affected. 
While the properties are still good at the normal heat treated density, 
further improvements are expected at the higher density. Free-standing 
pieces of the Sample VI coating (CoNiCrAlY plus Pt, Ta and Oxide) were 
made and subjected to heat treatments at higher temperatures. The results 
showed that the improvement in density is obtainable if the newly 
developed coating is combined with a heat treatment that is optimized to 
the coating. It is expected that the higher density obtained will enhance 
other coating properties, such as increasing oxidation and sulfidation 
resistance, increasing strain to failure, increasing yield strength and 
ultimate tensile strength, and increase the coatings' resistance to creep. 
The thermal expansion coefficient would not be expected to change. 
The improved coatings of this invention are also useful as bondcoats or 
undercoats in a thermal barrier coating system. In a thermal barrier, 
typically there is a three to ten mil thick bondcoat of a Sample I coating 
(Table 4), then a ten to twelve mil or more thick layer of 
yttria-stabilized zirconia coating. A blade was coated with a bondcoat 
using the Sample VI coating, followed by a 10 ml thick layer of 
yttria-stabilized zirconia. The same plasma torch was used for both 
layers, with just the powders and the operating conditions changed. The 
thermal barrier coated blade was heat treated 2 hours at 1100.degree. C. 
in vacuum, and the zirconia external surface smoothed in the vibratory 
finisher using abrasive alumina media. The microstructure of the thermal 
barrier coated blade showed a well bonded undercoat and oxide layer which 
is needed for successful barrier coating. 
It is to be understood that modifications and changes to the preferred 
embodiment of the invention herein shown and described can be made without 
departing from the spirit and scope of the invention. For example, a 
post-coating of aluminum or chromium by the pack cementation process over 
the coating of this invention could result in at least a portion of the 
aluminum or chromium diffusing into the coating. Another example is to 
have a coating of this invention coated with a top layer of zirconia to 
produce a good duplex thermal barrier coating.