Thermal barrier coating for substrates and process for producing it

A thermal barrier coating for substrates comprising zirconia partially stabilized by yttria and having a density greater than 88% of the theoretical density with a plurality of vertical macrocracks homogeneously dispersed throughout the coating to improve its thermal fatigue resistance. The invention also discloses a process for producing the thermal barrier coating.

FIELD OF THE INVENTION 
The invention relates to a thermal barrier coating and process for 
producing such a coating for substrates intended to operate in cyclic 
thermal environments, said coating comprising zirconia partially 
stabilized by yttria and having a substantial homogeneous dispersion of 
vertical macrocracks throughout the coating to improve its thermal fatigue 
resistance. 
BACKGROUND OF THE INVENTION 
Modern gas turbine engines operate in high temperature environment in 
excess of 2000.degree. F. in which hot gases are expanded across rows of 
turbine blades. Outer air seal or shroud segments circumscribe the turbine 
blades to minimize leakage of the gases over the tips of the blades. The 
use of thermal barrier coatings on gas turbine blades and surfaces such as 
such as shroud segments have been found to have several advantages. 
Through the use of thermal barrier coatings higher operating efficiency 
can be obtained because less cooling air is required to maintain blade or 
shroud temperatures. In addition, component life is extended since the 
rate of change of metal temperature is reduced by the insulating effect of 
the thermal barrier. 
Zirconia based thermal barrier coatings, because of their low thermal 
conductivity, are added to the surface of metal components to insulate 
them from the hot gas stream. Stabilized zirconia was developed and used 
as a thermal barrier coating for turbine and shroud components. Coatings 
such as CaO stabilized zirconia, MgO stabilized zirconia and Y.sub.2 
O.sub.3 stabilized ziconia have been tested with Y.sub.2 O.sub.3 partially 
stabilized zirconia providing the best results. 
U.S. Pat. No. 4,377,371 discloses an improved thermal shock resistance of a 
ceramic layer in which benign cracks are deliberately introduced to a 
plasma-sprayed ceramic layer. The benign cracks are generated by scanning 
a laser beam over the plasma-sprayed ceramic surface where the ceramic 
material immediately beneath the beam melts to produce a thin fused layer. 
Shrinkage accompanying cooling and solidification of the fused layer 
produces a network of microcracks in the fused layer that resists the 
formation and growth of a catastrophic crack during thermal shock 
exposure. Another method disclosed for introducing fine cracks on the 
surface of a ceramic coating is to quench the surface of the ceramic while 
it is hot with an ethanol saturated paper pad. 
An article published by the AIAA/SAE/ASME 16th Joint Propulsion Conference, 
June 30-July 2, 1980, Development of Improved-Durability Plasma Sprayed 
Ceramic Coatings for Gas Turbine Engines by I. E. Summer et al, discloses 
that the durability of plasma sprayed ceramic coatings subjected to cyclic 
thermal environment has been improved substantially by improving the 
strain tolerance of a ceramic structure and also by controlling the 
substrate temperature during the application of the coating. It further 
states that the improved strain tolerance was achieved by using ceramic 
structures with increased porosity, microcracking or segmentation. 
In an article published by J. Vac. Sci. Technol. A3 (6) November/December 
1985 titled Experience with MCrAl and Thermal Barrier Coatings Produced 
Via Inert Gas Shrouded Plasma Deposition, by T. A. Taylor et all, 
discloses the depositing of a ceramic oxide coating of ZrO.sub.2 -7 wt % 
Y.sub.2 O.sub.3 onto a coated substrate. The ceramic oxide coating is a 
thermal barrier coating which has intentionally imparted microcracks 
having an average spacing of about 15 microns and which are staggered from 
layer to layer of the coating. 
It is an object of the present invention to provide a thermal barrier 
coating for components intended to be used in cyclic thermal environments 
in which the thermal barrier coating has deliberately produced macrocracks 
homogeneous dispersed throughout the coating to improve its thermal 
fatigue resistance. 
It is another object of the present invention to provide a thermal barrier 
coating for components of turbine engines in which the coating is composed 
of zirconia partially stabilized by yttria and in which the coating has a 
density greater than about 88% of theoretical. 
It is another object of the present invention to provide a thermal barrier 
top coating over a bond coating of an alloy containing chrominum, 
aluminum, yttrium with a metal selected from the group consisting of 
nickel, cobalt and iron. 
It is another object of the present invention to provide a thermal barrier 
coating for gas turbine blades, vanes and seal surfaces exposed in the hot 
section of gas turbine engines. 
It is another object of the present invention to provide a process for 
producing a thermal barrier coating having good thermal fatigue 
resistance. 
SUMMARY OF THE INVENTION 
The invention relates to a thermal barrier coating for protecting a 
substrate such as blades, vanes and seal surfaces of gas turbine engines, 
said coating comprising zirconia partially stabilized by yttria, having a 
density greater than 88% of the theoretical density, and having a 
plurality of vertical macrocracks substantially homogeneously dispersed 
throughout the coating in which a cross-section area of the coating normal 
to the surface of substrate exposes a plurality of vertical macrocracks 
with at least 70%, preferably at least 90%, of said macrocracks extending 
at least 4 mils, preferably 8 mils, in length up to the thickness of the 
coating and having 20 to 200 vertical macrocracks, preferably from 75 to 
100 vertical macrocracks, per linear inch measured in a line parallel to 
the surface of the substrate and in a plane perpendicular to its 
substrate. The length of at least 70%, preferably 90%, of the vertical 
macrocracks should extend at least 4 mils so that they pass through at 
least 50 splats of the deposited powder. 
The invention also relates to a process for producing a thermal barrier 
coating having good thermal fatigue resistance comprising the steps: 
a) thermally depositing zironia-yttria powders onto a substrate to form a 
monolayer having at least two superimposed splats of the deposited powders 
on the substrate in which the temperature of a subsequent deposited splat 
is higher than the temperature of a previously deposited splat; 
b) cooling and solidifying said monolayer wherein said monolayer has a 
density of at least 88% of the theoretical density and wherein a plurality 
of vertical cracks are produced in the monolayer due to shrinkage of the 
deposited splats; 
c) repeating steps a) and b) at least once to produce an overall coated 
layer in which each monolayer has induced vertical cracks through the 
splats and wherein at least 70% of the vertical cracks in each monolayer 
are aligned with vertical cracks in an adjacent monolayer to form vertical 
macrocracks having a length of at least 4 mils up to the thickness of the 
coating and said coated layer having at least 20 vertical macrocracks per 
linear inch measured in a line parallel to the surface of the substrate. 
As used herein, a splat shall mean a single molten powder particle impacted 
upon the surface of the substrate wherein it spreads out to form a thin 
platelet. Generally these platelets are from 5 to 100 microns in diameter 
and 1 to 5 microns thick, more generally about 2 microns thick. 
As used herein, a vertical macrocrack is a crack in the coating if extended 
to contact the surface of the substrate will form and angle of from 
30.degree. to 0.degree. with a line extended from said contact point 
normal to the surface of the substrate. Preferably, the vertical 
macrocracks will form an angle of 10.degree. to 0.degree. with the normal 
line. In addition to vertical macrocracks, one or more horizontal 
macrocracks may develop in the coating. Preferably, the coating should 
have no horizontal macrocracks. A horizontal macrocrack is a crack forming 
an angle of from 10.degree. to 0.degree. with a plane bisecting said crack 
and diposed parallel to the surface of the substrate. If present, the 
horizontal macrocracks preferably should not extend to contact more than 
one vertical macrocrack since to do so could weaken the coating and 
subject the coating to spalling. The length dimension of the vertical 
macrocrack and the length dimension of the horizontal macrocrack is the 
straight line distance from one end of the crack to the opposite end of 
the crack. The length of the horizontal macrocrack, if present, could be 
from about 5 to 25 percent of the average length of the vertical 
macrocracks on both sides of the horizontal macrocrack. 
For most applications, the density of the coating preferably should be 
between 90% and 98% of the theoretical density and most preferably about 
92 percent of the theoretical density. The vertical macrocracks are formed 
in the coating by plasma depositing powders of the coating onto the 
surface of the substrate in discrete monolayers in which the thickness of 
each monolayer contains at least two superimposed splats of the deposited 
powder (about 0.16 mils) and preferably from about four to five splats of 
the deposited powder (from about 0.32 mils and 0.40 mils, respectively). 
Although not bound by theory, it is believed that the deposit of two or 
more superimposed splats of the powder will result in the second and 
subsequent splats being deposited at higher temperatures than the 
preceding splats. This is due to the fact that the first splat of the 
powder is deposited on a relatively colder substrate while the second and 
subsequent splats are deposted on preceding splats that are progressively 
hotter. Thus the overall deposit of two or more splats results in a 
temperature gradient with the higher temperature at the top surface. Upon 
cooling and solidification of the monolayer deposit, the second and 
subsequent splats shrink more than the preceding splats and form vertical 
microcracks through the deposited layer. Additional monolayers are 
superimposed on the substrate with each monolayer forming vertical 
macrocracks which have a tendency to align with the previously formed 
macrocracks in the preceding monolayers. This effectively produces some 
macrocracks that extend substantially through the thickness of the 
coating. The width of the vertical macrocracks, i.e., the distance between 
opposing faces defining the vertical macrocracks, is generally less than 
about 1 mil, preferably less than 1/2 mil. 
It has been found that if the density of coating is less than 88% of the 
theoretical density, the stress caused by the shrinkage of splats in the 
monolayer may be absorbed or compensated by the porosity of the coating. 
This will effectively prevent the formation of the macrocracks throughout 
the coating as is required according to this invention and prevent 
producing a coating with good thermal fatigue resistance. The substantial 
homogeneous distribution of vertical macrocracks throughout the coating as 
requied by this invention will reduce the modulus of elasticity of the 
coating structure thereby reducing the local stresses. This results in 
excellent thermal fatigue resistance for the coating that enables it to 
function without failure in cyclic thermal environments. 
The density of the vertical macrocracks should be preferably 75 or more, 
most preferably 100 or more, vertical macrocracks per linear inch taken in 
a cross-section plane of the coating along a line parallel to the surface 
of the substrate. This will insure that sufficient vertical macrocracks 
are present in the coating to provide good thermal fatigue resistance. To 
obtain the necessary vertical macrocracks in this coating, the plasma 
apparatus should be of high efficiency and stable over the period of 
depositing the coating. The spray torch should be positioned at a fixed 
distance from the substrate and the relative speed between the torch and 
the substrate should be controlled to insure that the monolayer instantly 
put down by one sweep of the torch will be sufficient to produce overlap 
of the deposited splats of powder in which the second and subsequent 
deposited splats are hotter than the preceding deposited splats for the 
reason discussed above. The overall thickness of the coating can vary 
depending on the end use application. For components of gas turbine 
engines, the coating thickness can vary from 0.003 to 0.10 inch. The 
preferred zirconia partially stablized by yttria would be 6 to 8 weight 
percent yttria with the balance zirconia and most preferably about 7 
weight percent yttria with the balance substantially zirconia. The thermal 
barrier coating of this invention is ideally suited as a top coat for a 
metallic bond coated substrate such as blades, vanes and seals of gas 
turbine engines. The preferred metallic bond coating would comprise an 
alloy containing chromium, aluminum, yttrium with a metal selected from 
the group consisting of nickel, cobalt and iron. This bond coating can be 
deposited using conventional plasma spray techniques or any other 
conventional technique. The substrate could be any suitable material such 
as a nickel-base, cobalt-base or iron-base alloy. 
While the preferred embodiment of the invention has been described, it will 
be appreciated that various modifications may be made to the thermal 
barrier coating without departing from the spirit or scope of the 
invention. 
Thermal Fatigue Test 
Cyclic thermal exposure can help distinguish between a number of candidate 
thermal barrier coatings with regard to thermal fatigue resistance. A good 
thermal barrier coating must be able to survive a large number of thermal 
cycles to high temperature without spalling if it is to be useful in 
service. 
To test the samples of this invention, a round metal alloy disc coated with 
a thermal barrier coating on one face was fabricated. The coated face was 
exposed to a high heat flux gas burner while the metal back face was 
allowed to cool by convection in air. The apparatus was automated with 
timers and a stepping motor which place the coated disc in the flame of a 
gas burner for a fixed time then move it out of the flame and to a second 
position where an air blast cools the coated face. The time at each 
position is adjustable, as well as the maximum temperature reached at the 
heating position. In the test work described herein the fixed variables of 
the test were as follows: 
20 seconds heating to 2550.degree. F. (average maximum temperature measured 
on the thermal barrier coated face) wherein the back metal face reaches 
about 1400.degree. F., followed by 
20 seconds blast air cooling to about 1500.degree. F., followed by 
40 seconds natural convection cooling to 850.degree. F. (average minimum 
temperature measured on the thermal barrier coated face) 
2000 heating/cooling cycles constitutes the full test. 
The thermal barrier coated layer thicknesses and compositions were as 
follows: 
6 to 8 mil thick bond coating of Co-32N-21Cr-8Al-0.5Y composition and 
43 to 47 mil thick topcoat of a thermal barrier layer of ZrO.sub.2 -6 to 8 
weight percent Y.sub.2 O.sub.3 composition. 
Before starting the thermal test, the edge of the coated disk was polished 
so that the thermal barrier coating would display any separation cracks 
that might be induced by the thermal test. These separation cracks are 
horizontal cracks within the thermal barrier layer that are visible at the 
polished edge. Usually, if a coating is susceptible to this cracking, a 
number of short horizontal crack segments are seen to grow and link up 
around the edge circumference of the thermal barrier layer. Usually the 
location of these cracks is within 5 to 15 mils of the bond coat 
interface. The lengths of these individual or linked cracks are measured 
after the thermal test. A stereoscopic microscope at 30.times. 
magnification is used to detect all such cracks. The total length of edge 
cracks is expressed as a percentage of the circumference length; i.e., 
100% edge cracking would have a visible crack fully around the entire edge 
circumference. In some cases where 100% edge cracking would occur, the 
thermalbarrier layer could spall off. In other cases, it remains bonded by 
uncracked areas deeper into the coating. In either case, 100% or other 
high percentage edge cracking results are taken as indicative of poor 
thermal fatigue resistance of that particular thermal barrier specimen. 
Thermal barrier coatings that have a low percentage of edge cracking at 
the conclusion of the test are considered to have good thermal fatigue 
resistance. Thermal barrier coatings that have zero percent edge cracking 
at the end of the test are considered to have outstanding thermal fatigue 
resistance.

EXAMPLE 1 
In this example, three different zirconium-yttrium oxide thermal barrier 
coatings (Samples A, B, and C) were prepared to have different macrocrack 
structures and then subjected to the thermal cycle test. All coatings were 
made from the same starting powder having the following characteristics 
shown in Table 1. 
TABLE 1 
______________________________________ 
Powder Characteristics 
______________________________________ 
Composition: 7.11 wt. % Y.sub.2 O.sub.3, 0.23 SiO.sub.2, 
0.15 TiO.sub.2, 0.07 Al.sub.2 O.sub.3, 0.09 Fe.sub.2 
O.sub.3, 
balance ZrO.sub.2 
Type powder: fused and crushed 
Size analysis: 
+200 mesh 0.0 wt. percent 
+230 mesh 0.0 wt. percent 
+325 mesh 18.55 wt. percent 
-325 mesh 81.45 wt. percent 
______________________________________ 
Using Microtrac.sup.@ analysis, the mean particle diameter size was found 
to be 40.95 microns. 
FNT .sup.@ Microtrac powder size analysis instrument, Model 7995-11 by Leeds 
and Northrup Co. 
All three coatings were deposited on 1 inch diameter.times.1/8 inch thick 
Inconel 718 discs. All sample discs had a 6 mil bond undercoat of a plasma 
sprayed alloy of Co-32Ni-21Cr-8Al-0.5Y. 
A number of specimens were made for each sample. A specimen of each sample 
was mounted on edge in epoxy resin, cured under pressure, then polished in 
cross-section so that the structure could be quantitatively analyzed. The 
high pressure epoxy cure allows penetration of epoxy into the somewhat 
porous zirconium-yttrium oxide layer which then better preserves the 
nature of the structure during abrasive polishing. The specimens were 
examined at 100.times. using a Leitz Orthoplan microscope, for an analysis 
of the microcrack structure. Separate specimens of the thermal barrier 
layer samples were carefully removed from the substrates and measured for 
their density. The density procedure using the water immersion method is 
described in ASTM B-328. All were sprayed with the same Union Carbide 
plasma torch, Model 1108. Certain torch operating parameters, standoff 
distance from torch to substrate and substrate velocity past the torch 
spray were changed in this example, to show how superior thermal fatigue 
resistance can be achieved. The properties and test data for each sample 
specimen are shown in Tables 2, 3, and 4. 
TABLE 2 
__________________________________________________________________________ 
Plasma Spray Conditions.sup.+ 
Total Torch Monolayer 
Final 
coating Powder* 
Current, 
Standoff 
Substrate** 
Height 
Coating 
Sample 
thickness, mils 
Feedrate 
(amps) 
(in) velocity 
(mils) 
Temperature 
__________________________________________________________________________ 
A 45 90 150 0.75 6,000 0.16 317.degree. F. 
B 45 90 150 0.75 12,000 
0.07 341.degree. F. 
C 45 50 170 0.87 2,750 0.34 472.degree. F. 
__________________________________________________________________________ 
*grams/minute 
**inches/minute 
.sup.+ The primary torch gas flows for each sample was: 90 cfh torch gas, 
90 cfh powder carrier (both argon), and 40 cfh auxiliary (hydrogen). 
TABLE 3 
__________________________________________________________________________ 
Thermal Barrier Layer Properties 
% of Vertical 
Vertical Horizontal 
Density 
Theoretical 
cracks Macrocracks per 
Branch cracks 
Sample 
(gm/cm.sup.3) 
Density* 
length, (mils) 
Inch laterally 
length, (mils) 
__________________________________________________________________________ 
A 5.471 
90.29 4-10 77.6 1-2 
B 5.485 
90.53 0 0.0 0 
C 5.539 
91.42 20-40 86.4 2-4 
__________________________________________________________________________ 
***Theoretical density is the density of the porefree materials, 6.059 
gm/cm.sup.3 for ZrO.sub.2 7.11 wt. % Y.sub.2 O.sub.3, as derived from 
Ingel and Lewis, "Lattice Parameters and Density for Y.sub.2 O.sub.3 
Stabilized ZrO.sub.2 ", J. Am. Ceramic Society, Vol. 69, No. 4, p. 325, 
April, 1986. 
TABLE 4 
______________________________________ 
Thermal Fatigue Test Results 
% Edge Cracks 
Sample after 2000 Cycles 
______________________________________ 
A1 32 
A2 0 
B1 100* 
B2 100 
C1 0 
______________________________________ 
*Failed early, after approximately 900 cycles. 
The test results showed Sample C had the best thermal fatigue resistance 
with no edge cracking after the test. Sample A was intermediate with 
specimen A1 having 32% edge cracking and specimen A2 having 0%. Sample B 
was the worst having 100% edge cracking, and specimen B1 failing even 
before the end of the test. 
Samples A and B can be compared to see the effect of the macrocracks in the 
coating structure. The densities of A and B are essentially the same. The 
torch operating parameters were the same, and the final coating 
temperatures were essentially the same. The substantial difference was 
that Sample A was coated with a 6,000 in/min substrate velocity and Sample 
B with 12,000 in/min. This was done to provide different buildup rates of 
the thermal barrier layers on the samples. The monolayer height of Sample 
A was 0.16 mils while that of Sample B was only 0.07 mils. The higher 
monolayer height on Sample A created sufficient stress in the ZrO.sub.2 
-Y.sub.2 O.sub.3 coating layer to produce macrocracks throughout the 
coating of Sample A. Sample A had about 77.6 cracks/inch on average, while 
Sample B, coated at the lower monolayer height had no cracks. With all 
other coating properties the same, the presence of the high number of 
macrocracks in Sample A is responsible for the much better thermal fatigue 
resistance, compared to Sample B which had no macrocracks. 
Sample C is a case where the results of Samples A and B were applied to 
further control the macrocrack structure. In this case slightly higher 
torch current was used in order to increase deposition efficiency so that 
a lower powder feedrate to the torch would produce about the same coating 
volume deposited per minute on the sample specimen. A substantial change 
was also made to substrate velocity in order to produce an even higher 
monolayer height which induced even more macrocracking in the coating. 
Sample C was coated at 2750 inches/minute substrate velocity. Sample C had 
a 0.34 mil monolayer height and 86.4 vertical macrocracks per inch on 
average. After the 2000 cycle thermal test no edge cracking was found upon 
examination. This study showed that macrocracks can be inducted into the 
ZrO.sub.2 -Y.sub.2 O.sub.3 coating by proper control of the coating 
parameters and that macrocracks are critical to the success of the coating 
in a thermal fatigue environment. 
EXAMPLE 2 
In this example, the coating parameters of Sample C of Example 1 were 
repeated with an entirely new setup. The powder characteristics of that 
coating material are shown in Table 5. 
TABLE 5 
______________________________________ 
Powder Characteristics 
______________________________________ 
Composition: 7.03 wt % Y.sub.2 O.sub.3, 0.33 SiO.sub.2, 
0.15 TiO.sub.2, 0.093 Al.sub.2 O.sub.3, 
0.09 Fe.sub.2 O.sub.3, balance ZrO.sub.2 
Type Powder: Fused and crushed 
Size Analysis: 
+200 mesh 0.0 wt percent 
+230 mesh 0.0 wt percent 
+325 mesh 19.09 wt percent 
-325 mesh 80.87 wt percent 
______________________________________ 
Using Microtrac analysis, the mean particle diameter size was found to be 
39.61 microns. 
Two of the same type 1-inch diameter disc substrates were coated, again 
with the same undercoat as in Example 1. The torch parameters were the 
same as for Sample C, Example 1. One sample was coated with a slight 
variation on the standoff distance from torch to substrate as shown in 
Table 6. 
TABLE 6 
______________________________________ 
Plasma Spray Conditions 
Powder Torch Substrate 
Feed Rate Current Standoff 
Velocity 
Sample (gm/min) (amps) (inches) 
(in/min) 
______________________________________ 
D 50 170 0.87 2750 
E 50 170 1.0 2800 
______________________________________ 
Cross-sectional polished areas of each sample prior to thermal testing were 
examined using the optical microscope. Coating density was measured on 
separate samples as before. The data obtained are shown in Table 7. The 
coating characteristics of Samples D and E are comparable to Sample C, 
Example 1, and show that the macrocracks homogeneously produced throughout 
the coating can be reproducibly obtained. 
TABLE 7 
______________________________________ 
Coating Characteristics 
Den- Vertical 
Vertical 
Horizontal 
sity Density macro- macrocrack 
branch crack 
Sam- (gm/ percent of 
cracks length length 
ple cm.sup.3) 
Theoretical 
per inch 
(mils) (mils) 
______________________________________ 
D** 5.55 91.6 79.7 23.7 4 
E 5.52 91.1 73.1 29.0 3 
______________________________________ 
*Crack length and spacing values are an average of 30 or more 
measurements. 
**Sample D also had several examples of horizontal branching cracks 
extending to contact two adjacent vertical macrocracks. 
As shown in Table 7, Sample D, coated at slightly closer standoff, obtained 
a slightly higher density, slightly more vertical macrocracks per inch, 
but also had slightly longer horizontal branching cracks connected to the 
vertical macrocracks. In fact, Sample D had several examples of horizontal 
branching cracks extending to contact two adjacent vertical macrocracks. 
The disc specimens were tested the same as in Example 1 using the thermal 
cycle test for 2000 cycles. The data obtained are shown in Table 8. 
TABLE 8 
______________________________________ 
Thermal Fatigue Test Results 
Sample % Edge Cracks after 2000 Cycles 
______________________________________ 
D 12 
E 1 
______________________________________ 
The results were again good for these samples prepared under thermal 
spraying conditions that would produce macrocracks. Any result of less 
than 15% edge cracking is considered excellent in this very severe thermal 
cycle test. 
The results for Sample D in the thermal cycle test are good but not as 
outstanding as for Sample E. Table 7 shows that Sample D and E are very 
similar in characteristics, except that Sample D had instances where it 
had horizontal branching cracks that extended to contact two adjacent 
vertical macrocracks. This observation leads to the conclusion that it is 
preferrable to minimize the extent of the horizontal cracks, in order to 
obtain excellent thermal fatigue resistance.