Porous thermal barrier coating

A thermal barrier coating for superalloy turbine engine vanes and blades that are exposed to high temperature gas is disclosed. The coating includes a ceramic layer applied to an Aluminide or MCrAlY bond coat by electron beam physical vapor deposition. The ceramic layer has a first portion having unstabilized porosity, a second portion, overlying the first portion, with stabilized porosity, and an outer portion wherein the pores are coated with a noble metal. The stabilized porosity portion along with the noble metal coating reduce the thermal conductivity of the ceramic layer. Stabilizing the porosity renders it more resistant to sintering densification at high temperatures.

TECHNICAL FIELD 
This invention relates generally to thermal barrier coatings for superalloy 
substrates and in particular to a multilayer, ceramic thermal barrier 
coating having low thermal conductivity for superalloy blades and vanes in 
gas turbine engines. 
BACKGROUND OF THE INVENTION 
As gas turbine engine technology advances and engines are required to be 
more efficient, gas temperatures within the engines continue to rise. 
However, the ability to operate at these increasing temperatures is 
limited by the ability of the superalloy turbine blades and vanes to 
maintain their mechanical strength when exposed to the heat, oxidation, 
and corrosive effects of the impinging gas. One approach to this problem 
has been to apply a protective thermal barrier coating which insulates the 
blades and vanes and inhibits oxidation and hot gas corrosion. 
Typically, thermal barrier coatings are applied to a superalloy substrate 
and include a bond coat and a ceramic top layer. The ceramic top layer is 
applied either by the process of plasma spraying or by the process of 
electron beam physical vapor deposition (EB-PVD). Use of the EB-PVD 
process results in the outer ceramic layer having a columnar grained 
microstructure. Gaps between the individual columns allow the columnar 
grains to expand and contract without developing stresses that could cause 
spalling. Strangman, U.S. Pat. Nos. 4,321,311, 4,401,697, and 4,405,659 
disclose thermal barrier coatings for superalloy substrates that contain a 
MCrAlY layer, an alumina layer, and an outer columnar grained ceramic 
layer. Duderstadt, et al., U.S. Pat. No. 5,238,752, and Strangman 
copending U.S. patent application Ser. No. 06/603,811 disclose a thermal 
barrier coating for a superalloy substrate that contains an aluminide 
layer, an alumina layer, and an outer columnar grained ceramic layer. 
A disadvantage to ceramic top layers applied by commercially available 
EB-PVD processes is that their thermal conductivity is about two times 
higher than the thermal conductivity of ceramic top layers applied by the 
plasma spray process. High thermal conductivity is undesirable and is 
believed to result from the deposition of relatively high density columnar 
grains with little internal microporosity. 
Accordingly, there is a need for a thermal barrier coating and method 
therefor to be applied by EB-PVD that has a lower thermal conductivity. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a superalloy article 
having a thermal barrier coating which can be applied by EB-PVD that has a 
lower thermal conductivity. 
Another object of the present invention is to provide a method for applying 
such a coating. 
The present invention achieves these objects by providing a thermal barrier 
coating that includes an aluminide or MCrAlY bond coat and a ceramic layer 
applied to the bond coat by electron beam physical vapor deposition. The 
ceramic layer has a first portion with unstabilized porosity, a second 
portion, overlying the first portion, with stabilized porosity, and an 
outer portion wherein the pores are coated with a noble metal. The 
stabilized porosity portion along with the noble metal coating reduce the 
thermal conductivity of the ceramic layer, rendering it more resistant to 
sintering densification at high temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, a base metal or substrate 10 is a nickel, cobalt 
or iron based high temperature alloy from which turbine airfoils are 
commonly made. Preferably, the substrate 10 is a superalloy having hafnium 
and/or zirconium such as MAR-M247 and MAR-M 509, the compositions of which 
are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Alloy Mo W Ta 
A1 
Ti Cr Co Hf 
V Zr C B Ni 
__________________________________________________________________________ 
Mar-M247 
.65 
10 3.3 
5.5 
1.05 
8.4 
10 1.4 
-- 
.055 
.15 
.15 
bal. 
Mar-M509 
-- 7.0 
3.5 
-- 
0.25 
23.4 
Bal. 
-- 
-- 
.5 .6 
-- 
10.0 
__________________________________________________________________________ 
A bond coat 12 lies over the substrate 10. The bond coat 12 is usually 
comprised of a MCrAlY alloy. Such alloys have a broad composition of 10 to 
35% chromium, 5 to 15% aluminum, 0.01 to 1% yttrium, or hafnium, or 
lanthanum, with M being the balance. M is selected from a group consisting 
of iron, cobalt, nickel, and mixtures thereof. Minor amounts of other 
elements such as Ta or Si may also be present. The MCrAlY bond coat is 
preferably applied by EB-PVD through sputtering, low pressure plasma or 
high velocity oxy fuel spraying or entrapment plating may also be used. 
Alternatively, the bond coat 12 can be comprised of an intermetallic 
aluminide such as nickel aluminide or platinum aluminide. The aluminide 
bond coat can be applied by standard commercially available aluminide 
processes whereby aluminum is reacted at the substrate surface to form an 
aluminum intermetallic compound which provides a reservoir for the growth 
of an alumina scale oxidation resistant layer. Thus the aluminide coating 
is predominately composed of aluminum intermetallic e.g., NiAl, CoAl, 
FeAl and (Ni, Co, Fe)Al phases! formed by reacting aluminum vapor species, 
aluminum rich alloy powder or surface layer with the substrate elements in 
the outer layer of the superalloy component. This layer is typically well 
bonded to the substrate. Aluminiding may be accomplished by one of several 
conventional prior art techniques, such as, the pack cementation process, 
spraying, chemical vapor deposition, electrophoresis, sputtering, and 
slurry sintering with an aluminum rich vapor, entrapment plating and 
appropriate diffusion heat treatments. Other beneficial elements can also 
be incorporated into diffusion aluminide coatings by a variety of 
processes. Beneficial elements include Pt, Pd, Si, Hf, Y and oxide 
particles, such as alumina, yttria, hafnia, for enhancement of alumina 
scale adhesion, Cr and Mn for hot corrosion resistance, Rh, Ta and Cb for 
diffusional stability and/or oxidation resistance and Ni, Co for 
increasing ductility or incipient melting limits. 
In the specific case of platinum modified diffusion aluminide coating 
layers, the coating phases adjacent to the alumina scale will be platinum 
aluminide and/or nickel-platinum aluminide phases (on a Ni-base 
superalloy). 
Through oxidation an alumina or aluminum oxide layer 14 is formed over the 
bond coat 12. This alumina layer 14 provides both oxidation resistance and 
a bonding surface for a ceramic coat 16. The alumina layer may be formed 
before the ceramic coat 16 is applied, during application of coat 16, or 
subsequently by heating the coated article in an oxygen containing 
atmosphere at a temperature consistent with the temperature capability of 
the superalloy, or by exposure to the turbine environment. The sub-micron 
thick alumina scale will thicken on the aluminide surface by heating the 
material to normal turbine exposure conditions. The thickness of the 
alumina scale is preferably sub-micron (up to about one micron). The 
alumina layer 14 may also be formed by chemical vapor deposition following 
deposition of the bond coat 12. 
Alternatively, the bond 12 can be eliminated if the substrate 10 is capable 
of forming a highly adherent alumina scale or layer 14. Examples of such 
substrates are PWA 1487 which contain 0.1% yttrium, Rene N5, and low 
sulphur versions of single crystal alloys SC180 or CMSX-3. 
The ceramic coat 16 may be any of the conventional ceramic compositions 
used for this purpose. A preferred composition is the yttria stabilized 
zirconia coating. The zirconia may be stabilized with CaO, MgO, CeO.sub.2 
as well as Y.sub.2 O.sub.3. Another ceramic believed to be useful as the 
columnar type coating material within the scope of the present invention 
is hafnia which can be yttria-stabilized. The particular ceramic material 
selected should be stable in the high temperature environment of a gas 
turbine. The thickness of the ceramic layer may vary from 1 to 1000 
microns but is typically in the 50 to 300 microns range. 
The ceramic coat 16 is applied by EB-PVD and as result has a columnar 
grained microstructure. The columnar grains or columns 18 are oriented 
substantially perpendicular to the surface of the substrate 10 and extend 
outward from the bond coat 12. Between the individual columns 18 are 
micron sized intercolumnar gaps 20 that extend from the outer surface of 
the ceramic coat 16 to the alumina layer 14. 
During EB-PVD the columnar grains 18, which are well bonded to the alumina 
layer 14, are grown by sequential deposition or condensation of submicron 
layers 22 of zirconia each time that the substrate 10 is rotated over the 
electron beam heated zirconia vapor source. As each layer is applied, 
small submicron pores form, primarily on the interfaces 24 between the 
layers 22. This naturally forming porosity is unstable with respect to 
sintering densification at high temperatures. The term porosity as used 
herein means the quality or state of being porous. 
In the portion 26 adjacent the alumina layer 14 of the grains 18, low 
levels of natural porosity is considered beneficial in achieving good 
bonding to the alumina layer 14. The portion 26 is preferably only a few 
microns in thickness. 
Overlying the portion 26 is a second portion 28 having a porosity that is 
greater than in the portion 26 and which is stabilized. This stable 
microporosity is achieved by introducing a small concentration of metallic 
vapor such as tungsten or molybdenum into the EB-PVD process. The tungsten 
or molybdenum can be introduced by several methods such as a second EB-PVD 
evaporation source, a sputtering target, a thermally decomposable gas 
containing one of these elements, or the addition of tungsten or 
molybdenum powder or wire to the stabilized zirconia evaporation source. 
The amount of tungsten or molybdenum added to the zirconia is a sufficient 
amount to produce isolated submicron particle (W or Mo atom clusters), on 
the zirconia layer interfaces 24 or in the layers 22 themselves. Because 
zirconia is permeable to oxygen, the atom clusters readily oxidize when 
exposed to a high temperature oxidizing environment. Typically, EB-PVD 
process occurs at a temperature in the range of 950.degree. C. to 
1100.degree. C. at an oxygen pressure of 0.5 to 2.5 millitorr. These 
conditions may be sufficient to nucleate and grow gas porosity during 
deposition. Alternatively, the porosity can be grown and stabilized during 
an oxidizing, post coating heat treatment. As the gaseous oxides of 
Molybdenum and tungsten are large molecules, the zirconia lattice hinders 
their diffusion. As a result, sintering densification is inhibited and the 
porosity is stabilized. Typically, the pores within the portion 28 will 
have a diameter of about 5.0 to 500.0 nanometers. 
The amount of tungsten or molybdenum added to the zirconia must be limited 
so as to prevent the extensive formation of channels interconnecting the 
pores. These channels reduce the strength of the grains, making them prone 
to erosion. Also, the gaseous oxides can escape through these channels 
into the atmosphere. 
Overlying the portion 28 is an outer portion 30 in which the reflectivity 
of the pores is increased by sputtering a noble metal (i.e., Pt, Au, Rh, 
Pd, Ir) concurrently with the deposition of the zirconia. The noble metal 
atoms are attracted to the tungsten and molybdenum to form intermetallic 
particles. After the tungsten and molybdenum oxidizes, the noble metal 
vapor coats the surfaces of the pores, enhancing their reflectivity. 
Preferably, the portion 30 has a thickness of about 5 to 25 microns. 
The stabilized porosity portion along with the noble metal coated pores 
reduce the thermal conductivity of the ceramic layer 16 rendering it more 
resistant to sintering densification at high temperatures. 
Various modifications and alterations to the above-described preferred 
embodiment will be apparent to those skilled in the art. Accordingly, this 
description of the invention should be considered exemplary and not as 
limiting the scope and spirit of the invention as set forth in the 
following claims.