The status of several coating programs is reviewed. These include efforts on protecting aircraft gas turbine engine materials from oxidation/corrosion and on protecting refractory metal reentry heat shields from oxidation

Grisaffe, S. J.

Merutka, J. P.

English

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NASA TECHNICAL
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COATINGS FOR AIRCRAFT GAS TURBINE ENGINES
AND SPACE SHUTTLE HEAT SHIELDS:
A REVIEW OF LEWIS RESEARCH CENTER PROGRAMS
by Salvatore J. Grisaffe and John P. Merutka
Lewis Research Center
Cleveland, Ohio
TECHNICAL PAPER proposed for presentation at
Nineteenth Refractory Composite Working Group sponsored
jointly by the National Aeronautics and Space Administration
and the Air Force Materials Laboratory
Houston, Texas, February 1-2, 1972
https://ntrs.nasa.gov/search.jsp?R=19720010928 2020-03-23T11:34:55+00:00Z
COATINGS FOR AIRCRAFT GAS TURBINE ENGINES AND SPACE SHUTTLE HEAT SHIELDS:
A REVIEW OF LEWIS RESEARCH CENTER PROGRAMS
By Salvatore J. Grisaffe and John P. Merutka
National Aeronautics and Space Administration
Lewis Research Center
. Cleveland, Ohio
ABSTRACT
The status of several Lewis Research Center in-house and contractual
coating programs is reviewed. This includes the efforts on protecting aircraft
gas turbine engine materials from oxidation/corrosion and on protecting refrac-
tory metal re-entry'heat shields from oxidation.
ii
COATINGS FOR AIRCRAFT GAS TURBINE ENGINES AND SPACE SHUTTLE HEAT SHIELDS:
A REVIEW OF LEWIS RESEARCH CENTER PROGRAMS
By Salvatore J. Grisaffe and John P. Merutka
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
SUMMARY
The Lewis Research Center is sponsoring a number of in-house and contractual
programs to develop protective coatings for aircraft gas turbine engine compo-
nents and for Space Shuttle re-entry heat shields. A Lewis duplex protection
system for superalloys involving a NiCrAISi clad, subsequently aluminized, has
shown very good oxidation resistance--about a factor of three better than
commercial aluminide coatings. The FS^85 alloy has been chosen.as the most
promising Cb alloy -for heat shields, and processing scale-up for full-sized
panels has progressed satisfactorily. The coatings for tantalum are definite
improvements over existing coatings, but their heat shield potential has yet
to be firmly established.
INTRODUCTION
Currently, the major areas of coating research being supported by the Lewis
Research Center include the development of protection systems for aircraft
gas turbine engine components and for Space Shuttle re-entry heat shields. The
gas turbine oriented studies are directed toward providing oxidation/corrosion
protection for nickel and cobalt turbine blade and vane alloys with some effort
on titanium compressor materials. The Space Shuttle oriented studies are
primarily directed toward providing 100 mission protection for columbium and
tantalum re-radiative heat shields which are candidates for the Orbiter vehicle.
Some studies also involve the evaluation and development of waterproof, high-
emittance coatings for fibrous ceramic external insulation heat shields.
The purpose of this paper is to provide an overview of the Lewis in-house
and contractual coating program and to highlight some of the areas of progress.
The table summarizes the total program, indicates the approximate time span for
each effort, and provides a rough estimate of the publication date for major
or final reports. The following sections will review.the programs listed in the
table and will identify.the progress being made.
COATINGS FOR AIRCRAFT GAS TURBINE ENGINES
Superalloys
Lewis has supported a continuing effort to characterize, evaluate, and
improve the available high temperature protection systems for superalloy turbine
components. One growing area of concern involves the influence of such coatings
on the increasing number of thin-walled, air-cooled turbine blades and vanes that
are being used in advanced engines; a situation where the coating thickness and
interdiffusion zone is a significant fraction of the total coating-substrate
'cross-section. An effort .is now underway at GE, West Lynn (NASJ-15557) "to quan-
titatively evaluate the influence of both pack aluminide • and CoCrAlY coatings
on 0.075, 0.11, and 0.15 cm (o.OJO, 0-0^ 5, and 0-06o in) thick cast sections of
Rene 80. The effects of such coatings on the metallurgical stability of the
substrate; on the tensile, stress-rupture, thermal and mechanical fatigue, and
ballistic impact properties of the composite before and after oxidation and hot
corrosion exposure; and on the stress-exposure behavior will be examined. These
studies are just beginning since prior effort was directed toward specimen fabri-
cation and coating.
Coating development studies are also progressing. Work on aluminide coating
development and evaluation has continued. Under NASJ-l^JOO (GE, Schenectady),
fused salt metalliding is being used to deposit aluminide coatings. Tests to-date
indicate that the deposition of only aluminum is more effective than depositing
aluminide coatings in which a second element such as Ti is incorporated. Pack
cementation variable effects are still being investigated at Lewis to develop
a better understanding of the relationship between activator, pack temperature,
and time. These studies are supported by an analytical effort which includes com-
parison of data obtained by electron microprobe (EMP) analysis and ion beam
mass spectrometric (IBMS) analysis. As expected, the EMP with a 1 micron beam
offers better microanalysis than the IBMS with a 300 micron beam which integrates
compositional variations. The IBMS, however, is quite sensitive to light elements
as well as heavy elements and supplements EMP coating characterization (l). Both
approaches, however, require standards of phases similar to those in the coatings
and of known composition before the results can be considered truly quantitative.
The evaluation of promising commercial, developmental, and experimental
coatings is continuing .at Lewis using the Mach 1 burner rig developed at the Center.
These efforts have two purposes. First, there is a definite need to develop
readily available, comparative data on the more promising systems. Second, there
is a need to develop standard test procedures employing both simple furnace
testing and more realistic burner rig exposures. The latest report in this
regard (2) has shown that the life of a developmental aluminide coating on B-1900
decreased by a factor of seven in cyclic furnace tests at 1090 C when the cooling
cycle frequency was increased from once each 20 hours to once each hour; raising
the exposure temperature by ko C from 1050 to 1090 C .decreased the 20-hour cyclic
life by a factor of ten. At 1090 C (l hour cycles) under the more severe Mach 1
burner rig test conditions, the coating on moderate thermal stressed regions of
the test specimens degraded about four times as rapidly as in one hour cyclic
furnace tests at 1090 C. At the highly thermal stressed leading edges, the
coating degraded about 20 times as rapidly as in comparative cyclic furnace
tests.
Alternate, improved protection systems are also under development at
Lewis. A nickel slurry coating has been developed that is sintered on nickel
and cobalt superalloy substrates. The result is a nickel surface layer and
enrichment of the superalloy surface in nickel. When this pre-coat is
subsequently aluminized, the resulting coating offers improved thermal fatigue
resistance for IN-100 and extended protection for WI-52 under Mach 1 burner
rig tests as compared to commercial aluminide coatings (3). The improved thermal
fatigue resistance' may be due to a reversal of the normal compositional gradients
in that here enrichment of the surface in nickel rather than surface depletion
occurs. Thus, the brittle carbide rich zone which generally forms in the nickel-
depleted region at the coating-substrate interface does not develop.
Lewis has also been conducting studies on metallic claddings for superalloys
as a way of evaluating, on an idealized basis, the protective ability of highly
oxidation resistant metal overlay systems (4,5). These claddings have shown
very promising furnace oxidation resistance but specimens tested in the Mach 1
rig showed cladding losses at the leading edges. However, a NiCrAISi cladding,
0.127 mm thick, subsequently aluminized to a depth of about one-third of the
cladding thickness,, protected IN-100 for over 800 hours in Mach 1 tests at 1090 C
(5)- This is the most protective system yet tested at Lewis and is a factor
of about three improvement over the best commercial coatings on IW-100 tested
to date.
Auxiliary in-house Lewis studies have identified a promising plasma sprayed
Ni-Cr system which when subsequently aluminized appears to offer potential for
oxidation protection of stainless steel--a material which has potential for use
in low-cost engines (6). Also, supporting studies on the degradation of aluminide
coatings have identified the presence of a martensitic transformation in the NiAl
coating phase after oxidation testing. The implications of this transformation
are currently under investigation (?)• In addition,•furnace oxidation studies on
bulk NiAl containing 0, 1, 3> and 10 atomic percent of Ti, Cr, and Si substituted
for Al have been conducted at 1100 C. At the 10 percent level, these additions
lowered the oxidation resistance of NiAl; the one percent Si may have offered a
slight improvement in oxidation resistance (8).
Thoria Dispersion-Strengthened Ni and Ni-Cr Materials
Two contractual efforts to develop diffusion-resistant coatings for dis-
persion-strengthened Ni and Ni-Cr materials for use above 1150 C are nearly
complete. Under NAS3-1^ 312 (Solar) no barrier prevented interdiffusion of FeCrAl
claddings'and the substrate materials while a tungsten-based slurry barrier limited
the inward diffusion of Al from NiCrAl coatings on TD-NiCr at 126o C. At this
temperature, loss of Al by surface oxidation, as well as edge attack, limited
the life of such a coating to less than kh hours in cyclic furnace tests. Within
this time period,.the mechanical properties of the substrate were generally
preserved. Under NASJ-lUjlU (GE, Evendale) cobalt alloys such as X-Uo. are
being deposited on dispersion-strengthened nickel and nickel-chromium materials
and are then aluminized. While tests to date in both programs indicate some
progress toward providing extended high temperature oxidation protection for
dispersion-strengthened materials, the coating lives do not yet approach the
stress-rupture lives at useful stress levels.
Titanium
An exploratory effort is underway at TRW uider NAS3-1^ 339 "to develop oxidation
protection (to 650 C) and hot salt stress corrosion protection (to 480 C) for
Ti-6Al-2Sn-J4-Zr-2Mo with spot checks on the all-beta alloy Ti-lJV-llCr-JAl. Many
of the coatings appear to minimize weight change in oxidation testing and some
also appear not to degrade the substrate mechanical properties after 100 hours
in air at 650 C. The full potential of these coatings containing Al, Si, and
additions of Mg, Cr, Ni, Fe, etc. are yet to be established however.
COATINGS FOR SPACE SHUTTLE THERMAL PROTECTION SYSTEMS (.TPS)
Columbium
The program at McDonnell-Douglas/Sylvania (NASJ-l^ JO?) has examined the
oxidation resistance; the mechanical properties before and after simulated re-
entry exposure; and the small panel performance under simulated lift-off and re-
entry conditions for R512E coated Cb-752, FS-85, and C129Y alloys. All coated
alloys showed over 150 cycles of resistance under both external and internal
pressure simulation before any evidence of local coating breakdown. Properly
coated rib-stiffened panels of FS-85 and Cb-752 survived 100 cycles of stress-
pressure-temperature simulation without local coating failure. Some C129Y panels
were thinly coated on the highly stressed ribs. While local coating breakdown
occurred at ko cycles under external pressure conditions (35 torr maximum), two
such panels survived an additional Vf cycles before structural failure. Under
internal pressure conditions (5 torr maximum), local coating breakdown occurred
after 70 cycles but no structural failures occurred within- 100 cycles. In this
program, considerable progress has also been made on process modifications which
allow full sized 51 x 51 cm (20" x 20") panels to be uniformly coated.
Based on the above tests, any of the three R512E coated alloys appeared
satisfactory for Space Shuttle TPS service. Additional considerations of alloy
manufacturing, ease of panel fabrication, and creep strength le'd to the selection
of FS-85 for the final coating program studies. Also, the R512E coating showed
slightly more diffusional stability on FS-85 than on the other alloys. (Recent
arc test data seem to support this choice of alloy-coating combination).
An advanced "fail-safe" TPS protection system is also "being sought. The
approach under study involves cladding the solid solution strengthened FS-85
with the more oxidation resistant but weaker B-l alloy (Cb-15Ti-10Ta-10W-2Hf-2.5Al),
and subsequently overcoating with an appropriate fused slurry silicide coating
(WAS3-155^ -6 - McDonnell-Douglas/Westinghouse/Sylvania). The program is in
its early stages wherein the roll cladding process is being developed to a.pply
B-l to FS-85 and the feasibility of the approach has yet to be demonstrated.
o
Tantalum
Two programs are being conducted to improve fused slurry silicide coatings
for tantalum alloys in order to better assess their potential for Space Shuttle
service (NAS3-14315, Solar/WAS3-l4j5l6, Lockheed). The goal of these programs
is 100 mission reuse at temperatures in the l425-15ljO°C range with a minimum
of refurbishment between flights. Several promising coating systems have been
identified in both programs for use at 1^ 25 C. For example., the Mn-Ti-Si coating
of Lockheed has protected Ta-lOW sheet for between 50 and more than 100 simulated
're-entry cycles at both 0.1 and 10 torr. At 15^ 0 C, however, 0.1 torr lives were
only 5 to 10 cycles and 10 torr lives ranged near ilO cycles. Tensile tests of '
the Mn-Ti-Si coated Ta-lOW after 1^ -25 C exposures indicated a general increase
in strength with cycles, but the elongation decreased only slightly from that of
the as-received sheet, even after 88 exposure cycles.
TABLE
PROGRAMS ON COATINGS SUPPORTED BY LEWIS RC
Program
designation
Aeronautics
NAS3-14300
(G.E . )
NAS 3-14312
(Solar)
NAS 3-14314
(G.E. )
NAS 3-15557
(G.E.)
NAS 3-14339
(T.R.W.)
YOG0603
(Lewis RC)
YOG3704
(Lewis RC)
Space shuttle
NAS 3-14307
(McD-D/
Sylvania)
NAS 3-15546
(McD-D/
West. /
Sylvania)
NAS 3-14315
(Solar)
NAS 3-14316
(Lockheed)
YOG3837)
(Lewis RC)
YOGS 100
(Lewis RC)
Program description
Fused salt bath aluminide coatings for IN- 100
and TD-NiCr
Alloyed coatings for TD -materials
Enriched aluminide coatings for dispersion
strengthened materials
Evaluation of coatings on thin-walled super-
alloys
Coatings for titanium
Evaluation and development of aluminide
coatings and metallic claddings for super-
alloys
Advanced coating concepts
Optimization/scale -up of coatings for Cb TPS
Outer skin protection for Cb TPS
Coatings for TATPS
Coatings forTA TPS
Evaluation of contractor coatings and Cb/TA
coating improvement
Characterization and coatings for RSI
Program duration
May '70 - Oct. '71
June '70 - Aug. '71
June '70 - June '72
June '71 - June '72
May '71 - Aug. '72
Continuing effort
Continuing effort
June '70 - Sept. '72
May '71 -Sept. '72
June '70 - June '72
June '70 - June '72
Continuing effort
•
Continuing effort
just started
Estimated date of
major/final report
Jan. '72
CR 120852
Jan. '72
Oct. '72
Oct. '72
Nov. '72
Several in '72
June '72
Nov. '73
Nov. '73
Sept. '72
Sept. '72
May '72
Dec. '72


Coatings for aircraft gas turbine engines and space shuttle heat shields: A review of Lewis Research Center programs

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