A 0.025 cm (0.010 in) thick thermal barrier coating (TBC) applied to turbine airfoils in a research gas turbine engine provided component temperature reductions of up to 190 C. These impressive temperature reductions can allow increased engine operating temperatures and reduced component cooling to achieve greater engine performance without sacrificing component durability. The significant benefits of TBCs are well established in aircraft gas turbine engine applications and their use is increasing. TBCs are also under intense development for use in the Low Heat Rejection (LHR) diesel engine currently being developed and are under consideration for use in utility and marine gas turbines. However, to fully utilize the benefits of TBCs it is necessary to accurately characterize coating attributes that affect the insulation and coating durability. The purpose there is to discuss areas in which nondestructive evaluation can make significant contributions to the further development and full utilization of TBCs for aircraft gas turbine engines and low heat rejection diesel engines

Brindley, William J.

Miller, Robert A.

English

NASA Technical Reports Server

SS %1
NASA Technical Memorandum 103708
Thermal Barrier Coating
Evaluation Needs
William J. Brindley and Robert A. Miller
Lewis Research Center
Cleveland, Ohio
Prepared for the
Conference on Nondestructive Evaluation of Modern Ceramics
cosponsored by the American Ceramic Society and the American
Society of Nondestructive Testing
Columbus, Ohio, July 9-12, 1990
NASA
https://ntrs.nasa.gov/search.jsp?R=19910006077 2020-03-19T19:17:55+00:00Z
Thermal Barrier Coating Evaluation Needs
William J. Brindley and Robert A. Miller
NASA Lewis Research Center
Cleveland, Ohio 44135
(216) 433-3274
A 0.025 cm (0.010 in) thick thermal barrier coating (TBC) applied to turbine airfoils in a
research gas turbine engine provided component temperature reductions of up to 190'C'.
These impressive temperature reductions can allow increased engine operating temperatures
and reduced component cooling to achieve greater engine performance without sacrificing
component durability. The significant benefits of TBCs are well established in aircraft gas
turbine engine applications and their use is increasing. TBCs are also under intense
development for use in the Low Heat Rejection (LHR) diesel engine currently being developed
and are under consideration for use in utility and marine gas turbines. However, to fully utilize
the benefits of TBCs it is necessary to accurately characterize coating attributes that affect the
insulation and coating durability. The purpose of this paper is to discuss areas in which
nondestructive evaluation can make significant contributions to the further development and
full utilization of TBCs for aircraft gas turbine engines and low heat rejection diesel engines.
TBC Concept
The Thermal Barrier Coating (TBC) concept involves placing a thermally insulating layer
between a cooled metallic component and the hot working gas to reduce heat transfer to the
component (Fig. 1). Reduced heat transfer translates to a reduced component steady state
temperature, less severe heating and cooling transients and a reduction in the severity of
temperature gradients. The insulating material for the majority of applications consists of a
ceramic layer. The actual design of a TBC, however, changes significantly with the specific
application. TBCs designed for use in aircraft turbine engines and in low heat rejection (LHR)
diesel engines will be reviewed here with a view to examining important aspects of TBCs that
must be characterized for quality control, in-service inspection and continued research and
development purposes.
Evaluation of As-Fabricated TBCs
TBCs for aircraft turbines and LHR diesel engines are quite different due to the differences
in environment for the two applications. The state-of-the-art TBC developed for aircraft gas
turbine engines incorporates an outer insulating layer (or top coat) of ZrO,-(6-8)wt%Y,O,
(partially stabilized zirconia or "PSZ") 0.013 to 0.038 cm thick and a 0.013 cm thick oxidation
resistant inner layer of MCrAIY (also called the bond coat), where the "M" is normally Ni, Co
or Ni + Co. The common method of application for both layers is plasma spraying, but
electron beam-physical vapor deposition (EB-PVD) of TBCs is gaining popularity, especially
for deposition of the PSZ layer'. These two methods of deposition produce coatings with
significantly different microstructures (Fig. 2).
TBCs developed for LHR diesel engines include a thick partially or fully stabilized zirconia
(all zirconia will be referred to as "PSZ" for brevity) layer to provide adequate insulation under
the low average heat flux conditions that exist in these engines. Thick PSZ layers tend to
w
Of
F—
w
w
Top Coat
COOLANT SUBSTRATE COATING HOT GAS
Figure 1. Schematic of the thermal barrier coating concept showing the hot gas heat source,
the insulating layer, the component (substrate) and the component coolant.
Bond Coat
200 µm	 10
Substrate
(a)	 (b)
Figure 2. Optical micrographs of (a) a plasma sprayed two layer TBC showing porosity and
cracks and (b) an EB-PVD two layer TBC showing a columnar structure.
Micrograph (b) was taken using differential interference contrast.
2
concentrate coefficient of thermal expansion (CTE) mismatch stresses at the top
coat/component interface and therefore have poor durability. Improved durability for thick
coatings is achieved by "grading" the CTE of the coating through the coating thickness.
Grading is accomplished by applying a 100% metallic bond coat layer followed by successive
layers of coating with decreasing fractions of bond coat material and increasing fractions of
PSZ ceramic. This type of coating achieves a bulk CTE that changes gradually from the
substrate to the outer surface. Current generation diesel TBCs achieve reasonable durability
with up to four grading layers from the bond coat to the top coat and a total coating thickness
of 0.25 cm or greater (Fig. 3).
The most important physical features of as-fabricated TBCs both for aircraft turbines and
LHR diesels are the thickness, density, microstructure, and residual stresses of the coating
layers S '. These features are important to the insulating value of the TBC and critical to the
durability of the TBC.
Increased thicknesses of the insulating layer will, of course, increase the insulation of the
underlying component. However, stresses concentrated at the PSZ/bond coat interface
increase with increasing thickness of the ceramic layer and will reduce the life of the PSZ
layers '. The thicknesses of the graded layers in a diesel TBC are also important to durability'.
However, coating thickness is often difficult to measure by normal means (micrometers,
calipers, etc) due to complex component shapes (Fig. 4). Complex component shapes can
also present problems in maintaining uniform coating thickness with the line-of-sight deposition
techniques used to fabricate TBCs. Therefore, it is the complex component shapes that most
need TBC thickness monitoring. Thus, one critical evaluation need for TBCs is a reliable
method for coating thickness measurement for both process control and quality control.
The durability of the top coat for aircraft TBCs is also a function of the thickness of the
oxidation resistant bond coat. Bond coats that are too thin are unable to provide oxidation
protection to the component for extended periods and will tend to provide shorter than desired
PSZ Payer life as well"'. Thus, close control of the bond coat thickness is also important to
achieving durable TBCs.
The density of a plasma sprayed top coat (as a fraction of the theoretical density) is critical
to the life of an aircraft engine TBC"' as shown in Figure 5, is suspected to be important to
the life of an EB-PVD aircraft TBC and may also be important to diesel engine TBCs'. Density
variations may occur as a result of process variations or may result from the combination of
part shape complexity and the line-of-sight deposition processes that are used. The effect of
density on plasma sprayed TBC durability is a result of the strain tolerance gained through
porosity and cracking in the top coat s ' (Fig. 2). The dependence of TBC life on the amount of
porosity is complex in that the ratio of equiaxed pores to flat crack-like pores changes with
density. Thus, it is not clear if top coat durability should be a strict function of density or a
function of both density and the ratio of the different types of pores. These points have not
been addressed due to the lack of adequate means of characterizing the pore content of a
TBC. Recent advances in metallographic techniques 9 have made possible metallographic
examination of the pore content of coatings and may lead to new information on pore effects
on TBC life. Until pore-durability correlations can be made, it is only certain that pore content
of a coating is critical to TBC durability. NDE methods that can characterize both density and
the pore content of a coating are certain to be useful for both quality control and for research
and development.
CATERPILLAR	 CUMMINS/UTRC
100% ZIRCONIA
100% ZIRCONIA
75% ZIRCONIA
25% NiCrA(Y 85% ZIRCONIA15% CoCr IY
40% ZIRCONIA
50% ZIRCONIA 60% COCrAIY
50% NiCrAfY NiCrAI
N i CrA
PISTON
CAP
PISTON
CAP
(a) (b)
Figure 3. Schematics of the four layer graded coating approach to fabricating LHR diesel
engine TBCs used by (a) Caterpillar and (b) Cummins/UTRC.
(a)	 (b)
Figure 4. An aircraft gas turbine engine second stage stator vane (a) and a piston cap (b)
are examples of components that can benefit from the use of TBCs.
4
Density also has an effect on the thermal conductivity of a plasma sprayed top coat material.
Pores in the top coat act as air gaps that help to reduce the conductivity of the top coat
material'° (Fig. 6). As for durability, the type of pores that are present have a bearing on the
conductivity. Flat pores with the major axis perpendicular to the direction of heat flow are more
effective in reducing the conductivity of a PSZ layer than are an equal volume of equiaxed
pores". Thus, knowledge of pore types and amounts of pores present in a coating could make
it possible to estimate the conductivity of a coating on a production part.
Another area of concern for TBCs is the presence of detrimental residual processing
stresses due to the high temperatures at which the coating processes are conducted. Methods
to minimize residual stresses in aircraft engine coatings have been widely adopted"' and
residual stress management is being investigated as a method to achieve higher durability for
LHR diesel TBCs 4 . However, reliable NDE methods to assess the residual stresses in TBCs
have not been established. A residual stress measurement technique would be useful in
monitoring coating quality, in particular assuring proper temperature control during processing,
and would be highly useful for research on controlling residual stresses during processing.
In-Service Evaluation of TBCs
The above discussion concentrated on the as-fabricated features that are of importance to
the durability and insulating ability of a TBC. The most pertinent features of in-service
evaluation of coatings are those that indicate degradation or imminent failure of the coating.
Coating degradation and failure for aircraft turbine engines and diesel engines are distinctly
different. Failure of aircraft engine TBCs is generally by delamination of the insulating PSZ top
coat from the bond coat, a process called spalling. The delamination process is clearly due to
progressive crack growth and link-up within the PSZ for plasma sprayed coatings and is
thought to be a process of sudden (rather than progressive) cracking through the AI,O, layer
for EB-PVD coatings. While the process of cracking in a plasma sprayed coating is
progressive, the actual spalling event is often catastrophic. This is a result of the crack growth
occurring at the top coat/bond coat interface and remaining essentially invisible until spalling
actually occurs. For this reason, NDE detection of delamination cracks is particularly attractive
for plasma sprayed TBCs. Such a technique could be used at regular inspection intervals to
determine how much delamination has occurred and if the coating is nearing the end of its
useful life. Some work has already been conducted in this critical area".
Spalling for EB-PVD coatings is thought to be sudden, without the progressive cracking that
occurs in plasma sprayed coatings. However, there is a tendency for the coating to spall in
much smaller sections than do plasma sprayed coatings and, therefore, a single spalling event
for an EB-PVD coating is not likely to be as detrimental to the component life as a spalling
event for a plasma sprayed coating. An EB-PVD coating will become more likely to spall in a
large section if the independent columns that provide strain tolerance (Fig. 2) become bonded
together by sintering. In this case, periodic density checks of EB-PVD coatings may signal the
occurrence of sintering that could lead to premature coating failure.
Other in-service inspection possibilities for aircraft TBCs include density examination of
plasma sprayed coatings to look for increases or decreases in density that may indicate
potential premature failure. The growth of an oxide between the top coat and the bond coat
is detrimental to the life of both types of coating and an examination technique that could
5
2.5^
z
2.0U
^ Y
z 1.50 EU \
Q 1.0
`-L' 0.5
4
1600
W
CY
::D 1400
Q
t 
0 1200
(/")w
U 1000
U
800
82	 86	 90	 94
DENSITY(% OF THEORETICAL)
Figure 5. Cyclic life of an aircraft engine two layer TBC as a function of the density of the top
coat'. The density is expressed as a percentage of the theoretical density.
0	 1	 1	 1	 1	 1
0	 10	 20	 30	 40	 50
PERCENT POROSITY
Figure 6. Variation in the conductivity (inverse of insulating ability) of a PSZ layer as a
function of PSZ porosity content'°.
6
determine oxide thicknesses below the PSZ on in-service parts may be helpful in determining
the remaining life of a part. Finally, both types of coating are subject to erosion due to their
strain tolerant structures and severe operation environments. Thus, monitoring of in-service
coating thickness will be essential in future, higher operating temperature aircraft turbines in
which the insulation provided by a TBC is critical to the short term life of the engine.
In-service inspection requirements for LHR diesel engine TBCs are expected to be
somewhat different than for gas turbines. The dominant mode of in-service coating failure is
yet to be identified for LHR TBCs but a few degradation mechanisms have been identified.
Shallow spalling, which is spalling of chips of PSZ that are a small fraction of the coating
thickness, has been observed for diesel coatings and is attributed to high cycle thermal
fatigue'. Shallow spalling could damage the insulating capability of the coating by reducing the
coating thickness. Delamination of TBCs, similar to that in aircraft engines, has also been
observed and could lead to spalling of significant fractions of the coating thickness. Both
shallow spalling and delamination may be amenable to detection during regular service
intervals for diesel engines.
CONCLUSIONS
Thermal barrier coatings offer the potential to significantly increase heat engine efficiency
or performance by allowing increased operation temperatures while maintaining component
durability. However, TBC durability and insulating ability are highly dependent on thickness,
density, microstructure and residual stresses. Therefore, establishment of coating evaluation
techniques that can handle the complexities of TBCs is critical to the continuing development
and use of TBCs for high performance heat engines.
REFERENCES
1. C.H. Liebert and F.S. Stepka, J. Aircraft, 14 [5] 487-493 (1977).
2. W.J. Brindley and R.A. Mlller, Adv. Mat. and Proc., 8, 29-33 (1989).
3. R.C. Brink, J. Eng. Gas Turbines and Power, 111, 570-577 (1989).
4. R.C. Novak, A.P. Matarese and R.P. Huston, Proc. of 26th Automotive Technology
Development Contractor's Coordination Meeting, Dearborn, Michigan, October 24-27, 1988.
5. R.A. Miller and C.E. Lowell, Thin Solid Films, 95, 265-273 (1982).
6. J.T. DeMasi, K.D. Sheffler and M. Ortiz, NASA Contractor Report CR-182230 (1989).
7. S. Stecura, NASA Technical Memorandum 86905 (1985).
8. T.E. Strangman, Thin Solid Films, 127, 93-105 (1985).
9. W.J. Brindley and T.A. Leonhardt, Mat. Characterization, 24, 93-101 (1990).
10. A.P Batakis and J.W.Vogan, NASA Contractor Report 175002 (1982).
11. T. E. Strangman, J. Neumann and A. Liu, NASA Contractor Report CR-179648 (1987).
7
National 	 and	 Report Documentation Page
Space Administration
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM-103708
4.	 Title and Subtitle 5. Report Date
Thermal Barrier Coating Evaluation Needs
6. Performing Organization Code
7. Author(s) 8. Performing Organization Report No.
William J. Brindley and Robert A. Miller E-5596
10. Work Unit No.
505-63-1A
9. Performing Organization Name and Address
11. Contract or Grant No.
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio	 44135-3191 13. Type of Report and Period Covered
Technical Memorandum12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration 14. Sponsoring Agency Code
Washington, D.C.	 20546-0001
15. Supplementary Notes
Prepared for the Conference on Nondestructive Evaluation of Modern Ceramics cosponsored by the American
Ceramic Society and the American Society of Nondestructive Testing, Columbus, Ohio, July 9-12, 1990.
Responsible person, William J. Brindley (216) 433-3274.
16. Abstract
A 0.025 cm (0.010 in) thick thermal barrier coating (TBC) applied to turbine airfoils in a research gas turbine
engine provided component temperature reductions of up to 190 °C i . These impressive temperature reductions
can allow increased engine operating temperatures and reduced component cooling to achieve greater engine
performance without sacrificing component durability. The significant benefits of TBCs are well established in
aircraft gas turbine engine applications and their use is increasing. TBCs are also under intense development for
use in the Low Heat Rejection (LHR) diesel engine currently being developed and are under consideration for use
in utility and marine gas turbines. However, to fully utilize the benefits of TBCs it is necessary to accurately
characterize coating attributes that affect the insulation and coating durability. The purpose of this paper is to
discuss areas in which nondestructive evaluation can make significant contributions to the further development
and full utilization of TBCs for aircraft gas turbine engines and low heat rejection diesel engines.
17. Key Words (Suggested by Author(s)) 18. Distribution Statement
Thermal barrier coatings; Nondestructive evaluation; Unclassified — Unlimited
Aircraft; Diesel; Plasma spray; Physical vapor deposition Subject Category 26, 27
19.	 Security Classif. (of this report) 20.	 Security Classif. (of this page) 21. No. of pages 22.	 Price'
Unclassified Unclassified 8 A02
NASA FORM 1626 OCT 86
	
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