Isothermal and cyclic oxidation of an air plasma-sprayed thermal barrier coating system Page: 1 of 10
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ISOTHERMAL AND CYCLIC OXIDATION OF AN AIR PLASMA-SPRAYED
THERMAL BARRIER COATING SYSTEM
J. Allen Haynes, Mattison K. Ferber, Wallace D. Porter
High Temperature Materials Laboratory
Oak Ridge National Laboratory
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Thermogravimetric methods for evaluating bond coat
oxidation in plasma-sprayed thermal barrier coating (TBC) systems
were assessed by high-temperature testing of TBC systems with air
plasma-sprayed (APS) Ni-22Cr-IOAI-lY bond coatings and yttria-
stabilized zirconia top coatings. High-mass thermogravimetric
analysis (at 1150'C) was used to measure bond coat oxidation
kinetics. Furnace cycling was used to evaluate APS TBC durability.
This paper describes the experimental methods and relative
oxidation kinetics of the various specimen types. Characterization
of the APS TBCs and their reaction products is discussed.
Ceramic thermal barrier coatings (TBCs) are commonly
applied to air-cooled gas turbine engine components in order to
extend service lifetimes by reducing metal temperatures and
decreasing the severity of hot spots. A typical TBC system
consists of a thermally insulating ceramic top coating (with good
strain tolerance) deposited over an oxidation-resistant metallic
bond coating. The most common ceramic thermal barrier material is
6 - 8 weight % yttria-stabilized zirconia (YSZ), applied by air
The thermal protection offered by the ceramic top coating also
has the potential to provide improved efficiencies to gas turbine
engines by permitting significant increases in turbine inlet
temperatures or decreases in cooling air. However, current TBC
systems eventually fail by spallation of the YSZ layer. Integration
of the TBC thermal benefit into advanced turbine designs would
involve substantial risk, since loss of the ceramic protection at
significantly elevated operating temperatures would result in rapid
degradation of the underlying metallic component. An improved
understanding of TBC degradation and failure mechanisms is
necessary for development of more reliable coating systems (Miller,
1987; Parks et. al, 1995).
It is generally agreed that TBC failure is primarily influenced
by, (1) thermal expansion mismatch stresses between the metal and
ceramic layers, and (2) the effects of various thermally activated
processes such as sintering, creep, diffusion, and bond coat
oxidation (Miller, 1984; Hillery et. al., 1988; DeMasi et. al. 1989;
Wortman et. al., 1989; Meier et. al., 1991). It has been reported that
high-temperature oxidation of the bond coating plays a major role in
TBC degradation (Miller, 1984; Wu et. al., 1989; Meier et. al.,
1991). However, the mechanisms by which oxidation affects TBC
life are still not well understood (Miller, 1989; Mevrel, 1989). It is
likely that as inlet temperatures and operating cycle lengths (in land-
based turbines) continue to increase, the role of thermally activated
degradation processes such as bond coat oxidation will also increase.
Bond coatings generally consist of an MCrAIY (M = Ni and/or
Co) alloy, and are applied by either APS or low pressure plasma
spraying (LPPS). During high temperature operation the bond coat
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Haynes, J.A.; Ferber, M.K.; Porter, W.D. & Rigney, E.D. Isothermal and cyclic oxidation of an air plasma-sprayed thermal barrier coating system, article, August 1, 1996; Tennessee. (digital.library.unt.edu/ark:/67531/metadc671291/m1/1/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.