Protective Coatings for Gas Turbines

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4.4.2
Protective Coatings for
Gas Turbines
Kang N Lee
Cleveland State University
NASA Glenn Research Center
Cleveland, OH 44135
Current Address:
Rolls-Royce Corpation
P.O. Box 420, Speed Code W-08
Indianapolis, IN 46206
phone: 317-230-4469
email: kang.n.lee@rolls-royce.com
419 419
4.4.2-1 Introduction
Economical and environmental concerns, i.e. improving effi ciency and
reducing emissions, are the main driving force behind the ever increasing demand
for higher gas turbine engine inlet temperatures. Technology improvements in
cooling, materials and coatings are required to achieve higher inlet temperatures1
.
Advances in the development of airfoil cooling designs have been achieved by
combining high convective cooling effi ciencies with fi lm cooling.
Material improvements have been dramatic during the past several decades. The
improvement in alloy composition and the development of directional and single
crystal casting technologies have allowed increased alloy operation temperatures,
and hence increased turbine inlet temperatures2. Improved high temperature
mechanical properties of alloys, however, have been made typically at the
expense of environmental resistance. This trend, combined with higher operating
temperatures, has resulted in environmental degradation of materials, deteriorating
the mechanical properties and shortening the service life of components3. The need
to protect alloys from environmental degradation motivated the development of
protective coatings. The idea to apply a layer with protective properties on the
surface of Ni-based superalloys was fi rst practiced in the 1960s4. Two types of
protective coatings have been most widely used: diffusion aluminide coatings
based on β-NiAl phase and MCrAlY (M = Ni, Co, or NiCo) overlay coatings based
on a mixture of β-NiAl and γ’-Ni3Al or γ phases5
.
As the temperature capability of Ni-based superalloys approaches their
intrinsic limit, further improvements in their temperature capability have become
increasingly diffi cult6. Therefore, during the past two decades, the emphasis in
gas turbine materials developments has shifted to thermal barrier coatings (TBC),
which are ceramic coatings with a very low thermal conductivity that reduce the
alloy surface temperature by insulating it from the hot gas. Current state-of-the-art
thermal barrier coatings comprise two layers: a diffusion aluminide or MCrAlY
bond coat and a low thermal conductivity partially stabilized zirconia (YSZ: 7 to 8
wt% Y2O3-ZrO2) top coat. Thermal barrier coatings were fi rst successfully tested in
a research turbine engine in mid 70s. By the early 80s they entered revenue service
on the vane platforms of aircraft engines, and today they are fl ying in revenue
service on vane and blade surfaces7. Thermal barrier coatings are expected to play
an increasingly signifi cant role in advanced gas turbine engines both in aero and
industrial applications in the future.
Major improvements in turbine inlet temperatures can be achieved by
replacing Ni-based superalloy hot section components with silicon-based ceramic
matrix composite (CMC) and silicon nitride (Si3N4) ceramics8. These materials
have superior high temperature mechanical properties, such as strength and creep
resistance, compared to Ni-based superalloys. They are also light and possess
excellent high temperature oxidation resistance in clean, dry air, due to the
formation of slow-growing, protective silica scale9. One major disadvantage of
these materials is the lack of environmental durability in combustion environments.
Water vapor, a combustion reaction product, reacts with the protective silica scale,
forming gaseous reaction products, such as Si(OH)4
10. In high pressure, high gas
velocity combustion environments, this reaction results in rapid recession of these
materials. These materials also suffer from severe hot corrosion in environments
contaminated by molten salt11
.
A new class of coatings, environmental barrier coating (EBC), has been
developed in the 90s to protect Si-based ceramics and ceramic composites from
the degradation by water vapor12. The current state-of-the-art environmental barrier
coating comprises three layers: a silicon bond coat, a mullite-based intermediate
coat, and a barium-strontium-aluminosilicate (1-xBaO·xSrO·Al2O3·2SiO2, 0 ≤ x ≤
1) top coat13. CMC combustor liners coated with the current state of the art EBC
were retrofi tted in a Solar Turbines’ industrial gas turbine engine and successfully
completed a 14,000 h fi eld test in the late 90s14
.
This paper will discuss the status of current thermal barrier coatings and
environmental barrier coatings, with the focus on key factors affecting their
performance.