Oxidation Resistance of Turbine Blades Made of ŻS6K Superalloy after Aluminizing by Low-Activity CVD and VPA Methods
Journal of Materials Engineering and Performance
_ Oxidation Resistance of Turbine Blades Made of ZS6K Superalloy after Aluminizing by Low-Activity CVD and VPA Methods
M. Zagula-Yavorska 0
P. Kocurek 0
M. Pytel 0
J. Sieniawski 0
0 M. Zagula-Yavorska , P. Kocurek, M. Pytel, and J. Sieniawski , Department of Materials Science, Rzeszow University of Technology , W. Pola 2 Str., 35-959 Rzeszow , Poland. Contact
Two aluminide layers (additive and interdiffusion) were deposited on a turbine blade made of Z_S6K superalloy by means of VPA and CVD methods. The additive and interdiffusion layers obtained by the VPA method consist of the NiAl phase and some carbides, while the additive layer deposited by the CVD method consists of the NiAl phase only. The residual stresses in the aluminide coating at the lock, suction side, and pressure side of the blade were tensile. The aluminide coating deposited by the CVD method has an oxidation resistance about 7 times better than that deposited by the VPA method. Al2O3 + HfO2 + NiAl2O4 phases were revealed on the surface of the aluminide coating deposited by the VPA method after 240 h oxidation. Al2O3 + TiO2 oxides were found on the surface of the aluminide coating deposited by the CVD method after 240 h oxidation. Increasing the time of oxidation from 240 to 720 h led to the formation of the NiO oxide on the surface of the coating deposited by the VPA method. Al2O3 oxide is still visible on the surface of the coating deposited by the CVD method. The residual stresses in the aluminide coating after 30 cycles of oxidation at the lock, suction side and pressure side of the turbine blade are compressive.
aerospace; coatings and paints; oxidation
1. Introduction
The increasing efficiency of advanced turbine engines
requires higher operation temperatures for more complete
combustion of fuels. At the high working temperature above
1000 C, surface oxidation becomes the life limiting factor (Ref
1). Diffusive nickel aluminide coatings are widely used to
protect nickel-based superalloys against oxidation (Ref 2).
Aluminide coatings rely on formation of the b-NiAl phase on
the surface of the alloys. Three major processes, by which
aluminide can be deposited include: pack cementation, vapor
phase aluminizing (VPA) and chemical vapor deposition
(CVD). One of the steps, which is common in these three
processes, is the generation of vapors containing aluminum or
the other metallic constituents of the coatings. The vapor phase
is transported to the chamber and reacts with the alloy which
forms the aluminide coating. In the pack process, the
component is embedded in, and therefore in intimate contact with a
pack mix in a heated retort (Ref 3). The pack generates the
halide vapors. In the vapor phase aluminizing process (VPA),
the component to be coated is inside the retort but not in contact
with the pack (Al-Cr alloy and Al2O3 filler). The halide vapors
are plumbed on to the accessible internal and external surfaces
of the component. In the CVD process, the halide vapor sources
are external in individual generators and vapors are plumbed on
to the component held inside a reactor in a heated retort (Ref 4,
5). The aluminum activity plays a critical role in determining
the predominant diffusing species. Depending on the content of
aluminum in the pack and the processing temperature, the
coating process is termed a ‘‘low-activity’’ or ‘‘high activity’’
one. When the aluminum activity is low and the temperature is
above 1000 C, the predominant diffusing species is nickel,
which diffuses out of the alloy, producing an outward
diffusion coating. If the aluminum activity is high and the
temperature is below 950 C, aluminum diffuses inwards,
resulting in an ‘‘inward diffusion’’ coating. Aluminum is also
the predominant diffusing species in the Ni2Al3 phase. In the
low-activity process, nickel is a predominant diffusing species,
which diffuses outwards and combines with aluminum to form
the external NiAl zone. Near the interface, the internal zone,
which is also called the interdiffusion zone, loses nickel. Many
of the alloying constituents of the alloy have very low solubility
in the NiAl phase formed in the inner zone. The total coating
thickness includes both the external and the interdiffusion
zones. Both of the zones consist of the NiAl phase, but only in
the interdiffusion zone carbides are formed. Aluminum is the
predominant diffusing species in the high-activity coating
process. The consequence of the higher inward diffusion of
aluminum to nickel is that the original surface becomes the
external surface of the coating (Ref 6).
Aluminum in the coating combines with the oxygen which
forms the protective a-Al2O3 oxide. When the oxide cracks and
spalls due to thermal cycling, additional aluminum from the
coating diffuses to the surface to reform the oxide. When the
aluminum content in the coatings drops to about 4 or 5% at., the
a-Al2O3 oxide can no longer be formed and rapid oxidation
occur (...truncated)