Magazine 138/139 - page 28-29

Test facility for thermo-mechanical testing
in synchrotron radiation
Turbines in modern aircraft engines have to cope with
extreme conditions. Combustion gases at temperatures of up to
1600 degrees Celsius enter the turbine, where they are guided
by alternating rows of fixed and rotating turbine blades, driving
the turbine and generating the aircraft thrust. The higher the
turbine entrance temperature, the greater the thermal efficiency.
Despite being made of high-strength nickel superalloys, the
turbine blades are only able to withstand these temperatures
because they are air cooled via internal labyrinthine ducts and
protected by an insulating ceramic coating on the outside. The
high temperature difference between the cool interior and hot
exterior produces a complex multi-axial state of stress in the
turbine blade wall. The rotor blades are exposed to centrifugal
forces that cause the material to creep, elongating the turbine
blades with exposure time. Furthermore, the alternating thermal
and mechanical loads during aircraft take-off and landing also
produce fatigue damages.
The best way to get between measuring stations is by bicycle. A complete circuit of the storage ring is over one
kilometre long. Thirty-five stations are available for scientists to work with high-energy, or hard, X-rays, permitting
unique images of technical materials and biological structures at the atomic dimension. Researchers granted
measurement times here are the chosen few, and a team from the DLR Institute of Materials Research was among
them. In cooperation with the University of Central Florida and the Argonne National Laboratory near Chicago it
was analysed, using synchrotron radiation, what happens in very thin protective layers of turbine blades, which are
under severe mechanical load and exposed to temperatures exceeding 1000 degrees Celsius.
Very thin layers under thermal mechanical load
By Marion Bartsch and Janine Wischek
Watching strain with X-ray vision
Team from the University of Central Florida together with Marion
Bartsch, Carla Meid and Janine Wischek from DLR (from the right).
At the DLR Institute of Materials Research in Cologne,
laboratory specimens are used to analyse these complex oper-
ating conditions in a specially developed test facility, allowing
the reproduction of the thermo-mechanical fatigue stresses, a
turbine blade and its protective coating are exposed to during
flight. It is therefore to be expected that these laboratory tests
will produce realistic damage. The purpose of this work is to
estimate the service life and damage characteristics of the
coated materials used in the aircraft turbines. If the protective
coatings of turbine blades fail, the blades are at risk of breaking,
which might lead to total engine breakdown.
When the protective coating system fails
Protective coating systems in turbine blades typically fail
by spalling. The transition between metal and ceramics is
particularly at risk. The protective coating system consists of
multiple layers, including an approximately 200-micron thick
ceramic thermal barrier coating made of zirconium oxide above
a roughly 100-micron thick metallic oxidation protection layer.
The zirconium oxide layer is highly porous, designed to provide
good thermal insulation. Its porosity renders it permeable to
oxygen contained in the hot combustion gases. For this reason,
an oxidation protection layer made of an aluminium-rich alloy is
applied. A thin layer of aluminium oxide forms during coating
and slows down any further oxidation. The aluminium oxide
layer initially possesses a thickness of between 0.3 and 0.5
microns, but it grows during flight operations, producing
mechanical stresses that ultimately cause the coating system
to fail.
At the boundary of the directly detectable
The damage that occurs in the test specimens after
several hundred to a few thousand thermo-mechanical load
cycles, particularly in the coating system, is examined microsco­
pically after the test is completed. However, the precise genesis
of the damage observed can only be explained once the local
MECHANICAL MATERIALS TESTING
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