Activity

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Ceramals for Gas Turbines
* EARTHY substances combined with
metals are the most promising alloys for
use in the turbine engines of tomorrow’s
airplanes.
These new substances are called “cera-
mals” and they are used in the blades on
the shafts of gas turbine engines where ex-
treme high temperatures are encountered.
Much research into alloys and other ma-
terials needed to withstand the high tem-
peratures within the gas turbine engine has
been under way during the past decade, and
particularly since the advent of the jet-
propelled airplane.
A leading part in such work has been
taken by the National Advisory Committee
for Aeronautics. A review of NACA re-
search on these ceramals has been issued by
the Society of Automotive Engineers.
The report is by G. Mervin Ault and G.
C. Deutsch, both of the Lewis Flight Propul-
sion Laboratory maintained by the NACA
at Cleveland. It covers work done by them
and others at this government institution
whose primary concern is aircraft engines
and fuels.
The high-temperature metal alloys now
used in the blades on the shafts of turbines
in both turbo-jet and turbo-prop engines are
operating close to their upper temperature
limits. Materials to withstand higher heat
are necessary.
Ceramic blades show favorable strength
at high temperatures, but they tend to frac-
ture with sudden drastic temperature
changes, a common occurrence in gas tur-
bines. Ceramics also are brittle and difficult
to handle without breakage.
The ceramals which were carefully investi-
gated by the NACA included boron carbide
ceramic to which iron was added, and
titanium carbide which was used separately
with cobalt, tungsten and molybdenum. The
boron ceramic is one of the strongest; the
titanium ceramic is the most resistant to
shock.
The boron carbide-iron ceramal, 36%
iron, has a strength consistently lower than
that of pure boron carbide, but the rate of
decrease in the ceramal’s strength with in-
crease in temperature is very low. At 2,400
degrees Fahrenheit, the ceramal lost only
27% of its room-temperature strength.
Titanium carbide ceramals containing
cobalt were investigated extensively because
cobalt was known to bond well with ce-
mented-carbide tool compositions. Ceramals
containing from 5% to 30% of the metal
were used. The purpose of this was to de-
termine the best mixture for the blading.
In bending tests, cobalt-bearing ceramals
had exceptional strength up to 2,000 degrees
Fahrenheit, but negligible strength at 2,400
degrees. The tungsten and molybdenum
compositions, on the other hand, have only
moderate strength at the lower temperatures
but considerably surpassed the cobalt cera-
mal at 2,400 degrees.
A titanium carbide ceramal with 20%
cobalt was found resistant to thermal shock.
The cobalt-bearing bodies were found re-
sistant to oxidation. From the tests, the
scientists decided that the titanium carbide
with 20% cobalt was the best to use in
actual operation tests in an engine.
Science News Letter, June 3. 1950