The potters herald. [volume] (East Liverpool, Ohio) 1899-1982, 1948-08-05

BY C. A. BOLE

| 80 - ePNb - ss1E Et9Nsh IN q
recent New Letter that "cermets'
give promise as a material for
use in jet engines and rocket lines
You probably are curious as to
what is meant by the term "cer-
NETS"
The word has been suggested To
describe new line of ceramic me-
tai compounds. The term is meant
to include not only oxides and metal
combinations, but any mixtures
such as oxides, carbides, nitrides,
borides, and silicates on the one
hand, and metals and alloys on the
other.
"Cermets" are the result of the
wedding of ceramics with the met
ais. the combination Of the two
takes place at high temperature
under controlled atmosphere. The
reaction of these two dissimilar
materials produces a product that
has properties quite different from
each alone. The metals are MALE
able, while ceramics are brittle.
The "cermets" are much less brit
tie than the ceramic constituent,
but distinctly less malleable than
the metals.
The ceramic components are
quite resistant to oxidation at high
temperatures, while the metals are
subject to oxidation. Intergranular
corrosion is a disease of metals at
abnormal temperatures, while the
ceramics are highly resistant to
high temperature breakdown.
"Cermets" are resistant to oxida-
tion and apparently resistant to in
tergranular corrosion.
The spall resistance of ceramics
is very poor, while the metals re-
sist heat shock ideally. The seer
NETS" lie between these two values.
They have some of the properties
of each. They will actually become
stronger on heat treatment, but
the best af the "cermets" have only
a fraction of the strength of the
metals at room temperatures. The
outstanding property of "cermets"
is that they have strength at high
temperatures in advance of the
constituent metals, and are nearly
as strong as the constituent oxides.
The techniques used in forming
the laboratory specimens more
nearil resemble those of powder
than ceramics. We have
found it necessary to control the
furnace atmospheres very careful
ly. These atmospheres vary all the
way from a slightly oxidizing to a
highly reducing atmosphere. Other
techniques are employed in vacuo.
The various gases used may be ar
gen, helium, nitrogen, hydrogen,
and water vapor, when a slightely
oxidizing atmosphere is desired.
All metals have a characteristic
and measurable vapor pressure far
below their melting points. Advan-
tage has been taken of this fact IN
the coating of both metals and OK
ides. In the laboratory, the reac-
tion is carried out under a bell
jar in nearly perfect vacuum. In
this way, an extremely even coating
can be produced. When an oxide
and metal are vaporizable at the
same temperature, coating can
be laid down constituting a Ger
met. Other techniques are employ
ed when the vapor pressure of the
oxide and the metal are at dlf-
ferent intensity.
One manufacturer is impregnat-
ing a ceramic with metal in the
liquid phase-thus producing a
'cermet" with a high strength at
elevated temperatures. The vapor
phase reaction is probably a fac
tor in this process, also.
Coatings have been developed
and bodies built up by flame spray |
techniques for laying down and
Tor flame curing "cermets".
From these illustrations, it is
evident that "cermets" can be pro-
luced in various ways, and it is
likely that many more processes
will be developed as time goes on.
Any one or all of these techni-
,ues give promise, each where par
ocular set of conditions is requir-
ID. Research in this field has been
developing so rapidly, that we have
lot had time to investigate all the
Iifferent procedures that have been
for the production OF
he "cermets."
Any method of forming, to be
uccessful, must take congnizance
f the idea that there must be a
ond between the ceramic and the
netal. This is the crux of the de-
elopment. Years ago the enamel
RS were faced with the same prob
en- of establishing bond. Recent
Y, company making enamel spec :.
ties, noted that the metal stock, +,