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National Advisory Committee for Aeronautics, Technical Notes - Sintering Mechanism Between Zirconium Carbide and Columbium

An investigation was made of the sintering process and the
sintering mechanism of a zirconium carbide - columbium ceramal
(12.5 percent by weight of columbium). The specimens used were
prepared by hot-pressing and the effects of sintering temperature
and time at temperature on the structures were determined. A bonding
study was also made of a hot—pressed zirconium carbide specimen and
columbium powder.

The results of the investigation indicated that the sintering
mechanism is one in which columbium atoms diffuse into the zirconium
carbide lattice, displace zirconium atoms, and form columbium carbide
and zirconium metal. This columbium carbide is completely soluble
in the matrix of zirconium carbide and a homogeneous solid solution
of the carbides is formed. At the sintering temperature of 3900° F,
the zirconium metal forms in the grain corners of the carbide struc-
ture.

Size and distribution of the metal phase could be controlled by
two sintering variables, temperature and time at temperature. The
specimen with a fine dispersion of metal has the highest strength.

Sintering is the fundamental process in the establishment of the
structure and properties of a ceramal; a study of the sintering mech-
anism is therefore helpful in the fabrication of a body with optimum
properties. Most of the published work concerning the theory of
sintering ceramals has been done on cemented carbides of the tungsten
carbide - cobalt type. With this material, sintering takes place in
the presence of a liquid and the metal facilitates the growth of
carbide crystals (reference 1). Other mechanisms of sintering a
ceramal depend on the materials and the area of the particular phase
diagram in which sintering takes place.

Metal additions to a ceramic are being considered because there is
a possibility of improving the thermal-shock resistance and of lowering
optimum fabrication temerature of the pure ceramic; there may, however,
be a loss in refractoriness. A ceramal consisting of zirconium carbide
and collmbium is of interest because both constituents have high melting
temperatures and, in addition, zirconium carbide has a high tensile
strength at 22000 F (reference 2). In order to study the sintering
mechanism of this ceramal, an investigation was therefore conducted at
the NACA Lewis laboratory.