Characterization and control of the fiber-matrix interface in ceramic ...

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of the specimen slowly decreased. This resulted in poor densification
of the lower regions of the composite closest to the water-cooled gas
injector.
Pyrometer sighting was accomplished through a right-angle glass
prism and a 6-mm-thick quartz window.
Corrections were made for the
prism and window absorptions and reflections.
Matched pairs of prisms
and windows were calibrated at the Oak Ridge National Laboratory (ORNL)
Metrology Research and Development Laboratory. The sets were installed
and the pyrometer reading was corrected using a black-body hole drilled
into the lid of a graphite preform holder.
The average loss resulting
from the quartz window and prism was 20 & 4" at 1475 K.
During the infiltration process, silicon carbide deposits on the
surface of the holders and closes the black-body hole; therefore, an
alternative sighting method was pursued. A point on the surface of the
.--
graphite holder was chosen as the sighting area. Once reactant gases
were introduced in the chamber and deposition proceeded, corrections were
provided for the emissivity difference of the silicon carbide coating.
The spectral emissivity factor for silicon carbide is between 0.75 and
0.85.
The optical pyrometer is equipped with an adjustable emissivity
factor; a value of 0.85 was used during the infiltration procedure.
The flow rates of the gases and vapors were controlled by mass flow
controllers.*
The hydrogen gas was purified by passage through a de-
oxygenating unit and a moisture-absorbing column containing drierite.
Argon gas was passed through a drying column to eliminate moisture but
*MKS instruments Inc. , Andover, Massachusetts.