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

73
further protection.
A 6.5-cm-diam by 10-cm-long graphite spacer can be
placed between the graphite holder and the water-cooled injector to
provide a uniform isothermal hot zone across tine preform.
9.2 Instrumentation
A schematic of the infiltration system is given in Figure 9.3. The
control systems have been automated to allow for unattended operation and
to eliminate daily cycling. The furnace operakes on either a time- or
pressure-control circuit. A photograph of the system is given In
Figure 9.4.
The correctxtd optical temperature at the top surface of the speci.nien
is measured and controlled by a single-wavelength automatic optical
*
pyrometer equi.pped with a time-proportioning power controller. The
operating temperature must be measured directly from the top surface of:
the specimen. As infiltration proceeds, t:ht? density and thermal conducti-vity
of the specimen increase and the heat flow through the
infiltrating composi.te increases. More heat is requi.red to maintain
the proper temperature and thermal gradient for deposition to continue.
This is demonstrzted by a steady increase in voltage and current being
suppl-ied to the furnace as densification proceeds, In the past, a
thermocouple placed in contact with the outs icle surface of the coati.ng
chnmlici: at the level of the specimen did not al.J.ow
for (.his change.
Furnace power levels were held constant and the top surface temperature
*
IRCON, Inc., Niles, Illinois