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

34
The selection of the coinponents of a composite is initially based on
mechanical. and physical properties of the individual constituents. When
the materials are cornbi-ned to form a composite, the system generally will.
not be at thermodynamic equilibrium, and thus, a driving force to change
the system exists. Thermodynamic data and calculations allow prediction
of the final equilibrium state of the composite material. These
calculations can also be used to determine possible interactions that may
occur between components during processing.
At relatively high temperatures (> 1000 K), chemical reactions
proceed rapidly and the equilibrium state is attained in a relatively
short the.
Equi.1.i.brium conditions can be predicted by employing basic
thermodynamic concepts to detc?rmi.ne the compositional state of a given
system having the minimum free energy (65).
Nuinerous phases wi.11 be
present in a system at elevated temperatures; therefore, the numerical
analysis becomes extremely complicated.
Computer programs have been
developed to simplify the analytical process.
The majority of the codes
are based on SOLGAS, a program created to examine systems containing
gaseous and pure condensed phases (65).
SOLGAS has been modified to
include pure solids, liquids, and gases and condensed phase solutions.
The program has evolved over the years into SOLGASMIX (06,67), which has
the ability to perform the calculations necessary to examine numerous
combinations of compounds and system conditions of temperature, pressure,
or volume.
Kinetic data including diffusivities are equally as important in the
analysis because these can be used to predict the rate at which the
system will change (68-70). It is obvious that chemical interactions