Microstructure and kinetics of formation of Si2N2O and Si3N4 into Si porous preforms by chemical vapor infiltration (CVI)

The kinetics of formation of Si2N2O and Si3N4 into Si porous preforms via chemical vapor infiltration (CVI) in N2 and N2–5% NH3, has been investigated. In addition, the effect of the following processing parameters on the phase, amount and product morphology was investigated: atmosphere, time, temperature, gas flow rate, particle size and porosity of Si porous preforms. A Taguchi experimental design allowed establishing that atmosphere is the parameter that most significantly influences the type of phase formed and that processing time and temperature are the parameters that most significantly affect the amount and morphology of the phases formed. In nitrogen Si2N2O is formed primarily with morphology of whiskers and fibers which grow with time and temperature. In N2–5% NH3, Si3N4 is formed predominantly in the form of coatings on the Si particles. Although thermodynamically, the reaction for formation of Si2N2O is more feasible than that for Si3N4, kinetically the reaction for formation of the latter occurs faster. The activation energy (E) for the reaction with pure N2 is 88.3 kJ/mol while the corresponding value for the reaction with N2–5% NH3 is 48.3 kJ/mol.