Microstructure and kinetics of formation of Si 2N 2O and Si 3N 4 into Si porous preforms by chemical vapor infiltration (CVI)

Abstract

The kinetics of formation of Si 2N 2O and Si 3N 4 into Si porous preforms via chemical vapor infiltration (CVI) in N 2 and N 2–5% NH 3, 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 Si 2N 2O is formed primarily with morphology of whiskers and fibers which grow with time and temperature. In N 2–5% NH 3, Si 3N 4 is formed predominantly in the form of coatings on the Si particles. Although thermodynamically, the reaction for formation of Si 2N 2O is more feasible than that for Si 3N 4, kinetically the reaction for formation of the latter occurs faster. The activation energy ( E) for the reaction with pure N 2 is 88.3 kJ/mol while the corresponding value for the reaction with N 2–5% NH 3 is 48.3 kJ/mol.