On the chemical vapor deposition of Si/B/C-Based coatings in various conditions of supersaturation

Abstract

The results of a thermodynamic and experimental study concerning the codeposition of three elements — silicon, boron and carbon — by a classical C.V.D. technique in a hot-wall reactor are reported. The thermodynamic approach has been described in a previous study and a first comparison was made with experiments carried out under conditions which favor the control by mass transport, at a relatively high temperature ( T = 1400 K) and low mass flow rate. Experimental conditions further from equilibrium have been searched for here for depositing coatings with both good thickness and chemical composition uniformity when the process is more kinetically-controlled. The codeposits were obtained on graphite from various initial gaseous mixtures which were composed of methyltrichlorosilane (MTS), boron trichloride and hydrogen, at a total pressure of 0.395 atm, a total flow rate between 0.1 and 1.0 g/min and a temperature varying in the range 1200–1400 K. Low temperatures and high mass flow rates are shown to favour the deposition of uniform coatings but different results can be obtained as a function of the inlet gaseous composition since different kinetic limitations can arise which favor a boron or a silicon excess in the coating.