Transient stages in the chemical vapor deposition of silicon carbide.

: Transient CVD experiments were simulated by varying continuously the deposition temperature or the initial gas flow rates (Q(MTS) or Q(H2)). Their consequences on the physicochemical properties of the coatings have been first examined. The adhesion of SiC/SiC bilayers containing these "transient interphases" (phi(Tr)) was investigated by scratch testing. For transient stages resulting from a decrease of Q(MTS) or T, free silicon can be co-deposited in proportions depending on alpha = Q(H2)/Q(MTS), T and P. This phenomenon is related to the high reactivity of the Si bearing species and is activated by high T and P and low a values. In this case, the continuous covalent bonding through the Si-rich interphases preserves the adhesion between the two SiC layers. Transient stages resulting from a decrease of Q(H2) lead first to larger and columnar SiC grains and finally to the deposition of anisotropic carbon, due to the formation of unsaturated hydrocarbons in the gas phase. The interphases with the highest carbon concentrations and thicknesses lead to delamination and local chipping of the outer SiC layer. The poor shear strength of these continuous and anisotropic layers is detrimental to the adherence of the bilayers.