The high-temperature oxidation of silicon carbide and chemically vapor-deposited silicon carbide coated graphite
One of the major challenges in the development of protective SiC coatings for graphite is preventing oxidation of the graphite substrate when cracks have formed in the SiC coating. There is evidence that the addition of boron results in the formation of a low melting oxide which either flows and covers the exposed carbon substrate or reacts with the silica scale to form a liquid borosilicate.^ The effect of boron additions on the oxidation of SiC has been investigated by comparing the oxidation behavior of sintered $\alpha$-SiC, which contains 0.5 wt% boron, with that of high purity CVD SiC. The boron in sintered $\alpha$-SiC does not significantly affect the growth rate of either amorphous silica or cristobalite between 1400$\sp\circ$C and 1600$\sp\circ$C, but does result in the formation of bubbles in the silica scale formed between 1230$\sp\circ$C and 1550$\sp\circ$C.^ The effect of boron on the oxidation of graphite through cracks in SiC coatings has been investigated by comparing the oxidation behavior of graphite coated with SiC, with boron-containing coatings and with double layer coatings consisting of a SiC outer layer and a boron-containing interlayer. Above 900$\sp\circ$C, the oxidation of cracked SiC coated graphite is limited by diffusion through the cracks, but below 900$\sp\circ$C, the oxidation rate is influenced by the chemical reaction. Boron-containing coatings alone decrease the oxidation rate of graphite. However, SiC coatings with a boron-containing interlayer can provide oxidation protection of graphite for up to 9 days at 1500$\sp\circ$C by forming a liquid borosilicate which covers the graphite substrate. ^
Engineering, Materials Science
Jeffrey Wayne Fergus,
"The high-temperature oxidation of silicon carbide and chemically vapor-deposited silicon carbide coated graphite"
(January 1, 1990).
Dissertations available from ProQuest.