THE EFFECT OF BORON ADDITIVE ON THE OXIDATION RESISTANCE OF SIC-PROTECTED GRAPHITE

To improve the oxidation resistance of graphite, SiC is often used as a coating material because of its stability at high temperatures. A protective SiC layer was produced on graphite by Si slurry dipping and reaction sintering at 1600-degrees-C for 4 h. The oxidation resistance of the treated graphite was influenced by the amount of boron added to the Si slurry. Instead of being determined by thermodynamics via the Si-B eutectic, the result of reaction sintering was greatly influenced by the wetting ability of Si-B on the graphite surface. The reaction sintered layer is a composite of SiC and the unreacted graphite matrix. As the coated sample encounters oxygen attack at high temperature, the main path of oxidation is through the unreacted matrix in the reaction layer. Values of oxidation activation energy were found to be, in order, 22.0, 14.0 and 6.5 kcal/mol, over the temperature range 700-1050-degrees-C. These changes in activation energy imply a mechanism change for graphite oxidation. The amount of boron added to the Si influenced the state of the sintered SiC layer. A transition temperature as high as 950-degrees-C was found in the case of 7.5 wt% boron addition. This means that oxygen transport through the SiC remained the controlling mechanism up to 950-degrees-C.