In this work, the hot-corrosion behavior of silicon nitride-bonded silicon carbide composite(SCN) in aluminum melt are studied especially on the similar environment to the actual working condition. The thermal shock resistance and cyclic thermal shock behavior of this composite are also studied.
The mass decrease of pre-oxidized SCN in the aluminum melt at 750℃ for 120 hr was greater than that of unoxidized one. Si$O_2$ formed during the pre-oxidizing step was corroded in aluminum melt. In case of pre-oxidized SCN, the hot-corrosion of Si$O_2$ was faster as the temperature of melt increased, but the amount of hot-corrosion of Si$O_2$ was same, because the amount of Si$O_2$ degraded was determined by the pre-oxidizing step.
By the hot-corrosion in the aluminum melt at 750℃ and 850℃, the compressive strength of unoxidized SCN was decreased to 4 % and 6 %, respectively. However, in case of pre-oxidized SCN, the compressive strength was decreased to 37 % and 53 %, respectively. The hot-corrosion of Si$O_2$ formed during the pre-oxidizing step caused the decrease of the compressive strength of SCN.
The change of mass increase showed parabolic behavior as time. The pore neck of porous silicon nitride was blocked by the Si$O_2$ in the oxidizing step at 1200℃, and the effective area of silicon nitride was decreased by the blocking. The activation energy of the oxidation of SCN was determined to be 223 kJ/mol, and this activation energy was thought to be that of the diffusion of oxygen through $SiO_2$, which was formed at the surface of silicon nitride during oxidation.
In the experiments of water quenching test of SCN composite, the initial four-point flexural strength was found to be 24.6 MPa, the critical thermal shock temperature difference was about 425℃, and remained four-point flexural strength was 18.5 MPa.
In case of reaction bonded silicon nitride(RBSN), the critical thermal shock temperature difference was determined to 253℃ by the calculation of theoretical equation. Due to presence of silicon carbide particles in SCN composite, the theoretical critical thermal shock temperature difference was increased to 336℃, because the thermal stress in the specimen was decreased by fast heat transfer through silicon carbide.
Experimental results obtained from cyclic thermal shock test of SCN composite show that the four-point flexural strength of composite was decreased as the number of cycle between 750℃ of aluminum melt and room temperature air was increased. However, after 100 cycles of thermal shock test, the four-point flexural strength was decreased to 13.7 MPa, which was 57 % of initial four-point flexural strength. The repeated thermal stress by the cyclic heating and cooling caused the decrease in strength.
