Synthesis of Novel Al₂O₃-Mullite-cBN Composites from a Mixture of Al₂O₃ with SiO₂ Nanolayer Coated cBN by Rotary CVD and Spark Plasma Sintering

In the present study, novel Al₂O₃-mullite-cBN composites were synthesized from a mixture of Al₂O₃ and SiO₂ coated on cBN (cBN/SiO₂) by spark plasma sintering. SiO₂ nanolayer was coated on cBN powder (cBN/SiO₂) by rotary CVD using tetraethyl orthosilicate as a precursor and O₂ as a reactive gas. The thickness of SiO₂ nanolayer (TS_SiO2) increased from 32 to 52 nm with increasing F_O2 from 0 to 0.42×10⁻⁶ m³.s⁻¹, and then decreased to 25 nm with increasing F_O2 to 0.42×10⁻⁶ m³.s⁻¹. The cBN/SiO2 powder was mixed with Al₂O₃ and consolidated by spark plasma sintering at 1473–1873 K for 0.6 ks. A uniform mullite (Al₆Si₂O₁₃) phase was formed at 1673 K by the reaction of SiO₂ nanolayer with Al₂O₃. At a sintering temperature of 1573 K and TS_SiO2=52 nm, the relative density and the hardness of Al₂O₃-mullite-cBN was 97% and 24.9 GPa, respectively. The obtained hardness was about 3.9 GPa higher than that of monolithic Al₂O₃, 12.4 GPa higher than that of SiO₂-cBN, and 8.9 GPa higher than that of SiAlON-cBN and mullite-cBN composites, suggesting that the as-obtained Al₂O₃-mullite-cBN composites could be possibly used for cutting and wear resistant applications.