Evaluation of CVD-Deposited SiO_2 as a Sintering Aid for Cubic Boron Nitride Consolidated with Alumina by Spark Plasma Sintering

摘要

A SiO_2 nanolayer, 1.0-2.8 mass% in content, was coated on cubic boron nitride (cBN) powder by rotary chemical vapor deposition (RCVD). The morphology of the SiO_2 nanolayer changed from isolated islands to a continuous layer with increasing supply rate of the precursor, tetraethyl orthosilicate. The effect of the SiO_2 coating on the sinterability and mechanical properties of cBN for machine-tool applications was evaluated. The SiO_2-coated cBN (cBN/SiO_2) powder was mixed with Al_2O_3 and consolidated by spark plasma sintering (SPS) at 1473-1873 K. The phase transformation temperature of cBN to hexagonal BN (hBN) in Al_2O_3-cBN/SiO_2 was 1773 K, 100 K higher than that in Al_2O_3-cBN composites. The highest density of the Al_2O_3-cBN/SiO_2 composites was 99.5% at 30 vol% cBN/SiO_2 content, at which the microhardness and fracture toughness were 28 GPa and 4.1 MPa-m~(1/2), respectively. This indicates that RCVD-deposited SiO_2 is a promising sintering aid for cBN and Al_2O_3-cBN/SiO_2 sintered bodies produced by SPS may be applicable as cutting tools.