Development of highly hard and ductile composite with surface-modified cBN and TiN-TiB2 eutectic

用RCVD方法将CBN(核)/SiO_2(壳)中的SiO_2含量(CBN粉末上的SiO_2厚度)改变到42wt%。研究了SiO_2含量对放电等离子烧结CBN-SiO_2复合材料致密化、相变和硬度的影响。采用放电等离子烧结工艺，在1873K下烧结0.3K，获得了致密化的CBN/SiO_2粉末。在CBN-SiO_2复合材料中没有观察到六方氮化硼(HBN)相，表明SiO_2纳米层抑制了CBN向hBN的相变。CBN-SiO_2的相对密度随SiO_2含量的增加而增加。当CSiO_2=38wt%，载荷为0.98n时，CBN-SiO_2复合材料的最高硬度为17.5 Gpa。采用放电等离子烧结(SPS)技术，在1773~1973K温度范围内，用CBN和SiO_2包覆的CBN(CBN(SiO_2))粉末制备了CBN-TiN-TiB_2复合材料。CBN(SiO_2)TiN-TiB_2复合材料中CBN的相变始于1973K，比未包覆的CBN的相变温度高200K。在1873K下烧结的CBN(SiO_2-TiN-TiB_2)复合材料的相对密度高达91.0~-96.2%。当CBN体积分数为10%时，CBN(SiO_2)-TiN-TiB_2复合材料的维氏硬度为21.0 Gpa。当CBN含量从10%增加到50%时，材料的断裂韧性从4.1 Mpa/2增加到7.3 Mpa/2。当CBN(SiO_2)含量为10%时，CBN(SiO_2)-TiN-TiB_2复合材料具有60~69W·m~(-1)K~(-1)的高导热系数。

The SiO2 content (SiO2 thickness on cBN powder) in cBN(core)/SiO2(shell) was changed up to 42wt% by RCVD. The effects of SiO2 content on the densification, phase transformation and hardness of the cBN-SiO2 composites by spark plasma sintering were investigated. The cBN/SiO2 powder was densified by spark plasma sintering at 1873 K for 0.3 ks. The hexagonal boron nitride (hBN) phase was not observed in the cBN-SiO2 composites, indicating that the SiO2 nanolayer depressed the phase transformation from cBN to hBN. The relative density of cBN-SiO2 increased with increasing SiO2 content (CSiO2). The highest hardness of the cBN-SiO2 composite was 17.5 GPa at CSiO2 = 38 wt% and a load of 0.98 N.The cBN-TiN-TiB2 composites were fabricated by spark plasma sintering (SPS) at 1773-1973 K using cBN and SiO2-coated cBN (cBN(SiO2)) powders. The phase transformation of cBN in cBN(SiO2）TiN-TiB2 composites started at 1973 K which was 200 K higher than that of using un-coated cBN. The cBN(SiO2-TiN-TiB2 composites containing 10-50 vol% cBN sintered at 1873 K showed high relative density of 91.0&-96.2%. The Vickers hardness of the cBN(SiO2)-TiN-TiB2 composites was 21.0 GPa at 10 vol% cBN. With increasing cBN content from 10 to 50 vol% the fracture toughness increased from 4.1 to 7.3 MPa m1/2. The cBN(SiO2)-TiN-TiB2 composite at 10 vol% cBN(SiO2) exhibited high thermal conductivity of 60 to 69 W m-1 K-1.