Preparation Of Hexagonal Boron Nitride Nanosheets By Borate Nitridation Method And Its Application In Ceramic Materials

Boron nitride nanosheets(BNNSs)can be used to prepare various composite materials due to their excellent performance.Adding BNNSs to ceramic matrix composites can not only improve their mechanical properties,but also improve their dielectric,thermal,and other properties.In this paper,BNNSs/MgO composite powders and BNNSs/Al2O3 composite powders were prepared by borate nitridation method,and BNNSs/MgO composite ceramics and BNNSs/Al2O3 composite ceramics were prepared by hot pressing sintering.The mechanical properties and fracture mechanism of BNNSs/MgO composite ceramics and BNNSs/Al2O3 composite ceramics were studied and the dielectric properties of BNNSs/MgO composite ceramics were analyzed.At the same time,BNNSs were prepared by magnesium borate nitridation method and used to toughen eutectic Al2O3/YAG/YSZ ceramics.The effect of BNNSs doping content on the mechanical properties of Al2O3/YAG/YSZ ceramics and the fracture mechanism of the ceramics were investigated.(1)In this paper,BNNSs/MgO composite powders were prepared by in-situ reaction using magnesium borate nitridation method,and BNNSs/MgO composite ceramics were prepared by hot pressing sintering.It was proved that BNNSs and MgO in the powders and ceramics were fully and uniformly mixed without introducing impurities.BNNSs/MgO composite ceramics have similar bending strength(201.4 MPa)and fracture toughness(3.78 MPa·m1/2)to pure MgO ceramics,but have lower bulk density(2.994 g/cm3),so they have higher ratio of strength-to-weight compared to pure MgO ceramics.BNNSs/MgO ceramics have a lower dielectric constant(about 7)and dielectric loss compared to pure MgO ceramics,so they have better wave transmission properties.The fracture mechanism of BNNSs/MgO ceramics is a joint mechanism of BNNSs pull-out,BNNSs bridging,and crack deflection.(2)In this paper,BNNSs/Al2O3 composite powders were prepared by in-situ reaction using aluminum borate nitridation method and BNNSs/Al2O3 composite ceramics were prepared by hot pressing sintering.It was proved that BNNSs and Al2O3 in the powders and ceramics were fully and uniformly mixed without introducing impurities.BNNSS/Al2O3 composite ceramics have excellent bending strength(537.9 MPa),fracture toughness(5.18 MPa·m1/2),and hardness(21.3 MPa).The doping of BNNSs can effectively reduce the grain size of Al2O3.The fracture mechanism of BNNSs/MgO ceramics is a mixture of transgranular and intergranular fracture modes of Al2O3 matrix,as well as a joint mechanism of BNNSs pull-out,BNNSs bridging,and crack deflection.(3)In this paper,BNNSs were successfully prepared by magnesium borate nitridation method and used to toughen eutectic-composition Al2O3/YAG/YSZ ceramics.The composite ceramics doped with 1.5 wt.%BNNSs have the highest bending strength(390.35 MPa)and fracture toughness(3.75 MPa-m1/2).The fracture mechanism of BNNSs doped eutectic-composition Al2O3/YAG/YSZ ceramics is a combination of BNNSs pull-out,BNNSs bridging,crack deflection,and particle toughening.