Preparation And Mechanical Properties Of BN Nanosheets/ZrO₂-Al₂O₃ Composite Ceramics

Hexagonal boron nitride（h-BN）is a layered substance with a graphite structure.A single layer or a few layers of h-BN is called BN nanosheets.It is a new type of two-dimensional material with excellent performance.The single layer BN nanosheet has a Young’s modulus of0.865-0.073TPa and a fracture strength of 70.5±5.5GPa.It also has good high temperature stability,good chemical corrosion resistance,excellent thermal conductivity and electrical insulation and many other properties.These excellent properties make it have very good application prospects in the fields of microelectronics,aerospace,defense military,automotive industry,new energy and high-performance composite materials.But compared to the preparation of graphene,BN nanosheets are more difficult to prepare BN nanosheets by top-down peeling due to their unique interlayer structure.At the same time,it is also difficult to post-process the prepared BN nanosheets to improve their dispersibility and prevent the BN nanosheets from aggregation.This article mainly carried out some research on the preparation and modification of BN nanosheets,and blended the prepared BN nanosheets into zirconia toughened alumina ceramic（ZTA）composite ceramics to prepare BNNSs/ZTA composite ceramic materials,discovered the further toughening effect of BN nanosheets on ZTA and explained its toughening mechanism.The specific research contents are:（1）A low-thickness（～5nm）and large-size（～5μm）BN nanosheet was prepared using a shear-grinding stirring mill,the preparation process is a purely physical way,the produced BN nanosheets have good morphology,uniform thickness,and the crystal form remains intact;The prepared BN nanosheets with amino groups on the edge were prepared by using urea as the modified material and by high energy ball milling,when the ball milling time is 10 hours and the rotation speed is 500 rpm,the prepared urea modified BN nanosheets have the best structure and modification effect.They exist in the form of colloids in water and have good dispersion.（2）A new type of composite material mixing method has been developed:using ultrasonic mixing,liquid nitrogen rapid freezing granulation,and finally freeze-drying into powder to prepare ZTA multiphase ceramic powder.Study different methods of mixing powder to prepare ZTA ceramic powder and block ceramics,think that this method is fast,efficient,energy-saving and environmentally friendly,the particle size of the prepared powder is moderate and uniform,the sintered ceramic block has good performance stability.Subsequently,the effects of different zirconia contents on the performance of ZTA ceramics were compared.It was found that the ZTA ceramics with 10vol%zirconia content had the best performance,the MSP strength,Young’s modulus,hardness and fracture toughness were 628.03MPa,399.25GPa,21.75GPa and5.02MPa·m^0.5.（3）Adding urea-modified BN nanosheets to 10ZTA composite ceramics,the study found that when the content of BN nanosheets was 1vol%,the fracture toughness of the composite material increased by 11.35%,from 5.02 MPa·m^0.5to 5.59 MPa·m^0.5.The main reason for the improvement of fracture toughness is the pull-out of BN nanosheets,the deflection of cracks and the bridging effect when the composite fractures.These mechanisms of action greatly consume the energy required for crack propagation.