Preparation And Mechanical Properties Of Boron Nitride Nanosheets/Silicon Carbide Composites

Due to its light weight,high temperature resistance and high temperature stability,Silicon carbide ceramic matrix composites are widely used in aerospace and other military fields,especially in aircraft and rocket engines.Silicon carbide ceramic matrix composites have become an excellent substitute for alloys in engines,and the engine can improve the operating temperature and reduce energy consumption,which is of great significance in energy saving and environmental protection.However,the problem of insufficient toughness of SiC ceramics has limited its wide application in various fields.Therefore,this paper proposes to introduce a two-dimensional nanosheet material?boron nitride nanosheets?as a reinforcing phase in silicon carbide ceramic matrix to prepare silicon carbide ceramic matrix composites to improve the mechanical properties of the silicon carbide ceramic,including bending strength and toughness.The main research contents are as follows:?1?Boron nitride nanosheets?BNNSs?were obtained by vertical agitator ball milling.The effects of different ball milling processes on the yield,morphology,size and thickness of BNNSs were systematically studied.The isopropanol is used as a dispersing agent in this paper.Isopropanol play a key role in hindering the recombination of BNNSs after stripping and dispersing BNNSs.The stripped BNNSs can be separated by centrifugation.The BNNSs prepared by stripping had superior mechanical properties and greater flexural modulus than BN.The final optimization determines the grinding process conditions as follows:a 2 mm diameter ball is selected;the rotational speed is 200 rpm;and the ball milling time is 8 h.Under the above ball milling process conditions,the BNNSs nanosheets have the largest yield,the thinnest thickness,complete morphology and less surface defects.Through the ball milling process research,it is found that the influence of BNNSs is different with the change of conditions,which is mainly reflected in the shear force generated by the ball.For example,when using a small ball and the rotational speed is low due to the shearing force is insufficient,so the size of BNNSs is small and the yield is low.When a large ball is used and the rotation speed is low due to the shearing force is sufficient,so that the size of BNNSs is appropriate and the yield is high.?2?The boron nitride nanosheets?BNNSs?and silicon carbide?SiC?powders were mixed by ball milling,and the composite powders were obtained by rotary evaporation and drying,and then incubated at 1800°C and 50 MPa for 5 min by SPS sintering technology.Sintering prepared BNNSs reinforced SiC ceramic composites.The effects of different contents of BNNSs on the flexural strength?four-point bending test?,fracture toughness?three-point fracture toughness test?and hardness of SiC composites were investigated.The distribution of BNNSs in composites was studied by means of SEM,TEM and other analytical methods,and its performance was analyzed in depth.The results show that the composite SiC ceramics have the highest flexural strength when the 1wt.%BNNSs are added,reaching 507 MPa,which is 123%higher than that of the SiC ceramic matrix?227 MPa?;the fracture toughness reaches 5.41 MPa·M¹/2/2 was 41.6%higher than the matrix(3.82 MPa·m ^1/2).However,when 1.5wt.%of BNNSs are added,the flexural strength and fracture toughness begin to decrease.From the SEM analysis of the composite cross section,it can be seen that the reason for the decrease in strength is mainly due to the agglomeration of BNNSs.