Research On Microstructure And Properties Of BN-SiC Prepared By Sol Gel Pressureless Sintering

Boron nitride ceramic is a ideal aerospace material for those performance of light weight,high temperature resistance,insulation,strong acid and alkali resistance.However,for hexagonal boron nitride,it is easy to produce cracks owing to the sheet-like accumulation and the weak acting force between atomic shells,which causes that intensity and hardness of the material is not high.In addition,there is using a covalent bond to connect B and N,which requires a high temperature to provide energy when sintering.In this paper,BN-SiC composite ceramics were fabricated by pressureless sintering process,and sintering additives(YAG)are added by using co-precipitation method to improve the densification degree of BN-SiC composite ceramicsand reduce the sintering temperature.In this paper,the relative density,bending strength and microstructure of BN-SiC composite ceramics are compared under different preparation techniques.Finally,the optimized sintering process of BN-SiC composite that are fabricated by pressureless sintering process is ultimately identified.In addition,the paper also compares the effects of the preparation methods of the two kind of powder(the mechanical mixing method and the sol-gel method)on the microstructures and mechanical properties of BN-SiC composite ceramics.In the paper,in order to study the optimal sintering temperature,the paper should firstly ensure that other factors affecting the properties of BN-SiC composite ceramics won’t be changed.The green body of BN-SiC composite ceramics is sintered at different temperatures such as1650℃,1700℃,1750℃,and 1800℃,which finds that effect of sintering is the most ideal under the temperature of 1700℃.The best results.After determining the optimal sintering temperature,researching on the effect of holding time(10min,30 min,60min and 90min)on the properties of BN-SiC composite ceramics.By comparison with the experimental test,the best bending strength andsintering performance are optimal when the holding time is 30 min.Under the condition of having the optimal sintering temperature,holding time and sintering aid content,study the content of the second phase(SiC).When the content of the second phase is 20wt%,it is optimal,and the bending strength and relative density of BN-SiC composite ceramics are 38.30 MPa,45.15% respectively.In order to further optimize the preparation process of BN-SiC composite ceramics,the paper studies the influence of sintering additive(YAG)content(6wt%、8wt%、10wt% and 12wt%)as well.The study shows that the performance of BN-SiC composite ceramics is the best when the content of YAG is 8wt%.Because there is not enough liquid phase to promote sintering when the content of YAG is small,and the ceramic performance is low.On the contrary,when the content of YAG is too large,the bending strength and relative density of the ceramics show a decrease,so the YAG content is optimal when it is 8wt%.The paper comparative analysises the mechanical mixing and sol-gel about the influence of preparation method of those two kind of raw material powders on sintering character and mechanical property.The result of the study shows that the sol-gel is more suitable for the preparation of the raw material powders of BN-SiC multiphase ceramic.