The Mechanical Properties,Thermal Shock And Ablative Properties Of BAS/BN Composite Ceramic

Hexagonal boron nitride ceramic has a series of advantages,such as outstanding thermal stability,excellent dielectric properties and good processability,thus it becomes the candidate material for the next generation high-speed aircraft radome.But in the meantime it has drawbacks such as difficulty in sintering,low strength,poor resistance to ionic erosion and weathering,which limits its application in some degree.BAS glass ceramics has a good strength and hardness,as well as a relative low dielectric coefficient and dielectric loss.So a composite material with excellent comprehensive performance can be fabricated by introducing BAS to BN ceramic matrix.In this paper,h-BN powder,BaCO3 powder,Al2O3 powder and SiO2 powder were treated as raw material to fabricate four BAS/BN composite ceramics of different composition content by hot-press sintering method.The effects of BAS contents on microstructure,mechanical properties,thermal properties,dielectric properties,thermal shock resistance and ablative properties of BAS/BN composite ceramics have been investigated.The results show that BAS/BN composite ceramics prepared by hot-press sintering were mainly composed of h-BN and hexacelsian.With the increasment of BAS content,the density and mechanical properties of BAS/BN composite ceramic were improved obviously.The comprehensive mechanical property of 40vol%BAS/BN composite ceramic was the best,of which the flexural strength,elastic modulus,fracture toughness,vickers hardness were 175 ± 3MPa,114.0 ± 1.2GPa,2.02 ± 0.10MPa·m1/2,1.08 ±0.01 GPa.With the increasment of BAS content,the dielectric coefficient and the dielectric loss of BAS/BN composite ceramics both enlarged,of which the dielectric properties were reduced.The dielectric property of 10vol%BAS/BN composite ceramic was most excellent.In 21~40GHz range,the average dielectric coefficient and dielectric loss of 10vol%BAS/BN composite ceramic were 4.71 and 3.55×10-3.With the rise of temperature,the specific heat capacity of four BAS/BN composite ceramics of different composition content increased firstly and then decreased.Thermal diffusivity and thermal conductivity of them showed a monotone decrease.The thermal conductivity of 20vol%BAS/BN composite ceramic was higher than the thermal conductivity of other BAS/BN composite ceramic,which ranged from 14.2 W·m-1·K-1 to 27.5W·m-1·K-1.With the increasment of BAS content,the thermal expansion coefficient of BAS/BN composite ceramics increased.From 300? to 700?,the average thermal expansion coefficient of 10vol%BAS/BN composite ceramic was the lowest,which was 1.51×10-6K-1After thermal shock,the BAS/BN composite ceramics were still composed of hexagonal boron nitride and hexacelsian.The thermal shock resistanc of 20vol%BAS/BN composite ceramic was most excellent.With the increased of thermal shock temperature difference,flexural strength went up at first and declined finally.When the thermal shock temperature difference was 600? or 800?,the flexural strength of 20vol%BAS/BN composite ceramic was higher than the flexural strength in room temperature,which could reach 173 MPa at most in the thermal shock temperature difference of 800?.The oxidation of the sample surface was exacerbated with the increased of thermal shock temperature difference.With the increasment of BAS content,the ablative rate decreased firstly then increased.The 20vol%BAS/BN composite ceramic has the most excellent ablative property.The mass ablative rate and line ablative rate of 20vol%BAS/BN composite ceramic were lowest,which were 0.0027g/s and 0.021mm/s;The 40vol%BAS/BN composite ceramic has the worst ablative property.The mass ablative rate and line ablative rate of 40vol%BAS/BN composite ceramic were highest,which were 0.0225g/s and 0.055mm/s;The ablative surface was made up of the ablative zone,transition zone and heat-affected zone.The ablative surface of BAS/BN composite ceramics include hBN,hexacelsian and B2O3.The ablative mechanisms of the BAS/BN composite ceramics were oxidation,gasification and high-temperature airflow scouring.