Preparation And Properties Of BNNSs/α-Si₃N₄ Composite Ceramics Coatings On Porous Silicon Nitride Surface

Porous Si₃N₄ ceramics have broad application prospects in the field of aerospace wave transmitting materials due to the excellent comprehensive properties including low density,easy processing,high temperature resistance,good thermal insulation,low dielectric constant and loss.However,the porous structure on the surface also has some problems in the application process,such as moisture absorption,unstable dielectric properties and reduction in mechanical properties etc.Therefore,it is particularly important to prepare a dense ceramic coating with good mechanical properties and thermal shock resistance on the surface of porous Si₃N₄.In this paper,α-Si₃N₄,boron nitride nanosheets（BNNSs）,Y-Si-Al-O glass powder are used as raw materials,the BNNSs/α-Si₃N₄ composite ceramic coatings were prepared on porous Si₃N₄ by a combination of slurry brushing and liquid phase melting and infiltration technology.Introducing BNNSs into the coating aims to improve the mechanical properties and thermal shock resistance of composite ceramic coating.Effects of sintering temperatures and BNNSs content parameters on the microstructure,waterproof and mechanical properties of composite ceramic coatings were investigated in detail,and Effects of thermal shock temperature and shocks times on the microstructure and performance of the coatings were explored.In addition,the toughening mechanism of BNNSs in the coating and the microstructure evolution process of the composite coatings during the thermal shock were clarified.Firstly,the effects of the technological parameters including sintering temperatures（1450℃～1550℃）and BNNSs contents（0～10 vol%）on the microstructures and properties of the coatings were investigated.It can see that as-prepared coatings containedα-Si₃N₄,O’-Sialon,h-BN andβ-Si₃N₄phase were relatively dense,and possessed a good waterproof ability,after preparing the coatings,water absorptions of porous Si₃N₄ were only 4.7%～10.5%,and decreased 1.7～5 times.With the increase of the sintering temperature and BNNSs contents,fracture toughness of the coatings obviously increased and then decreased,although the hardness of the coatings decreased slightly,which indicated that the introduction of BNNSs into the coatings could significantly improve the toughness of the coatings.When the sintering temperature was 1500℃and the content of BNNSs was 5 vol%,it is the best preparation condition for the coatings.The water absorption of the composite coating prepared under this condition was only 5.3%,the hardness was 6.78 GPa and the fracture toughness reached to 6.12MPa·m^1/2.This indicated that the coating had good waterproof and mechanical properties,and can protect the porous substrates.By observing the fracture morphology of the coatings,it could be concluded that the introduced BNNSs can improve the toughness of the coating through some toughening mechanisms such as three-dimensional crack deflection,nanosheets pull-out,bridging,tearing,microcracks deflection and branching.According to the application background of the coating,thermal shock resistance of the BNNSs/α-Si₃N₄ composite coating was studied.The results showed that when the number of thermal shocks was 5 times,the oxidation of BNNSs intensified and the surface defects of the coatings increased with the increase of thermal shock temperature.When the thermal shock temperature exceeded 1100℃,the penetrating cracks appeared in the coatings,resulting in a sharp decrease in the waterproof ability and hardness of the coating.When the thermal shock temperature is 1100°C,with the increase of the thermal shocks times,the defects of the coating increased and the penetrating cracks appeared,thus resulting in the decrease in the waterproof and hardness of the coating.In the process of thermal shock,BNNSs could improve the toughness of the coating through toughening mechanisms such as three-dimensional crack deflection,nanosheets pull-out,bridging,tearing,and crack branching,thereby improving the thermal shock resistance of the coating.At the same time,Si₃N₄ particles and BNNSs could be oxidized into glass at high temperature,which had a certain healing effect on the coating and further improved the thermal shock resistance of the coating.