Preparation And Properties Of Environmental Barrier Coatings On Porous Silicon Nitride Surface

Porous Si₃N₄ ceramics have widly application prospects in aeronautics and spaceflight due to their excellent mechanical and dielectric properties and thermal insulation.However,high porosity and porous structure will result in unstability of dielectric properties and decrease of thermal insulation and mechanical properties of porous Si₃N₄,which limits its practical applications.Therefore,it is imperative to fabricate an environmental barrier coating?EBC?on porous Si₃N₄ ceramics.The coating not only has the ability of moisture-proof and rain-resistant,but also has a better resistance to thermal shock.In this paper,?-Si₃N₄ and Y-Si-Al-O glass powder and SiC nanowires were used as raw materials,the multiphase ceramic coating toughened by SiC nanowires was prepared on porous Si₃N₄by the combination of slurry coating and reactive infiltration.The effects of sintering temperature and the content of SiC nanowires on the microstructure,waterproof and mechanical properties of the coatings were studied,and the thermal shock resistance of the coatings was studied in detail.In the preparation process of the coating,the effects of sintering temperature and nanowire content on the microstructure,water resistance and mechanical properties of the coating were investigated.Studies results showed that as-prepared coatings contained?-Si₃N₄,o'-Sialon,?-Si₃N₄,SiC and glass phases.The coatings were dense and had an excellent water resistance.As the sintering temperature increased to 1600°C,pores appeared in the coating due to the infiltration of liquid phase into the substrate.In addition,hardness and fracture toughness of the coating increased and then decreased with the increase of the sintering temperature,and reached the optimum value at 1550°C.When the content of SiC nanowires was 0wt%,5wt%and 10wt%,as-prepared coatings were dense.When the content of SiC nanowires was more than 15wt%,the micropores appeared on the surface of coatings.The hardness and fracture toughness of the coatings increased and then decreased with the increase of SiC nanowire content,and reached the optimum value when the content of nanowires was 10wt%.When the sintering temperature was 1550°C and the SiC nanowires content was 10wt%,mechanical properties and waterproof abilitiy of the coating reached optimum values,the hardness and fracture toughness of the coated porous Si₃N₄ were678HV and 6.27 MPa·M^1/2,respectively,and the water absorption was only 4.54%.This indicated that the coating had good protective effect and moisture-proof ability.The damage behaviors of the coatings under different thermal shock temperatures and times were studied.The results showed that when the thermal shock time was 5 times,the surface of the coating will be oxidized into a dense SiO₂ film with the increase of thermal shock temperature to 1000°C,which leaded to decreace of hardness and inprovment of the waterproof ability.When the thermal shock temperature was up to 1200°C,cracks occurred on the coating,resulting in a decrease in the hardness and waterproof properties of the coating.With the increase of thermal shock times,cracks appeared on the coating surface and hardness decreased gradually.When the thermal shock temperature was 1000°C,the microcracks appeared on the surface of the coatings with the increase of the thermal shock times,and the crack width became wider and the hardness of the coatings decreaced.