Study On Preparation And Properties Of Zr₂Al₄C₅-SiC-SiB₆ Complex Ceramics

Ultra-high temperature ceramic is an important structural material of spacecraft.In ultra-high temperature ceramic system,there are a series of ternary layered carbide and nitride ceramics with the excellent characteristics,such as: low density,low cost,high modulus,etc.,thus it has wide application prospects.In this paper,Zr₂Al₄C₅-SiC-SiB₆ three-phase composite ceramics were prepared by two steps sintering method.In the first step,Zr₂Al₄C₅ three ceramic material were synthesized in-situ synthesis.The raw materials are zirconium carbide powder,aluminium powder and carbon black powder.However,its oxidation resistance cannot meet the requirements of the application.In view of this,the second step sintering uses SiC and SiB₆ particles as the second phase and the third phase,Zr₂Al₄C₅-SiC-SiB₆ three-phase composite ceramics were prepared by hot pressing.Mechanical properties,crack healing properties and oxidation resistance of the composites were studied in detail using modern analyzing and testing technologies.First,The Zr₂Al₄C₅ ternary ceramics were prepared by pressureless sintering at 1700℃ for 60 min,Then add 20 vol.%SiC and different volume ratios of Si B₆,Zr₂Al₄C₅-20 vol.%SiC-Xvol.%SiB₆ three phase composite ceramics were prepared by hot pressing sintering at 1800℃ for 60 min,sintering process parameters were 30 MPa pressure and argon protection.Throuth testing the mechanical properties and density of three kinds of three-phase composite ceramics.It was found that with the increasing of SiB₆ particles,the densification,room temperature bending strength,fracture toughness and Vickers hardness showed an increasing trend,the room temperature flexural strength and fracture toughness can reach 554 Mpa and 5.70 MPa m1/2 respectively.Through observing the fracture surface morphology and fracture toughness crack propagation path of the material and according to theoretical analysis,we found that the strengthening mechanisms of the material were densification and grain reinforcing,the fracture mode of single-phase ceramic Zr₂Al₄C₅ was transcrystalline fracture,its grain was relatively large,the fracture surface was relatively rough.With the addition of SiC and SiB₆,the grain size became smaller gradually,the fracture surface became relatively smooth,and the fracture mode had changed into intergranular and intergranular fracture modes;the toughening mechanism was the crack deflection and the crack bifurcation,in the process of crack propagation,there were crack deflection and bifurcation,so that released more energy in the dissemination process of the crack,the expansion was blocked,leading to higher toughness of the material.Secondly,in order to explore the crack-bridging ability of the material,a pre-oxidation experiment was designed to investigate the effect of pre-oxidation on the bridging ability of composite ceramics.The experimental results showed that the low temperature pre oxidation in the air can effectively improve the room temperature bending strength,the effect was best at 1100℃ and 1100℃.Considering the surface morphology of the oxide layer and the X-ray diffraction,the improvement of bending strength was mainly attributed to the effect of pre oxidation on bridging and passivation of cracks and other defects on the surface of material.Finally,the oxidation resistance of three-phase composite ceramics was studied,we investigated static oxidation resistance of three-phase composite ceramics use muffle furnace,the mass change and thickness of oxidized layer followed the parabolic rule.The results showed that Zr₂Al₄C₅-20 vol.% SiC-8vol.% SiB₆ three-phase composite ceramics had a small mass gain and oxide thickness,indicating that the material had good oxidation resistance.In addition,the oxidation weight gained and thickness of oxidized layer of the addition of 2vol.% SiB₆ three-phase composite ceramics were much higher than the addition of 5vol.% SiB₆ and 8vol.% SiB₆ three-phase composite ceramics at 1000-1200℃,indicating that SiB₆ can improve the oxidation resistance of the material at 1000-1200℃.By analyzing the microstructure of the oxidized layer,it was found that SiB₆ can produce B2O3,which can form Glass phase B2SiO5 and Al2SiO5 with SiO2 and Al2O3.At last,the static oxidation mechanism of the material was investigated by analyzing the contents of the three-phase composite ceramics with 8vol.% SiB₆ in different temperature,learned that Zr₂Al₄C₅ began to oxidize to form ZrO2 and Al2O3 at 700-800℃.SiB₆ was oxidized to form SiO2 and B2O3 at 900-1000℃.SiC began oxidize form SiO2 at 1200℃,and Intensified at 1300℃.