Investigation On The Mechanical And Oxidation Resistance Of NB-doped Al₂O₃/Ti₃SiC₂ Composites

Al₂O₃/Ti₃SiC₂ composites combine the advantages of two matrix materials,such as good electrical conductivity,thermal conductivity,fracture toughness,thermal shock resistance of Ti₃SiC₂,and excellent hardness,strength,high temperature resistance,oxidation resistance of Al₂O₃.However,the mechanical and oxidation resistance of the composites need to be improved in order to fulfill the requirement of high temperature structural materials.Based on the research status that Nb doping can improve the oxidation resistance of Ti₃SiC₂ in composites,the Nb doped Al₂O₃/Ti₃SiC₂ composites were prepared by vacuum hot-pressing sintering method in this paper.The mechanical properties and oxidation resistance of Nb-doped Al₂O₃/Ti₃SiC₂ composites were studied by mechanical properties tests and marker oxidation experiment.The effects of Nb doping on phase composition,microstructure,mechanics and oxidation resistance of Al₂O₃/Ti₃SiC₂ composites were studied by XRD,SEM and EDS.The optimal doping amount of Nb was selected to optimize the comprehensive performance of Al₂O₃/Ti₃SiC₂ composite material,and adapt to more harsh service environment in future applications.In this work,the samples 0-5 with Al₂O₃:Ti₃SiC₂ volume ratio 4:6 and Nb content of 0,1,2,3,4,5 vol.%were obtained by vacuum hot-pressing sintering process at 1450?,30 MPa,holding pressure for 1 h.By comparing the microstructure and mechanical properties of the samples 0-5,it was found that Nb doping changed the phase composition of the composites because the Ti-Si bond with the weakest bond strength was easy to break at high temperature and high pressure,and the Si separated from Ti-Si preferred to combine with Nb to form Nb₃Si.Part of Ti₃SiC₂ in the matrix was decomposed into Ti C and Ti₅Si₃ after the absence of Si,which reduced the extra pores formed by Ti₃SiC₂ bridging,resulting in the decrease of apparent porosity and densification of the Al₂O₃/Ti₃SiC₂ composites,the significant increase of bending strength and hardness,and the slight decrease of fracture toughness.Compared with sample 0,the apparent porosity of sample 5 decreased by 31.4%,hardness increased by 8.1%,bending strength increased by 23%,fracture toughness decreased by 4%,and the apparent porosity of sample 1 decreased by 10.5%,hardness increased by 2%,bending strength increased by 9.7%,fracture toughness decreased by 0.2%.In terms of mechanical properties,the optimal Nb content in this experiment was 1 vol.%,which could improve the bending strength and hardness while maintaining the fracture toughness of the composite.The samples were cut into cubes with side length of 5 mm,sprayed with gold on the surface as in-situ markers,and the oxidation experiment was carried out in the air at 1400?.Comparing the microstructures and oxidation resistance of the oxide layer after oxidation for15 h,it was found that the doping of Nb made the oxide layer of the composite from the inner and outer double layers into a thinner mixed layer,and the structure was more compact.Compared with sample 0,the oxidation weight gain per unit area of sample 5 decreased by93.9%,and the oxide layer thickness decreased by 70.4%.It was concluded that Nb doping led to the formation of new phase Nb₂O₅ in Al₂O₃/Ti₃SiC₂ composites after oxidation.Nb₂O₅generated Ti vacancy with TiO₂ and consumed Ti interstitial and O vacancy in the matrix.The Nb enriched on the surface of the sample can inhibit the outward diffusion of Ti atoms and reduce the inward diffusion rate of O atoms,acting as a diffusion barrier.Therefore,the Nb doping in the range of 0-5 vol.%in this experiment could enhance the oxidation resistance of Al₂O₃/Ti₃SiC₂ composites.