Study On The Processing And Properties Of ZrB₂-Mo₅SiB₂ Composites

Zirconium diboride（ZrB₂）ceramic has high melting point,high strength,high hardness,good thermal conductivity and other excellent comprehensive properties,which is one of the most promising ultra-high temperature ceramics（UHTCs）family of materials.However,its disadvantages such as difficult sintering,poor toughness and poor oxidation resistance at high temperature also limit the application and development of ZrB₂ ceramics.This paper mainly aims at the above three disadvantages of ZrB₂ ceramics and adopts composite technology to improve its performance and promote its application and development in the field of aerospace.This paper select Mo,Si and B powder as the starting material,Mo alloy is obtained by mechanical alloying process Si mixed powder B,then mix with the ZrB₂powder,through the hot pressing sintering process to fabricate ZrB₂-Mo₅SiB₂ ceramic matrix composites.Mo₅SiB₂ can improve the toughness of ZrB₂ ceramics,but also has higher melting point,under the condition of high temperature oxidation after one of the product of SiO₂ can adhesion ZrB₂ ceramic surfaces,increase its oxidation resistance.Thesis mainly studied the factors such as raw materials,sintering process parameters of ZrB₂-Mo₅SiB₂ ceramic matrix composites phase composition,microstructure,mechanical properties and the influence law of oxidation resistance,with the help of X-ray diffractometer（XRD）and scanning electron microscope（SEM）to the preparation of material phase composition analysis and microstructure observation.The relative density,vickers hardness,bending strength,fracture toughness and other room temperature mechanical properties of the samples were tested respectively.High temperature static oxidation method was used to test the high temperature oxidation resistance of samples and explore its oxidation mechanism.It provides technical support and theoretical reference for the preparation of high-performance ZrB₂-Mo₅SiB₂ ceramic matrix composites.The main research results are as follows.Mo powder,Si powder and B powder were mixed according to the molar ratio of5:1:2,and only Mo peak was found in the XRD pattern of the powders after ball milling,without Si peak and B peak,indicating that Mo solid solution（Si,B）was obtained at different ball milling time.Moreover,with the increase of ball grinding time,Mo peak tends to shift to the left,indicating that the proportion of Si and B solid solution Mo is higher and higher,and the lattice distortion of Mo is larger and larger.The results of SEM observation of Mo-Si-B mixed powders with different ball milling time showed that the particle size of Mo-Si-B mixed powders decreased with the increase of ball milling time.However,when the ball milling time was longer than 24h,the agglomeration phenomenon of the mixed powders was obvious.ZrB₂-Mo₅SiB₂ ceramic matrix composites have been fabricated by hot pressing at 1800°C,1850°C and 1900°C for 60 min under a pressure of 30 MPa in argon（Ar）condition.For two groups of composite materials sintered at 1800°C and 1850°C,the relative density,vickers hardness,bending strength and fracture toughness of the sintered samples increased with the amount of Mo-Si-B increasing.The samples with40wt.%Mo-Si-B achieve maximum performance.For the samples sintered at 1900°C,the relative density,vickers hardness,bending strength and fracture toughness first increased and then decreased with the amount of Mo-Si-B increasing.The sintered sample with 30wt.%Mo-Si-B achieve maximum performance.the relative density of the ZrB₂-40wt.%Mo₅SiB₂ sample sintered at 1850°C by hot pressing sintering is98.40%,and its vickers hardness value was 17.06±1.02 GPa,bending strength value of 400±16 MPa,fracture toughness value of 7.2±0.24 MPa·m^1/2.The oxidation resistance of ZrB₂-Mo₅SiB₂ ceramic matrix composites was determined by static oxidation method.Selected 1300°C and 1400°C and 1500°C three oxidation temperature points,after 30 min of oxidation,test the quality of the composite samples after different temperature oxidation and residual flexural strength changes,using SEM,EDS observation samples after oxidation surface microstructure and oxidation layer of cross section.The weight of ZrB₂-Mo₅SiB₂ ceramic composites decreased with the increase of the co-content of Mo-Si-B.With the increase of oxidation temperature,the quality change of samples after oxidation increases accordingly.In the process of high-temperature oxidation,the thickness of silicate glass phase formed by oxidation on the surface of samples increases with the increase of co-content of Mo-Si-B,which acts as an obstacle to the diffusion of oxygen elements and improves the oxidation resistance of ZrB₂ ceramic material at high temperature.The flake ZrO₂ grain mixxed with B₂O₃ and SiO₂ glass phase cover the surface of the samples oxidated at 1400 ^oC and 1500°C,which can also act as an obstacle to the diffusion of oxygen elements and improves the oxidation resistance of ZrB₂ ceramic material at high temperature.