Preparation And Mechanical Properties Of High-Entropy Diboride Ceramics

As a kind of ultra-high temperature ceramics,transition metal diboride ceramics are potential candidates in the fields of aerospace and cutting tools because of their high melting point,high hardness,wear resistance and corrosion resistance.The appearance of high entropy expands the variety of ultra-high temperature ceramics and provides a new idea for designing materials with better properties.High-entropy diboride ceramics are developed on this basis and have attracted much attention.In this work,firstly,(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂high-entropy diboride powder was synthesized by boro/carbothermal reduction reaction,and the effects of synthesized temperature and raw material ratio on phase composition and morphology were investigated.Secondly,(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂high-entropydiborideceramics（HEB）and(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-20 vol%SiC high-entropy diboride composites（HEB-20SiC）were prepared by spark plasma sintering（SPS）.And then,the effects of sintering temperatures and SiC particles on relative density,phase and structure were researched.Finally,samples of HEB and HEB-20SiC were prepared by hot pressing.Their bending strength,fracture toughness and Vickers hardness were tested,and the mechanisms were also analyzed.The main results of this paper are as follows:（1）(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂high-entropy diboride powder was synthesized from TiO₂,ZrO₂,HfO₂,Nb₂O₅,Ta₂O₅and B₄C by boro/carbothermal reduction reaction.Based on thermodynamic calculation and the experience of synthesizing transition metal diboride powders,single-phase high-entropy diboride powder was synthesized at 1600°C with a B₄C excess of 20wt%.The synthesized powder has a hexagonal structure with an average particle size of about1.40μm.（2）(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂high-entropy diboride ceramics（HEB）and(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-20 vol%SiC high-entropy diboride composites（HEB-20SiC）were prepared by SPS at 1600°C for 10 min.The phase composition,microstructure and elements distribution were analyzed by XRD,SEM and EDS,respectively.The results show that:a)The relative densities of those ceramics are all above 97%.And they are negatively correlated with sintering temperatures.b)In HEB ceramics,there are main high-entropy diboride phase and a small amount of（Zr,Hf）O₂phase.Ti,Zr,Hf,Nb and Ta elements are homogeneously distributed in the high-entropy phase.The average grain size of high-entropy phase increases with the increase of sintering temperatures.c)In HEB-20SiC ceramics,the second phase SiC is homogeneously distributed in the high-entropy phase matrix.Vickers hardness is increased with the addition of SiC particles due to the grain growth inhibition effect.On the other hand,the improvement in fracture toughness is attributed to the particulate toughening mechanisms such as crack deflection and branching by the added SiC particles.（3）HEB and HEB-20SiC samples were prepared by hot pressing for analyzing mechanical properties.The results show that:a)The four-point bending strength of HEB and HEB-20SiC ceramics is 339±17 MPa and 447±45 MPa,respectively.The improvement of bending strength of HEB-20SiC ceramics is owing to the effect of grain refinement.b)The fracture toughness of HEB and HEB-20SiC ceramics is 3.81±0.40 MPa·m^1/2and 4.85±0.33 MPa·m^1/2,respectively.The addition of SiC particles improved the fracture toughness by about 27%through crack deflection and branching.c)The Vickers hardness Hν_0.2of HEB and HEB-20SiC ceramics is 23.7±0.7 GPa and 24.8±1.2 GPa,respectively.With the increase of indentation load,the hardness of the sintered ceramics decreases rapidly until the load reaches about 49 N,due to the indentation size effect.