Preparation And Properties Of High-entropy （Ca，Sr，Ba）ZrO₃-hBN Ceramics

Vacuum induction melting technology is considered as an effective way to smelt titanium alloys.However,titanium alloys will react with common refractory materials at high temperatures due to the lively chemical properties of titanium alloy.The quality of titanium alloy is also affected by this phenomenon.Therefore,it is a key scientific problem to develop a new crucible material with excellent high temperature stability and corrosion resistance in the process of titanium alloy melting.High entropy（Ca,Sr,Ba）ZrO₃ceramics exhibited excellent corrosion resistance to Ti Ni alloy melt.,but its poor thermal shock resistance could not meet the requirements of crucible materials for titanium alloy melting.In order to improve the thermal shock resistance,high entropy（Ca,Sr,Ba）ZrO₃-hBN ceramic was designed based on the concept of the composite material.Ca ZrO₃,Sr ZrO₃,Ba ZrO₃and hBN were selected as raw materials.High entropy（Ca,Sr,Ba）ZrO₃ceramic powder was firstly synthesized by pressureless sintering,then（Ca,Sr,Ba）ZrO₃-hBN was prepared by spark plasma sintering.X-ray diffractometer（XRD）,X-ray energy spectrometer（EDS）and scanning electron microscope（SEM）were used to analyze the phase composition,microstructure morphology and elemental distribution uniformity of the high entropy（Ca,Sr,Ba）ZrO₃-hBN ceramics.The mechanical properties,thermal properties and corrosion resistance of the high entropy ceramics were also studied.The main results are as follows:（1）Ca ZrO₃,Sr ZrO₃and Ba ZrO₃were mixed in equal molar ratio.High entropy（Ca,Sr,Ba）ZrO₃ceramic powder with tetragonal perovskite structure was synthesized by pressureless sintering at 1600 ^oC.Then high entropy（Ca,Sr,Ba）ZrO₃-hBN ceramics was successfully prepared by spark plasma sintering.The XRD results show that the main phases of（Ca,Sr,Ba）ZrO₃-hBN were（Ca,Sr,Ba）ZrO₃and hBN.The addition of hBN did not affect the existence of（Ca,Sr,Ba）ZrO₃.（2）The mechanical and thermal properties of high entropy（Ca,Sr,Ba）ZrO₃-hBN ceramics was changed with the content of hBN increasing.For the samples sintered by spark plasma sintering at 1800 ^oC,when the contents of hBN was 10 wt.%,15wt.%and 20 wt.%,the relative density of the samples was 93.6%,90.3%and 87.1%,respectively.The hardness of the samples was 5.02 GPa,4.24 GPa and 2.21 GPa,respectively.The flexural strength of the samples was 335.92 MPa,177.61MPa and145.04 MPa,respectively.The relative density,hardness and flexural strength of the sample sintered by pressureless sintering at 1600 ^oC was 97.4%,8.6 GPa and 147.75MPa,respectively.After comparison,the relative density and hardness of high entropy（Ca,Sr,Ba）ZrO₃-hBN ceramics decreased slightly.The sample with 10 wt.%hBN had the biggest flexural strength.（3）The thermal properties of high entropy（Ca,Sr,Ba）ZrO₃-hBN ceramics was changed with the content of hBN increasing.For the samples sintered by spark plasma sintering at 1800 ^oC,when the contents of hBN was 10 wt.%,15 wt.%and 20wt.%,the thermal conductivity of the samples was 7.53 W/m K,13.35 W/m K and19.59 W/m K,respectively.The thermal expansion coefficient of the samples was9.73×10^-6/K,9.04×10^-6/K and 8.54×10^-6/K,respectively.The retention rate of bending strength of the samples was 18.84%,42.96%and 54.47%,respectively.The thermal conductivity,thermal expansion coefficient and the retention rate of bending strength of the sample sintered by pressureless sintering at 1600 ^oC was 1.58 W/m K,11.8×10^-6/K and 7.05%,respectively.After comparison,the thermal properties of high entropy（Ca,Sr,Ba）ZrO₃-hBN ceramics was obviously improved.（4）High entropy（Ca,Sr,Ba）ZrO₃-hBN ceramics exhibited excellent corrosion resistance for TC4 alloy melt.There is no obvious corrosion layer at the interface between high entropy ceramics and TC4 alloy.This could be explained by two points.First,from the perspective of thermodynamics,the Gibbs free energy of high entropy（Ca,Sr,Ba）ZrO₃ceramics was lower than that of titanium oxide,therefore,high entropy ceramics will not react with titanium alloy melt at high temperature.As the second phase,hBN is also the ideal crucible material for titanium alloy melting,which is stable at high temperature.Secondly,from the perspective of dynamics,elements in the high entropy ceramics were not difficult to diffuse into alloy layers due to the diffusion hysteresis effect.