The Thermal Shock And Ablative Properties Of HfB₂/SiBCN Multiphase Ceramic

SiBCN ceramic materials have attracted the attention of researchers in the aerospace field because of its excellent tissue stability and excellent high temperature oxidation resistance.However,its performance in the service under high-temperature and harsh conditions can also be improved.Among the ultra-high temperature materials,Hf B2 has a broad application direction due to its excellent ablation resistance,high hardness,and high mechanical strength in the harsh service environment of spacecraft.This article intends to introduce Hf B2 into the SiBCN ceramic matrix to produce a composite ceramic material with excellent overall performance.In this paper,c-Si,h-BN,C,Hf and B were used as raw materials.SiBCN powder and Hf B2 powder were prepared by mechanical alloying method respectively.Hf B2/SiBCN composite ceramic materials with different volume fractions were prepared by hot-pressing sintering.The influence of the content of Hf B2 on the microstructure,mechanical properties,thermal shock resistance and ablation resistance of the composite ceramics was investigated.The results show that the main phase of Hf B2/SiBCN composite ceramics prepared by hot-pressing sintering at 1900�C/60MPa/30min/8bar Ar is mainly composed of ?-Si C,?-Si C,BN(C),Hf B2 and Hf C.With the increase of Hf B2 content,the density of Hf B2/SiBCN composite ceramics gradually increases,the porosity decreases,the density increases,and the comprehensive mechanical properties increase significantly.The SHB30 specimen has a maximum density of 4.91 g/cm3 and a minimum porosity of 1.59%.The composite mechanical properties are optimal at room temperature,and the measured values of its bending strength,elastic modulus,fracture toughness,and Vickers hardness are respectively 176.1�3.7 MPa,270.1�7.6 GPa,4.17�0.24 MPa�m1/2,and 7.18�0.6 GPa.After the thermal shock,the surface phases of the Hf B2/SiBCN composite ceramics are mainly Hf O2,Hf B2,Si C and Hf Si O4 phases.With the increase of thermal shock temperature,the intensity of the diffraction peak of Hf B2 decreases,and the intensity of the diffraction peaks of Hf O2 and Hf Si O4 increases.The residual flexural strength of 30vol% Hf B2/SiBCN composite ceramics prepared by hot-pressing sintering at 1900�C/60MPa/30min/8 bar Ar was higher than that of other groups.The 20vol% Hf B2/SiBCN composite ceramic samples have higher retention of residual flexural strength after thermal shock.With the increase of thermal shock temperature difference,the more intense the surface of the sample was oxidized,the surface density and finish decreased.When the thermal shock temperature is 1200�C,a large number of pores and obvious cracks appear on the surface of the sample.With the increasing of the volume fraction of Hf B2,the maximum temperature that the surface of the sample can reach during the ablation process also increases.After the ablation,the surface of the sample is mainly divided into three parts: the ablation central zone,the ablation transition zone and the ablative heat affected zone.The ablation pit on the surface of SHB30 specimen has the shallowest depth,the smallest area,the higher surface roughness,and the higher degree of densification.With the increase of Hf B2 content,the mass ablation rate of the sample gradually increases and the ablation rate of the wire gradually decreases.The ablation rate of the SHB30 sample line is a minimum of 5.5�10-4 mm/s.However,because of its highest density,the mass ablation rate is 1.851�10-4 g/s,and its ablation performance is better.The surface of the sample after ablation is mainly Hf B2,Hf O2,Si O2,Si C,Hf Si O4 and BN(C)phases.The ablation mechanism of Hf B2/SiBCN multiphase ceramics mainly includes mechanical ablation,oxidative ablation and high-temperature gas flow erosion.