| SiBCN ceramics have excellent structural stability,high temperature oxidation resistance and anti-ablation properties,and have gradually became research hot-spots in the aerospace field.It is easy to crystallize for the amorphous SiO2 layer formed by the oxidization of SiBCN ceramics under high-temperature oxidation circumstances,and result in the cracking of the oxidized layer and reduce the protective effect for the substrate.During the ultra-high temperature ablation process,the violent volatilization of SiO2lost its protective effect for the substrate.Therefore,there is still room for improving their oxidation resistance and ablation resistance in high-temperature environments.According to the above problems,this research focused the introduction of SiC,HfC,and HfB2 to improve the high-temperature oxidation resistance and ablation resistance of SiBCN ceramics,the main research findings are as follows:（1）Amorphous SiBCN powder was prepared by high energy ball milling-mechanical alloying method.In order to solve the problems of poor sintering activity of amorphous SiBCN powder and difficulty in densification,SiC powder was introduced into SiBCN powder SiC/SiBCN composite ceramics with flexural strength of 353±16 MPa were prepared by hot pressing sintering.The oxidation resistance of SiC/SiBCN ceramics was studied by static oxidation method.The oxidation kinetics process of SiC/SiBCN ceramic composites at 1500℃ can be divided into two stages,and their oxidation rate constants are k1-5h=10.8μm2/h and k5-20h=5.4μm2/h,respectively.（2）The ultra-high temperature ceramic phase HfC was introduced into SiBCN ceramics,and HfC/SiBCN ceramic composites with excellent oxidation and ablation resistance were prepared by hot pressing sintering process.The optimized mechanical properties of HfC/SiBCN ceramic composites with the flexural strength and fracture toughness of 323±12 MPa and 6.0±0.2 MPa·m1/2,respectively,were obtained,when the addition content of HfC was 15mol%.The introduced HfC promoted the SiC and BN（C）grain growth in SiBCN ceramics,and the grain size grew to 200-500 nm.Due to the toughening effect of HfC deflection cracks,the fracture toughness of SiBCN ceramics was improved.The oxidation resistance of HfC/SiBCN ceramic composites was studied by static oxidation method.HfC/SiBCN ceramic composites exhibited better oxidation resistance than SiBCN ceramics.The anti-oxidant mechanism can be attributed to:the formation of HfSiO4 originated from the reaction between SiO2 and HfO2,inhibited the crystallization of amorphous SiO2.HfO2 and HfSiO4 were uniformly distributed in the amorphous SiO2,which compensated for the mismatch of thermal expansion coefficients,maintained the continuous integrity of the oxide layer,and improved the oxidation resistance.The oxidation kinetics of SiBCN-5mol%HfC ceramic composites at 1500oC can be divided into two stages,and their oxidation rate constants are k1-5h=8.1μm2/h and k5-20h=1.1μm2/h（which is 20%of SiBCN）.The ablation properties and mechanism of HfC/SiBCN composites under atmospheric plasma flame with a heat flux of 4.02 MW/m2 were investigated.As the addition of HfC increased,the mass ablation rate of HfC/SiBCN decreased first and then increased.When the addition content of HfC was 10 mol%,the mass ablation rate was the lowest(0.23×10-4mg/mm2·s),which was an order of magnitude lower than that of pure SiBCN ceramics.HfO2,SiC,BN（C）nanoparticles were homogenously distributed in the silica layer of the network structure,which provided the strength support for the oxide layer.With the HfC addition increased,the size and number of pores on the surface of the oxide layer gradually decreased,and finally a continuously sealed oxide layer was formed,which provided protection for the substrate.（3）In order to optimize the mechanical properties of SiBCN ceramics,HfC/SiBCN ceramic composites were prepared by spark plasma sintering（SPS）.With the increase of HfC content,the flexural strength and fracture toughness of HfC/SiBCN first increased and then decreased.The optimized flexural strength and fracture toughness were 420±65 MPa、6.7±0.5 MPa·m1/2,respectively,when the HfC addition content was 5 mol%.The flexural strength of SiBCN-5mol%HfC ceramic composites at 1600℃ reached 195 MPa,which was 44%of the room temperature strength.The anti-ablation performance of HfC/SiBCN ceramic composites prepared by SPS was evaluated by atmospheric plasma flame.With the ablation distance was 65 mm,the surface temperature of SiBCN-5mol%HfC ceramic composites reached 2600℃ after ablation for 60s.The mass ablation rate of SiBCN-5mol%HfC was 1.59×10-4 mg/mm2·s,which was 33%of SiBCN.The SiC phase in SiBCN ceramics was decomposed,and the thickness of the oxide layer was 65μm.The silica thickness of the SiBCN-5mol%HfC was about 35μm,which was about half of that of SiBCN ceramics.When the ablation distance was 70 mm,the linear ablation rate of HfC/SiBCN ceramic composites gradually decreased with the increase of HfC addition.The line ablation rate of SiBCN-20mol%HfC was only-0.06μm/s;the mass loss rate of SiBCN-10mol%HfC was the smallest,which was 0.29×10-4 mg/mm2·s,which was 20%of that of SiBCN ceramics.SiO2 and HfO2 melt into a liquid phase under a high-temperature plasma flame,and formed a high-viscosity,scour-resistant Hf-Si-O liquid-phase protective layer,which provided protection for the substrate.（4）In order to optimize the anti-ablation performance of ceramics,HfB2 was introduced into SiBCN,and high-temperature ablation-resistant HfB2/SiBCN ceramic composites were prepared by SPS.The optimized Flexural strength of HfB2/SiBCN ceramic composites reached 328±10 MPa,when the addition content of HfB2 was 15mol%.The flexural strength of HfB2/SiBCN at 1600℃ was still 247 MPa,which was88%of the room temperature bending strength.The anti-ablation performance of HfB2/SiBCN ceramic composites was studied by atmospheric plasma flame.When the ablation distance was 65 mm,the mass ablation rate of HfB2/SiBCN showed a trend of decreasing first and then increasing with the addition of HfB2.When the HfB2 content was 15 mol%,the mass ablation rate of HfB2/SiBCN was the smallest(1.02×10-4mg/mm2·s),which was 20%of that of SiBCN;and its line ablation rate was 0.08μm/s,which was 2.5%of that of SiBCN.When the ablation distance was 70mm and the ablation time was 60s,the mass ablation rate of SiBCN-10mol%HfB2 was the smallest(0.61×10-4 mg/mm2·s),which was 40%of that of SiBCN ceramics.The line ablation rate of SiBCN-15mol%HfB2 was the smallest,which was-0.28μm/s.HfB2/SiBCN ceramic composites exhibited superior anti-ablation properties than SiBCN and HfC/SiBCN.The anti-ablation mechanism was as follows.The rapid heat conduction of HfB2 in the ceramic composites reduced the surface temperature of sample from 2600℃to 1900℃,which effectively improved the ablation resistance. |
