Investigations On Synthesis And Properties Of Novel HfC_xN_1-x Ceramics And Modifications Through Compositing And High Entropy

In the field of aeronautics and astronautics,hypersonic vehicles have received a lot of promotion around the world.In recent years,as their broad applications in civil and military fields.Due to its sharp airframe,the tip（nose cap and leading edge）parts of these hypersonic vehicles encounter severe extreme environment during the atmosphere fight,including ultra-high temperature（>1600℃）,oxidation/ablation etc.Therefore,designing and developing of ultra-high temperature structural materials for prolonged service above 1600℃ in oxidizing and rapidly heating environments is of great scientific and engineering importance.Ultra-high temperature ceramics（UHTCs）,due to their high melting point,low density,good high temperature mechanical properties,excellent oxidation resistance and good thermophysical properties,are potential for the applications in rocket and hypersonic vehicle.UHTCs are mainly carbides,nitrides and borides of transition group metals.In particular,carbides,nitrides and borides of Hf and Zr are currently of great interest due to their extremely high melting points and excellent high temperature thermal stability,and good oxidation resistance due to the formation of extremely stable HfO₂（melting point 2758℃）or ZrO₂（2700℃）on their surfaces in ultra-high temperature oxidizing environments.It is well known that the properties of compound ceramics can be modified by adding additional elements to form solid solutions.For example,carbides and nitrides of the same metal combine to form a carbon-nitride solid solution with better mechanical and oxidation resistance properties compared with either single phase carbide or nitride.In recent years,based on the calculations of the maximum melting points of various selected materials,it is predicted that Hf-C-N system is a potential candidate of UHTCs.Firstly,a large number of experiments have shown that HfC is the best one of UHTCs in terms of overall performances.Secondly,according to the phase diagram,HfC and HfN can form a continuous solid solution.However,although some theoretical calculations have been carried on Hf-C-N system,there is a lack of experimental verification.It is needed to further explore the preparation process and relevant properties of Hf-C-N ceramics.Therefore,in the research work of this thesis,HfC_xN_1-x（x=0.3,0.4,0.5,0.6,0.7）powders were firstly synthesized by solid phase reaction in the atmosphere of flowing nitrogen at 2400℃ and then the corresponding bulk materials were prepared by hot press sintering under the condition of 1950 ℃/30 MPa/30 min.In order to further improve the ultra-high temperature oxidation resistance of HfC_xN_1-x ceramics,HfC_0.5N_0.5-20vol%SiC and HfC_0.5N_0.5-20vol.%SiB₆ composites were prepared by hot press sintering.Meanwhile,two novel high entropy ceramics,i.e.(Hf_0.2Zr_0.2Ti_0.2TavNb_0.2)C_0.6N_0.4 and(Hf_0.2V_0.2Ti_0.2Ta_0.2Nb_0.2)C_0.6N_0.4 with polycationic and polyanionic structures,were designed and synthesized of to improve the overall performances of HfC_xN_1-x at ultrahigh temperature.By using XRD,SEM/EDS,TEM and XPS etc.,the phase compositions,morphologies,microstructures and elemental distributions of the as-prepared powder and bulk materials were characterized,and their thermal and electrical conductivities,room temperature mechanical properties（hardness,flexural strength,fracture toughness）,high temperature mechanical properties（high temperature flexural strength）and resistance to ultra-high temperature oxidation resistances were investigated,the effects of N content were disscussed.As-prepared HfC_xN_1-x（x=0.3,0.4,0.5,0.6,0.7）powders were single-phase solid solution with almost no impurities.HfC_xN_1-x had a NaCl type Fm-3m（225）crystal structure.HfC_xN_1-x（x=0.3,0.4,0.5,0.6,0.7）bulk materials were also a single-phase solid solution with a relative density of 96%-97%.In the range from room temperature to 1600℃,the thermal conductivities of HfC_xN_1-x ceramics increased with increasing temperature.The thermal conductivities of HfC_xN_1-x ceramics in this temperature range was mainly determined by the heat capacity.When the content of N in HfC_xN_1-x increased at the same temperature,the role of electrons in heat conduction increased,so the thermal conductivity tended to increase.As the content of N increased,the conductivity of the sample also increased.The room temperature mechanical properties（hardness,flexural strength,fracture toughness）and high temperature mechanical properties（high temperature flexural strength）of as-prepared HfC_xN_1-x（x=0.3,0.4,0.5,0.6,0.7）ceramics were determined.The results indicate that with the increase of the content of N,Hf-C covalent bonds were gradually replaced by Hf-N covalent bonds with lower bond strength.Therefore,HfC_0.7N_0.3 ceramic possessed the highest room temperature flexural strength and hardness,while HfC_0.3N_0.7 ceramic had the highest fracture toughness.The high temperature flexural strength of HfC_xN_1-x decreased with increasing temperature.It should be noted that,HfC_0.3N_0.7 ceramic showed plasticity at 2000℃.Under this condition,kinking and cracking of large grains became the main deformation mechanism.The oxidation tests of HfC_xN_1-x（x=0.3,0.4,0.5,0.6,0.7）ceramics in atmospheric air at 1800 ℃ were conducted in the ultra-high temperature induction heating oxidation test apparatus established in our laboratory.The results show that the weight gain curves of HfC_xN_1-x（x=0.3,0.4,0.5,0.6,0.7）in atmospheric air at 1800℃ followed parabolic law.The weight gain of HfC_xN_1-x decreased with increasing the content of N,therefore,the higher the content of N in HfC_xN_1-x,the lower its oxidation rate.Compared to HfC_0.5N_0.5,the oxidation rates of HfC0.sN0.5-20SiC and HfC_0.5N_0.5-20SiB₆ composites were significantly decreased in atmospheric air at 1800℃.This is because that during oxidation,SiC was oxidized to form molten SiO₂,it futher reacted with HfO₂ to form HfSiO4,which could fill the pores and cracks in the HfO₂ oxide layer and reduce the channels for inward diffusion of oxygen,resulting in the significant improvement of the oxidation resistance of the composites at 1800℃,The effect of air pressure on the oxidation behavior of HfC_0.5N_0.5-20SiC composite in low pressure air（5,10 and 20 kPa）at 1800℃ has been systematically investigated.It is shown that as the gas pressure decreased,SiC underwent active oxidation to form SiO gaseous,SiO easily evaporated to the surrounding environment,resulting in a gradual decrease of the protective oxide layer,even finally no SiO₂ existed in the oxide layer.As the air pressure decreased,SiO gaseous evaporated more severely.Two novel high entropy creamics,i.e.(Hf_0.2Zr_0.2Ti_0.2TavNb_0.2)C_0.6N_0.4 and(Hf_0.2V_0.2Ti_0.2Ta_0.2Nb_0.2)C_0.6N_0.4 with polycation and polyanion structure,have been prepared by hot press sintering.These two high entropy ceramics were dense and homogeneous.Their thermal conductivities,mechanical properties and ultra-high temperature oxidation resistance were investigated.It was found that the high-entropy carbonitride ceramics have lower thermal conductivity and slightly improved fracture toughness and hardness,especially resistance to ultra-high temperature oxidation,compared to the single-component carbide and nitride ceramics.