Preparation And Hardness Characteristics Of Nano-polycrystalline Boron Carbon Compound?B₄C?

A material with a Vickers hardness greater than 40 GPa is defined as a superhard material,with excellent properties such as high hardness,strong wear resistance and strong chemical stability.They have played an irreplaceable role in industrial processing,national defense construction,and production of high-precision instruments.Typical superhard materials that have been widely used include diamond and cubic boron nitride,which have some defects.For example,diamond has poor thermal stability and easily reacts with iron-based metals;cubic boron nitride easily reacts with water at high temperatures.The development of new superhard materials has important application value and scientific significance.B₄C is a ceramic material with a hardness second only to super-hard materials,and has a low density,stable properties and abrasion resistance.It is widely used in military armor,nuclear power control,industrial processing and other fields.How to improve the hardness of B₄C and prepare a new type of multifunctional superhard material hard ceramics has always been a research hotspot in materials science.According to the Hall-Petch effect,nano-polycrystallization is an effective means to improve the hardness of materials.Due to the low self-diffusion coefficient of B₄C,it is very difficult to prepare densified nano-polycrystalline B₄C.In this thesis,HPHT methods are used to study the preparation and hardness characteristics of nano-polycrystalline B₄C.This research is of great significance for expanding the scope of B₄C industrial applications and understanding the intrinsic properties of nano-polycrystalline materials.In order to prepare and study the properties of nano-polycrystalline B₄C,this experiment uses two methods to prepare nano-scale B₄C as a precursor.After the preparation of precursors,micro-to-nanoscale B₄C polycrystals were prepared by means of HPHT,and the samples were tested by XRD,Raman,TEM,SEM,hardness and density.The results are as follows:1?Micron-level B₄C body materials are prepared under HPHT,and Vickers hardness can be as high as 35.6 GPa.Under the conditions of 5 GPa and different temperature conditions,the grains are prepared as B₄C polycrystals in the order of micrometers.The physical property test results show that when the temperature is lower than 1800?,the hardness of B₄C polycrystals increases with the temperature sintering,And tends to a fixed value,the density gradually increases and approaches the theoretical density;the hardness reaches a peak of-35.6 GPa at 1600?,and the density is 99.2%at this time;when sintering above 2000?,some B₄C decomposes,The precipitation of graphite phase leads to a decrease in the density and hardness of the polycrystal.2?Nano-polycrystalline B₄C containing a small amount of graphite was prepared by secondary sintering under high pressure,with hardness up to 35.2 GPa.We use the HPHT method to synthesize nano B₄C under different conditions using graphite of different particle sizes powder and 100 nm boron powder mixed as raw materials under different conditions.Using 500 nm graphite as the raw material at 5GPa,1900?,60 min,the purity of nano-polycrystalline B₄C with a particle size of500 nm and a hardness of 27.1 GPa was synthesized.With 30 nm graphite as the raw material,nano-polycrystalline B₄C with a particle size of 300 nm can be synthesized at 5 GPa,1900?,and 30 min.This result indicates that reducing the particle size of the raw carbon powder may reduce the reaction barrier of the B₄C synthesis,increase the chemical reaction rate,and effectively reduce the sample size.In order to further reduce the particle size of B₄C polycrystalline,improve its compactness and hardness,using 100 nm boron powder and 30 nm graphite as raw materials at 5 GPa,1600°C,and 30 min,B₄C with a particle size of 50 nm and incomplete reaction was prepared.In order to make the prepared 50 nm B₄C react more fully and improve its density and hardness,it was sintered at 20 GPa,1600°C,and 60 min for 60 minutes to prepare a high particle size of 100 nm with a small amount of graphite nano-polycrystalline B₄C,hardness value up to 35.2 GPa.3?The shear stress environment was constructed in the high-pressure sample cavity,and combined with the centrifugal separation method,a high-purity nano B₄C powder was successfully prepared.Using micron-level B₄C as a raw material,a high-hardness single crystal substance is placed in a quasi-hydrostatic high-pressure sample chamber.In the compression process of the high-pressure sample chamber,due to the huge difference between the elastic constant of the high-hardness single crystal and the sample,the difference in compression rate causes the quasi-static hydraulic environment inside the high-pressure sample chamber during the pressurization process to give samples around the high-hardness single crystal Provide shear stress environment.The shear force is used to break the micron-sized B₄C grains multiple times.The average particle size of B₄C decreases with the number of pressure loads.Subsequently,the prepared nano-B₄C powder was extracted by centrifugal separation method,and the B₄C powder with uniform particle size and smaller than 100 nm was successfully obtained.4?Use the prepared nano-B₄C powder to prepare nano-polycrystalline B₄C.The pure nano-polycrystalline B₄C was prepared under the conditions of 20 GPa,1500?and 15 min,and its hardness was measured to be 30.6 GPa?4.9 N?,increased particle size to 150 nm.The main reason for the low hardness is the low density of the polycrystalline B₄C prepared from this raw material.This result indicates that the preparation of high-density nano-polycrystalline B₄C may require higher sintering conditions,and excessively high sintering temperature will cause grain growth.Using different precursors,nano-polycrystalline B₄C was successfully prepared under HPHT conditions.The sintering temperature was the main factor affecting the polycrystalline particle size.These results provide new ideas for the preparation of B₄C nano-polycrystals and new data for the development of new superhard materials.