| Superhard materials(such as diamond,boron suboxide B6 O,cubic boron nitride c BN and polycrystalline diamond composites)are generally composed of boron(B),carbon(C),nitrogen(N),oxygen(O)and other light elements.These elements have small atomic radius and strong bonding energy,which can form high atomic density,three-dimensional network and super covalent bond dense structure,and have high resistance to external compression / shear stress.Although superhard materials have high hardness,their toughness is poor.Once they are subjected to strong impact load,they are prone to fracture,resulting in the overall performance degradation or even failure.By comparison,superhard composites can effectively improve the toughness and enhance the comprehensive properties of the material.Polycrystalline diamond composite has excellent physical and chemical properties,and has been widely used in mechanical processing,mining,geological exploration,oil and gas mining and other industrial fields.In recent years,a lot of researches have been done on the preparation of polycrystalline diamond composites,but most of them are focused on the preparation and static performance characterization.As one of the polycrystalline diamond composites,the diamond-Si C composite,has become more and more attractive.From the perspective of crystallography,diamond and Si C have similar structure.They can be combined to form bonds at high temperature and high pressure,which makes diamond-Si C composite have super interface strength and the material has high hardness,strength and toughness.Usually,diamond-Si C composite was prepared by chemical vapor deposition or high temperature and high pressure reaction.However,in the chemical vapor deposition method,the properties of diamond-Si C composites are unstable because the reaction speed is not easy to control,and it is difficult to get bulk materials.Because diamond-Si C composite has excellent properties,and can be used as ablation material and high performance armor material,the research on its preparation and dynamic performance is significant not only in academic studies,but also in engineering applications.The main contents,findings and innovations of this dissertation are as follows:(1)By the high pressure and temperature reaction method,we have synthesized the micron diamond-Si C composite,with this composite,the shock dynamic compression measurement have been carried out.Although the impact compression experiments of micro diamond-Si C composites have been carried out in our early studies,the previous composites contain tungsten carbide impurities,and also the maximum impact pressure is limited by 110 GPa.Importantly,the Hugoniot elastic limit of micro diamond particles yet has been observed.In this thesis,the diamondSi C composites without impurities were prepared by using 1400 tons hexahedral press under high temperature and high pressure.Then,the impact response of two-phase micro crystalline diamond-Si C composite was studied in the range of 22-195 GPa on the two-stage light gas gun dynamic high pressure device in the laboratory of the author.The impact results show that the Hugoniot elastic limit of micron diamond particles in Si C matrix can reach ~ 169 GPa is about twice of diamond single crystal.Meanwhile,the double elastic shock wave phenomenon was observed again,which verified the physical mechanism proposed by us,explains that the micron diamond particles how to have the ultra-high Hugoniot elastic limit in Si C matrix: when the impact pressure exceeds the Hugoniot elastic limit of Si C,the Si C matrix becomes damage and wrapped around the diamond particles,such a behavior significantly releases the heavy shear stresses and protests the diamond from damage.As a result,the impact resistance of diamond particles is greatly enhanced.(2)Submicron diamond-Si C composites were prepared under high temperature and high pressure.The impact response characteristics of submicron diamond-Si C composites were studied by using a two-stage light gas gun.The experimental results show that although the static mechanical properties of nano polycrystalline diamond(NPD)are much higher than those of submicron diamond-Si C composites(the hardness of NPD is more than twice that of submicron diamond-Si C composites),the Hugoniot elastic limit of submicron diamond-Si C composites(~ 200 GPa)is equivalent to Hugoniot elastic limit of NPD reported by Japanese scholars of(Physics Review Letters,2020).In terms of preparation process and technical difficulty,submicron diamond Si C composites are easier to prepare large-size bulk materials,and the preparation cost is much lower than NPD.The pressure of synthesizing diamond-Si C composite is about 5.5 GPa.According to the existing domestic technology level of static high pressure synthesis with hexahedral top and large cavity,the submicron diamond silicon carbide composite block with diameter of 3 inches(~80 mm)can be prepared;while the synthesis pressure of NPD is more than 15 GPa,only 10 GPa can be prepared at present.In addition,the fabrication cost is more than100 times of that of submicron diamond-Si C composites.In other words,compared with nano polycrystalline diamond bulk materials,the preparation efficiency of submicron diamond Si C composites is higher and the cost is lower.Therefore,submicron diamond-Si C composites with excellent impact properties have a broader engineering application prospect.(3)The impact characteristics of three kinds of polycrystalline diamond composite bulk samples with different microstructures were studied and compared.The three samples are: a)micron grain diamond silicon carbide composite;b)submicron grain diamond silicon carbide composite;c)micron grain polycrystalline diamond(MPD)block.In the experimental study on the impact compression of diamond-Si C composites with the same composition,we found that the micron polycrystalline diamond bulk samples with silicon carbide matrix can exhibit double elastic shock waves,while the diamond grain size is submicron,the double elastic shock wave phenomenon disappears and only a single elastic wave is observed.In addition,it is observed that the hardness and Hugoniot elastic limit of MPD are equivalent to that of diamond single crystal because of the micro crystal diamond framework formed by strong D-D bonding.This result is interesting,because the hardness of MPD is more than twice that of diamond-Si C composites,but the Hugoniot elastic limit of the former is only half that of the latter.Our research results show that the microstructure of polycrystalline diamond can effectively control its impact response performance,in other words,the materials with specific impact response characteristics can be obtained through microstructure design. |
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