Preparation And Properties Of Silicon Nitride Ceramic Hollow Buoyancy Ball

There is a lot of resources and rich biodiversity in deep sea,besides,it may contain the key to unravel the mystery about the origin of life on earth.Since the beginning of the 21 st century,various countries were attaching great importance to deep-sea exploration.Deep sea exploration requires deep diving equipment,which need quality materials to make it reliable to be used for exploration.Silicon nitride?Si₃N₄?is a kind of structural ceramic material with excellent performance which has been widely used in many fields such as defense and aerospace industry.Si₃N₄ ceramic hollow flotation spheres is a kind of excellent performance deep-sea buoyant material with high hydrostatic compressive strength,which could be lower density and more stable in seawater environment than the other buoyant materials at the same grade.This thesis extensively discussed the design of the ceramic hollow flotation spheres;the research about the forming process of the hemispherical shell;and the quality control method and pressure test of ceramic hollow flotation spheres.Si₃N₄ ceramic is the best choice for ceramic hollow flotation spheres for its higher modulus of elasticity and theoretical compressive strength,and its density is relatively low at the same time.The theoretical calculation shows that the ultimate compressive strength and buckling strength are 286.8 and 532.2 MPa respectively while the Si₃N₄ ceramic hollow flotation spheres got 100 mm inner diameter and 2mm wall thickness,which satisfies the safety factor of SF?2?which the design pressure is 115MPa?.The ultimate compressive strength and buckling strength of Si₃N₄ ceramic hollow flotation spheres could be pretty close in theoretical value which has been calculated by the ABAQUS finite element analysis software,and the maximum principal stress distribution of the spheres is uniform which proved that it got great pressure resistant structure.Compared with the ABAQUS analysis of Si₃N₄ ceramic canister,it is found that the structural symmetry of the ceramic canister is lower than the hollow spheres,which got more stress concentration areas that have a great influence on the structure strength.The cause of “Nereus” unmanned submersible crash may not only from the ceramic hollow flotation spheres implosion,but the ceramic canister was related.In the research of the gel-casting and 3D printing rapid prototyping of Si₃N₄ ceramic hemispherical shell,it could be found that the densities of Si₃N₄ ceramic sintered parts formed by gel-casting did not meet the requirements,and plenty of pinholes in green and sintered bodies could be observation by the microscopic observation of fracture;the sintered 3D printing hemispherical shells which can achieve 99.7% theoretical density with more than 1000 MPa bending strength and the Weibull modulus 12,shows high performance and reliability,besides,the Si₃N₄ ceramic hollow flotation spheres could been fabricated by 3D printing technology stability,nevertheless,the wall thickness and outer diameter of 3D printing ceramic hemispherical shell exists uneven phenomenon,due to the shrinkage uneven in the post-processing process.A corresponding quality control method has been formed while the main parameters of the ceramic hollow flotation spheres were measured.The qualified sintered density of Si₃N₄ ceramic hemispherical shell should reach 3.23g/cm³;ceramic hollow flotation spheres with a nominal diameter of 100 mm in the density of 0.33,0.39,0.45 g/cm³ with different thickness has be formed in laboratory products series,the buoyancy performance of these three series of products are better than the current deep sea buoyancy material for 10000 meters diving.The average implosion compressive strength of Si₃N₄ ceramic hollow flotation spheres with a density of 0.33 g/cm³ was 203.8MPa,but the dispersion of the measured data were large,indicating that the preparation process was still needed to be further optimized.The Si₃N₄ ceramic hollow flotation spheres with a density of 0.39 g/cm³ exhibited a higher safety factor while its implosion compressive strength can reach 300 MPa.The Si₃N₄ ceramic hollow flotation spheres has got no failure with the static pressure fatigue test at a holding pressure of 145 MPa for 10 hours and 1000 cycles of fatigue test at 115 MPa.The research on the implosion process of the ceramic hollow spheres shows that the ceramic hollow spheres accumulates a large amount of elastic strain energy during the compression stage;then some of the energy is converted into material fracture energy while the explosion occurring,and the other part is converted into kinetic energy,which shows the siphon effect on the center of the spheres;then energy and matter begin to spread to the surroundings at the moment,the system achieve balance finally.