| In this thesis,based on the fabrication of Si3N4-hBN ceramic composite,Si3N4-hBN/Mo laminated composite was designed and prepared for the application requirements of engineering ceramics in various fields.The physical,mechanical and tribological behaviors under artificial seawater environment with different salinity and loads of Si3N4-hBN/Mo laminated composite are studied.Besides,the friction and wear characteristics of layered composite under the marine atmospheric environment are explored.Moreover,the composition and forming principle of the layer interface materials,as well as the wear mechanism and the lubrication mechanism of the friction pairs under the seawater environment are discussed.The physical and mechanical properties of Si3N4-hBN/Mo laminated composite were tested by Archimedes drainage method,three-point bending method,microhardness test and single edged notch beam.Meanwhile,a friction test platform with the MMW-1 friction and wear tester was set up for tribological performance testing.The fracture,corrosion morphology and wear surface after testing were observed by scanning electron microscope(SEM)and 3D topography.The phase composition of the layer interface and wear surface were analyzed by EDS and X-ray diffractometer(XRD).This thesis mainly includes such following research work:(1)In this thesis,the physical and mechanical properties of Si3N4-hBN ceramic composite materials were studied,moreover,the influence of the content of hBN on the tribological properties of Si3N4-hBN/Si3N4 and Si3N4-hBN/TC4 friction pairs under the artificial seawater environment with multi-salinity were explored.The results showed that the physical and mechanical properties of Si3N4-hBN ceramic composite decreased slightly due to the addition of hBN.For tribological behavior,different counterpart materials had obtained opposite test results under seawater environment with the change of salinity.When the counterpart material was Si3N4,the friction and wear characteristics of Si3N4-hBN ceramic composite increased as the seawater salinity increased.Moreover,the ceramic composite with hBN obtained a smoother friction surface in the friction process,which further reflected the better tribological performance than the ceramics without hBN additive.Besides,the wear mechanism of Si3N4-hBN and Si3N4 sliding pair under seawater environment was abrasive wear mainly.When the counterpart material was TC4,the effect of seawater salinity and the addition of hBN on the friction coefficient of Si3N4-hBN ceramic composite were not significant.However,high salinity of artificial seawater and high hardness of ceramics promoted the wear of TC4.In addition,the wear mechanism of Si3N4-hBN and Si3N4 friction pair in seawater was a mixture of abrasive wear and adhesive wear.(2)In this thesis,for improving the low fracture toughness of Si3N4-hBN ceramic composite,the layered structure was proposed to promote the properties of the composite ceramics.Based on that,the Si3N4-hBN/Mo laminated composite was prepared by adding Mo to Si3N4-hBN ceramic composite.The physical and mechanical properties of Si3N4-hBN/Mo laminated composite were studied.The tribological properties of Si3N4/Mo and TC4 friction pair under the artificial seawater environment with multi-salinity and variable load were also studied.Besides,the effect of rotational speed on Si3N4-hBN/Mo laminated composite was preliminarily explored.For the physical and mechanical properties,the layered structure did improve the fracture toughness of Si3N4-hBN/Mo laminated composite.When the crack extended to the interface of the layered material,this crack deflected and finally formed a "ladder" fracture path.However,the chemical reaction at the layer interface mainly produced Mo5Si3 that is a molybdenum-silicon compound with many inherent cracks,this multi-crack substance led to greatly reducing of compactness and bending strength.For tribological behavior,the increase of salinity and load in artificial seawater caused great damage to TC4 disc specimen sliding against Si3N4/Mo laminated composite,but the change of Mo content had no obvious effect on the tribological properties of Si3N4/Mo laminated composite.Moreover,the wear mechanism of Si3N4/Mo layered material and TC4 titanium alloy sliding pair was a mixture of abrasive wear and adhesive wear under different salt content and loads.In addition,the friction coefficient and wear rate of Si3N4-hBN/Mo laminated composite sliding against TC4 titanium alloy did no change trend signifcantly under the influence of rotating speed in the marine atmosphere.(3)In this thesis,the friction mechanism of Si3N4/Mo laminated composite under artificial seawater lubrication was discussed by comparing with Si3N4 ceramic composite under the same environment.The results showed that the tribological properties of Si3N4/Mo laminated composite were worse than that of Si3N4 ceramic composite.The reason can be explored that some substances,the molybdenum with low hardness and fragile Mo5 Si3 in the metal layer area,fell off the wear surface in the friction process.This situation led to smaller actual contact area between the friction pairs and larger contact pressure,which caused severe wear of TC4 disc.(4)In this thesis,the material distribution at the interface of Si3N4-hBN/Mo laminated composite was studied.The spontaneous chemical reaction at the interface was detected based on the calculated Gibbs free energy.It can be found that there were a series of chemical reactions between Si3N4 and Mo at the interface of layered composite materials during sintering and forming.The distribution of these reaction products is Si3N4-hBN,MoSi2,Mo5Si3,Mo3Si and Mo from interface to the middle of metal layer.In addition,according to the calculation of Gibbs free energy,these continuous chemical reactions can spontaneous at the sintering temperature.Futhermore,the fact that the reaction that produced Mo5Si3 was more spontaneous proved why there was so much of it in the metal layer.The research in this thesis shows that the layered structure of Si3N4-hBN/Mo composite can improve the fracture toughness of the ceramic.However,compared with bulk ceramics,the tribological properties of Si3N4-hBN/Mo laminated composite were poor under seawater environment,which was related to the products from continuous chemical reactions at the layer interface.Through these studies,it can not only improve the theoretical system of silicon nitride-based materials in laminated ceramic materials,but also enrich the test guidance data of the practical application of silicon nitride based layered materials in the field of marine lubrication. |
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