| This thesis,which is based on National Natural Science Foundation: Self lubricating film（51405278）,aimed at study the preparation process（conventional pressure sintering,gas pressure sintering,hot pressing sintering）,physical properties,mechanical properties,tribological properties and corrosion resistance properties of the Si3N4-hBN composite ceramics.The Si3N4-hBN composite ceramics was sintered by three sintering processes respectively,and its the microstructure and properties were analyzed and observed so as to demonstrate and optimize the sintering process of Si3N4-hBN composite ceramics.The tribological properties and corrosion resistance properties of（nano and micron）Si3N4-h BN composite ceramics of hot pressing sintering were investigated.Among them,the comprehensive influence of h BN on properties was systematically researched and its mechanism was deeply studied.In order to explicit the sintering mechanism of the different preparation process and its effected on the densification,the relationship among preparation process,microstructure and properties was synthetically analyzed.Besides,the MMW-1 pin-disc wear and friction tester was used to carry out the friction test of Si3N4-h BN composite ceramic sliding against bearing steel（GCr15）and stainless steel 0Cr18Ni9Ti（ASS）.The static immersion method was used to carry out the corrosion test of Si3N4-hBN composite ceramics in different corrosive environment（acid and cryolite molten salt）.At the same time,the phase and morphology of the ceramic specimens were analyzed and observed by using X-ray diffraction（XRD）,energy dispersive spectrometer（EDS）,scanning electron microscope（SEM）and laser scanning microscope（LSM）.The results showed as follows:（1）The Si3N4-h BN composite ceramics prepared by conventional sintering process（sintering temperature was 1450℃）didn’t have a phase transition from α-Si3N4 to β-Si3N4,and its texture was loose and mechanical properties was deteriorated.The Si3N4-hBN composite ceramics prepared by gas pressure sintering process（sintering temperature was 1800℃,N2）were mainly composed of β-Si3N4 and hBN,and its physical and mechanical properties had a certain extent increase with respect to conventional sintering.Among them,the density of Si3N4-h BN composite ceramics was improved significantly.Particularly,the porosity of Si3N4 reached 0.32,the flexural strength and Vickers hardness reached 467 MPa and 13.6GPa,respectively.At the same time,the mechanical properties of Si3N4-hBN composite ceramics decreased with the increase of hBN.（2）The mechanical properties and tribological properties of Si3N4-hBN composite ceramics prepared by hot pressure sintering process（sintering temperature is 1800℃,N2,sintering pressure is 30MPa）were significantly improved.Among them,the mechanical properties of ceramics decreased with the increase of hBN.Specially,the mechanical properties was the lowest when the hBN was 30%,but it was the same as that of the Si3N4 ceramics of gas pressure sintering.Under the condition of dry friction(speed was 500r·min-1,load was 10N),the friction coefficient of the Si3N4-20%hBN and GCr15 tribocouples was 0.36,and the wear coefficients of pins were 10-8mm3·N-1m-1.The friction coefficient of the Si3N4-30%h BN and ASS tribocouples was 0.23,and the wear coefficients of pins were 10-11 mm3·N-1m-1.（3）The physical and mechanical properties and tribological properties of nano Si3N4-hBN composite ceramics prepared by hot pressing sintering process were improved slightly compared with that of micron Si3N4-hBN composite ceramics.Among them,the density,flexural strength and fracture toughness of Si3N4 ceramics increased to 3.2g·cm-3,818 MPa and 7.36MPa·m1/2,respectively.Except for Si3N4 ceramics,the physical and mechanical properties of Si3N4-5%hBN,Si3N4-10%hBN,Si3N4-20%hBN and Si3N4-30%h BN composite ceramics were basically the same as those of the micron Si3N4-hBN composite ceramics.Besides,the mechanical properties of Si3N4-hBN composite ceramics decreased with the increase of hBN.Under the condition of dry friction,the friction coefficient of the Si3N4-20%h BN and GCr15 tribocouples was 0.31,and the wear coefficients of pins were 10-8mm3·N-1m-1.The friction coefficient of the Si3N4-30%hBN and ASS tribocouples was 0.28,and the wear coefficients of pins were 10-11mm3·N-1m-1.The reason for this is that the friction surface form a smooth thin layer,which contains a solid lubricant hBN,so that play a good role in reducing friction and lubrication.In the corrosion experiment,the corrosion of hydrofluoric acid and cryolite molten salt on samples was the most serious.The reason for this is that the loss of Si3N4 is caused by the chemical reaction between Si3N4 and corrosive medium.Furthermore,the higher density ceramics and the more uniform distribution of hBN in ceramics can significantly reduce or block the contact between the ceramic and the corrosive medium,thus slowing down the corrosion process.（4）The Si3N4-hBN composite ceramics prepared by hot pressure sintering process have good physical and mechanical properties.The reason for this is that the chemical reaction of Si O2 and sintering aids（Y2O3 and Al2O3）to form a sufficient amount of Y-Al-Si-O euetectic liquid phase,so that the mass transfer process can be carried out smoothly.At the same time,the pressure of hot pressing sintering process can greatly accelerate the mass transfer process,so that the holes of green body are effectively filled,so as to improve the density and mechanical properties.In addition,the morphology of the Si3N4-hBN composite ceramics prepared by hot pressing sintering is changed from compact to porous with the increase of hBN.Altogether,the physical and mechanical properties of the ceramics are significantly improved by liquid phase sintering,and decreased with the increase of hBN.（5）The densification of ceramics is determined by the degree of the mass transfer process in the sintering process.The three kinds of sintering process are occurred in liquid phase sintering,whose degree is different.Among them,the conventional sintering is the lowest,and gas pressing sintering is as the center and hot pressing sintering is the highest.Therefore,the hot pressing sintered Si3N4-h BN composite ceramics have good properties. |
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