| As it is well known,alumina ceramics with rich source and low cost are often used in wear-resistant materials and devices due to their excellent characteristics such as high hardness,high temperature oxidation resistance,corrosion resistance and high strength.With the continuous development of science and technology,the requirements for material properties become more stringent.The brittleness and high wearability of alumina ceramics limit their industrial applications and stability.Thus,how to prepare alumina ceramics with excellent properties such as high toughness,high strength and low wear is a hot and difficult point of current research.In this thesis,a series of alumina ceramic matrix composites reinforced with graphene,silicon carbide whiskers and aluminum borate whiskers were prepared by a hot-pressing sintering method,and the regulation mechanism of structural performance of alumina ceramic matrix composites was systematically studied and explored.Furthermore,oil-coated rollers for glass fiber production were successfully developed by the use of aluminum borate whisker-reinforced composite ceramics demonstrating the application prospects of low-wear alumina ceramic matrix composites in wear-resistant devices.The main research contents and conclusions are as follows:(1)Preparation and performance of Graphene-reinforced alumina ceramic matrix composites.For the first time,an improved stepwise feeding method was used to prepare a graphene alumina mixed powder.The results show that the stepwise feeding method was more conducive to graphene powder dispersion than the one-step feeding method,effectively solved the problem of graphene powder aggregation which may lead to large pores in the microstructure of the fired ceramics.Secondly,graphene-reinforced alumina matrix composites were prepared by the hot-pressing sintering method.The effects of sintering temperature,sintering pressure and graphene content on the mechanical properties of the material were systematically investigated.The results show that the structure regulation through the change of sintering temperature and sintering pressure can significantly affect the mechanical properties such as bending strength,fracture toughness and Vickers hardness of the prepared alumina matrix composites.At a sintering temperature of 1550°C and a sintering pressure of 40 MP,the bending strength,fracture toughness and Vickers hardness increased first and then decreased with the increase of graphene content.The optimal properties can be approached by the composites with 1.0 wt%of graphene,which were7.50 MPa·m1/2,854.50 MPa and 21.30 Gpa for the bending strength,fracture toughness and Vickers hardness,respectively.When the graphene content was 0.5 wt%,the composites had the lowest wear rate of 2.91×10-6 mm3/(N·m).Based on the theoretical model of fracture strength,which can be established by considerable critical defect size and effect of porosity rate,and the experimental results,the variation trend of the fracture strength of the composites with sintering temperature and graphene content and its inherent control mechanism were systematically analyzed.The fracture strength of the composites mainly depends on the largest grain size in the material microstructure,which has not been reported in previous studies on the control mechanism of fracture behavior in alumina ceramic matrix composites.The Hall-Petch relationship between the hardness and the grain size and the experimental results were used to analyze the influence factors of the material hardness.The increase in fracture toughness of the composites is mainly due to the deflection,bending and bifurcation that occurs during crack propagation,the transformation of fracture mode,the extraction and bridging of graphene,and the laminated structures and the strong and weak interfaces introduced by graphene.The improvement of wear performance of the composites was mainly due to lubricating effect of graphene on the contact surfaces.In addition,the results indicate that 4.76 wt%Zr O2 brought into the material system by the Zr O2 grinding balls during the raw material ball-milling process did not adversely affect the alumina ceramic matrix composites in this thesis.(2)Preparation and performance of silicon carbide whisker-reinforced alumina ceramic matrix composites.Silicon carbide whisker-reinforced alumina matrix composites were prepared by the hot-pressing sintering method.The effects of the content of silicon carbide whiskers on the bending strength,fracture toughness,Vickers hardness and wear rate were systematically studied.The results indicate that with the increased content of silicon carbide whiskers,the bending strength and fracture toughness had a trend of first increase and then decrease.When the content was 20%,the bending strength and fracture toughness reached the highest value of 875.00 MPa and 5.40 MPa·m1/2,while the friction coefficient and wear rate reached the minimum and the latter was 3.82×10-6 mm3/(N·m).The influence of silicon carbide whiskers on the strength and toughness based on the mechanism of grain refinement,crack deflection and the pull out of the whiskers.The wear resistance mainly depends on the synergy between the hardness and toughness of materials.(3)Preparation and performance of aluminum borate whisker-reinforced alumina ceramic matrix composites.For the first time,aluminum borate whisker-reinforced alumina ceramic matrix composites were prepared by the hot-pressing sintering process.The effects of aluminum borate whisker content,sintering temperature and sintering method on the mechanical properties of alumina ceramic matrix composites were systematically analyzed.The results show that alumina ceramic matrix composites with the best performance can be obtained through controlled microstructure of the material by changing the content of aluminum borate whiskers,sintering temperature and sintering method.With the increase of the volume fraction of the aluminum borate whiskers or the hot-pressing sintering temperature,the bending strength and hardness of the composites tended to increase first and then decrease while the fracture toughness increased gradually.With the increase of the volume fraction of the aluminum borate whiskers,the wear rate of the composites decreased first and then increased.The bending strength of the alumina ceramic matrix composites reached the highest of 767.00 Mpa when the volume fraction of aluminum borate whisker was 20%.The fracture toughness reached 4.43 MPa·m1/2 with the volume fraction of aluminum borate whiskers of 30%.When the volume fraction of aluminum borate whiskers was 10%,the wear rate of reached the smallest of 1.50×10-6mm3/(N·m).The fracture strength of composites mainly depends on the size of the largest grains or clusters in the microstructures.The increase in fracture toughness is mainly due to whisker reinforcement.The liquefied aluminum borate whiskers were extruded by the grains during sintering and recrystallized on the grain surface to form needle-like whiskers,while bridging occurred in some whiskers.It can be known that the fracture toughness of composites increases with the increase of whisker volume fraction according to the model that the bridging whiskers would lead to an increase in fracture toughness.At the same time,crack deflection and bridging due to reinforcing whiskers could also improve the fracture toughness.The wear properties of the composites were mainly affected by the combination of material strength,hardness and toughness.In addition,it was found that the liquefied aluminum borate whiskers were reduced by solid phase C from graphite mold,carbon paper and partially vaporized C from graphite heater,carbon felt,and then reacted with Zr O2 at the internal grain boundary to form Zr B2 particles to further achieve the role of toughening and strengthening.(4)Application research of alumina ceramic matrix wear-resistant composites reinforced by aluminum borate whiskers.In order to present the application prospects of low-wear aluminum borate whisker-reinforced alumina ceramic matrix composites,oil-coated rollers for glass fiber production were successfully produced by using aluminum borate whisker-reinforced alumina ceramic matrix composites,and performance comparison between the oil-coated rollers for glass fiber production and commercial graphite oil-coated rollers was also carried out.The results show that the oil-coated rollers prepared with aluminum borate whisker-reinforced alumina ceramic matrix composites not only have greater hardness but also excellent wear resistance comparing with the graphite oil-coated rollers.The high performance oil-coated rollers prepared with aluminum borate whisker-reinforced alumina ceramic matrix composites are capable for solution of serious charging on insulating glass fiber,and they have been applied to a world-renowned glass fiber company opening up a broad market prospect of wear-resistant materials and devices. |
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