Preparation And Properties Of SiC Nano Whiskers/Fibers Reinforced SiC Ceramic Matrix Composites By Reactive Melt Infiltration

SiC ceramics are widely used in safety protection,aerospace and electronics fields due to their low density,high strength,high temperature resistance and oxidation resistance.However,due to its disadvantage such as high brittleness and poor reliability,limit its application as a high-temperature structural material.Generally,reinforcements such as whiskers and fibers are introduced into the SiC ceramics to prepare a SiC ceramic matrix composites to improve their mechanical properties.In this paper,SiC nano whiskers(SiCnw)and SiC nano fibers(SiCnf)are used to prepare SiCnw/SiCnf reinforced SiC ceramic matrix composites by reactive melt infiltration.The main research work is as follows(1)PyC/BN-coating of SiCnw/SiCnf reinforcement,BN-coating of SiCnw/SiCnf by using boric acid and urea impregnation reaction process.Research indicates:the layered BN interface phase is formed on the SiCnw/SiCnf surface,and its thickness is about several tens to several hundreds of nanometers.PyC-coating of SiCnw/SiCnf by pyrolysis of phenolic resin.Research indicates:after PyC-coated,the uniform and rough PyC interface phase was formed on the surface of BN-coated SiCnw/SiCnf,and its thickness was about several hundred nanometers.(2)The preparation of SiCnw/SiC composites,the effects of carbon black content on the microstructure and mechanical properties of SiCnw/SiC composites were investigated,and the mechanism of SiCnw/SiC composites strengthening and toughening was analyzed.Research indicates:when the carbon black content is 12 wt%,the flexural strength and fracture toughness reach the highest values of 273 MPa and 4.3 MPa·m1/2,respectively,which is 20%higher than that of SiC ceramics,and the fracture toughness is increased by 15%;when the carbon black content exceeds 12 wt%,due to the agglomeration and uneven distribution of carbon black,the further improvement of its mechanical properties is restricted;there is obvious SiCnw pullout at the fracture of SiCnw/SiC composites,it is indicated that PyC/BN-coated of SiCnw can avoid the erosion of Si during high temperature reactive melt infiltration;the improvement in mechanical properties of SiCnw/SiC composites is mainly due to whisker debonding,whisker bridging,crack deflection and whisker pullout.These mechanisms need to consume a large amount of fracture energy,and thus play a role of strengthening and toughening.(3)The preparation of SiCnf/SiC composites,based on the research of SiCnw/SiC composites,optimize and improve the preparation process.The effects of different content phenolic resin as C-course coated SiCnf on the mechanical properties and microstructure of SiCnf/SiC composites were investigated,and the mechanism of SiCnf/SiC composites strengthening and toughening was analyzed.Research indicates:when the phenolic resin content is 16 wt%,the maximum bending strength is 294 MPa,which is 29%higher than that of SiC ceramics;when the content of phenolic resin is 12 wt%,the fracture toughness reaches a maximum value of 4.74 MPa·m1/2,which is 22%higher than that of SiC ceramics;when the content of the phenolic resin is 16 wt%,the bonding strength of the PyC interface is strong,so that the SiCnf is not easily to debond and pull out,and the fracture toughness of the composite material is decreased;the improvement in mechanical properties of SiCnf/SiC composites is mainly due to fiber debonding,fiber bridging,crack deflection and fiber pullout.All of the above mechanisms need to absorb the fracture energy,and thus play a role of strengthening and toughening.In a word,compared with SiCnw,the mechanical performance of composites prepared by using SiCnf as a reinforcement is more excellent.The strength and toughness are significantly improved.At the same time,the research still has the following problems to be improved:Coating SiCnw/SiCnf with PyC/BN composite interface phase can prevent it from being attacked by Si during high temperature reactive melt infiltration,but PyC/BN-coating of SiCnw/SiCnf by using boric acid and urea impregnation reaction process and pyrolysis of phenolic resin,the interface bonding strength is difficult to adjust,and the coating effect is not ideal.In addition,SiCnw/SiCnf is easy to agglomerate and difficult to disperse evenly,and it is easy to destroy the morphology of SiCnw/SiCnf during the dispersion process to shorten the fiber.These factors restrict the further improvement of the mechanical properties of the composite.