Microstructure And Mechanical Properties Of Joints For C_f/SiC Composites By Reaction Bonding Process

Carbon fiber reinforced silicon carbide matrix composites?C_f/SiC?have received considerable attention for their excellent high-temperature performances,excellent corrosion resistance,high specific rigidity,high thermal conductivity and exhibits low density.Thus,C_f/SiC composites are possible materials in many areas,such as high-temperature structural components,aerospace,national defense,and energy and so on.The leap-forward development in the fields of aerospace,military,and energy has placed an urgent application demand for complex internal profile structures and large-sized C_f/SiC composite structural members.The use of joining technology to join relatively simple members is an effective method for preparing complex and large-sized C_f/SiC composite components.Therefore,the joining technology is the key technology to solve engineering application of the complex structure and the large-scale C_f/SiC composite.This paper aims to develop a reaction joining technology that enables stable joint.A stable carbide-based ceramic joint of the C_f/SiC composite is obtained by reaction joining using organic resin as a carbon source.This is of great significance for advancing the engineering application of C_f/SiC composites.Obtaining a porous carbon preform with controlled pore size and a uniform distribution is the basis and premise of the reaction joining.In this paper,the phenolic resin-alcohol solvent system was used to prepare porous carbon with controlled pore size by adding active materials such as divalent metal salts and boric acid.The addition of divalent metal salts and boric acid can achieve that the pore size of the resulting porous carbon material can be regulated in the range of nm-?m.Research shows:?a?the addition of iron favors the formation of porous pores with large pore size,fine carbon skeleton,and uniformly distributed pore structure.With?Fe increasing from 0%to 1%,the average pore size and apparent porosity of the porous carbons increase from14 to 190 nm and from 25.6%to 63.3%,respectively.Iron ions can be used as a template and catalyst to change the pore structure of porous carbons.?b?Boric acid was used as a source of complexing agent to change phase separation kinetics and dynamics of the resin-glycol system to regulate the pore structure of porous carbon.The results show that the addition of H₃BO₃ in the resin mixtures can change the polymerization dynamics during curing of resin-glycol mixtures.For the complexation of H₃BO₃ to diols,the size of the ethylene glycol-rich phase produced during the curing of the resin mixture increase with the increase of the content of H₃BO₃.Similarly,the pore size of porous carbon after pyrolysis increases with the increase of H₃BO₃ content.The average pore size of resulting porous carbon can be regulated in the range from 15 to 2754 nm.These results reveal that H₃BO₃ exhibited obvious effects in changing the pore structure of resin-glycol system during pyrolysis.In this paper,an improved joining technique by reaction bonded technology was studied.The C_f/C preform was joined before the silicon infiltration process and the joining process was applied at the stage of the siliconization.The final densification and joining of the reaction-sintered composites were simultaneously completed in the reaction bonding process.The most important advantage of this technology is that the joining of the reaction-sintered composite by a one-step Si infiltration process,avoiding the repeated silicon infiltration process.The effects of porous carbon preform density,pore size and the addition of inert filler on the microstructure and properties of the joint were investigated.Research shows:?a?the density of the porous carbon preform has a great influence on the content of free silicon in the joint,and is suitably selected from0.7 to 0.9 g�cm-3;the pore size of the porous carbon preform mainly affects the size of free silicon in the joint,and is suitable for 0.2⁰.6?m.?b?The addition of inert filler?-SiC effectively reduces the volumetric shrinkage of the porous C/SiC preform and reduces defects such as cracks caused by shrinkage.For the samples joined with 10-50wt%SiC fillers in the precursor slurry,and SiC and Si phase trend to be homogeneously distributed with increase of filler addition and the highest flexural strength reaches 216�44 MPa when filler addition is 50 wt.%.?c?The joint obtained by the one-step silicon infiltration reaction has a more uniform microstructure,the joint flexural strength is increased to 203 MPa,and the strength retention rate is 96%.The results show that a transition layer of 2³?m transition layer formed between the interlayer and the C_f/SiC substrate,which is composed of SiC crystal grains of about 0.5¹?m.The formation of the transition layer is due to the carbon concentration difference at the interface between interlayer and substrate,resulting in the diffusion of carbon during the Si-C reaction.Random chopped short carbon fibers(C_sf)/phenol-formaldehyde resin?PF?/SiC powder mixtures are used as filler for the joining of Cf/SiC composites to obtain SiC interlayer at the joining region.It has been demonstrated that joints with fine interfacial bonding and high flexural strength can be achieved by reaction bonding method using C_sf/PF-ET/SiC mixture as filler material.The preform with C_sf shows much more uniform microstructures due to the formation of porous carbon-bonded carbon fiber skeleton.The chopped carbon fiber is converted into silicon carbide by silicon infiltration reaction.The addition of the random chopped carbon fiber is advantageous in decreasing the residual silicon contents of the interlayer.The decrease of residual silicon content is mainly because of the increase of newly formed?-SiC with increasing carbon fiber content,and the high volume expansion occurred during the Si-C reaction between the C_sf and the molten silicon.As the carbon fiber content reaches to 30 wt.%,the flexural strength of joint increased to the highest level of 232�33 MPa due to the reduced of residual silicon content and formation of the nano-sized SiC particles.