Study On The Fabrication Of Oxidation Resistance C_f/C Composites By Rapid Chemical Liquid Deposition Techniques And Its Welding Properties

Since 1980s,the C_f/C composite has more and more applications in many industries such as aerospace,chemistry,medicine,etc,due to its low density,high specific strength,excellent wear and ablation resistance.However,the high cost,low oxidation resistance at high temperature and complexity in preformed parts' fabrication of this kind of material restrict its further application.In this work,rapid fabrication of C_f/C composites were successfully achieved by using rapid chemical liquid deposition technology,and C_f/C composites with a density up to 1.7 g/cm³ were fabricated.The surfaces of C_f/C composites were modified with electrochemical method by using concentrated nitric acid.The effects of two different modification methods on the microstructure and properties were compared by using SEM and three point bending test.The ideal surface modification procedure is as follows:electrochemical method,1.2 V,5 minutes.Effect of density of fibrous body and deposition time on the density and microstructure of C_f/C composite were investigated.The results show that the C matrix surrounds the C fibers in multiple circular layers,with the number of layers related to thickness and density of fibrous body.The density of C_f/C composite depends on its deposition time.With fibrous body with densities of 0.12 and 0.58 g/cm³,the density of C_f/C composite increased rapidly with deposition time at beginning.After 4 hours deposition,the density of C_f/C composite increased very slowly.And after 5 hours deposition,the density of C_f/C composite did not change any more.With fibrous body with a density of 0.66 g/cm³,the density of C_f/C composite kept increasing during the initial 6 hours of deposition.The density of C_f/C composite relate to the density of fibrous body.With a constant deposition time,the density of C_f/C composite decreased with increasing the density of fibrous body.The resistivity and scleroscope hardness of the resultant C_f/C composite were measured.The resistivities fall between 87.5Ω·μm and 119.5Ω·μm,with an average value of 103.5Ω·μm.With three kinds of fibrous body with different densities(0.12 g/cm³,0.58 g/cm³ and 0.66g/cm³),the hardnesses of C_f/C composites are 78,90 and 96 HSD,respectively。The bending strength and compressive strength were measured by using universal testing machine.The maximum bending strength and compressive strength are 122.51 MPa and 102.57 MPa respectively.Various factors affecting the strength of C_f/C and the characteristic of stress-strain curve were analyzed.The fracture appearances of C_f/C composites were investigated by using SEM,and the fracture mechanism for C_f/C was analyzed.It is indicated that the bending strengths of C_f/C composites increased with the increasing of density of fibrous body.The fracture appearances of C_f/C composites with high interface bonding strength exhibit a typical brittle characteristic,and those of C_f/C composites with a modest interface bonding strength exhibit pseudo-plastic characteristics.A layer containing C and Si series chemical compound were deposited on C_f/C composite through secondary deposition method.The deposition parameters were optimized by Orthogonal design,the optimal parameters were obtained.Silicone oil with a Si fraction of 20%was used as source of silicon.The rise time for 1 volt is 60 minutes.The maximum density is 1.94 g/cm³,and the maximum porosity is 17.07%. The morphology and distribution of various phases were analyzed with XRD,SEM and EDS.The results show that C and Si series chemical compound were deposited together into the surface pore of C_f/C composite.The bending strength and compressive strength of C_f/C-Si series chemical compound composites were measured.The maximum bending strength is 119.54 MPa,lower by 5%～10%compared with that of C_f/C composite.The maximum compressive strength is 80.02 MPa,higher by 10%～15%.The oxidation behaviors of C_f/C-Si series chemical compound composites were investigated with thermogravimetry,DSC and oxidation kinetics.Compared with C_f/C composite,the oxidation resistance of C_f/C-Si series chemical compound composite increases by 4-5 times.This is attributed to that the Si series chemical compound surface layer can effectively prevent the composite from oxidizing.The initial oxidation temperature of C_f/C-Si series chemical compound composites rised up to 684.0℃.The effect of oxidation on the structure and maco-dimension.The isothermal oxidation weight loss curve is linear curve.With increasing temperature,the weight loss rates due to oxidation increase.By means of SEM,the effect of the density of fibrous body on micro-morphology of oxidized C_f/C composite and weight loss rate were investigated.The results show that pyrolytic carbon is more liable to oxidation. With increasing the density of fibrous body,the weight loss rates of C_f/C decrease.Systematic studies were conducted on vacuum brazing of C_f/C composite. Microstructures and properties of C_f/C/Cu-Ti/ C_f/C brazing joints under various brazing parameters were investigated.Cu-based and Al-based brazes with different content of Ti were designed.The wet abilities of the brazes were measured by specific contact area method.The results show that the wet abilities of both Cu-based and Al-based brazes on C_f/C composites were improved with increasing content of Ti.In case of 15%of Ti,both Cu-based and Al-based brazes on C_f/C composites exhibit optimal wet abilities.Ti in molten braze diffuse towards interface between C_f/C and braze,and react with C to form TiC,reducing the interfacial energy and improve the wet ability.With low Ti content,only few isolated TiC particles form on the interface and thus the wet abilities are not acceptable.However,with too high Ti content,a lot ofβ-TiCu₄ chemical compounds were found,the wet abilities are not acceptable.The microstructure of joint brazed with Cu-Ti consists of C_f/C,TiC,Cu-Ti eutecticβ-TiCu₄+α-Cu.Based on the structure of joint,a physical model for brazing process of C_f/C composite and a mechanism for interface structure formation were developed.The constituents in the brazing joint do not vary with the brazing process parameter, only the fraction of each constituent(C_f/C,TiC,Cu-Ti eutectic andβ-TiCu₄+α-Cu) and the thickness of TiC change.With increasing of brazing temperature and soak time, the thickness of TiC increase,and the amount of Cu-Ti eutectic decease.With a brazing temperature of 1050℃for 30min,joints with a shear strength of 21MPa were obtained.