Microstructure Design and Performance Control of C/SiC Composites Fabricated by LA-CVI Process
Supervisor:cheng lai fei liu yong sheng
Chemical Vapor Infiltration(CVI)method owe many excellent performances such as little damage to fiber,high purity of matrix,simple process equipment.Which make CVI emerged as one of the few major manufacturing technologies for the industrialization of continuous carbon fiber reinforced carbide composites(C/SiC).However,with the development of new aeronautics and astronautics technology,in order to maintain the properties of components under the mechanical and aerodynamic loading conditions,we are facing the challenges of increasing the thickness of components(>4mm)and the closed structure,which puts forward higher requirements for the ability of penetration during CVI process.How to realize the rapid densification and performance controlling of the thicker and complex components of C/SiC composite is a difficult problem to be solved.Therefore,this paper is focus on the improvement the permeability and toughening properties of C/SiC composites,a new idea of femtosecond laser assisted CVI(LA-CVI)method is proposed.The influence of femtosecond laser on the preparation of mass transfer channel was studied systematically,and the regulation mechanism of the mass transfer channel on the deposition rate,density uniformity and performance of the composite material was discussed,combined with the mass transfer model and the corresponding simulation calculation,which could improve the permeability and density of the composite,and regulated the strength of the composite material.Based on this study,the mass transfer channel sealed with second phase was studied to expand its functionality.The main contents and results are exhibited as follow:(1)Compared to the single ring line mode,the helical lines scanning mode was more suitable to use in holes machining,which could obtain high quality holes.While,using helical lines scanning mode of fixed focus machining C/SiC composite,only got certain blind holes.When the laser power was less than 4 W,only a certain depth of blind holes could be obtained;when laser power was higher than 6 W,high quality holes can be got.However,overtop power might cause serious oxidation and even ablated the edge area.When the scanning speed was less than 600 mm/s,the number of equivalent spot increased,and the overlapping coefficient added,which could get a better quality hole;contrary to the conclusion,when the scanning speed was more than600 mm/s,the number of the spots decreased,and the taper of the hole would be larger.When the scanning distance is less than 6μm,the coupling rate of spots could be larger,and the laser energy could repeated in the processing area,and the high quality through hole could be obtained;when the scanning distance is increased to 15μm,the absorbed energy in the area of action would be gradually lower than the material processing threshold,and it could only formed a blind hole with certain depth,while the diameter the entrance of the hole was basically unchanged.(2)The regulation of the microstructure of the C/SiC composites by the mass transfer channel was studied,and the mass transfer model was used to verify the results.Results showed that,the effect of densification was obvious.There were three typical natural features in the LA-CVI-C/SiC composite,which were compact strip,densification coating and SiC particle.And the width of the densification coating is about 100μm whatever the diameter of the mass transfer channel was.With the decrease of mass transfer channel spacing,the densification effect of composites increased,and the extent of densification increased.Along with the increase of the longitudinal rows of the mass transfer channel,the densification range of the composites increases,and the effect of density uniformity was remarkable.The mass transfer channel is prepared when the initial density of the preform was 1.5 g/cm~3,and the densification effect was obviously higher than that of the preform with the initial density of 1.8 g/cm~3.(3)The mechanical properties and fracture failure behavior of LA-CVI-C/SiC composites were studied.The regulation of mass transfer channel structure on the uniformity and properties of C/SiC composites was analyzed in combination with the crack propagation failure mode and the section structure.Results showed that,the smaller of the mass transfer channel was,more strengthening effect got.When the diameter of the channel was larger than 0.9mm,the effect of strengthening was less than the effect of stress concentration,which led to the decrease of the bending property.With the increase of the distance between the mass transfer channels,the effect of control on the mechanical properties decreased.When the longitudinal rows of 3,the bearing capacity and bending strength of the composite was the best.When the longitudinal rows of 5,the average stress level of the minimum cross section increased and caused to reach the level of failure damage.When the initial density of the preform was 1.5 g/cm~3,the effective permeability of SiC the deposition rate were higher in the unit time,thus increasing the effective bearing capacity.The mechanical properties of LA-CVI-C/SiC composites prepared by the optimized process are higher than that of CVI-C/SiC composites,and the bending strength,the compressive strength,the shear strength were increased by 16.17%,13.8%,respectively.The tensile strength was basically unchanged.(4)The process and properties of LA-CVI-C/SiC composites with SiCw/CNTs sealed mass transfer channel were studied.Results showed that,because SiCw introduced as the second phase,which owned the high strength and high thermal conductivity.The LA-CVI-C/SiC+SiCw+SiC composites,which prepared by the vacuum impregnation method combined with the CVI method,the flexural strength of the composite material was improved by 13.6%before unsealed.The thermal conductivity of the composite was controlled,which was 4 times higher than that of the CVI-C/SiC composite.As the CNTs which owned high strength and high conductive scattering between the reinforced fiber and the matrix.The LA-CVI-C/SiC+CNTs+SiC composites,which prepared by vacuum impregnation combined with CVI method,the flexural strength of the composite material was improved by 13.2%before unsealed.The electrical conductivity was up to 260 times higher than that of the CVI-C/SiC composite.The overall effective thermal conductivity follows the composite principle,and the thermal conductivity is 25 times higher than that of the CVI-C/SiC composite.