Study On Preparation And Mechanical Properties Of C/C Composite Bolts

Carbon/Carbon(C/C)composites have a broad market prospect and have become a hot spot for research in various countries,and are known as the"black gold"of the 21st century.In practice,the manufacture of large and complex C/C composite components often requires the use of bolted connections for integrated assembly.In order to reduce costs and improve efficiency,there is an urgent need for high efficiency and low cost manufacturing processes for C/C composite bolts as well as strengthening and modification studies.In this paper,three-dimensional needle-punched carbon fibre preforms were selected and the effect of the precursor ratio on increasing the densification rate of C/C composites during the Chemical Vapor Infiltration(CVI)process was investigated using renewable energy ethanol and methane as precursors.The CVI combined with the Precursor Infiltration and Pyrolysis(PIP)process was used to prepare C/C composites with different densities to reveal the mechanical failure mechanism of C/C composites and the bolts,and the silicon infiltration process was used to strengthen the C/C bolts to reveal the toughening mechanism of the C/C bolts by gaseous silicon infiltration,thus providing important data and theoretical support for the preparation and mechanical research of C/C bolts and promoting the application of C/C composite material applications.The main research contents and results are as follows:(1)The effects of methane and ethanol precursors on the densification rate and structural properties of C/C composites were investigated,and the mechanism of mixed deposition of methane and ethanol was revealed.It was shown that the increase in the proportion of methane in the precursor components did not change the weave of pyrolytic carbon,both of which were high performance rough layer structures.The highest deposition efficiency of the specimens was achieved when the molar precursor ratio of ethanol and methane was 7:3,and the density of C/C composites could reach 1.37 g/cm~3after 20 h of mixed deposition.(2)The influence law of density change of C/C composites and bolts on their mechanical properties was studied,and the correlation between the mechanical properties of C/C composites themselves and bolts was established to reveal the failure mechanism of C/C bolts.The study shows that as the density of C/C composites increases,the pores inside the C/C composites decrease and the path of crack expansion is restricted,thus making the crack expansion consume more energy and the interface bond between the fibres and the matrix becomes tighter,thus increasing the strength of C/C composites.the threads of C/C composites can cause damage to the carbon fibres after mechanical processing,and the defects formed will reduce the load-bearing capacity of the threads.When the C/C composite density reaches 1.5g/cm~3,the C/C bolt thread profile is more complete and the strength is higher.tensile failure of the C/C bolt is thread pull-out,with the highest C/C bolt at 1.5g/cm~3being a tensile strength of 43.49MPa.in the double-sided shear test,the inflection point load occurs,with the maximum inflection point load of 4630.04N for the 1.7g/cm~3 bolt.double shear Failure was thread crush and stud shear,with a maximum double-sided shear strength of50.96 MPa for 1.8 g/cm~3 C/C bolts.(3)The effect of the gaseous silicon infiltration process on the strengthening of C/C bolts was investigated,and the effect of the silicon infiltration process on the microstructure and mechanical properties of C/C bolts was discussed,revealing the mechanism of the toughening of C/C bolts by gaseous silicon infiltration.The study shows that the gaseous silicon infiltration process produced a mixed layer of Si C and Si on the surface of C/C bolts,and this mixed layer increased the tensile strength and double-sided shear strength of the bolts to 55.16 MPa and 54.73 MPa,respectively,which were 26.83%and 24.73%higher compared to C/C bolts.The failure mode of the modified C/C bolts changed from pull-off of the threads to fracture of the studs The failure mechanisms of the bolts included matrix cracking,fibre debonding and fibre bridging.