Strengthening,toughening And High-temperature Stability Of C/C-LAS Joints

Carbon/carbon(C/C)composites have been considered as the most promising strategic materials in aerospace vehicles and their power systems due to their excellent performance at elevated temperatures.Lithium aluminosilicate(LAS)glass ceramics possess unique dielectric properties in the high-temperature environment.But the inherent brittleness and low strength significantly limit their applications for the difficult processing and molding of large size and complex components.Reliable joining of C/C composites and LAS glass ceramics would be benefit from the excellent mechanical properties at high temperature provided by C/C composites as well as the functional features(e.g.microwave absorption property)provided by LAS glass ceramics,realizing the integration of structural and functional performances.In this dissertation,Si C-based ceramics modified C/C composites were joined to LAS glass ceramics using glassy brazing.Nano composite interfacial layer and wavelike Si C/MAS interface were designed and their effects on the joining performance were investigated at room temperature,300oC and 600oC.Thermal cycling strengthening mechanism of the C/C-LAS joints was also discussed.One-dimensional nano reinforcements CNTs array and Si C nanowires(Si Cnw)were synthesized on Si C-based transition layer by injection chemical vapor deposition(ICVD)and precursor impregnation and pyrolysis(PIP),respectively.Combined with hot-pressing sintering,C/C-LAS joints with nano composite interfacial layer were achieved.Shear strength of these joints followed a 2-parameter Weibull distribution.For the joints with MAS interlayer before and after reinforced by CNTs array,Weibull modulus m and Weibull strength(also called scale parameter)?were 8.81(5.96)and 22.36(31.94)MPa.For the joints without and with PIP Si Cnw,Weibull modulus m and Weibull strength?were 6.90(7.02)and 25.86(35.80)MPa,respectively.Improvements of 43%and 39%in Weibull strength were achieved by constructing the nanocomposite interlayer.Moreover,toughness of these joints was significantly enhanced.Fracture work of the joints before and after reinforced by ICVD CNTs were 5.42±1.51 k J/m~2and 8.44±2.25 k J/m~2,improved by 56%.After the joints were reinforced by Si C-Si transition layer,fracture work was increased from 5.79±1.09 k J/m~2 to 8.16±1.04 k J/m~2,improved by 41%.For the neat joints,fractured failure always occurred around the Si C/MAS interface.But for the reinforced joints,failure path ran through Si C transition layer,MAS interlayer,and LAS substrate because of the comprehensive effects of joggle,debonding,pulling-out,bridging and crack deflection of ICVD CNTs array and PIP Si Cnw.Distributions of residual thermal stress around the Si C-Si/MAS interface with different geometry and size were calculated by COMSOL Multiphysics simulation software.The effects of the wavelike Si C-Si/MAS interface on the joint properties were subsequently investigated by experimental study,and the mechanisms of the heterogeneous interfacial structure on the joint performance were also analyzed.In this work,oxidative foaming phenomenon of Si C-based ceramics at elevated temperature was utilized to fabricate porous structure in the surface of the Si C-Si transition layer.Wavelike Si C-Si/MAS interface was achieved by filling glassy interlayer into these pores through hot-pressing sintering of C/C-LAS joint.Experimental results showed that the wavelike interface could facilitate to form a mechanical snap-in structure in the joint,enlarge the effective joining area and relieve residual thermal stress.As a result,shear strength of the joints with wavelike Si C-Si/MAS interface achieved an average value of 32.46±1.35 MPa,which was 45%higher than the ones with flat Si C/MAS interface.High-temperature stability and joining strength of the C/C-LAS joints with and without reinforcements were evaluated and the performance evolution and failure mechanisms were revealed.After suffering several hours of heat treatment at 1000oC,the joints with MAS interlayer exhibited improved shear strength as the result of positive annealing effect,while those with YAS interlayer kept relatively stable.When the joints were shear tested at 300oC and600oC,residual shear strength of reinforced MAS joints were 51%and 31%,while those of YAS joints were 73%and 65%.At high temperatures,the performance of YAS joints was superior to the MAS joints.The degeneration of MAS joints was mainly due to the poor structural stability of MAS interlayer at high temperatures,and the decreasing joining performance of the YAS joints was caused by the degradation of the glass ceramics themselves.Thermal cycling strengthening phenomena of C/C-LAS joints were summarized and their strengthening mechanism was discussed.Strengthening effect was realized as the synergetic results of heat treatment factor and high-low sudden change factor during the thermal cycling process.Herein,the high-temperature treatment could strengthen the joints,while sudden change of the temperature from high level to low level weakened the joints.As a result of the coupling effects,shear strength of the joints presented firstly increasing trend and then decreasing during the thermal shock cycles.The realization was not only closely related to the inherent properties of the materials themselves,but also affected by the number of cycles and the thermal shock temperatures.