Study Of Joining And Properties Improvement Of Silicon Carbide Ceramics Under Low Temperature And Low Pressure

SiC ceramic materials have excellent mechanical properties at room temperature and high temperature,corrosion resistance,neutron radiation resistance,and low neutron absorption cross-section.However,the brittleness and low toughness of SiC ceramics result in the poor machinability.By means of the introduction of joining technology,the fabrication of various complex shapes of SiC ceramics has been realized in the previous works,therefore,it is widely applied in aerospace,marine engineering,nuclear energy,and other fields.However,the joining of SiC ceramics still requires harsh joining condition(high temperature and high pressure)and joint properties cannot meet the extreme environment,especially in the field of nuclear energy.In order to solve above-mentioned problems,the low temperature/pressure joining,and properties of SiC joints was carried out by using nano-infiltration and transient eutectic-phase(NITE),Ti metal,and Al-based brazing filler as the interlayer in this thesis.The detailed findings were described as follows:(1)The SiC ceramic joined by NITE process has excellent joining properties,but it requires high temperature and high pressure during the NITE joining process of SiC ceramics.The low temperature NITE process joining of SiC ceramics was explored by introducing the novel sintering aids and adjusting the thickness of the joining layer.Firstly,the effects of the joining process on the microstructure and joining strength of SiC joints were discussed by using SiC-Al2O3-Ho2O3 as the interlayer.It showed that the joining strength of 157.8 MPa was obtained when joining at 1700?,while the shear strength was only 53.9 MPa when joining at 1500?,which showed a weak interface between the SiC matrix and interlayer.Based on the above research,the joining process of SiC ceramics were optimized by using SiC-Al2O3-CeO2 as an interlayer and adjusting the thickness of the interlayer.It showed that the joining strength was recorded to be 163.9 MPa when joining at 1700?,while the joining strength decreased to 57.4 MPa and the binding force between SiC matrix and interlayer decreased as decreasing joining temperature to 1500?.However,the shear strength of the SiC joint,which had a strong binding force in the interface,was increased to 113.2 MPa by reducing the thickness of the interlayer by half.(2)Through the above research,although the SiC ceramic joining temperature was reduced from 1700? to 1500?,the joining temperature was still very high and a relatively high joining pressure was required.In order to reduce the joining temperature and pressure,Ti metal was introduced as the interlayer,and the low-temperature and low-pressure joining of SiC ceramics was studied.Firstly,the effects of the joining pressure and joining atmosphere on the phase composition,microstructure,and joining performance of SiC joints were investigated,and the residual stress was analyzed by finite element simulation.It showed that Ar atmosphere was conducive than the vacuum environment to join SiC ceramics.The shear strength was measured to be 109.3±4.5 MPa with the joining pressure of 30 MPa.At the sam e time,Ar joining atmosphere promoted the formation of the YAG phase and suppressed the generation of brittle phase(Ti5Si3),and the element distribution in the joined SiC joints was homogeneous,which was beneficial for the reduction of the residual stress of SiC joints.Accordingly,the solid-phase diffusion joining by using Ti powder as the interlayer was studied under low temperature(1200-1300?)without pressure.The effect of the joining process on phase composition,microstructure,and joining strength of SiC joints were discussed,simultaneously,the residual stress was analyzed.Owing to the high reactivity and porous structure of Ti powder,the diffusion-reaction was enhanced between the SiC matrix and interlayer,therefore,the pressureless joining of SiC ceramics at low temperature was realized.At the same time,compared with the increase in joining temperature,appropriately extending the holding time was conducive to the control of the interface reaction and thus improve the interfacial shear strength.The SiC joints with a shear strength of 41±4 MPa andresidual stress of 200.0 MPa could be obtained when the joining condition was performed at 1200? for 30 min.The intermediate products consisted of Ti5Si3 and TiC phases in the SiC joints.(3)Based on the above Ti metal joining research,SiC ceramics was joined at 1200?without pressure,but the joining strength of the joints was a little weak.And the joining temperature was still a little high for the nuclear SiC ceramics.In order to improve the joining strength and further reduce the joining temperature,a tough SiC joint was joined by using Al-based brazing filler as the interlayer at low temperature without pressure,and the hydrothermal corrosion resistance as well as residual stress of the joint were analyzed.Firstly,the Al-Si brazing joining was carried out using a high vacuum brazing furnace.The effect of Si content on the phase composition,microstructure,and properties was studied.It showed that the structure and joining properties of the SiC joint could be optimized by adjusting the content of Si powder within the interlayer at 1200 oC without pressure.The joining properties of SiC joints were improved when the Si powder content was 30 wt%,where the shear strengths of SiC joints at room temperature and 1200? were 105.2±21.0 MPa and 33.8±1.7 MPa,respectively.At the same time,the residual stress of the SiC joint was 369.4 MPa.The weight gain of SiC joints was found after hydrothermal corrosion for 720 h at 360?.In addition,cracks were observed in the SiC joint filled with fibrous SiO2 and abnormally grown Al2O3 grains,which were derived from the corrosion products of the interlayer.On this basis,the Al air brazing joining was carried out by using Al metal as the interlayer.The thickness of the interlayer,joining process,and heat treatment process were adjusted to achieve a tough SiC joint under a low joining temperature of 900? and a short time of heat treatment(10 h).At the same time,the residual stress of the SiC joint was studied.It showed that the extension of the dwelling time would promote the transformation of the composition of the interlayer from Al to Al2O3 by using two-layers Al foils as the interlayer.However,the formation of the brittle Al4C3 phase was observed after heating at 1000? for 100 h.Therefore,both thicknesses of the interlayer and joining temperature were adjusted to reduce the heat treatment time.Consequently,the conversion of the interlayer from Al to Al2O3 was nearly completed.On the other hand,the shear strength of SiC joints at room temperature and 1200? were 117.0±18.7 MPa and 13.7±1.6 MPa,respectively.The residual stress of the SiC joint was 244.5 MPa.However,the SiC joint cracked after the hydrothermal corrosion resistance at 360? for 720 h.