SiC陶瓷在钎焊过程中的6H到3C多型相变.pdf

第 52 卷 第 8 期2023 年 8 月人工晶体学报JOURNALOFSYNTHETICCRYSTALSVol.52 No.8August,20236H to 3C Polytypic Transformation in SiC CeramicsDuring Brazing ProcessSHI Haojiang1,ZHANG Ruiqian1,LI Ming1,YAN Jiazhen2,LIU Zihao2,BAI Dong2(1.State Key Laboratory of Reactor Fuel and Materials,Nuclear Power Institute of China,Chengdu 610213,China;2.School of Mechanical Engineering,Sichuan University,Chengdu 610065,China)Abstract:In this paper,pure Ni foil was used as an intermediate layer to achieve brazing connection of 6H-SiC at 1 100 1 245.The microstructure of the brazed joint and the interface between the brazed joint and the 6H-SiC substrate werestudied to investigate the effect of brazing process on the crystal structure of SiC ceramics and provide theoretical andexperimental data supports for brazing process design.The results show that a small amount of Ni atoms diffuse into the6H-SiC ceramics during the brazing process and exist in solid solution form,which reduces the dislocation energy of 6H-SiC.The residual stress at the 6H-SiC/brazed joint interface increases with the increase of brazing temperature,and when thebrazing temperature reaches 1 245,the(0001)plane of 6H-SiC at the interface slips along the 1/3 direction,and the 6H-SiC is sheared to form 3C-SiC.Therefore,SiC ceramics can undergo phase transformation under the influence ofstress and brazing material composition during the brazing process,and the effect of brazing process on their crystal structureand properties should be considered for SiC ceramics used in special environments.Key words:6H-SiC;3C-SiC;phase transformation;brazing;residual stressCLC number:O469Document code:AArticle ID:1000-985X(2023)08-1516-07SiC 陶瓷在钎焊过程中的6H 到3C 多型相变石浩江1,张瑞谦1,李 鸣1,颜家振2,刘自豪2,白 冬2(1.中国核动力研究设计院,反应堆燃料及材料国家重点实验室,成都 610213;2.四川大学机械工程学院,成都 610065)摘要:为探究钎焊过程对 SiC 陶瓷晶体结构的影响,为钎焊工艺设计提供理论及试验数据支撑,本研究采用纯 Ni 箔作为中间层在 1 100 1 245 下实现了 6H-SiC 的钎焊连接,并研究了焊缝以及 6H-SiC 基体与焊缝界面处的微观形貌。研究结果表明,少量 Ni 原子在钎焊过程中会扩散进入 6H-SiC 陶瓷,并以固溶形式存在,降低了 6H-SiC 层错能。随着钎焊温度升高,6H-SiC/焊缝界面处的焊后残余应力增大,当钎焊温度达到 1 245 时,界面处的 6H-SiC 的(0001)面沿1/3 方向产生滑移,6H-SiC 切变形成3C-SiC。因此,SiC 陶瓷在钎焊过程中受应力和钎料组成元素的作用发生相变,针对特殊环境使用的 SiC 陶瓷需要斟酌钎焊工艺对其晶体结构及性能的影响。关键词:6H-SiC;3C-SiC;相变;钎焊;残余应力 Received date:2023-01-03 Foundation item:Research on Accident Tderant Fuel Technology for Nuclear Power(Phase)Biography:SHI Haojiang(1994),male,from Sichuan province,doctor,assistant research fellow.E-mail: Corresponding author:YAN Jiazhen,doctor,associate professor.E-mail:yanjiazhen 0 IntroductionSiC has a variety of crystal structures,such as hexagonal structure,cubic structure,and rhomboidal structure.Among them,the cubic SiC has only one stacking sequence,that is ABCABC along the 0001 packing axis.SiCwith this structure is called 3C-SiC,which is also known as-SiC.All the other SiC crystals are collectively referredto as-SiC.The hexagonal SiC with a stacking sequence of ABCACB along the packing axis is called 6H-SiC,第 8 期SHI Haojiang et al:6H to 3C Polytypic Transformation in SiC Ceramics During Brazing Process1517and it is the most commonly seen SiC crystal in the-SiC.SiC polytypes can transform to each other under certainconditions.Vlaskina et al1-2found that 3C-SiC spontaneously transforms to 6H-SiC at 2 000.Parish et al3alsoobserved the transformation from 3C-SiC to 6H-SiC at 1 440 under 9 dpa neutron irradiation.Due to the irradiationinfluence,the transformation temperature from 3C-SiC to 6H-SiC can be significantly reduced to 1 440.Inconclusion,-SiC can transform to-SiC through simple heat treatment at elevated temperatures.However,the transformation from-SiC to-SiC cannot be achieved by merely heat treatment.Zhu et al4investigated the deformation behavior and phase transformation in 4H-SiC during nanoindentation process viamolecular dynamics simulation.The simulation results show that it takes 9 GPa of the shear stress for 4H-SiC totransform to 3C-SiC,where the atoms on(0001)plane of 4H-SiC slips in 1/3 direction,resulting in thephase transformation from 4H to 3C.Yang et al5conducted a microindentation at 1 170 with a load of 300 gand a dwell time of 30 s on the 6H-SiC,and the indented region shows that a 6H to 3C polytypic transformationoccurs.The result confirms that SiC phase transformation can be induced by an applied stress.Based on the aboveinvestigations,a conclusion can be drawn that though it is difficult,the transformation from-SiC to-SiC ispossible when enough stress is applied.Due to the significant differences in thermal conductivity,electrical conductivity,and radiation swellingresistance between-SiC and-SiC,it is necessary for SiC ceramics to maintain structural stability in certainspecial service environments,such as structural materials in reactors and conductive materials in semiconductorcomponents.Therefore studying the effect of the brazing process on the crystal structure of SiC ceramics is of greatsignificance when SiC ceramics applications are often companied with joining.Nickel element is a commoncomponent element of SiC ceramic brazing fillers,and the reaction mechanism between nickel and SiC ceramic by anickel foil as the brazing interlayer is reported in our previous work6.In this paper,the polytypic transformationfrom 6H-SiC to 3C-SiC during brazing process is reported.The formation mechanism of the 3C-SiC is investigatedvia HRTEM and SAED and well elaborated.1 Experimental sectionPressureless sintered 6H-SiC ceramic blocks with dimension of 15 mm 10 mm 5 mm were brazed by a0.1 mm-thick pure nickel foil.The schematic diagram of the assembled SiC joint is shown in Fig.1(a).Thedetailed description about the SiC ceramics and nickel foil can be found in our previous work6.The assembledspecimens were heated to 1 100,1 180,1 245,respectively,at a rate of 15 /min with initial pressure of3.5 10-3Pa in vacuum furnace,and held for 10 min.Then the brazed samples were cooled down to roomtemperature in the furnace.The brazing heating curve is displayed in Fig.1(b).Fig.1 Schematic diagram of the assembled SiC joint(a)and the brazing heating curve(b)The detailed morphologies and compositions of the brazed seams were examined by field emission scanningelectron microscope(FESEM,JSM-7500 F)equipped with an energy dispersive spectrometer(EDS,Ultim Max).The EDS analysis using XPP correction method was operated under an accelerating voltage of 15 kV,a set of1518研究论文人 工 晶 体 学 报 第 52 卷physical standards including SiO2,pure Ni and pure C are used for the analysis of element Si,Ni and C,respectively.Only the atomic fractions of Si and Ni are compared to characterize the reaction products in the joiningseam due to the fact that carbon has low solubility in Ni-Si phases and can hardly be quantitatively analyzed.Tofurther identify the phase structure of the joined seam,lift-out method is applied to the target location of interestusing focused ion beam(FIB)on FEI Helios Nanolab 600i.The thickness of the FIB sample decreases to 50 nmwith a voltage range from 2 kV to 30 kV,and then analyzed by FEI Talos F200X and FEI Tecnai G2 F20,operatedat 200 kV,with an extreme field emission gun(X-FEG)electron source.The target FIB area locates at theinterface between the transition zone and the 6H-SiC.2 Results and discussionFig.2 shows the microstructure of the SiC joints brazed at 1 100,1 180,and 1 245,respectively.Asshown in Fig.2(b)and(d),the brazed seam and the 6H-SiC ceramics are distinctly separated in the SiC jointsbrazed at 1 100 and 1 180.The interfaces are obvious and clear.When the brazing temperature increases to1 245,the morphology of the interface changes,a transition zone forms between the brazed seam and the6H-SiC.The transition zone consists of three phases,the island-like-Ni2Si,scattered graphite flakes,and the3C-SiC substrate6.The chemical composition of the 3C-SiC was analyzed under SEM mode,and the EDS resultsshow that the mole fractions of C,Si and Ni in the target area are 62.6%,35.7%and 1.7%,respectively.Thereaction process and mechanism between the nickel foil and 6H-SiC can be found in our previous work6.Fig.2 Microstructure of the SiC joints brazed at 1 100(a),(b),1 180(c),(d),and 1 245 (e),(f)held for 10 min 第 8 期SHI Haojiang et al:6H to 3C Polytypic Transformation in SiC Ceramics During Brazing Process1519Fig.3 shows the bright field images and corresponding HRTEM and SAED images of the FIB sample.3C-SiC,6H-SiC and-Ni2Si are identified.EDS results are displayed in Table 1.Notably,3C-SiC and 6H-SiC both contain0.1%(atomic fraction)Ni.Since no nickel silicides are found in the 3C-SiC and 6H-SiC,Ni atoms should be ina form of solid solution atoms.Based on the HRTEM and SAED images after Fourier transform,it can be seen that3C-SiC and 6H-SiC share an obvious orientation relationship,the(0001)plane of the 6H-SiC and the(111)plane of the 3C-SiC are parallel to each other.The interface shown in Fig.3(f)is not crystallographically perfect,dislocations and terraces on(0110)6Hplane can be found along the interface.The interface morphologies indicatethat compared to the possibility of 3C-SiC nucleating and growing on 6H-SiC,it is more likely that 3C-SiC is formedthrough the 6H-SiC(0001)plane slipping along a specific direction.Table 1 EDS analysis results of each area in Fig.3LocationComposition/%NiSiCPhaseP10.135.964.06H-SiCP20.165.634.33C-SiCP368.032.0-Ni2SiP465.035.0-Ni2SiTransformation from 6H-SiC to 3C-SiC requires a certain shear stress applied on the slipping plane.As can beseen in Fig.2,no transition layer forms at the brazing temperature of 1 100 and 1 180,while it forms at a highertemperature of 1 245.So it is highly possible that the shear stress comes from the residual stress that generatesinterface due to the CTE mismatch between the brazed seam and 6H-SiC.The residual stress at the interface isestimated as=ESERES+ER(R-S)T(1)where represents the residual stress,S stands for SiC,R stands for the reaction products near the interface,Erepresents the elastic modulus,represents the linear expansion coefficient,and T represents the differencebetween the brazing temperature and the room temperature.The products near the interface are mainly Ni2Si andgraphite,which are mixed up with each other and are uniformly distributed.So,the mixture of Ni2Si and graphitecan be considered as a composite phase.The elastic modulus E of this composite phase is calculated according tothe Voigt-Reuss formula7Ecomposite=38EUL+58ELL(2)EUL=VNEN+VGEG(3)ELL=EGENVNEG+VGEN(4)where N stands for Ni2Si,G stands for graphite,V and E stands for the volume modulus and elastic modulus,respectively.UL and LL represent the so-called upper and lower limit of the elastic modulus,respectively.Thelinear expansion coefficient of this composite phase is calculated using a more sophisticated formula,where theimpact of stress is considered.The formula is written ascomposite=NVNKN+GVGKGVNKN+VGKG(5)where V stands for volume fraction,K stands for the bulk modulus.Since graphite is anisotropic,therefore itsmorphology characteristics should be considered when selecting the physical property data of graphite.Thegraphites physical properties along C axis are used since the graphite flake is nearly vertical to the brazed seam.All the physical properties used in these calculations are displayed in Table 2.The results calculated according to1520研究论文人 工 晶 体 学 报 第 52 卷the above formula show that the residual stress generated at the interface is 1.76 GPa when the joint cools downfrom 1 245,and the residual stress is 1.55 GPa when the joint cools down from 1 100.The residual stressincreases with the increase of brazing temperature,providing a stronger force for-SiC to transform.Fig.3(a)TEM bright field image of the sample;(b),(c)HRTEM images of the 3C/6H-SiC interface;(d)SAED image of the3C/6H-SiC interface;(e),(f)higher magnification HRTEM images of the 3C/6H-SiC interfaceTable 2 Physical properties of-SiC,Ni2Si and graphite used in this paperPhaseElastic modulus,E/GPaShear modulus,K/GPaCoefficient of linear expansion,/(10-6K-1)Volume fraction/%-SiC401.38823483.58Ni2Si16891899131051.3Graphite133.11153.311311248.7The calculated residual stress is much lower than the shear stress calculated by Zhu et al4.However,there isno consensus on the critical value of stress that-SiC requires to transform to-SiC.Jepps et al13find thatimpurity element in SiC have a significant effect on the isomerism transition of SiC.When there are elements suchas B and Al in SiC,-SiC is more stable,and-SiC can hardly transform to-SiC even under large shear stress.However,when the SiC contains N or P element,the stability of-SiC is stronger,and only a small shear stress isrequired for-SiC to transform to-SiC.Whitney et al14conducts a same experiment with the same conditions asSokhor et al15reports,but fail to reproduce the-SiC to-SiC transformation under the ambient pressure of 3 7 GPaat 1 200 1 400.But after Whitney et al adds BN into-SiC,the transformation from-SiC to-SiC occurredunder the same circumstance.Therefore,impurity element has a significant effect on the(0001)6Hslip activationenergy of-SiC.As mentioned before,the presence of Ni element in SiC near the interface is confirmed,the existence of Nishould lower the magnitude of the residual stress that-SiC needs to transform to-SiC.Yuryeva et al16-17investigates the influence of Ni as an impurity element on the SiC bonding energy using first-principlescalculations.They find that regardless of whether Ni exists in the form of a substitutional atom,an interstitial atom,or a more complex defect in SiC,the Si and C atoms around Ni atom are slightly displaced,resulting in the obviousweakening of the binding energy of the SiC covalent bond.This means that the isomeric transformation of SiC 第 8 期SHI Haojiang et al:6H to 3C Polytypic Transformation in SiC Ceramics During Brazing Process1521becomes easier.As shown in Fig.4,twins are found in the transformed SiC near the interface.The formation oftwins in-SiC indicates low stacking fault energy,which provides a high possibility for-SiC to transform to-SiCthrough(0001)6Hplane sliding along the 1/3 direction during the cooling process,turning the stackingsequence of ABCACB into ABCABC,as shown in Fig.5.Fig.4(a)TEM bright field image of the sample;(b),(c)HRTEM images of the twin grain in the 3C-SiC;(d)corresponding FFT pattern of the twin grain in the 3C-SiCFig.5 HRTEM images of 6H-SiC(a)and 3C-SiC(b),and the schematic diagram of 6H-SiC transforming to 3C-SiC(c)1522研究论文人 工 晶 体 学 报 第 52 卷3 ConclusionThe change of the SiC crystal structure during brazing process was investigated,and a polytypic transformationof 6H-SiC to 3C-SiC is reported in this paper.When 6H-SiC ceramics was brazed using a 0.1 mm-thick pure nickelfoil as the interlayer at 1 245,the transformation of 6H-SiC to 3C-SiC is observed near the interface between thebrazed seam and ceramic substrate.The microstructure and chemical composition of transformed SiC was analyzedand the 6H-SiC to 3C-SiC transformation mechanism is summarized as follow:1)A slight amount of Ni element exsi