【作者】 贺艳明；

【导师】 张杰；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2012， 博士

【摘要】 氮化硅陶瓷是一种常用的结构陶瓷，研究该类陶瓷的连接意义重大。当采用传统的合金钎料连接氮化硅时，由于母材与钎料合金之间较大的热膨胀系数差异，容易在连接后的接头内产生高的残余应力。本文在Ag-Cu-Ti合金钎料内添加一定体积分数的SiCp或Mo颗粒，以降低钎料热膨胀系数，缓解接头内的应力。在此基础上，研究了Si₃N₄陶瓷与钎料之间的界面反应过程和接头内反应相的形成机制；并采用数字图像相关法研究了接头在添加颗粒后性能得到提升的根本原因。研究发现，采用Ag-Cu-Ti+SiCp复合钎料连接Si₃N₄陶瓷时，当连接温度较低或保温时间较短时，界面反应不能充分进行。而当钎焊温度过高或保温时间过长时，钎料内SiCp反应完全。在本研究范围内，当钎料内含4wt.%Ti时，随着接头内SiCp含量的增加，接头弯曲强度先增加后降低；当钎料内含5vol.%SiCp时，随着接头内Ti含量的提升，接头弯曲强度也是先增加后降低。当钎料成分为(Ag₇₂Cu₂₈)₉₂Ti₈+5vol.%SiCp，在900℃保温10min下，接头最高的三点弯曲强度达到了506MPa。采用Ag-Cu-Ti+Mo复合钎料连接Si₃N₄陶瓷时，接头的力学性能随着钎焊温度的升高先增加后降低（840-950℃），而随着保温时间的延长逐渐降低（10-40min）；当复合钎料内Mo含量从0vol.%增加到20vol.%时（Ti含量为4wt.%），接头弯曲强度先增加后降低，高Mo含量接头内出现大量的Ti-Cu金属间化合物；当钎料合金中Ti含量从2wt.%增加到6wt.%时（Mo为5vol.%），接头弯曲强度也是先增加后降低。当钎料成分为Ag₇₂Cu₂₈)₉₂Ti₄+5vol.%Mo，在900℃保温10min下，接头的平均三点弯曲强度达到了429MPa。使用Ag-Cu-Ti+SiCp复合钎料连接Si₃N₄陶瓷的连接机理为：当钎料熔化后，活性元素Ti与Si₃N₄陶瓷发生反应，在陶瓷/钎料界面处生成了晶粒尺寸约30-50nm的TiN；反应释放的Si原子往液态钎料内扩散，在TiN/钎料界面处生成晶粒尺寸约200nm的Ti₅Si₃。HRTEM表明TiN沿着Si₃N₄陶瓷母材的某些晶面外延生成。钎缝内Ti与SiCp的反应逐步进行，首先生成Ti₃SiC₂和Ti₅Si₃；当SiCp完全分解后，Ti₃SiC₂继续与Ti反应生成TiC和Ti₅Si₃。Si从Ti₃SiC₂中的脱嵌引起了Ti₃SiC₂的分解。使用Ag-Cu-Ti+Mo复合钎料连接Si₃N₄陶瓷的连接机理为：母材/钎料界面层结构和形成机制与含SiCp的复合钎料一致；Ag基和Cu基固溶体组织构成钎缝的主体组织，在两类固溶体上弥散分布着Mo颗粒及Ti-Cu金属间化合物。钎料层内出现了多种类型的Ti-Cu化合物，Ti-Cu化合物的吉布斯生成自由能计算以及钎料层内的透射分析结果均证实了这一点。纳米压痕测试表明Ti-Cu化合物具有高于Ag或Cu的杨式模量和硬度。本文研究发现：当在接头内复合一定含量的添加相（SiCp或Mo）时（5vol.%），均可获得优质的钎焊接头。以SiCp为例，采用DIC揭示了该条件下的增强机制为：添加颗粒后引起接头内宏观残余应力水平的降低；钎料层内发生早期的塑性变形以及大的变形程度导致了接头内微观残余应力水平的下降；颗粒的增强效应。此外，研究发现当采用SiCp作为添加相时，在合适的工艺和钎料成分下，得到接头最高的弯曲强度（506MPa）高于含Mo复合钎料所获得的最高弯曲强度（429MPa）。分析认为含SiCp的复合钎料中可通过灵活调整接头内颗粒含量及Ti含量，使得在钎焊完成后的接头中得到一个缓解应力效应的最佳结构；而含Mo的复合钎料接头内生成的Ti-Cu脆性金属间化合物在性能上比TiC和Ti₅Si₃等陶瓷颗粒相差很远，不能起到复合材料中增强体的作用，成为钎缝中缺陷的来源。在前述分析的基础上，本研究采用了Ag-Cu-Ti+Mo复合钎料连接Si₃N₄陶瓷和42CrMo钢，发现当钎料内Mo含量为10vol.%，Ti含量为4wt.%时，得到的接头最高的连接强度达到了587MPa。更多还原

【Abstract】 Silicon nitride ceramics are very common structural ceramic materials. It iscritically important to investigate the Si₃N₄ceramic bonding. When the traditionalmetal brazing alloy was used to join Si₃N₄ceramic, the high residual stresses wereusually induced due to a large coefficient of thermal expansion （CTE） between theceramic substrates and brazing alloy. In this research, in order to decrease the CTEof the brazing alloy, Ag-Cu-Ti brazing alloy incorporated with SiCp or Mo particleswas introduced into joining the Si₃N₄ceramic. On that basis, the formationmechanism in the joint was analyzed. Furthermore, by incorporating a definiteamount of SiCp or Mo in the joint, a high joint strength could be obtained and thestrengthening mechanism was revealed by Digital Image Correlation technique.The Ag-Cu-Ti+SiCp composite filler was introduced into joining the Si₃N₄ceramic. When the brazing temperature was lower or holding time shorter, a lowerjoint bend strength was obtained due to insufficient reaction on the interface. Whilerising the brazing temperature or prolonging the holding time, the incorporatedSiCp in the brazing layer reacted with Ti completely, the microstructure obtainedshowed bad ability in decreasing CTE of the filler alloy, and thus resulting in alower joint strength. In this study, when the content of Ti was4wt.%in the joint, thejoint bend strength increased and then decreased with elevating the SiCp content inthe composite filler. In addition, when the content of SiCp was5vol.%in the joint,the joint bend strength also increased and then decreased with increasing the Ticontent in the composite filler. The highest joint strength （506MPa） was receivedwhile the Si₃N₄ceramic was brazed with (Ag₇₂Cu₂₈)₉₂Ti₈+5vol.%SiCp at900℃for10min.The Ag-Cu-Ti+Mo composite filler was introduced into joining the Si₃N₄ceramic. With elevating brazing temperature, the joint bend strength was increasedand then decreased. However, they decreased with prolonging holding time. Thejoint bend strength increased and then decreased with increasing the Mo particlescontent from0vol.%to20vol.%（Ti:4wt.%）. Lots of Ti-Cu intermetallics occurredin the joint while the higher content of Mo particles was incorporated, impairing theplasticity in the filler alloy and causing the lower joint bend strength. The joint bendstrength increased and then decreased while the content of Ti increased from2wt.% to5wt.%（SiCp:5vol.%）. The highest joint bend strength （429MPa） could beobtained while the Si₃N₄ceramic was brazed with （Ag72Cu28）96Ti4+5vol.%Mocomposite filler at900℃for10min.The formation mechanism in the Si₃N₄ceramic joint brazed withAg-Cu-Ti+SiCp composite filler was as follow: during heating, the brazing alloymelted and the active Ti element diffused towards Si₃N₄ceramic and react withthem. A TiN reaction layer with grain size of30-50nm was formed at the interface.The released Si atoms diffused towards the brazing alloy, a Ti₅Si₃reaction layerwith the grain size of200nm was produced at the TiN/brazing alloy interface.HRTEM analysis revealed that TiN reaction phases were formed along some crystalsurfaces of Si₃N₄ceramic. In the brazing layer, SiCp also reacted with Ti andformed Ti₃SiC₂and Ti₅Si₃. Ti₃SiC₂could be transformed into TiC and Ti₅Si₃owingto the reaction between Ti₃SiC₂and Ti, which was essentially caused by thede-intercalation of Si from Ti₃SiC₂due to the weak bonding between Ti and Si inthe Ti₃SiC₂.The formation mechanism in the Si₃N₄ceramic joint brazed with Ag-Cu-Ti+Mocomposite filler was as follow: the reaction products at the Si₃N₄/brazing alloyinterface were similar to the joint brazed with Ag-Cu-Ti+SiCp composite filler. Thecentral part of joint was mainly composed of Ag and Cu based solid solution, inwhich Mo particles and Ti-Cu intermetallics were dispersed. We observed that manykinds of Ti-Cu intermetallics precipitated in the joint, which was also validated bystandard Gibbs free energy of Ti-Cu intermetallics and TEM analysis in the brazinglayer. In addition, by nanoindentaion technique, the elastic modulus and hardnessfor Ti-Cu intermetallics are higher.In the research, a high quality joint was received while a definite amount ofSiCp or Mo was incorporated （5vol.%）. Here, SiCp were used as an example andstrengthening mechanism was revealed by DIC method. We believed that thefollowing three factors contribute the large joint strength: the lower macroscopicalresidual stresses level in the joint due to SiCp addition, the lower microscopicalresidual stresses due to an earlier and larger plastic deformation in the brazing layer,and the reinforcement effect of SiCp. In addition, the highest joint bend strengthcould be obtained in the joint brazed with Ag-Cu-Ti+SiCp composite filler at thesuitable brazing parameters and compositions, which were higher than the jointstrength brazed with the Ag-Cu-Ti+Mo composite filler. We believed that the optimum structure that relaxed the residual stresses to a large extent could beobtained while the Ag-Cu-Ti+SiCp were introduced by adjusting the content ofSiCp and Ti in the composite filler flexibly. However, the ability in adjusting theCTE mismatch between the joined materials by using Mo particles as theincorporation was limited. In addition, the in-situ formation of Ti-Cu intermetallicsdisplays the poorer mechanical properties than that of TiC and Ti₃SiC₂, so theycould not play the role of reinforcement in the joint and might become the source ofcracks during bending tests.In the research, the Ag-Cu-Ti+Mo composite filler was also used to join Si₃N₄ceramic and42CrMo steel. The miximum joint bend strength （587MPa） while thejoint was brazed with （Ag72Cu28）96Ti4+10vol.%Mo composite filler.更多还原