【作者】 郭伟；

【导师】 何鹏；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2018， 博士

【摘要】 蓝宝石是一种单晶状态的Al₂O₃陶瓷,它具有强度高、硬度大、熔点高、绝缘性好、结构稳定性好、耐磨、耐蚀性能好以及透光性能优异等特点,被广泛用作导弹和战斗机等高速飞行器的窗口材料。现有的技术无法实现大尺寸蓝宝石的制备,要实现大尺寸蓝宝石窗口的制备往往需要连接技术将其装配成型,因此,实现蓝宝石的有效连接是扩展其应用、发挥其结构和功能材料作用的关键。目前,蓝宝石最主要的连接方法为金属活性钎焊法,但其连接过程需要真空和高温环境,此外还存在蓝宝石母材与金属钎料热膨胀系数不匹配而导致的接头应力问题。如果采用与蓝宝石热膨胀系数更为匹配的低熔点玻璃在大气环境下实现蓝宝石的低温连接,则可以大大降低成本,并且有利于缓解接头应力问题。本文制备并采用低熔点铋酸盐玻璃对蓝宝石进行了连接试验,通过工艺参数和玻璃成分的优化实现了蓝宝石的有效连接和对接头界面反应的有效控制,从而降低了连接温度,缓解了蓝宝石接头的残余应力。基于玻璃与蓝宝石母材的热膨胀系数具有良好匹配性和玻璃软化点低的设计思想,本文采用熔融-急冷法制备了成分分别为50Bi₂O₃-30B₂O₃-20Zn O、50Bi₂O₃-40B₂O₃-10Zn O、50Bi₂O₃-50B₂O₃和40Bi₂O₃-60B₂O₃的铋酸盐玻璃,随后对玻璃的热学性能和力学性能等进行了表征。试验发现,随着玻璃B₂O₃/Bi₂O₃比例的提高,玻璃的转变温度（T_g）、软化温度（T_f）、维氏硬度（H_v）和三点弯曲强度（σ）逐渐降低,玻璃的热膨胀系数（α）逐渐增加,玻璃的析晶倾向逐渐降低。在对玻璃性能研究基础上,制定了蓝宝石连接工艺,首先采用铋硼锌玻璃对蓝宝石进行了连接试验。研究发现,当采用50Bi₂O₃-40B₂O₃-10Zn O玻璃连接蓝宝石时,在600℃、625℃和635℃连接温度下,接头内部产物分别为块状Bi₄B₂O₉、长条状Bi₂₄B₂O₃₉以及颗粒状Zn Al₂O₄。当温度高于635℃时,随着连接温度提高和保温时间延长,Zn Al₂O₄颗粒长大,当温度为675℃时,颗粒开始团聚,随着温度提高,团聚现象加剧。随着连接温度的提高,接头剪切强度先升高后降低,接头强度最大值为85MPa。在650℃连接蓝宝石时,随着保温时间增加,接头剪切强度逐渐降低。高硬度、高模量的Zn Al₂O₄颗粒在接头中原位形成,实现了对接头的弥散强化,并通过诱导裂纹偏转和增加裂纹扩展路径,促使裂纹断裂能升高。当采用50Bi₂O₃-30B₂O₃-20Zn O玻璃连接蓝宝石时,接头内部生成的Zn Al₂O₄颗粒较大、团聚现象较为严重,接头的剪切强度较低。此外,采用50Bi₂O₃-40B₂O₃-10ZnO玻璃对纯度分别为100wt.%、99wt.%和95wt.%的多晶Al₂O₃陶瓷（分别记作100Al₂O₃、99Al₂O₃和95Al₂O₃）进行了连接试验。研究发现,存在玻璃向多晶Al₂O₃陶瓷的渗透现象,随着多晶Al₂O₃陶瓷母材纯度的降低,玻璃的渗透现象加剧,玻璃向母材的渗透破坏了玻璃/Al₂O₃界面,导致接头力学性能下降。通过对95Al₂O₃连接试验发现,随着温度的升高,接头强度先升高后降低,当连接温度为625℃时,接头强度达到最大值,为58MPa。通过对玻璃成分进行优化,随后采用了与蓝宝石热膨胀系数更为匹配的铋硼玻璃对蓝宝石进行了连接试验。研究发现,当采用50Bi₂O₃-50B₂O₃玻璃在600-750℃连接蓝宝石时,蓝宝石接头内部生成了Al₄B₂O₉晶须,随着温度升高,Al₄B₂O₉晶须长大,接头强度先升高后降低。随着保温时间增加,Al₄B₂O₉晶须逐渐长大,接头强度逐渐降低。接头中原位生成的Al₄B₂O₉晶须增加了接头焊缝区域与蓝宝石的热膨胀匹配性,并阻碍了剪切裂纹扩展,促使接头剪切强度最高可达95MPa。当采用40Bi₂O₃-60B₂O₃玻璃在700℃连接蓝宝石时,较采用50Bi₂O₃-50B₂O₃玻璃连接接头相比,Al₄B₂O₉晶须尺寸较大,由于50Bi₂O₃-50B₂O₃玻璃与蓝宝石热膨胀系数更为匹配,因此接头强度也较高。随后,采用了50Bi₂O₃-50B₂O₃玻璃对不同纯度的多晶Al₂O₃陶瓷进行了连接试验。研究发现,在多晶Al₂O₃接头内部均生成了Al₄B₂O₉晶须,随着多晶Al₂O₃陶瓷纯度的降低,玻璃向多晶Al₂O₃陶瓷的渗透加剧,接头强度逐渐降低。更多还原

【Abstract】 Sapphire,a single crystal form of pure alumina ceramics,has been widely used as large surveillance windows and high speed missile nose cones due to excellent mechanical and optical properties.However,such applications are often constrained by their physical size,because big size sapphire could not be obtained by avaliable manufacturing technology in the present.Thus,joining is a technology of choice used to mitigate the size limits,expanding its practical uses.In the present,the most commonly used method is active metal brazing.However,the joining process needs vacuum and high temperature,besid es,the CTE mismatch between metal braze and sapphire could lead to the innerstress in the joint.Realizing brazing sapphire at low temperature with a more compatible braze could save cost and release the interness in the joint.In our work,bismuth glass with low melting point was designed and used to join sapphire.Based on reaction characteristic of the bismuth glass/sapphire interfce,we aim to control the interfacial reaction through alting joining parameters and composition of the glass.Firstly,bismuth glasses with chemical composition of 50Bi₂O₃-30B₂O₃-20 Zn O,50Bi₂O₃-40B₂O₃-10 Zn O,50Bi₂O₃-50B₂O₃and40Bi₂O₃-60B₂O₃ were prepared by trational melting-quenching method.Thermal and mechanical properties of prepared bismuth glasses were characterized.The results showed that with ratio of B₂O₃/Bi₂O₃ increasing,the glass transition temperature（T_g）,softening temperature（T_f）,hardness（H_v）and three bending strength（σ）increased,the thermal expansion coefficient（α）and crystrallization trendency of the glass decreased.Based on the properties and wetting experiment analysis,sapphire joining process was determined,thus,the prepared glasses were employed to join sapphire.The results showed that when joining sapphire with 50Bi₂O₃-40B₂O₃-10 Zn O glass,the phases in the sapphire joints bonded at 600℃,625℃and 635℃were Bi₄B₂O₉,Bi₂₄B₂O₃₉and ZnAl₂O₄,respectively.With increasing of joining temperature,the size of Zn Al₂O₄ increased,and particle aggreration occurred when the temperature was≥675℃,and the aggreration was becoming more and more serious.With the joining temperature increasing,the shear strength of the joint first increased and then decreased,the maximum value of 85 MPa was achieved at 650℃.With holding time increasing at joining temperature of 650℃,the shear strength decreased.The in-stitu formation of dispersed Zn Al₂O₄ particles enhance the joint,could induced crack propagation and increase propagation path.When joining polycrystalline Al₂O₃ ceramics,the infiltration of glass into polycrystalline Al₂O₃ ceramics was observed.With the alumina purities decreasing,the infiltration was more and more serious,the joint shear strength decreased.The influence of joining temperature on microstructure and shear strength of the 95Al₂O₃ joint was then studied.The results showed that with the temperature increasing,the infiltration of the glass into95Al₂O₃ was becoming more and more serious,the joint strength first increased and then decreased,maximum value of 58 MPa was obtained when the joining temperature was 625℃.Through alting glass composition,bismuth borate glass,whose CTE is more compatible with sapphire was also used to as a joining media.In our work,we mainly studied joining characteristic of 50Bi₂O₃-50B₂O₃ glass/sapphire system.The results showed that when joining sapphire with 50Bi₂O₃-50B₂O₃ glass at tempreture of 600-750℃,Al₄B₂O₉ whiskers were formed in the joint due to a reaction of sapphire and the glass.With joining temperature and holding time increasing,the size of the Al₄B₂O₉ whisker increased,the joint strength first increased and then decreased.The in-stitu formation of Al₄B₂O₉ whiskers in the joint enhanced the CTE match between the seam and sapphire,and exhibited effective resistance to the cracks growth.The maximum shear strength of 95 MPa was obtained then.When bonding sapphire with 40Bi₂O₃-60B₂O₃ glass at 700℃for 20 min,compared with the sapphire joint bonded with 50Bi₂O₃-50B₂O₃ glass at the same temperature,the size of Al₄B₂O₉ whisker was larger,and the joint shear strength was higher.The influence of Al₂O₃ crystal form on microstructure and mechanical properties of the joint was studied.The infiltration of glass into polycrystalline Al₂O₃ could be observed,and with Al₂O₃ purities decreased,the infiltration was more and more serious and the joint shear strength decreased.更多还原