基于界面结构梯度化设计的SiC陶瓷接头高温力学行为研究

In order to broaden the application of ceramic materials, it is significant to obtain the reliable and stable ceramic joint for high temperature use. Based on the facts, the following program was designed to fabricate the SiC ceramic joint. The bonding surface of SiC parent ceramic was prefabricated with porous SiC coating, and then Ni-Cr alloy was used to join the pretreated SiC ceramic through brazing. A gradient transition of coefficient of thermal expansion in the joint was formed from SiC parent ceramic to brazing alloy due to the buffering of SiC coating, lowering the overall residual stress level in the joint and subsequently improving the bonding strength and stability of the joint at high temperature. In addition, the research focused on the following issues who influence the mechanical behavior of ceramic joint at high temperature from the perspectives of stress accumulation and variation of joining materials properties: effect of the structure of bonding area on the magnitude and distribution of residual stresses in the ceramic joint; internal stress and strain cumulative effect in the joint under cyclic temperature during service; effect of microstructure evolution in the bonding area after thermal cycling on the mechanical properties of the joint. The key factor affecting the reliability of high-temperature ceramic joints was ultimately revealed based on the researches results of mechanical behavior of ceramic joint at high temperature. The research results showed a great significance in designing and selecting the appropriate joining structure and materials for high temperature use.

制备稳定可靠的耐高温陶瓷接头对扩大陶瓷材料的应用具有重要的意义。基于此，本研究设计了如下的方案制备耐高温SiC陶瓷接头：首先在SiC母材被连接表面预制多孔SiC涂层，采用Ni-Cr合金以钎焊方式连接预处理SiC陶瓷。通过SiC涂层的缓冲作用，接头内热膨胀系数形成从SiC母材→SiC涂层→钎料合金的梯度过渡，降低了接头内整体的应力水平，提高接头在高温下的连接强度和稳定性。此外，本研究从应力累积和连接材料性能变化角度对影响接头高温力学行为的如下关键问题开展深入的研究：陶瓷接头内连接区域组成影响接头内应力大小和分布的关系；接头服役时在温度循环下其内应力和应变的累积损伤效应；连接区材料经过温度循环后显微组织演化与接头力学性能之间的对应关系。通过对接头高温力学行为的研究将最终揭示出影响陶瓷接头高温可靠性的关键因素。本研究结果对设计和选择合适的高温连接结构和材料具有重要意义。

制备稳定可靠的耐高温SiC陶瓷/Hastelloy N合金接头对于实现熔盐堆中燃料包壳和结构支撑件的连接具有重要意义。本研究基于界面结构梯度化设计理念，利用激光表面处理方法制备了高品质的SiC陶瓷/Hastelloy N合金钎焊接头。基于此，本研究首先探究了Hastelloy N合金组织和力学性能的高温稳定性。采用热模拟技术获得了焊接热影响区组织，并研究热模拟峰值温度对合金室温和高温力学性能的影响。研究发现：当热循环峰值温度大于1300℃时合金内析出层片状组织，使得合金高温塑性下降，而对强度无明显影响。随后探讨了热处理工艺（保温温度700-1100℃，时间120-360h）对热模拟前后的Hastelloy N合金显微组织演化和高温力学性能的影响。结果表明：原始合金随热处理温度增加，晶粒长大和MoC颗粒大量析出，延伸率和抗拉强度略微下降；随保温时间延长，MoC析出相数量增加，延伸率先降低后增加。热模拟合金随热处理温度增加，层片状组织粗化，细小MoC和碳化铬析出。随保温时间增加，延伸率先增加后减降低；抗拉强度和延伸率略微下降。热模拟后的合金与原始合金在在相同热处理条件下性能接近，表现出较好的稳定性。为实现界面梯度化，本研究采用激光表面处理技术在Hastelloy N合金表面分别制备了含SiC和Si涂层。采用纳米压痕确定了涂层内的增强相的杨氏模量和硬度值，研究了涂层高温下的稳定性。热处理后，含SiC涂层内Mo6Ni6C枝晶明显粗化，析出大量的Cr3C2和纳米级Mo6Ni6C，增强相总体积分数达到55%；含Si涂层内Ni-Si化合物逐渐转变为Mo6Ni6C，Mo6Ni6C枝晶发生分裂和球化，增强相总的体积分数达到85%。最后，本研究采用AgCuTi钎料对SiC陶瓷和预处理的Hastelloy N合金进行钎焊，获得了残余应力低的高品质SiC/Hastelloy N合金接接头。本研究提出的界面结构梯度化设计SiC陶瓷/Hastelloy N合金钎焊接头可为熔盐堆的发展提供了技术支持和理论依据。