中文摘要：

氮化铝陶瓷在低温工程中有着良好的应用前景,但这方面的研究报道甚少.本文对氮化铝陶瓷的低温断裂行为进行了系统研究,以液氮和干冰作为制冷剂,测试低温环境下材料的抗弯强度和断裂韧性.当测试温度从293 K降低到77 K时,氮化铝陶瓷抗弯强度从364.6±29.2MPa增加至415.3±21.7 MPa,断裂韧性从3.98±0.19 MPa m1/2增加到4.59±0.28 MPa m1/2,同时发现穿晶断裂比例从7.3%增加至14.5%.基于实验结果,分析了陶瓷中第二相引起的残余应力对三叉晶界处氮化铝晶粒以及裂纹扩展方式的影响,结果表明,低温下第二相导致氮化铝陶瓷断裂模式发生改变;通过调控第二相在基体中的分布状况可以改善氮化铝陶瓷性能,对于氮化铝陶瓷的低温工程应用具有参考价值.

英文摘要：

Aluminum nitride （A1N） ceramics has received increasing interest for their superior properties, high thermal conductivity, including low dielectric constant, wide-band gap and low thermal expansion coefficient. A1N ceramics have been evaluated as a promising candidate for cryogenic engineering, eg. conduction-cooled high temperature superconductor （HTS）, but related research is limited. In this paper, the fracture behavior of A1N ceramics at cryogenic temperatures was studied. A1N ceramics with 5 wt% Y2O3 were fabricated by pressureless sintering at 1850℃. The bending strength of A1N ceramics was measured by three-point bending on 3 mmx4 mm×36 mm test bars. The fracture toughness of the resultant materials was measured by SENB technique on 4 mrn×6 mm×36 mm test bars. With liquid nitrogen and dry ice as refrigerants, bending strength and fracture toughness were measured at different temperatures. When test temperature was reduced from 293 to 77 K, the bending strength increased from 364.6 ±29.2 to 415.3 ± 21.7 MPa and the fracture toughness increased from 3.98 ± 0.19 to 4.59 ±0.28 MPa m1/2. Scanning electron microscope （SEM） images of cross-sections formed at different temperatures were investigated to explain the phenomenon. Intergranular fracture shows sharp grain shapes and transgranular fracture shows cleavage planes. The results shown the proportion of transgranular fracture increased from 7.3% to 14.5%. For A1N ceramics, neither transformation nor elongated grains exist and the toughening mechanism at cryogenic temperatures, so the mechanical properties might be influenced by residual stress. The effects of the residual stresses on A1N grains were thus investigated. Surprisingly and interestingly, based on backscattered electron images, it was found that transgranular fractures appear principally near the secondary phase. The thermal expansion coefficient （TEC） of secondary phase （YAG, 8.0×104 K-1） is higher than that of A1N grains （3.5×10-6 K-1）. Thus, a larger shrinkage w