Evolution behavior of Al2O3 nanoparticles reinforcements during reactive air brazing and its role in improving the joint strength

Publication date: 15 October 2017
Source:Materials & Design, Volume 132
Author(s): Xiaoqing Si, Jian Cao, Xiaoguo Song, Zhijie Wang, Zhiquan Wang, Jicai Feng
Nanoparticles have not been employed as reinforcements of composite filler for reactive air brazing (RAB) until now. It is crucial to investigate the evolution behavior and the strengthening mechanism of nanoscale reinforcements during RAB. In this study, the Al2O3 nanoparticle reinforced Ag-CuO-Al2O3 composite filler developed before was used to reactive air braze YSZ ceramic. Al2O3 nanoparticles were sintered together during RAB, forming microscale particles. TEM observations discovered that some Al2O3 reinforcements still remained at nanoscale. Fine crack bifurcations were readily formed between nanoscale and microscale Al2O3 particles during crack propagation. This process absorbed more fracture energy than the case of only microscale Al2O3. Higher brazing temperature reduced the viscosity of braze melt, which facilitated the mobility of Al2O3 particles. Therefore, the aggregation of Al2O3 nanoparticles was enhanced. So that fewer nanoscale Al2O3 reinforcements remained in the joint. Consequently, the joint properties would be weakened. The maximum shear strength (62MPa) was obtained when the joints were brazed at 1050°C for 30min. High-temperature oxidation test (800°C for 100h and 150h) showed that the morphology and distribution of Al2O3 reinforcements underwent few changes. These joints still maintained the high-temperature (800°C) shear strength above 38MPa.

Graphical abstract

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