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The objective of this study is to investigate the effect of LSM coating on the creep properties of a SOFC joint between a glass-ceramic sealant and an interconnect steel with no and 1000-h thermal aging in air. The materials used are a GC-9 glass-ceramic sealant developed at the Institute of Nuclear Energy Research and a commercial Crofer 22 H ferritic steel. The creep test is conducted by applying a constant load (shear or tensile mode) on the joint at 800 °C. Comparison of the joint strength and creep properties between LSM-coated and non-coated specimens is also made for the non-aged condition.
With the LSM coating, the shear strength of the non-aged joint specimen is reduced by 78% and 92% at RT and 800 °C, respectively. On the other hand, the tensile strength of the joint is reduced by 59% and 72% at RT and 800 °C, respectively. During the joining process, La diffuses out from LSM film and reacts with GC-9 to form a rare-earth oxyapatite ceramic. Volume shrinkage during crystallization of oxyapatite and thermal mismatch at high operation temperature lead to the formation of microcracks and the fracture is mainly related to this oxyapatite phase.
The creep rupture time of LSM-coated joint is increased with a decrease in the applied constant shear and tensile loading at 800 °C regardless of thermal aging condition. The shear creep strength of non-aged joint at 1000 h in air is about 42% of the average shear strength, while the tensile creep strength at 1000 h is only about 3% of the average tensile strength. Compared to the non-aged, non-coated specimens, the creep strength at 1000 h for the LSM-coated shear and tensile specimens is significantly reduced by 85% and 89% at 800 °C, respectively. For both non-aged shear and tensile specimens with a short creep rupture time less than 100 h, fracture mainly takes place in the oxyapatite layer. For a medium-term creep rupture time (100 h < tr < 1000 h), fracture site changes from the oxyapatite interlayer to the mixed oxyapatite/BaCrO4 layer with an increase of creep rupture time to exceed 100 h.
A thermal aging treatment at 800 °C for 1000 h significantly enhances the joint strength of shear specimen at RT and 800 °C. However, a similar thermal aging treatment enhances the joint strength of tensile specimen at RT but degrades it at 800 °C. For tensile loading mode, fracture site changes from the interface between GC-9/oxyapatite and BaCrO4 to the interface between GC-9/oxyapatite and (Cr,Mn)3O4 when the testing temperature increases from RT to 800 °C. The interface between GC-9/oxyapatite and (Cr,Mn)3O4 becomes the weakest path when subjected to tensile loading at high temperature.
After 1000-h thermal aging, the shear and tensile creep strength at 1000 h of the thermally aged joint is enhanced by 13% and 216%, respectively, compared to the non-aged counterparts. Oxyapatite and BaCrO4 dominate the creep failure mechanism for 1000 h-aged shear specimens, while the (Cr,Mn)3O4 spinel layer becomes thicker after thermal aging and might also play a role in the creep failure of 1000-h aged tensile specimens in addition to the oxyapatite and BaCrO4 phases. | en_US |