Advanced PVD protective coatings for SOFC interconnects

In an effort to enable inexpensive ferritic steels as interconnects (bi-polar plates) in planar solid oxide fuel cell (SOFC) stacks, advanced large area filtered arc physical vapor deposition (LAFAD) of MCrAlYO ceramic protective coatings was investigated (M=TiM=Ti, Co and/or Mn). This manuscript presents three unique LAFAD MCrAlYO coatings, which were applied to 430 ferritic steel specimens and subjected to various SOFC relevant exposures. The coatings’ microstructure, composition, area specific resistance (ASR) and Cr volatility were evaluated as a function of exposure to 800∘C air for up to 1000 h. Significant improvement in simulated SOFC interconnect performance was observed with coated vs. uncoated specimens (e.g., increased thermal stability and decreased ASR and Cr volatility). Outward Mn transport (from steel substrate) was observed (to varying extents) in all coatings. This appeared to promote surface crystallization and lower ASR values with extended exposure time. Coatings containing Mn and/or Co as-deposited exhibited the lowest and most stable ASR values, and more-uniform surface crystallization compared with coatings without. Coatings without Mn and/or Co as-deposited retained an amorphous surface structure, apart from local regions of high Mn outward transport, which evolved surface crystallites concentrated at topological features on the substrate steel surface (e.g., roll marks). ASR values for these coatings decreased significantly with exposure time. The engineering of LAFAD MCrAlYO coating processes to enhance SOFC interconnect performance of ferritic steels is presented and discussed.