Fracture mechanism of metallic film with nano to sub-micron thickness on polycrystalline substrate

The coating-then-forming process offers a promising route to improve the manufacturing efficiency of metallic bipolar plates in proton exchange membrane fuel cells. However, fracture of ultrathin coatings during forming presents a formidable challenge to the corrosion resistance of coated metallic sheets. To provide theoretical guidance for addressing this issue, this work investigates the coating fracture behavior during the micro-channel forming process through experiments and numerical analysis. Coating cracks were revealed to be concentrated within the fillet and flat regions of the channel ridge, highly sensitive to local strain levels governed by both channel geometry and substrate microstructure. Therefore, a full-field simulation methodology was developed to predict the coating fracture during forming, balancing between computational efficiency with accuracy. Within this numerical framework, hybrid modeling of the metallic sheet substrate was employed, combining local implementation of crystal plasticity in the critical region with homogeneous materials elsewhere. For coating on the critical region, varying failure parameters correlated with substrate microstructure were determined by the microscopic fracture mechanism. Experimental validation confirms that the developed model can precisely predict both coating fractures and formed profiles, notably achieving an 80 % accuracy improvement in crack density compared to conventional approaches. Based on the simulation results, process windows were established to correlate coating fracture and channel geometry with forming parameters, thereby guiding optimization toward the minimization of crack density under geometrical constraints. Following parameter optimization, coating cracks were successfully eliminated at the flat region, demonstrating an effective strategy for maintaining the corrosion resistance after forming.

Nanocrystalline coatings are critical for extensive applications, yet their fracture on polycrystalline metallic substrates severely deteriorates the performance. Nevertheless, the underlying coating fracture mechanism correlated with inhomogeneous substrate plasticity remains ambiguous, and accurately predicting the crack formation is challenging. To address these issues, this study comprehensively characterized 100-nm niobium coating cracks on stainless-steel sheets and developed a multiscale model to predict coating fracture dominated by substrate plasticity. In particular, different coating cracks were identified and classified into three patterns based on their locations: on intragranular slip bands, grain boundaries, and twin boundaries of the substrate. Crystallographic calculations and statistical analyses demonstrated that the coating fractures were induced by grain and sub-grain scale strain localization of the substrate, which was incorporated within a multiscale modeling framework. For nanocrystalline coatings, molecular dynamics simulations were employed to derive the cohesive zone model in the extended finite element method. The coating fracture was subsequently simulated on a representative volume element of the substrate containing discrete slip bands, which was developed based on crystal plasticity and calibrated using slip steps. Microscopic substrate slips with Burgers vectors oriented at 30° to 50° relative to the surface were revealed to trigger coating cracks, which were generalized with a fracture parameter to be efficiently implemented in macroscopic simulations. Compared to traditional homogeneous models, the developed model enabled precise identification of all coating crack patterns in practical samples. This multiscale modeling procedure and these in-depth insights facilitate the prevention of failure in engineered components with nano-coatings.

Metallic bipolar plates (BPPs) manufactured by stamping have great commercialization prospects in proton exchange membrane fuel cells (PEMFCs) owing to their significant advantages of low density, small volume, high mechanical strength, and low cost. The development of coatings on metallic BPPs is essential for its application in PEMFCs. However, both the design and preparation of metallic BPP coatings face great challenges of increasing requirements, low efficiency, insufficient performance, and high cost. In present work, the design, manufacture and evaluation of coatings on metallic BPPs are extensively reviewed and explored. The changing requirements including corrosion resistance, conductivity, hydrophobicity, and mechanical properties for metallic BPP coatings and corresponding evaluation methods are comprehensively discussed. Especially, improved mechanical properties are critical for the emerging precoated metallic BBPs. Numerous coatings previously developed are quantitatively compared to provide guidance for the coating material selection. For highly efficient development of high-performance and low-cost coating, the material-structure-properties relationship of coatings is thoroughly summarized as the coating design principle. Moreover, different design methods including experimental, calculation, and machine learning are respectively in depth explored and compared, suggesting that machine learning combining decreased experiments and material calculation is a high-efficiency and effective method for coating design.