Enhancing fracture resistance in advanced micro-ceramics via interface engineering
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High-precision 2.5D and 3D micro-/nano-scale fabrication techniques enable micro-ceramic-based metamaterials with unprecedented mechanical performance, unattainable through traditional methods, for advanced micro-devices (e.g., MEMS, micro-lenses, ultra-light, and stiff materials). However, their reliability remains largely unexplored, requiring a fundamental understanding of fracture mechanisms. Therefore, this study investigates the mechanical behavior of silica based micro-ceramics (crucial materials for many micro-devices) and explores the role of interface engineering in optimizing their fracture resistance. Building upon the innovative work by Bauer et al. [1], the mechanical properties of sinterless silica upon two-photon polymerization and pyrolysis were investigated [2] to understand the efficacy of the production route on fracture toughness. Results show the consistent elastic modulus, hardness, and fracture toughness (0.588 ± 0.064 MPa·m^1/2) of the sinterless silica with literature value for bulk silica. Increasing the processing temperature from 650 °C to 1000 °C enhanced toughness to 0.684 ± 0.052 MPa·m^1/2, confirming the feasibility of sinterless silica fabrication with preserved mechanical integrity. At the core of this study, the influence of interface engineering through atomic layer deposition (ALD) of ultra-thin films on silica and glassy carbon micro-pillars was explored to fine tune the fracture toughness. A 50 nm Al₂O₃ coating on silica at 200 °C increased fracture toughness by 134%, while a 100 nm coating at 300 °C further enhanced it by 165%. We used 3D printed glassy carbon as a comparative system. In this case, ALD Al₂O₃ at 200 °C reduced fracture toughness by 35%. These differences stem from crack morphology: tensile residual stress in film enhances toughness in median cracks by inducing compressive stress in the substrate, while in Palmquist-crack morphology—dominated by surface effects—it promotes embrittlement rather than toughening, as confirmed by finite element simulations. These findings demonstrate that reliability of micro-ceramics could be fine engineered trough the deposition of conformal thin coatings having precise values of residual stresses based on the interplay between the stress-state and the substrate properties.
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