3D-printed micro- and nano-architected ceramic metamaterials are susceptible to reduced mechanical performance in high-humidity environments. Prior studies have evidenced a decrement of 20% in fracture toughness at RH > 60% compared to baseline low-humidity conditions.
To address this challenge, the current study explores the efficacy of surface modification to enhance these materials' environmental durability. Utilizing two-photon polymerization–direct laser writing (TPP-DLW) technology, pyrolytic glassy carbon micro-pillars and uniform 2.5D structures of 5.8 μm diameter were fabricated and subsequently coated with Al₂O₃ thin films through Atomic Layer Deposition (ALD). The thickness of the ALD films was consistently maintained at 50 nm. These ALD films' impact on the micro-pillars' fracture toughness was rigorously evaluated under extreme relative humidity conditions (below 5% and above 60%). This was coupled with the measurement of localized residual stresses. The ALD-coated pillars exhibited an enhanced fracture toughness (2.38 \pm 0.2 \, \text{MPa}\sqrt{\text{m}} at RH <5% and 2.35 \pm 0.2 \, \text{MPa}\sqrt{\text{m}} at RH >60%), independent of the humidity conditions, compared to the uncoated pillars tested under high humidity (1.86 \pm 0.3 \, \text{MPa}\sqrt{\text{m}}). Notably, tensile residual stresses within the coatings could interact with crack propagation, acting as an additional energy reservoir. This may explain the incomplete recovery in fracture toughness in non-coated samples at low RH.
A comparative pillar-splitting analysis via cube-corner was conducted using silica-based micro-pillars to substantiate these findings further and eliminate confounding factors such as the superelastic behavior of glassy carbon during indentation and cracking. These pillars, identical in diameter to the TPP-DLW glassy carbon structures, were subjected to similar ALD coatings and fabricated via deep reactive ion etching (DRIE).
Complementing the experimental approach, simulations incorporating cohesive-zone elements were conducted. These provided a comprehensive parametric investigation into the effects of residual stress states in the ALD-deposited films, the elastoplastic energy during indentation, and the influence of film thickness.
This research not only underscores the significance of surface coatings in augmenting the environmental reliability of 3D-printed ceramic metamaterials but also illuminates the intricate dynamics of crack propagation and stress distribution within these materials.