Ceramic materials, characterized by high melting points, hardness, wear resistance, and oxidation stability, serve critical roles as structural, cutting, and mold materials. However, their inherent brittleness limits widespread application. To overcome this, polymer matrices are introduced to form ceramic-resin composites. Yet, increasing ceramic content typically reduces fracture toughness, compromising safety. Nature offers a solution through mollusk shell nacre—a structure combining high strength and exceptional toughness despite comprising 95 vol% brittle aragonite (CaCO3) platelets. This review explores freeze casting technology for fabricating nacre-mimetic composites, emphasizing process-structure-property relationships.
Structural Inspiration from Nacre
Nacre’s “brick-and-mortar” architecture comprises microscale aragonite platelets (5–8 μm diameter, 200–900 nm thickness) bonded by nanoscale organic layers (10–50 nm thickness). This multi-scale design achieves a fracture toughness (~10 MPa·m1/2) 40× higher than monolithic aragonite (0.25 MPa·m1/2). Key toughening mechanisms include:
- Crack deflection: Organic interfaces divert cracks, increasing propagation path length.
- Platelet pull-out: Energy dissipation via frictional sliding during platelet extraction.
- Mineral bridging: Aragonite nanofibrils bridge platelets, enhancing crack resistance.
- Nanoscale asperities: Surface roughness induces inelastic shear deformation.
These mechanisms synergistically absorb fracture energy, providing a blueprint for synthetic composites.
Freeze Casting Process Fundamentals
Freeze casting (ice templating) enables scalable fabrication of nacre-like lamellar structures. The casting process involves four stages:
- Slurry Preparation: Aqueous ceramic suspensions (e.g., Al2O3, SiC) with binders/dispersants.
- Directional Freezing: Controlled cooling (typically -1 to -30°C/min) induces vertical ice-crystal growth, expelling ceramic particles to interlamellar regions.
- Freeze Drying: Sublimation of ice crystals under vacuum, leaving porous lamellar scaffolds.
- Polymer Infiltration: Epoxy, PMMA, or other polymers impregnate pores, forming dense composites.
Key process parameters include:
| Parameter | Effect on Microstructure | Typical Range |
|---|---|---|
| Cooling Rate | ↓ Lamellar spacing with ↑ rate; dendritic structures at high rates | -1 to -30°C/min |
| Solid Loading | ↑ Lamellar thickness with ↑ concentration | 15–40 vol% |
| Additives (SCMC*) | ↑ Viscosity enables 3D interlocking bridges | 0.1–1.0 wt% |
Structure-Property Relationships
Lamellar Thickness and Spacing
Finer lamellae enhance crack deflection. For SiC/PMMA composites, reducing lamellar thickness from 35 μm to 7 μm via faster cooling (-10°C/min) increased flexural strength by 30%:
$$ \sigma_f \propto \frac{1}{\sqrt{d}} $$
where \(\sigma_f\) = flexural strength and \(d\) = lamellar thickness. Smaller spacings promote crack branching and energy dissipation.
| Lamellar Thickness (μm) | Flexural Strength (MPa) | Fracture Toughness (kJ/m2) |
|---|---|---|
| 35 | 110 ± 12 | 0.45 |
| 15 | 135 ± 15 | 0.67 |
| 7 | 148 ± 18 | 0.85 |
Ceramic Bridges and Surface Engineering
Introducing mineral bridges between lamellae mimics nacre’s “brick-bridge-mortar” structure. Adding nano-Al2O3 (100 nm) to suspensions creates bridges and surface asperities. Bridge fracture energy (\(G_b\)) follows:
$$ G_b = \frac{K_{IC}^2}{E} \cdot \phi_b $$
where \(K_{IC}\) = fracture toughness, \(E\) = Young’s modulus, and \(\phi_b\) = bridge volume fraction. 3D-interlocked Al2O3/cyanate ester composites achieve specific strengths of 162 MPa·cm3/g.
Polymer Matrix Selection
Matrix stiffness governs stress distribution. Hard polymers (e.g., PMMA) suppress stress concentrations at bridges:
| Matrix Polymer | Flexural Strength (MPa) | Fracture Toughness (MPa·m1/2) |
|---|---|---|
| PLMA† | 120 | 1.8 |
| PMMA | 182 | 2.4 |
| PUA-PHEMA‡ | 168 | 3.4 |
Interfacial Bonding
γ-MPS§ grafting onto Al2O3 scaffolds before PMMA infiltration enhances interfacial strength by 70%. The interfacial shear stress (\(\tau_i\)) is critical for platelet pull-out work (\(W_p\)):
$$ W_p = \frac{\tau_i \cdot A_p \cdot l_p}{2} $$
where \(A_p\) = platelet area and \(l_p\) = pull-out length.
Conclusions and Outlook
Freeze casting enables scalable fabrication of nacre-inspired composites with tunable lamellar architectures. Key advances include:
- Precise control of lamellar dimensions via casting process parameters.
- Integration of mineral bridges and nanoscale roughness for multi-scale toughening.
- Optimized polymer matrices and interfaces for stress redistribution.
Future directions involve multi-material architectures, in situ polymerization, and high-temperature matrices. This casting process holds promise for next-generation lightweight structural materials requiring exceptional damage tolerance.