Innovations in engineering ceramic processing

This thesis integrates four peer-reviewed studies on engineering ceramics, unified by the principle that mechanical and functional performance are governed by microstructural control. The research addresses three persistent challenges in ceramic processing. First, alumina and related oxides require sintering above 1500–1600 °C to achieve ≥97 % relative density, resulting in high energy demand. Second, monolithic ceramics exhibit limited fracture toughness, typically 2–4 MPa·m½ , which is an order of magnitude lower than engineering metals. Third, additive manufacturing often produces heterogeneous microstructures with pore fractions of 30–40 % and pore sizes of 50–200 µm, compromising reliability.

The first study investigates co-doping of alumina with manganese oxide and titania. Relative densities of ~98 % were achieved at 1300 °C, compared with ~1500 °C for undoped alumina. Grain refinement from ~5 µm to ~2 µm was observed, with transient liquid-phase sintering suggested as a mechanism. Mechanical testing showed hardness of 14.8 ± 0.3 GPa and modulus of 370 ± 5 GPa, though formation of aluminium titanate (Al₂TiO₅) above 1350 °C introduced thermal instability.

The second study evaluates nano-reinforcement using multi-walled carbon nanotubes (MWCNTs) in alumina and yttria-stabilised zirconia. At 1 wt.% loading, yttria-stabilised zirconia composites showed the lowest penetration depths under 200 m s⁻¹ impact, linked to nanotube crack bridging and pull-out. In alumina, weaker bonding led to agglomeration and reduced benefit. Loadings of 3–5 wt.% decreased relative density from ~99 % to ~94 %, confirming that reinforcement is effective only at ≤1 wt.%.

The third study compares slip casting and fused deposition modelling of yttria-stabilised zirconia. Slip-cast samples achieved 99.4 % relative density, average grain size of ~65 nm, hardness of 15.26 ± 0.4 GPa, fracture toughness of 5.78 ± 0.5 MPa·m½ , and compressive strength of 510 ± 10 MPa. Fused deposition modelling samples reached 96.2 % relative density with finer grains (~20 nm), hardness of 13.79 ± 0.3 GPa, toughness of 5.02 ± 0.3 MPa·m½ , and compressive strength of 346 ± 12 MPa. While strength was reduced, interconnected porosity offers advantages for biomedical scaffolds.
The fourth study examines internal architecture in fused deposition modelling-printed alumina. Linear infill patterns delivered the highest hardness (23.5 ± 0.3 GPa), while hexagonal infill improved fracture toughness (4.5 ± 0.3 MPa·m½ ) through crack deflection and stress redistribution. These results show that fracture pathways can be deliberately manipulated through geometry without altering chemistry.

Taken together, the findings establish a multi-scale framework for microstructural engineering. At the atomic level, dopants lower sintering temperature. At the nanoscale, reinforcements modify crack–matrix interactions. At the microscale, processing routes control density and porosity. At the mesoscale, architecture governs fracture behaviour. The collective contribution demonstrates that deliberate microstructural control enables reduced processing energy, enhanced toughness, and application-specific responses in structural and biomedical ceramics.