Phase Transformation Toughening
Principle: The metastable tetragonal phase (t-ZrO₂) undergoes a diffusionless phase transformation under the stress field at the crack tip, transforming into the monoclinic phase (m-ZrO₂), accompanied by approximately 4%-5% volume expansion. This expansion generates compressive stress at the crack tip, offsetting some of the external stress and hindering crack propagation.
Characteristics: Significant toughening effect; this is the most important toughening mechanism for zirconia ceramics, but phase transformation conditions (such as temperature and stress field) must be controlled to avoid excessive phase transformation leading to material failure.
Particle Toughening
Principle: High-strength, high-elastic-modulus particles (such as Al₂O₃, SiC, etc.) are added as toughening agents to ZrO₂ ceramic powder. The particles toughen through the following mechanisms:
* Crack Deflection: Particles hinder the straight propagation of cracks, causing them to bypass the particles and form a longer path.
* Crack Bridging: Particles connect both sides of the crack, bearing part of the load and reducing stress concentration at the crack tip.
Features: Simple process, low cost, but the toughening effect is weaker than whiskers or fibers, requiring optimization of particle type, particle size (usually submicron level), and content (generally less than 20%).
Fiber Toughening
Principle: Fibers (such as carbon fibers, SiC fibers) toughen through the following mechanisms:
* Crack bridging: Fibers cross cracks, bearing tensile stress and preventing crack propagation.
* Pull-out effect: Fibers consume energy when pulled out of the matrix, increasing fracture work.
* Crack deflection: Fibers cause a change in crack propagation direction, increasing the path length.
Features: Toughening effect is better than particles, but fiber dispersion uniformity and interfacial bonding strength need strict control, resulting in higher cost.
Self-Toughening
Principle: By controlling the sintering process of zirconia ceramics, columnar crystals or long grains are formed inside the material. During fracture, the crack propagates along the grain boundaries, deflecting upon encountering columnar crystals, increasing the propagation path and consuming more energy.