The high strength and toughness of zirconia ceramics are essentially derived from their unique crystal structure and dynamic phase transformation ability, which breaks through the inherent perception of traditional ceramics that "high strength must be brittle". Compared with ceramics such as alumina and silicon nitride, the balance between the variability and stability of its crystal structure constitutes the core of the performance difference.
Zirconia ceramics have three reversible crystal forms: monoclinic phase (m-ZrO₂) at room temperature, tetragonal phase (t-ZrO₂) when heated to 1170℃, and cubic phase (c-ZrO₂) above 2370℃. This crystal form transformation is accompanied by volume change - when the tetragonal phase transforms to the monoclinic phase, the volume expands by about 3%-5%. This characteristic seems to cause the material to crack, but it becomes a key mechanism for toughening.
When zirconia ceramics are impacted by external forces or cracks extend, the stress at the crack tip induces the surrounding tetragonal phase grains to transform into the monoclinic phase. This "stress-induced phase change" process absorbs a lot of energy, and the volume expansion caused by the phase change will form a compressive stress on the crack, hindering the further extension of the crack. This "crack control by change" characteristic is the core reason why its toughness is much higher than other ceramics. The crystal structure of traditional ceramics (such as the corundum structure of alumina) is stable and has no such phase change. Once the crack is generated, it will expand rapidly.
In order to make the tetragonal phase stable at room temperature, stabilizers (such as yttrium oxide and cerium oxide) need to be introduced. The stabilizer will enter the zirconia lattice to form a solid solution, lower the transition temperature from the tetragonal phase to the monoclinic phase, and keep part of the tetragonal phase in a metastable state at room temperature. This "metastable structure" is like an "energy storage device" that triggers phase change only under stress, which not only ensures high strength at room temperature (derived from the stable lattice structure), but also retains the phase change toughening ability when impacted. This is the exquisiteness of its structural design.
The high strength of zirconia ceramics also comes from its fine and uniform grain structure. After optimizing the sintering process, its grain size can be controlled within 1-5 microns, and the grain boundaries are tightly bonded. The integrity of the lattice arrangement of the tetragonal phase grains enables it to withstand higher external stress (bending strength can reach more than 1000MPa). In contrast, although alumina ceramics can also improve strength by refining the grains, the layered structure of its corundum phase (α-Al₂O₃) is prone to fracture along the grain boundaries when subjected to stress. The upper limit of strength is usually around 600MPa, and there is no phase change buffer, and the toughness is far less than that of zirconia.
Compared with silicon nitride ceramics, although both are high-toughness ceramics, the toughening mechanism is completely different. The toughness of silicon nitride comes from the "bridging" and "pulling out" effects of long columnar grains - when the crack expands, the columnar grains will span the two ends of the crack, or pull out from the matrix to consume energy, and its crystal structure (hexagonal phase) itself is stable without phase change participation. The toughness of zirconia depends on dynamic phase change. This "active response" toughening is more efficient under instantaneous impact, especially in low-temperature environments, where the phase change speed is accelerated and the toughness advantage is more obvious.
In essence, the difference between zirconia ceramics and other ceramics lies in the "dynamic adaptability" of the crystal structure: the crystal structure of other ceramics is statically stable, and only the strength of the lattice itself resists damage when subjected to force. Once the limit is exceeded, it will fracture brittlely; while the crystal structure of zirconia can actively "dissipate" stress through phase change, converting external impact energy into phase change internal energy, and achieving a synergistic improvement in strength and toughness. This structural characteristic makes it far superior to traditional ceramic materials in scenarios that require both wear resistance and impact resistance (such as medical devices and precision knives).
