The strongest zirconia phase is the tetragonal phase, particularly in yttria-stabilized zirconia (YSZ) when processed at optimal temperatures. This phase exhibits high fracture strength, reaching about 1280 MPa when fired at approximately 1500°C to 1550°C.
Explanation:
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Tetragonal Phase in Yttria-Stabilized Zirconia (YSZ): YSZ is a high-performance material known for its excellent mechanical properties and biocompatibility. Zirconia exists in three allotropic forms: monoclinic, tetragonal, and cubic. The tetragonal phase is metastable at room temperature, meaning it can transform into the monoclinic phase under certain conditions such as external stresses or thermal aging. This transformation is crucial as it induces compressive stresses that help close the tips of any advancing cracks, enhancing the material's resistance to fracture.
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Optimal Processing Temperature: The strength of zirconia is highly dependent on the processing temperature. Studies indicate that firing YSZ at 1500°C to 1550°C results in maximum strength. Deviations from this temperature range, even by as little as 150°C, can significantly reduce the strength of zirconia due to undesirable grain growth. For instance, the strength can drop from 1280 MPa at 1500°C to 980 MPa at 1600°C and further to 600 MPa at 1700°C.
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Applications and Performance: The superior mechanical properties of tetragonal YSZ make it suitable for various applications, including dental implants, abutments, inlays, onlays, and crowns, as well as orthopedic implants like hip joint heads. The material's high flexural strength, often exceeding 800 MPa, further classifies it as a Class 5 zirconia, indicating its reliability and durability in medical and dental applications.
In summary, the tetragonal phase of yttria-stabilized zirconia, when processed at the optimal temperature range, exhibits the highest strength and is thus considered the strongest zirconia phase. This phase's unique properties, including its ability to transform and induce compressive stresses, make it superior to other dental ceramics and suitable for demanding applications in both dentistry and orthopedics.
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