Why Zirconia Blocks Dental are the Go-to Dental Material in 2024
2024-07-16
2024-10-31
With the continuous development of dental restorative technology, the types of dental restorative materials have gradually increased. Ceramics have been widely used in various fields of dental restorative materials due to their good mechanical properties, biocompatibility and structural stability. According to the different types of ceramics, dental all-ceramic materials are hot die cast porcelain, glass ceramics and zirconia ceramics. Hot-die cast porcelain and glass ceramics are relatively low in strength. In contrast, zirconia ceramics have higher mechanical properties due to the toughening of the monoclinic and tetragonal phases and are more suitable for use as dental materials. In addition, zirconia has several advantages: structural stability, non-reactivity in the oral environment, good biocompatibility and relatively high light transmission. Therefore, more than 95 per cent of all-ceramic crowns and bridges are made of zirconia ceramics.
I. Basic properties of zirconia ceramics
High purity zirconia powder is white, zirconia ceramics are chalky. Relative molecular mass 123.223g/mol, density 5.85g/cm3, melting point 2715℃. Zirconia has three crystal structures, monoclinic phase, tetragonal phase and cubic phase. These three crystal structures have different morphologies at different melting points and are transformed under certain temperature conditions. The temperature at which the monoclinic and tetragonal phases transform into each other is around 1150°C, and the temperature at which the tetragonal and cubic phases transform into each other is around 2370°C. During the transformation of tetragonal phase zirconia to monoclinic phase zirconia, martensitic phase transformation occurs, and is accompanied by volume expansion.
II. Toughening of zirconia ceramics
Compared with metals, the fracture toughness of ceramic materials is usually 1 to 2 orders of magnitude lower. Zirconia ceramics can be toughened in different ways to improve its fracture toughness, the main toughening mechanisms are: stress-induced phase transition toughening, microcracking toughening, microcracking bending, bifurcation and bridging toughening, whisker toughening, diffuse toughening, fine crystal strengthening, fibre toughening, etc., in practice, zirconia ceramics material toughness is often a variety of toughness enhancement mechanism is often a common result of the action of. At present, the laboratory measurement of zirconia ceramic fracture toughness of the most widely used methods are: unilateral incision beam method and indentation method.
Zirconia ceramic toughness research has begun as early as the 1950s, after 1975 with the discovery of the phase transition phenomenon, some researchers believe that stress-induced phase transition toughening of zirconia ceramics due to the external stress effect of the crack, the crack tip of the stress can be induced by the t →m martensitic phase transition, the volume expansion generated by the phase transition grains will inhibit the crack expansion, thereby improving the toughness of the material. However, in the initial phase of the phase transition, the expansion deformation existing within the 120° angle of the crack tip will cause a decrease in the toughness of zirconia, after which the volume expansion will inhibit the crack expansion, so that the toughness is rapidly improved, and the fracture toughness grows slowly when the crack expands to 5~10h.
III.The low temperature oxidation of zirconia ceramics
Under the low-temperature humid environment, zirconia undergoes t-m phase transition aging is essentially a martensitic phase transition: a non-thermodynamic, non-diffusive crystal structure change. Low-temperature aging first occurs on the surface of the material t-m phase transition, the phase transition is accompanied by volume expansion so that the surface of the material to produce bumps and micro-cracks, aesthetic properties degradation; subsequently, water molecules along the micro-cracks infiltrated into the interior of the substrate, caused by the material inside the zirconia t-m phase transition, resulting in the generation of macro-cracks, and ultimately a decline in the mechanical properties, and even cause sudden failure. After a large number of experimental studies, the characteristics of the low-temperature aging process mainly include four points:
1) Low-temperature ageing is an autocatalytic process without thermal conductivity, and t-m phase transition ageing proceeds through a nucleation-growth (N-G) mechanism of the m-phase;4) Stabiliser content and grain size directly affect zirconia resistance to low temperature ageing.
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