Dental Asia Jul/Aug 2018

62 DENTAL ASIA JULY / AUGUST 2018 In depth with -Nacera (DOCERAM Medical Ceramics GmbH) Market Overview The dental market is o ơ ered a broad portfolio of zirconium oxide in terms of sizes, strengths, and aesthetics (colour). Additionally, a distinction is also made in the yttrium oxide content that leads to: Low Translucent Zirconia (LT), High Translucent Zirconia (HT) and Ultra High Translucent Zirconia (UHT, which is the highest concentration). However, higher concentrations lead to changes in the crystal structure that reduce ƪexural strength. Relation of stabilisation, translucency and ƪexural strength The zirconia (ZrO 2 ) used for dental applications is a manufactured structure. Pure ZrO 2 has an unusable monoclinic crystal structure at room temperature. The crystal structure refers to the physical orientation and distances between atoms. Di ơ erences in structure a ơ ect density and other properties. Themonoclinic structuredensity is 5.56g / cm 3 , cubic structure is approx. 5.68-5.91 g / cm 3 , while tetragonal ZrO 2 has the highest density, at 6.1 g / cm³. These changes in structure a r e t he r esu l t s o f small deviations in the orientation of the oxygen atoms (Fig. 1). Themost critical structural transformation is the phase shift from tetragonal to monoclinic that results in a volume expansion of 5-8%. Stretching the bonds between atoms weakens the material. For this reason, yttria oxide (Y 2 O 3 ) is added to the ZrO 2 in a process called, “doping,” to prevent the phase shift to monoclinic. The addition of yttria oxide stabilises the newly formed stronger structures of zirconium oxide, cubic and tetragonal. As the yttrium content increases, the proportion of cubic phase in the material also increases. The greatest advantage for dental purposes is the stabilisation of the tetragonal phase that maximises ƪexural strength. Flexural strength & microfractures 3Y-TZP stabilised ZrO 2 is measured by its highest ƪexural strength. However, in the presence of a microfracture, the tetragonal structure undergoes a transformation (phase shift) to the monoclinic structure that includes a 5-8% particle expansion. Tensile stresses can induce cracks that propagate in the microstructure, depending on the rate of crack growth. As the crack travels through the structure, the adjacent tetragonal ZrO 2 grains are converted to the monoclinic phase. The resulting volume expansion compresses the crack, called “phase toughening,” and dissipates much of the crack energy to contain the fracture and increase ƪexural strength (Fig. 2). This e ơ ect also enhances 5 and 6mol% stabilised ZrO 2 . However, in contrast to 3Y-TZP, there is a high proportion of cubic grains in the structure of the 5 & 6 Y-PSZ (partly stabilised zirconia). Because cubic grains are not converted into the monoclinic phase in the presence of a microfracture, much of the crack-inhibiting e ơ ect is lost (Fig. 3). As a result, cubic microfractures extend deeper and decrease strength. Nacera Blue X: Translucency-Enhancing Liquid for 3Y-TZP Zirconia Fig. 1: from left to righ: A= modiƤcations cubic, B= tetragonal, C= monoclinic crystal lattice; bright spheres = Zr, dark spheres = O (Source: Brevier) Fig. 2: 3Y-TZP Zirconia Fig. 4: Phase transformation Fig. 3: 5Y- and 6Y-PSZ Zirconia Translucency Translucency is influenced by various aspects of the structure (grain sizes, pore distribution,etc.). I n a d d i t i o n t o t h e absorption and the reƪection of the light beam by grain boundaries and porosity, light refraction’s dependence upon crystal boundary and structure also plays a major role. Most systems show an anisotropic structure that splits the incident light beam and lowers translucency, a process called, birefringence.

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