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The aim of this project was to evaluate and compare the mechanical properties of commercial yttria partially stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics, which have generated interest in restorative dentistry because of their high strength and high resistance to fracture. Mechanical properties of three commercial Y-TZP ceramics (Lava™, Cercon® and Invizion™) were investigated including the biaxial flexural strength, hardness, fatigue, and subcritical crack growth. Crown shaped specimens (Kavo Everest ZS-blanks with IPS e.max® Ceram and IPS e.max® Zirpress) were submitted to the fatigue followed by biaxial flexural strength test. The microstructure was analysed using AFM, and elemental analysis and fracture pattern were analysed via SEM. Additionally, stress induced ransformation toughening was studied using Vickers indentation (load 20-50 kg) and Raman spectroscopy. The phase transformation was also analysed on fracture surfaces of specimens that failed after biaxial flexural strength and subcritical crack growth test. The results of this project showed no significant difference in the mean biaxial flexural strength of uncoloured and coloured Lava™; however, Cercon® (823±115 MPa) and Invizion™ (828±87 MPa) had significantly lower flexural strength than Lava™ groups (~1100 MPa). All groups had similar hardness values (>1300 Hv). The survival strength of three commercial ceramics ranged from 60 to 70% of their mean biaxial flexural strength at 500,000 cycles. The types of sintering processes (heat pressed and sintered methods) did not influence the flexural strength of crown shaped specimens after fatigue for 50,000 cycles. The subcritical crack growth parameter ranged from 30-197. There were differences in the appearance of the microstructure and the amount of elemental components between the three Y-TZP ceramics. The XRD showed that the as-received surface had three phases, which were tetragonal cubic1 and cubic 2 (slightly larger than cubic 1). The polished surface contained two phases, which were tetragonal and cubic 1. The Raman spectroscopy revealed that the variation of loads had little effect on the level of transformation toughening per unit area; however, the transformation was varied by the distance from the centre to outside the indentation area. The cubic phase had the highest level at the centre while the tetragonal phase was found at a higher level outside the indentation. The monoclinic phase was only detected within the indentation area. For the fractured specimens, monoclinic and cubic phase were found at a higher level at the fracture origin, tension and compression sites. Further study examined the biaxial flexural strength and shear bond strength of zirconia containing 1, 2 and 5 wt% titanium phosphate glasses in order to identify whether glass affected the flexural and bond strengths. The results showed that the addition of glass 5 wt% or greater lead to the dramatic decrease of the biaxial flexural strength. There was also no significant difference in biaxial flexural strength and shear bond strength between titanium phosphate glass zirconia 0 wt%, 1 wt% and 2 wt% glasses zirconia etched for 2 and 5 minutes. Therefore, this project concluded that there was a difference in properties and phase transformation between Lava™ Cercon® and Invizion™ groups.

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