OBJECTIVES: The objective of this study is to assess the effects of different surface treatments on flexural strength, surface roughness, and crystal structure of zirconia with different yttria concentrations on YZ ceramics are to be evaluated. Moreover, the effects yttria stabilization on the mechanical properties of four variants of highly translucent yttria-stabilized zirconia (VITA YZ ® Zahnfabrik) and porcelain-fused YZ.
MATERIALS AND METHODS: Four different monolithic zirconia ceramics: VITA-Zahnfabrik VITA-YZ T, VITA-YZ HT, VITA-YZ ST, VITA-YZ XT, as well as porcelain-fused zirconia (PFZ) were used in this study. Disc specimens were prepared by core-drilling and sectioning the bulk discs, which were sintered according to the manufacturer’s recommendations. These sintered zirconia specimens were subsequently subjected to surface treatments with diamond discs of various fineness parameters such as 1μm, 50μm, 125μm, and without surface treatment as a control group. Surface roughness was evaluated at three different locations of five polished specimens from each group of the five different materials by using a contact profilometer Mitutoyo Surftest SJ-201 with a 2μm diamond stylus. A Bruker D2 Phaser diffractometer was used for X-ray diffraction (XRD) measurements. Crystallographic phase identification and analysis were performed through Rietveld refinements on XRD raw data using GSAS-II software implemented by Python. An Instron universal testing machine was used to test biaxial flexural strength of the materials, in which a ball-on-three-balls configuration was set up to perform biaxial flexural strength testing with a speed of 1 mm/min and a load cell capacity of 10 kN. The microstructures of materials were analyzed by using a SU6600 Hitachi field emission scanning electron microscope (FESEM). Descriptive statistics such as surface roughness, phase contents, grain size, and flexural strength were indicated as mean and standard deviation (SD). Statistical analysis was performed using multi-way ANOVA and linear regression model followed by the post hoc Tukey test.
RESULTS: XRD analyses for all control, as well as treated specimens, indicate the presence of multiple phases of zirconia in each specimen, which include tetragonal, cubic, monoclinic, and rhombohedral crystalline phases. Some of the control samples have no detectable monoclinic phase of zirconia while all surface treated specimens have various amounts of monoclinic phases and rhombohedral phases. The presence of these phases has affected the flexural strength as well as the crack toughening characteristics. The YZ T samples have the highest flexural strength while YZ XT have the lowest flexural strength. The sample surface roughness depends on the type of treatment. The surface treatment affects each group of specimens in a different way depending upon its various pristine crystal phases. The surface treatment with different fineness parameters such as 1μm, 50μm, and 125μm was found to induce the stress-induced phase transformation in the surface layers of the specimens, which includes tetragonal to monoclinic phase transformation. This phase transformation seriously affects the flexural strength and crack propagation of zirconia due to the transformation toughening mechanism. The results indicated that grain size was not a crucial factor influencing either the transformation toughening mechanism or the tetragonal phase content in the control groups in each monolithic material system. In case of the porcelain fused zirconia (PFZ), Several factors can affect the flexural strength of the veneering materials.
CONCLUSIONS: The higher yttria content over 4 mol% decreases the flexural strength of the ceramics studied. Moreover, the surface polishing treatments seem to enhance the flexural strength in all cases. The biaxial flexural strength of porcelain-fused zirconia (PFZ) was significantly lower than monolithic zirconia (YZ)
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/44243 |
| Date | 15 April 2022 |
| Creators | Mohammad, Bebi |
| Contributors | Giordano II, Russell, Fan, Yuwei |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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