Fracture anisotropy in Ls2 under mixed-mode loading

Fracture anisotropy in Ls2 under mixed-mode loading

e62 d e n t a l m a t e r i a l s 3 1 S ( 2 0 1 5 ) e1–e66 Paffenbarger Award finalists P1 Fracture anisotropy in Ls2 under mixed-mode loading M. Wen...

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d e n t a l m a t e r i a l s 3 1 S ( 2 0 1 5 ) e1–e66

Paffenbarger Award finalists P1 Fracture anisotropy in Ls2 under mixed-mode loading M. Wendler 1,2,∗ , R. Belli 1 , L.H. Da Silva 3 , J.I. Zorzin 1 , A. Petschelt 1 , U. Lohbauer 1 1

Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Erlangen, Germany 2 Faculty of Dentistry, University of Concepción, Concepción, Chile 3 Department of Biomaterials and Oral Biology, Faculty of Dentistry, University of Sao Paulo, Sao Paulo, Brazil Purpose: The aim of this study was to induce fracture anisotropy in lithium disilicate (LS2) glass-ceramic specimens under mixed-mode loading by changing the crystalline orientation. An isotropic material (3Y-TZP) was used as reference. The bar under 3-point bending set-up was validated using monolithic dental bridge structures. Methods and materials: LS2 bar specimens (20 × 5.33 × 4.5 mm) were produced by the heat-pressed technique, using PMMA precursors and following two pressing orientations: with the sprues placed in a “V” disposition perpendicular to the long-axis of the beam or one sprue parallel to its long-axis. After pressing, specimens were divided in 5 subgroups and notched using razor blades and abrasive pastes at different distances (d) from the midplane. 3Y-TZP bars were cut from CAD-CAM blocks and notched prior to sintering using a steel dented saw at the same d as for LS2 groups. Specimens were tested in three-point bending and the mode-mixity was calculated in terms of fracture energy (Gc), using the relation between the mode-I (KI, tensile) and mode-II (KII, shear) stress intensity components at each d. The deflection angle of the crack path from the original precrack was assessed post- mortem. Three-unit dental bridges were produced for both materials using CAD-CAM processing, with the connectors located at 4.5 mm (mesial) and 2.5 mm (distal) from the midspan. Bridges were loaded with a flat piston on their midspan until failure. Fracture origins and crack propagation angles were analyzed by fractography and profilometry. Results: Due to the higher contribution of the shear component, a constant decrease in Gc was observed for 3Y-TZP as d increased. Conversely, LS2 specimens showed increases in Gc at higher mode-mixities, with fracture energy values surpassing those of 3Y-TZP at KI = KII. Deflection angles for LS2 showed negative values at lower KII/KI while fracture angles of 3Y-TZP followed the predicted path for isotropic materials. Fracture patterns in the bridges matched those in the beam geometry, as crack deflection angles followed similar paths for both materials and pressing configurations. Conclusion: The tailoring of crystal alignment in LS2 glass-ceramics can lead to anisotropic fracture behavior

and increased fracture energy. This seems a promising reinforcement strategy for structural applications such as dental bridge constructs. P2 Growth of crystalline titanium oxide films in different acid electrolytes S. Jain ∗ , R.S. Williamson, M.D. Roach University of Mississippi Medical Center, USA Purpose: Titanium implants are often anodized to enhance the surface oxide layer for improved osseointegration. Both Anatase (A) and Rutile (R) phases of titanium oxide have shown antimicrobial effects and enhanced bioactivity levels. However, it is not currently known which A/R phase ratio promotes the best results. A previous study in our laboratories showed that specific A/R phase ratios in the anodized layer can be controlled using acid electrolyte mixtures and a stepped potentiostatic forming voltage. However, the changes in oxide layer thickness, surface chemistry, surface morphology, and crystallinity that occur at increasing voltage steps have not been fully characterized. The objective of the present study was to compare and contrast the development of anodized layers at each voltage step in acid electrolyte mixtures. Methods and materials: Commercially pure titanium grade 4 samples were cut from 2.00-mm sheet to 1 in 2 samples and ultrasonically cleaned in alcohol. Samples were dipped in a nitric-hydrofluoric acid solution for a period of 30 s and rinsed with distilled water. Dipped samples were anodized using potentiostatic 12 V steps for 10 s per step in electrolyte mixtures of sulfuric acid, phosphoric acid, oxalic acid, and hydrogen peroxide. Samples were analyzed for anatase and rutile phases using thin film X-ray diffraction (XRD). Oxide thickness and surface morphology were examined using scanning electron microscopy (SEM). Surface chemistry was determined using a combination of energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Electron backscattered diffraction (EBSD) was used to show the spatial distribution of crystalline oxide phases. Results: XRD results showed anatase to form first in each electrolyte once a threshold voltage had been achieved. Rutile phase forms at higher forming voltages, and its threshold voltage was also electrolyte dependent. Anodized layer thickness showed exponential growth with increasing voltage steps in each electrolyte. The surface morphologies and chemistries were shown to change at particular voltage steps. EBSD analyses provided additional insight into the mechanisms of oxide growth in relation to the changes in surface chemistry. Conclusion: Anodized layer thickness showed exponential growth in each electrolyte with increasing voltage. The surface chemistry, surface morphology, and crystallinity for each anodized layer were found to be dependent on both the electrolyte mixture and the applied voltage. The detailed understanding of growth mechanisms and surface characteristics of each anodized layer is anticipated to provide valuable