Influence of hydrothermal and mechanical conditions on the strength of zirconia

Low temperature degradation and mechanical and thermal cycling may decrease the strength of zirconia and jeopardize the long-term success of dental restorations made of this material. The objective of this study was to reveal the influence of different environmental and loading conditions on the strength of 3 mol.% yttria-stabilized polycrystalline tetragonal zirconia (3Y-TZP). A total of 144 disk specimens were produced from each of two 3Y-TZP materials, and subjected to one of the following conditions: (A) no further treatment (control); (B and C) 10⁶ and 5×10⁶ mechanical cycles, respectively, with an upper load limit of 100 N; (D) 10⁴ thermal cycles between 5 and 55°C; (E) 200 days storage in water at 36°C; (F) a successive combination of conditions B, D and E; (G) storage in water at 80°C for 64 days; (H) storage in an autoclave at 134°C for 8 h. Monoclinic phase content was evaluated by X-ray diffraction (XRD) analysis. Specimen strength was determined in a biaxial bending test. The two ceramics exhibited average strengths of 995 and 1239 MPa, respectively. No statistically significant influence of any treatment on strength was demonstrated for either material. However, XRD measurements revealed a substantial increase in monoclinic phase content, from an initial 2% (control) to up to 10%, according to storage conditions. As a consequence of hydrothermal loading a tetragonal to monoclinic phase transformation took place at the surface of the 3Y-TZP materials investigated, but, like thermal and mechanical cycling, this did not lead to significant changes in bulk strength.

Influence of cooling rate on zirconia/veneer interfacial adhesion

Slow cooling firing schedules have recently been introduced by some manufacturers to reduce chipping complications in zirconia-based core/veneer composites. The aim of this study was to test the hypothesis that these firing schedules may influence the bond strength between the zirconia core and veneering ceramic. Four different veneering ceramics recommended for zirconia (Lava Ceram, Triceram, VM9 and Zirox) were fired onto rectangular shaped Y-TZP specimens (Lava Frame) and cooled using a rapid or a slow cooling rate. The resulting bilayer specimens were notched, loaded in a four-point bending test and load-displacement curves were recorded. The critical load to induce stable crack extension at the core/veneer interface was determined, in order to calculate the strain energy release rate (G, Jm⁻²). Additionally, dilatometric measurements of the veneering ceramics were performed to determine the coefficient of thermal expansion (α, ppm.K⁻¹) between 50 and 450°C (α₁) and in the temperature region above the glass transition temperature (α₂). Discrepancies between α₂ and α₁ (Δα) were calculated. For all core/veneer compositions G values were lower for the slowly cooled specimens than for the rapidly cooled specimens. Significant differences with respect to the firing schedule were found in the Triceram and VM9 groups (P<0.05). The reductions in G values correlated with Δα. The bond strength between the zirconia core and the veneer decreased with the slow cooling rate. These results indicate that slow cooling of zirconia restorations may increase the risk of adhesive delamination failures between the core and veneer.

Von Fluorsilylamiden zu Element-Silylamiden und Iminosilanen

Ethyltrifluorosilane reacts with tert-butyllithium in a molar ratio 1:2 to give Et(CMe3)2SiF (1), which forms the aminosilane, Et(CMe3)2SiNH2 (2) in the reaction with NaNH2. The lithium salt of 2 and (Me3C)2SiF2 gives (Me3C)2SiF-NHSi(CMe3)2Et (3). 3 reacts with BuLi to give the lithium salt, (Me3C)2 SiF-NLi-Si(CMe3)2Et (4). tert-Butyl-(di-tert-butylfluoro)-silylaminoalanes (Me3C)2SiF-NCMe3-AlR2, R = Me (5), Et (6) are formed in the reaction of (Me3C)2SiF-NLi CMe3 (I) and ClAlR2. The lithium salt (Me3C)2SiF-NLi-Si(CMe3)2Me (III) reacts with BF3 under formation of the bis(silyl)aminodifluoroborane (Me3C)2SiF-N(BF2) Si(CMe3)2Me (7). In the reaction of I with F3SiPh rotamers of (Me3C)2SiF-NCMe3 SiF2Ph (8) are obtained.

Stannyl-silylamines are prepared in reactions of the lithium salts (Me3C)2SiF-NLiR, R = CMe3 (I), SiF(CMe3)2 (II); Si(CMe3)2Me (III) and the chlorostannanes ClSnMe3 (9,10,11) and Cl2SnPh2 (12), (Me3C)2SiF-NSnMe3-Si(CMe3)2R (9: R = F, 10: R = Me), (Me3C)2SiF-NSnMe3CMe3 (11), (Me3C)2SiF-NCMe3SnClPh2 (12). The NMR spectra of 9 and 10 prove the existence of rotamers.

The reaction of III with Me3SiOSO2 CF3 in n-hexane leads to the formation of the tris(silyl)amine 13, (Me3C)SiF-NSiMe3)2Si(CMe3)2Me. However, the analogous reaction of III and 4 with Me3SiOSO2CF3 in thf leads to the formation of the stable iminosilanes 14 and 15, (Me3C)2Si==N- Si(CMe3)R, R = Me (14), Et (15) crystal structures of 7 and 12 are presented.