Two tetragonal (t) zirconia-alumina composites were prepared by adding Al2O3 (20 vol%) into t-ZrO2 solid solutions doped with 3.0 mol% Y2O3, 1.6 mol% Nb2O5, 3.6 mol% CeO2 (Al20Ce) and 5.3 mol% Y2O3, 4.6 mol% Nb2O5 (Al20) as the femoral heads for the total hip replacements. The effect of machining and annealing on the hydrothermal stability and surface roughness of the composites was evaluated by comparing with the commercial 3 mol% Y2O3 stabilized t-ZrO2 (3Y-TZP). Machining was conducted by using the high-precision machine, where the spindle speeds (rpm 9000~18000) and the electroplated wheels (#400, 800) can be easily adjusted. The machined specimens were annealed at temperatures from 900 to 1300 oC and subsequently autoclaved for 5 days at 120oC under 0.3 MPa water vapor pressure. Although the phase stability and surface roughness of the machined composites were found to be inferior, the hydrothermal stability of the annealed and subsequently autoclaved composites was determined to be excellent as compared with that of the commercial 3Y-TZP. The lower phase stability of the machined composites was likely to be due to numerous machining damages (surface defects) of the composites than those of 3Y-TZP.
The green and hard machining characteristics of dental ceramics are of great interest to dental industry. The green bodies of TZP/ Al 2 O 3 composites were prepared by the cold isostatic pressing, and machined on the CNC lathe using PCD (polycrystalline diamond) insert under various machining conditions. With increasing nose radius of PCD insert, surface roughness initially increased due to increased cutting resistance, but decreased by the onset of sliding fracture. The lowest surface roughness was obtained at spindle speed of 1,300 rpm and lowest feed rate. Hard bodies were prepared by pressureless sintering the machined green bodies at several temperatures. The grinding test for sintered hard body was conducted using electroplated diamond bur with different grit sizes. During grinding, grain pull out in the composite was occurred due to thermal expansion mismatch between the alumina and zirconia. The strength of the composite decreased with alumina contents, due to increased surface roughness and high monoclinic phase transformed during grinding process. The final polished samples represented high strength by the elimination of a phase transformation layer.
Yttria and iron co-doped tetragonal zirconia polycrystals ((Y, Fe)-TZPs) were investigated to evaluate the feasibility for the ceramic bracket application. Presintered the (Y,Fe)-TZPs brackets were machined, abraded and coated with granule spray containing 10~50 wt% of flux. Finally, the base-coated brackets were sintered. Roughness of the coated surface was decreased to 13 um and 10 um when the flux content was 30 wt% and 50 wt%, respectively. It may be due to the fusion and the collapse of the granules as a result of the high amount of flux. Better adhesion to the base surface of the brackets and irregular shape were observed by raising the amount of flux in the slurries. Wetting angle of ethylene glycol droplet on the (Y,Fe)-TZP bare surface was determined to be 41.85°, however, the angle became 0° after the granule spraying treatments. The (Y,Fe)-TZP brackets exhibited uniform embossed base and good wetting. The newly developed base-coated bracket could be highly applicable to enhance retention and to reduce adhesive resin remnant during the bracket debonding.
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