The aim of this work was to evaluate the effects of application time of an acid mixture solution on the surface roughness, phase transformation, and biaxial flexural strength of 3Y-TZP after sintering or air abrasion. For the biaxial flexural strength measurement, 220 3Y-TZP disk-shaped specimens were prepared after as-sintering or air abrasion. The etching solution comprised a mixture of hydrofluoric acid, sulfuric acid, hydrogen peroxide, methyl alcohol, and purified water. The samples were divided into 11 subgroups according to the etching times (Control, 1, 2, 3, 5, 8, 10, 12, 15, 20, and 30 min). The results showed that acid treatment on both as-sintered and air-abraded 3Y-TZP surfaces increased the surface roughness. However, it had no significant effects on the monoclinic phase or flexural strength of as-sintered zirconia. The monoclinic phase and flexural strength of air-abraded zirconia increased sharply after air abrasion; however, they gradually decreased after acid treatment, to a similar level to the case of the untreated surface. Surface treatment with acid mixture increased the roughness, but the lack of increase of monoclinic phase is thought to be because the loose monoclinic particles remaining on the surface were removed through the etching process.
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.
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