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Literature regarding the etching mechanism of zirconia is veryscarce. It seems that there has been only one attempt to describe thedissolution of ZrO2 in Hydrofluoric acid , but the study, based on the Pourbaixspeciation diagrams, was essentially theoretical. Besides, the caseof 3Y-TZP is more complex because of the presence of yttrium oxide.Based on the experimental results presented above, an attemptto describe the etching mechanism of 3Y-TZP in HF 40%, summa-rizing previous observations, is presented here: F dissolves zirconium oxide and yttrium oxide. Fluoride, oxide,and hydroxide complexes are formed. Etching is slightly preferential at the grain border, but also occurs inside the grains. Yttrium complexes have very low solubility. From the beginningof the etching process, yttrium trifluoride (YF3) octahedral crys-tals precipitate on the surface. Zirconium complexes are partially soluble. After a certain time,an “adhered layer” composed of yttrium, zirconium and fluorineprecipitates, probably because the saturation threshold for zir-conium fluoride complexes is reached. It may be formed at leastpartially by agglomerated textured polycrystalline needles.

The main outcome of all these observations is that Hydrofluoric acid etch-ing makes easy to tailor a surface with the desired smooth–roughtransition, fractal dimension and roughness parameters. Thisconstitutes an interesting result given the high sensitivity ofosteoblasts to roughness at different scales and thestrong influence of fractal dimension on osteoblastic adhesion anddifferentiation. Regarding the roughness parameters, althoughthe lack of standardization in the measurements makes compari-son difficult with other studies, the values of Sa, Sdr, Sdsand Sciwhich were obtained here fall within the range of the reported val-ues for commercial dental implants with proved high success rate. On the other hand, the limited evolution of Sa, Sdr, SdsandSciafter one hour of etching tends to indicate that inferior etchingtimes are more appropriate for the treatment of dental implants.Unfortunately most of the studies in the literature omitthe otherroughness parameters and therefore their influence on the boneresponse is currently not well documented.

The present work shows that Hydrofluoric acid(HF) etching of zirconia is a complexphenomenon involving the dissolution of zirconium and yttriumoxides and the precipitation of fluoride crystals, which is reportedfor the first time. The formation of these precipitates on the surfacehighlights the importance of the cleaning step, since their effect onthe bonding between implant and bone and on the patient healthis unknown. At room temperature and within the limits of thisstudy, a concentration of 40% leads to the fastest and most uni-form etching, and appears therefore to be the most appropriatefor the treatment of zirconia dental implants. On the other hand,monitoring the etching time allows producing surfaces with controlled roughness, smooth–rough transition and fractal dimension.The roughness analysis was exhaustive and evidenced that a tran-sition was taking place around one hour of etching, after whichthe evolution of the roughness parameters known to be importantfor osseointegration was limited. Chemical changes at the surfacewere moderate and not time related. This work could constitutea sound basis for future biological studies aiming at determiningthe influence of the topography of zirconia on cell response andosseointegration.