Potassium fluoride,anhydrous
           Potassium fluoride,extra pure
           Potassium fluoride,Granular
           Silicon Dioxide
           Hydrofluoric acid
           Synthetic Cryolite
           Potassium Fluoaluminate
           Ammonium bifluoride
           Potassium Bifluoride
           Aluminium fluoride
           Sodium fluoride
           Potassium Fluorosilicate
           Fluorosilicic Acid
           Sodium silicofluoride
           Potassium Hydroxide Flakes
           Magnesium Fluoride
           Magnesium fluorosilicate
           Barium Fluoride
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Concentrated Hydrofluoric Acid (HF 40% QP Panreac, Spain)was used to prepare aqueous solutions with respective mass con-centrations of 5%, 20% and 40%. Etching was performed at roomtemperature on zirconia disks in high-density polyethylene flasks.The volume of acid was 1 mL by sample. The disks were placedwith the polished side upwards. Just after being removed from HF,they were rinsed with DI water in order to stop the reaction. 

Unlessotherwise specified, after etching samples were cleaned twice tenminutes with fresh DI water in an ultrasonic bath in order to removeany remaining product of the reaction from the surface.In order to identify the soluble products of the reaction, 1 mL ofthe solution resulting from the etching of a zirconia disk withHydrofluoric Acid HF40% was evaporated. The dry residue was dissolved in 1 mL of DIwater with 0.1% of formic acid (HCOOH). Electrospray ionizationFourier transform mass spectrometry (ESI-FTMS) of the resultingsolution was carried out on a LTQ OrbitrapTMVelos mass spectrom-eter coupled with a Thermo Scientific AccelaTMHigh-PerformanceLiquid Chromatography system. The solution was introduced bythe flow injection analysis method (flow rate: 100 μL/min, mobilephase: H2O/CH3CN (1:1) with 0.1% of formic acid, injected volume:5 μL). Mass spectra were acquired in both positive and negativemode. For the calculation of molecular formulas the restrictionsdetailed were applied.

To understand the etching mechanism and to identify the precipitates which formed during the treatment, zirconia disks wereimmersed for different times in Hydrofluoric Acid 40%. To avoid removing the reac-tion products from the surface, after etching the samples were notsubjected to sonication but only rinsed with DI water. In the rest ofthis work, this state of the surface will be referred to as “as etched”. The “as etched” surfaces were observed by scanning electronmicroscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) wasused for the elemental analysis of the etching products. To deter-mine the elemental composition and the chemical state of thesurface, X-ray photoelectron spectroscopy (XPS) was performedwith a SPECS system equipped with an Al anode XR50 source oper-ating at 150 W and a Phoibos 150 MCD?9 detector XP. Spectrawere recorded with pass energy of 25 eV, 0.1 eV steps and a pres-sure below 7.5 × 10?9mbar. Binding energies were referred to theadventitious C 1s signal and background was subtracted. The iden-tification of the local bonding environment of each element wasperformed by comparing the experimental peak positions with thedata from the NIST Standard Reference Database 20, Version 4.1.