The aim of this study was to prepare and to characterize the structure of Al2O3–3YSZ composites with 5% TiO2 addition as well as the surface modification upon treatments with SnF2 and NaBF4, respectively. SEM micrographs showed the controlled densification of the composites as an effect of 3YSZ and TiO2 addition to alumina matrix. By FTIR and XRD, the characteristics of Al-O and Zr-O vibrations, respectively, the diffractions lines related to a-corundum and zirconia in tetragonal phase were discussed. Qualitative and quantitative results obtained by XPS and ATR FTIR demonstrated that the proposed materials are more sensitive to SnF2 than to NaBF4 treatment.
Surface Modification of Alumina/ Zirconia CeramicsUpon Different Fluoride-Based Treatments
Simona Cavalu* and Florin Banica
Faculty of Medicine and Pharmacy, University of Oradea, Oradea, Romania
Viorica Simon
Faculty of Physics & Institute of Interdisciplinary Research in Bio-Nano-Sciences,Babes-Bolyai University, Cluj-Napoca, Romania
Ipek Akin and Gultekin Goller
Metallurgical & Materials Engineering Department, Istanbul Technical University, Istanbul, Turkey
The aim of this study was to prepare and to characterize the structure of Al2O3–3YSZ composites with 5% TiO2 addi-tion as well as the surface modification upon treatments with SnF2 and NaBF4, respectively. SEM micrographs showed thecontrolled densification of the composites as an effect of 3YSZ and TiO2 addition to alumina matrix. By FTIR and XRD,
the characteristics of Al-O and Zr-O vibrations, respectively, the diffractions lines related to a-corundum and zirconia intetragonal phase were discussed. Qualitative and quantitative results obtained by XPS and ATR FTIR demonstrated that theproposed materials are more sensitive to SnF2 than to NaBF4 treatment.
Int. J. Appl. Ceram. Technol., 1–10 (2013)DOI:10.1111/ijac.12075
Introduction
Ceramics have a great potential in the biomedicalfield, thanks to their biocompatibility, strength, andwear resistance. The two dominant ceramic materials inclinical use today as bearing surfaces are still alumina(Al2O3) and zirconia (ZrO2).
1–3 Alumina exhibitsexcellent hardness and wear properties; fracture tough-ness values are lower than those of the metals used inorthopedic surgery. However, it is a brittle material,with low resistance to the propagation of cracks. Zirco-nia was introduced to overcome the limitations of alu-mina. It is well known that transformation tougheningimproves the mechanical properties of zirconia ceram-ics, as their major drawback is the strength reduction,due to an unfavorable tetragonal to monoclinic mar-tensitic phase transformation during the contact withphysiological fluids.4–7 Zirconia, in contrast to alumina,is an unstable material, existing in three crystallinephases: monoclinic, tetragonal, and cubic. The tetrago-nal phase that is the most resistant tends to transforminto the monoclinic phase under certain conditions(aging, thermal treatment). The addition of stabilizingmaterials such as Y2O3 or CeO2 during manufacturecan control the phase transformation of zirconia.5–7
Therefore, the ideal ceramic for orthopedic and dentalapplications is a high-performance biocomposite mate-rial that combines the excellent material properties ofalumina in terms of chemical stability, hydrothermalstability, biocompatibility, and extremely low wear andof zirconia with its superior mechanical strength andfracture toughness.8–11 On the other hand, the surfacesmodification and postsynthesis treatment also influencethe performances of the bioceramics designed to dentalapplications.12,13 It was demonstrated that the adminis-tration of complex fluorides as compared with NaFsuggests the possibility of using them as effective agentsin dental caries prevention in human populations.14,15
For example, stannous fluoride converts the calciummineral apatite into fluorapatite, which makes toothenamel more resistant to bacteria generated acid attacks.In toothpastes containing calcium minerals, sodiumfluoride becomes ineffective over time, while stannousfluoride remains effective in strengthening toothenamel. Stannous fluoride has been shown to be moreeffective than sodium fluoride in reducing the incidenceof dental caries and controlling gingivitis.16 Furtheraspects related to the action of these new bioceramicsupon different surface treatments on dentinal tissue are
to be analyzed, to be properly used by professionals, sothat they can make the best of properties during clini-cal applications.17 Even if increasing attention has beenpaid to elucidating the influence of fluoride chemistryin tooth mineralization, there are also some debatesabout the use of fluoride in osteoporosis treatment, par-ticularly concerning the beneficial effects on bone massand quality.18 NaF has been known to be one of themost effective agents for the treatment of vertebral oste-oporosis by its stimulating effect on new bone forma-tion.19 In this study, we are focused on the possiblebeneficial effect of fluorination with respect to dentalbioceramics. The surface modifications of alumina andalumina/zirconia bioceramics are investigated upon dif-ferent treatments with sodium tetrafluoroborate andstannous fluoride, respectively. The proposed bioceram-ics are designed for orthopedic or dental implants,being prepared by Spark Plasma Sintering.20 Usingcomplementary spectroscopic tools such as AttenuatedTotal Reflection Fourier Transform Infrared Spectros-copy (ATR FTIR) and X-rays Photoelectron Spectros-copy (XPS), the chemical changes on the surfaceinduced by fluoride treatment are discussed in terms oftheir effectiveness.
Materials and Methods
Preparation and Structural Characterization ofAlumina and Alumina/Zirconia Specimens
Al2O3 (Baikowski grade SM8, an average particlesize of 0.6 lm), 3 mol% yttria stabilized ZrO2 (3YSZ,Tosoh grade, an average particle size of 0.1 lm), andTiO2 (Merck, an average particle size of 1 lm) pow-ders were used as starting materials. The raw materialswere weighed in appropriate quantities, ball milled inethanol for 24 h and then dried. A graphite die 5 mminner diameter was used in the sintering process. Al2O3
and Al2O3–3YSZ composites with 5% (wt) TiO2 addi-tion were prepared using a spark plasma sinteringmethod (SPS apparatus SPS-7.40 MK-VII Syntex, FujiElectronic Industrial, Saitama, Japan) at 1350°C for5 min with a heating rate of 100°C/min in vacuum,under a pressure of 40 MPa, resulting three differentspecimens with the chemical composition as follows:specimen 1- monolithic Al2O3; specimen2- 80%Al2O3 � 20%3YSZ; specimen 3- 80%Al2O3
� 20%3YSZ + 5%TiO2. The specific Al2O3/3YSZratio was chosen because it was previously demon-
2 International Journal of Applied Ceramic Technology—Cavalu, et al. 2013
strated that zirconia has a reinforcing effect up to30%.20 In the same time, as a result of our optimiza-tion studies (not presented here), it was found that5 wt% TiO2 addition had a remarkable effect withrespect to their mechanical properties. Structural char-acterization of the specimens was made by FTIR spec-troscopy (BXII spectrometer using K Br pellettechnique, resolution of 2/cm, at room temperature;Perkin-Elmer, Waltham, MA), and X-ray diffractionanalysis carried out with a Shimadzu XRD- 600 diffrac-tometer, using Cu-Ka radiation (k = 1.5418
��A) withNi-filter. The morphology of the specimen surface (onfracture) was investigated by scanning electron micros-copy (JSM 7000F, JEOL, Tokyo, Japan).
Fluoride Surface Treatment and SurfaceInvestigation of the Specimens
High purity stannous fluoride (Tin II fluoride) andsodium tetrafluoroborate (Sigma Aldrich, St. Louis,MO) were used to prepare saturated solutions (0.4 g/mL and 1 g/mL, respectively) for surface treatment ofthe specimens by conventional anodization during 2 hat 12V. Upon the anodization treatment, the specimenswere ultrasonically treated for 90 min to remove thedeposits, then air-dried. The modifications of samplessurface upon both fluoride treatment were investigatedby ATR FTIR spectroscopy using ATR Miracle device(single reflection with ZnSe crystal) and XPS measure-ments performed with SPECS PHOIBOS 150 MCDsystem equipped with monochromatic Al-Ka source(250W, hm = 1486.64 eV) and Epass = 50 eV, with aresolution of 1 eV/step. The vacuum in the analysischamber during the measurements was kept in therange 10�9–10�10 mbar.
All binding energies were referenced to the C 1 speak arising from adventitious carbon at 284.6 eV. Thepeak areas combined with the appropriate sensitivityfactors allowed to quantify the elemental compositionat the surface. The depth of analysis was about 5 nm.
Results
Structural Investigation of the Specimens by SEM,FTIR, XRD Spectroscopy
The morphological characteristics and the detailsof the fractured surfaces of the proposed specimenswere evidenced by SEM analysis and presented in
Fig. 1. The details including the size and shape of thealumina (micron size, gray) and zirconia (submicron,bright white) grains clearly demonstrate that SparkPlasma Sintering makes possible the densification ofAl2O3 based composites at a lower temperature and ina shorter time compared with some other conventionaltechniques.9,11,21 Furthermore, the microstructure andgrain size can be controlled by a fast heating rate andshorter processing time. The structural details wereobserved from the analysis of the FTIR spectrarecorded between 400 and 1400/cm and presented inFig. 2. The FTIR spectra are dominated by absorptionlines arising from Al2O3 phase (1088/cm, 780 and797/cm). The addition of zirconia phase clearly modi-fies the relative intensity of these bands. The vibrationof Zr-O bond in tetragonal phase is visible at 518 and580/cm. A superposition of the characteristic absorp-tion bands occurs in the spectral region 500–650/cmupon TiO2 addition to alumina/zirconia matrix and, asa consequence, Ti-O vibration band cannot be distin-guished. The XRD patterns of the proposed specimensare presented in Fig. 3 showing the characteristic peaksof a-corundum (JCPDS: 30-0415) and tetragonalzirconia (JCPDS: 42-1164). The reflection lines occur-ring from crystallographic planes related to a-corundumare clearly marked at 2h = 25.6; 35.2; 37.9; 43.4;57.5; 61.3; 66.4; 68.2; 76.9, and 80.7°, while the iden-tification of tetragonal zirconia is assigned to2h = 29.9; 49.9; 59.7, and 62.5° in specimen 2 and 3.The pattern show highest tetragonal intensities of (111)planes at 2h = 29.9° and (220) planes at 2h = 59.7°and lower intensities of (113) and (311) at 2h = 62.5°.The presence of rutile TiO2 is assigned to small peaksat 2h = 26.4 and 36° in specimen 3. No monoclinicphase of ZrO2 was detected from the XRD results.
Surface Modification Upon Fluoride TreatmentsInvestigated by ATR FTIR and XPS Spectroscopy
In Fig. 4a are presented the vibrational ATR FTIRdetails of both fluoride as received from the supplier(crystalline powder). The fingerprints of SnF2 areobserved at 492, respectively, 548/cm and assigned tosymmetric and asymmetric stretch mode, whereas forNaBF4, the marker bands in the selected region are443, 472, 498, and 575/cm, as the [BF4] speciesbelongs to a symmetry group with four normal modesof vibration. For the wide range of tetrafluoroboratesand other [XF4] compounds (X = C, Si, Al, Ge, N, P,
etc.), the position of the normal modes follows thetrend: m3 > m1 > m4 > m2.
Upon the fluoride treatment, the surface of thespecimens was strongly affected as revealed by the ATRFTIR spectra presented in Fig. 4(b-d). The markerbands of both SnF2 and NaBF4 can be observed alongwith the characteristic features of Al-O stretching vibra-tions at 435/cm and, respectively, Zr-O at 526/cm.The survey XPS spectra recorded on the surface of thespecimens before and after fluoride treatment are pre-sented comparatively in Fig. 5. The main photoelectron
peaks in the spectra of the specimens before treatmentsare assigned to Al 2s (117.9 eV), Al 2p (74.3 eV), O1s (531.8 eV) (specimen 1), Zr 3d (180 eV), and Ti2p (456 eV) (specimen 2 and 3 respectively). AfterSnF2 treatment, a strong peak at 487.1 eV indicatesthe contribution of Sn 3d electrons, while the presenceof fluorine is proved by F 1s photoelectrons peak at685 eV. These marker peaks are strongly visible for allthe specimens, but as presented in Table 1, the atomicconcentration of the elements shows a higher percentof Sn on the surface of composites (specimen 2 and 3)
(a) (b)
(c) (d)
(e) (f)
Fig. 1. SEM micrographs recorded on the fractured surface of the specimens, with different details and magnifications along with theEDAX spectrum: specimen 1 (a, b); specimen 2 (c, d); and specimen 3(e, f).
4 International Journal of Applied Ceramic Technology—Cavalu, et al. 2013
compared with the monolithic Al2O3. With respect tothe NaBF4 treatment, the marker peaks in this case areF1s at 685.7 eV and Na 1s at 1072 eV, but this treat-ment shows a less effectiveness compared with SnF2.Anyway, the maximum effect in this case is observedtoward the specimen 3. The results obtained by bothXPS and ATR FTIR spectroscopy show a good correla-
tion from the standpoint of qualitative and quantitativeaspects.
Discussion
To overcome the low toughness of alumina andthe aging sensitivity of zirconia, alumina-zirconia, com-posites have been proposed for biomedical applications.The toughening mechanism in ZTA ceramics (zirconiatoughened alumina) is related to structural propertiesof these materials, conferred especially by zirconia dueto its versatile structural properties. The details pre-sented in Fig. 1 demonstrate that the presence of zirco-nia as a second phase is beneficial with respect to theinhibition of grain growth. Fine zirconia particleslocated on the boundaries inhibit the movement andprevent the grain growth of alumina (about 50%reduction in alumina grain size was observed). It hasbeen previously demonstrated that the zirconia additionto alumina matrix promotes composites with higherdensities, higher flexural strength, and fracture tough-ness.11,21 Moreover, as shown in Fig. 1 (e, f), addingTiO2 particles is more effective, as the size of aluminagrains is reduced by comparison with Fig.1 (c, d). Aspecial behavior with respect to the evolution of thestructural units present in these samples was observedfrom the analysis of the FTIR spectra recorded between400 and 1400 cm (Fig. 2). The correlation between IR
absorption bands and different types of aluminate poly-hedral is based on previous results obtained for alumi-nate crystals.22–25
The Al-O stretching vibrations of tetrahedral AlO4
groups are related to the broad, strong band at 1088/cm with the shoulder at 1168/cm and to the doublet at780 and 797/cm. The aluminum atoms are differentlycoordinated, usually by four or six oxygen atoms, andless likely by five oxygens. The absorption bands andshoulders recorded in the spectral region between 465and 648/cm are assigned to six coordinated aluminumwhich are associated with stretching modes of AlO6
octahedra. The addition of zirconia phase clearly modi-fies the relative intensity of these bands. In some previ-
ous studies on zirconia structural characteristics, theauthors mentioned absorption bands at 410, 445, 500,572, 740, 1104, and 1187/cm.26 Other studies27,28
reported FTIR bands at 740/cm corresponding to Zr-Ovibrations in monoclinic ZrO2 and bands at 510/cmand 590/cm corresponding to Zr-O vibrations intetragonal ZrO2. In our spectra, the vibrations of Zr-Oin tetragonal phase are visible at 518 and 580/cm.Moreover, upon TiO2 addition to alumina-zirconiamatrix, the relative intensity of 648/617/cm is consider-ably modified, as a superposition of the characteristicsabsorption bands occurs in this region.29 The analysisof XRD patterns (Fig. 3) led to results that are inagreement with previously reported studies with respect
(a) (b)
(c) (d)
Fig. 4. (A) Attenuated total reflection fourier transform infrared spectroscopy (ATR FTIR) spectra of SnF2 and NaBF4 powders asreceived from the supplier (a), and ATR FTIR spectra recorded on specimen surface before and after treatment using SnF2 and NaBF4:specimen 1 (b), specimen 2 (c), and specimen 3 (d).
6 International Journal of Applied Ceramic Technology—Cavalu, et al. 2013
to the effect of zirconia content on properties of Al2O3
–ZrO2 (Y2O3) composites.30–32 As expected, the con-straint exerted by the alumina matrix on the zirconia
particles maintains them in tetragonal state. In the sametime, the intensity ratio of the main peaks for aluminaand zirconia is in agreement with the ZrO2 content insamples. The results demonstrate that the high densityof the matrix correlated with the optimization of thezirconia particles microstructure can assure the parame-ters of better material performances.33
According to their interaction with surroundingtissue, bioceramics can be categorized as “bioinert” or“bioactive.” Tough and strong ceramics like zirconia,alumina, or alumina-zirconia composites are not capa-ble of creating a biologically adherent interface layerwith bone due to the chemically inert nature of thesetwo stable oxides.34 It has been demonstrated that sur-face morphology and bone–implant interactions deter-mine the predictability of endosseous implant boneintegration.13,35 Different surface treatments such as
Table 1. Atomic concentration of Sn, F, and Na onthe surface of the specimens after fluoride treatmentdetermined from X-rays photoelectron spectroscopy
Fig. 5. X-rays photoelectron spectroscopy (XPS) survey spectra of specimen 1 (a), specimen 2 (b), and specimen 3 (c) before and aftertreatment with SnF2 and NaBF4.
surface blasting or acid etching can increase the rateand amount of new bone formation on the implantsurface. Sandblasting procedure may be performedusing either medium or large grit Al2O3 particles,whereas acid-etching process can employ hydrofluoricacid/nitric acid. Some authors36 evaluated and reportedthe apatite-forming ability of a zirconia/alumina nano-composite (10Ce-TZP/Al2O3) in SBF as a result of theformation of ZrAlOH groups on the surface afterchemical treatment of the material in H3PO4, H2SO4,HCl, and NaOH at 95°C for 4 days. Hence, many dif-ferent techniques are currently in use to condition thesurfaces of abutments and fixtures of implants.37 Sev-eral in vitro and in vivo studies have demonstrated thatthe surface structure of implant abutments influencesboth the orientation and proliferation of connective tis-sue cells and inhibits epithelial downgrowth.38 In thisstudy, the surface modifications of the proposed alu-mina and alumina/zirconia ceramics upon differentfluoride treatments are emphasized by complementarytechniques ATR FTIR and XPS spectroscopy. TheATR FTIR spectra recorded on the specimens’ surface(Fig. 4) clearly demonstrate that the surface is beingtreated, emphasized by the presence of the markerbands of both SnF2 and NaBF4 according to their spe-cific vibration modes.39,40 By comparing with theFTIR spectra of the specimens before fluoride treat-ments (Fig. 2), the changes are evident. On the otherhand, taking account of the relative intensities of thefluoride marker bands with respect to each specimen,one can observe that, even after the removal of thesurface deposits, different fluoride concentration canbe detected on the surface. To obtain more details,XPS survey spectra were recorded on the specimens’surface before and after fluoride treatment (Fig. 5). Insome previous studies, XPS has been successfully usedto investigate the surface chemistry of the commercialzirconia implants, showing substantial differences frombulk.41 After sandblasting procedure performed by themanufacturer, large differences in the XPS elementalcomposition were identified for the collar and threadedroot of the commercial implants. These values mayimply that the residual Al2O3 particles are aggregatedin a thinner superficial layer. Other studies related toXPS analysis of tin oxide on glass surface demon-strated the presence of several valences of tin that gaverice to Sn 3d3/2 and Sn 3d5/2 typical peaks at494.70 eV and 486.24 eV, along with two additionalpeaks at 493.13 eV and 484.71 eV.42–45 The binding
energy of the doublet at 495.5 and 487.1 eV is ingood agreement with the data reported for In2O3–SnO2 films prepared using as starting material for tinoxide the hydrated stannic chloride (SnCl4 9
5H2O).43 By comparing the results presented in Fig. 5(a-c), we can notice that all the specimens present ahigh sensitivity to the SnF2 treatment. These resultsare in a good agreement with those obtained by ATRFTIR spectroscopy. To our knowledge, this is the firststudy dealing with the aspects of different fluoridetreatment applied to alumina/zirconia-sintered compos-ites. Although it is known that fluoride is responsiblefor the regulation of biomineralization process, thechemical process that combines zirconia dental ceram-ics with fluorine is still unexplained, as mentioned ina very recently published report on dental ZrO2-basedmaterials.46 The most well-documented effect of fluo-ride is that this ion substitutes for an hydroxyl in theapatite structure, giving rise to a reduction in crystalvolume and, consequently, a more stable structure.47
Free fluoride ions in solutions can react with apatitecrystal or biomaterial in several different ways, depend-ing on their concentrations and solution composition.Of course, further in vitro tests are required to be per-formed to establish a correlation between the effective-ness of surface treatment in improving the bioactivityof alumina/zirconia composites.
Conclusions
The composites investigated in this study aredesigned for orthopedic and dental implants, being pre-pared by Spark Plasma Sintering. The structural prop-erties of alumina and alumina/zirconia composites weredetermined by SEM analysis, X-ray diffraction, andFTIR spectroscopy. As showed by SEM micrographs,the grain growth of alumina particles was suppressed bythe addition of zirconia. No monoclinic phase of ZrO2
was detected from the XRD results, as supported alsoby the FTIR spectra. The samples were fluorinated toimprove the performances of these bioceramics as con-sidered for dental applications. The surface modifica-tion of the specimens upon different treatments withsodium tetrafluoroborate and stannous fluoride, respec-tively, was investigated by ATR FTIR and XPS. Quali-tative and quantitative results obtained by XPS andATR FTIR demonstrated that the proposed materialsare more sensitive to SnF2 than to NaBF4 treatment
8 International Journal of Applied Ceramic Technology—Cavalu, et al. 2013
for samples fluorination. These results support otherpreviously reported studies justifying the long-termeffectiveness of topical fluoride treatment in dentistryand maxillofacial applications.
Acknowledgments
This work was supported by the RomanianNational Authority for Scientific Research CNCS-UE-FISCDI, project PNII-ID-PCE 2011-3-0441 contract237/2011 and Bilateral Cooperation between Romaniaand Turkey 2012-2013.
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