Surface Modification of Alumina/ Zirconia Ceramics Upon Different Fluoride-Based Treatments

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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.

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Surface Modification of Alumina/ Zirconia Ceramics Upon Different Fluoride-Based Treatments

  1. 1. Surface Modification of Alumina/ Zirconia Ceramics Upon 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 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. *scavalu@rdslink.ro © 2013 The American Ceramic Society Int. J. Appl. Ceram. Technol., 1–10 (2013) DOI:10.1111/ijac.12075
  2. 2. Introduction Ceramics have a great potential in the biomedical field, thanks to their biocompatibility, strength, and wear resistance. The two dominant ceramic materials in clinical use today as bearing surfaces are still alumina (Al2O3) and zirconia (ZrO2).1–3 Alumina exhibits excellent hardness and wear properties; fracture tough- ness values are lower than those of the metals used in orthopedic 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 toughening improves 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 with physiological fluids.4–7 Zirconia, in contrast to alumina, is an unstable material, existing in three crystalline phases: monoclinic, tetragonal, and cubic. The tetrago- nal phase that is the most resistant tends to transform into the monoclinic phase under certain conditions (aging, thermal treatment). The addition of stabilizing materials such as Y2O3 or CeO2 during manufacture can control the phase transformation of zirconia.5–7 Therefore, the ideal ceramic for orthopedic and dental applications is a high-performance biocomposite mate- rial that combines the excellent material properties of alumina in terms of chemical stability, hydrothermal stability, biocompatibility, and extremely low wear and of zirconia with its superior mechanical strength and fracture toughness.8–11 On the other hand, the surfaces modification and postsynthesis treatment also influence the performances of the bioceramics designed to dental applications.12,13 It was demonstrated that the adminis- tration of complex fluorides as compared with NaF suggests the possibility of using them as effective agents in dental caries prevention in human populations.14,15 For example, stannous fluoride converts the calcium mineral apatite into fluorapatite, which makes tooth enamel more resistant to bacteria generated acid attacks. In toothpastes containing calcium minerals, sodium fluoride becomes ineffective over time, while stannous fluoride remains effective in strengthening tooth enamel. Stannous fluoride has been shown to be more effective than sodium fluoride in reducing the incidence of dental caries and controlling gingivitis.16 Further aspects related to the action of these new bioceramics upon different surface treatments on dentinal tissue are to be analyzed, to be properly used by professionals, so that they can make the best of properties during clini- cal applications.17 Even if increasing attention has been paid to elucidating the influence of fluoride chemistry in tooth mineralization, there are also some debates about the use of fluoride in osteoporosis treatment, par- ticularly concerning the beneficial effects on bone mass and quality.18 NaF has been known to be one of the most 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 possible beneficial effect of fluorination with respect to dental bioceramics. The surface modifications of alumina and alumina/zirconia bioceramics are investigated upon dif- ferent treatments with sodium tetrafluoroborate and stannous fluoride, respectively. The proposed bioceram- ics are designed for orthopedic or dental implants, being prepared by Spark Plasma Sintering.20 Using complementary spectroscopic tools such as Attenuated Total Reflection Fourier Transform Infrared Spectros- copy (ATR FTIR) and X-rays Photoelectron Spectros- copy (XPS), the chemical changes on the surface induced by fluoride treatment are discussed in terms of their effectiveness. Materials and Methods Preparation and Structural Characterization of Alumina and Alumina/Zirconia Specimens Al2O3 (Baikowski grade SM8, an average particle size of 0.6 lm), 3 mol% yttria stabilized ZrO2 (3YSZ, Tosoh grade, an average particle size of 0.1 lm), and TiO2 (Merck, an average particle size of 1 lm) pow- ders were used as starting materials. The raw materials were weighed in appropriate quantities, ball milled in ethanol for 24 h and then dried. A graphite die 5 mm inner diameter was used in the sintering process. Al2O3 and Al2O3–3YSZ composites with 5% (wt) TiO2 addi- tion were prepared using a spark plasma sintering method (SPS apparatus SPS-7.40 MK-VII Syntex, Fuji Electronic Industrial, Saitama, Japan) at 1350°C for 5 min with a heating rate of 100°C/min in vacuum, under a pressure of 40 MPa, resulting three different specimens with the chemical composition as follows: specimen 1- monolithic Al2O3; specimen 2- 80%Al2O3 À 20%3YSZ; specimen 3- 80%Al2O3 À 20%3YSZ + 5%TiO2. The specific Al2O3/3YSZ ratio was chosen because it was previously demon- 2 International Journal of Applied Ceramic Technology—Cavalu, et al. 2013
  3. 3. strated that zirconia has a reinforcing effect up to 30%.20 In the same time, as a result of our optimiza- tion studies (not presented here), it was found that 5 wt% TiO2 addition had a remarkable effect with respect to their mechanical properties. Structural char- acterization of the specimens was made by FTIR spec- troscopy (BXII spectrometer using K Br pellet technique, resolution of 2/cm, at room temperature; Perkin-Elmer, Waltham, MA), and X-ray diffraction analysis carried out with a Shimadzu XRD- 600 diffrac- tometer, using Cu-Ka radiation (k = 1.5418 A) with Ni-filter. The morphology of the specimen surface (on fracture) was investigated by scanning electron micros- copy (JSM 7000F, JEOL, Tokyo, Japan). Fluoride Surface Treatment and Surface Investigation of the Specimens High purity stannous fluoride (Tin II fluoride) and sodium tetrafluoroborate (Sigma Aldrich, St. Louis, MO) were used to prepare saturated solutions (0.4 g/ mL and 1 g/mL, respectively) for surface treatment of the specimens by conventional anodization during 2 h at 12V. Upon the anodization treatment, the specimens were ultrasonically treated for 90 min to remove the deposits, then air-dried. The modifications of samples surface upon both fluoride treatment were investigated by ATR FTIR spectroscopy using ATR Miracle device (single reflection with ZnSe crystal) and XPS measure- ments performed with SPECS PHOIBOS 150 MCD system equipped with monochromatic Al-Ka source (250W, hm = 1486.64 eV) and Epass = 50 eV, with a resolution of 1 eV/step. The vacuum in the analysis chamber during the measurements was kept in the range 10À9 –10À10 mbar. All binding energies were referenced to the C 1 s peak arising from adventitious carbon at 284.6 eV. The peak areas combined with the appropriate sensitivity factors allowed to quantify the elemental composition at 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 details of the fractured surfaces of the proposed specimens were evidenced by SEM analysis and presented in Fig. 1. The details including the size and shape of the alumina (micron size, gray) and zirconia (submicron, bright white) grains clearly demonstrate that Spark Plasma Sintering makes possible the densification of Al2O3 based composites at a lower temperature and in a shorter time compared with some other conventional techniques.9,11,21 Furthermore, the microstructure and grain size can be controlled by a fast heating rate and shorter processing time. The structural details were observed from the analysis of the FTIR spectra recorded between 400 and 1400/cm and presented in Fig. 2. The FTIR spectra are dominated by absorption lines arising from Al2O3 phase (1088/cm, 780 and 797/cm). The addition of zirconia phase clearly modi- fies the relative intensity of these bands. The vibration of Zr-O bond in tetragonal phase is visible at 518 and 580/cm. A superposition of the characteristic absorp- tion bands occurs in the spectral region 500–650/cm upon TiO2 addition to alumina/zirconia matrix and, as a consequence, Ti-O vibration band cannot be distin- guished. The XRD patterns of the proposed specimens are presented in Fig. 3 showing the characteristic peaks of a-corundum (JCPDS: 30-0415) and tetragonal zirconia (JCPDS: 42-1164). The reflection lines occur- ring from crystallographic planes related to a-corundum are 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 to 2h = 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 peaks at 2h = 26.4 and 36° in specimen 3. No monoclinic phase of ZrO2 was detected from the XRD results. Surface Modification Upon Fluoride Treatments Investigated by ATR FTIR and XPS Spectroscopy In Fig. 4a are presented the vibrational ATR FTIR details of both fluoride as received from the supplier (crystalline powder). The fingerprints of SnF2 are observed at 492, respectively, 548/cm and assigned to symmetric and asymmetric stretch mode, whereas for NaBF4, the marker bands in the selected region are 443, 472, 498, and 575/cm, as the [BF4] species belongs to a symmetry group with four normal modes of vibration. For the wide range of tetrafluoroborates and other [XF4] compounds (X = C, Si, Al, Ge, N, P, www.ceramics.org/ACT Alumina Zirconia Bioceramics 3
  4. 4. etc.), the position of the normal modes follows the trend: m3 m1 m4 m2. Upon the fluoride treatment, the surface of the specimens was strongly affected as revealed by the ATR FTIR spectra presented in Fig. 4(b-d). The marker bands of both SnF2 and NaBF4 can be observed along with 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 the specimens before and after fluoride treatment are pre- sented comparatively in Fig. 5. The main photoelectron peaks in the spectra of the specimens before treatments are assigned to Al 2s (117.9 eV), Al 2p (74.3 eV), O 1s (531.8 eV) (specimen 1), Zr 3d (180 eV), and Ti 2p (456 eV) (specimen 2 and 3 respectively). After SnF2 treatment, a strong peak at 487.1 eV indicates the contribution of Sn 3d electrons, while the presence of fluorine is proved by F 1s photoelectrons peak at 685 eV. These marker peaks are strongly visible for all the specimens, but as presented in Table 1, the atomic concentration of the elements shows a higher percent of 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 the EDAX 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
  5. 5. compared with the monolithic Al2O3. With respect to the NaBF4 treatment, the marker peaks in this case are F1s 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 observed toward the specimen 3. The results obtained by both XPS and ATR FTIR spectroscopy show a good correla- tion from the standpoint of qualitative and quantitative aspects. Discussion To overcome the low toughness of alumina and the aging sensitivity of zirconia, alumina-zirconia, com- posites have been proposed for biomedical applications. The toughening mechanism in ZTA ceramics (zirconia toughened alumina) is related to structural properties of these materials, conferred especially by zirconia due to 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 the inhibition of grain growth. Fine zirconia particles located on the boundaries inhibit the movement and prevent the grain growth of alumina (about 50% reduction in alumina grain size was observed). It has been previously demonstrated that the zirconia addition to alumina matrix promotes composites with higher densities, higher flexural strength, and fracture tough- ness.11,21 Moreover, as shown in Fig. 1 (e, f), adding TiO2 particles is more effective, as the size of alumina grains is reduced by comparison with Fig.1 (c, d). A special behavior with respect to the evolution of the structural units present in these samples was observed from the analysis of the FTIR spectra recorded between 400 and 1400 cm (Fig. 2). The correlation between IR (a) (b) (c) Fig. 2. Fourier transform infrared spectroscopy (FTIR) spectra of alumina and alumina-zirconia specimens: (a) specimen 1, (b) specimen 2, and (c) specimen 3. 0 10 20 30 40 50 60 70 80 90 100 0 500 1000 1500 2000 T T A A AZ AA A Z A A Z Z Z A A A A AA AA A (b) (a) Intensity(a.u.) 2 theta (degrees) (c) Fig. 3. XRD patterns of specimen 1 (a), specimen 2 (b), and specimen 3 (c). www.ceramics.org/ACT Alumina Zirconia Bioceramics 5
  6. 6. 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 at 780 and 797/cm. The aluminum atoms are differently coordinated, usually by four or six oxygen atoms, and less likely by five oxygens. The absorption bands and shoulders recorded in the spectral region between 465 and 648/cm are assigned to six coordinated aluminum which 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, the authors 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-O vibrations in monoclinic ZrO2 and bands at 510/cm and 590/cm corresponding to Zr-O vibrations in tetragonal ZrO2. In our spectra, the vibrations of Zr-O in tetragonal phase are visible at 518 and 580/cm. Moreover, upon TiO2 addition to alumina-zirconia matrix, the relative intensity of 648/617/cm is consider- ably modified, as a superposition of the characteristics absorption bands occurs in this region.29 The analysis of XRD patterns (Fig. 3) led to results that are in agreement 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 as received 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
  7. 7. 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 same time, the intensity ratio of the main peaks for alumina and zirconia is in agreement with the ZrO2 content in samples. The results demonstrate that the high density of the matrix correlated with the optimization of the zirconia particles microstructure can assure the parame- ters of better material performances.33 According to their interaction with surrounding tissue, 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 layer with bone due to the chemically inert nature of these two stable oxides.34 It has been demonstrated that sur- face morphology and bone–implant interactions deter- mine the predictability of endosseous implant bone integration.13,35 Different surface treatments such as Table 1. Atomic concentration of Sn, F, and Na on the surface of the specimens after fluoride treatment determined from X-rays photoelectron spectroscopy (XPS) survey spectra Specimen Elemental composition (at %) Sn F NaSnF4 NaBF4 1 3.4 4.9 3.2 2.1 2 12.8 3.9 2.4 1.9 3 12.4 3.3 6.8 4.2 1200 1000 800 600 400 200 0 O1s F1s O2s Al2s Al2p F1s Al2p OAuger Na1s O1s C1s Intensity(a.u) Binding Energy (eV) Sn3d Al2s O2s Sn4dF2sNa2p Specimen 1 SnF2 NaBF4 1200 1000 800 600 400 200 0 F1s Al2s Zr3d Al2p C1s N1s O1s Sn4dZr4pF2s Sn3p 1 Sn3d Zr3d N1s F1s Al2p Na1s O1s C1s Intensity(a.u) Binding Energy (eV) Sn3p 3 Al2s OAuger Zr4p Specimen 2 SnF2 NaBF4 1200 1000 800 600 400 200 0 Sn4d,Zr4p,F2s,Na2p,Ti3p B1s FAuger Al2s Al2p OAuger F1s Al2p C1s Ti2p O1sO1s Sn3p 1 Sn3d F1s Na1s O1s C1s Intensity(a.u.) Binding energy (eV) Al2s Zr3d Ti2p OAuger Sn3p 3 SnAuger Specimen 3 SnF2 NaBF4 (a) (b) (c) Fig. 5. X-rays photoelectron spectroscopy (XPS) survey spectra of specimen 1 (a), specimen 2 (b), and specimen 3 (c) before and after treatment with SnF2 and NaBF4. www.ceramics.org/ACT Alumina Zirconia Bioceramics 7
  8. 8. surface blasting or acid etching can increase the rate and amount of new bone formation on the implant surface. Sandblasting procedure may be performed using either medium or large grit Al2O3 particles, whereas acid-etching process can employ hydrofluoric acid/nitric acid. Some authors36 evaluated and reported the apatite-forming ability of a zirconia/alumina nano- composite (10Ce-TZP/Al2O3) in SBF as a result of the formation of ZrAlOH groups on the surface after chemical 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 the surfaces of abutments and fixtures of implants.37 Sev- eral in vitro and in vivo studies have demonstrated that the surface structure of implant abutments influences both the orientation and proliferation of connective tis- sue cells and inhibits epithelial downgrowth.38 In this study, the surface modifications of the proposed alu- mina and alumina/zirconia ceramics upon different fluoride treatments are emphasized by complementary techniques ATR FTIR and XPS spectroscopy. The ATR FTIR spectra recorded on the specimens’ surface (Fig. 4) clearly demonstrate that the surface is being treated, emphasized by the presence of the marker bands of both SnF2 and NaBF4 according to their spe- cific vibration modes.39,40 By comparing with the FTIR spectra of the specimens before fluoride treat- ments (Fig. 2), the changes are evident. On the other hand, taking account of the relative intensities of the fluoride marker bands with respect to each specimen, one can observe that, even after the removal of the surface deposits, different fluoride concentration can be detected on the surface. To obtain more details, XPS survey spectra were recorded on the specimens’ surface before and after fluoride treatment (Fig. 5). In some previous studies, XPS has been successfully used to investigate the surface chemistry of the commercial zirconia implants, showing substantial differences from bulk.41 After sandblasting procedure performed by the manufacturer, large differences in the XPS elemental composition were identified for the collar and threaded root of the commercial implants. These values may imply that the residual Al2O3 particles are aggregated in a thinner superficial layer. Other studies related to XPS analysis of tin oxide on glass surface demon- strated the presence of several valences of tin that gave rice to Sn 3d3/2 and Sn 3d5/2 typical peaks at 494.70 eV and 486.24 eV, along with two additional peaks at 493.13 eV and 484.71 eV.42–45 The binding energy of the doublet at 495.5 and 487.1 eV is in good agreement with the data reported for In2O3– SnO2 films prepared using as starting material for tin oxide 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 a high sensitivity to the SnF2 treatment. These results are in a good agreement with those obtained by ATR FTIR spectroscopy. To our knowledge, this is the first study dealing with the aspects of different fluoride treatment applied to alumina/zirconia-sintered compos- ites. Although it is known that fluoride is responsible for the regulation of biomineralization process, the chemical process that combines zirconia dental ceram- ics with fluorine is still unexplained, as mentioned in a very recently published report on dental ZrO2-based materials.46 The most well-documented effect of fluo- ride is that this ion substitutes for an hydroxyl in the apatite structure, giving rise to a reduction in crystal volume and, consequently, a more stable structure.47 Free fluoride ions in solutions can react with apatite crystal 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 bioactivity of alumina/zirconia composites. Conclusions The composites investigated in this study are designed for orthopedic and dental implants, being pre- pared by Spark Plasma Sintering. The structural prop- erties of alumina and alumina/zirconia composites were determined by SEM analysis, X-ray diffraction, and FTIR spectroscopy. As showed by SEM micrographs, the grain growth of alumina particles was suppressed by the addition of zirconia. No monoclinic phase of ZrO2 was detected from the XRD results, as supported also by the FTIR spectra. The samples were fluorinated to improve the performances of these bioceramics as con- sidered for dental applications. The surface modifica- tion of the specimens upon different treatments with sodium tetrafluoroborate and stannous fluoride, respec- tively, was investigated by ATR FTIR and XPS. Quali- tative and quantitative results obtained by XPS and ATR FTIR demonstrated that the proposed materials are more sensitive to SnF2 than to NaBF4 treatment 8 International Journal of Applied Ceramic Technology—Cavalu, et al. 2013
  9. 9. for samples fluorination. These results support other previously reported studies justifying the long-term effectiveness of topical fluoride treatment in dentistry and maxillofacial applications. Acknowledgments This work was supported by the Romanian National Authority for Scientific Research CNCS-UE- FISCDI, project PNII-ID-PCE 2011-3-0441 contract 237/2011 and Bilateral Cooperation between Romania and Turkey 2012-2013. References 1. I. Denry and J. R. Kelly, “State of the Art of Zirconia for Dental Applica- tions,” Dent. Mater., 24 299–307 (2008). 2. J. A. D’Antonio and K. Sutton, “Ceramic Materials as Bearing Surfaces for Total Hip Arthroplasty,” J. Am. Acad. Orthop. Surg., 17 63–68 (2009). 3. L. W. Hobbs, V. B. Rosen, S. P. Mangin, M. Treska, and G. Hunter, “Oxidation Microstructures and Interfaces in the Oxidized Zirconium Knee,” Int. J. Appl. Ceram. Technol., 2 221–246 (2005). 4. T. Kosmac, A. Dakskobler, C. Oblak, and P. 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