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Methacrylate bonding to zirconia by in situ silica nanoparticle surface deposition Aline Oliveira-Ogliari a , Fabrício M. Collares b , Victor P. Feitosa c , Salvatore Sauro d,∗ , Fabrício A. Ogliari e , Rafael R. Moraes a a
Biomaterials Development and Control Center, School of Dentistry, Federal University of Pelotas, Pelotas, Brazil Dental Materials Laboratory, School of Dentistry, Federal University of Rio Grande do Sul, Porto Alegre, Brazil c School of Dentistry, Federal University of Ceará, Fortaleza, Brazil d Dental Biomaterials and Minimally Invasive Dentistry, Departamento de Odontología, Universidad CEU-Cardenal Herrera, Valencia, Spain e School of Materials Engineering, Federal University of Pelotas, Pelotas, Brazil b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. This study introduces an innovative method to enhance adhesion of methacrylate-
Received 25 February 2014
based cements to yttria-stabilized zirconia (Y-TZP) by means of a silica-nanoparticle
Received in revised form
deposition process.
23 July 2014
Methods. Two alkoxide organic precursors, tetraethyl-orthosilicate (TEOS) and zirconium
Accepted 14 November 2014
tert-butoxide (ZTB) were diluted in hexane at different concentrations in order to obtain several experimental materials to enhance deposition of a SiOx reactive layer to Y-TZP. This deposition was attained via sintering alkoxide precursors directly on pre-sintered zirconia
Keywords:
(infiltration method—INF) or application on the surface of fully sintered zirconia (coating
Biomaterials
method—COA). Untreated specimens and a commercial tribochemical silica coating were
Bonding
also tested as controls and all the treated Y-TZP specimens were analyzed using SEM-EDX.
Ceramics
Specimens were bonded using silane, adhesive and dual-cure luting cement and submitted
Silica-coating
to shear bond strength test after different water storage periods (24 h, 3-, 6- and 12-months).
Yttria-stabilized polycrystalline
Results. SEM-EDX revealed Y-TZP surface covered by silica nanoclusters. The morphology
zirconia
of silica-covered Y-TZP surfaces was influenced by sintering method, employed to deposit nanoclusters. High bond strength (MPa) was observed when using COA method; highest TEOS percentage achieved the greatest bond strengths to Y-TZP surface (36.7 ± 6.3 at 24 h). However, bonds stability was dependent on ZTB presence (32.9 ± 9.7 at 3 months; 32.3 ± 7.1 at 6 months). Regarding INF method, the highest and more stable resin–zirconia bond strength was attained when using experimental solutions containing TEOS and no ZTB. Both sintering methods tested in this study were able to achieve a bonding performance similar to that of classic tribochemical strategies.
∗
Corresponding author. Tel.: +34 96 136 90 00x64302; fax: +34 96 136 90 00x64302. E-mail addresses: [email protected], [email protected] (S. Sauro).
http://dx.doi.org/10.1016/j.dental.2014.11.011 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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Significance. This study demonstrates that it is possible to achieve a reliable and long-lasting bonding between yttria-stabilized zirconia ceramic and methacrylate-based cements when using this novel, simple, and cost-effective bonding approach. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The use of yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) ceramics increases every year in biomedical fields. In dentistry, Y-TZP bioceramics can be employed as bone implants due to their excellent osseointegration and biocompatibility properties. Moreover, Y-TZP ceramics are commonly used for indirect dental restorations as they possess excellent esthetics and mechanical properties (i.e. high fracture toughness) as well as chemical inertia. The introduction of novel CAD-CAM technologies has simplified laboratory procedures of Y-TZP for dental prosthesis with fewer occurrences of prefails during processing. A significant shortcoming of Y-TZP ceramics may be attributed to a lack of bonding performance when using methacrylate-based resin cements. This limits the use of YTZP ceramics in dental preparations with reduced frictional retention (e.g. teeth with short or conical abutments) such as inlays, onlays and full dental crowns [2]. Two main factors are responsible: (1) The homogeneous single-phase structure of Y-TZP is highly dense; hence impeding formation of selective micro-retentions [1]; (2) the absence of silica (glass-phase) in YTZP structure which avoids formation of chemical bonds when using organo-silanes. Although alternative methods have been advocated to enhance adhesion to Y-TZP ceramics, a reliable, practical, and cost-effective method is still needed to overcome this issue entirely. Undeniably, one of the most recent methods to improve bonding between Y-TZP ceramics and resin-based materials is based on inclusion of functional phosphoric-acid ester monomers within resin cements composition which have some chemical interaction with zirconia. Nevertheless, despite good initial adhesion results reported, bonding longevity is still unreliable and requires further improvements [3–5]. A further approach based on tribochemical deposition of silica on zirconia surface by means of air-blasting devices has shown good results, but occasionally inconsistent as a significant drop in bond strength has been reported after aging [6,7]. Alternative methods such as vapor deposition of SiCl4 [8] and SiO2 fusion by plasma treatment [9,10] have been proposed; however, these are still expensive, complex, and require specific equipment and a high level of proficiency. It has been shown that reactivity of Y-TZP ceramics with resin cements may be addressed by depositing silica layer onto Y-TZP surface [11,12] as well as deposition of a silicacontaining layer through application and calcination of a silane-based solution making it effective in providing early bonding to Y-TZP ceramics [13]. Therefore, the aim of the present study was to generate an innovative strategy to provide a long-lasting bonding to Y-TZP ceramics. The method evaluated in this study consisted of a simplified direct and
Table 1 – Formulation of the experimental solutions (% mass). Solution
100:00 75:25 50:50 25:75
Alkoxide precursor TEOS
ZTB
5 3.75 2.5 1.25
0 1.25 2.5 3.75
Hexane
95 95 95 95
TEOS: tetraethyl orthosilicate; ZTB: zirconium tert-butoxide.
Fig. 1 – Molecular structure of the organic-metallic precursors tested. TEOS: tetraethyl-orthosilicate; ZTB: zirconium tert-butoxide.
indirect nano-silica-coating method using organic silica (Si) and zirconia (Zr) alkoxy precursors at different concentrations. The hypothesis tested was that the use of organic Si/Zr precursors would induce silica deposition on zirconia surface and enhance bonding performance of resin cements to Y-TZP ceramic.
2.
Materials and methods
2.1. Preparation of experimental solutions and sintering methods Four experimental solutions were prepared using tetraethyl orthosilicate (TEOS) and zirconium tert-butoxide (ZTB) diluted in hexane. The use of zirconia precursors mixed with silica precursors was tested in order to increase the compatibility of surface coating with zirconia substrate. All tested compositions are shown in detail in Table 1, while molecular structures of metallic precursors are depicted in Fig. 1. Pre-sintered blocks (40 × 19 × 19 mm) of a Y-TZP dental ceramic (Zircon-CAD; Angelus, Londrina, PR, Brazil) were used in this study. They were initially cut into smaller blocks (10 × 9 × 9 mm dimensions) using a diamond saw, and then
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Fig. 2 – SEM micrographs of treated and untreated zirconia surfaces. The control surface was clean, with grooves due to the polishing procedure (A and B). The surface treated with Rocatec Plus had a rough particle deposition (C and D). The group COA 100:00 (E and F) had surfaces that were entirely covered by silica nanoparticle clusters, while the formation of nanoparticle clusters covering the zirconia surface had irregularity and voids for the group INF 100:00 (G and H).
polished with no. 320, 400, 600, and 1200 SiC-papers under water-cooling (Aropol E; Arotec, Cotia, SP, Brazil). The following silica-coating processes were performed before or after Y-TZP ceramic specimens sintering.
Brazil) using the protocol recommended by the manufacturer: heating rate of 100 ◦ C/h until reaching 1350 ◦ C and maintained constant for 2 h.
2.1.1.
2.1.2.
Silica coating before zirconia sintering (INF method)
Pre-sintered zirconia blocks were immersed in the experimental solutions for 5 min in order to allow maximum infiltration of organic precursors into the Y-TZP, as observed in a pilot study. Subsequent to infiltration period, the zirconia-infiltrated specimens were fully sintered in a computer-controlled furnace (FEZ-1600/4; INTI, São Carlos, SP,
Silica coating after zirconia sintering (COA method)
Zirconia blocks were fully sintered as previously described. The surface of blocks was coated with ∼100 L of prepared solutions with organic precursors, followed by a heat treatment consisting of heating rate of 10 ◦ C/min until 800 ◦ C and maintained constant for 2 h in order to condensate the SiOx network and evaporate the solvent.
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A negative control group (untreated sintered zirconia specimens) as well as sintered zirconia specimens treated with a commercial tribochemical silica deposition method (Rocatec Plus; 3M ESPE, St. Paul, MN, USA) were also tested; the latter was applied according to the manufacturer’s instructions.
2.2.
SEM-EDX analysis
Specimens treated with different solutions and infiltration/sintering methods were non-sputter coated or sputtercoated with carbon 10 s without significantly affect the surface morphology of specimens and compositional analysis. The microstructural evaluation was performed using a scanning electron microscopy (SEM-SSX-550; Shimadzu, Tokyo, Japan) along with an x-ray energy dispersive spectroscopy (EDX) for elemental analysis.
2.3.
Bond strength test and failure analysis
Specimens of all groups tested in this study received a layer of organo-silane (Silano; Angelus), which was applied onto the entire surface and solvent evaporated following manufacturer’s instructions. Subsequently, a layer of solvent-free adhesive (Scotchbond Multipurpose; 3M ESPE) was applied with a microbrush and polyvinylsiloxane molds (thickness 0.5 mm, diameter 1.5 mm) were placed onto the surface of zirconia blocks. The adhesive was light-cured for 20 s using a light-emitting diode curing unit (Radii-Cal; SDI, Bayswater, Australia) with 1200 mW/cm2 irradiance, and the molds were filled with a regular, dual-cure resin cement (RelyX ARC; 3M ESPE). A polyester strip and glass slide were placed onto the filled molds, and the cement was light-cured for 40 s. For each group, 10 resin–cement cylinders were built up on ceramic surfaces. The samples were stored in distilled water at 37 ◦ C for 24 h, 3 months, 6 months and 1 year; water was replaced every month. A stainless steel wire (diameter 0.2 mm) was looped around each cement cylinder, aligned with bonding interface and shear test performed using a mechanical testing machine (DL500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until failure. The fractured specimens were observed using a stereomicroscope (40×). Failures were classified as mixed failure (remnants of cement left on ceramic) or adhesive failure (interfacial debonding). Bond strength values were calculated in MPa and data analyzed using two-way ANOVA (surface treatment × storage time). All pairwise multiple comparison procedures were performed by the Student–Newman–Keuls’ method (˛ = 0.05).
3.
Results
3.1.
SEM-EDX analysis
SEM micrographs of untreated zirconia specimens, those treated with Rocatec Plus, and zirconia ceramics subjected to experimental coatings (COA and INF methods) using silica organic precursors (100:00) are illustrated in Fig. 2. The control surfaces (Fig. 2A and B) presented a highly homogeneous and dense appearance with characteristic grooves induced by
Fig. 3 – SEM micrograph of a treated zirconia surface (group COA 50:50). Densely clustered nanoparticles were observed, and the silica composition was confirmed by the EDX analysis.
polishing process. The specimens treated with Rocatec Plus (Fig. 2C and D) showed a heterogeneously dispersed particle deposition, which were apparently poorly bound to substrate. In experimental COA treatment (Fig. 2E and F), the surface was entirely and homogeneously covered by silica nanoparticles, whilst several irregularities and voids on coating were noticed in specimens treated using the INF method (Fig. 2G and H). Fig. 3 demonstrates densely agglomerated nanoparticle clusters formation, approximately 140 nm to 300 nm in size, that were deposited on surface of zirconia treated with experimental COA method. The EDX analysis confirmed that nanoagglomerates were mainly composed by silica (Fig. 4) deposited on entire zirconia surface, whereas the untreated zirconia presented no conspicuous Si content. In Fig. 5, it is possible to see SEM micrographs comparing COA and INF methods when using silica precursors at the lowest content (25:75). The COA method (Fig. 5A and B) had a typical profile of phase separation between silica and zirconia precursors (also shown in Fig. 4 for COA 50:50), with silica dispersed as nanoparticles coated by a layer composed of zirconia. Conversely, this was not observed for the INF method (Fig. 5C and D), which revealed a more homogeneous layer.
3.2.
Bond strength test and failure analysis
The results of strength test are shown in Table 2. The factors ‘surface treatment’ and ‘storage time’ were both statistically significant (p < 0.001), as well as interaction between the two factors (p = 0.022). Higher bond strengths were generally observed for experimental groups and Rocatec Plus when compared to control group. For the COA method, the solution with highest silica content (100:00) showed higher shortterm bond strength, although bond stability was dependent on presence of zirconia precursors in solution. The shortterm bonding potential of INF method was generally lower compared to COA method. For the INF method, more stable zirconia bonds were observed with solutions with lowest silica content (25:75). Results for failure analysis are shown in
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Fig. 4 – The EDX analysis confirmed that the Si is deposited all over the treated zirconia surface (method COA 50:50 in this example), whereas the control, untreated surface had no appreciable Si content. Table 2 – Means (standard deviations) of shear bond strength to Y-TZP in MPa (n = 10). Storage timeb
Groupa 24 h Control COA 100:00 COA 75:25 COA 50:50 COA 25:75 INF 100:00 INF 75:25 INF 50:50 INF 25:75 Rocatec Plus a
b
3 months AB,c
14.0 (8.0) 36.7 (6.3)A,a 24.6 (5.6)A,b 33.8 (6.4)A,a 23.7 (8.5)A,b 23.2 (10.6)A,b 15.4 (5.2)AB,bc 29.1 (6.0)A,ab 20.3 (2.0)A,bc 33.3 (9.0)A,a
AB,c
14.9 (6.2) 32.0 (9.7)A,a 22.7 (7.3)A,abc 31.9 (7.0)A,a 19.2 (8.0)A,abc 21.3 (10.3)A,bc 21.0 (9.8)A,bc 25.7 (7.9)AB,ab 20.6 (6.7)A,bc 24.7 (8.3)C,ab
6 months A,c
17.7 (6.4) 32.3 (7.1)A,ab 23.2 (8.6)A,c 31.7 (6.4)A,ab 21.6 (8.3)A,c 26.6 (6.9)A,abc 21.5 (6.5)A,c 23.9 (6.8)AB,bc 19.3 (7.7)A,c 33.0 (7.5)AB,a
1 year 7.1 (4.4)B,d 23.2 (7.9)B,ab 15.9 (5.7)A,bc 25.7 (6.2)A,a 22.7 (9.6)A,ab 27.0 (4.8)A,a 12.0 (6.4)B,cd 19.5 (9.3)B,abc 18.6 (8.8)A,abc 26.8 (6.0)BC,a
Groups labeled as COA: experimental silica coating was carried out after full zirconia sintering; groups labeled as INF: experimental silica coating was carried out before zirconia sintering. Uppercase letters in a same row indicate significant differences between storage times. Lowercase letters in a same column indicate significant differences between surface treatments (˛ = 0.05).
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Fig. 5 – SEM micrographs comparing the COA (A and B) and INF (C and D) methods with the lowest content of silica precursors (25:75). In (A) and (B), the coating method showed a typical profile of phase separation between the silica and zirconia precursors, with the silica dispersed as nanoparticles coated by a layer composed of zirconia with characteristics of a brittle material. A more homogeneous coating is observed in (C) and (D).
Fig. 6 – Frequency distribution of failure modes. A predominance of mixed failures was observed. The INF method generally had more adhesive failures than the COA method in the 24-h and 3-month analyses.
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Fig. 6 where a predominance of mixed failures was observed with INF method which mainly failed in adhesive mode; COA method failed prevalently in mixed mode after 24-h and 3month aging.
4.
Discussion
The use of Y-TZP ceramics in dentistry has become one of the main choices for restorative procedures requiring laboratory process to achieve a structural integrity and excellent esthetic. The mechanical properties of such dental ceramics, in particular fracture toughness, are much higher than other dental ceramics commonly used in prosthodontics (i.e. feldspathic and alumina). Y-TZP ceramics have become suitable alternatives to metal-ceramics for posterior and anterior restorations; as suitable performance of zirconia restorations have been demonstrated in clinical studies [14–16]. However, due to this structural integrity and chemical inertia [17,18], the main concern of Y-TZP dental ceramics is bonding performance of methacrylate-based resin cements when employed for luting purposes. The present study showed evidence of bonding between resin cement and untreated zirconia. This was likely due to micro-retentions created during polishing procedures in specimen preparation that allowed formation of a bonding interface based on micro-mechanical interlocking. This hypothesis is supported by SEM results attained for this group (untreated control) which showed prevalently mixed failures. These results are in accordance with those of a previous study [19] that showed bonding potential of resin cements to micro/macro-abraded Y-TZP ceramics. A further factor which may have also contributed in achieving these bond strength values may by attributed to the use of organo-silane methacrylates. Although chemical coupling to zirconia is not expected in this case, the silane might have improved wettability of resin components on substrate and increasing the bond strength [13]. Nevertheless, the control group showed a severe reduction in bond strength after 12 months of water storage indicating “weakness” of bonding interaction formed between organosilane and zirconia substrate. The impact of water storage on zirconia bond observed in the present study is in accordance with previous research reports [13,20]. Consequently, the hypothesis tested in this study was accepted since the method proposed and tested in this experimental project led to significant improvement of bonding to zirconia. Irrespective of silica deposition method tested (COA, INF, or Rocatec), an initial increase in bond strength values was generally attained as well as bond stability after water storage. This finding can be easily explained as surface treatments performed in this study present greater resistance to hydrolysis due to high strength energy (111 kcal/mol) of Si O Si siloxane bonds formed between silanol groups of organo-silane and silica layer deposited on zirconia surface; the ionic interactions between silane and zirconia is much weaker (less than 10 kcal/mol) [21]. A heterogeneous dispersion of silica particles was observed on zirconia surface treated with Rocatec Plus [7,17]. The roughness and silica deposition attained during this
latter treatment relies on air-abrasion (physical method) of silica-coated alumina particles. Conversely, the experimental COA method was able to uniformly coat the zirconia surface with silica nano-clusters (particle size between 140 nm and 300 nm) promoting an increase in surface area of glass phase available for bonding. Assuming a uniform spherical shape distribution of silica particles, it is possible to obtain a theoretical increase of bonding surface area (∼57%) with no need for further mechanical abrasion in order to attain better micro-mechanical interlocking. The profile of silica nanoparticles deposited on zirconia in experimental COA group is comparable to that obtained both when using hexamethyldisilazane and plasma on polymeric substrates of polyethylene naphthalate [22] as well as when using spray drying techniques [12]. Conversely, the INF method induced an inhomogeneous deposition of silica on zirconia surface and lower short-term bond strength compared with COA groups; INF resulted higher technique-sensitive when compared to COA method. This is likely due to the fact that only those silica precursors penetrating pre-sintered zirconia blocks and remained embedded on zirconia surface after sintering process, making it available for silane bonding. However, when lowering silica precursors content in COA method, a clear phase separation was observed on Y-TZP ceramic. This is probably due to a thermal incompatibility caused by different condensation rates between organometallic precursors of zirconia and silica during thermal processes; this phase-separation was not observed for INF method. Remarkably, coating method with only silica precursor (COA 100:00) achieved higher initial bond strength than method containing zirconia precursor. However, zirconia precursor presence provided more resistance against aging, in particular when used at 75% (COA 25:75). Similarly in INF method, stable zirconia bond strength (one year water storage) was observed when using solution with lowest silica and higher ZTB content. These results support the importance of ZTB to obtain more stable bonds. A feasible hypothesis is that ZTB might act as a ligand which allows better wetting of TEOS on zirconia surface creating a surface coating more resistant to hydrolytic degradation. Although the interaction is mainly physical and occurrence of covalent bonds at zirconia-silica interface is not expected, it is possible to consider establishment of secondary bonds such as Van der Waals. Considering the nanostructured characteristic of silica layer, interaction of resin-based materials may be also enhanced by larger contact area available. The zirconia substrate was not sandblasted before coating or infiltration as performed in previous studies [23–25]; this was to exclude further variability, but assess only the potential effects of proposed treatments. The need for sandblasting prior to usage of organo-metallic precursors might be considered a drawback of the technique once it has been reported that air-abrasion could affect long-term reliability of oxide ceramics [26]. Overall, INF method attained lower initial bonding potential compared to COA method. These outcomes might be attributed to a less homogeneous surface coating, as observed in SEM analysis, and less silica present on zirconia surface (Fig. 4C and D). Application of organo-metallic precursor solutions before zirconia sintering (INF method) could
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theoretically improve coating retention to ceramic structure since precursors and zirconia frame sintering occurs simultaneously; hence better physical entanglement of silica layer and zirconia would be expected. However, INF method seems to be more sensitive to processing variables, as uncoated areas or areas inconsistently coated were often observed. A -Raman analysis (data not shown) was also conducted in an attempt to investigate the thickness and in-depth homogeneity of silica layer. The analysis was conducted up to 10 m deep into coating layer and failed to identify any significant differences in silica concentration. It is postulated that coating had a thickness above 10 m, though further investigation is required to assess actual thickness of silica coat deposited on zirconia by methods proposed in this study. This information is essential since it may interfere with adaptation of ceramic structure on abutment teeth clinically in case a very thick silica layer is formed. It is also expected that silica coating thickness may be controlled by concentration of organometallic precursors used in treatment solution as well as through the volume of solution used. Further techniques such as spincoating or dip-coating, despite showing great efficiency in control of film thickness, would be difficult to reproduce in laboratory and clinical applications. Previous studies proposed silica films deposition on zirconia by magnetron sputtering in a custom-made vacuum chamber using a target of ultra-pure Si [27,28]. Although this technique is different from methods presented in this study, the morphology of silica layer formed as well as improved adhesion to zirconia were very similar to those observed in the present study. By comparison however, the present methods seem to be simpler and more feasible for application in dental laboratories. An additional issue concerning the technology proposed whether INF method could significantly affects structural integrity and sintering cycle of zirconia. From a structural point of view, INF method is somewhat audacious as it could alter the environmental conditions to which zirconia is subjected and also its mechanical properties. Studies evaluating dynamic and static mechanical strength should be performed in future investigations to clarify this condition. Notably, COA method appears to address the problem of yielding adhesion to zirconia ceramic clearer by using an easy and economical approach. In fact, this technique may be a feasible alternative to deposit silica onto zirconia in laboratory to improve subsequent adhesion with methacrylate-based materials. Furthermore, clinical studies are needed before broad indication of the techniques proposed in this study in order to assess their actual effectiveness, since any in vitro study has a number of limitations in predicting clinical performance of materials and techniques.
5.
Conclusion
Present study introduces a novel, simple, and cost-effective method to provide adhesion of methacrylates to yttriastabilized zirconia ceramic. The reagents used are safe, non-toxic, and reasonably inexpensive. The application technique used is also convenient as it demands same clinical and laboratory time employed in processing of oxide bioceramics.
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The sintering time and temperature used to process or glazing veneering ceramic could be used for SiOx condensation and networking when COA method is used, while sintering cycle of zirconia blocks could be used for INF method. Consequently, it may be concluded that experimental methods tested in this study may be useful alternative simplified low-cost approach to commercial tribochemical silicatization since they are able to provide a stable bond between resin cement and zirconia. The methods proposed could also allow bonding of other types of polymeric materials for varied applications of Y-TZP ceramics as biomaterials.
Acknowledgments A.O.O. is grateful to CAPES/Brazil for a MSc scholarship. R.R.M. is grateful for the support from CNPq/Brazil (protocol 308404/2011-4). F.A.O. is grateful for the support from FINEP/Brazil (grant 01.10.0709.00). The authors thank Angelus S/A for donating the zirconia blocks used in the study.
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[22] Petersen J, Bardon J, Dinia A, Ruch D, Gherardi N. Organosilicon coatings deposited in atmospheric pressure Townsend discharge for gas barrier purpose: effect of substrate temperature on structure and properties. ACS Appl Mater Interfaces 2012;4:5872–82. [23] Akin H, Ozkurt Z, Kirmali O, Kazazoglu E, Ozdemir AK. Shear bond strength of resin cement to zirconia ceramic after aluminum oxide sandblasting and various laser treatments. Photomed Laser Surg 2011;29:797–802. [24] Bhargava S, Doi H, Kondo R, Aoki H, Hanawa T, Kasugai S. Effect of sandblasting on the mechanical properties of Y-TZP zirconia. Biomed Mater Eng 2012;22:383–98. [25] Gomes AL, Castillo-Oyague R, Lynch CD, Montero J, Albaladejo A. Influence of sandblasting granulometry and resin cement composition on microtensile bond strength to zirconia ceramic for dental prosthetic frameworks. J Dent 2013;41:31–41. [26] Zhang Y, Lawn BR, Malament KA, Van Thompson P, Rekow ED. Damage accumulation and fatigue life of particle-abraded ceramics. Int J Prosthodont 2006;19:442–8. [27] Queiroz JR, Massi M, Nogueira Jr L, Sobrinho AS, Bottino MA, Ozcan M. Silica-based nano-coating on zirconia surfaces using reactive magnetron sputtering: effect on chemical adhesion of resin cements. J Adhes Dent 2013;15:151–9. [28] Queiroz JRC, Nogueira Junior L, Massi M, Silva AM, Bottino MA, Sobrinho ASS, et al. Si-based thin film coating on Y-TZP: influence of deposition parameters on adhesion of resin cement. Appl Surf Sci 2013;282:245–52.
Available online at www.sciencedirect.com
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Methacrylate bonding to zirconia by in situ silica nanoparticle surface deposition Aline Oliveira-Ogliari a , Fabrício M. Collares b , Victor P. Feitosa c , Salvatore Sauro d,∗ , Fabrício A. Ogliari e , Rafael R. Moraes a a
Biomaterials Development and Control Center, School of Dentistry, Federal University of Pelotas, Pelotas, Brazil Dental Materials Laboratory, School of Dentistry, Federal University of Rio Grande do Sul, Porto Alegre, Brazil c School of Dentistry, Federal University of Ceará, Fortaleza, Brazil d Dental Biomaterials and Minimally Invasive Dentistry, Departamento de Odontología, Universidad CEU-Cardenal Herrera, Valencia, Spain e School of Materials Engineering, Federal University of Pelotas, Pelotas, Brazil b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. This study introduces an innovative method to enhance adhesion of methacrylate-
Received 25 February 2014
based cements to yttria-stabilized zirconia (Y-TZP) by means of a silica-nanoparticle
Received in revised form
deposition process.
23 July 2014
Methods. Two alkoxide organic precursors, tetraethyl-orthosilicate (TEOS) and zirconium
Accepted 14 November 2014
tert-butoxide (ZTB) were diluted in hexane at different concentrations in order to obtain several experimental materials to enhance deposition of a SiOx reactive layer to Y-TZP. This deposition was attained via sintering alkoxide precursors directly on pre-sintered zirconia
Keywords:
(infiltration method—INF) or application on the surface of fully sintered zirconia (coating
Biomaterials
method—COA). Untreated specimens and a commercial tribochemical silica coating were
Bonding
also tested as controls and all the treated Y-TZP specimens were analyzed using SEM-EDX.
Ceramics
Specimens were bonded using silane, adhesive and dual-cure luting cement and submitted
Silica-coating
to shear bond strength test after different water storage periods (24 h, 3-, 6- and 12-months).
Yttria-stabilized polycrystalline
Results. SEM-EDX revealed Y-TZP surface covered by silica nanoclusters. The morphology
zirconia
of silica-covered Y-TZP surfaces was influenced by sintering method, employed to deposit nanoclusters. High bond strength (MPa) was observed when using COA method; highest TEOS percentage achieved the greatest bond strengths to Y-TZP surface (36.7 ± 6.3 at 24 h). However, bonds stability was dependent on ZTB presence (32.9 ± 9.7 at 3 months; 32.3 ± 7.1 at 6 months). Regarding INF method, the highest and more stable resin–zirconia bond strength was attained when using experimental solutions containing TEOS and no ZTB. Both sintering methods tested in this study were able to achieve a bonding performance similar to that of classic tribochemical strategies.
∗
Corresponding author. Tel.: +34 96 136 90 00x64302; fax: +34 96 136 90 00x64302. E-mail addresses: [email protected], [email protected] (S. Sauro).
http://dx.doi.org/10.1016/j.dental.2014.11.011 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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Significance. This study demonstrates that it is possible to achieve a reliable and long-lasting bonding between yttria-stabilized zirconia ceramic and methacrylate-based cements when using this novel, simple, and cost-effective bonding approach. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The use of yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) ceramics increases every year in biomedical fields. In dentistry, Y-TZP bioceramics can be employed as bone implants due to their excellent osseointegration and biocompatibility properties. Moreover, Y-TZP ceramics are commonly used for indirect dental restorations as they possess excellent esthetics and mechanical properties (i.e. high fracture toughness) as well as chemical inertia. The introduction of novel CAD-CAM technologies has simplified laboratory procedures of Y-TZP for dental prosthesis with fewer occurrences of prefails during processing. A significant shortcoming of Y-TZP ceramics may be attributed to a lack of bonding performance when using methacrylate-based resin cements. This limits the use of YTZP ceramics in dental preparations with reduced frictional retention (e.g. teeth with short or conical abutments) such as inlays, onlays and full dental crowns [2]. Two main factors are responsible: (1) The homogeneous single-phase structure of Y-TZP is highly dense; hence impeding formation of selective micro-retentions [1]; (2) the absence of silica (glass-phase) in YTZP structure which avoids formation of chemical bonds when using organo-silanes. Although alternative methods have been advocated to enhance adhesion to Y-TZP ceramics, a reliable, practical, and cost-effective method is still needed to overcome this issue entirely. Undeniably, one of the most recent methods to improve bonding between Y-TZP ceramics and resin-based materials is based on inclusion of functional phosphoric-acid ester monomers within resin cements composition which have some chemical interaction with zirconia. Nevertheless, despite good initial adhesion results reported, bonding longevity is still unreliable and requires further improvements [3–5]. A further approach based on tribochemical deposition of silica on zirconia surface by means of air-blasting devices has shown good results, but occasionally inconsistent as a significant drop in bond strength has been reported after aging [6,7]. Alternative methods such as vapor deposition of SiCl4 [8] and SiO2 fusion by plasma treatment [9,10] have been proposed; however, these are still expensive, complex, and require specific equipment and a high level of proficiency. It has been shown that reactivity of Y-TZP ceramics with resin cements may be addressed by depositing silica layer onto Y-TZP surface [11,12] as well as deposition of a silicacontaining layer through application and calcination of a silane-based solution making it effective in providing early bonding to Y-TZP ceramics [13]. Therefore, the aim of the present study was to generate an innovative strategy to provide a long-lasting bonding to Y-TZP ceramics. The method evaluated in this study consisted of a simplified direct and
Table 1 – Formulation of the experimental solutions (% mass). Solution
100:00 75:25 50:50 25:75
Alkoxide precursor TEOS
ZTB
5 3.75 2.5 1.25
0 1.25 2.5 3.75
Hexane
95 95 95 95
TEOS: tetraethyl orthosilicate; ZTB: zirconium tert-butoxide.
Fig. 1 – Molecular structure of the organic-metallic precursors tested. TEOS: tetraethyl-orthosilicate; ZTB: zirconium tert-butoxide.
indirect nano-silica-coating method using organic silica (Si) and zirconia (Zr) alkoxy precursors at different concentrations. The hypothesis tested was that the use of organic Si/Zr precursors would induce silica deposition on zirconia surface and enhance bonding performance of resin cements to Y-TZP ceramic.
2.
Materials and methods
2.1. Preparation of experimental solutions and sintering methods Four experimental solutions were prepared using tetraethyl orthosilicate (TEOS) and zirconium tert-butoxide (ZTB) diluted in hexane. The use of zirconia precursors mixed with silica precursors was tested in order to increase the compatibility of surface coating with zirconia substrate. All tested compositions are shown in detail in Table 1, while molecular structures of metallic precursors are depicted in Fig. 1. Pre-sintered blocks (40 × 19 × 19 mm) of a Y-TZP dental ceramic (Zircon-CAD; Angelus, Londrina, PR, Brazil) were used in this study. They were initially cut into smaller blocks (10 × 9 × 9 mm dimensions) using a diamond saw, and then
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Fig. 2 – SEM micrographs of treated and untreated zirconia surfaces. The control surface was clean, with grooves due to the polishing procedure (A and B). The surface treated with Rocatec Plus had a rough particle deposition (C and D). The group COA 100:00 (E and F) had surfaces that were entirely covered by silica nanoparticle clusters, while the formation of nanoparticle clusters covering the zirconia surface had irregularity and voids for the group INF 100:00 (G and H).
polished with no. 320, 400, 600, and 1200 SiC-papers under water-cooling (Aropol E; Arotec, Cotia, SP, Brazil). The following silica-coating processes were performed before or after Y-TZP ceramic specimens sintering.
Brazil) using the protocol recommended by the manufacturer: heating rate of 100 ◦ C/h until reaching 1350 ◦ C and maintained constant for 2 h.
2.1.1.
2.1.2.
Silica coating before zirconia sintering (INF method)
Pre-sintered zirconia blocks were immersed in the experimental solutions for 5 min in order to allow maximum infiltration of organic precursors into the Y-TZP, as observed in a pilot study. Subsequent to infiltration period, the zirconia-infiltrated specimens were fully sintered in a computer-controlled furnace (FEZ-1600/4; INTI, São Carlos, SP,
Silica coating after zirconia sintering (COA method)
Zirconia blocks were fully sintered as previously described. The surface of blocks was coated with ∼100 L of prepared solutions with organic precursors, followed by a heat treatment consisting of heating rate of 10 ◦ C/min until 800 ◦ C and maintained constant for 2 h in order to condensate the SiOx network and evaporate the solvent.
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A negative control group (untreated sintered zirconia specimens) as well as sintered zirconia specimens treated with a commercial tribochemical silica deposition method (Rocatec Plus; 3M ESPE, St. Paul, MN, USA) were also tested; the latter was applied according to the manufacturer’s instructions.
2.2.
SEM-EDX analysis
Specimens treated with different solutions and infiltration/sintering methods were non-sputter coated or sputtercoated with carbon 10 s without significantly affect the surface morphology of specimens and compositional analysis. The microstructural evaluation was performed using a scanning electron microscopy (SEM-SSX-550; Shimadzu, Tokyo, Japan) along with an x-ray energy dispersive spectroscopy (EDX) for elemental analysis.
2.3.
Bond strength test and failure analysis
Specimens of all groups tested in this study received a layer of organo-silane (Silano; Angelus), which was applied onto the entire surface and solvent evaporated following manufacturer’s instructions. Subsequently, a layer of solvent-free adhesive (Scotchbond Multipurpose; 3M ESPE) was applied with a microbrush and polyvinylsiloxane molds (thickness 0.5 mm, diameter 1.5 mm) were placed onto the surface of zirconia blocks. The adhesive was light-cured for 20 s using a light-emitting diode curing unit (Radii-Cal; SDI, Bayswater, Australia) with 1200 mW/cm2 irradiance, and the molds were filled with a regular, dual-cure resin cement (RelyX ARC; 3M ESPE). A polyester strip and glass slide were placed onto the filled molds, and the cement was light-cured for 40 s. For each group, 10 resin–cement cylinders were built up on ceramic surfaces. The samples were stored in distilled water at 37 ◦ C for 24 h, 3 months, 6 months and 1 year; water was replaced every month. A stainless steel wire (diameter 0.2 mm) was looped around each cement cylinder, aligned with bonding interface and shear test performed using a mechanical testing machine (DL500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until failure. The fractured specimens were observed using a stereomicroscope (40×). Failures were classified as mixed failure (remnants of cement left on ceramic) or adhesive failure (interfacial debonding). Bond strength values were calculated in MPa and data analyzed using two-way ANOVA (surface treatment × storage time). All pairwise multiple comparison procedures were performed by the Student–Newman–Keuls’ method (˛ = 0.05).
3.
Results
3.1.
SEM-EDX analysis
SEM micrographs of untreated zirconia specimens, those treated with Rocatec Plus, and zirconia ceramics subjected to experimental coatings (COA and INF methods) using silica organic precursors (100:00) are illustrated in Fig. 2. The control surfaces (Fig. 2A and B) presented a highly homogeneous and dense appearance with characteristic grooves induced by
Fig. 3 – SEM micrograph of a treated zirconia surface (group COA 50:50). Densely clustered nanoparticles were observed, and the silica composition was confirmed by the EDX analysis.
polishing process. The specimens treated with Rocatec Plus (Fig. 2C and D) showed a heterogeneously dispersed particle deposition, which were apparently poorly bound to substrate. In experimental COA treatment (Fig. 2E and F), the surface was entirely and homogeneously covered by silica nanoparticles, whilst several irregularities and voids on coating were noticed in specimens treated using the INF method (Fig. 2G and H). Fig. 3 demonstrates densely agglomerated nanoparticle clusters formation, approximately 140 nm to 300 nm in size, that were deposited on surface of zirconia treated with experimental COA method. The EDX analysis confirmed that nanoagglomerates were mainly composed by silica (Fig. 4) deposited on entire zirconia surface, whereas the untreated zirconia presented no conspicuous Si content. In Fig. 5, it is possible to see SEM micrographs comparing COA and INF methods when using silica precursors at the lowest content (25:75). The COA method (Fig. 5A and B) had a typical profile of phase separation between silica and zirconia precursors (also shown in Fig. 4 for COA 50:50), with silica dispersed as nanoparticles coated by a layer composed of zirconia. Conversely, this was not observed for the INF method (Fig. 5C and D), which revealed a more homogeneous layer.
3.2.
Bond strength test and failure analysis
The results of strength test are shown in Table 2. The factors ‘surface treatment’ and ‘storage time’ were both statistically significant (p < 0.001), as well as interaction between the two factors (p = 0.022). Higher bond strengths were generally observed for experimental groups and Rocatec Plus when compared to control group. For the COA method, the solution with highest silica content (100:00) showed higher shortterm bond strength, although bond stability was dependent on presence of zirconia precursors in solution. The shortterm bonding potential of INF method was generally lower compared to COA method. For the INF method, more stable zirconia bonds were observed with solutions with lowest silica content (25:75). Results for failure analysis are shown in
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Fig. 4 – The EDX analysis confirmed that the Si is deposited all over the treated zirconia surface (method COA 50:50 in this example), whereas the control, untreated surface had no appreciable Si content. Table 2 – Means (standard deviations) of shear bond strength to Y-TZP in MPa (n = 10). Storage timeb
Groupa 24 h Control COA 100:00 COA 75:25 COA 50:50 COA 25:75 INF 100:00 INF 75:25 INF 50:50 INF 25:75 Rocatec Plus a
b
3 months AB,c
14.0 (8.0) 36.7 (6.3)A,a 24.6 (5.6)A,b 33.8 (6.4)A,a 23.7 (8.5)A,b 23.2 (10.6)A,b 15.4 (5.2)AB,bc 29.1 (6.0)A,ab 20.3 (2.0)A,bc 33.3 (9.0)A,a
AB,c
14.9 (6.2) 32.0 (9.7)A,a 22.7 (7.3)A,abc 31.9 (7.0)A,a 19.2 (8.0)A,abc 21.3 (10.3)A,bc 21.0 (9.8)A,bc 25.7 (7.9)AB,ab 20.6 (6.7)A,bc 24.7 (8.3)C,ab
6 months A,c
17.7 (6.4) 32.3 (7.1)A,ab 23.2 (8.6)A,c 31.7 (6.4)A,ab 21.6 (8.3)A,c 26.6 (6.9)A,abc 21.5 (6.5)A,c 23.9 (6.8)AB,bc 19.3 (7.7)A,c 33.0 (7.5)AB,a
1 year 7.1 (4.4)B,d 23.2 (7.9)B,ab 15.9 (5.7)A,bc 25.7 (6.2)A,a 22.7 (9.6)A,ab 27.0 (4.8)A,a 12.0 (6.4)B,cd 19.5 (9.3)B,abc 18.6 (8.8)A,abc 26.8 (6.0)BC,a
Groups labeled as COA: experimental silica coating was carried out after full zirconia sintering; groups labeled as INF: experimental silica coating was carried out before zirconia sintering. Uppercase letters in a same row indicate significant differences between storage times. Lowercase letters in a same column indicate significant differences between surface treatments (˛ = 0.05).
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Fig. 5 – SEM micrographs comparing the COA (A and B) and INF (C and D) methods with the lowest content of silica precursors (25:75). In (A) and (B), the coating method showed a typical profile of phase separation between the silica and zirconia precursors, with the silica dispersed as nanoparticles coated by a layer composed of zirconia with characteristics of a brittle material. A more homogeneous coating is observed in (C) and (D).
Fig. 6 – Frequency distribution of failure modes. A predominance of mixed failures was observed. The INF method generally had more adhesive failures than the COA method in the 24-h and 3-month analyses.
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Fig. 6 where a predominance of mixed failures was observed with INF method which mainly failed in adhesive mode; COA method failed prevalently in mixed mode after 24-h and 3month aging.
4.
Discussion
The use of Y-TZP ceramics in dentistry has become one of the main choices for restorative procedures requiring laboratory process to achieve a structural integrity and excellent esthetic. The mechanical properties of such dental ceramics, in particular fracture toughness, are much higher than other dental ceramics commonly used in prosthodontics (i.e. feldspathic and alumina). Y-TZP ceramics have become suitable alternatives to metal-ceramics for posterior and anterior restorations; as suitable performance of zirconia restorations have been demonstrated in clinical studies [14–16]. However, due to this structural integrity and chemical inertia [17,18], the main concern of Y-TZP dental ceramics is bonding performance of methacrylate-based resin cements when employed for luting purposes. The present study showed evidence of bonding between resin cement and untreated zirconia. This was likely due to micro-retentions created during polishing procedures in specimen preparation that allowed formation of a bonding interface based on micro-mechanical interlocking. This hypothesis is supported by SEM results attained for this group (untreated control) which showed prevalently mixed failures. These results are in accordance with those of a previous study [19] that showed bonding potential of resin cements to micro/macro-abraded Y-TZP ceramics. A further factor which may have also contributed in achieving these bond strength values may by attributed to the use of organo-silane methacrylates. Although chemical coupling to zirconia is not expected in this case, the silane might have improved wettability of resin components on substrate and increasing the bond strength [13]. Nevertheless, the control group showed a severe reduction in bond strength after 12 months of water storage indicating “weakness” of bonding interaction formed between organosilane and zirconia substrate. The impact of water storage on zirconia bond observed in the present study is in accordance with previous research reports [13,20]. Consequently, the hypothesis tested in this study was accepted since the method proposed and tested in this experimental project led to significant improvement of bonding to zirconia. Irrespective of silica deposition method tested (COA, INF, or Rocatec), an initial increase in bond strength values was generally attained as well as bond stability after water storage. This finding can be easily explained as surface treatments performed in this study present greater resistance to hydrolysis due to high strength energy (111 kcal/mol) of Si O Si siloxane bonds formed between silanol groups of organo-silane and silica layer deposited on zirconia surface; the ionic interactions between silane and zirconia is much weaker (less than 10 kcal/mol) [21]. A heterogeneous dispersion of silica particles was observed on zirconia surface treated with Rocatec Plus [7,17]. The roughness and silica deposition attained during this
latter treatment relies on air-abrasion (physical method) of silica-coated alumina particles. Conversely, the experimental COA method was able to uniformly coat the zirconia surface with silica nano-clusters (particle size between 140 nm and 300 nm) promoting an increase in surface area of glass phase available for bonding. Assuming a uniform spherical shape distribution of silica particles, it is possible to obtain a theoretical increase of bonding surface area (∼57%) with no need for further mechanical abrasion in order to attain better micro-mechanical interlocking. The profile of silica nanoparticles deposited on zirconia in experimental COA group is comparable to that obtained both when using hexamethyldisilazane and plasma on polymeric substrates of polyethylene naphthalate [22] as well as when using spray drying techniques [12]. Conversely, the INF method induced an inhomogeneous deposition of silica on zirconia surface and lower short-term bond strength compared with COA groups; INF resulted higher technique-sensitive when compared to COA method. This is likely due to the fact that only those silica precursors penetrating pre-sintered zirconia blocks and remained embedded on zirconia surface after sintering process, making it available for silane bonding. However, when lowering silica precursors content in COA method, a clear phase separation was observed on Y-TZP ceramic. This is probably due to a thermal incompatibility caused by different condensation rates between organometallic precursors of zirconia and silica during thermal processes; this phase-separation was not observed for INF method. Remarkably, coating method with only silica precursor (COA 100:00) achieved higher initial bond strength than method containing zirconia precursor. However, zirconia precursor presence provided more resistance against aging, in particular when used at 75% (COA 25:75). Similarly in INF method, stable zirconia bond strength (one year water storage) was observed when using solution with lowest silica and higher ZTB content. These results support the importance of ZTB to obtain more stable bonds. A feasible hypothesis is that ZTB might act as a ligand which allows better wetting of TEOS on zirconia surface creating a surface coating more resistant to hydrolytic degradation. Although the interaction is mainly physical and occurrence of covalent bonds at zirconia-silica interface is not expected, it is possible to consider establishment of secondary bonds such as Van der Waals. Considering the nanostructured characteristic of silica layer, interaction of resin-based materials may be also enhanced by larger contact area available. The zirconia substrate was not sandblasted before coating or infiltration as performed in previous studies [23–25]; this was to exclude further variability, but assess only the potential effects of proposed treatments. The need for sandblasting prior to usage of organo-metallic precursors might be considered a drawback of the technique once it has been reported that air-abrasion could affect long-term reliability of oxide ceramics [26]. Overall, INF method attained lower initial bonding potential compared to COA method. These outcomes might be attributed to a less homogeneous surface coating, as observed in SEM analysis, and less silica present on zirconia surface (Fig. 4C and D). Application of organo-metallic precursor solutions before zirconia sintering (INF method) could
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theoretically improve coating retention to ceramic structure since precursors and zirconia frame sintering occurs simultaneously; hence better physical entanglement of silica layer and zirconia would be expected. However, INF method seems to be more sensitive to processing variables, as uncoated areas or areas inconsistently coated were often observed. A -Raman analysis (data not shown) was also conducted in an attempt to investigate the thickness and in-depth homogeneity of silica layer. The analysis was conducted up to 10 m deep into coating layer and failed to identify any significant differences in silica concentration. It is postulated that coating had a thickness above 10 m, though further investigation is required to assess actual thickness of silica coat deposited on zirconia by methods proposed in this study. This information is essential since it may interfere with adaptation of ceramic structure on abutment teeth clinically in case a very thick silica layer is formed. It is also expected that silica coating thickness may be controlled by concentration of organometallic precursors used in treatment solution as well as through the volume of solution used. Further techniques such as spincoating or dip-coating, despite showing great efficiency in control of film thickness, would be difficult to reproduce in laboratory and clinical applications. Previous studies proposed silica films deposition on zirconia by magnetron sputtering in a custom-made vacuum chamber using a target of ultra-pure Si [27,28]. Although this technique is different from methods presented in this study, the morphology of silica layer formed as well as improved adhesion to zirconia were very similar to those observed in the present study. By comparison however, the present methods seem to be simpler and more feasible for application in dental laboratories. An additional issue concerning the technology proposed whether INF method could significantly affects structural integrity and sintering cycle of zirconia. From a structural point of view, INF method is somewhat audacious as it could alter the environmental conditions to which zirconia is subjected and also its mechanical properties. Studies evaluating dynamic and static mechanical strength should be performed in future investigations to clarify this condition. Notably, COA method appears to address the problem of yielding adhesion to zirconia ceramic clearer by using an easy and economical approach. In fact, this technique may be a feasible alternative to deposit silica onto zirconia in laboratory to improve subsequent adhesion with methacrylate-based materials. Furthermore, clinical studies are needed before broad indication of the techniques proposed in this study in order to assess their actual effectiveness, since any in vitro study has a number of limitations in predicting clinical performance of materials and techniques.
5.
Conclusion
Present study introduces a novel, simple, and cost-effective method to provide adhesion of methacrylates to yttriastabilized zirconia ceramic. The reagents used are safe, non-toxic, and reasonably inexpensive. The application technique used is also convenient as it demands same clinical and laboratory time employed in processing of oxide bioceramics.
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The sintering time and temperature used to process or glazing veneering ceramic could be used for SiOx condensation and networking when COA method is used, while sintering cycle of zirconia blocks could be used for INF method. Consequently, it may be concluded that experimental methods tested in this study may be useful alternative simplified low-cost approach to commercial tribochemical silicatization since they are able to provide a stable bond between resin cement and zirconia. The methods proposed could also allow bonding of other types of polymeric materials for varied applications of Y-TZP ceramics as biomaterials.
Acknowledgments A.O.O. is grateful to CAPES/Brazil for a MSc scholarship. R.R.M. is grateful for the support from CNPq/Brazil (protocol 308404/2011-4). F.A.O. is grateful for the support from FINEP/Brazil (grant 01.10.0709.00). The authors thank Angelus S/A for donating the zirconia blocks used in the study.
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