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Evolution of silica coating layer on titanium surface and the effect on the bond strength between titanium and porcelain
Applied Surface Science 276 (2013) 723–730
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Evolution of silica coating layer on titanium surface and the effect on the bond strength between titanium and porcelain Aili Wang a , Chaoqun Ge a , Hengbo Yin a,∗ , Yu Gao b , Tao Jiang b , Chunlin Xia a , Gang Wu b , Zhanao Wu b,∗ a b
Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China Department of Stomatology, Chinese PLA 359 Hospital, Zhenjiang 212001, China
a r t i c l e
i n f o
Article history: Received 30 November 2012 Received in revised form 18 March 2013 Accepted 25 March 2013 Available online 1 April 2013 Keywords: Titanium Porcelain Bond strength SiO2 interlayer Pre-oxidation Coating
a b s t r a c t SiO2 coating layers were uniformly anchored at the surfaces of sandblasted/pre-oxidized commercially pure titanium (CP-Ti) substrates by the chemical deposition method using Na2 SiO3 as the SiO2 precursor at the pH values of 8−10 with the Na2 SiO3 concentrations of 0.05–0.5 mol/L. The SiO2 coating layers were composed of small-sized SiO2 nanoparticles with the average particle sizes ranging from 18.0 to 20.5 nm. After firing porcelain (Ti-22) on SiO2 -coated sandblasted/pre-oxidized CP-Ti substrates, the bond strengths of CP-Ti and porcelain ranged from 33.56 to 40.43 MPa, which were detected by the three-point flexure bend test method. In the absence of SiO2 interlayer, the bond strength of sandblasted/pre-oxidized CP-Ti and porcelain was 25.6 MPa. The bond strengths in the presence of SiO2 interlayer were higher than that in the absence of SiO2 interlayer. On the other hand, when the CP-Ti substrates were only treated by hydrochloric acid pickling, the bond strengths of SiO2 -coated acid-pickled CP-Ti and porcelain ranged from 12.99 to 16.59 MPa. The chemical interaction between the SiO2 interlayers and the oxidized CP-Ti surfaces probably played an important role in increasing the bond strength of CP-Ti and porcelain. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Noble metals, such as Au and Pt, have been successfully used for metal–ceramic restorations in stomatology due to their excellent castability and fittingness. With the increase in the price of noble metals, nickel–chromium based alloys were developed for metal–ceramic restorations [1]. However, the allergic and carcinogenic properties of Ni and Cr in dental prostheses, implants, and artificial organs are controversial [1,2]. Titanium and its alloys are gaining acceptance for dental use since they exhibit excellent biocompatibility, corrosion resistance, low specific gravity, good mechanical property, and low cost [3–6]. Titanium–ceramic restorations combine the high esthetic property of ceramic materials and the mechanical strength of metallic frameworks. Nevertheless, one of the main problems in titanium–ceramic restorations is often related to the adhesion between the different ceramic veneer and the titanium framework, leading to fracture of the ceramics with or without metal exposure. On the other hand, the bond strength of titanium and porcelain
∗ Corresponding author. Tel.: +86 511 88787591; fax: +86 511 88791800. E-mail address: [email protected] (Z. Wu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.160
was lower than that of NiCr alloy and porcelain [7], limiting its application in dental prostheses. Titanium suffers from its violent reactivity with oxygen at high temperatures [8], yielding an excessive thick layer of TiO2 and subsequently causing difficulty with porcelain bonding [1,9]. To solve the weak bond strength problem, several methods were proposed, such as grinding and sandblasting [7,10], electrical discharge machining [7], base and acid pickling [11], nitriding [9,12], SiO2 coating [13], and TiO2 –SiO2 coating [14,15]. Among the above mentioned methods, coating SiO2 and TiO2 –SiO2 on titanium surfaces by magnetron sputtering [13] and sol–gel deposition methods [14,15] effectively prevented the excessive oxidation of titanium and increased the bond strength of titanium and porcelain. After deposition of a SiO2 or TiO2 –SiO2 interlayer on titanium surfaces, the bond strengths between commercially pure titanium and low-fusing dental porcelain were reported in wide ranges of 17.22−24.91 [13], 25.54−35.86 [14], and 27.98−28.84 MPa [15], respectively. In most cases, the bond strength was slightly higher than the ISO 9693 bond strength standard of 25 MPa [16]. Therefore, it is worth of further investigation to discover how to improve the bond strength between titanium and porcelain by SiO2 interlayer modification. In our present work, we report the deposition of SiO2 coating layers on commercially pure titanium (CP-Ti) surfaces by the
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Ti
2. Experimental
Rutile TiO2 Titanium oxide
2.1. Materials
Intensity (a.u.)
d
Commercially pure Ti (CP-Ti) strips were supplied by BAOTI Group Co. Ltd. and the CP-Ti strips were abraded into uniform dimensions of 25 mm × 3 mm × 0.5 mm according to the ISO 9693 standard [16]. Ti-22 low-fusing porcelain for titanium was purchased from Noritake. Other chemicals, such as sodium silicate (Na2 SiO3 ·9H2 O, 99%), acetone, anhydrous ethanol, sodium hydroxide, hydrochloric acid (36%), and sulfuric acid (98%) were of reagent grade and were used as received without further purification. Distilled water was used throughout all of the experiments.
c
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a 2.2. Surface treatment of CP-Ti and coating SiO2 layer
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Fig. 1. XRD patterns of (a) the pure CP-Ti, (b) SiO2 -coated acid-pickled CP-Ti, (c) sandblasted/pre-oxidized CP-Ti, and (d) SiO2 -coated sandblasted/pre-oxidized CPTi samples.
chemical deposition method using Na2 SiO3 as the SiO2 precursor. The effects of acid pickling and sandblasting/pre-oxidizing CP-Ti surfaces on the bond strength of SiO2 -coated CP-Ti and porcelain were investigated. High bond strength between SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain, ranging from 33.56 to 40.43 MPa, was achieved.
Firstly, the CP-Ti strips were carefully polished with silicon carbide papers up to 1200 grits. Then the CP-Ti strips were sandblasted with 150 m Al2 O3 particles at the pressure of 0.2 MPa. After sandblasting, the CP-Ti strips were ultrasonically rinsed in deionized water, acetone, and anhydrous ethanol for 15 min, respectively, and dried in air. The sandblasted CP-Ti strips were oxidized in a furnace with a heating rate of 65 ◦ C/min, where they remained for 1 min at 795 ◦ C without vacuuming to obtain sandblasted/pre-oxidized CP-Ti substrates. After the pre-oxidation treatment, five pieces of the sandblasted/pre-oxidized CP-Ti substrates were perpendicularly fixed at the bottom of a 2000 mL beaker. 400 mL of distilled water was added into the beaker and heated to 80 ◦ C in a water
Fig. 2. SEM images of the SiO2 -coated acid-pickled CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. The CP-Ti substrates were only pickled with hydrochloric acid before coating SiO2 layers.
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Table 1 Bond strength of SiO2 -coated acid-pickled CP-Ti and porcelain.a Sample no.
1 2 3 4
Coating SiO2 on CP-Ti
Bond strength (MPa)
pH
Na2 SiO3 (mol/L)
8 8 10 10
0.1 0.5 0.05 0.1
12.99 16.59 15.47 13.44
± ± ± ±
1.59 0.69 1.33 1.16
a The CP-Ti substrates were pickled with hydrochloric acid before coating SiO2 layers.
Fig. 4. SEM image of the cross-section of the porcelain fired SiO2 -coated acidpickled CP-Ti sample.
For comparison, SiO2 -coated acid-pickled CP-Ti samples were also prepared. The CP-Ti substrates were only pickled in hydrochloric acid (20%) at room temperature for 20 min and washed with deionized water. The washed CP-Ti substrates were coated with SiO2 layers through the same processes as mentioned above.
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bath under continuous stirring. The aqueous solution was adjusted to specified pH values of 8, 9, and 10, by adding NaOH (0.5 mol/L) aqueous solution. Then, 250 mL of Na2 SiO3 (0.05, 0.1, 0.3, and 0.5 mol/L) aqueous solution and diluted sulfuric acid (1%) aqueous solution were added into the beaker with two constant flow pumps. The flow rate of Na2 SiO3 aqueous solution was kept constant and the feeding time was fixed at 3 h. The pH value of the reaction solution was kept constant during the coating process by adjusting the flow rate of H2 SO4 aqueous solution. After feeding the Na2 SiO3 aqueous solution, the resultant suspension was aged at 80 ◦ C for 3 h under continuous stirring. The SiO2 -coated sandblasted/pre-oxidized CP-Ti samples were taken out from the suspension and washed with distilled water. The washed samples were dried in an electric oven at 120 ◦ C for 24 h in order to remove water and strengthen the bonding between SiO2 coating layers and sandblasted/pre-oxidized CP-Ti substrates.
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Fig. 3. Particle size distributions of the SiO2 nanoparticles in the SiO2 -coated acid-pickled CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. The CP-Ti substrates were only pickled with hydrochloric acid before coating SiO2 layers. D, average particle size.
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2.3. Firing porcelain on SiO2 -coated CP-Ti
2.4. Characterization
The Ti-22 low-fusing porcelain was fired in the middle of the SiO2 -coated CP-Ti substrates. According to the manufacturer’s recommendations, the porcelain was deposited in the following order: bonding porcelain, opaque porcelain, and dentin porcelain. The parameters for porcelain firing were controlled automatically with a dental porcelain furnace (Dentsply Multimant 99). The low-fusing dental titanium porcelain layer was about 8 mm × 3 mm × 1 mm. In order to compare the effect of the SiO2 interlayer as well as the surface treatment of CP-Ti on the bond strength of CP-Ti and porcelain, CP-Ti/porcelain samples were also prepared by firing the Ti-22 dental porcelain on the sandblasted/pre-oxidized CP-Ti and the SiO2 -coated acid-pickled CP-Ti substrates.
The crystal structures of the pure CP-Ti, SiO2 -coated acidpickled CP-Ti, sandblasted/pre-oxidized CP-Ti, and SiO2 -coated sandblasted/pre-oxidized CP-Ti samples were analyzed by powder X-ray diffraction (XRD) on a D8 Super Speed Bruke-AEX diffractometer using Cu K␣ radiation with a scanning rate of 2◦ min−1 . The surface morphology of the SiO2 -coated CP-Ti samples and the cross-section of the CP-Ti/porcelain samples were examined on a scanning electron microscope (JSM7001F) operating at 10 kV. The bond strength of CP-Ti and porcelain was measured by the three-point flexure bend test on a universal test machine (Shimadzu AGS-10KNG, Japan). The CP-Ti was positioned on the two supports (20 mm span distance) with the porcelain facing down,
Fig. 5. SEM images of the SiO2 -coated sandblasted/pre-oxidized CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations.
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Fig. 5. (continued ).
opposite to the applied load. A vertical force was applied at the midline of the titanium face by a round loading rod at a loading speed of 1 mm/min. The radius of the supports and the loading rod was 1 mm. The load was applied until the disruption in the load–deflection curve, which indicated bond failure. The threepoint bond strength was determined by the failure load (Ffail ) multiplied with a coefficient (K) according to ISO 9693 standard [16]. The coefficient (K) was read from the graph, given in ISO 9693. The results were analyzed by one-way ANOVA and Dunnett-t test (a = 0.05) with SPSS 12.0 software (Statsoft Inc., USA).
No diffraction peaks of crystal silica were found in the XRD spectra of the SiO2 -coated samples, meaning that silica coating layers existed in amorphous phase. However, after coating SiO2 layer on the surface of sandblasted/pre-oxidized CP-Ti, the intensity of the XRD peak at 26.7◦ was obviously increased, indicating that a strong chemical interaction between the SiO2 coating layer and the preoxidized CP-Ti surface probably occurred. While SiO2 coating layer was formed on the surface of acid-pickled CP-Ti, a weak XRD peak at 26.7◦ was also detected, indicating that a weak chemical interaction between the SiO2 coating layer and the pure CP-Ti surface probably occurred.
3. Results and discussion 3.1. Crystal structures of SiO2 -coated acid-pickled CP-Ti and SiO2 -coated sandblasted/pre-oxidized CP-Ti samples Fig. 1 shows the XRD patterns of the pure CP-Ti, SiO2 -coated acid-pickled CP-Ti, sandblasted/pre-oxidized CP-Ti, and SiO2 coated sandblasted/pre-oxidized CP-Ti samples. The SiO2 -coated acid-pickled CP-Ti and SiO2 -coated sandblasted/pre-oxidized CP-Ti samples were prepared with 0.5 mol/L Na2 SiO3 aqueous solution at 80 ◦ C and the pH values of 8 and 9, respectively. The XRD peaks of the pure CP-Ti sample appearing at (2) 35.1◦ , 38.4◦ , 40.2◦ , 53.0◦ , 63.0◦ , 70.7◦ , 76.2◦ , and 77.4◦ were ascribed to those of metallic titanium (JCPDS 44-1294). The XRD peaks of the sandblasted/preoxidized CP-Ti sample appearing at 27.4◦ , 36.1◦ , 41.2◦ , 54.4◦ , and 56.6◦ were ascribed to those of rutile TiO2 (JCPDS 21-1276). Additionally, a XRD peak appearing at 26.7◦ indicated that another type of titanium oxide was also formed during pre-oxidation process (Fig. 1c). Pre-oxidation resulted in the oxidation of CP-Ti surface.
3.2. Morphology of SiO2 -coated acid-pickled CP-Ti and bond strength of SiO2 -coated acid-pickled CP-Ti and porcelain Fig. 2 shows the SEM images of the SiO2 -coated acid-pickled CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. The CP-Ti substrates were only pickled with hydrochloric acid before coating SiO2 layers. SEM images show that SiO2 nanoparticles constructed dense coating layers on the CP-Ti surfaces. At a pH value of 8, the average particle sizes of the SiO2 nanoparticles on CP-Ti surfaces were ca. 20 nm and the particle sizes ranged from 12 to 34 nm (Fig. 3). When the pH value was increased to 10, the average particle sizes of the SiO2 nanoparticles were ca. 16 nm and the particle sizes ranged from 10 to 25 nm. Increasing the pH value of the reaction solution decreased the SiO2 nanoparticle sizes and the size distributions. But Na2 SiO3 concentration did not significantly affect the average particle sizes. It can be explained as being that at a low pH value, rapid hydrolysis of sodium silicate caused the formation of a large number of siliceous
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micelles and then these siliceous micelles condensed to form largesized SiO2 nanoparticles on the surfaces of CP-Ti substrates. While at a high pH value, the relatively low hydrolysis rate of sodium silicate resulted in the formation of small-sized SiO2 nanoparticles. The bond strengths of SiO2 -coated acid-pickled CP-Ti and porcelain were in a range of 12.99–16.59 MPa (Table 1). The particle size of the SiO2 nanoparticles had no significant effect on the bond strength of CP-Ti and porcelain. The bond strengths of SiO2 -coated acid-pickled CP-Ti and porcelain were less than the ISO 9693 bond strength standard of 25 MPa. The SEM image of the cross-section of the porcelain fired SiO2 -coated CP-Ti sample shows that there are gaps between the CP-Ti surface and the porcelain layer (Fig. 4). Combined with the XRD analysis, it is reasonable to suggest that
the chemical interaction between the acid-pickled CP-Ti and the SiO2 coating layer was weak, causing the fracture between CP-Ti and porcelain layer. 3.3. Morphology of SiO2 -coated sandblasted/pre-oxidized CP-Ti and bond strength of SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain SEM images show that the sandblasted/pre-oxidized CP-Ti substrates had rough surfaces (Fig. 5). The sandblasted/pre-oxidized CP-Ti surfaces were completely covered by SiO2 coating layers under the present experimental conditions. The SiO2 coating layers were composed of SiO2 nanoparticles. The average particle sizes of
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Fig. 6. Particle size distributions of the SiO2 nanoparticles in the SiO2 -coated sandblasted/pre-oxidized CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. D, average particle size.
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A. Wang et al. / Applied Surface Science 276 (2013) 723–730
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the SiO2 nanoparticles ranged from 18.0 to 20.5 nm and the particle size distributions were in a range of 10−34 nm (Fig. 6). The pH value of the reaction solution had little effect on the particle sizes of the SiO2 nanoparticles at the surfaces of the sandblasted/pre-oxidized CP-Ti substrates. In general, the content of SiO2 nanoparticles increased with the increase in the Na2 SiO3 concentrations. The bond strengths of SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain ranged from 33.56 to 40.43 MPa, which are 1.34−1.62 times the ISO 9693 bond strength standard value (Table 2). The SEM image of the cross section of the CP-Ti/porcelain sample shows that the porcelain was tightly combined at the CP-Ti surface after firing (Fig. 7). On the other hand, the bond strength of sandblasted/pre-oxidied CP-Ti and porcelain was only 25.6 MPa, indicating that the presence of the thin SiO2 interlayers on the surfaces of the sandblasted/pre-oxidized CP-Ti substrates enhanced the bonding between CP-Ti and porcelain.
The effect of SiO2 interlayers on improving the bonding extent between CP-Ti and porcelain can be explained as follows. As reported in our previous work [17,18], SiO2 coating layers can tightly anchor at rutile TiO2 surfaces via Ti O Si bonds when SiO2 coating layers are deposited on rutile TiO2 surfaces by the chemical deposition method starting from Na2 SiO3 at 80−90 ◦ C and the pH values of 8−10. In our present work, SiO2 interlayers were coated on the surfaces of sandblasted/pre-oxidized CP-Ti under the same experimental conditions as those in our previous work [17,18]. Therefore, combined with the XRD analysis,
Table 2 Bond strength of SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain. Sample no.
1 2 3 4 5 6 7 8 9 10
Coating SiO2 on sandblasted/pre-oxidized CP-Ti pH
Na2 SiO3 (mol/L)
/ 8 8 8 9 9 9 10 10 10
/ 0.05 0.1 0.5 0.1 0.3 0.5 0.1 0.3 0.5
Bond strength (MPa)
25.60 40.03 33.56 39.45 40.43 35.03 35.92 39.74 39.89 38.57
± ± ± ± ± ± ± ± ± ±
1.20 1.18 1.20 1.17 2.75 2.65 1.17 2.06 0.15 0.30
Fig. 7. SEM image of the cross-section of the porcelain fired SiO2 -coated sandblasted/pre-oxidized CP-Ti sample.
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porcelain. The present work was financially supported by Chinese PLA 359 Hospital. References
Scheme 1. Evolution mechanism of SiO2 coating layer on the pre-oxidized CP-Ti substrate and porcelain firing.
it is reasonable to suggest that the SiO2 interlayers can tightly anchor at the sandblasted/pre-oxidized CP-Ti surfaces via Ti O Si bonds because rutile TiO2 was formed on CP-Ti surfaces during the pre-oxidation treatment. The formation of SiO2 interlayers on the surfaces of sandblasted/pre-oxidized CP-Ti increased the bonding strength of CP-Ti and porcelain by chemical interaction. The bonding mechanism of porcelain on the SiO2 -coated sandblasted/pre-oxidized CP-Ti is illustrated in Scheme 1. 4. Conclusions SiO2 coating layers were uniformly formed on the surfaces of the acid-pickled and the sandblasted/pre-oxidized CP-Ti substrates by the chemical deposition method starting from Na2 SiO3 . The presence of SiO2 interlayers highly improved the bonding extent between the sandblasted/pre-oxidized CP-Ti and the porcelain (Ti22). The interaction between the SiO2 interlayer and the rutile TiO2 on the CP-Ti surface played an important role for enhancing the combination between CP-Ti and porcelain. Acknowledgements The authors sincerely thank Prof. K. Chen of Jiangsu University for supporting the SEM analysis. The authors also sincerely thank Ms. L. Shen of School of Stomatology, Fourth Military Medical University for the measurement of the bond strength of CP-Ti and
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Evolution of silica coating layer on titanium surface and the effect on the bond strength between titanium and porcelain Aili Wang a , Chaoqun Ge a , Hengbo Yin a,∗ , Yu Gao b , Tao Jiang b , Chunlin Xia a , Gang Wu b , Zhanao Wu b,∗ a b
Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China Department of Stomatology, Chinese PLA 359 Hospital, Zhenjiang 212001, China
a r t i c l e
i n f o
Article history: Received 30 November 2012 Received in revised form 18 March 2013 Accepted 25 March 2013 Available online 1 April 2013 Keywords: Titanium Porcelain Bond strength SiO2 interlayer Pre-oxidation Coating
a b s t r a c t SiO2 coating layers were uniformly anchored at the surfaces of sandblasted/pre-oxidized commercially pure titanium (CP-Ti) substrates by the chemical deposition method using Na2 SiO3 as the SiO2 precursor at the pH values of 8−10 with the Na2 SiO3 concentrations of 0.05–0.5 mol/L. The SiO2 coating layers were composed of small-sized SiO2 nanoparticles with the average particle sizes ranging from 18.0 to 20.5 nm. After firing porcelain (Ti-22) on SiO2 -coated sandblasted/pre-oxidized CP-Ti substrates, the bond strengths of CP-Ti and porcelain ranged from 33.56 to 40.43 MPa, which were detected by the three-point flexure bend test method. In the absence of SiO2 interlayer, the bond strength of sandblasted/pre-oxidized CP-Ti and porcelain was 25.6 MPa. The bond strengths in the presence of SiO2 interlayer were higher than that in the absence of SiO2 interlayer. On the other hand, when the CP-Ti substrates were only treated by hydrochloric acid pickling, the bond strengths of SiO2 -coated acid-pickled CP-Ti and porcelain ranged from 12.99 to 16.59 MPa. The chemical interaction between the SiO2 interlayers and the oxidized CP-Ti surfaces probably played an important role in increasing the bond strength of CP-Ti and porcelain. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Noble metals, such as Au and Pt, have been successfully used for metal–ceramic restorations in stomatology due to their excellent castability and fittingness. With the increase in the price of noble metals, nickel–chromium based alloys were developed for metal–ceramic restorations [1]. However, the allergic and carcinogenic properties of Ni and Cr in dental prostheses, implants, and artificial organs are controversial [1,2]. Titanium and its alloys are gaining acceptance for dental use since they exhibit excellent biocompatibility, corrosion resistance, low specific gravity, good mechanical property, and low cost [3–6]. Titanium–ceramic restorations combine the high esthetic property of ceramic materials and the mechanical strength of metallic frameworks. Nevertheless, one of the main problems in titanium–ceramic restorations is often related to the adhesion between the different ceramic veneer and the titanium framework, leading to fracture of the ceramics with or without metal exposure. On the other hand, the bond strength of titanium and porcelain
∗ Corresponding author. Tel.: +86 511 88787591; fax: +86 511 88791800. E-mail address: [email protected] (Z. Wu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.160
was lower than that of NiCr alloy and porcelain [7], limiting its application in dental prostheses. Titanium suffers from its violent reactivity with oxygen at high temperatures [8], yielding an excessive thick layer of TiO2 and subsequently causing difficulty with porcelain bonding [1,9]. To solve the weak bond strength problem, several methods were proposed, such as grinding and sandblasting [7,10], electrical discharge machining [7], base and acid pickling [11], nitriding [9,12], SiO2 coating [13], and TiO2 –SiO2 coating [14,15]. Among the above mentioned methods, coating SiO2 and TiO2 –SiO2 on titanium surfaces by magnetron sputtering [13] and sol–gel deposition methods [14,15] effectively prevented the excessive oxidation of titanium and increased the bond strength of titanium and porcelain. After deposition of a SiO2 or TiO2 –SiO2 interlayer on titanium surfaces, the bond strengths between commercially pure titanium and low-fusing dental porcelain were reported in wide ranges of 17.22−24.91 [13], 25.54−35.86 [14], and 27.98−28.84 MPa [15], respectively. In most cases, the bond strength was slightly higher than the ISO 9693 bond strength standard of 25 MPa [16]. Therefore, it is worth of further investigation to discover how to improve the bond strength between titanium and porcelain by SiO2 interlayer modification. In our present work, we report the deposition of SiO2 coating layers on commercially pure titanium (CP-Ti) surfaces by the
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Ti
2. Experimental
Rutile TiO2 Titanium oxide
2.1. Materials
Intensity (a.u.)
d
Commercially pure Ti (CP-Ti) strips were supplied by BAOTI Group Co. Ltd. and the CP-Ti strips were abraded into uniform dimensions of 25 mm × 3 mm × 0.5 mm according to the ISO 9693 standard [16]. Ti-22 low-fusing porcelain for titanium was purchased from Noritake. Other chemicals, such as sodium silicate (Na2 SiO3 ·9H2 O, 99%), acetone, anhydrous ethanol, sodium hydroxide, hydrochloric acid (36%), and sulfuric acid (98%) were of reagent grade and were used as received without further purification. Distilled water was used throughout all of the experiments.
c
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a 2.2. Surface treatment of CP-Ti and coating SiO2 layer
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Fig. 1. XRD patterns of (a) the pure CP-Ti, (b) SiO2 -coated acid-pickled CP-Ti, (c) sandblasted/pre-oxidized CP-Ti, and (d) SiO2 -coated sandblasted/pre-oxidized CPTi samples.
chemical deposition method using Na2 SiO3 as the SiO2 precursor. The effects of acid pickling and sandblasting/pre-oxidizing CP-Ti surfaces on the bond strength of SiO2 -coated CP-Ti and porcelain were investigated. High bond strength between SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain, ranging from 33.56 to 40.43 MPa, was achieved.
Firstly, the CP-Ti strips were carefully polished with silicon carbide papers up to 1200 grits. Then the CP-Ti strips were sandblasted with 150 m Al2 O3 particles at the pressure of 0.2 MPa. After sandblasting, the CP-Ti strips were ultrasonically rinsed in deionized water, acetone, and anhydrous ethanol for 15 min, respectively, and dried in air. The sandblasted CP-Ti strips were oxidized in a furnace with a heating rate of 65 ◦ C/min, where they remained for 1 min at 795 ◦ C without vacuuming to obtain sandblasted/pre-oxidized CP-Ti substrates. After the pre-oxidation treatment, five pieces of the sandblasted/pre-oxidized CP-Ti substrates were perpendicularly fixed at the bottom of a 2000 mL beaker. 400 mL of distilled water was added into the beaker and heated to 80 ◦ C in a water
Fig. 2. SEM images of the SiO2 -coated acid-pickled CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. The CP-Ti substrates were only pickled with hydrochloric acid before coating SiO2 layers.
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Table 1 Bond strength of SiO2 -coated acid-pickled CP-Ti and porcelain.a Sample no.
1 2 3 4
Coating SiO2 on CP-Ti
Bond strength (MPa)
pH
Na2 SiO3 (mol/L)
8 8 10 10
0.1 0.5 0.05 0.1
12.99 16.59 15.47 13.44
± ± ± ±
1.59 0.69 1.33 1.16
a The CP-Ti substrates were pickled with hydrochloric acid before coating SiO2 layers.
Fig. 4. SEM image of the cross-section of the porcelain fired SiO2 -coated acidpickled CP-Ti sample.
For comparison, SiO2 -coated acid-pickled CP-Ti samples were also prepared. The CP-Ti substrates were only pickled in hydrochloric acid (20%) at room temperature for 20 min and washed with deionized water. The washed CP-Ti substrates were coated with SiO2 layers through the same processes as mentioned above.
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Particle size distribution (%)
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bath under continuous stirring. The aqueous solution was adjusted to specified pH values of 8, 9, and 10, by adding NaOH (0.5 mol/L) aqueous solution. Then, 250 mL of Na2 SiO3 (0.05, 0.1, 0.3, and 0.5 mol/L) aqueous solution and diluted sulfuric acid (1%) aqueous solution were added into the beaker with two constant flow pumps. The flow rate of Na2 SiO3 aqueous solution was kept constant and the feeding time was fixed at 3 h. The pH value of the reaction solution was kept constant during the coating process by adjusting the flow rate of H2 SO4 aqueous solution. After feeding the Na2 SiO3 aqueous solution, the resultant suspension was aged at 80 ◦ C for 3 h under continuous stirring. The SiO2 -coated sandblasted/pre-oxidized CP-Ti samples were taken out from the suspension and washed with distilled water. The washed samples were dried in an electric oven at 120 ◦ C for 24 h in order to remove water and strengthen the bonding between SiO2 coating layers and sandblasted/pre-oxidized CP-Ti substrates.
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Fig. 3. Particle size distributions of the SiO2 nanoparticles in the SiO2 -coated acid-pickled CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. The CP-Ti substrates were only pickled with hydrochloric acid before coating SiO2 layers. D, average particle size.
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2.3. Firing porcelain on SiO2 -coated CP-Ti
2.4. Characterization
The Ti-22 low-fusing porcelain was fired in the middle of the SiO2 -coated CP-Ti substrates. According to the manufacturer’s recommendations, the porcelain was deposited in the following order: bonding porcelain, opaque porcelain, and dentin porcelain. The parameters for porcelain firing were controlled automatically with a dental porcelain furnace (Dentsply Multimant 99). The low-fusing dental titanium porcelain layer was about 8 mm × 3 mm × 1 mm. In order to compare the effect of the SiO2 interlayer as well as the surface treatment of CP-Ti on the bond strength of CP-Ti and porcelain, CP-Ti/porcelain samples were also prepared by firing the Ti-22 dental porcelain on the sandblasted/pre-oxidized CP-Ti and the SiO2 -coated acid-pickled CP-Ti substrates.
The crystal structures of the pure CP-Ti, SiO2 -coated acidpickled CP-Ti, sandblasted/pre-oxidized CP-Ti, and SiO2 -coated sandblasted/pre-oxidized CP-Ti samples were analyzed by powder X-ray diffraction (XRD) on a D8 Super Speed Bruke-AEX diffractometer using Cu K␣ radiation with a scanning rate of 2◦ min−1 . The surface morphology of the SiO2 -coated CP-Ti samples and the cross-section of the CP-Ti/porcelain samples were examined on a scanning electron microscope (JSM7001F) operating at 10 kV. The bond strength of CP-Ti and porcelain was measured by the three-point flexure bend test on a universal test machine (Shimadzu AGS-10KNG, Japan). The CP-Ti was positioned on the two supports (20 mm span distance) with the porcelain facing down,
Fig. 5. SEM images of the SiO2 -coated sandblasted/pre-oxidized CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations.
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Fig. 5. (continued ).
opposite to the applied load. A vertical force was applied at the midline of the titanium face by a round loading rod at a loading speed of 1 mm/min. The radius of the supports and the loading rod was 1 mm. The load was applied until the disruption in the load–deflection curve, which indicated bond failure. The threepoint bond strength was determined by the failure load (Ffail ) multiplied with a coefficient (K) according to ISO 9693 standard [16]. The coefficient (K) was read from the graph, given in ISO 9693. The results were analyzed by one-way ANOVA and Dunnett-t test (a = 0.05) with SPSS 12.0 software (Statsoft Inc., USA).
No diffraction peaks of crystal silica were found in the XRD spectra of the SiO2 -coated samples, meaning that silica coating layers existed in amorphous phase. However, after coating SiO2 layer on the surface of sandblasted/pre-oxidized CP-Ti, the intensity of the XRD peak at 26.7◦ was obviously increased, indicating that a strong chemical interaction between the SiO2 coating layer and the preoxidized CP-Ti surface probably occurred. While SiO2 coating layer was formed on the surface of acid-pickled CP-Ti, a weak XRD peak at 26.7◦ was also detected, indicating that a weak chemical interaction between the SiO2 coating layer and the pure CP-Ti surface probably occurred.
3. Results and discussion 3.1. Crystal structures of SiO2 -coated acid-pickled CP-Ti and SiO2 -coated sandblasted/pre-oxidized CP-Ti samples Fig. 1 shows the XRD patterns of the pure CP-Ti, SiO2 -coated acid-pickled CP-Ti, sandblasted/pre-oxidized CP-Ti, and SiO2 coated sandblasted/pre-oxidized CP-Ti samples. The SiO2 -coated acid-pickled CP-Ti and SiO2 -coated sandblasted/pre-oxidized CP-Ti samples were prepared with 0.5 mol/L Na2 SiO3 aqueous solution at 80 ◦ C and the pH values of 8 and 9, respectively. The XRD peaks of the pure CP-Ti sample appearing at (2) 35.1◦ , 38.4◦ , 40.2◦ , 53.0◦ , 63.0◦ , 70.7◦ , 76.2◦ , and 77.4◦ were ascribed to those of metallic titanium (JCPDS 44-1294). The XRD peaks of the sandblasted/preoxidized CP-Ti sample appearing at 27.4◦ , 36.1◦ , 41.2◦ , 54.4◦ , and 56.6◦ were ascribed to those of rutile TiO2 (JCPDS 21-1276). Additionally, a XRD peak appearing at 26.7◦ indicated that another type of titanium oxide was also formed during pre-oxidation process (Fig. 1c). Pre-oxidation resulted in the oxidation of CP-Ti surface.
3.2. Morphology of SiO2 -coated acid-pickled CP-Ti and bond strength of SiO2 -coated acid-pickled CP-Ti and porcelain Fig. 2 shows the SEM images of the SiO2 -coated acid-pickled CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. The CP-Ti substrates were only pickled with hydrochloric acid before coating SiO2 layers. SEM images show that SiO2 nanoparticles constructed dense coating layers on the CP-Ti surfaces. At a pH value of 8, the average particle sizes of the SiO2 nanoparticles on CP-Ti surfaces were ca. 20 nm and the particle sizes ranged from 12 to 34 nm (Fig. 3). When the pH value was increased to 10, the average particle sizes of the SiO2 nanoparticles were ca. 16 nm and the particle sizes ranged from 10 to 25 nm. Increasing the pH value of the reaction solution decreased the SiO2 nanoparticle sizes and the size distributions. But Na2 SiO3 concentration did not significantly affect the average particle sizes. It can be explained as being that at a low pH value, rapid hydrolysis of sodium silicate caused the formation of a large number of siliceous
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micelles and then these siliceous micelles condensed to form largesized SiO2 nanoparticles on the surfaces of CP-Ti substrates. While at a high pH value, the relatively low hydrolysis rate of sodium silicate resulted in the formation of small-sized SiO2 nanoparticles. The bond strengths of SiO2 -coated acid-pickled CP-Ti and porcelain were in a range of 12.99–16.59 MPa (Table 1). The particle size of the SiO2 nanoparticles had no significant effect on the bond strength of CP-Ti and porcelain. The bond strengths of SiO2 -coated acid-pickled CP-Ti and porcelain were less than the ISO 9693 bond strength standard of 25 MPa. The SEM image of the cross-section of the porcelain fired SiO2 -coated CP-Ti sample shows that there are gaps between the CP-Ti surface and the porcelain layer (Fig. 4). Combined with the XRD analysis, it is reasonable to suggest that
the chemical interaction between the acid-pickled CP-Ti and the SiO2 coating layer was weak, causing the fracture between CP-Ti and porcelain layer. 3.3. Morphology of SiO2 -coated sandblasted/pre-oxidized CP-Ti and bond strength of SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain SEM images show that the sandblasted/pre-oxidized CP-Ti substrates had rough surfaces (Fig. 5). The sandblasted/pre-oxidized CP-Ti surfaces were completely covered by SiO2 coating layers under the present experimental conditions. The SiO2 coating layers were composed of SiO2 nanoparticles. The average particle sizes of
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Fig. 6. Particle size distributions of the SiO2 nanoparticles in the SiO2 -coated sandblasted/pre-oxidized CP-Ti samples prepared at different pH values and Na2 SiO3 concentrations. D, average particle size.
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the SiO2 nanoparticles ranged from 18.0 to 20.5 nm and the particle size distributions were in a range of 10−34 nm (Fig. 6). The pH value of the reaction solution had little effect on the particle sizes of the SiO2 nanoparticles at the surfaces of the sandblasted/pre-oxidized CP-Ti substrates. In general, the content of SiO2 nanoparticles increased with the increase in the Na2 SiO3 concentrations. The bond strengths of SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain ranged from 33.56 to 40.43 MPa, which are 1.34−1.62 times the ISO 9693 bond strength standard value (Table 2). The SEM image of the cross section of the CP-Ti/porcelain sample shows that the porcelain was tightly combined at the CP-Ti surface after firing (Fig. 7). On the other hand, the bond strength of sandblasted/pre-oxidied CP-Ti and porcelain was only 25.6 MPa, indicating that the presence of the thin SiO2 interlayers on the surfaces of the sandblasted/pre-oxidized CP-Ti substrates enhanced the bonding between CP-Ti and porcelain.
The effect of SiO2 interlayers on improving the bonding extent between CP-Ti and porcelain can be explained as follows. As reported in our previous work [17,18], SiO2 coating layers can tightly anchor at rutile TiO2 surfaces via Ti O Si bonds when SiO2 coating layers are deposited on rutile TiO2 surfaces by the chemical deposition method starting from Na2 SiO3 at 80−90 ◦ C and the pH values of 8−10. In our present work, SiO2 interlayers were coated on the surfaces of sandblasted/pre-oxidized CP-Ti under the same experimental conditions as those in our previous work [17,18]. Therefore, combined with the XRD analysis,
Table 2 Bond strength of SiO2 -coated sandblasted/pre-oxidized CP-Ti and porcelain. Sample no.
1 2 3 4 5 6 7 8 9 10
Coating SiO2 on sandblasted/pre-oxidized CP-Ti pH
Na2 SiO3 (mol/L)
/ 8 8 8 9 9 9 10 10 10
/ 0.05 0.1 0.5 0.1 0.3 0.5 0.1 0.3 0.5
Bond strength (MPa)
25.60 40.03 33.56 39.45 40.43 35.03 35.92 39.74 39.89 38.57
± ± ± ± ± ± ± ± ± ±
1.20 1.18 1.20 1.17 2.75 2.65 1.17 2.06 0.15 0.30
Fig. 7. SEM image of the cross-section of the porcelain fired SiO2 -coated sandblasted/pre-oxidized CP-Ti sample.
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porcelain. The present work was financially supported by Chinese PLA 359 Hospital. References
Scheme 1. Evolution mechanism of SiO2 coating layer on the pre-oxidized CP-Ti substrate and porcelain firing.
it is reasonable to suggest that the SiO2 interlayers can tightly anchor at the sandblasted/pre-oxidized CP-Ti surfaces via Ti O Si bonds because rutile TiO2 was formed on CP-Ti surfaces during the pre-oxidation treatment. The formation of SiO2 interlayers on the surfaces of sandblasted/pre-oxidized CP-Ti increased the bonding strength of CP-Ti and porcelain by chemical interaction. The bonding mechanism of porcelain on the SiO2 -coated sandblasted/pre-oxidized CP-Ti is illustrated in Scheme 1. 4. Conclusions SiO2 coating layers were uniformly formed on the surfaces of the acid-pickled and the sandblasted/pre-oxidized CP-Ti substrates by the chemical deposition method starting from Na2 SiO3 . The presence of SiO2 interlayers highly improved the bonding extent between the sandblasted/pre-oxidized CP-Ti and the porcelain (Ti22). The interaction between the SiO2 interlayer and the rutile TiO2 on the CP-Ti surface played an important role for enhancing the combination between CP-Ti and porcelain. Acknowledgements The authors sincerely thank Prof. K. Chen of Jiangsu University for supporting the SEM analysis. The authors also sincerely thank Ms. L. Shen of School of Stomatology, Fourth Military Medical University for the measurement of the bond strength of CP-Ti and
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