- Email: [email protected]
ZrO2 film on the bond strength of commercially pure titanium to porcelain
Effect of a magnetron-sput tered ZrSiN/ ZrO2 film on the bond strength of commercially pure titanium to porcelain Guowei Wang, MD,a Xiaojing Wang, MD,b Yimin Zhao, PhD,c and Tianwen Guo, PhDd School of Stomatology, Fourth Military Medical University, Xi’an, Shaanxi, China Statement of problem. The excessively thick and nonadherent titanium oxide layer formed during the porcelain sintering process can cause bonding problems between titanium and porcelain. Purpose. The purpose of this study was to evaluate the effect of a magnetron-sputtered ZrSiN/ZrO2 composite film on the bond strength of commercially pure titanium (CP Ti) to porcelain. Material and methods. Sixty-eight cast titanium specimens were prepared according to the ISO 9693 standard and then divided into 2 coated and 2 noncoated groups (n=17). The ZrSiN/ZrO2 composite film was deposited on specimens of the 2 coated groups by magnetron sputtering. A low-fusing porcelain was applied on 1 coated group and 1 noncoated group. A surface profilometer, surface roughness tester, energy-dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), and x-ray diffraction (XRD) were used to examine the characteristics of the film and the interfacial properties, while the bond strength of titanium-porcelain specimens was analyzed with the 3-point bend test. The results were analyzed with an independent samples t test (α=.05). Results. The mean bond strength of ZrSiN/ZrO2-coated CP Ti to porcelain (43.67 ±2.08 MPa) was significantly higher than that of the noncoated group (35.44 ±3.56 MPa). A generally cohesive failure mode was observed in the coated group, but the failure mode in the noncoated group was adhesive. EDS data showed that the ZrSiN/ZrO2 film effectively prevented the intrusion of oxygen into the Ti substrate. Conclusions. The data suggested that the magnetron-sputtered ZrSiN/ZrO2 film could significantly improve the bond strength of CP Ti to porcelain and this may have clinical significance. (J Prosthet Dent 2013;109:313-318)
Clinical Implications
Clinical delamination of titanium ceramic restorations may be reduced by the application of a magnetron-sputtered ZrSiN/ZrO2 film. Titanium (Ti) has been widely used as a dental material for its excellent biocompatibility, high corrosion resistance, desirable physical mechanical properties, and low cost.1-5 However, Ti exhibits strong reactivity with oxygen, especially during porcelain sintering at high temperatures, which creates an excessively thick and nonadherent Ti oxide layer.6,7 This oxi-
dation layer can cause problems such as poor bonding, even leading to the failure of the Ti-to-porcelain bond.8-10 Many approaches have been reported to control Ti oxidation.11-15 A well-recognized method is to coat the Ti substrate with an intermediate film prior to porcelain sintering.16-19 The film should firmly bond with both the Ti substrate and the porcelain after
sintering.17,20,21 A magnetron-sputtered amorphous ZrSiN film has been reported to exert an oxidation-resistant ability at high temperature.18,22,23 However, it should be noted that a phase evolution of the ZrSiN film might occur with the increasing partial pressure of nitrogen (pN2). Considering the preferential bonding between the elements, a nanocomposite
This work was supported by the National Natural Science Foundation of China (No. 30371557). Lecturer, Department of Prosthodontics. Lecturer, Department of Prosthodontics. c Professor, Department of Prosthodontics. d Professor, Department of Prosthodontics. a
b
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Volume 109 Issue 5 structure composed of ZrSi2 grains embedded within an amorphous Si3N4 matrix with low pN2 (pN2<0.05 Pa) is assumed to develop. This is similar to the TiSiN film synthesized by Veprek et al.24 Another consequence of high pN2 is an increasing amount of Si3N4, which eventually becomes the dominant component (pN2 > 0.05 Pa) of the film. ZrSiN film, which consists of Si3N4, ZrSix, and ZrNx, exhibits an amorphous-like structure as determined by x-ray diffraction (XRD). In general, the structure of ZrSiN film can change from a crystalline (pN2< 0.05 Pa) one to an x-ray amorphous one (pN2>0.05 Pa).25-29 Furthermore, Song et al30 have demonstrated that a preferable diffusion barrier of ZrSiN film was formed with a pN2 of 0.09 Pa. However, in an aerobic high temperature environment, the ZrSiN film may release nitrogen gas and form a thick and unstable layer of ZrO2,22 which may compromise the Ti-to-porcelain bond. The present investigation was designed to improve this ZrSiN film with a composite magnetron-sputtered ZrSiN/ZrO2 film. The hypothesis was that this optimized composite film, ZrSiN/ZrO2, may eliminate the adverse effects of the byproducts of the ZrSiN film created during the porcelain sintering process but still retain the oxidation-resistant ability of previously used ZrSiN film. The null hypothesis of the study was that the magnetron-sputtered ZrSiN/ZrO2 composite film would not improve the bond strength of CP Ti to porcelain. MATERIAL AND METHODS Specimen preparation Sixty-eight CP Ti specimens (ASTM Grade II; Northwest Institute for Nonferrous Metal Research, Xi’an, China) were cast (25 × 3 × 0.5 mm) according to the International Organization of Standardization (ISO) 9693 standard31 and polished with silicon carbide papers under wet conditions. All of the specimens were airborneparticle abraded with 70 µm Al2O3
under 0.2 MPa pressure at a 45-degree angle. The source of the Al2O3 particles was 10 mm away from the Ti substrates. The specimens were then ultrasonically rinsed in deionized water, acetone, and ethanol for 15 minutes each and dried in air. A ZrSiN film was magnetron-sputtered by a multitarget magnetron sputtering system (ASC-4000-C4; ULVAC Inc, Kanagawa, Japan) onto 34 Ti substrates in an Ar/N2 (7/3) gas mixture with a base pressure of 2 × 10-5 Pa. Si and Zr disks (purity 99%, diameter 73.0 × 3.0 mm; Northwest Institute for Nonferrous Metal Research) were used as the targets. The substrate-to-target distance was 60.0 mm. Before sputtering, Ti substrates were cleaned by bombardment with Ar ions. The ZrSiN film was deposited with a target power for Zr of 100 W (direct current, DC) and a target power for Si of 11 W (radio frequency, RF). The substrate bias voltage was -100 V, working pressure 0.3 Pa, and deposition time 1 hour. Subsequently, a ZrO2 film was deposited under the following conditions: gas mixture, Ar/ O2 (7/3); target power, 120 W (DC); bias voltage, -100 V; working pressure, 0.3 Pa and deposition time, 1 hour. Water circulation was used as a cooling treatment. After the deposition, the substrates were subjected to steam and ultrasonic cleaning. To study the structure of the composite film more easily, 5 silicon substrates were also processed with the same surface treatments as previously described and then tested with XRD (DMAX 1200; Tigaku, Tokyo, Japan). To measure the film thickness, 10 silicon plates (3.0 × 1.0 cm) were covered by other smaller silicon plates (1.0 × 1.0 cm) in the central part. After they were coated with ZrSiN/ZrO2 film, the small central silicon plates were carefully removed. The cross-sections of the magnetron-sputtered ZrSiN/ZrO2 film were therefore exposed on the 3.0 × 1.0 cm silicon plates. The thickness of the film was tested with a surface profilometer (Dektak IIA; Veeco Instruments Inc, Plainview, NY). After ultrasonic cleaning, a low-
The Journal of Prosthetic Dentistry
fusing porcelain (Ti-22; Noritake Co, Nagoya, Japan) was then brushed onto the central region (8.0 × 3.0 mm) of 34 specimens (17 coated with ZrSiN/ZrO2 film and 17 noncoated) and sintered according to the manufacturer’s recommendations. The thickness of the bonding agent and the opaque layer were 0.2 mm; the dentin coating was 0.6 mm. Surface and interfacial characterization With an assumed minimum of 80% power and 2-sided type α error of .05, the sample size of 8 was calculated on the surface roughness (Ra) value from the preliminary experiment (the difference between coated and uncoated ZrSiN/ZrO2 group was 0.3, and the pooled variance was 0.02), with 4 in each group. For Ra measurements in the current study, 24 specimens (12 coated and 12 noncoated with ZrSiN/ ZrO2 film) were selected. The Ra value was measured with a surface roughness tester (TR240; Beijing Time Group Inc, Beijing, China). To study the thermal stability of the film, 10 titanium specimens (5 coated with ZrSiN/ZrO2 film and 5 noncoated) were subjected to simulated porcelain sintering thermocycles (SPSTs) in a dental furnace (Multimat 99; Dentsply Intl, York, Pa). They were not coated with porcelain layers. Ten porcelain sintered specimens (5 coated and 5 noncoated with ZrSiN/ZrO2 film) were selected to study the cross-sections. SEM and EDS (S-4800; Hitachi Ltd, Tokyo, Japan) were used to analyze the surface structures and components. Evaluation of bond strength With an assumed minimum of 80% power and 2-sided type α error of .05, the sample size of 14 was calculated on the bond strength from the preliminary experiment (the difference between coated and uncoated ZrSiN/ZrO2 group was 6, and the pooled variance was 16), with 7 in each group. In the current study, the
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May 2013 tion peaks of the silicon-containing and nitrogen-containing compound were not detected. Surface roughness and bond strength Table I shows the mean values and standard deviations of the CP Ti to porcelain bond strength and the surface roughness. The results revealed that both the Ra value of the composite film and the bond strength of the ZrSiN/ZrO2-coated group were significantly higher than the noncoated group (P<.05).
1 Schematic diagram of 3-point bend test. 160 140
Thermal stability and fracture mode
Intensity (Counts)
120 100 ZrO2
80
Si
60 40 20 0
20
30
40
50
70
60
80
90
2-Theta (degrees)
2 XRD pattern of ZrSiN/ZrO2 film deposited on Si substrate.
Table I. Results of CP Ti to porcelain bond strength and roughness parameters (n=12). Ra is arithmetic mean Bond Strength (MPa)
Ra (µm)
Noncoated
35.44 ±3.56
1.05 ±0.09
Coated
43.67 ±2.08
1.41 ±0.15
Groups
bond strength of 24 porcelain sintered specimens (12 coated and 12 noncoated with ZrSiN/ZrO2 film) was evaluated by a 3-point bend test with a universal mechanical testing machine (DSS-25T; Shimadzu Corp, Kyoto, Japan). The schematic diagram is displayed in Figure 1. The results were analyzed with an independent samples t test (α = .05) with statistical software (SPSS v11.0; IBM Corp, Chicago, Ill).
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RESULTS Film thickness and Phase composition The mean thickness of the ZrSiN/ ZrO2 film was 650 ±21 nm. Figure 2 shows the XRD pattern of the ZrSiN/ ZrO2 composite film deposited on a silicon substrate. Only a silicon phase (substrate) and a ZrO2 phase (surface coating) were detected. The diffrac-
Scanning electron microscope (SEM) and energy-dispersive x-ray analysis (EDS) were used to study the surfaces of the coated and noncoated Ti substrates that had been subjected to simulated porcelain sintering thermocycles (SPSTs) in the dental furnace. A thick oxidized layer was formed on the surfaces of noncoated Ti substrates. However, no cracks or flaws were observed on coated surfaces, and the film appeared to fully adhere to the Ti substrates. The EDS results revealed that the oxygen atom concentration increased significantly in the noncoated group after SPSTs, but almost no change in the ZrSiN/ ZrO2-coated group was noted. Figures 3A and B show the SEM results of the delaminated surfaces of a selected specimen after the 3-point bend test. The EDS test revealed that the bright areas corresponded to the residual porcelain, while the dark ones were interlayer or exposed Ti surfaces. On the noncoated substrate, no residual ceramic was retained, and only a substantial amount of Ti was detected by EDS, indicating an adhesive titanium-ceramic bond failure mode (Fig. 3A). However, a mixed adhesive and cohesive failure mode was observed on the surface of the ZrSiN/ZrO2-coated substrate, which had sporadic porcelain remnants remaining (Fig. 3B).
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A
B
3 SEM images of delaminated noncoated (A) and coated (B) surfaces (×150). Porcelain remnants could be identified as light areas. Dark regions represented metal substrates. EDS analyses of elements of these remnants were consistent with bonding porcelain composition. A, Noncoated group; minimal residual porcelain retained on surface. B, Coated group; sporadic porcelain remnants retained on surface.
O
Si
Zr
Ti
80 60 40 20 0 80 60 40 20 0 80 60 40 20 0 80 60 40 20 0
O
Si
Zr
Ti 0
50
100
150
200
250
80 60 40 20 0 80 60 40 20 0 80 60 40 20 0 80 60 40 20 0
0
50
Distance (µm)
100
150
200
250
Distance (µm)
4 SEM and line scanning EDS results of cross-sectioned noncoated (A) and coated (B) titanium-porcelain systems (×150). Region in which oxidation occurred is marked by black arrows (A) according to EDS results.
Interfacial characterization The SEM and line scanning EDS examinations of the cross-sectioned, post-sintering specimens are shown in Figure 4. No apparent oxide layer
was observed at the interface of Ti and porcelain in either group. In the noncoated group (Fig. 4A), several precracks were observed, and the EDS result showed that the oxygen atom concentration increased, while that of
The Journal of Prosthetic Dentistry
titanium decreased at the same location on the Ti substrate (black arrows, Fig. 4A). In the coated group, the oxygen atom concentration stayed at a low level on the Ti substrate (Fig. 4B).
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May 2013 DISCUSSION The null hypothesis was rejected based on the study results. The data demonstrated that the magnetronsputtered ZrSiN/ZrO2 film increased the bond strength of CP Ti to porcelain. In the current study, the results of 3-point bend tests revealed that the bond strength of ZrSiN/ZrO2-coated CP Ti to porcelain was significantly higher than that of the noncoated group. The SEM images of delaminated surfaces provide further evidence. In addition, EDS analysis of the crosssectioned ZrSiN/ZrO2-coated CP Ti after porcelain sintering showed that the ZrSiN/ZrO2 film effectively prevented the intrusion of oxygen into the Ti substrate. Furthermore, the SPSTs experiment verified the thermal stability of the ZrSiN/ZrO2-coated CP Ti during the porcelain sintering process. The XRD pattern of the ZrSiN/ ZrO2 film coated silicon was silicon phase (substrate) and ZrO2 phase (surface coating) only, without the detection of silicon-containing and nitrogen-containing compound. The crystallized phase was not evidenced by XRD since it was present in the film as a small quantity of grains or as ultrafine grains dispersed in a large amount of amorphous tissue. The pN2 in this study was 0.09 Pa. This means that the ZrSiN/ZrO2 film should consist of a crystallized ZrO2 layer and a ZrSiN layer, which might exist in an amorphous/ nanocrystalline state. This is in agreement with previous studies.25,30 Moreover, surface characteristics have been documented to exert a remarkable influence on the bonding force,16 and surface roughness, which results from differences in the substrate treatment, has been proven to correlate highly to the bond strength of metal to porcelain.13,14 A high degree of surface roughness facilitates efficient mechanical interlocking between Ti and porcelain.32 The results suggested that the increased surface roughness of the ZrSiN/ ZrO2-coated group might have contribute to the increased bond strength of CP Ti to porcelain.
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The SEM and EDS results revealed that the oxygen atom concentration increased significantly in the noncoated group after SPSTs, but almost no change in the ZrSiN/ZrO2-coated group was observed. The data suggested that the surfaces of the Ti substrates without coating were oxidized, while the surfaces of the ZrSiN/ ZrO2-coated Ti substrates remained stable. The ZrSiN/ZrO2 composite film appeared to successfully inhibit the Ti substrates from being oxidized, which prevented the formation of a nonadherent oxide film on the Ti surface, thereby effectively improving the bond strength. Furthermore, the ceramic remnants on the ZrSiN/ZrO2-coated specimens were significantly larger and more frequent than on the noncoated specimens. This could indicate the superior adhesion of coated-Ti to porcelain.33 However, Papazoglou and Brantley34 considered that no correlation existed between the percentage of ceramic retained and the failure bond. In addition, the results of interfacial characterization indicated that the oxidation might occur inside the Ti substrate in the control group. According to the EDS results of the coated group, no obvious oxidation occurred inside the Ti substrate (Fig. 4B). A comparison of the EDS results revealed that the oxygen atoms penetrated the Ti surface to a greater depth in the case of the noncoated group.19 Preventing the formation of oxidation on titanium surfaces with a binary transition metal nitride film, ZrN, was widely used in the past.35 And to further improve its thermal stability and oxidation resistance, the addition of silicon was recommended, and ZrSiN film was applied.18,23 However, some major problems such as nitrogen gas and unstable ZrO2 arose from the use of ZrSiN film during the sintering process, and, for this reason, a series of metal oxygen films composite with ZrSiN film was devised to address the issue. In the present study, the magnetron-sputtered ZrSiN/ZrO2
composite film was selected not only because the strong stability of magnetron-sputtered ZrO2 film36 could effectively prevent the formation of the byproducts of ZrSiN film during the porcelain sintering at high temperature, but also because the intrinsic affinity of ZrO2 to porcelain could greatly increase the bonding force of CP Ti to porcelain. The excessively thick and nonadherent titanium oxide layer formed during the porcelain sintering process can cause severe bonding problems,8,9 which must be solved. The present investigation was designed to improve the titanium-ceramic bond with a composite magnetron-sputtered ZrSiN/ZrO2 film. The results suggested that this intermediate film significantly increased the bond strength of CP Ti to porcelain. However, a limitation of this study was that only XRD, EDS, and SEM techniques were used to study the structure, elemental compositions, and surface morphologies. In order to further analyze the ZrSiN/ZrO2 film and investigate its oxidation-resistant ability, the x-ray photoelectron spectroscopy (XPS) atomic emission spectrometer (AES) and other technologies should be included. Furthermore, future research work should focus on facilitating the clinical application of ZrSiN/ZrO2 coated titanium.
CONCLUSIONS Within the limitations of this investigation and for the materials evaluated in this study, the following conclusions can be drawn: 1. The magnetron-sputtered ZrSiN/ZrO2 film significantly increased the bond strength of CP Ti to porcelain. 2. The magnetron-sputtered ZrSiN/ZrO2 diffusion barrier exhibited superior thermal stability during the porcelain sintering process.
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Volume 109 Issue 5 REFERENCES 1. Lautenschlager EP, Monaghan P. Titanium and titanium alloys as dental materials. Int Dent J 1993;43:245-53. 2. Zinelis S, Barmpagadaki X, Vergos V, Chakmakchi M, Eliades G. Bond strength and interfacial characterization of eight low fusing porcelains to cp Ti. Dent Mater 2010;26:264-73. 3. Eliopoulos D, Zinelis S, Papadopoulos T. The effect of investment material type on the contamination zone and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2005;94:539-48. 4. Zinelis S. Effect of pressure of helium, argon, krypton, and xenon on the porosity, microstructure, and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2000;84:575-82. 5. Suansuwan N, Swain MV. Adhesion of porcelain to titanium and a titanium alloy. J Dent 2003;31:509-18. 6. Adachi M, Mackert JR, Jr., Parry EE, Fairhurst CW. Oxide adherence and porcelain bonding to titanium and Ti-6Al-4V alloy. J Dent Res 1990;69:1230-5. 7. Tróia MG, Jr., Henriques GE, Nóbilo MA, Mesquita MF. The effect of thermal cycling on the bond strength of low-fusing porcelain to commercially pure titanium and titanium-aluminium-vanadium alloy. Dent Mater 2003;19:790-6. 8. Papadopoulos TD, Spyropoulos KD. The effect of a ceramic coating on the cpTiporcelain bond strength. Dent Mater 2009;25:247-53. 9. Atsü S, Berksun S. Bond strength of three porcelains to two forms of titanium using two firing atmospheres. J Prosthet Dent 2000;84:567-74. 10.Tholey MJ, Waddell JN, Swain MV. Influence of the bonder on the adhesion of porcelain to machined titanium as determined by the strain energy release rate. Dent Mater 2007;23:822-8. 11.Wang RR, Fung KK. Oxidation behavior of surface-modified titanium for titaniumceramic restorations. J Prosthet Dent 1997;77:423-34. 12.Zhang XM, Yue TM, Man HC. Enhancement of ceramic to metal adhesive bonding by excimer laser surface treatment. Mater Lett 1997;30:327-32. 13.Wagner WC, Asgar K, Bigelow WC, Flinn RA. Effect of interfacial variables on metalporcelain bonding. J Biomed Mater Res 1993;27:531-7. 14.Gilbert JL, Covey DA, Lautenschlager EP. Bond characteristics of porcelain fused to milled titanium. Dent Mater 1994;10:134-40.
15.Papadopoulos T, Tsetsekou A, Eliades G. Effect of aluminium oxide sandblasting on cast commercially pure titanium surfaces. Eur J Prosthodont Restor Dent 1999;7:15-21. 16.Zhang ZC, Zhang P, Guo LT, Guo TW, Yang JF. Effect of TiO2-SiO2 sol-gel coating on the cpTi-porcelain bond strength. Mater Lett 2011;65:1082-5. 17.Yamada K, Onizuka T, Endo K, Ohno H, Swain MV. The influence of Goldbonder and pre-heat treatment on the adhesion of titanium alloy and porcelain. J Oral Rehabil 2005;32:213-20. 18.Zhang H, Guo TW, Song ZX, Wang XJ, Xu KW. The effect of ZrSiN diffusion barrier on the bonding strength of titanium porcelain. Surf Coat Technol 2007;210:5637-40. 19.Wang SS, Xia Y, Zhang LB, Guang HB, Shen M, Zhang FM. Effect of NbN and ZrN films formed by magnetron sputtering on Ti and porcelain bonding. Surf Coat Technol 2010;205:1886-91. 20.Guo LT, Liu XC, He ZY, Gao JQ, Yang JF, Guo TW. Effect of fluoride corrosion on the bonding strength of Ti-porcelain. Mater Lett 2008;62:2204-6. 21.Guo LT, Liu XC, Zhu YB, Xu C, Gao JQ, Guo TW. Effect of oxidation and SiO2 coating on the bonding strength of Ti-porcelain. J Mater Eng Perform 2010;19:1189-92. 22.Song ZX, Xu KW, Chen H. The characterization of Zr-Si-N diffusion barrier films with different sputtering bias voltage. Thin Solid Films 2004;468:203-7. 23.Zeman P, Musil J. Difference in high-temperature oxidation resistance of amorphous Zr-Si-N and W-Si-N films with a high Si content. Appl Surf Sci 2006;252:8319-25. 24.Veprek S, Niederhofer A, Moto K, Bolom T. Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si3N4/a- and ncTiSi2 nano-composites with HV=80 to ≥ 105 Gpa. Surf Coat Technol 2000;152:133-4. 25.Daniel R, Musil J, Zeman P, Mitterer C. Thermal stability of magnetron sputtered Zr-Si-N films. Surf Coat Technol 2006;201:3368-76. 26.Mae T, Nose M, Zhou M, Nagae T, Shimamura K. The effects of Si addition on the structure and mechanical properties of ZrN thin films deposited by an r.f. reactive sputtering method. Surf Coat Technol 2001;142:954-8. 27.Pilloud D, Pierson JF, Marques AP, Cavaleiro A. Structural changes in ZrSiN films vs. their silicon content. Surf Coat Technol 2004;180:352-6.
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28.Barna PB, Adamik M, Lábár J, Kövér L, Tóth J, Dévényi A et al. Formation of polycrystalline and microcrystalline composite thin films by codeposition and surface chemical reaction. Surf Coat Technol 2000;125:147-50. 29.Nose M, Chiou WA, Zhou M, Mae T, Meshii M. Microstructure and mechanical properties of ZrSiN films prepared by rf-reactive sputtering. J Vac Sci Technol A 2002;20:823-8. 30.Song ZX, Xu KW, Chen H. The effect of nitrogen partial pressure on Zr-Si-N diffusion barrier. Microelecton Eng 2004;71:28-33. 31.International Organization for Standardization. ISO 9693-1:2012. Dentistry--Compatibility testing--Part 1: Metal-ceramic systems. Geneva: International Organization for Standardization; 2012. Available at http://www.iso.org/iso/store/htm 32.Papadopoulos T, Tsetsekou A, Eliades G. Effect of aluminium oxide sandblasting on cast commercially pure titanium surfaces. Eur J Prosthodont Restor Dent 1999;7:15-21. 33.Ozcan I, Uysal H. Effects of silicon coating on bond strength of two different titanium ceramic to titanium. Dent Mater 2005;21:773-9. 34.Papazoglou E, Brantley WA. Porcelain adherence vs force to failure for palladiumgallium alloys: a critique of metal-ceramic bond testing. Dent Mater 1998;14:112-9. 35.Bhuvaneswari HB, Reddy VR, Chandramani R, Rao GM. Annealing effects on zirconium nitride films. Appl Surf Sci 2004;230:88-93. 36.Ko JH, Kim SH, Jee SH, Yoon YS. Characteristics of ZrO2 thin films deposited by reactive magnetron sputtering. J Korean Phys Soc 2007;50:1843-7. Corresponding author: Dr Tianwen Guo Department of Prosthodontics School of Stomatology Fourth Military Medical University Xi’an 710032, Shaanxi P.R. CHINA Fax: +86 29 84776252 E-mail: [email protected] Acknowledgments The authors thank Prof. Zhongxiao Song and Dr Guohua He, State-Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, China, for their technical support. Copyright © 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications
Clinical delamination of titanium ceramic restorations may be reduced by the application of a magnetron-sputtered ZrSiN/ZrO2 film. Titanium (Ti) has been widely used as a dental material for its excellent biocompatibility, high corrosion resistance, desirable physical mechanical properties, and low cost.1-5 However, Ti exhibits strong reactivity with oxygen, especially during porcelain sintering at high temperatures, which creates an excessively thick and nonadherent Ti oxide layer.6,7 This oxi-
dation layer can cause problems such as poor bonding, even leading to the failure of the Ti-to-porcelain bond.8-10 Many approaches have been reported to control Ti oxidation.11-15 A well-recognized method is to coat the Ti substrate with an intermediate film prior to porcelain sintering.16-19 The film should firmly bond with both the Ti substrate and the porcelain after
sintering.17,20,21 A magnetron-sputtered amorphous ZrSiN film has been reported to exert an oxidation-resistant ability at high temperature.18,22,23 However, it should be noted that a phase evolution of the ZrSiN film might occur with the increasing partial pressure of nitrogen (pN2). Considering the preferential bonding between the elements, a nanocomposite
This work was supported by the National Natural Science Foundation of China (No. 30371557). Lecturer, Department of Prosthodontics. Lecturer, Department of Prosthodontics. c Professor, Department of Prosthodontics. d Professor, Department of Prosthodontics. a
b
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Volume 109 Issue 5 structure composed of ZrSi2 grains embedded within an amorphous Si3N4 matrix with low pN2 (pN2<0.05 Pa) is assumed to develop. This is similar to the TiSiN film synthesized by Veprek et al.24 Another consequence of high pN2 is an increasing amount of Si3N4, which eventually becomes the dominant component (pN2 > 0.05 Pa) of the film. ZrSiN film, which consists of Si3N4, ZrSix, and ZrNx, exhibits an amorphous-like structure as determined by x-ray diffraction (XRD). In general, the structure of ZrSiN film can change from a crystalline (pN2< 0.05 Pa) one to an x-ray amorphous one (pN2>0.05 Pa).25-29 Furthermore, Song et al30 have demonstrated that a preferable diffusion barrier of ZrSiN film was formed with a pN2 of 0.09 Pa. However, in an aerobic high temperature environment, the ZrSiN film may release nitrogen gas and form a thick and unstable layer of ZrO2,22 which may compromise the Ti-to-porcelain bond. The present investigation was designed to improve this ZrSiN film with a composite magnetron-sputtered ZrSiN/ZrO2 film. The hypothesis was that this optimized composite film, ZrSiN/ZrO2, may eliminate the adverse effects of the byproducts of the ZrSiN film created during the porcelain sintering process but still retain the oxidation-resistant ability of previously used ZrSiN film. The null hypothesis of the study was that the magnetron-sputtered ZrSiN/ZrO2 composite film would not improve the bond strength of CP Ti to porcelain. MATERIAL AND METHODS Specimen preparation Sixty-eight CP Ti specimens (ASTM Grade II; Northwest Institute for Nonferrous Metal Research, Xi’an, China) were cast (25 × 3 × 0.5 mm) according to the International Organization of Standardization (ISO) 9693 standard31 and polished with silicon carbide papers under wet conditions. All of the specimens were airborneparticle abraded with 70 µm Al2O3
under 0.2 MPa pressure at a 45-degree angle. The source of the Al2O3 particles was 10 mm away from the Ti substrates. The specimens were then ultrasonically rinsed in deionized water, acetone, and ethanol for 15 minutes each and dried in air. A ZrSiN film was magnetron-sputtered by a multitarget magnetron sputtering system (ASC-4000-C4; ULVAC Inc, Kanagawa, Japan) onto 34 Ti substrates in an Ar/N2 (7/3) gas mixture with a base pressure of 2 × 10-5 Pa. Si and Zr disks (purity 99%, diameter 73.0 × 3.0 mm; Northwest Institute for Nonferrous Metal Research) were used as the targets. The substrate-to-target distance was 60.0 mm. Before sputtering, Ti substrates were cleaned by bombardment with Ar ions. The ZrSiN film was deposited with a target power for Zr of 100 W (direct current, DC) and a target power for Si of 11 W (radio frequency, RF). The substrate bias voltage was -100 V, working pressure 0.3 Pa, and deposition time 1 hour. Subsequently, a ZrO2 film was deposited under the following conditions: gas mixture, Ar/ O2 (7/3); target power, 120 W (DC); bias voltage, -100 V; working pressure, 0.3 Pa and deposition time, 1 hour. Water circulation was used as a cooling treatment. After the deposition, the substrates were subjected to steam and ultrasonic cleaning. To study the structure of the composite film more easily, 5 silicon substrates were also processed with the same surface treatments as previously described and then tested with XRD (DMAX 1200; Tigaku, Tokyo, Japan). To measure the film thickness, 10 silicon plates (3.0 × 1.0 cm) were covered by other smaller silicon plates (1.0 × 1.0 cm) in the central part. After they were coated with ZrSiN/ZrO2 film, the small central silicon plates were carefully removed. The cross-sections of the magnetron-sputtered ZrSiN/ZrO2 film were therefore exposed on the 3.0 × 1.0 cm silicon plates. The thickness of the film was tested with a surface profilometer (Dektak IIA; Veeco Instruments Inc, Plainview, NY). After ultrasonic cleaning, a low-
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fusing porcelain (Ti-22; Noritake Co, Nagoya, Japan) was then brushed onto the central region (8.0 × 3.0 mm) of 34 specimens (17 coated with ZrSiN/ZrO2 film and 17 noncoated) and sintered according to the manufacturer’s recommendations. The thickness of the bonding agent and the opaque layer were 0.2 mm; the dentin coating was 0.6 mm. Surface and interfacial characterization With an assumed minimum of 80% power and 2-sided type α error of .05, the sample size of 8 was calculated on the surface roughness (Ra) value from the preliminary experiment (the difference between coated and uncoated ZrSiN/ZrO2 group was 0.3, and the pooled variance was 0.02), with 4 in each group. For Ra measurements in the current study, 24 specimens (12 coated and 12 noncoated with ZrSiN/ ZrO2 film) were selected. The Ra value was measured with a surface roughness tester (TR240; Beijing Time Group Inc, Beijing, China). To study the thermal stability of the film, 10 titanium specimens (5 coated with ZrSiN/ZrO2 film and 5 noncoated) were subjected to simulated porcelain sintering thermocycles (SPSTs) in a dental furnace (Multimat 99; Dentsply Intl, York, Pa). They were not coated with porcelain layers. Ten porcelain sintered specimens (5 coated and 5 noncoated with ZrSiN/ZrO2 film) were selected to study the cross-sections. SEM and EDS (S-4800; Hitachi Ltd, Tokyo, Japan) were used to analyze the surface structures and components. Evaluation of bond strength With an assumed minimum of 80% power and 2-sided type α error of .05, the sample size of 14 was calculated on the bond strength from the preliminary experiment (the difference between coated and uncoated ZrSiN/ZrO2 group was 6, and the pooled variance was 16), with 7 in each group. In the current study, the
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May 2013 tion peaks of the silicon-containing and nitrogen-containing compound were not detected. Surface roughness and bond strength Table I shows the mean values and standard deviations of the CP Ti to porcelain bond strength and the surface roughness. The results revealed that both the Ra value of the composite film and the bond strength of the ZrSiN/ZrO2-coated group were significantly higher than the noncoated group (P<.05).
1 Schematic diagram of 3-point bend test. 160 140
Thermal stability and fracture mode
Intensity (Counts)
120 100 ZrO2
80
Si
60 40 20 0
20
30
40
50
70
60
80
90
2-Theta (degrees)
2 XRD pattern of ZrSiN/ZrO2 film deposited on Si substrate.
Table I. Results of CP Ti to porcelain bond strength and roughness parameters (n=12). Ra is arithmetic mean Bond Strength (MPa)
Ra (µm)
Noncoated
35.44 ±3.56
1.05 ±0.09
Coated
43.67 ±2.08
1.41 ±0.15
Groups
bond strength of 24 porcelain sintered specimens (12 coated and 12 noncoated with ZrSiN/ZrO2 film) was evaluated by a 3-point bend test with a universal mechanical testing machine (DSS-25T; Shimadzu Corp, Kyoto, Japan). The schematic diagram is displayed in Figure 1. The results were analyzed with an independent samples t test (α = .05) with statistical software (SPSS v11.0; IBM Corp, Chicago, Ill).
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RESULTS Film thickness and Phase composition The mean thickness of the ZrSiN/ ZrO2 film was 650 ±21 nm. Figure 2 shows the XRD pattern of the ZrSiN/ ZrO2 composite film deposited on a silicon substrate. Only a silicon phase (substrate) and a ZrO2 phase (surface coating) were detected. The diffrac-
Scanning electron microscope (SEM) and energy-dispersive x-ray analysis (EDS) were used to study the surfaces of the coated and noncoated Ti substrates that had been subjected to simulated porcelain sintering thermocycles (SPSTs) in the dental furnace. A thick oxidized layer was formed on the surfaces of noncoated Ti substrates. However, no cracks or flaws were observed on coated surfaces, and the film appeared to fully adhere to the Ti substrates. The EDS results revealed that the oxygen atom concentration increased significantly in the noncoated group after SPSTs, but almost no change in the ZrSiN/ ZrO2-coated group was noted. Figures 3A and B show the SEM results of the delaminated surfaces of a selected specimen after the 3-point bend test. The EDS test revealed that the bright areas corresponded to the residual porcelain, while the dark ones were interlayer or exposed Ti surfaces. On the noncoated substrate, no residual ceramic was retained, and only a substantial amount of Ti was detected by EDS, indicating an adhesive titanium-ceramic bond failure mode (Fig. 3A). However, a mixed adhesive and cohesive failure mode was observed on the surface of the ZrSiN/ZrO2-coated substrate, which had sporadic porcelain remnants remaining (Fig. 3B).
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A
B
3 SEM images of delaminated noncoated (A) and coated (B) surfaces (×150). Porcelain remnants could be identified as light areas. Dark regions represented metal substrates. EDS analyses of elements of these remnants were consistent with bonding porcelain composition. A, Noncoated group; minimal residual porcelain retained on surface. B, Coated group; sporadic porcelain remnants retained on surface.
O
Si
Zr
Ti
80 60 40 20 0 80 60 40 20 0 80 60 40 20 0 80 60 40 20 0
O
Si
Zr
Ti 0
50
100
150
200
250
80 60 40 20 0 80 60 40 20 0 80 60 40 20 0 80 60 40 20 0
0
50
Distance (µm)
100
150
200
250
Distance (µm)
4 SEM and line scanning EDS results of cross-sectioned noncoated (A) and coated (B) titanium-porcelain systems (×150). Region in which oxidation occurred is marked by black arrows (A) according to EDS results.
Interfacial characterization The SEM and line scanning EDS examinations of the cross-sectioned, post-sintering specimens are shown in Figure 4. No apparent oxide layer
was observed at the interface of Ti and porcelain in either group. In the noncoated group (Fig. 4A), several precracks were observed, and the EDS result showed that the oxygen atom concentration increased, while that of
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titanium decreased at the same location on the Ti substrate (black arrows, Fig. 4A). In the coated group, the oxygen atom concentration stayed at a low level on the Ti substrate (Fig. 4B).
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May 2013 DISCUSSION The null hypothesis was rejected based on the study results. The data demonstrated that the magnetronsputtered ZrSiN/ZrO2 film increased the bond strength of CP Ti to porcelain. In the current study, the results of 3-point bend tests revealed that the bond strength of ZrSiN/ZrO2-coated CP Ti to porcelain was significantly higher than that of the noncoated group. The SEM images of delaminated surfaces provide further evidence. In addition, EDS analysis of the crosssectioned ZrSiN/ZrO2-coated CP Ti after porcelain sintering showed that the ZrSiN/ZrO2 film effectively prevented the intrusion of oxygen into the Ti substrate. Furthermore, the SPSTs experiment verified the thermal stability of the ZrSiN/ZrO2-coated CP Ti during the porcelain sintering process. The XRD pattern of the ZrSiN/ ZrO2 film coated silicon was silicon phase (substrate) and ZrO2 phase (surface coating) only, without the detection of silicon-containing and nitrogen-containing compound. The crystallized phase was not evidenced by XRD since it was present in the film as a small quantity of grains or as ultrafine grains dispersed in a large amount of amorphous tissue. The pN2 in this study was 0.09 Pa. This means that the ZrSiN/ZrO2 film should consist of a crystallized ZrO2 layer and a ZrSiN layer, which might exist in an amorphous/ nanocrystalline state. This is in agreement with previous studies.25,30 Moreover, surface characteristics have been documented to exert a remarkable influence on the bonding force,16 and surface roughness, which results from differences in the substrate treatment, has been proven to correlate highly to the bond strength of metal to porcelain.13,14 A high degree of surface roughness facilitates efficient mechanical interlocking between Ti and porcelain.32 The results suggested that the increased surface roughness of the ZrSiN/ ZrO2-coated group might have contribute to the increased bond strength of CP Ti to porcelain.
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The SEM and EDS results revealed that the oxygen atom concentration increased significantly in the noncoated group after SPSTs, but almost no change in the ZrSiN/ZrO2-coated group was observed. The data suggested that the surfaces of the Ti substrates without coating were oxidized, while the surfaces of the ZrSiN/ ZrO2-coated Ti substrates remained stable. The ZrSiN/ZrO2 composite film appeared to successfully inhibit the Ti substrates from being oxidized, which prevented the formation of a nonadherent oxide film on the Ti surface, thereby effectively improving the bond strength. Furthermore, the ceramic remnants on the ZrSiN/ZrO2-coated specimens were significantly larger and more frequent than on the noncoated specimens. This could indicate the superior adhesion of coated-Ti to porcelain.33 However, Papazoglou and Brantley34 considered that no correlation existed between the percentage of ceramic retained and the failure bond. In addition, the results of interfacial characterization indicated that the oxidation might occur inside the Ti substrate in the control group. According to the EDS results of the coated group, no obvious oxidation occurred inside the Ti substrate (Fig. 4B). A comparison of the EDS results revealed that the oxygen atoms penetrated the Ti surface to a greater depth in the case of the noncoated group.19 Preventing the formation of oxidation on titanium surfaces with a binary transition metal nitride film, ZrN, was widely used in the past.35 And to further improve its thermal stability and oxidation resistance, the addition of silicon was recommended, and ZrSiN film was applied.18,23 However, some major problems such as nitrogen gas and unstable ZrO2 arose from the use of ZrSiN film during the sintering process, and, for this reason, a series of metal oxygen films composite with ZrSiN film was devised to address the issue. In the present study, the magnetron-sputtered ZrSiN/ZrO2
composite film was selected not only because the strong stability of magnetron-sputtered ZrO2 film36 could effectively prevent the formation of the byproducts of ZrSiN film during the porcelain sintering at high temperature, but also because the intrinsic affinity of ZrO2 to porcelain could greatly increase the bonding force of CP Ti to porcelain. The excessively thick and nonadherent titanium oxide layer formed during the porcelain sintering process can cause severe bonding problems,8,9 which must be solved. The present investigation was designed to improve the titanium-ceramic bond with a composite magnetron-sputtered ZrSiN/ZrO2 film. The results suggested that this intermediate film significantly increased the bond strength of CP Ti to porcelain. However, a limitation of this study was that only XRD, EDS, and SEM techniques were used to study the structure, elemental compositions, and surface morphologies. In order to further analyze the ZrSiN/ZrO2 film and investigate its oxidation-resistant ability, the x-ray photoelectron spectroscopy (XPS) atomic emission spectrometer (AES) and other technologies should be included. Furthermore, future research work should focus on facilitating the clinical application of ZrSiN/ZrO2 coated titanium.
CONCLUSIONS Within the limitations of this investigation and for the materials evaluated in this study, the following conclusions can be drawn: 1. The magnetron-sputtered ZrSiN/ZrO2 film significantly increased the bond strength of CP Ti to porcelain. 2. The magnetron-sputtered ZrSiN/ZrO2 diffusion barrier exhibited superior thermal stability during the porcelain sintering process.
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Volume 109 Issue 5 REFERENCES 1. Lautenschlager EP, Monaghan P. Titanium and titanium alloys as dental materials. Int Dent J 1993;43:245-53. 2. Zinelis S, Barmpagadaki X, Vergos V, Chakmakchi M, Eliades G. Bond strength and interfacial characterization of eight low fusing porcelains to cp Ti. Dent Mater 2010;26:264-73. 3. Eliopoulos D, Zinelis S, Papadopoulos T. The effect of investment material type on the contamination zone and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2005;94:539-48. 4. Zinelis S. Effect of pressure of helium, argon, krypton, and xenon on the porosity, microstructure, and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2000;84:575-82. 5. Suansuwan N, Swain MV. Adhesion of porcelain to titanium and a titanium alloy. J Dent 2003;31:509-18. 6. Adachi M, Mackert JR, Jr., Parry EE, Fairhurst CW. Oxide adherence and porcelain bonding to titanium and Ti-6Al-4V alloy. J Dent Res 1990;69:1230-5. 7. Tróia MG, Jr., Henriques GE, Nóbilo MA, Mesquita MF. The effect of thermal cycling on the bond strength of low-fusing porcelain to commercially pure titanium and titanium-aluminium-vanadium alloy. Dent Mater 2003;19:790-6. 8. Papadopoulos TD, Spyropoulos KD. The effect of a ceramic coating on the cpTiporcelain bond strength. Dent Mater 2009;25:247-53. 9. Atsü S, Berksun S. Bond strength of three porcelains to two forms of titanium using two firing atmospheres. J Prosthet Dent 2000;84:567-74. 10.Tholey MJ, Waddell JN, Swain MV. Influence of the bonder on the adhesion of porcelain to machined titanium as determined by the strain energy release rate. Dent Mater 2007;23:822-8. 11.Wang RR, Fung KK. Oxidation behavior of surface-modified titanium for titaniumceramic restorations. J Prosthet Dent 1997;77:423-34. 12.Zhang XM, Yue TM, Man HC. Enhancement of ceramic to metal adhesive bonding by excimer laser surface treatment. Mater Lett 1997;30:327-32. 13.Wagner WC, Asgar K, Bigelow WC, Flinn RA. Effect of interfacial variables on metalporcelain bonding. J Biomed Mater Res 1993;27:531-7. 14.Gilbert JL, Covey DA, Lautenschlager EP. Bond characteristics of porcelain fused to milled titanium. Dent Mater 1994;10:134-40.
15.Papadopoulos T, Tsetsekou A, Eliades G. Effect of aluminium oxide sandblasting on cast commercially pure titanium surfaces. Eur J Prosthodont Restor Dent 1999;7:15-21. 16.Zhang ZC, Zhang P, Guo LT, Guo TW, Yang JF. Effect of TiO2-SiO2 sol-gel coating on the cpTi-porcelain bond strength. Mater Lett 2011;65:1082-5. 17.Yamada K, Onizuka T, Endo K, Ohno H, Swain MV. The influence of Goldbonder and pre-heat treatment on the adhesion of titanium alloy and porcelain. J Oral Rehabil 2005;32:213-20. 18.Zhang H, Guo TW, Song ZX, Wang XJ, Xu KW. The effect of ZrSiN diffusion barrier on the bonding strength of titanium porcelain. Surf Coat Technol 2007;210:5637-40. 19.Wang SS, Xia Y, Zhang LB, Guang HB, Shen M, Zhang FM. Effect of NbN and ZrN films formed by magnetron sputtering on Ti and porcelain bonding. Surf Coat Technol 2010;205:1886-91. 20.Guo LT, Liu XC, He ZY, Gao JQ, Yang JF, Guo TW. Effect of fluoride corrosion on the bonding strength of Ti-porcelain. Mater Lett 2008;62:2204-6. 21.Guo LT, Liu XC, Zhu YB, Xu C, Gao JQ, Guo TW. Effect of oxidation and SiO2 coating on the bonding strength of Ti-porcelain. J Mater Eng Perform 2010;19:1189-92. 22.Song ZX, Xu KW, Chen H. The characterization of Zr-Si-N diffusion barrier films with different sputtering bias voltage. Thin Solid Films 2004;468:203-7. 23.Zeman P, Musil J. Difference in high-temperature oxidation resistance of amorphous Zr-Si-N and W-Si-N films with a high Si content. Appl Surf Sci 2006;252:8319-25. 24.Veprek S, Niederhofer A, Moto K, Bolom T. Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si3N4/a- and ncTiSi2 nano-composites with HV=80 to ≥ 105 Gpa. Surf Coat Technol 2000;152:133-4. 25.Daniel R, Musil J, Zeman P, Mitterer C. Thermal stability of magnetron sputtered Zr-Si-N films. Surf Coat Technol 2006;201:3368-76. 26.Mae T, Nose M, Zhou M, Nagae T, Shimamura K. The effects of Si addition on the structure and mechanical properties of ZrN thin films deposited by an r.f. reactive sputtering method. Surf Coat Technol 2001;142:954-8. 27.Pilloud D, Pierson JF, Marques AP, Cavaleiro A. Structural changes in ZrSiN films vs. their silicon content. Surf Coat Technol 2004;180:352-6.
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28.Barna PB, Adamik M, Lábár J, Kövér L, Tóth J, Dévényi A et al. Formation of polycrystalline and microcrystalline composite thin films by codeposition and surface chemical reaction. Surf Coat Technol 2000;125:147-50. 29.Nose M, Chiou WA, Zhou M, Mae T, Meshii M. Microstructure and mechanical properties of ZrSiN films prepared by rf-reactive sputtering. J Vac Sci Technol A 2002;20:823-8. 30.Song ZX, Xu KW, Chen H. The effect of nitrogen partial pressure on Zr-Si-N diffusion barrier. Microelecton Eng 2004;71:28-33. 31.International Organization for Standardization. ISO 9693-1:2012. Dentistry--Compatibility testing--Part 1: Metal-ceramic systems. Geneva: International Organization for Standardization; 2012. Available at http://www.iso.org/iso/store/htm 32.Papadopoulos T, Tsetsekou A, Eliades G. Effect of aluminium oxide sandblasting on cast commercially pure titanium surfaces. Eur J Prosthodont Restor Dent 1999;7:15-21. 33.Ozcan I, Uysal H. Effects of silicon coating on bond strength of two different titanium ceramic to titanium. Dent Mater 2005;21:773-9. 34.Papazoglou E, Brantley WA. Porcelain adherence vs force to failure for palladiumgallium alloys: a critique of metal-ceramic bond testing. Dent Mater 1998;14:112-9. 35.Bhuvaneswari HB, Reddy VR, Chandramani R, Rao GM. Annealing effects on zirconium nitride films. Appl Surf Sci 2004;230:88-93. 36.Ko JH, Kim SH, Jee SH, Yoon YS. Characteristics of ZrO2 thin films deposited by reactive magnetron sputtering. J Korean Phys Soc 2007;50:1843-7. Corresponding author: Dr Tianwen Guo Department of Prosthodontics School of Stomatology Fourth Military Medical University Xi’an 710032, Shaanxi P.R. CHINA Fax: +86 29 84776252 E-mail: [email protected] Acknowledgments The authors thank Prof. Zhongxiao Song and Dr Guohua He, State-Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, China, for their technical support. Copyright © 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
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