| Titanium (Ti) has been widely used as a dental material for its excellent biocompatibility, high corrosion resistance, desirable physical mechanical properties, and low cost. However, Ti exhibits strong reactivity with oxygen, especially during porcelain sintering at high temperatures, which creates an excessively thick and nonadherent layer of Ti oxide. Of note, this oxidation layer can cause severe problems such as poor bonding strength and sometimes even lead to the failure of bonding between Ti and porcelain.Since the major hurdle in the process of titanium sintered to porcelain is the control of oxidation, a variety of approaches were under intense investigations to prevent the forming of oxidation layer. A well-recognized approach is to coat the Ti substrate with an intermediate film prior to porcelain sintering, which should bond firmly with both the Ti substrate and the porcelain after sintering. To date, some progress has been made in this field with active reports that an amorphous ZrSiN film could exert an oxidation-resistant ability at high temperatures. However, a potential risk is that high temperature along with aerobic environment may lead to the release of nitrogen and the formation of an unstable layer of ZrO2from ZrSiN film. This nitrogen gas and unstable ZrO2can severely influence the intensity of bonding between Ti and porcelain. Aimed to eliminate the adverse influence aroused from these by-products of ZrSiN film at high temperature, the present investigation dedicated to optimize the film with a composite magnetron-sputtered ZrSiN/ZrO2. The aim of this study is to evaluate the effect of a ZrSiN/ZrO2composite film deposited by magnetron sputtering on the bonding strength of cpTi to porcelain.CpTi specimens (ASTM Grade II, Northwest Institute for Nonferrous Metal Research, Xi’an, China) was cast (25mm×3mm×0.5mm) according to the ISO9693standard and polished with silicon carbide papers carefully. All of the specimens were sandblasted with70μm AI2O3particles under0.2MPa pressure at a45°angle. The source of AI2O3particles was10mm away from the Ti substrates. The specimens were then ultrasonically rinsed in deionized water, acetone and ethanol for15minutes respectively, and dried in air.A ZrSiN film was then magnetron-sputtered onto34Ti substrates in an Ar/N2(7/3) gas mixture with a base pressure of2×10-5Pa. Si and Zr discs (purity:99%; Φ73.0mm×3.0mm) were used as the targets. The substrate-to-target distance was60.0mm. Prior to sputtering, Ti substrates were cleaned by bombardment with Ar ions. The ZrSiN film was deposited with a target power for Zr of100W (direct-current, DC) and a target power for Si of11W (radio-frequency, RF). The substrate bias voltage, working pressure and deposition time were-100V,0.3Pa and1hour respectively. Subsequently, a ZrO2film was deposited under the following conditions:gas mixture, Ar/O2(7/3); target power,120W (DC); bias voltage,-100V; working pressure,0.3Pa and deposition time,1hour. Water circulation was carried out as a cooling treatment. After the deposition, the substrates were subjected to steam and ultrasonic cleaning. For ease of studying the structure of the composite film,5silicon substrates were also processed under the same surface treatments as described above and then were tested by XRD (DMAX1200, Tigaku, Tokyo, Japan).A low-fusing porcelain (Ti-22, Noritake, Nagoya, Japan) was then brushed onto the central region (8.0mm×3.0mm) of34specimens (17coated and17noncoated) and sintered according to manufacturer’s recommendation. The thickness of the bonding agent, the opaque layer and the dentin coat were0.2,0.2and0.6mm, respectively. The Ra value was measured using a surface roughness tester (TR240, Beijing Time Group Inc, Beijing, China).In order to study the thermal stability of the film, Specimens were subjected to simulated porcelain sintering thermocycles (SPSTs) in a dental furnace but that were not coated with porcelain layers.Sintered specimens were selected randomly to study the cross-sections. SEM and EDS (S-4800, Hitachi, Tokyo, Japan) were employed to analyze the surface structures and components.The bonding strength of porcelain sintered samples was evaluated by a three-point bending test using a universal mechanical testing machine (DSS-25T, Shimadzu, Kyoto, Japan). The schematic diagram are displayed in.The results were analyzed by an independent samples t-test (a=0.05) with SPSS11.0software (Statsoft Inc. USA).The results were shown as following:(1) The XRD pattern of the ZrSiN/ZrO2composite film deposited on a silicon substrate. Only a silicon phase (substrate) and a ZrO2phase (surface coating) were detected. The diffraction peaks of the silicon-containing and nitrogen-containing compound were not detected. Taking into account the strong hindering effect of an amorphous Si3N4phase, crystallized phase is not evidenced by XRD since it is present in the film as a small quantity of grains or as ultra-fine grains dispersed in a high amount of an amorphous tissue. It means that the ZrSiN/ZrO2film consisted of a crystallized ZrO2layer and a ZrSiN layer which might exist in an amorphous/nanocrystalline state.(2) The mean values and standard deviations of the cpTi-porcelain bonding strength and the surface roughness. The results revealed that both the Ra value of the composite film and the bonding strength of the ZrSiN/ZrO2-coated group were significantly increased compared with the noncoated group (p<0.05). Our results suggested that the increased surface roughness of the ZrSiN/ZrO2-coated group might contribute to the increased bonding strength of cpTi to porcelain to some extent.(3) SEM and EDS were applied to study the surfaces of the coated and noncoated Ti substrates that had been subjected to simulated porcelain sintering thermocycles (SPSTs) in a 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 there was almost no change in the ZrSiN/ZrO2-coated group. These results 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/ZrO2composite film appeared to successfully inhibited the Ti substrates from being oxidized, which prevented the formation of a nonadherent oxide film on the Ti surface, and thereby effectively improved the bonding strength.(4) No apparent oxide layer was observed at the interface of Ti and porcelain in either group. In the noncoated group several precracks were observed, and the EDS result showed that the oxygen atom concentration increased while that of titanium decreased at the same location on the Ti substrate. These results indicated that oxidation might occur inside the Ti substrate. According to the EDS results of the coated group, no obvious oxidation occurred inside the Ti substrate. 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. In the very beginning, to prevent the oxidation being happened, ZrN, a binary transition metal nitride film, had been widely used as a protective coating. And to further improve its thermal stability and oxidation resistance, the addition of silicon was recommended, and ZrSiN film was applied. However, some major problems such as nitrogen gas and unstable ZrO2were aroused from the use of ZrSiN film during the sintering process and for this reason, a series of metal oxygen films composite with ZrSiN film were devised to address the issue. In our study, the magnetron-sputtered ZrSiN/ZrO2composite film was selected not only because that the strong stability of magnetron-sputtered ZrO2film could effectively prevent the formation of byproducts of mere ZrSiN film during the sintering at high temperature, but also because that the intrinsic affinity of ZrO2to porcelain could greatly increase the bonding force of cpTi to porcelain.Conclusion(1) The ZrSiN/ZrO2composite film consisted of a crystalline ZrO2layer and an amorphous/nanocrystalline ZrSiN layer.(2) After the previous ZrSiN film was optimized and replaced by a composite ZrSiN/ZrO2film, a much improved diffusion barrier was achieved.(3) This ZrSiN/ZrO2composite film effectively prevented the formation of nitrogen gas and unstable ZrO2caused by the high-temperature sintering process.(4) As a type of porcelain, ZrO2itself has an intrinsic affinity to porcelain and thereby is able to accomplish a better binding effect.(5) Combining all of the data provided the evidence that how the magnetron-sputtered ZrSiN/ZrO2composite film achieved a significantly high bonding strength when it was applied as an intermediate layer between cpTi and porcelain. |
PDF Full Text Request