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RESEARCH REPORT |
s1,*
1 University of Debrecen, Medical and Health Science Center, Institute of Dental Science, Department of Prosthetic Dentistry, Debrecen, Hungary Nagyerdei krt. 98, H-4012, Hungary; and
2 University of Debrecen, Department of Solid State Physics, Hungary;
*corresponding author, hegedus{at}fogaszat.dote.hu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: transmission electron microscopy • nickel-chromium alloy • dental ceramics
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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The bond strength between metal and ceramic is determined by the properties of the different phases emerging in the diffusion zone during the firing process. The formation and growth of these phases indicate that very complex reduction-oxidation reactions take place in the interface region. These oxide layers play a fundamental role in adherence, i.e., in solid-state diffusion bonding (Klomp, 1987). Time and temperature of firing strongly affect the quality of bonding relative to the different interlayer structures.
In these complex systems, diffusion bonding is established not only by mutual interdiffusion but also by simultaneous solid-state reactions. Comparison of the results of microstructural investigations of the diffusion zone and the macroscopic bond-strength tests can clarify the role of various phases in the adherence process. Previously, most of the microstructural studies in the literature were based on optical microscopy (OM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Several studies have been reported on oxidation and ceramic coatings of NiCr alloys (Anusavice et al., 1977a, b; Williams et al., 1978; Ringle et al., 1979; Pask and Tomsia, 1988; Watanabe et al., 1989; Mizutani, 1990; Inoue et al., 1992). The limited space resolution of the abovementioned methods has made it impossibile for an effective investigation of the diffusion zone in the submicron range to be conducted. The use of a cross-sectional transmission electron microscopy (CTEM) technique has made possible, on a finer scale, the study of the reaction layer between metal and glass.
The aim of the present work is to carry out a detailed microstructural investigation and to compare the reaction layer developed among three different brands of dental ceramics and NiCr alloy under different firing conditions.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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For CTEM investigations, 1.5 x 1.5 mm plates were cut from the samples. These plates were embedded into an aluminum disc (3 mm in diameter) so that the metal-ceramic interface was perpendicular to the plane of the disc. The discs were ground and polished from both sides and dimpled from one side. The minimum thickness of the sample was about 0.02 mm after dimpling. The final thinning of the sample was performed by low-angle Ar-ion milling with 10 kV ion energy. In the final stage of the milling process, ion energy was reduced to 5 kV, decreasing the possibility that preparation artifacts would be formed. The TEM investigations were performed with the use of a JEOL 2000FX-II microscope (JEOL Ltd., Tokyo, Japan) with an Oxford Link-Isis EDS system (Oxford Instruments Ltd., United Kingdom).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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. Two different phases are visible in the diffusion zone. The bright bubble-shaped inclusions (highly electron-transparent phase), marked by ‘A’ in the Fig., grew into the original flat metal surface. The corresponding insert shows the EDS spectra of this area. (In this and in the following EDS spectra, the observable copper signals are scattered from the sample holder. Because of the beryllium window of the detector, the lines of the light elements [Z < 11] are absent from the spectra.) According to the electron diffraction results, this phase has an amorphous structure. Near the directly observed silicon, oxides of light elements such as Li and B may also be present in the inclusions. The amorphous structure and the presence of silicon as the main component indicate that this phase is probably a glassy silicate structure with multiple oxides.
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) follows the line of the original metal surface. As revealed by the EDS measurements (insert B in Fig. 1), this phase has a very high chromium content. The electron diffraction patterns are in accordance with the structure of the rhombohedral Cr2O3. A selected area diffraction pattern is shown in the insert in Fig. 1.
The morphology, structure, and composition of the observed phases in the NiCr-Vita system after the firing process recommended by the manufacturer are quite similar to those presented above for the NiCr-Carat system. On the other hand, in the case of the NiCr-Vision system, a different structure is visible (Fig. 2) after the firing process recommended by the manufacturer. Three different phases were observed at the metal/ceramic interface. A small amount of bright amorphous inclusions (A) with a large potassium content was grown at the metal surface. Phase B is a polycrystalline Cr2O3 layer similar to the former cases. The coarse-grain (~ 150 nm) oxide phase (denoted by ‘C’) has high Ni and Cr contents. The ratio of the Ni to Cr atoms is nearly 2:1 in this phase. According to the CBED pictures, its structure corresponds to an f.c.c. lattice with a lattice parameter of 0.82 nm (insert in Fig. 2 is a representative pattern from the [310] zone axis).
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. Near the well-known glassy inclusions (A) and polycrystalline Cr2O3 layer (B), a new crystalline phase (C) developed. According to the EDS results, it is a complex phase composed of Ti, Cr, and Ni oxides.
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The phase structure of the NiCr-Vision system did not change after the elevated firing process. The extent of the glassy inclusions increased considerably. The potassium content of the crystalline phases was also slightly higher than after the shorter firing process.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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To summarize the common and diverging features of the above processes: It is plausible to assume that, at the start of the firing process, a very thin Cr2O3 layer already exists on the surfaces of the NiCr samples (as is usual in the case of high-chromium alloys). The high surface-segregation ability and high chemical activity of chromium can justify the above supposition. In addition, it was observed (Mizutani, 1990) that the composition of the oxide formed after atmospheric heat treatments was sensitive to the original Cr content. Thus, with about 30-40 wt% Cr, it was mainly the Cr2O3 phase that formed, while at a lower Cr content, not only this phase but also NiO and NiCr2O were observed. In our case, the lower oxygen pressure in the vacuum furnace can explain why only Cr2O3 was formed.
The above nanocrystalline chromium-oxide layer produces a diffusion barrier between the metal and the glass. Only the grain and phase boundaries form fast diffusion paths for the diffusing atoms. At higher temperatures, there is an intermixing by diffusion of the metal atoms of the CrNi phase and the species of the ceramics (Fig. 4a). The intrinsic diffusion coefficient of a given species, D, has the following form:
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is Darken's thermodynamic factor,
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is the activity coefficient and x is the molar fraction of the given species.
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and the diffusion flux J (J = -D dc/dx, where dc/dx is the concentration gradient of a given species) can be very high in the case of Cr because of its high chemical activity. In the early stage of the firing process from the metal side, the chromium current is the dominant factor. On the other side, the SiO2 molecules are the most mobile components (their molar weight is the smallest compared with that of other oxides in the ceramics). Obviously, molecules of Li or B oxides can be similarly fast diffusers, but these light elements cannot be detected by our EDS system. Thus, during intermixing, the Cr can reduce (at least partially) the SiO2 (Conforto and Schmid, 2001). The decrease in the dimensions of the SiO2 molecule, due to the partial reduction and the presence of free oxygen in the glass melt, and owing to the introduction of additives such as TiO2 and ZrO2 (Iwamoto et al., 1987), can further enhance the diffusion current of SiO2 and Cr. As a result, silicon oxide bubbles appear (with an approximate composition of SiOx, where 1 < x < 2) between the metal and the Cr2O3 layer (see Fig. 1), and the chromium oxide increases/grows further. Similar considerations can be valid for other oxides present in the ceramics. Although there is almost 1 wt% Si present in the initial NiCr-based alloy, such a small amount of Si alone cannot be responsible for the formation of the bubble-like inclusions (see, e.g., Fig. 1, where they are almost continuously present along the interface). In the later stage of firing, the concentration gradient and consequent diffusion flux of the Cr decrease, which results in the relative enhancement of the Ni flux. This makes the formation of complex oxide phases with relatively high Ni content possible (see Figs. 3 and 4b). It is not clear why potassium instead of silicon was observed in the amorphous inclusions in the case of the NiCr-Vision system. Supposedly, the details of the above-described processes can be strongly affected by the additives of the glass and the exact circumstances of the firing process (e.g., residual atmosphere, water vapor pressure in the furnace).
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ACKNOWLEDGMENTS |
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Received May 14, 2001; Last revision February 27, 2002; Accepted March 18, 2002
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REFERENCES |
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Anusavice KJ, Ringle RD, Fairhurst CW (1977b). Bonding mechanism evidence in ceramic-nonprecious alloy system. J Biomed Mater Res 11:701–709.[Medline]
Conforto E, Schmid PE (2001). Pt-Si reaction through interfacial native silicon oxide layers. Phil Mag 81(A):61–82.
Inoue K, Murakami T, Terada Y (1992). The bond strength of porcelain to Ni-Cr alloy–the influence of thin or chromium plating. Int J Prosthodont 5:262–268.[Medline]
Iwamoto N, Umesaki N, Haibara Y (1987). Silicon nitride joining with glass soldier in the CaO-SiO2-TiO2 system; proceedings of the First Seiken International Symposium on Interface Structure, Properties and Diffusion Bonding, Tokyo, Japan. In: Studies in physical and theoretical chemistry. Ishida Y, editor. Tokyo: Elsevier, pp. 89-105.
Klomp JT (1987). Interface chemistry and structure of metal-ceramics interfaces. Proceedings of the First Seiken International Symposium on Interface Structure, Properties and Diffusion Bonding, Tokyo, Japan. In: Studies in physical and theoretical chemistry. Ishida Y, editor. Tokyo: Elsevier, pp. 3-24.
Mizutani M (1990). Study on high temperature oxidation of Ni-Cr ceramic alloys. Aichi-Gakuin J Dent Sci 28:59–78.
Pask JA, Tomsia AP (1988). Oxidation and ceramic coatings on 80Ni20Cr alloys. J Dent Res 67:1164–1171.
Ringle RD, Fairhurst CW, Anusavice KJ (1979). Microstructures in non-precious alloys near the porcelain-metal interaction zone. J Dent Res 58:1987–1993.
Smithells CJ, editor (1976). Metals reference book. London: Butterworth's, p. 862.
Watanabe K, Okawa S, Miyakawa O, Nakano S, Shiokawa N, Kobayashi M (1989). Reactions at the nonprecious metal-ceramic interface during porcelain firing (3). A commercial alloy forming much Cr oxide during firing. Shika Zairo Kikai 9:453–462.
Williams TRP, Johnson CE, Winchell PG, Philips RW (1978). Be, Li, and Na redistribution near a porcelain/Ni alloy interface shown by ion microprobe mass analysis. J Dent Res 57:233–236.
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