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Strengthening of a Pd-free high gold dental alloy for porcelain bonding by a pre-firing heat treatment
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Strengthening of a Pd-free high gold dental alloy for porcelain bonding by a pre-firing heat treatment W.B. Liu, J.N. Wang ∗ School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The purpose of the present investigation was to report a Pd-free high gold alloy
Received 11 November 2005
for porcelain bonding based on the ternary system of Au–Pt–Zn with a nominal composition
Received in revised form
of 86Au–11Pt–1.5Zn–0.5In–0.7Rh–0.2Fe–0.1Ir (wt.%). Emphasis was put on the effect of a pre-
28 March 2006
firing heat treatment on the mechanical properties of the alloy.
Accepted 22 June 2006
Methods. The strengthening effect of the pre-firing heat treatment was investigated by means of hardness measurement, tensile testing, X-ray diffraction, optical microscopy and scanning electron microscopy.
Keywords:
Results. Experimental results showed that both the hardness and tensile strength of the alloy
Dental alloy
can be significantly improved after heat treatment at the temperature of 980 ◦ C for 15 min.
Strengthening
The strengthening could be attributed to homogenization of microstructure and alloying
Heat treatment
elements and precipitation of new fine particles.
Porcelain-fused-to-metal
Significance. The cast framework of the present new Pd-free alloy could be heat treated before
Homogenization
actual firing, and this would improve the processing properties of the alloy during firing.
Precipitation
1.
© 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction
In restorative dentistry, aesthetic care has led to the application of porcelain-fused-to-metal (PFM) techniques. For such applications, various types of alloy have been developed. However, noble metal alloys have become the primary selection because a restoration with good aesthetics, good biocompatibility and high toughness can be achieved. Most of the noble metal alloys currently used for the PFM technique are high gold alloys based on the ternary system of Au (80–86%), Pt (10–15%) and In (1–2%) with the addition of Pd (1–2%) (all compositions in this paper are in weight percentage). Excellent biocompatibility is obtained through the inclusion of high contents of the two noble metal elements, and good hardening is achieved with the addition of In [1]. The addition of Pd permits, due to its narrow melting range with Au and Pt, a sufficient amount of In to be
∗
homogeneously dissolved in the alloy. As a result, the mechanical strength of the alloy can be increased to a level for practical utilization [2]. In addition to Au-based alloys containing Pd, Pd-based alloys for metal–ceramic restorations are also in widespread clinical use. The Pd alloys are cheaper than the Au alloys, although there was considerable Pd price volatility several years ago. The much greater specific gravity of the Au alloys provided another positive economic factor for the Pd alloys. However, some side effects of Pd, such as allergies, were lately found, and the risk of using Pd in dental casting alloys was raised [3]. As a matter of fact, lingering concerns about the biocompatibility of Pd alloys remain a hindrance for their clinical selection by dental laboratories and practicing dentists, although the risk from these alloys appears to be low [3]. For this reason, Pd-free dental alloys for porcelain bonding are being developed.
Corresponding author. Tel.: +86 21 62932015; fax: +86 21 62932587. E-mail address: [email protected] (J.N. Wang). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.06.048
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
It is also well known that high-gold PFM alloys often give rise to complaints because of their soft surface, which makes it difficult to trim them after casting. Overlapping on the surface after grinding leads to porosities at the metal–ceramic interface [4]. In addition, alloys with a very high gold content have insufficient creep resistance after casting, and show poor hardness and processing properties. Distortions of the framework due to creep [5–8] lead to an inaccurate fit of the bridge on the master model. Therefore, it is desirable to improve the hardness and strength of the as-cast alloy by heat treatment prior to firing the ceramic onto the alloy. The aim of the present investigation was to report a Pdfree PFM alloy based on the ternary system of Au–Pt–Zn with minor additions of In, Fe, Rh, and Ir. Emphasis was put on the effect of a pre-firing heat treatment on the mechanical properties of the alloy. It was particularly shown that both the room-temperature hardness and tensile strength of the alloy could be significantly improved after treatment at the temperature usually used for firing.
2.
Materials and methods
The alloy used in the present study was an experimental high-gold alloy with a nominal composition of 86Au–11Pt–1.5Zn–0.5In–0.7Rh–0.2Fe–0.1Ir. This composition is different from commercial ones based on Au–Pt–In–Pd. The addition of Zn and In is to improve the bonding strength between porcelain and metal and the mechanical properties of the metallic framework. The addition of other elements is primarily for grain refinement and thus improvement of mechanical properties. The alloy was prepared by vacuum arcmelting in an argon atmosphere. To minimize compositional segregation and inhomogeneity, the alloy was melted at least three times. The alloy ingot was cut into small pieces using electrical discharge machining (EDM). They were then remelted and cast by the lost wax casting technique with a universal investment material (Deguvest® impact, Degussa dental GmbH & Co. KG, Germany) and a centrifugal casting machine (Multicast® Compact, Degussa, Germany). The molds were cooled in air before divesting. The specimen obtained was either an alloy cylinder with a diameter of 6 mm and a length of 40 mm (to be cut into plate pieces for hardness testing and X-ray diffraction (XRD)) or one with a gauge diameter of 3 mm and a gauge length of 15 mm (for tensile testing in accordance with ISO 9693: 1999(E)). Some specimens were left in the as-cast state; but others were heat treated at 980 ◦ C for 15 min. They were then bench cooled as a pre-firing heat treatment according to ISO 9693: 1999(E). This treatment was applied both to the plate specimens with a diameter of 6 mm and a thickness of 1.0 mm cut from the alloy cylinder and to the specimens for tensile testing. After pre-firing treatment, the hardness test was carried out using a Vickers microhardness tester (HX-1000, Shanghai, China) with a load of 300 gf and a dwell time of 45 s. For each alloy state, the mean hardness value was calculated from at least five indentations. The tensile specimens were
1137
then precision ground to the end dimensions prescribed by the standard with an accuracy of ±0.01 mm. The tensile test was performed with a universal tensile testing machine (Zwick/Roell, Zwick, Germany) at a constant cross-head speed of 1.00 mm/min corresponding to a constant strain rate of 1.67 × 10−3 s−1 . Plate specimens of the same size as for the hardness testing were prepared according to the above-described procedure. Xray diffraction (XRD) experiments were conducted using an X-ray diffractometer (Bruker D8 Advance, Bruker AXS, Germany). The X-ray diffractometer was operated at 40 kV and 40 mA. Nickel-filtered Cu K␣ radiation was used in the incident beam. The specimens employed in the hardness testing were also used for metallographic determinations. They were mounted in epoxy-resin. After polishing down to 1m with diamond paste, the specimens were etched with a mixture of 20 ml of 37% hydrochloric acid and 10 ml of 67% nitric acid. Microstructure was observed using an optical microscope (Leica MEF4M, Germany). The microstructures and fracture surfaces of the specimens were studied with a field emission scanning electron microscope (FESEM) (Sirion 200, Philips, Holland). Energy dispersive X-ray spectroscopy (EDX) was also used to identify the chemical compositions of the phases in specimens. The approximate spot size of the X-ray beam on the specimen surface was about 2 m.
3.
Results
3.1.
X-ray diffraction
Fig. 1 gives the XRD spectrums of the present alloy in the ascast and heat-treated states. The alloy in both states showed ␣1 and ␣2 reflections, which were identified as the Au-rich ␣1 phase and Pt-rich ␣2 -phase according to ASTM PDF cards (PDF number 04-0784 for ␣1 -phase and PDF number 04-0802 for ␣2 phase), respectively. Additionally, there were two extra reflections at 2Â positions of about 39.2◦ and 45.4◦ for the as-cast specimen. The corresponding phase of these two reflections was regarded as having a face-centered cubic (FCC) structure, and was termed as -phase for the convenience of description. To identify the -phase, its lattice constant was estimated to be a = 0.3995 nm from the X-ray diffraction data [9]. This lattice constant is very close to the reported lattice constant of a = 0.3992 nm for the binary Pt3 In-phase with a Cu3 Au-type superstructure [10]. Thereby the -phase is suggested to be Pt3 In, considering the fact that the present alloy contained Pt and In.
3.2.
Microstructure
The metallographic micrographs of the present alloy in its ascast and heat-treated states are given in Fig. 2. The matrix phase in these two states was identified as a Au-rich ␣1 -phase, whereas the other phase, which appeared to be small particles (∼2–10 m), was regarded as a Pt-rich ␣2 -phase (arrowed in Fig. 2). These results were confirmed by EDX analysis. EDX results revealed that the ␣1 -phase contained 88–93% Au, 4–7% Pt and some Zn, while the ␣2 -phase contained about 55–65%
1138
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
Fig. 1 – XRD spectra of the present alloy in the as-cast (a) and the heat-treated states (b).
Pt and 20–30% Au with some Rh and Fe. Some particles were found to include Pt and In, which may correspond to the phase detected by XRD. The grain size of the ␣1 -phase was approximately identical in both the as-cast and heat-treated states (∼30–50 m). Similarly, no distinct difference was found in grain size of the ␣2 -phase between the two different states. The as-cast specimen, however, showed an inhomogeneous microstructure within individual ␣1 grains. With the imposition of heat treatment, the microstructure became homogeneous, and more ␣2 particles were observed. These fine particles appeared not only within grains but also at grain boundaries (Fig. 2d).
3.3.
Mechanical properties
The results of Vickers hardness, yield strength (at 0.2% strain), and tensile strength are depicted in Fig. 3. Compared to that in the as-cast state, the specimen in the heat-treated state showed higher levels of hardness and strength. For example, the yield strength increased from 370 MPa to 550 MPa. With such a significant increase, the tensile elongation decreased from 8% to 5%. This result demonstrates that the alloy was distinctly strengthened with a slight loss of ductility after the pre-firing heat treatment.
Fig. 2 – Optical micrographs of the present alloy in the as-cast (a and b) and the heat-treated states (c and d).
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
1139
Fig. 3 – Vickers hardness (a), yield strength (0.2% strain) (b), and tensile strength (c) of the present alloy in the as-cast and heat-treated states.
3.4.
Fracture surfaces
Representative fracture surfaces of the alloy after tensile testing are shown in Fig. 4. The fractographs revealed that specimens in the two states fractured with complex mechanisms. Obviously, the fracture surface of the as-cast specimen showed composite characteristics of various sizes of dimples and intergranular fracture. For the specimen after heat treatment, finer uniform dimples were observed. Furthermore, fracture also occurred along the phase boundary in both states. More importantly, fine heterogeneous particles were observed in grain boundaries or large-sized dimples in the ascast specimen (Fig. 4a).
4.
Discussion
The present Pd-free PFM alloy shows a clear improvement in hardness and tensile strength from the as-cast state to the heat-treated state (Fig. 3). This improvement is directly
induced by the modifications of microstructure with the imposition of heat treatment at high temperature. The following modifications can be noted: (1) the microstructure within individual grains became homogeneous (Fig. 2c and d); (2) the original -phase was no longer present (Fig. 1b); and (3) more fine ␣2 particles appeared both within matrix grains and at their boundaries (Fig. 2c and d). The microstructure homogeneity was also seen from the fine and uniform dimples on the fracture surface (Fig. 4b). The immiscibility of Au and Pt in solid state is well known [11]. After casting a solid solution nucleates from the melt. During cooling the alloy enters the miscibility gap, separating into an Au-rich and a Pt-rich phase, and thus hardening. With the heat treatment at 980 ◦ C, the alloy enters the single phase zone between the miscibility gap and the solidus line. At this temperature a solid solution is thermodynamically stable with a random distribution of the atoms in the crystal lattice. The inhomogeneous texture formed after casting therefore is not thermodynamically stable. Owing to the high energy content at high temperature the atoms are able to diffuse in
1140
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
By this treatment the alloy will be hardened before being trimmed, thus improving further processing [15]. Additionally, casting stresses will be relaxed without deformation of the cast work, thus improving the fit of the reconstruction [16–18]. While firing the ceramic, the alloy is heated for several times to 890–980 ◦ C and maintained at the temperature for 1–5 min. The present heat treatment could be considered as a firing simulation according to ISO 9693: 1999(E). This is because the heat treatment was conducted at 980 ◦ C, the highest temperature used for firing, for 15 min, the longest time that could possibly be used for firing. In terms of such consideration, it may be deduced that the present alloy would not experience degradation during actual firing.
5.
Fig. 4 – SEM photographs of fracture surfaces: (a) the as-cast state and (b) the heat-treated state.
the crystal lattice and distribute randomly on the atomic sites. Subsequently the alloy homogenizes. Due to the homogeneous distribution the precipitates of the Pt-rich phase formed during cooling are very small and well distributed in the alloy, resulting in an increased hardness and tensile strength. The improvement of hardness and strength may also be related to the elimination of the -phase by heat treatment at high temperature. It is well known that the inclusion of In in the ternary system of Au–Pt–In is beneficial to hardening [1]. The addition of Pd to this system is to increase the solubility of In in the alloy matrix [2]. Therefore, the dissolution of the phase of Pt3 In in the present alloy, after heat treatment, could give rise to hardening. New ␣2 precipitates were observed in the present alloy, especially at the grain boundaries, after heat treatment. A similar phenomenon was found in some other dental alloys, and a hardening effect was detected. But these alloys were subject to solid solution treatment followed by a long period of isothermal aging [9,12,13]. The present experiment shows that precipitation of new particles at the grain boundaries could also take place after solid solution treatment followed directly by cooling in air. The precipitation of new particles might have contributed to the observed strengthening although this may not be the primary cause [14]. Based on the results of this study, for the practical application of the present PFM alloy, it is advisable to perform a pre-firing heat treatment. This may be done by annealing the cast framework at 980 ◦ C for 15 min immediately after divesting and cleaning the surface, without removing the sprues.
Conclusion
A Pd-free high gold dental alloy for porcelain bonding is reported. This alloy was based on Au–Pt–Zn with a nominal composition of 86Au–11Pt–1.5Zn–0.5In–0.7Rh–0.2Fe–0.1Ir (wt.%). It was found that both the hardness and tensile strength of the alloy were significantly increased after heat treatment at the temperature of 980 ◦ C for 15 min. The strengthening mechanisms may include homogenization of microstructure and alloying elements and precipitation of new fine particles. Based on these results, it is concluded that the cast framework of such alloy should be heat treated first and the processing properties could then be improved during actual firing.
Acknowledgements This investigation was supported by The Dental Materials Company of Shanghai and The National Natural Science Foundation of China.
references
[1] Leinfelder KF, O’Brien WJ, Ryge G, Fairhurst CW. Hardening of high-fusing gold alloys. J Dent Res 1966;45:292–396. ¨ [2] Fischer J, Guo HK, Salk M. Die Bedeutung des ternaren ¨ die Entwicklung aufbrennfahiger ¨ Systems Au–Pt–In fur Edelmetall-Legierungen (The significance of the ternary system Au–Pt–In for the development of previous metal alloys for the ceramic-fused-to-metal technique). Dtsch ¨ Zahnarztl Z 1994;49:636–42. [3] Wataha JC, Hanks CT. Biological effects of palladium and risk of using palladium in dental casting alloys. J Oral Rehab 1996;23:309–20. [4] Yamamoto M. The metal ceramics. Tokyo: Quintessence; 1982. [5] Anusavice KJ, Carroll JE. Effect of incompatibility stress on the fit of metal-ceramic crowns. J Dent Res 1987;66:1341–5. [6] Tuccillo JJ, Nielsen JP. Creep and sag properties of a porcelain–gold alloy. J Dent Res 1967;46:579–83. [7] Bryant RA, Nicholls JI. Measurement of distortion in fixed partial dentures resulting from degassing. J Prosthet Dent 1979;42:515–20. [8] Bertolotti RL. Alloys for porcelain-fused-to-metal restorations. In: O’Brien WJ, editor. Dental materials:
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
[9]
[10] [11] [12] [13]
properties and selection, vol. 323. Chicago: Quintessence; 1989. Shiraishi T. Age-hardening behaviors and grain boundary discontinuous precipitation in a Pd-free gold alloy for porcelain bonding. J Mater Sci: Mater Med 2002;13:979–83. Harris IR, Norman M, Bryant AW. J Less-Common Met 1968;16:427–40. Massalski TB, editor. Binary alloy phase diagrams. 2nd ed. Metals Park, OH: American Society for Metals; 1990. Leinfelder KF, Taylor DF. Hardening of gold-based alloys. J Dent Res 1977;56:335–45. Kim HI, Jang MI, Kim MS. Age-hardening associated with grain boundary precipitation in a commercial dental gold alloy. J Oral Rehab 1999;26:215–22.
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[14] O’Brien WJ. Precious metal casting alloys. In: O’Brien WJ, editor. Dental materials and their selection. 3rd ed. Chicago: Quintessence; 2002. p. 192–209. [15] Fischer J, Fleetwood PW. Improving the processing of high-gold metal-ceramic frameworks by a pre-firing heat treatment. Dent Mater 2000;16:109–13. [16] Campbell SD, Pelletier LB. Thermal cyclic distortion of metal ceramics. Part II. Etiology. J Prosthet Dent 1992;68:284–9. [17] Campbell SD, Sirakian A, Pelletier LB, Giordano RA. Effect of firing cycle and surface finishing on distortion of metal ceramic castings. J Prosthet Dent 1995;74:476–81. [18] Anusavice KJ, Shen C, Hashinger D, Twiggs SW. Interactive effect of stress and temperature on creep of PFM alloys. J Dent Res 1985;64:1094–9.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Strengthening of a Pd-free high gold dental alloy for porcelain bonding by a pre-firing heat treatment W.B. Liu, J.N. Wang ∗ School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The purpose of the present investigation was to report a Pd-free high gold alloy
Received 11 November 2005
for porcelain bonding based on the ternary system of Au–Pt–Zn with a nominal composition
Received in revised form
of 86Au–11Pt–1.5Zn–0.5In–0.7Rh–0.2Fe–0.1Ir (wt.%). Emphasis was put on the effect of a pre-
28 March 2006
firing heat treatment on the mechanical properties of the alloy.
Accepted 22 June 2006
Methods. The strengthening effect of the pre-firing heat treatment was investigated by means of hardness measurement, tensile testing, X-ray diffraction, optical microscopy and scanning electron microscopy.
Keywords:
Results. Experimental results showed that both the hardness and tensile strength of the alloy
Dental alloy
can be significantly improved after heat treatment at the temperature of 980 ◦ C for 15 min.
Strengthening
The strengthening could be attributed to homogenization of microstructure and alloying
Heat treatment
elements and precipitation of new fine particles.
Porcelain-fused-to-metal
Significance. The cast framework of the present new Pd-free alloy could be heat treated before
Homogenization
actual firing, and this would improve the processing properties of the alloy during firing.
Precipitation
1.
© 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction
In restorative dentistry, aesthetic care has led to the application of porcelain-fused-to-metal (PFM) techniques. For such applications, various types of alloy have been developed. However, noble metal alloys have become the primary selection because a restoration with good aesthetics, good biocompatibility and high toughness can be achieved. Most of the noble metal alloys currently used for the PFM technique are high gold alloys based on the ternary system of Au (80–86%), Pt (10–15%) and In (1–2%) with the addition of Pd (1–2%) (all compositions in this paper are in weight percentage). Excellent biocompatibility is obtained through the inclusion of high contents of the two noble metal elements, and good hardening is achieved with the addition of In [1]. The addition of Pd permits, due to its narrow melting range with Au and Pt, a sufficient amount of In to be
∗
homogeneously dissolved in the alloy. As a result, the mechanical strength of the alloy can be increased to a level for practical utilization [2]. In addition to Au-based alloys containing Pd, Pd-based alloys for metal–ceramic restorations are also in widespread clinical use. The Pd alloys are cheaper than the Au alloys, although there was considerable Pd price volatility several years ago. The much greater specific gravity of the Au alloys provided another positive economic factor for the Pd alloys. However, some side effects of Pd, such as allergies, were lately found, and the risk of using Pd in dental casting alloys was raised [3]. As a matter of fact, lingering concerns about the biocompatibility of Pd alloys remain a hindrance for their clinical selection by dental laboratories and practicing dentists, although the risk from these alloys appears to be low [3]. For this reason, Pd-free dental alloys for porcelain bonding are being developed.
Corresponding author. Tel.: +86 21 62932015; fax: +86 21 62932587. E-mail address: [email protected] (J.N. Wang). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.06.048
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
It is also well known that high-gold PFM alloys often give rise to complaints because of their soft surface, which makes it difficult to trim them after casting. Overlapping on the surface after grinding leads to porosities at the metal–ceramic interface [4]. In addition, alloys with a very high gold content have insufficient creep resistance after casting, and show poor hardness and processing properties. Distortions of the framework due to creep [5–8] lead to an inaccurate fit of the bridge on the master model. Therefore, it is desirable to improve the hardness and strength of the as-cast alloy by heat treatment prior to firing the ceramic onto the alloy. The aim of the present investigation was to report a Pdfree PFM alloy based on the ternary system of Au–Pt–Zn with minor additions of In, Fe, Rh, and Ir. Emphasis was put on the effect of a pre-firing heat treatment on the mechanical properties of the alloy. It was particularly shown that both the room-temperature hardness and tensile strength of the alloy could be significantly improved after treatment at the temperature usually used for firing.
2.
Materials and methods
The alloy used in the present study was an experimental high-gold alloy with a nominal composition of 86Au–11Pt–1.5Zn–0.5In–0.7Rh–0.2Fe–0.1Ir. This composition is different from commercial ones based on Au–Pt–In–Pd. The addition of Zn and In is to improve the bonding strength between porcelain and metal and the mechanical properties of the metallic framework. The addition of other elements is primarily for grain refinement and thus improvement of mechanical properties. The alloy was prepared by vacuum arcmelting in an argon atmosphere. To minimize compositional segregation and inhomogeneity, the alloy was melted at least three times. The alloy ingot was cut into small pieces using electrical discharge machining (EDM). They were then remelted and cast by the lost wax casting technique with a universal investment material (Deguvest® impact, Degussa dental GmbH & Co. KG, Germany) and a centrifugal casting machine (Multicast® Compact, Degussa, Germany). The molds were cooled in air before divesting. The specimen obtained was either an alloy cylinder with a diameter of 6 mm and a length of 40 mm (to be cut into plate pieces for hardness testing and X-ray diffraction (XRD)) or one with a gauge diameter of 3 mm and a gauge length of 15 mm (for tensile testing in accordance with ISO 9693: 1999(E)). Some specimens were left in the as-cast state; but others were heat treated at 980 ◦ C for 15 min. They were then bench cooled as a pre-firing heat treatment according to ISO 9693: 1999(E). This treatment was applied both to the plate specimens with a diameter of 6 mm and a thickness of 1.0 mm cut from the alloy cylinder and to the specimens for tensile testing. After pre-firing treatment, the hardness test was carried out using a Vickers microhardness tester (HX-1000, Shanghai, China) with a load of 300 gf and a dwell time of 45 s. For each alloy state, the mean hardness value was calculated from at least five indentations. The tensile specimens were
1137
then precision ground to the end dimensions prescribed by the standard with an accuracy of ±0.01 mm. The tensile test was performed with a universal tensile testing machine (Zwick/Roell, Zwick, Germany) at a constant cross-head speed of 1.00 mm/min corresponding to a constant strain rate of 1.67 × 10−3 s−1 . Plate specimens of the same size as for the hardness testing were prepared according to the above-described procedure. Xray diffraction (XRD) experiments were conducted using an X-ray diffractometer (Bruker D8 Advance, Bruker AXS, Germany). The X-ray diffractometer was operated at 40 kV and 40 mA. Nickel-filtered Cu K␣ radiation was used in the incident beam. The specimens employed in the hardness testing were also used for metallographic determinations. They were mounted in epoxy-resin. After polishing down to 1m with diamond paste, the specimens were etched with a mixture of 20 ml of 37% hydrochloric acid and 10 ml of 67% nitric acid. Microstructure was observed using an optical microscope (Leica MEF4M, Germany). The microstructures and fracture surfaces of the specimens were studied with a field emission scanning electron microscope (FESEM) (Sirion 200, Philips, Holland). Energy dispersive X-ray spectroscopy (EDX) was also used to identify the chemical compositions of the phases in specimens. The approximate spot size of the X-ray beam on the specimen surface was about 2 m.
3.
Results
3.1.
X-ray diffraction
Fig. 1 gives the XRD spectrums of the present alloy in the ascast and heat-treated states. The alloy in both states showed ␣1 and ␣2 reflections, which were identified as the Au-rich ␣1 phase and Pt-rich ␣2 -phase according to ASTM PDF cards (PDF number 04-0784 for ␣1 -phase and PDF number 04-0802 for ␣2 phase), respectively. Additionally, there were two extra reflections at 2Â positions of about 39.2◦ and 45.4◦ for the as-cast specimen. The corresponding phase of these two reflections was regarded as having a face-centered cubic (FCC) structure, and was termed as -phase for the convenience of description. To identify the -phase, its lattice constant was estimated to be a = 0.3995 nm from the X-ray diffraction data [9]. This lattice constant is very close to the reported lattice constant of a = 0.3992 nm for the binary Pt3 In-phase with a Cu3 Au-type superstructure [10]. Thereby the -phase is suggested to be Pt3 In, considering the fact that the present alloy contained Pt and In.
3.2.
Microstructure
The metallographic micrographs of the present alloy in its ascast and heat-treated states are given in Fig. 2. The matrix phase in these two states was identified as a Au-rich ␣1 -phase, whereas the other phase, which appeared to be small particles (∼2–10 m), was regarded as a Pt-rich ␣2 -phase (arrowed in Fig. 2). These results were confirmed by EDX analysis. EDX results revealed that the ␣1 -phase contained 88–93% Au, 4–7% Pt and some Zn, while the ␣2 -phase contained about 55–65%
1138
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
Fig. 1 – XRD spectra of the present alloy in the as-cast (a) and the heat-treated states (b).
Pt and 20–30% Au with some Rh and Fe. Some particles were found to include Pt and In, which may correspond to the phase detected by XRD. The grain size of the ␣1 -phase was approximately identical in both the as-cast and heat-treated states (∼30–50 m). Similarly, no distinct difference was found in grain size of the ␣2 -phase between the two different states. The as-cast specimen, however, showed an inhomogeneous microstructure within individual ␣1 grains. With the imposition of heat treatment, the microstructure became homogeneous, and more ␣2 particles were observed. These fine particles appeared not only within grains but also at grain boundaries (Fig. 2d).
3.3.
Mechanical properties
The results of Vickers hardness, yield strength (at 0.2% strain), and tensile strength are depicted in Fig. 3. Compared to that in the as-cast state, the specimen in the heat-treated state showed higher levels of hardness and strength. For example, the yield strength increased from 370 MPa to 550 MPa. With such a significant increase, the tensile elongation decreased from 8% to 5%. This result demonstrates that the alloy was distinctly strengthened with a slight loss of ductility after the pre-firing heat treatment.
Fig. 2 – Optical micrographs of the present alloy in the as-cast (a and b) and the heat-treated states (c and d).
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1136–1141
1139
Fig. 3 – Vickers hardness (a), yield strength (0.2% strain) (b), and tensile strength (c) of the present alloy in the as-cast and heat-treated states.
3.4.
Fracture surfaces
Representative fracture surfaces of the alloy after tensile testing are shown in Fig. 4. The fractographs revealed that specimens in the two states fractured with complex mechanisms. Obviously, the fracture surface of the as-cast specimen showed composite characteristics of various sizes of dimples and intergranular fracture. For the specimen after heat treatment, finer uniform dimples were observed. Furthermore, fracture also occurred along the phase boundary in both states. More importantly, fine heterogeneous particles were observed in grain boundaries or large-sized dimples in the ascast specimen (Fig. 4a).
4.
Discussion
The present Pd-free PFM alloy shows a clear improvement in hardness and tensile strength from the as-cast state to the heat-treated state (Fig. 3). This improvement is directly
induced by the modifications of microstructure with the imposition of heat treatment at high temperature. The following modifications can be noted: (1) the microstructure within individual grains became homogeneous (Fig. 2c and d); (2) the original -phase was no longer present (Fig. 1b); and (3) more fine ␣2 particles appeared both within matrix grains and at their boundaries (Fig. 2c and d). The microstructure homogeneity was also seen from the fine and uniform dimples on the fracture surface (Fig. 4b). The immiscibility of Au and Pt in solid state is well known [11]. After casting a solid solution nucleates from the melt. During cooling the alloy enters the miscibility gap, separating into an Au-rich and a Pt-rich phase, and thus hardening. With the heat treatment at 980 ◦ C, the alloy enters the single phase zone between the miscibility gap and the solidus line. At this temperature a solid solution is thermodynamically stable with a random distribution of the atoms in the crystal lattice. The inhomogeneous texture formed after casting therefore is not thermodynamically stable. Owing to the high energy content at high temperature the atoms are able to diffuse in
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By this treatment the alloy will be hardened before being trimmed, thus improving further processing [15]. Additionally, casting stresses will be relaxed without deformation of the cast work, thus improving the fit of the reconstruction [16–18]. While firing the ceramic, the alloy is heated for several times to 890–980 ◦ C and maintained at the temperature for 1–5 min. The present heat treatment could be considered as a firing simulation according to ISO 9693: 1999(E). This is because the heat treatment was conducted at 980 ◦ C, the highest temperature used for firing, for 15 min, the longest time that could possibly be used for firing. In terms of such consideration, it may be deduced that the present alloy would not experience degradation during actual firing.
5.
Fig. 4 – SEM photographs of fracture surfaces: (a) the as-cast state and (b) the heat-treated state.
the crystal lattice and distribute randomly on the atomic sites. Subsequently the alloy homogenizes. Due to the homogeneous distribution the precipitates of the Pt-rich phase formed during cooling are very small and well distributed in the alloy, resulting in an increased hardness and tensile strength. The improvement of hardness and strength may also be related to the elimination of the -phase by heat treatment at high temperature. It is well known that the inclusion of In in the ternary system of Au–Pt–In is beneficial to hardening [1]. The addition of Pd to this system is to increase the solubility of In in the alloy matrix [2]. Therefore, the dissolution of the phase of Pt3 In in the present alloy, after heat treatment, could give rise to hardening. New ␣2 precipitates were observed in the present alloy, especially at the grain boundaries, after heat treatment. A similar phenomenon was found in some other dental alloys, and a hardening effect was detected. But these alloys were subject to solid solution treatment followed by a long period of isothermal aging [9,12,13]. The present experiment shows that precipitation of new particles at the grain boundaries could also take place after solid solution treatment followed directly by cooling in air. The precipitation of new particles might have contributed to the observed strengthening although this may not be the primary cause [14]. Based on the results of this study, for the practical application of the present PFM alloy, it is advisable to perform a pre-firing heat treatment. This may be done by annealing the cast framework at 980 ◦ C for 15 min immediately after divesting and cleaning the surface, without removing the sprues.
Conclusion
A Pd-free high gold dental alloy for porcelain bonding is reported. This alloy was based on Au–Pt–Zn with a nominal composition of 86Au–11Pt–1.5Zn–0.5In–0.7Rh–0.2Fe–0.1Ir (wt.%). It was found that both the hardness and tensile strength of the alloy were significantly increased after heat treatment at the temperature of 980 ◦ C for 15 min. The strengthening mechanisms may include homogenization of microstructure and alloying elements and precipitation of new fine particles. Based on these results, it is concluded that the cast framework of such alloy should be heat treated first and the processing properties could then be improved during actual firing.
Acknowledgements This investigation was supported by The Dental Materials Company of Shanghai and The National Natural Science Foundation of China.
references
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