Mechanical properties, fracture surface characterization, and microstructural analysis of six noble dental casting alloys

Mechanical properties, fracture surface characterization, and microstructural analysis of six noble dental casting alloys Yurdanur Ucar, DDS, MS, PhD,a William A. Brantley, PhD,b William M. Johnston, PhD,c and Tridib Dasgupta, MSd Faculty of Dentistry, Cukurova University, Adana, Turkey; College of Dentistry, The Ohio State University, Columbus, Ohio; Ivoclar Vivadent, Amherst, NY Statement of problem. Because noble dental casting alloys for metal ceramic restorations have a wide range of mechanical properties, knowledge of these properties is needed for rational alloy selection in different clinical situations where cast metal restorations are indicated. Purpose. The purpose of this study was to compare the mechanical properties and examine both the fracture and polished surfaces of 6 noble casting alloys that span many currently marketed systems. Five alloys were designed for metal ceramic restorations, and a sixth Type GPT has Type IV alloy for fixed prosthodontics (Maxigold KF) was included for comparison. Material and methods. Specimens (n=6) meeting dimensional requirements for ISO Standards 9693 and 8891 were loaded to failure in tension using a universal testing machine at a crosshead speed of 2 mm/min. Values of 0.1% and 0.2% yield strength, ultimate tensile strength, elastic modulus, and percentage elongation were obtained. Statistical comparisons of the alloy mechanical properties were made using 1-way ANOVA and the REGW multiple-range test (α=.05). Following fracture surface characterization using scanning electron microscopy (SEM), specimens were embedded in epoxy resin, polished, and again, examined with the SEM. Results. When the multiple comparisons were considered, there were generally no significant differences in the elastic modulus, 0.1% and 0.2% offset yield strength, and ultimate tensile strength for the d.SIGN 91 (Au-Pd), d.SIGN 59 (Pd-Ag), Capricorn 15 (Pd-Ag-Au) and Maxigold KF (Au-Ag-Pd) alloys, except that the ultimate tensile strength was significantly lower (P<.05) for Maxigold KF than these other 3 alloys. These 4 mechanical properties were generally significantly lower (P<.05) for Aquarius XH (Au-Pt-Pd) and Brite Gold XH (Au-Pt). The d.SIGN 59 (14.6%) and Capricorn 15 (13.8%) alloys had the highest values of mean percentage elongation, which were not significantly different. Aquarius XH (6.0%) and Maxigold KF (4.2%) had the lower mean values of percentage elongation, which were also not significantly different. The polished and etched surfaces for all alloys revealed equiaxed, fine-grain microstructures, and all fracture surfaces contained casting porosity. Incomplete solidification suggestive of dendritic structures was observed for some alloys. Fracture surfaces were complex, with characteristic features of both brittle and ductile fracture. Precipitate particles on the fracture surfaces indicated the multi-phase character of the alloys. Conclusions. For the important mechanical property of yield strength, there were generally no significant differences among the Au-Pd, Pd-Ag, Pd-Ag-Au and Au-Ag-Pd alloys. Wide variation was found in percentage elongation, with the Pd-Ag and Pd-Ag-Au alloys having the highest values and the Au-Pd-Pt and Au-Ag-Pd alloys having the lowest values. (J Prosthet Dent 2011;105:394-402)

None of the authors besides Tridib Dasgupta has an association with Ivoclar Vivadent or any financial interest in the dental casting alloys selected for this study. Assistant Professor, Department of Prosthodontics, Faculty of Dentistry, Cukurova University. Professor and Director of the Graduate Program in Dental Materials Science, Division of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University. c Professor, Division of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University. Division of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University. d Director of Research and Development, Ivoclar Vivadent. a

b

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Clinical Implications

Significant variations in yield strength suggest that the prosthodontist should consider type of restoration and in vivo functional forces, such as a long-span fixed prosthesis or a restoration subjected to potentially high stress when selecting a noble alloy. Significant differences in elastic modulus and percentage elongation, along with yield strength, may also be important for adjustment of restorations prepared from these alloys. The mechanical properties of dental alloys have been studied1,2 using tensile tests. The clinically important mechanical properties of elastic modulus, (offset) yield strength, and ultimate tensile strength can be calculated using the stress-strain curve obtained when the specimen is loaded to fracture.3 The percentage elongation can be obtained by fitting the fractured specimen halves together and microscopically measuring the new separation between the original gauge length marks, or by using a breakaway extensometer to record the strain at specimen fracture. From basic metallurgical principles, the differences in mechanical properties among the different types of dental alloys arise from compositional differences that result in different microstructural phases. During the dental casting procedure, alloys undergo rapid solidification, and microstructures may deviate far from the multi-component equilibrium state.4 Moreover, the multi-component nature of noble alloy compositions can result in a variety of secondary microstructural phases that influences mechanical properties. For example, with the palladium-silver alloys, examination of the individual binary phase diagrams reveals that besides the solid solution matrix based on the PdAg system, intermetallic compounds, which are based on Pd-In, Pd-Sn, AgIn and Ag-Sn phases, may also occur5 and affect mechanical properties.6 A further complicating factor, reported for a high-palladium alloy when cast specimens simulating the coping for a maxillary central incisor restoration7

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were compared to cast tensile bars,8 is that the microstructures for some cast alloys can be dependent upon specimen size and geometry. Fractography, the analysis of the fracture surfaces, is a useful tool for obtaining information about the failure initiation site and the nature of the fracture process for alloys.9 For cast dental alloys, fractographic observations often correlate with laboratory measurements of relative ductility. Microstructural examination of fracture surfaces can be performed using reflected light with an optical microscope or a scanning electron microscopy (SEM).10 Gapido et al11 used SEM to examine the fracture surfaces of Co-Cr and Ag-Pd-Cu-Au cast occlusal rests and identified the casting defects. Li et al12,13 compared the fatigue limits and fracture characteristics for highpalladium and palladium-silver alloys. Complementary information to the fracture surface analysis can be obtained by characterizing the bulk microstructure of polished and etched surfaces of cast dental alloys.13 Defects in the bulk alloy formed during casting can also be readily studied with SEM.14 The objectives of this study were to: (1) compare the mechanical properties, (2)  examine the fracture surfaces, and (3) characterize the polished and etched surfaces of 6 representative noble alloys of the Au-Pt-Pd, Au-Pt, Pd-Ag-Au, Pd-Ag, Au-Pd, and Au-Ag-Pd types. Five of these 6 alloy types encompass a wide variety of currently marketed noble alloys for metal ceramic applications. A sixth Type GPT has Type IV

alloy for fixed prosthodontic applications was included for comparison. The hypothesis of this study was that there would be a difference between the 6 selected alloys when mechanical properties, fracture surfaces, and microstructures are compared. Information about comparative mechanical properties of these alloys, along with complementary knowledge of their fracture behavior at high loads, and the relationships of such properties with alloy microstructures, enable the prosthodontist to make a rational alloy selection for the given clinical situation.

MATERIAL AND METHODS Six noble casting alloys were investigated (Table I): 2 low-fusing highgold ceramic alloys, a gold-platinumpalladium alloy (Aquarius XH) and a gold-platinum alloy (Brite Gold XH); a gold-palladium ceramic alloy (IPS d.SIGN 91); a palladium-silver ceramic alloy (IPS d.SIGN 59); a palladiumsilver-gold ceramic alloy (Capricorn 15); and a copper-free Type IV fixed prosthodontic alloy (Maxigold KF). All alloys were developed by Ivoclar Vivadent (Amherst, NY). The first 5 alloys were selected to span many of the noble alloy types (not including high-palladium alloys) that are currently used for metal ceramic restorations, and the Type IV alloy for cast metal restorations was included for comparison. Using a custom-fabricated specimen mold, 6 wax patterns were prepared for each group according to ISO specifications 969315 and 889116 for metal ceramic restoration alloys and cast metal restoration alloys, respectively.

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Table I. Compositions (wt. %) of 6 casting alloys selected for study Alloy

Au

Pd

Pt

Aquarius XH

82.8

5.0

Brite Gold XH

88.9

IPS d.SIGN 91

60.0

Ag

In

Sn

9.0

2.5

<1.0

9.0

<1.0

<1.0

30.55

IPS d.SIGN 59

59.2

8.4 <1.0

27.9

2.7

Capricorn 15

15.0

51.9

23.0

8.0

Maxigold KF

56.6

8.4

29.0

5.0

Ga

Zn

1.0 8.2

1.3 2.0 1.0

Note: Aquarius XH, Brite Gold XH and Maxigold KF contain <1% of grain-refining element iridium. IPS d.Sign 91, IPS d.SIGN 59, and Capricorn 15 contain <1% of grain-refining elements rhenium and ruthenium.

Following the recommendations of the manufacturer, the patterns were invested and cast using a multi-orifice gas-oxygen torch and a broken-arm centrifugal casting machine. Specimens were bench-cooled to room temperature. The investment was removed with a sharp instrument and by airborne-particle abrasion using 50-μm aluminum oxide powder. Simulated porcelain firing cycles were performed using the manufacturer’s protocol for IPS Classic porcelain (Ivoclar Vivadent) for the 5 ceramic alloys. Type IV Maxigold KF alloy specimens were not heat treated. The 6 specimens for each alloy were loaded in tension at a crosshead speed of 2 mm/minute according to ISO specification 9693 and 8891 using a mechanical testing machine (Model 4204; Instron, Norwood, Mass). Values of elastic modulus, 0.1% and 0.2% yield strength (YS), and ultimate tensile strength (UTS) were obtained with the aid of the mechanical testing machine software. The initially measured diameter for each specimen was input to the software prior to each test. The percentage elongation was obtained by fitting the fractured specimen halves together and microscopically measuring the new separation between the original gauge length marks. Statistical com-

parisons of the mechanical properties for the alloys were performed by 1-way analysis of variance (ANOVA) and the Ryan-Einot-Gabriel-Welsch (REGW) multiple range test (α=.05). Three of the fractured bar specimens for each alloy used for the tensile mechanical property comparisons were subsequently used for microstructural analysis. Initially, the fracture surfaces of the test specimens were examined with SEM (Quanta 200; Philips-FEI, Eindhoven, The Netherlands) using secondary electron images at a wide range of magnifications. After completion of the fracture surface characterization, the percentage elongation was determined. The specimens were embedded in epoxy resin (Fast Cure Acrylic Powder and Liquid; MetLab Corp, Niagara Falls, NY), wet polished, and etched using different dilutions of laboratory-prepared aqua regia solutions.7,8,13 The polished and etched specimens were coated with a thin conducting layer of graphite prior to the SEM examination.14 A wide range of magnifications was used to examine the etched alloy microstructures using secondary electron (SE) imaging and backscattered electron (BSE) imaging. The latter mode provided insight into microstructures from atomic number contrast.17

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RESULTS According to ISO specification 969315, the 0.2% offset YS for metal ceramic dental restorative alloys should be at least 250 MPa, while the percentage elongation after fracture should be no less than 3%. All 5 metal ceramic alloys used in the current study met these requirements. Maxigold KF, the metal restoration alloy, met the requirements in ISO specification 889116 (0.2% offset YS greater than 450 MPa and percentage elongation greater than 3% for Type IV alloys). Table II presents the comparison of the tensile mechanical properties for the 6 noble dental alloys. Significant differences were found among the 6 alloys for all mechanical properties, as noted by the REGW analysis. No significant difference in UTS was found for the d.SIGN 59 alloy (782 ±21 MPa) compared to the d.SIGN 91 (778 ±19 MPa) (P=1.0) and Capricorn 15 (771 ±38 MPa) (P=.98) alloys; nor was a statistically significant difference found between UTS of d.SIGN 91 and Capricorn 15 (P=.998). All 3 alloys had significantly higher UTS compared to Maxigold KF (626 ±39 MPa) and Aquarius XH (592 ±17) (P<.0001 for all pairwise comparisons), which were not sig-

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Table II. Comparison of tensile mechanical properties of 6 noble dental alloys Ultimate Tensile Strength (MPa)

Elastic Modulus (GPa)

0.1% Yield Strength (MPa)

0.2% Yield Strength (MPa)

% Elongation

Aquarius XH

592 (17) B*

103 (6) C

470 (19) B

488 (19) C

6.0 (1.4) D, E

Brite Gold XH

518 (8) C

90 (13) C

409 (13) C

427 (9) D

8.6 (1.4) C, D

Capricorn 15

771 (38) A

130 (19) A, B

508 (49) A, B

535 (49) A, B

13.8 (2.1) A, B

Maxigold KF

626 (39) B

108 (10) B, C

512 (27) A, B

535 (25) A, B

4.2 (1.9) E

d.SIGN 59

782 (21) A

130 (21) A, B

492 (11) B

512 (8) B, C

14.6 (1.6) A

d.SIGN 91

778 (19) A

136 (10) A

548 (16) A

570 (10) A

11.0 (2.1) B, C

Alloy

*Different upper case letters within each column represent significant differences (P<.05) among different alloys. Note: Mean values and standard deviations (in parentheses) rounded to nearest whole number for elastic modulus, yield strength, and ultimate tensile strength, and to 1 decimal place for percent elongation.

nificantly different from each other (P=.246). Both of the latter alloys had significantly higher UTS compared to Brite Gold XH (518 ±8 MPa) (P<.001 for both pairwise comparisons). No significant differences in elastic modulus were found for the d.SIGN 91 (136 ±10 GPa), d.SIGN 59 (130 ±21 GPa), and Capricorn 15 (130 ±19 GPa) alloys (P=.981, P=.972, and P=1.000, respectively, when d.SIGN 91 was compared to d.SIGN 59, d.SIGN 91 was compared to Capricorn 15, and d.SIGN 59 was compared to Capricorn 15). Likewise, there were no significant differences in the elastic modulus for d.SIGN 59 and Maxigold KF (108 ± 10 GPa) (P=.139) and Capricorn 15 and Maxigold KF (P=.157). The Maxigold KF alloy had a significantly lower elastic modulus than d.SIGN 91 (P=.033), but there were no significant differences in elastic modulus for Maxigold KF compared to Aquarius XH (103 ±6 GPa) (P=.992) and Brite Gold XH (90 ±13

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GPa) (P=.299). The values of elastic modulus for Aquarius XH and Brite Gold XH were also not significantly different (P=.583). There were no significant differences in 0.1% yield strength for the d.SIGN 91 (548 ±16 MPa), Maxigold KF (512 ±27 MPa), Capricorn 15 (508 ±49 MPa), and d.SIGN 59 (492 ±11 MPa) alloys. A significant difference was found between the values of 0.1% yield strength for d.SIGN 91 and Aquarius XH (470 ±19 MPa) (P<.001), but there were no significant differences in the values of 0.1% yield strength for Maxigold KF compared to Capricorn 15 (P=1.00), d.SIGN 59 (P=.794), and Aquarius XH (P=.107). No significant differences were found between Capricorn 15 and d.SIGN 59 (P=.875), between Capricorn 15 and Aquarius XH (P=.132), and between d.SIGN 59 and Aquarius XH (P=.676). All other alloys had significantly higher 0.1% yield strength than Brite Gold XH (409 ±13 MPa)

(P=.004 when Aquarius XH was compared to Brite Gold XH, and P<.001 when Brite Gold XH was compared to the other 4 alloys). There were no significant differences in 0.2% yield strength when d.SIGN 91 (570 ±10 MPa) was compared to Capricorn 15 (535 ±49 MPa) (P=.182) and Maxigold KF (535 ±25 MPa) (P=.220). There were also no significant differences (P=1.0 and P=.586) in 0.2% yield strength when Capricorn 15 was compared to Maxigold KF and d.SIGN 59 (512 ±8 MPa), respectively. No significant difference was found between Maxigold KF and d.SIGN 59 (P=.639). d.SIGN 59 had significantly lower 0.2% yield strength than d.SIGN 91 (P=.004). The 0.2% yield strength for Aquarius XH (488 ±19 MPa) was significantly lower than the 0.2% yield strength for d.SIGN 91 (P<.001), Capricorn 15 (P=.025), and Maxigold KF (P=.037) and significantly higher than the 0.2% yield strength for Brite Gold XH (427 ±9

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A

B

C

D

E

F

1 Fracture surfaces for alloys. All alloys are in heat-treated condition, except for Maxigold KF, which is in as-cast condition. A, Aquarius XH: cleavage due to brittle fracture at ridges (bright areas) and regions of ductile fracture are characterized by dimples (x2000 original magnification); B, Aquarius XH: crack, which probably formed during solidification (x1000 original magnification); C, Capricorn 15: region of cleavage (brittle) fracture (x4000 original magnification); D, Maxigold KF: large pore (x80 original magnification); E, d.SIGN 59: precipitates on floor of local dendritic structure (x5000 original magnification); F, d.SIGN 91: flat surface at bottom formed during terminal stage of fracture due to increased porosity (x100 original magnification). MPa) (P=.002). The Brite Gold XH alloy had the lowest values for UTS, elastic modulus (but not significantly different from Maxigold KF and Aquarius XH), and 0.1% and 0.2% yield strength. The values of percentage elongation (%) at fracture were the highest for d.SIGN 59 (14.6 ±1.6) and Capricorn 15 (13.8 ±2.1), and were not significantly different (P=.972). The values of percentage elongation were also not significantly different (P=.105) for Capricorn 15 and d.SIGN 91 (11.0 ±2.1). There was also no significant difference (P=.189) in percentage elongation for d.SIGN 91 and Brite Gold XH (8.6 ±1.4). Moreover, there was no significant difference (P=.165) in percentage elongation for Brite Gold XH and Aquarius XH (6.0 ±1.4), and there was no significant difference (P=.523) in percentage elongation for Aquarius XH and Maxigold KF (4.2 ±1.9), which

had the lowest values. Similar fracture surfaces were observed for all 6 alloys (Fig. 1). The fracture surfaces for all alloys typically contained casting porosity (Fig. 1A-D) as shown in some black circles, which was considered the major cause of fracture initiation. The pore diameters ranged in size from tens of micrometers to as large as 1/2 mm (Fig. 1D). Several microstructural features that have an angular nature, indicated by black circles in Figures 1A and 1B, are considered to be former sites of secondary phases that were lost during the fracture process, rather than porosity. Figure 1 also shows complex fracture surfaces, with all alloys having characteristic features of both brittle and ductile fracture. Flat facets, the characteristic feature of brittle fracture (cleavage areas),10 were most

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clearly observed for Capricorn 15 (Fig. 1C) as shown with black arrows. Representative low-magnification SEM images of the flat areas at the peripheries of the fractured specimens can be seen in Figures 1D and 1F (outlined with black dots). Ductile fracture, characterized by dimpled rupture,10 is particularly evident in Figures 1A-C (white circles), where numerous microvoids can be seen. Precipitate particles, as shown in black circles, typically found on the fracture surfaces, were indicative of the multiphase character of the alloys5 and may have had roles in the fracture processes (Fig. 1E). The presence of incompletely formed grains at the upper center and right side of Figure 1E suggests some dendritic character to the microstructure of d.SIGN 59, and the complex structures of small precipitates within the black circles are

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A

B

C

D

E

F

G

H

2 Microstructures for polished and etched surfaces of alloys. All alloys are in heat-treated condition, except for Maxigold KF, which is in as-cast condition, and had equiaxed fine-grain microstructures: A, Aquarius XH (SE image, x2000 original magnification); B, Aquarius XH (BSE image, x2000 original magnification); C, Brite Gold XH (BSE image, x1000 original magnification). Note small secondary phase particles located at center of each grain in (A), (B) and (C). D, Capricorn 15 (SE image, x3047 original magnification), showing platelike precipitates at grain boundaries removed during etching; E, Maxigold KF (SE image, x2000 original magnification), showing shrinkage porosity at grain boundaries; F, d.SIGN 59 (SE image, x4000 original magnification), showing platelike precipitates and irregular precipitates; G, d.SIGN 91 (BSE image, x1000 original magnification), showing shrinkage porosity at grain boundaries; and (H) d.SIGN 91 (SE image, x4000 original magnification), showing complex substructure at higher magnification, where secondary phase particles were lost during etching. evident. A greater amount of casting porosity was present on the fracture surfaces of Maxigold KF specimens (Fig. 1D), which was consistent with the lower percentage elongation (approximately 4%), compared to the other 5 alloys studied. The effect of

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cooling during alloy solidification was suggested on the fracture surface of d.SIGN 91 by a series of flat terraces (Fig. 1F) that may correspond to successive stages of the solidification process. Cracks of varying sizes associated with the solidification process

can also be seen in Figure 1B (black arrows). There were many similarities in the microstructures of the polished and etched surfaces for the 6 alloys (Fig. 2). While all alloys had equiaxed, fine-grain microstructures, differences

400 in the microstructures at the grain boundary areas were evident. Aquarius XH (Fig. 2A, B) and Brite Gold XH (Fig. 2C) had thin and homogenous grain boundaries containing precipitates that were generally removed during the etching. Although one might view these features as porosities, the authors consider them precipitates since porosities would not be located equidistantly, and the shapes of the porosities would not be so similar. While the dimensions of the grain boundary precipitates were similar for the alloys, small precipitates at the centers of the Brite Gold XH grains (shown with black arrows) were not observed in Aquarius XH. The BSE image in Figure 2C showed that the precipitates at the centers of the Brite Gold XH grains had a different composition from the matrix grains. Heat-treated Capricorn 15 had an equiaxed grain microstructure (Fig. 2D). The later solidifying grain boundary regions were wide and had a fine-scale texture. Platelike precipitates at the grain boundaries were removed by the etching procedure. Different zones were observed inside the equiaxed grains of etched Maxigold KF (Fig. 2E). At least 3 zones are evident in this SE image, starting from the center of each grain to the periphery. The compositional differences among these layers were confirmed using BSE images (not shown), with the zone at the center of some grains apparently having a lower atomic number than the surrounding middle zone.17 The BSE contrast for these regions in Maxigold KF was opposite to that for Figure 2G to follow. The thicker, irregular nature of the grain boundary regions of d.SIGN 59 (Fig. 2F) showed that the porcelain-firing heat treatment did not completely homogenize the microstructure of this alloy, and granular precipitates and dendrites were observed at the grain boundaries. Shrinkage porosity that formed during cooling was also observed at grain boundaries. The d.SIGN 91 microstructure (Fig. 2G) was also composed of equiaxed

Volume 105 Issue 6 grains, but without well-defined grain boundaries, and suggestive of some dendritic character to the microstructure, similar to the Maxigold KF microstructure in Figure 2E. Unlike Maxigold KF, the centers of the d.SIGN 91 grains were always composed of higher atomic number elements compared to the grain peripheries. The BSE image in Figure 2G shows shrinkage porosity at not sharply or “poorly”defined grain boundaries (shown with black arrows), which presumably arise because of local dendritic character. A higher magnification SE image of d.SIGN 91 (Fig. 2H) showed the complex substructure, where secondary phase particles were lost during etching.

DISCUSSION Based on the results of this study, the hypothesis that there would be significant differences between the mechanical properties, fracture surfaces, and microstructures of the 6 selected noble alloys was accepted. The high mechanical properties for d.SIGN 91 might be attributed to the relatively high indium content, which may have caused the formation of the palladium-indium precipitates5 which appeared in the etched microstructures observed with the SEM. No significant differences were found between d.SIGN 91 and d.SIGN 59 for UTS, elastic modulus, and 0.1% yield strength. These high mechanical property values for d.SIGN 59 might also be attributed to palladium-tin and palladium-indium precipitates, in addition to silver-tin and silver-indium precipitates. Guo et al6 reached the same conclusion for palladiumsilver alloys. No significant difference was found when the mechanical properties of Capricorn 15 were compared with d.SIGN 91 or d.SIGN 59. These results might be expected because the indium content is similar for Capricorn 15 and d.SIGN 91. The alloy d.SIGN 59 contains a similar amount of tin (8.2% ) with less indium (2.7%). The indium in Capricorn 15 might have formed strengthening interme-

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tallic compounds with both palladium and silver. Similarly, the higher UTS, elastic modulus, and 0.1% and 0.2% yield strength of Aquarius XH when compared with Brite Gold XH might be explained by the addition of 2.5 % indium to Aquarius XH alloy, whereas indium was added only in trace amounts to Brite Gold XH. Furthermore, the lower indium content of Aquarius XH compared to d.SIGN 91, Capricorn 15, and Maxigold KF, as well as the trace amount of tin, might have resulted in the lower UTS, elastic modulus, 0.1% yield strength and 0.2% yield strength for this alloy. Results from this study suggest that mechanical properties of the 6 alloys are more affected by concentrations of minor elements that can form secondary phases, rather than the concentrations of the major elements. For example, even though the composition of d.SIGN 91 (gold-palladium alloy) was different from the composition of Capricorn 15 (palladium-silver alloy with 15 wt % gold), their mechanical properties were similar. This may be due to their similar indium contents of 8.4 wt % and 8 wt %, respectively. Even though the indium content (2.7 wt %) of d.SIGN 59 was lower than for d.SIGN 91 and Capricorn 15, the addition of tin (8.2 wt %) in the former alloy may have resulted in the similar mechanical properties observed. The comparable mechanical properties for Maxigold KF (goldsilver alloy) to those for d.SIGN 91, d.SIGN 59, and Capricorn 15, and the higher mechanical property values compared with Aquarius XH and Brite Gold XH, may arise from the 5 wt % indium in Maxigold KF. Further study of these 6 alloys using transmission electron microscopy, with energydispersive spectroscopic analysis to determine precipitate compositions, is necessary to verify these hypotheses about strengthening roles of the secondary alloying elements. The size of a cast specimen has a potential effect on the as-cast microstructure and mechanical properties. The near-surface eutectic in

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June 2011 simulated maxillary incisor coping specimens for the Liberty Pd-CuGa alloy7 was absent in much larger cast tensile test specimens.8 This was presumably due to the lower rate of heat loss and solidification with the thicker specimens. Similar to the findings from the 2 previously mentioned studies, results from the current study confirm that relatively less porosity was observed in the larger tensile bar specimen microstructure compared to the smaller cast alloy microstructure on the cast-to specimens (unpublished data). However, other than the amount of porosity, there was no difference between the smaller and larger specimen microstructures. The fracture surface characterization of the 6 alloys in this study was in accordance with the previous findings of Li et al,13 where casting defects (such as porosity) and other microstructural defects were considered to be responsible for the low fatigue limit of cast dental Pd-Ag alloys. This would be expected, since the regions around pores have localized stress concentrations,9 and stress concentrations would exist at other microstructural defects. Some porosity is inevitable in the microstructure of a cast dental alloy and can arise from the sprue design, as well as improper heating of the alloy and an inappropriate mold temperature.3,4 Although localized shrinkage most commonly occurs near the spruecasting junction, it may be observed at the grain boundaries since these are the last parts in the cast alloy to solidify and can contain secondary phases with much lower solidification temperatures than the matrix grains. The initially solidifying regions of the cast alloy are the central portions of these grains. Besides casting porosity, precipitates of varying sizes and shapes were observed with the SEM on the fracture surfaces of the 6 alloys, and these secondary phase particles might have a key role in fracture initiation. BSE images of the polished and etched surfaces clearly indicate zones of microsegregation within grains. The

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simulated porcelain-firing heat treatment is insufficient to homogenize the microstructures of the as-cast alloys completely. An interesting concluding matter is the presence of flat regions (Fig. 1D and 1F) at the periphery of some alloy fracture surfaces. This may be due to localized cleavage at planes of facile fracture in certain microstructural phases, but further study is needed to verify this hypothesis. Clinical alloy selection should be based upon the type of restoration and mechanical property requirements in vivo. The major influence of casting porosity on the fracture process emphasizes the importance of careful control of alloy melting and the casting procedure. All 6 alloys were composed of more than one phase, and a full understanding of these secondary phases in regards to mechanical properties and fracture requires further research. One-way ANOVA followed by the REGW multiple range test (α=.05) were used for statistical analysis. The REGW comparisons of means assume that variances for test sample groups are nearly equal, which was not the case for UTS (P=.048), 0.1% yield strength (P=.003) and 0.2% yield strength (P<.001) where variances ranged over an order of magnitude, as shown in Table II. These parametric statistical methods might have reduced the power of the analysis and should be considered a limitation of the study. It is imperative to emphasize knowledge of the mechanical properties for rational clinical selection among the different types of noble alloys. For example, high elastic modulus is important for long-span fixed prostheses where alloy bending deflection under in vivo forces should be minimized, and high alloy yield strength is essential for restorations that will be subjected to substantial functional loads. In addition, laboratory manipulation of these alloys for adjustment of castings will be dependent upon complex relationships between elastic modulus, yield strength,

and percentage elongations that require further study.

CONCLUSIONS The mechanical properties of the 6 gold and palladium alloys differed widely, which should be considered for their clinical selection for fixed prosthodontics. Differences in mechanical properties are directly related to their compositional differences, particularly secondary elements, such as indium and tin. These can form intermetallic phases to provide strengthening, as well as to the alloy microstructures, which can have equiaxed grains as well as local dendritic character.

REFERENCES

1. Oilo G, Gjerdet NR. Dental casting alloys with a low content of noble metals: physical properties. Acta Odontol Scand 1983;41:111-6. 2. Cheng AC, Chai JY, Gilbert J, Jameson LM. Mechanical properties of metal connectors soldered by gas torch versus an infrared technique. J Prosthodont 1993;2:103-9. 3. Powers JM, Sakaguchi RL. Craig’s Restorative Dental Materials. 12th ed. St. Louis: Mosby Elsevier; 2006. p. 55-60, 431-5. 4. Anusavice KJ. Phillips’ Science of Dental Materials. 11th ed. Philadelphia: Saunders; 2003. p.124-30, 337-46. 5. Massalski TB. Binary alloy phase diagrams, Vol. 1 and 2. Ohio: American Society for Metals, 1990. p. 34, 54, 71, 1390, 1874. 6. Guo WH, Brantley WA, Li D, Clark WA, Monaghan P, Heshmati RH. Annealing study of palladium-silver dental alloys: Vickers hardness measurements and SEM microstructural observations. J Mater Sci Mater Med 2007:18:111-8. 7. Carr AB, Brantley WA. New high-palladium casting alloys: 1. Overview and initial studies. Int J Prosthodont 1991;4:265-75. 8. Papazoglou E, Wu Q, Brantley WA, Mitchell JC, Meyrick G. Comparison of mechanical properties for equiaxed fine-grained and dendritic high-palladium alloys. J Mater Sci Mater Med 2000;11:601-8. 9. Dieter GE. Mechanical Metallurgy. 3rd ed. New York: McGraw-Hill; 1986. p. 61-3, 242-3, 254-6, 329-32. 10.ASM Handbook Committee. Metals Handbook, 9th Edition, Volume 12: Fractography. Ohio: ASM International; 1987. p. 91-178. 11.Gapido CG, Kobayashi H, Miyakawa O, Kohno S. Fatigue resistance of cast occlusal rests using Co-Cr and Ag-Pd-Cu-Au alloys. J Prosthet Dent 2003;90:261-9.

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12.Li D, Brantley WA, Mitchell JC, Daehn GS, Monaghan P, Papazoglou E. Fatigue studies of high-palladium dental casting alloys: Part I. Fatigue limits and fracture characteristics. J Mater Sci Mater Med 2002;13:361-7. 13.Li D, Brantley WA, Guo W, Clark WA, Alapati SB, Heshmati RH, Daehn GS. Fatigue limits and SEM/TEM observations of fracture characteristics for three Pd-Ag dental casting alloys. J Mater Sci Mater Med 2007;18:119-25. 14.Ucar Y, Brantley WA, Bhattiprolu SN, Johnston WM, McGlumphy EA. Characterization of cast-to implant components from five manufacturers. J Prosthet Dent 2009;102:216-23.

15.ISO Standard 9693. Metal-ceramic dental restorative systems. International Organization for Standardization, Geneva, Switzerland (1999E). 16.ISO Standard 8891. Dental casting alloys with noble metal content of at least 25% but less than 75%. International Organization for Standardization, Geneva, Switzerland (1998E). 17.Goldstein J, Newbury D, Joy D, Lyman C, Echlin P, Lifshin E, et al. Scanning Electron Microscopy and X-Ray Microanalysis. 3rd ed. New York: Springer; 2003. p.404-5, 613-4.

Corresponding author: Dr Yurdanur Ucar Faculty of Dentistry Cukurova University 01330 Balcalı, Adana TURKEY Fax: +90-322-3387331 E-mail: [email protected] Copyright © 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.

Noteworthy Abstracts of the Current Literature Candida albicans adherence on silicone elastomers: effect of polymerisation duration and exposure to simulated saliva and nasal secretion Kurtulmus H, Kumbuloglu O, Ozcan M, Ozdemir G, Vural C. Dent Mater 2010;26:76-82. Objectives. The surfaces of maxillo-facial prostheses made of silicone elastomers exposed to soft tissues may interact with saliva and nasal secretion. These body fluids may lead to colonisation of microorganisms on their surfaces leading to their degradation or infection. This study investigated Candida albicans adhesion onto commercial maxillo-facial silicone elastomers based on different polymerisation processes. Methods. Room-temperature polymerised maxillo-facial silicone elastomers (N = 48) (10 mm x 10 mm x 2 mm) processed at different durations [VerSilTal VST-30 (20 min), VST-50 (12 h overnight), VST-50F (6 h)] were studied. C. albicans was chosen as a model organism for this study. The specimens were randomly divided into two subgroups and incubated in either 1.5 ml simulated saliva or nasal secretion containing C. albicans (ATCC 60193, set to 0.5 OD, 540 nm in advance) for 2 h. Candida assays and adherence assays were made by inoculating C. albicans into Mueller Hinton Broth, Fluka® added 500 mmol sucrose overnight. After fixation, specimens were stained by using sterilised Methylene Blue stain (Merck®) and evaluated under optical microscope and SEM. For each material, on each specimen 15 different areas (mm2) were counted. Data were analysed using one-way ANOVA, paired sample t-test and Tukey’s HSD (α = 0.05). Results. Material type (p < 0.05) and exposure media (p < 0.05) showed a significant influence on the C. albicans adherence. VST-30 material showed the most C. albicans adherence in both saliva and nasal secretion (mean rank: 99.84 and 53.47, respectively) (p < 0.05) and VST-50 had the least colonisation in both media (10.35 and 5.57, respectively). Microscopic evaluation showed clusters of blastospore cells of C. albicans being more spread out on VST-30 whereas cells were more localised on VST-50 and VST-50F. Conclusion. Among the tested materials, 12 h room-temperature polymerised silicone elastomer resulted in less C. albicans adherence in both artificial saliva and nasal secretion. Reprinted with permission of the Academy of Dental Materials.

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