- Email: [email protected]
Effect of ice-quenching after oxidation treatment on hardening of a Pd-Cu-Ga-Zn alloy for bonding porcelain
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
Contents lists available at ScienceDirect
Journal of the Mechanical Behavior of Biomedical Materials journal homepage: www.elsevier.com/locate/jmbbm
Effect of ice-quenching after oxidation treatment on hardening of a Pd-CuGa-Zn alloy for bonding porcelain Min-Jung Kim, Hye-Jeong Shin, Hyung-Il Kim, Yong Hoon Kwon, Hyo-Joung Seol
T
⁎
Department of Dental Materials, Institute of translational dental science and BK21 PLUS Project, School of Dentistry, Pusan National University, Beomeo-Ri, Mulgeum-Eup, Yangsan-Si, Gyeongsangnam-Do 626-814, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Porcelain firing simulation Pd-Cu-based alloy Ice-quenching Precipitation Softening
This study examined the effect of ice-quenching after oxidation treatment on hardness change of a Pd-Cu-Ga-Zn metal-ceramic alloy during porcelain firing simulation. Although not statistically significant, the alloy was softened slightly during porcelain firing simulation with conventional slow cooling rate. On the other hand, the hardness increased significantly by ice-quenching instead of the slow cooling after oxidation (p < 0.001). The gap in the final hardness depending on ice-quenching occurred in the matrix and plate-like precipitates but not in the particle-like structure without plate-like precipitates (p < 0.05). The mechanism of ice-quenching after oxidation to prevent softening and induce hardening during porcelain firing simulation involved the more active precipitation and retardation of microstructural coarsening. In conclusion, for practical work on Pd-Cu-Ga-Zn alloys, the oxidation treatment followed by ice-quenching instead of slow cooling is recommended for the simultaneous oxidation and hardening effects on the alloy.
1. Introduction Dental metal-ceramic alloys are used as a substructure of porcelain superstructures to improve the durability of metal-ceramic prostheses. Pd-based metal-ceramic alloys are used as alternative alloys for Au-Ptbased alloys. Among them, Pd-Ag-based alloys have been used as typical alternative metal-ceramic alloys. Pd-Ag-based alloys have high elastic modulus and sag resistance, easy to solder, and excellent corrosion resistance (O'Brien, 2002; Roberts et al., 2009). On the other hand, because of their high silver contents, porcelain greening and furnace contamination can result during laboratory procedures, unless the porcelain is carefully selected (Rosenstiel et al., 2016). Pd-Cu-based alloys were developed as economical alternatives to the Au-Pt-based alloys in the 1980s, and these alloys have casting accuracy comparable with that of the high-gold content metal-ceramic alloys (Rosenstiel et al., 2016). Pd-based alternative alloys are relatively harder than the Au-based metal-ceramic alloys in the as-cast state (Fischer and Fleetwood, 2000; Liu and Wang, 2007; Li et al., 2010). On the other hand, softening occurs in some cases in the Pd-based metal substructure during the porcelain firing process (Guo et al., 2003; Li et al., 2010), which is undesirable because it may affect the durability of the final prosthesis. In the study with a Pd-Au-In-Ag alloy, performing a solution treatment by heating the alloy at approximately 1000 °C and then ice-quenching
⁎
(quenching into ice brine) before firing porcelain was reported to be effective in solving this problem (Jeon et al., 2014). In that study, the hardness of the alloy was reduced by the solution treatment, but this recovered rapidly from the next firing step for bonding porcelain. In addition, unlike the untreated alloys, softening did not occur in the solution-treated alloy during the remaining firing process. Despite this, the solution treatment of the alloy is troublesome because it must be carried out separately from the porcelain firing process. The first step in the porcelain firing process is oxidation, which usually takes place within 10 min at temperatures around 1000 °C and the alloy is then bench cooled. This process is similar to the solution treatment, only with different cooling rates. Therefore, if the oxidation treatment is followed by ice-quenching instead of conventional slow cooling, the oxidation and the solution treatment effects can be obtained simultaneously without an additional solution treatment. In this case, softening of the alloy necessarily occurs immediately after oxidation. Therefore, it is important to determine if the hardness recovers rapidly during the subsequent firing process. In a previous study with a Pd-Au-based metal-ceramic alloy without silver, softening during porcelain firing simulation did not occur regardless of ice-quenching after degassing treatment, but the ice-quenching after degassing improved the final hardness slightly (Shin et al., 2017). For Pd-Cu-based alternative alloys, the changes in hardness during porcelain firing process have not reported, but softening of the alloys can be expected during the firing
Corresponding author. E-mail address: [email protected] (H.-J. Seol).
https://doi.org/10.1016/j.jmbbm.2017.12.014 Received 12 April 2017; Received in revised form 16 November 2017; Accepted 14 December 2017 Available online 15 December 2017 1751-6161/ © 2017 Elsevier Ltd. All rights reserved.
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Table 1 Chemical composition of the specimen alloy.
Table 3 Cooling rate during simulated firing.
Composition
Pd
Cu
Ga
Zn
In
Ru
Ag
Cooling rate
Icequenching
Stage 0
Stage 1
Stage 2
Stage 3
wt% at%
79.0 71.4
8.0 12.1
4.9 6.8
4.5 6.6
3.0 2.5
0.4 0.4
0.2 0.2
Condition
Cooled by rapid quenching into ice brine
Firing chamber moves immediately to upper end position
Firing chamber opens about 70 mm
Firing chamber opens about 50 mm
Firing chamber remains closed
process. Therefore in the present study, a Pd-Cu-based alloy was investigated to determine the softening behavior during the porcelain firing process and if it is the case, whether the oxidation treatment followed by ice-quenching prevents softening without an additional solution treatment. For this, the effect of an oxidation treatment followed by ice-quenching on the change in hardness of a Pd-Cu-Ga-Zn alloy during a porcelain firing simulation was characterized by analyzing the changes in hardness, microstructure, crystal structure, and elemental distribution.
time of 10 s using a Vickers micro-hardness tester (MVK-H1, Akashi, Japan) for the porcelain firing simulated specimens. The hardness results were recorded as the mean of five measurements. In addition, to measure the hardness of each microstructure (particle-like structure and matrix) separately, the load was reduced to 10 gf. The regions covering the precipitate-generated particle-like structure were measured with a load of 25 gf. The hardness results were recorded as the mean of at least seven measurements.
2. Materials and methods 2.1. Specimen alloy
2.4. Field emission scanning electron microscopy (FE-SEM) The alloy used in this study was a Pd-Cu-Ga-Zn alternative alloy for metal-ceramic (silfree79, Aurium research, USA). The melting range of the alloy according to the manufacturer is 1160 to 1250 °C and the casting temperature is 1375 °C. Table 1 lists the composition of the alloy. The atomic ratio (at%) was calculated from the weight ratio (wt %) supplied by the manufacturer. A phosphate-based investment was used to cast the plate specimens (10 × 10 × 0.5 mm3 in size). The alloy was melted with an oxygen-gas torch and cast using a centrifugal casting machine (Casting machine, Osung, South Korea). The as-cast specimens were bench cooled to room temperature and washed with an ultrasonic cleaner (Bransonic, Branson, USA) for 30 min.
The microstructural changes in the specimens during the porcelain firing simulation were examined by FE-SEM (JSM-6700F, JEOL, Japan). The porcelain firing simulated specimens were polished metallographically and etched in an aqueous solution containing 10% KCN (potassium cyanide) and 10% (NH4)2S2O8 (ammonium persulfate). The surfaces of the specimens were observed by FE-SEM at 15 kV. 2.5. X-ray diffraction (XRD) XRD (XPERT-PRO, Philips, Netherlands) was performed to examine the present phases in each step of the porcelain firing simulation. The porcelain firing simulated specimens for XRD analyses were polished metallographically and etched in an aqueous solution, as done for the FE-SEM observation. The XRD profile was recorded at 30 kV and 40 mA using Ni-filtered Cu Kα radiation as the incident beam. The scanning rate of the goniometer was 1° (2θ/min).
2.2. Porcelain firing simulation The as-cast specimens were firing simulated according to the steps in Table 2. First, an oxidation treatment was performed at 1010 °C for 5 min in a porcelain furnace (Multimat 2 torch, Dentsply, Germany). The specimen was then cooled at various cooling rates to determine the most effective cooling rate for alloy hardening. The cooling rate was divided into 5 stages (Ice-quenching; quenching into ice brine, Stage 0, Stage 1, Stage 2, and Stage 3) in the order of cooling speed, as shown in Table 3. The remaining firing steps in Table 2 were then carried out and cooled at the most effective cooling rate for alloy hardening. The simulated porcelain firing cycles in Table 2 correspond to a slight modification of the firing cycles of the specific dental porcelain product (VITA VMK Master, VITA Zahnfabrik, Germany); the modification was done to simplify the complex firing process.
2.6. Field emission electron probe microanalysis (FE-EPMA) The porcelain firing simulated specimens were polished metallographically and etched in an aqueous solution, as done for the FE-SEM observation. The elemental distribution of the porcelain firing simulated specimens was examined using a field emission electron probe microanalyzer (JXA-8530F, JEOL, Japan) at 15 kV. 2.7. Energy-dispersive X-ray spectrometry (EDS)
2.3. Hardness test
The porcelain firing simulated specimens were polished metallographically and etched in an aqueous solution, as done for the FE-SEM observation. The elemental content in each microstructure of the porcelain firing simulated specimens was examined using an energy dispersive X-ray spectrometer (INCA x-sight, Oxford Instruments Ltd., UK) attached to the FE-SEM at 15 kV.
The Vickers hardness was measured at a load of 300 gf and a dwell Table 2 Simulated porcelain firing cycles. Firing cycles
Oxidation Wash Opaque Main bake Correction Glaze
Predrying (min)
Heat rate (°C/ min)
Start temp. (°C)
Final temp. (°C)
Time at final temp. (min)
Vacuum time (min)
Vacuum level (hpa)
2.8. Statistical analysis
0 2 2 4 4 0
70 70 70 70 70 70
550 550 550 550 550 550
1010 960 930 920 910 900
5 1 1 1 1 0
0 6:51 6:26 6:17 6:09 0
0 70 70 70 70 0
Hardness data were analyzed using a statistical program SPSS 23.0 (Statistical Product and Service Solutions 23.0, IBM Co., USA); a statistical significance level of 0.05 was considered significant for all tests. The hardness measured as a function of the cooling rate after oxidation was subjected to the one-way ANOVA, followed by a Tukey HSD test for multiple comparisons. The hardness variation in each firing step with cooling rate was analyzed by a RMANOVA (Repeated Measure 84
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Fig. 1. Change in hardness according to the cooling rates after oxidation with the standard deviation marked in.
Fig. 2. Change in hardness of the specimens during the porcelain firing simulation with the standard deviation marked in.
ANOVA), followed by tests of within-subjects contrasts. The hardness corresponding to each microstructure (matrix, particle-like structure without plate-like precipitates) during the porcelain-firing simulation was analyzed using a generalized linear model (GLM), as the hardness values did not show a normal distribution. The hardness corresponding to each microstructure obtained at the final stage of firing was analyzed by the Kruskal-Wallis H test, followed by the Bonferroni-Dunn test for multiple comparisons as the hardness values did not show a normal distribution.
Table 5-1 The statistics of the hardness as a function of cooling rate and firing step.
To determine the most effective cooling rate for alloy hardening, the specimens were cooled at various cooling rates after oxidation, as listed in Table 3. Fig. 1 shows the difference in hardness as a function of the cooling rate. Table 4 shows the statistics of the hardness as a function of the cooling rate analyzed by a one-way ANOVA, followed by a Tukey HSD test for multiple-comparisons. The hardness results were recorded as the mean of five measurements. In Fig. 1 and Table 4, the hardness increased with decreasing cooling rate; the highest hardness was obtained at Stage 2 and 3 (p=0.000). After oxidation, ice-quenched specimen and specimen cooled at Stage 3 (the most effective cooling rate for hardening) were prepared. The remaining firing steps in Table 2 were then carried out to observe the changes in hardness during porcelain firing simulation (Fig. 2). At this time, the cooling rate was set to Stage 3 for each specimen, and the hardness test was carried out with a load of 300 gf. The hardness results were recorded as the mean of five measurements. Tables 5-1, 5-2 shows the statistics of the measured hardness as a function of the cooling rate Table 4 The statistics of the hardness as a function of the cooling rates after oxidation.
Hardness (M ± SD)
a
257.36 ( ± 2.78)
Stage 0 b
267.32 ( ± 1.8)
Stage 1 b
270.52 ( ± 2.68)
Stage 2 c
277.74 ( ± 2.49)
Cooling rate Firing step Cooling rate×Firing step
16.363 (0.004)* 20.167 (< 0.001)* 43.860 (< 0.001)*
and firing step as analyzed by a RMANOVA (Repeated Measure ANOVA), followed by tests of within-subjects contrasts. As seen in Table 5-1, the cooling rate (Stage3, Ice-quenching) (p = 0.004) and firing step (Oxidation-Glaze) (p < 0.001) were found to affect the hardness; the interaction between the cooling rate and the firing step also affected the hardness (p < 0.001). As seen in Tables 5-2, the hardness values corresponding to the oxidation and wash steps were statistically higher for the specimen cooled at Stage 3 than that for the ice-quenched specimen. However, the hardness values observed at the opaque, main bake, correction, and glaze steps showed the opposite results (p < 0.001). Comparing the hardness values (measured as a function of the firing step) for the ice-quenched specimen, the hardness corresponding to the wash step was significantly higher than that corresponding to the oxidation step, but was not significantly different from those corresponding to the opaque and main bake steps. Additionally, the hardness corresponding to the correction step in the icequenched specimen was significantly higher than that observed for the main bake step, but was not significantly different from that observed for the glaze step. Thus, in the ice-quenched specimen, the hardness increased steadily after oxidation, through the remaining firing steps; the highest hardness was observed at the correction and glaze steps. On the other hand, in the specimen cooled at Stage 3, the hardness at the oxidation step was not significantly different from that observed for the remaining firing steps. To analyze the cause of the change in hardness in more detail, the specimens were etched before measuring the hardness. Fig. 3 shows the hardness which was measured with the load of 10 gf or 25 gf according to the microstructure. The hardness results were recorded as the mean of at least seven measurements. All specimens consisted of matrix and particle-like structure, and the complete firing simulated specimens had plate-like precipitates in the particle-like structure, as will be seen in the FE-SEM images in Fig. 4. Tables 6-1, 6-2 shows the statistics of the hardness as a function of the cooling rate and firing step in matrix and
3.1. Hardness changes
Ice-quenching
F (p)
* Statistically significant difference (p < 0.05). Statistical significance was analyzed by a RMANOVA (Repeated Measure ANOVA) at a=0.05.
3. Results
Cooling rate
Factor
Stage 3 279.86c ( ± 1.59)
The values are expressed as mean ± standard deviation. Statistical significance was analyzed by a one-way ANOVA at a=0.05, followed by a Tukey HSD test for multiplecomparisons. a,b,c Statistically significant difference in the hardness. Same lowercase letters indicate that there are no statistical differences among groups.
85
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Table 5-2 The statistics of the hardness as a function of cooling rate and firing step. Hardness (M ± SD)
Stage3 Ice-quenching
Oxidation
Wash
Opaque
Main bake
Correction
Glaze
279.86Ba ( ± 1.59) 257.36Aa ( ± 2.78)
279.98Ba ( ± 2.89) 277.30Ab ( ± 3.98)
277.34Aa ( ± 2.98) 280.44Bb ( ± 4.21)
276.48Aa ( ± 1.73) 282.56Bb ( ± 1.61)
275.52Aa ( ± 2.97) 286.58Bc ( ± 2.38)
272.10Aa ( ± 4.01) 287.74Bc ( ± 3.48)
The values are expressed as mean ± standard deviation. Statistical significance was analyzed by a RMANOVA (Repeated Measure ANOVA) at a=0.05, followed by tests of within subjects contrasts. A,B Statistically significant difference in the cooling rate (Stage3, Ice-quenching). a,b,c Statistically significant difference for different firing steps. Same uppercase letters indicate that there are no statistical differences between the cooling rate (Stage3, Ice-quenching), and same lowercase letters indicate that there are no statistical differences among firing steps (OxidationGlaze).
quenching the as-cast specimen after oxidation (IQ-O), the fine dot-like precipitates in the matrix disappeared, and the boundaries of the particle-like structure became smoother than those in the as-cast specimen due to homogenization. On the other hand, by cooling the specimen at Stage 3 after oxidation (S3-O), fine dot-like precipitates re-appeared, resulting in hardening. In the complete firing simulated specimens (IQG, S3-G), plate-like precipitates were generated in the particle-like structure, and fine dot-like precipitates covered the matrix and the plate-like precipitates regardless of ice-quenching after oxidation. However, the dot-like precipitates were much coarser in the S3-G specimen. 3.3. Phase transformation The phase transformation associated with changes in hardness was observed by XRD (Fig. 5). The IQ-O specimen consisted of a face-centered cubic (f.c.c.) α phase with a lattice constant of a200=3.862 Å and a CsCl-type β phase with a lattice constant of a200=3.047 Å. The 110β and 200α peaks were separated by Kα1 and Kα2, without extra peaks from another phase. Considering that the regions of the particle-like structure were wider than those of the matrix (Fig. 4), the α phase with a relatively high peak intensity and the β phase were confirmed to be the particle-like structure and the matrix, respectively. In the S3-O specimen and the complete firing simulated IQ-G and S3-G specimens, the 200α' and 110β' peaks were newly formed in the lower angle side of the 200α peak and in the higher angle side of the 110β peak respectively, due to precipitation. In the FE-EPMA result (Fig. 6) of the IQ-O specimen, Pd, In, and Ag were concentrated in the particle-like structure (P), whereas Cu, Ga, and Zn were concentrated in the matrix (M). Such a result was also obtained in the complete firing simulated IQ-G specimen. In the IQ-G specimen, the elemental distribution of the plate-like precipitates (Ppt) that were formed in the particle-like structure corresponded to that of the matrix. Such a result corresponded to the EDS result, which was carried out on the S3-G specimen (Fig. 7, Table 8). Compared to the content in the particle-like structure (P), the Pd content decreased with increasing Zn, Cu and Ga content in both the matrix (M) and plate-like precipitates (Ppt), even though there was a contamination by the constituents of the fine dot-like precipitates which covered the matrix and plate-like precipitates. The Ag and Ru content was not detected by EDS due to their relatively small amounts.
Fig. 3. Change in hardness in each microstructure (matrix, particle-like structure with or without plate-like precipitates) during porcelain firing simulation with the standard deviation marked in: ice-quenched after oxidation (IQ-O), cooled at Stage 3 after oxidation (S3-O), complete firing simulation of IQ-O (IQ-G), and complete firing simulation of S3-O (S3-G).
particle-like structure without plate-like precipitates analyzed using the GLM. As seen in Tables 6-1, in both the matrix and particle, the firing step (Oxidation, Glaze) and the interactions between the cooling rate and the firing step affected the hardness (p < 0.001). The cooling rate affected the hardness of the particle only (p < 0.001). Table 6-2 shows the statistics of the hardness corresponding to each microstructure. As seen in Table 6-2, in the matrix, the hardness of IQ-O was significantly lower (by 44.04 HV) (p < 0.001) than that of S3-O, while the hardness of IQ-G was significantly higher (by 44.21 HV) (p < 0.001) than that of S3-G. In the particle, the hardness of IQ-O was significantly lower than that of S3-O (by 22.41 HV) (p < 0.001), while the hardness values of IQ-O and S3-G were not significantly different (p= 0.337). Table 7 shows the statistics of the hardness corresponding to each microstructure obtained at the final stage of firing (Glaze) and analyzed by the Kruskal-Wallis H test, followed by the Bonferroni-Dunn test for multiple-comparisons (M:Matrix, P:Particle without plate-like precipitates, P+:Particle with plate-like precipitates). The hardness values of IQ-G (M), S3-G (M), and IQ-G (P+) were significantly higher than those of IQ-G (P), S3-G (P) and S3-G (P+) (p < 0.001).
4. Discussion Pd-Cu-based alloys were developed as economical alternatives to the Au-Pt-based alloys in the 1980s, and these alloys have casting accuracy comparable with that of the high-gold content metal-ceramic alloys (Rosenstiel et al., 2016). In dental Pd-based metal-ceramic alloys, there are cases where a decrease in hardness occurs during porcelain
3.2. Microstructural changes In Fig. 4, the as-cast specimen had a particle-like structure surrounded by the matrix, and the matrix had fine dot-like precipitates. The boundaries of the particle-like structure were irregular. By ice86
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Fig. 4. Microstructural changes during porcelain firing simulation: ascast, ice-quenched after oxidation (IQ-O), cooled at Stage 3 after oxidation (S3-O), complete firing simulation of IQ-O (IQ-G), and complete firing simulation of S3-O (S3-G) at magnifications of ×4000 (left), ×30,000 (right), M: matrix, P: particle-like structure.
firing process (Guo et al., 2003; Li et al., 2010). As for Pd-Cu-based alternative alloys, the changes in hardness during the porcelain firing process have not been reported, but softening of the alloys can be expected during the firing process. The introduction of a solution treatment before porcelain firing process was reported to be effective in solving this problem (Jeon et al., 2014). The first step in the firing process for bonding porcelain is an oxidation treatment, which is
similar to the solution treatment only with different cooling rates. Therefore, the present study examined the softening behavior of the PdCu-based alternative alloy during porcelain firing process and if it is the case, whether the oxidation treatment followed by ice-quenching is effective in preventing softening instead of solution-treatment. First, to minimize the softening of the alloy during the porcelain firing process, the most effective cooling rate for alloy hardening was found. As a 87
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Table 6-1 The statistics of the hardness as a function of cooling rate and firing step in matrix and particle-like structure without plate-like precipitates. Factor
Wald-chi-square (p)
Cooling rate Firing step Cooling rate×Firing step
Matrix
Particle
0.000 (0.989) 12.321 (< 0.001)* 43.860 (< 0.001)*
17.699 (< 0.001)* 16.171 (< 0.001)* 30.425 (< 0.001)*
* Statistically significant difference (p < 0.05). Statistical significance was analyzed using a general linear model (GLM). Table 6-2 The statistics of the hardness as a function of cooling rate and firing step in matrix and particle-like structure without plate-like precipitates. Specimens
Difference (p) Matrix
S3-O " " IQ-O " S3-G
IQ-O S3-G IQ-G S3-G IQ-G IQ-G
Particle *
44.04 (< 0.001) 22.51 (0.021)* −21.70 (0.030)* −21.53 (0.003)* −65.74 (< 0.001)* −44.21 (< 0.001)*
22.41 (< 0.001)* 3.44 (0.287) 0.43 (0.899) −18.96 (< 0.001)* −21.98 (< 0.001)* −3.02 (0.337)
Fig. 5. Change in the XRD patterns during porcelain firing simulation: ice-quenched after oxidation (IQ-O), cooled at Stage 3 after oxidation (S3-O), complete firing simulation of IQ-O (IQ-G), and complete firing simulation of S3-O (S3-G).
* Statistically significant difference (p < 0.05). Statistical significance was analyzed using a general linear model (GLM).
In the FE-SEM images (Fig. 4) of complete firing simulated specimens (IQ-G, S3-G), the matrix was covered with fine dot-like precipitates, and the plate-like precipitates were formed in the particle-like structure regardless of ice-quenching after oxidation. The plate-like precipitates had the same composition as the matrix (Fig. 6, Fig. 7, Table 8), and were covered with fine dot-like precipitates, as it was in the matrix in both the specimens. However, the dot-like precipitates were formed more actively in the ice-quenched specimen (IQ-G), as reported in the study with a Pd-Au-based alloy which was ice-quenched after degassing treatment (Shin et al., 2017). In addition, the dot-like precipitates were less coarsened in the ice-quenched specimen (IQ-G) than in the specimen cooled at Stage 3 (S3-G). The precipitation hardening has been reported to be the common hardening mechanism in various dental alloys (Hisatsune et al., 1990, 1997; Vermilyea et al., 1996; Kim et al., 2012, 2015; Jeon et al., 2014). However, the coarsening of the precipitates by overaging reduces the interface with the matrix, thereby eliminating the internal stresses caused by lattice distortion, which causes softening (Udoh et al., 1984; Guo et al., 2007; Pan and Wang, 2007; Kim et al., 2015). Therefore, although the ice-quenched specimen after oxidation showed lower hardness than the specimen cooled at Stage 3 after oxidation, the final hardness after complete firing simulation was reversed (Table 5–2, p < 0.001). The XRD results (Fig. 5) of the ice-quenched specimen after oxidation (IQ-O) revealed that the particle-like structure was composed of the f.c.c. α phase with a200=3.862 Å and the matrix was composed of the CsCl-type β phase with a200=3.047 Å. In the complete firing simulated specimens (IQ-G, S3-G), the α and β phases underwent a phase transformation to the α' phase with a 0.003 Å larger lattice constant (a200=3.865 Å) and β' phase with a 0.005 Å smaller lattice constant (a200=3.042 Å), respectively, regardless of ice-quenching after oxidation. FE-EPMA analysis (Fig. 6) and the EDS result (Fig. 7, Table 8) showed that the Pd content decreased with increasing Zn, Cu and Ga content in both the matrix and plate-like precipitates compared to that in the particle-like structure. These results, together with the XRD results (Fig. 5), showed that the particle-like structure was composed of the f.c.c. Pd-rich phase and the matrix and plate-like precipitates were composed of the CsCl-type ordered phase containing Pd, Cu, Ga, and Zn in the complete firing simulated specimens. The reported lattice constant of the Pd2Zn phase with a CsCl-type is 3.055 Å (Villars and Calvert, 1985), which is similar to the lattice constant of the matrix. Cu
Table 7 The statistics of the hardness as a function of cooling rate in each microstructure obtained at the glaze step. Microstructure
Hardness (M ± SD)
IQ-G S3-G IQ-G S3-G IQ-G S3-G
425.40 ± 21.64 b 381.19 ± 22.35 b 244.95 ± 6.35 a 241.93 ± 5.14 a 282.68 ± 11.36 b 256.50 ± 4.04 a
(M) (M) (P) (P) (P+) (P+)
M: Matrix, P: Particle without plate-like precipitates, P+: Particle with plate-like precipitates. IQ-O: ice-quenched after oxidation, S3-O: cooled at Stage 3 after oxidation, IQ-G: complete firing simulation of IQ-O, S3-G: complete firing simulation of S3-O. The values are expressed as mean ± standard deviation. Statistical significance was analyzed by a Kruskal-Wallis H at a=0.05, followed by a Bonferroni-Dunn test for multiple-comparisons. a,b Statistically significant difference in the hardness. Same lowercase letters indicate that there are no statistical differences among groups.
result, the hardening effect was stronger at a slower cooling rate (Fig. 1, Table 4, p=0.000), which was different from the results obtained from the Pd-Au-Zn, Pd-Au-In, and Pd-Ag-Sn alloys with melting points similar to that of the present alloy (Jeon et al., 2014; Yu et al., 2016; Shin et al., 2017). Although not statistically significant, slight softening was unavoidable even with firing simulation at the most effective cooling rate (Stage 3) for alloy hardening (Fig. 2). On the other hand, in the case that the oxidation treatment was followed by ice-quenching instead of cooling at Stage 3, the hardness increased during the remaining firing process (Fig. 2, Table 5–2, p < 0.001). Therefore, the change in hardness in each microstructure during the firing process was examined to determine the cause of difference in the final hardness in more detail. As a result, a gap in the final hardness depending on ice-quenching occurred in the matrix and plate-like precipitates but not in the particle-like structure without plate-like precipitates (Fig. 3, Table 7, p < 0.05). 88
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Fig. 6. Element distribution by FE-EPMA: ice-quenched specimen after oxidation (a: IQ-O), complete firing simulated specimen of IQ-O (b: IQ-G).
89
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
having f.c.c. structure to minimize the lattice misfit (Porter et al., 2009). The plate-like precipitates are coarser than the dot-like precipitates in the glaze step, so the lattice distortion must have been released by the reduced interphase boundaries between the coarsened plate-like precipitates and the surrounding Pd-rich particle-like structures in the glaze step, while the lattice distortion by the dot-like precipitates caused hardening in the surrounding matrix and plate-like precipitates (Udoh et al., 1984; Guo et al., 2007; Pan and Wang, 2007; Kim et al., 2015). The limitation of the present study was that the effect of oxides on the hardness of the alloys could not be measured. Alloy hardening due to firing was more pronounced when the alloy was oxidized and icequenched instead of using slow cooling after oxidation. For clinical practice of the ice-quenching step after oxidation, further studies are needed to resolve whether the ice-quenching step after oxidation increases the oxide film thickness, likely resulting in easier separation of the alloy from porcelain.
Fig. 7. FE-SEM image of complete firing simulated specimen at a cooling rate of Stage 3 (S3-G, M: matrix, P: particle-like structure, Ppt: plate-like precipitates).
5. Conclusions Table 8 EDS analysis at the regions marked in Fig. 7. Region (at%)
Pd
Cu
Ga
Zn
In
Ru, Ag
M1 M2 P1 P2 Ppt 1 Ppt 2
61.7 61.3 73.2 74.0 61.4 61.3
18.3 18.5 11.8 11.0 18.6 19.5
8.7 9.4 6.0 6.7 9.3 7.9
9.3 9.4 6.5 6.0 8.8 10.0
2.0 1.4 2.5 2.3 1.9 1.3
0 0 0 0 0 0
Although not statistically significant, the alloy was softened slightly during porcelain firing simulation with conventional slow cooling rate. On the other hand, the hardness increased significantly by icequenching instead of the slow cooling after oxidation (p < 0.001). The mechanism of ice-quenching after oxidation to prevent softening and induce hardening during porcelain firing process involved more active precipitation and retardation of microstructural coarsening. In conclusion, for practical work on the Pd-Cu-Ga-Zn alloy, an oxidation treatment followed by ice-quenching instead of conventional slow cooling is recommended to simultaneously obtain an oxidation effect and a hardening effect of the final prosthesis.
and Ga have a similar atomic size to Zn, and the combined Zn, Cu and Ga content was more than half of the Pd content in the matrix and plate-like precipitates (Fig. 7, Table 8). Considering this fact, the matrix and plate-like precipitates were thought to be the Pd2(Cu,Ga,Zn) phase of the CsCl-type. From the above, as the oxidation-treated specimen at 1010 °C was fired several times at a lower temperature, Cu, Ga, and Zn, which were contained in the Pd-rich α particle-like structure, precipitated with Pd to form the β' plate-like precipitates in the α' particlelike structure. The Cu-, Ga-, and Zn-depleted α' phase had a slightly larger lattice constant than the parent α phase because Cu, Ga, and Zn have a relatively small atomic size compared to the other components of the alloy (Cullity, 1978). Although the dot-like precipitates that were formed in the matrix could not be detected by EPMA or EDS due to the finer nature, In, which has a negligible solubility in Ga and Zn, was believed to have precipitated with Pd to form the α' dot-like precipitates in the β' matrix (Massalski, 1990). In the present study, the content of Cu was the highest after that of Pd in the alloy, but the precipitation reaction observed was consistent with the result predicted from the Pd-Zn binary phase diagram, and not the Pd-Cu binary phase diagram (Massalski, 1990). According to the PdZn binary phase diagram, when the Zn content is about 20–30 at%, the Pd-Zn alloy is divided into the Pd-rich phase and the Pd2Zn phase by eutectic reaction in the temperature range 700–1340 °C. In this range, as the temperature is lowered, the solubility limit for each other decreases, and therefore precipitation occurs. In the present study, the Zn content in the alloy did not correspond to 20–30 at%; nevertheless, the combined Zn, Cu, and Ga content was within that range. The firing started at 1010 °C and gradually decreased to 900 °C; after each firing step, the alloy was cooled to room temperature. However, since the cooling was relatively fast, phase equilibrium did not occur. Thus, the Pd-rich and Pd2(Cu,Ga,Zn) phases, which are stable in the temperature range 700–1340 °C, still existed after cooling, and precipitation occurred due to the decrease in solubility limit for each other during the porcelain firing simulation. Plate-like precipitates composed of the CsCl-type Pd2(Cu,Ga,Zn) phase appear to have grown along a preferred crystallographic direction within the Pd-rich particle-like structure
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant number: NRF-201728410001) References Cullity, B.D., 1978. Elements of X-ray Diffraction, second ed. Addison-Wesley Publishing, Massachusetts, pp. 506–507. Fischer, J., Fleetwood, P.W., 2000. Improving the processing of high-gold metal-ceramic frameworks by a pre-firing heat treatment. Dent. Mater. 16 (2), 109–113. Guo, W.H., Brantley, W.A., Clark, W.A.T., Monaghan, P., Mills, M.J., 2003. Transmission electron microscopic investigation of a Pd-Ag-In-Sn dental alloy. Biomaterials 24 (10), 1705–1712. Guo, W.H., Brantley, W.A., Li, D., Clark, W.A.T., Monaghan, P., Heshmati, R.H., 2007. Annealing study of palladium-silver dental alloys: vickers hardness measurements and SEM microstructural observations. J. Mater. Sci. Mater. Med. 18 (1), 111–118. Hisatsune, K., Hasaka, M., Sosrosoedirdjo, B.I., Udoh, K., 1990. Age-hardening behavior in a palladium-base dental porcelain-fused alloy. Mater. Charact. 25 (2), 177–184. Hisatsune, K., Tanaka, Y., Udoh, K., Yasuda, K., 1997. Ageing reactions in a high carat gold alloy for dental porcelain bonding. J. Mater. Sci. Mater. Med. 8 (5), 277–282. Jeon, B.W., Kim, S.M., Kim, H.I., Kwon, Y.H., Seol, H.J., 2014. Hardening effect of preand post-firing heat treatment for a firing simulated Au-Pd-In metal-ceramic alloy. Gold Bull. 47 (4), 255–261. Kim, S.M., Kim, H.I., Kwon, Y.H., Seol, H.J., 2015. Change in hardness of an as-cast and softening heat-treated low-gold-content alloy for bonding porcelain by simulated porcelain firing and its mechanism. Gold Bull. 48 (1), 39–46. Kim, Y.H., Niinomi, M., Nakai, M., Akahori, T., Kanno, T., Fukui, H., 2012. Mechanism of unique hardening of dental Ag-Pd-Au-Cu alloys in relation with constitutional phases. J. Alloy. Compd. 519 (5), 15–24. Li, D., Baba, N., Brantley, W.A., Alapati, S.B., Heshmati, R.H., Daehn, G.S., 2010. Study of Pd-Ag dental alloys: examination of effect of casting porosity on fatigue behaviour and microstructural analysis. J. Mater. Sci. Mater. Med. 21 (10), 2723–2731. Liu, W.B., Wang, J.N., 2007. Strengthening of a Pd-free high gold dental alloy for porcelain bonding by a pre-firing heat treatment. Dent. Mater. 23 (9), 1136–1141. Massalski, T.B., 1990. Binary Alloy Phase Diagrams, second ed. ASM International, Materials Park, pp. 1454–1456 (1811-1814, 2316-2319, 3068-3070). O'Brien, W.J., 2002. Dental Materials and their Selection, third ed. Quintessence Publishing Co., Chicago, pp. 358–359.
90
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Mater. J. 36 (2), 142–148. Udoh, K., Hisatsune, K., Yasuda, K., Otha, M., 1984. Isothermal age-hardening behavior in commercial dental gold alloys containing palladium. Dent. Mater. J. 3 (2), 253–261. Vermilyea, S.G., Cai, Z., Brantley, W.A., Mitchell, J.C., 1996. Metallurgical structure and microhardness of four new palladium-based alloys. J. Prosthodont. 5 (4), 288–294. Villars, P., Calvert, L.D., 1985. Pearson's Handbook of Crystallographic Data for Intermetallic Phases. American Society for Metals, Metals Park, pp. 3027. Yu, Y.J., Seol, H.J., Cho, M.H., Kim, H.I., Kwon, Y.H., 2016. Hardening by cooling rate control and post-firing heat treatment in Pd-Ag-Sn alloy for bonding porcelain. Dent. Mater. J. 35 (2), 309–315.
Pan, L.G., Wang, J.N., 2007. Age-hardening behavior of a low-gold dental alloy. J. Mater. Sci. Mater. Med. 18 (1), 171–177. Porter, D.A., Easterling, K.E., Sherif, M.Y., 2009. Phase Transformations in Metals and Alloys, third ed. CRC Press/Taylor and Francis Group, Boca Raton (FL) (Chapter 5.5.1). Roberts, H.W., Berzins, D.W., Moore, B.K., Charlton, D.G., 2009. Metal-ceramic alloys in dentistry: a review. J. Prosthodont. 18 (2), 188–194. Rosenstiel, S.F., Land, M.F., Fujimoto, J., 2016. Contemporary Fixed Prosthodontics, fifth ed. Elsevier, St. Louis, pp. 538. Shin, H.J., Kim, M.J., Kim, H.I., Kwon, Y.H., Seol, H.J., 2017. Effect of ice-quenching on the change in hardness of a Pd-Au-Zn alloy during porcelain firing simulation. Dent.
91
Contents lists available at ScienceDirect
Journal of the Mechanical Behavior of Biomedical Materials journal homepage: www.elsevier.com/locate/jmbbm
Effect of ice-quenching after oxidation treatment on hardening of a Pd-CuGa-Zn alloy for bonding porcelain Min-Jung Kim, Hye-Jeong Shin, Hyung-Il Kim, Yong Hoon Kwon, Hyo-Joung Seol
T
⁎
Department of Dental Materials, Institute of translational dental science and BK21 PLUS Project, School of Dentistry, Pusan National University, Beomeo-Ri, Mulgeum-Eup, Yangsan-Si, Gyeongsangnam-Do 626-814, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Porcelain firing simulation Pd-Cu-based alloy Ice-quenching Precipitation Softening
This study examined the effect of ice-quenching after oxidation treatment on hardness change of a Pd-Cu-Ga-Zn metal-ceramic alloy during porcelain firing simulation. Although not statistically significant, the alloy was softened slightly during porcelain firing simulation with conventional slow cooling rate. On the other hand, the hardness increased significantly by ice-quenching instead of the slow cooling after oxidation (p < 0.001). The gap in the final hardness depending on ice-quenching occurred in the matrix and plate-like precipitates but not in the particle-like structure without plate-like precipitates (p < 0.05). The mechanism of ice-quenching after oxidation to prevent softening and induce hardening during porcelain firing simulation involved the more active precipitation and retardation of microstructural coarsening. In conclusion, for practical work on Pd-Cu-Ga-Zn alloys, the oxidation treatment followed by ice-quenching instead of slow cooling is recommended for the simultaneous oxidation and hardening effects on the alloy.
1. Introduction Dental metal-ceramic alloys are used as a substructure of porcelain superstructures to improve the durability of metal-ceramic prostheses. Pd-based metal-ceramic alloys are used as alternative alloys for Au-Ptbased alloys. Among them, Pd-Ag-based alloys have been used as typical alternative metal-ceramic alloys. Pd-Ag-based alloys have high elastic modulus and sag resistance, easy to solder, and excellent corrosion resistance (O'Brien, 2002; Roberts et al., 2009). On the other hand, because of their high silver contents, porcelain greening and furnace contamination can result during laboratory procedures, unless the porcelain is carefully selected (Rosenstiel et al., 2016). Pd-Cu-based alloys were developed as economical alternatives to the Au-Pt-based alloys in the 1980s, and these alloys have casting accuracy comparable with that of the high-gold content metal-ceramic alloys (Rosenstiel et al., 2016). Pd-based alternative alloys are relatively harder than the Au-based metal-ceramic alloys in the as-cast state (Fischer and Fleetwood, 2000; Liu and Wang, 2007; Li et al., 2010). On the other hand, softening occurs in some cases in the Pd-based metal substructure during the porcelain firing process (Guo et al., 2003; Li et al., 2010), which is undesirable because it may affect the durability of the final prosthesis. In the study with a Pd-Au-In-Ag alloy, performing a solution treatment by heating the alloy at approximately 1000 °C and then ice-quenching
⁎
(quenching into ice brine) before firing porcelain was reported to be effective in solving this problem (Jeon et al., 2014). In that study, the hardness of the alloy was reduced by the solution treatment, but this recovered rapidly from the next firing step for bonding porcelain. In addition, unlike the untreated alloys, softening did not occur in the solution-treated alloy during the remaining firing process. Despite this, the solution treatment of the alloy is troublesome because it must be carried out separately from the porcelain firing process. The first step in the porcelain firing process is oxidation, which usually takes place within 10 min at temperatures around 1000 °C and the alloy is then bench cooled. This process is similar to the solution treatment, only with different cooling rates. Therefore, if the oxidation treatment is followed by ice-quenching instead of conventional slow cooling, the oxidation and the solution treatment effects can be obtained simultaneously without an additional solution treatment. In this case, softening of the alloy necessarily occurs immediately after oxidation. Therefore, it is important to determine if the hardness recovers rapidly during the subsequent firing process. In a previous study with a Pd-Au-based metal-ceramic alloy without silver, softening during porcelain firing simulation did not occur regardless of ice-quenching after degassing treatment, but the ice-quenching after degassing improved the final hardness slightly (Shin et al., 2017). For Pd-Cu-based alternative alloys, the changes in hardness during porcelain firing process have not reported, but softening of the alloys can be expected during the firing
Corresponding author. E-mail address: [email protected] (H.-J. Seol).
https://doi.org/10.1016/j.jmbbm.2017.12.014 Received 12 April 2017; Received in revised form 16 November 2017; Accepted 14 December 2017 Available online 15 December 2017 1751-6161/ © 2017 Elsevier Ltd. All rights reserved.
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Table 1 Chemical composition of the specimen alloy.
Table 3 Cooling rate during simulated firing.
Composition
Pd
Cu
Ga
Zn
In
Ru
Ag
Cooling rate
Icequenching
Stage 0
Stage 1
Stage 2
Stage 3
wt% at%
79.0 71.4
8.0 12.1
4.9 6.8
4.5 6.6
3.0 2.5
0.4 0.4
0.2 0.2
Condition
Cooled by rapid quenching into ice brine
Firing chamber moves immediately to upper end position
Firing chamber opens about 70 mm
Firing chamber opens about 50 mm
Firing chamber remains closed
process. Therefore in the present study, a Pd-Cu-based alloy was investigated to determine the softening behavior during the porcelain firing process and if it is the case, whether the oxidation treatment followed by ice-quenching prevents softening without an additional solution treatment. For this, the effect of an oxidation treatment followed by ice-quenching on the change in hardness of a Pd-Cu-Ga-Zn alloy during a porcelain firing simulation was characterized by analyzing the changes in hardness, microstructure, crystal structure, and elemental distribution.
time of 10 s using a Vickers micro-hardness tester (MVK-H1, Akashi, Japan) for the porcelain firing simulated specimens. The hardness results were recorded as the mean of five measurements. In addition, to measure the hardness of each microstructure (particle-like structure and matrix) separately, the load was reduced to 10 gf. The regions covering the precipitate-generated particle-like structure were measured with a load of 25 gf. The hardness results were recorded as the mean of at least seven measurements.
2. Materials and methods 2.1. Specimen alloy
2.4. Field emission scanning electron microscopy (FE-SEM) The alloy used in this study was a Pd-Cu-Ga-Zn alternative alloy for metal-ceramic (silfree79, Aurium research, USA). The melting range of the alloy according to the manufacturer is 1160 to 1250 °C and the casting temperature is 1375 °C. Table 1 lists the composition of the alloy. The atomic ratio (at%) was calculated from the weight ratio (wt %) supplied by the manufacturer. A phosphate-based investment was used to cast the plate specimens (10 × 10 × 0.5 mm3 in size). The alloy was melted with an oxygen-gas torch and cast using a centrifugal casting machine (Casting machine, Osung, South Korea). The as-cast specimens were bench cooled to room temperature and washed with an ultrasonic cleaner (Bransonic, Branson, USA) for 30 min.
The microstructural changes in the specimens during the porcelain firing simulation were examined by FE-SEM (JSM-6700F, JEOL, Japan). The porcelain firing simulated specimens were polished metallographically and etched in an aqueous solution containing 10% KCN (potassium cyanide) and 10% (NH4)2S2O8 (ammonium persulfate). The surfaces of the specimens were observed by FE-SEM at 15 kV. 2.5. X-ray diffraction (XRD) XRD (XPERT-PRO, Philips, Netherlands) was performed to examine the present phases in each step of the porcelain firing simulation. The porcelain firing simulated specimens for XRD analyses were polished metallographically and etched in an aqueous solution, as done for the FE-SEM observation. The XRD profile was recorded at 30 kV and 40 mA using Ni-filtered Cu Kα radiation as the incident beam. The scanning rate of the goniometer was 1° (2θ/min).
2.2. Porcelain firing simulation The as-cast specimens were firing simulated according to the steps in Table 2. First, an oxidation treatment was performed at 1010 °C for 5 min in a porcelain furnace (Multimat 2 torch, Dentsply, Germany). The specimen was then cooled at various cooling rates to determine the most effective cooling rate for alloy hardening. The cooling rate was divided into 5 stages (Ice-quenching; quenching into ice brine, Stage 0, Stage 1, Stage 2, and Stage 3) in the order of cooling speed, as shown in Table 3. The remaining firing steps in Table 2 were then carried out and cooled at the most effective cooling rate for alloy hardening. The simulated porcelain firing cycles in Table 2 correspond to a slight modification of the firing cycles of the specific dental porcelain product (VITA VMK Master, VITA Zahnfabrik, Germany); the modification was done to simplify the complex firing process.
2.6. Field emission electron probe microanalysis (FE-EPMA) The porcelain firing simulated specimens were polished metallographically and etched in an aqueous solution, as done for the FE-SEM observation. The elemental distribution of the porcelain firing simulated specimens was examined using a field emission electron probe microanalyzer (JXA-8530F, JEOL, Japan) at 15 kV. 2.7. Energy-dispersive X-ray spectrometry (EDS)
2.3. Hardness test
The porcelain firing simulated specimens were polished metallographically and etched in an aqueous solution, as done for the FE-SEM observation. The elemental content in each microstructure of the porcelain firing simulated specimens was examined using an energy dispersive X-ray spectrometer (INCA x-sight, Oxford Instruments Ltd., UK) attached to the FE-SEM at 15 kV.
The Vickers hardness was measured at a load of 300 gf and a dwell Table 2 Simulated porcelain firing cycles. Firing cycles
Oxidation Wash Opaque Main bake Correction Glaze
Predrying (min)
Heat rate (°C/ min)
Start temp. (°C)
Final temp. (°C)
Time at final temp. (min)
Vacuum time (min)
Vacuum level (hpa)
2.8. Statistical analysis
0 2 2 4 4 0
70 70 70 70 70 70
550 550 550 550 550 550
1010 960 930 920 910 900
5 1 1 1 1 0
0 6:51 6:26 6:17 6:09 0
0 70 70 70 70 0
Hardness data were analyzed using a statistical program SPSS 23.0 (Statistical Product and Service Solutions 23.0, IBM Co., USA); a statistical significance level of 0.05 was considered significant for all tests. The hardness measured as a function of the cooling rate after oxidation was subjected to the one-way ANOVA, followed by a Tukey HSD test for multiple comparisons. The hardness variation in each firing step with cooling rate was analyzed by a RMANOVA (Repeated Measure 84
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Fig. 1. Change in hardness according to the cooling rates after oxidation with the standard deviation marked in.
Fig. 2. Change in hardness of the specimens during the porcelain firing simulation with the standard deviation marked in.
ANOVA), followed by tests of within-subjects contrasts. The hardness corresponding to each microstructure (matrix, particle-like structure without plate-like precipitates) during the porcelain-firing simulation was analyzed using a generalized linear model (GLM), as the hardness values did not show a normal distribution. The hardness corresponding to each microstructure obtained at the final stage of firing was analyzed by the Kruskal-Wallis H test, followed by the Bonferroni-Dunn test for multiple comparisons as the hardness values did not show a normal distribution.
Table 5-1 The statistics of the hardness as a function of cooling rate and firing step.
To determine the most effective cooling rate for alloy hardening, the specimens were cooled at various cooling rates after oxidation, as listed in Table 3. Fig. 1 shows the difference in hardness as a function of the cooling rate. Table 4 shows the statistics of the hardness as a function of the cooling rate analyzed by a one-way ANOVA, followed by a Tukey HSD test for multiple-comparisons. The hardness results were recorded as the mean of five measurements. In Fig. 1 and Table 4, the hardness increased with decreasing cooling rate; the highest hardness was obtained at Stage 2 and 3 (p=0.000). After oxidation, ice-quenched specimen and specimen cooled at Stage 3 (the most effective cooling rate for hardening) were prepared. The remaining firing steps in Table 2 were then carried out to observe the changes in hardness during porcelain firing simulation (Fig. 2). At this time, the cooling rate was set to Stage 3 for each specimen, and the hardness test was carried out with a load of 300 gf. The hardness results were recorded as the mean of five measurements. Tables 5-1, 5-2 shows the statistics of the measured hardness as a function of the cooling rate Table 4 The statistics of the hardness as a function of the cooling rates after oxidation.
Hardness (M ± SD)
a
257.36 ( ± 2.78)
Stage 0 b
267.32 ( ± 1.8)
Stage 1 b
270.52 ( ± 2.68)
Stage 2 c
277.74 ( ± 2.49)
Cooling rate Firing step Cooling rate×Firing step
16.363 (0.004)* 20.167 (< 0.001)* 43.860 (< 0.001)*
and firing step as analyzed by a RMANOVA (Repeated Measure ANOVA), followed by tests of within-subjects contrasts. As seen in Table 5-1, the cooling rate (Stage3, Ice-quenching) (p = 0.004) and firing step (Oxidation-Glaze) (p < 0.001) were found to affect the hardness; the interaction between the cooling rate and the firing step also affected the hardness (p < 0.001). As seen in Tables 5-2, the hardness values corresponding to the oxidation and wash steps were statistically higher for the specimen cooled at Stage 3 than that for the ice-quenched specimen. However, the hardness values observed at the opaque, main bake, correction, and glaze steps showed the opposite results (p < 0.001). Comparing the hardness values (measured as a function of the firing step) for the ice-quenched specimen, the hardness corresponding to the wash step was significantly higher than that corresponding to the oxidation step, but was not significantly different from those corresponding to the opaque and main bake steps. Additionally, the hardness corresponding to the correction step in the icequenched specimen was significantly higher than that observed for the main bake step, but was not significantly different from that observed for the glaze step. Thus, in the ice-quenched specimen, the hardness increased steadily after oxidation, through the remaining firing steps; the highest hardness was observed at the correction and glaze steps. On the other hand, in the specimen cooled at Stage 3, the hardness at the oxidation step was not significantly different from that observed for the remaining firing steps. To analyze the cause of the change in hardness in more detail, the specimens were etched before measuring the hardness. Fig. 3 shows the hardness which was measured with the load of 10 gf or 25 gf according to the microstructure. The hardness results were recorded as the mean of at least seven measurements. All specimens consisted of matrix and particle-like structure, and the complete firing simulated specimens had plate-like precipitates in the particle-like structure, as will be seen in the FE-SEM images in Fig. 4. Tables 6-1, 6-2 shows the statistics of the hardness as a function of the cooling rate and firing step in matrix and
3.1. Hardness changes
Ice-quenching
F (p)
* Statistically significant difference (p < 0.05). Statistical significance was analyzed by a RMANOVA (Repeated Measure ANOVA) at a=0.05.
3. Results
Cooling rate
Factor
Stage 3 279.86c ( ± 1.59)
The values are expressed as mean ± standard deviation. Statistical significance was analyzed by a one-way ANOVA at a=0.05, followed by a Tukey HSD test for multiplecomparisons. a,b,c Statistically significant difference in the hardness. Same lowercase letters indicate that there are no statistical differences among groups.
85
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Table 5-2 The statistics of the hardness as a function of cooling rate and firing step. Hardness (M ± SD)
Stage3 Ice-quenching
Oxidation
Wash
Opaque
Main bake
Correction
Glaze
279.86Ba ( ± 1.59) 257.36Aa ( ± 2.78)
279.98Ba ( ± 2.89) 277.30Ab ( ± 3.98)
277.34Aa ( ± 2.98) 280.44Bb ( ± 4.21)
276.48Aa ( ± 1.73) 282.56Bb ( ± 1.61)
275.52Aa ( ± 2.97) 286.58Bc ( ± 2.38)
272.10Aa ( ± 4.01) 287.74Bc ( ± 3.48)
The values are expressed as mean ± standard deviation. Statistical significance was analyzed by a RMANOVA (Repeated Measure ANOVA) at a=0.05, followed by tests of within subjects contrasts. A,B Statistically significant difference in the cooling rate (Stage3, Ice-quenching). a,b,c Statistically significant difference for different firing steps. Same uppercase letters indicate that there are no statistical differences between the cooling rate (Stage3, Ice-quenching), and same lowercase letters indicate that there are no statistical differences among firing steps (OxidationGlaze).
quenching the as-cast specimen after oxidation (IQ-O), the fine dot-like precipitates in the matrix disappeared, and the boundaries of the particle-like structure became smoother than those in the as-cast specimen due to homogenization. On the other hand, by cooling the specimen at Stage 3 after oxidation (S3-O), fine dot-like precipitates re-appeared, resulting in hardening. In the complete firing simulated specimens (IQG, S3-G), plate-like precipitates were generated in the particle-like structure, and fine dot-like precipitates covered the matrix and the plate-like precipitates regardless of ice-quenching after oxidation. However, the dot-like precipitates were much coarser in the S3-G specimen. 3.3. Phase transformation The phase transformation associated with changes in hardness was observed by XRD (Fig. 5). The IQ-O specimen consisted of a face-centered cubic (f.c.c.) α phase with a lattice constant of a200=3.862 Å and a CsCl-type β phase with a lattice constant of a200=3.047 Å. The 110β and 200α peaks were separated by Kα1 and Kα2, without extra peaks from another phase. Considering that the regions of the particle-like structure were wider than those of the matrix (Fig. 4), the α phase with a relatively high peak intensity and the β phase were confirmed to be the particle-like structure and the matrix, respectively. In the S3-O specimen and the complete firing simulated IQ-G and S3-G specimens, the 200α' and 110β' peaks were newly formed in the lower angle side of the 200α peak and in the higher angle side of the 110β peak respectively, due to precipitation. In the FE-EPMA result (Fig. 6) of the IQ-O specimen, Pd, In, and Ag were concentrated in the particle-like structure (P), whereas Cu, Ga, and Zn were concentrated in the matrix (M). Such a result was also obtained in the complete firing simulated IQ-G specimen. In the IQ-G specimen, the elemental distribution of the plate-like precipitates (Ppt) that were formed in the particle-like structure corresponded to that of the matrix. Such a result corresponded to the EDS result, which was carried out on the S3-G specimen (Fig. 7, Table 8). Compared to the content in the particle-like structure (P), the Pd content decreased with increasing Zn, Cu and Ga content in both the matrix (M) and plate-like precipitates (Ppt), even though there was a contamination by the constituents of the fine dot-like precipitates which covered the matrix and plate-like precipitates. The Ag and Ru content was not detected by EDS due to their relatively small amounts.
Fig. 3. Change in hardness in each microstructure (matrix, particle-like structure with or without plate-like precipitates) during porcelain firing simulation with the standard deviation marked in: ice-quenched after oxidation (IQ-O), cooled at Stage 3 after oxidation (S3-O), complete firing simulation of IQ-O (IQ-G), and complete firing simulation of S3-O (S3-G).
particle-like structure without plate-like precipitates analyzed using the GLM. As seen in Tables 6-1, in both the matrix and particle, the firing step (Oxidation, Glaze) and the interactions between the cooling rate and the firing step affected the hardness (p < 0.001). The cooling rate affected the hardness of the particle only (p < 0.001). Table 6-2 shows the statistics of the hardness corresponding to each microstructure. As seen in Table 6-2, in the matrix, the hardness of IQ-O was significantly lower (by 44.04 HV) (p < 0.001) than that of S3-O, while the hardness of IQ-G was significantly higher (by 44.21 HV) (p < 0.001) than that of S3-G. In the particle, the hardness of IQ-O was significantly lower than that of S3-O (by 22.41 HV) (p < 0.001), while the hardness values of IQ-O and S3-G were not significantly different (p= 0.337). Table 7 shows the statistics of the hardness corresponding to each microstructure obtained at the final stage of firing (Glaze) and analyzed by the Kruskal-Wallis H test, followed by the Bonferroni-Dunn test for multiple-comparisons (M:Matrix, P:Particle without plate-like precipitates, P+:Particle with plate-like precipitates). The hardness values of IQ-G (M), S3-G (M), and IQ-G (P+) were significantly higher than those of IQ-G (P), S3-G (P) and S3-G (P+) (p < 0.001).
4. Discussion Pd-Cu-based alloys were developed as economical alternatives to the Au-Pt-based alloys in the 1980s, and these alloys have casting accuracy comparable with that of the high-gold content metal-ceramic alloys (Rosenstiel et al., 2016). In dental Pd-based metal-ceramic alloys, there are cases where a decrease in hardness occurs during porcelain
3.2. Microstructural changes In Fig. 4, the as-cast specimen had a particle-like structure surrounded by the matrix, and the matrix had fine dot-like precipitates. The boundaries of the particle-like structure were irregular. By ice86
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Fig. 4. Microstructural changes during porcelain firing simulation: ascast, ice-quenched after oxidation (IQ-O), cooled at Stage 3 after oxidation (S3-O), complete firing simulation of IQ-O (IQ-G), and complete firing simulation of S3-O (S3-G) at magnifications of ×4000 (left), ×30,000 (right), M: matrix, P: particle-like structure.
firing process (Guo et al., 2003; Li et al., 2010). As for Pd-Cu-based alternative alloys, the changes in hardness during the porcelain firing process have not been reported, but softening of the alloys can be expected during the firing process. The introduction of a solution treatment before porcelain firing process was reported to be effective in solving this problem (Jeon et al., 2014). The first step in the firing process for bonding porcelain is an oxidation treatment, which is
similar to the solution treatment only with different cooling rates. Therefore, the present study examined the softening behavior of the PdCu-based alternative alloy during porcelain firing process and if it is the case, whether the oxidation treatment followed by ice-quenching is effective in preventing softening instead of solution-treatment. First, to minimize the softening of the alloy during the porcelain firing process, the most effective cooling rate for alloy hardening was found. As a 87
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Table 6-1 The statistics of the hardness as a function of cooling rate and firing step in matrix and particle-like structure without plate-like precipitates. Factor
Wald-chi-square (p)
Cooling rate Firing step Cooling rate×Firing step
Matrix
Particle
0.000 (0.989) 12.321 (< 0.001)* 43.860 (< 0.001)*
17.699 (< 0.001)* 16.171 (< 0.001)* 30.425 (< 0.001)*
* Statistically significant difference (p < 0.05). Statistical significance was analyzed using a general linear model (GLM). Table 6-2 The statistics of the hardness as a function of cooling rate and firing step in matrix and particle-like structure without plate-like precipitates. Specimens
Difference (p) Matrix
S3-O " " IQ-O " S3-G
IQ-O S3-G IQ-G S3-G IQ-G IQ-G
Particle *
44.04 (< 0.001) 22.51 (0.021)* −21.70 (0.030)* −21.53 (0.003)* −65.74 (< 0.001)* −44.21 (< 0.001)*
22.41 (< 0.001)* 3.44 (0.287) 0.43 (0.899) −18.96 (< 0.001)* −21.98 (< 0.001)* −3.02 (0.337)
Fig. 5. Change in the XRD patterns during porcelain firing simulation: ice-quenched after oxidation (IQ-O), cooled at Stage 3 after oxidation (S3-O), complete firing simulation of IQ-O (IQ-G), and complete firing simulation of S3-O (S3-G).
* Statistically significant difference (p < 0.05). Statistical significance was analyzed using a general linear model (GLM).
In the FE-SEM images (Fig. 4) of complete firing simulated specimens (IQ-G, S3-G), the matrix was covered with fine dot-like precipitates, and the plate-like precipitates were formed in the particle-like structure regardless of ice-quenching after oxidation. The plate-like precipitates had the same composition as the matrix (Fig. 6, Fig. 7, Table 8), and were covered with fine dot-like precipitates, as it was in the matrix in both the specimens. However, the dot-like precipitates were formed more actively in the ice-quenched specimen (IQ-G), as reported in the study with a Pd-Au-based alloy which was ice-quenched after degassing treatment (Shin et al., 2017). In addition, the dot-like precipitates were less coarsened in the ice-quenched specimen (IQ-G) than in the specimen cooled at Stage 3 (S3-G). The precipitation hardening has been reported to be the common hardening mechanism in various dental alloys (Hisatsune et al., 1990, 1997; Vermilyea et al., 1996; Kim et al., 2012, 2015; Jeon et al., 2014). However, the coarsening of the precipitates by overaging reduces the interface with the matrix, thereby eliminating the internal stresses caused by lattice distortion, which causes softening (Udoh et al., 1984; Guo et al., 2007; Pan and Wang, 2007; Kim et al., 2015). Therefore, although the ice-quenched specimen after oxidation showed lower hardness than the specimen cooled at Stage 3 after oxidation, the final hardness after complete firing simulation was reversed (Table 5–2, p < 0.001). The XRD results (Fig. 5) of the ice-quenched specimen after oxidation (IQ-O) revealed that the particle-like structure was composed of the f.c.c. α phase with a200=3.862 Å and the matrix was composed of the CsCl-type β phase with a200=3.047 Å. In the complete firing simulated specimens (IQ-G, S3-G), the α and β phases underwent a phase transformation to the α' phase with a 0.003 Å larger lattice constant (a200=3.865 Å) and β' phase with a 0.005 Å smaller lattice constant (a200=3.042 Å), respectively, regardless of ice-quenching after oxidation. FE-EPMA analysis (Fig. 6) and the EDS result (Fig. 7, Table 8) showed that the Pd content decreased with increasing Zn, Cu and Ga content in both the matrix and plate-like precipitates compared to that in the particle-like structure. These results, together with the XRD results (Fig. 5), showed that the particle-like structure was composed of the f.c.c. Pd-rich phase and the matrix and plate-like precipitates were composed of the CsCl-type ordered phase containing Pd, Cu, Ga, and Zn in the complete firing simulated specimens. The reported lattice constant of the Pd2Zn phase with a CsCl-type is 3.055 Å (Villars and Calvert, 1985), which is similar to the lattice constant of the matrix. Cu
Table 7 The statistics of the hardness as a function of cooling rate in each microstructure obtained at the glaze step. Microstructure
Hardness (M ± SD)
IQ-G S3-G IQ-G S3-G IQ-G S3-G
425.40 ± 21.64 b 381.19 ± 22.35 b 244.95 ± 6.35 a 241.93 ± 5.14 a 282.68 ± 11.36 b 256.50 ± 4.04 a
(M) (M) (P) (P) (P+) (P+)
M: Matrix, P: Particle without plate-like precipitates, P+: Particle with plate-like precipitates. IQ-O: ice-quenched after oxidation, S3-O: cooled at Stage 3 after oxidation, IQ-G: complete firing simulation of IQ-O, S3-G: complete firing simulation of S3-O. The values are expressed as mean ± standard deviation. Statistical significance was analyzed by a Kruskal-Wallis H at a=0.05, followed by a Bonferroni-Dunn test for multiple-comparisons. a,b Statistically significant difference in the hardness. Same lowercase letters indicate that there are no statistical differences among groups.
result, the hardening effect was stronger at a slower cooling rate (Fig. 1, Table 4, p=0.000), which was different from the results obtained from the Pd-Au-Zn, Pd-Au-In, and Pd-Ag-Sn alloys with melting points similar to that of the present alloy (Jeon et al., 2014; Yu et al., 2016; Shin et al., 2017). Although not statistically significant, slight softening was unavoidable even with firing simulation at the most effective cooling rate (Stage 3) for alloy hardening (Fig. 2). On the other hand, in the case that the oxidation treatment was followed by ice-quenching instead of cooling at Stage 3, the hardness increased during the remaining firing process (Fig. 2, Table 5–2, p < 0.001). Therefore, the change in hardness in each microstructure during the firing process was examined to determine the cause of difference in the final hardness in more detail. As a result, a gap in the final hardness depending on ice-quenching occurred in the matrix and plate-like precipitates but not in the particle-like structure without plate-like precipitates (Fig. 3, Table 7, p < 0.05). 88
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Fig. 6. Element distribution by FE-EPMA: ice-quenched specimen after oxidation (a: IQ-O), complete firing simulated specimen of IQ-O (b: IQ-G).
89
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
having f.c.c. structure to minimize the lattice misfit (Porter et al., 2009). The plate-like precipitates are coarser than the dot-like precipitates in the glaze step, so the lattice distortion must have been released by the reduced interphase boundaries between the coarsened plate-like precipitates and the surrounding Pd-rich particle-like structures in the glaze step, while the lattice distortion by the dot-like precipitates caused hardening in the surrounding matrix and plate-like precipitates (Udoh et al., 1984; Guo et al., 2007; Pan and Wang, 2007; Kim et al., 2015). The limitation of the present study was that the effect of oxides on the hardness of the alloys could not be measured. Alloy hardening due to firing was more pronounced when the alloy was oxidized and icequenched instead of using slow cooling after oxidation. For clinical practice of the ice-quenching step after oxidation, further studies are needed to resolve whether the ice-quenching step after oxidation increases the oxide film thickness, likely resulting in easier separation of the alloy from porcelain.
Fig. 7. FE-SEM image of complete firing simulated specimen at a cooling rate of Stage 3 (S3-G, M: matrix, P: particle-like structure, Ppt: plate-like precipitates).
5. Conclusions Table 8 EDS analysis at the regions marked in Fig. 7. Region (at%)
Pd
Cu
Ga
Zn
In
Ru, Ag
M1 M2 P1 P2 Ppt 1 Ppt 2
61.7 61.3 73.2 74.0 61.4 61.3
18.3 18.5 11.8 11.0 18.6 19.5
8.7 9.4 6.0 6.7 9.3 7.9
9.3 9.4 6.5 6.0 8.8 10.0
2.0 1.4 2.5 2.3 1.9 1.3
0 0 0 0 0 0
Although not statistically significant, the alloy was softened slightly during porcelain firing simulation with conventional slow cooling rate. On the other hand, the hardness increased significantly by icequenching instead of the slow cooling after oxidation (p < 0.001). The mechanism of ice-quenching after oxidation to prevent softening and induce hardening during porcelain firing process involved more active precipitation and retardation of microstructural coarsening. In conclusion, for practical work on the Pd-Cu-Ga-Zn alloy, an oxidation treatment followed by ice-quenching instead of conventional slow cooling is recommended to simultaneously obtain an oxidation effect and a hardening effect of the final prosthesis.
and Ga have a similar atomic size to Zn, and the combined Zn, Cu and Ga content was more than half of the Pd content in the matrix and plate-like precipitates (Fig. 7, Table 8). Considering this fact, the matrix and plate-like precipitates were thought to be the Pd2(Cu,Ga,Zn) phase of the CsCl-type. From the above, as the oxidation-treated specimen at 1010 °C was fired several times at a lower temperature, Cu, Ga, and Zn, which were contained in the Pd-rich α particle-like structure, precipitated with Pd to form the β' plate-like precipitates in the α' particlelike structure. The Cu-, Ga-, and Zn-depleted α' phase had a slightly larger lattice constant than the parent α phase because Cu, Ga, and Zn have a relatively small atomic size compared to the other components of the alloy (Cullity, 1978). Although the dot-like precipitates that were formed in the matrix could not be detected by EPMA or EDS due to the finer nature, In, which has a negligible solubility in Ga and Zn, was believed to have precipitated with Pd to form the α' dot-like precipitates in the β' matrix (Massalski, 1990). In the present study, the content of Cu was the highest after that of Pd in the alloy, but the precipitation reaction observed was consistent with the result predicted from the Pd-Zn binary phase diagram, and not the Pd-Cu binary phase diagram (Massalski, 1990). According to the PdZn binary phase diagram, when the Zn content is about 20–30 at%, the Pd-Zn alloy is divided into the Pd-rich phase and the Pd2Zn phase by eutectic reaction in the temperature range 700–1340 °C. In this range, as the temperature is lowered, the solubility limit for each other decreases, and therefore precipitation occurs. In the present study, the Zn content in the alloy did not correspond to 20–30 at%; nevertheless, the combined Zn, Cu, and Ga content was within that range. The firing started at 1010 °C and gradually decreased to 900 °C; after each firing step, the alloy was cooled to room temperature. However, since the cooling was relatively fast, phase equilibrium did not occur. Thus, the Pd-rich and Pd2(Cu,Ga,Zn) phases, which are stable in the temperature range 700–1340 °C, still existed after cooling, and precipitation occurred due to the decrease in solubility limit for each other during the porcelain firing simulation. Plate-like precipitates composed of the CsCl-type Pd2(Cu,Ga,Zn) phase appear to have grown along a preferred crystallographic direction within the Pd-rich particle-like structure
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant number: NRF-201728410001) References Cullity, B.D., 1978. Elements of X-ray Diffraction, second ed. Addison-Wesley Publishing, Massachusetts, pp. 506–507. Fischer, J., Fleetwood, P.W., 2000. Improving the processing of high-gold metal-ceramic frameworks by a pre-firing heat treatment. Dent. Mater. 16 (2), 109–113. Guo, W.H., Brantley, W.A., Clark, W.A.T., Monaghan, P., Mills, M.J., 2003. Transmission electron microscopic investigation of a Pd-Ag-In-Sn dental alloy. Biomaterials 24 (10), 1705–1712. Guo, W.H., Brantley, W.A., Li, D., Clark, W.A.T., Monaghan, P., Heshmati, R.H., 2007. Annealing study of palladium-silver dental alloys: vickers hardness measurements and SEM microstructural observations. J. Mater. Sci. Mater. Med. 18 (1), 111–118. Hisatsune, K., Hasaka, M., Sosrosoedirdjo, B.I., Udoh, K., 1990. Age-hardening behavior in a palladium-base dental porcelain-fused alloy. Mater. Charact. 25 (2), 177–184. Hisatsune, K., Tanaka, Y., Udoh, K., Yasuda, K., 1997. Ageing reactions in a high carat gold alloy for dental porcelain bonding. J. Mater. Sci. Mater. Med. 8 (5), 277–282. Jeon, B.W., Kim, S.M., Kim, H.I., Kwon, Y.H., Seol, H.J., 2014. Hardening effect of preand post-firing heat treatment for a firing simulated Au-Pd-In metal-ceramic alloy. Gold Bull. 47 (4), 255–261. Kim, S.M., Kim, H.I., Kwon, Y.H., Seol, H.J., 2015. Change in hardness of an as-cast and softening heat-treated low-gold-content alloy for bonding porcelain by simulated porcelain firing and its mechanism. Gold Bull. 48 (1), 39–46. Kim, Y.H., Niinomi, M., Nakai, M., Akahori, T., Kanno, T., Fukui, H., 2012. Mechanism of unique hardening of dental Ag-Pd-Au-Cu alloys in relation with constitutional phases. J. Alloy. Compd. 519 (5), 15–24. Li, D., Baba, N., Brantley, W.A., Alapati, S.B., Heshmati, R.H., Daehn, G.S., 2010. Study of Pd-Ag dental alloys: examination of effect of casting porosity on fatigue behaviour and microstructural analysis. J. Mater. Sci. Mater. Med. 21 (10), 2723–2731. Liu, W.B., Wang, J.N., 2007. Strengthening of a Pd-free high gold dental alloy for porcelain bonding by a pre-firing heat treatment. Dent. Mater. 23 (9), 1136–1141. Massalski, T.B., 1990. Binary Alloy Phase Diagrams, second ed. ASM International, Materials Park, pp. 1454–1456 (1811-1814, 2316-2319, 3068-3070). O'Brien, W.J., 2002. Dental Materials and their Selection, third ed. Quintessence Publishing Co., Chicago, pp. 358–359.
90
Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 83–91
M.-J. Kim et al.
Mater. J. 36 (2), 142–148. Udoh, K., Hisatsune, K., Yasuda, K., Otha, M., 1984. Isothermal age-hardening behavior in commercial dental gold alloys containing palladium. Dent. Mater. J. 3 (2), 253–261. Vermilyea, S.G., Cai, Z., Brantley, W.A., Mitchell, J.C., 1996. Metallurgical structure and microhardness of four new palladium-based alloys. J. Prosthodont. 5 (4), 288–294. Villars, P., Calvert, L.D., 1985. Pearson's Handbook of Crystallographic Data for Intermetallic Phases. American Society for Metals, Metals Park, pp. 3027. Yu, Y.J., Seol, H.J., Cho, M.H., Kim, H.I., Kwon, Y.H., 2016. Hardening by cooling rate control and post-firing heat treatment in Pd-Ag-Sn alloy for bonding porcelain. Dent. Mater. J. 35 (2), 309–315.
Pan, L.G., Wang, J.N., 2007. Age-hardening behavior of a low-gold dental alloy. J. Mater. Sci. Mater. Med. 18 (1), 171–177. Porter, D.A., Easterling, K.E., Sherif, M.Y., 2009. Phase Transformations in Metals and Alloys, third ed. CRC Press/Taylor and Francis Group, Boca Raton (FL) (Chapter 5.5.1). Roberts, H.W., Berzins, D.W., Moore, B.K., Charlton, D.G., 2009. Metal-ceramic alloys in dentistry: a review. J. Prosthodont. 18 (2), 188–194. Rosenstiel, S.F., Land, M.F., Fujimoto, J., 2016. Contemporary Fixed Prosthodontics, fifth ed. Elsevier, St. Louis, pp. 538. Shin, H.J., Kim, M.J., Kim, H.I., Kwon, Y.H., Seol, H.J., 2017. Effect of ice-quenching on the change in hardness of a Pd-Au-Zn alloy during porcelain firing simulation. Dent.
91