investment casting behavior of palladium-based alloys · investment casting behavior of...

�May 2008

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

BattainiInvestment Casting Behavior

of Palladium-Based Alloys

Paolo Battaini8853 S.p.A.

Milano, Italy

IntroductionThe development of dental alloys with significant palladium (Pd) content has taken place since the 1970s. These alloys, used for the making of prostheses by the lost-wax method, were of the Pd-Ag type, with the palladium content ranging from 40 to about 60%. In the 1980s, dental alloys with a palladium content from 70 to 85% were developed. These were the so-called palladium-based alloys and the users were required to set up working cycles appropriate to the new physi-cal properties of the material, which were unknown to those who normally dealt with gold alloys. At the beginning, some difficulties arose regarding their launch into the market, due to the unusual properties of palladium.

Palladium-based alloys are developing now in the goldsmith field. In this case the palladium content is equal to 95% (950‰). Even though this concentration is higher than that typical of dental alloys, similar problems can be expected in the metal working of both kinds of materials.

This work is aimed at presenting the main physical and chemical properties of palladium-based dental alloys and transferring the experience acquired in the dental field to the goldsmith’s.

Physical Properties of PalladiumPalladium is a face-centered cubic (f.c.c.) crystal lattice metal, like copper, plati-num, gold and silver. Table 1 shows some of its most important physical proper-ties, which are also compared with those of platinum. See the different density values (palladium’s are significantly lower than platinum’s)—and Mean Linear Thermal Expansion Coefficient. Palladium has the highest linear expansion coef-ficient among the platinum group metals.

� Investment Casting Behavior of Palladium-Based Alloys

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Table 1 Some physical properties of Pd compared to those of Pt.1

PdDensity (g/cm3)

Melting Point

M.L.T.E.C. (x 10-6/°C) (*)

Volume Change on Melting (%) (+)

at 20°C (68°F) 12.0

1552°C (2825.6°F)

13.6 (20 ÷ 1000°C) 7.0

at 1552°C (2825.6°F) 10.49 (liq.)

Pt

at 20°C (68°F) 21.45

1769°C (3216.2°F)

11.31 (20 ÷ 1500°C) 6.0

at 1769°C (2825.6°F) 19.0 (liq.)

(+) Computed using the density variation(*) M.L.T.E.C.: Mean Linear Thermal Expansion Coefficient

Palladium-based Dental Alloys The Vickers hardness of pure palladium is about 42HV, whereas its tensile strength is 180MPa.2 As a consequence, palladium must be alloyed with other metals in order to improve its mechanical and physical properties and make it usable. Figures 1 and 2 show how the hardness of palladium changes by alloying it with some elements.2,3

Figure 1 Change of the Pd Vickers Figure 2 Change of the Pd Vickers hardness as a function of the atomic hardness as a function of the weight concentration of some alloy elements. concentration of some alloy elements.

Table 2 shows the chemical composition of the most common kinds of palladium-based dental alloys.4,5 As shown, the most used alloy elements are copper (Cu), indium (In), and gallium (Ga). Dental alloys of the Pd-Ag and Pd-Au type exist as well. In such cases the palladium weight concentration is usually lower than 40%, so these alloys are not going to be considered in the present work.

Table 2 Main properties of the most common kinds of Pd-based dental alloys

Alloy Pd Au Ag In Cu Ga Ru OthersMelting Range

°C (°F) HV

(as-cast)

1 79.0 2.0 9.8 9.0 x1130–1230

(2066–2246)350

2 75.0 6.0 6.0 6.0 6.0 x Sn1160–1300

(2120–2372)240

3 75.0 2.0 6.5 10.0 6.0 x Zn, Sn1140–1240

(2084–2264)350

4 74.0 5.0 14.5 1.6 Sn 4.91160–1275

(2120–2330)250

A major property of the dental alloys in Table 2 is the large width of the melting range, which is between 100 and 150°C (212 and 302°F). This is an important dif-ference between palladium-based dental alloys and the most common platinum alloys normally used in investment casting (see Table 3). Since the width of the melting range strongly affects the solidification process,6 different behavior dur-ing casting operations must be expected for the two kinds of alloy.

Table 3 Melting ranges of some goldsmiths’ 950 platinum alloys

Melting Range °C (°F)

Pt-5% Cu 1725–1745 (3137–3173)

Pt-5% Co 1750–1765 (3182–3209)

Pt-5% Au 1740–1770 (3164–3218)

Pt-5% W 1830–1845 (3326–3353)

Pt-5% Ru 1780–1795 (3236–3263)

The most common melting techniques for dental alloys are induction melting, in air or in argon protective atmosphere, and the usual oxygen-propane torch melting. A phosphate-bonded investment, graphite-free, is always used, with a pre-heating temperature of the mold between 800°C (1472°F) and 900°C (1652°F), according to the alloy. Quartz or ceramic (melted silica or alumina) crucibles are employed, even though silicon may give rise to brittleness problems, as discussed below. Graphite crucibles are absolutely avoided.

In palladium-based dental alloys the large number of alloy elements gives rise to a complex microstructure, with many phases and structural constituents. The alloys are non-homogeneous at a microscopic level (see Figures 3–6 for example). Particularly, lamellar eutectic constituents are present (see Figures 7 and 8).

Investment Casting Behavior of Palladium-Based Alloys �May 2008

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Table 1 Some physical properties of Pd compared to those of Pt.1

PdDensity (g/cm3)

Melting Point

M.L.T.E.C. (x 10-6/°C) (*)

Volume Change on Melting (%) (+)

at 20°C (68°F) 12.0

1552°C (2825.6°F)

13.6 (20 ÷ 1000°C) 7.0

at 1552°C (2825.6°F) 10.49 (liq.)

Pt

at 20°C (68°F) 21.45

1769°C (3216.2°F)

11.31 (20 ÷ 1500°C) 6.0

at 1769°C (2825.6°F) 19.0 (liq.)

(+) Computed using the density variation(*) M.L.T.E.C.: Mean Linear Thermal Expansion Coefficient

Palladium-based Dental Alloys The Vickers hardness of pure palladium is about 42HV, whereas its tensile strength is 180MPa.2 As a consequence, palladium must be alloyed with other metals in order to improve its mechanical and physical properties and make it usable. Figures 1 and 2 show how the hardness of palladium changes by alloying it with some elements.2,3

Figure 1 Change of the Pd Vickers Figure 2 Change of the Pd Vickers hardness as a function of the atomic hardness as a function of the weight concentration of some alloy elements. concentration of some alloy elements.

Table 2 shows the chemical composition of the most common kinds of palladium-based dental alloys.4,5 As shown, the most used alloy elements are copper (Cu), indium (In), and gallium (Ga). Dental alloys of the Pd-Ag and Pd-Au type exist as well. In such cases the palladium weight concentration is usually lower than 40%, so these alloys are not going to be considered in the present work.

Table 2 Main properties of the most common kinds of Pd-based dental alloys

Alloy Pd Au Ag In Cu Ga Ru OthersMelting Range

°C (°F) HV

(as-cast)

1 79.0 2.0 9.8 9.0 x1130–1230

(2066–2246)350

2 75.0 6.0 6.0 6.0 6.0 x Sn1160–1300

(2120–2372)240

3 75.0 2.0 6.5 10.0 6.0 x Zn, Sn1140–1240

(2084–2264)350

4 74.0 5.0 14.5 1.6 Sn 4.91160–1275

(2120–2330)250

A major property of the dental alloys in Table 2 is the large width of the melting range, which is between 100 and 150°C (212 and 302°F). This is an important dif-ference between palladium-based dental alloys and the most common platinum alloys normally used in investment casting (see Table 3). Since the width of the melting range strongly affects the solidification process,6 different behavior dur-ing casting operations must be expected for the two kinds of alloy.

Table 3 Melting ranges of some goldsmiths’ 950 platinum alloys

Melting Range °C (°F)

Pt-5% Cu 1725–1745 (3137–3173)

Pt-5% Co 1750–1765 (3182–3209)

Pt-5% Au 1740–1770 (3164–3218)

Pt-5% W 1830–1845 (3326–3353)

Pt-5% Ru 1780–1795 (3236–3263)

The most common melting techniques for dental alloys are induction melting, in air or in argon protective atmosphere, and the usual oxygen-propane torch melting. A phosphate-bonded investment, graphite-free, is always used, with a pre-heating temperature of the mold between 800°C (1472°F) and 900°C (1652°F), according to the alloy. Quartz or ceramic (melted silica or alumina) crucibles are employed, even though silicon may give rise to brittleness problems, as discussed below. Graphite crucibles are absolutely avoided.

In palladium-based dental alloys the large number of alloy elements gives rise to a complex microstructure, with many phases and structural constituents. The alloys are non-homogeneous at a microscopic level (see Figures 3–6 for example). Particularly, lamellar eutectic constituents are present (see Figures 7 and 8).


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 3 Microstructure of Alloy 1 (Table 2) in the as-cast condition. The alloy shows equiaxic crystal grains whose mean size is about 35µm, and is multi-phasic.


Figure 5 Microstructure of Alloy 3 (Table 2) in the as-cast condition. The alloy is biphasic, with a lamellar eutectic structural constituent.



Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini






Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 7 Detail of Figure 3 (Alloy 1 in Table 2). Precipitates within the grains and along the grain boundaries are visible. Lamellar

eutectic structural constituents can be observed as well.

Figure 8 Detail of Figure 6 (Alloy 4 in Table 2). The lamellar eutectic structural constituent is clearly visible.


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Figure 7 Detail of Figure 3 (Alloy 1 in Table 2). Precipitates within the grains and along the grain boundaries are visible. Lamellar

eutectic structural constituents can be observed as well.

Figure 8 Detail of Figure 6 (Alloy 4 in Table 2). The lamellar eutectic structural constituent is clearly visible.

High-palladium dental alloys are usually fine-grained due to the presence of grain-refining elements (typically ruthenium or rhenium). However, alloys with a dendritic as-cast microstructure are also employed.7

The metallurgical aspects of palladium-based dental alloys have been partly stud-ied. Generally, it can be said that these alloys are multi-phasic, with a primary face-centered cubic phase based on the palladium crystal lattice, and secondary body-centered or simply cubic phases, whose crystal structure is similar to that of the Cu3Ga and PdGa intermetallic compounds.8

The dark lamellae, visible in Figures 7 and 8, are regions where the chemical attack is more effective and are said to be made of a Pd2Ga-type phase.9 The material between the lamellae is made of the primary face-centered cubic phase based on the palladium crystal lattice and identical with the matrix of the equiaxic crystal grains.

Because of the multi-component nature of the alloys in Table 2, the Pd2Ga phase and the palladium solid solution matrix have complex compositions and contain some amount of each component element in the alloy under equilibrium conditions. Accordingly, the matrix chemical composition changes from one alloy to the other. Figure 9 shows the morphological detail of the lamellar eutectic constituent.

Figure 9 SEM image of the micro-structural detail of Alloy 4 (Table 2 and Figure 8) as it appears from a metallographic specimen.


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Other authors10 report on a tetragonal face-centered phase of Pd3Ga or Pd3GaxCu1-x type. The presence of these phases is responsible for the high hardness of high-pal-ladium dental alloys, both in the as-cast condition and after thermal treatment.

However, it should be taken into account that the microstructure of these alloys may change as a function of thermal treatments and different solidification rates, which may differ from casting to casting. Dissolution, at least partial, of the lamellar eutectic phase is usually observed as a consequence of high tempera-ture treatments. See Figures 10 and 11 as examples. Clearly, solid-state diffusion phenomena, due to the temperature rise, lead to the partial dissolution of these phases and to a better micro-structural homogeneity. In fact, high-palladium dental alloys are commonly subjected to homogenizing treatments at about 950°C (1742°F) for 10 minutes followed by rapid cooling. Corrosion resistance in an oral environment also benefits from these treatments, since it is already raised by the high palladium content anyway.

Figure 10 Microstructure of Alloy 3. On the left: as-cast condition. The as-cast condition can change according to the different casting temperatures, solidification

rate or mold-cooling rate. This can be understood by comparing Figure 10 with Figure 5, which belongs to another casting. On the right: after heat treatment

at 950°C (1742°F) for 10 minutes and rapid quenching. The alloy remains biphasic. However, the eutectic phase tends to disappear.

Figure 11 Microstructure of Alloy 2. On the left: after heat treatment at 950°C (1742°F) for 10 minutes. On the right: as-cast condition.

Figure 12 Microstructure of Alloy 1 in Table 2 without any ruthenium addition. Typically, it consists of large dendritic grains. See Figure 3 for comparison.


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Figure 11 Microstructure of Alloy 2. On the left: after heat treatment at 950°C (1742°F) for 10 minutes. On the right: as-cast condition.

Figure 12 Microstructure of Alloy 1 in Table 2 without any ruthenium addition. Typically, it consists of large dendritic grains. See Figure 3 for comparison.

�0 Investment Casting Behavior of Palladium-Based Alloys

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Generally, high-palladium dental alloys reveal inter-dendritic or grain boundary segregation of copper, gallium and indium—if present. In other words, the first material to solidify is rich in palladium. Ruthenium is generally added to the alloy as a grain refiner at weight concentrations lower than 0.3% and does not have a primary role in the hardness increase. However, its effect is clear by comparing Figures 12 and 3. Both refer to Alloy 1 in Table 2. Figure 12 shows the microstruc-ture without any addition of ruthenium. Crystal grains are typically large and dendritic.

The phase diagrams of Pd-Cu, Pd-Ag and Pd-Au alloys (Figures 13, 14, 15) indi-cate that these systems crystallize through a continuous series of solid solutions. Pd-Au and Pd-Cu show solid-state transformations at low temperatures and within some composition ranges. Furthermore, the melting range is small in each case and does not go beyond 10°C (50°F) or so.

The Pd-Ga, Pd-In, Pd-Ru and Pd-Sn systems are the most complex (Figures 16 to 19).1, 2, 11, 12 Particularly, Pd-In and Pd-Ga alloys contain a lot of inter-metallic com-pounds. Not only is gallium one of the causes of the multi-phasic structure of pal-ladium dental alloys, but it also forms a Pd-Ga eutectic phase at 1000°C (1832°F), which persists even if gallium is partly replaced by indium. In fact, indium acts as a substitute for gallium, so the eutectic temperature is not changed.

With palladium weight concentration of about 95%, the melting ranges of Pd-Ga, Pd-Sn and Pd-Ru alloys are quite wide—see the phase diagrams of Figures 18, 19 and 20 and compare to the platinum alloys’ data (Table 3). Hence, these three ele-ments can broaden the melting range of palladium alloys.

What has been observed so far can be summarized by stating that the addition of indium, gallium and tin increases the hardness and improves the mechanical properties of palladium, lowering its casting temperature at the same time. Lastly, it is to be expected that the 950 palladium alloys for jewelry be mainly mono-pha-sic, as the alloy elements’ concentrations are below those typical of dental alloys (Figure 21).

Figure 13 Cu-Pd phase diagram. Figure 14 Ag-Pd phase diagram. Low temperature solid-state The Ag solubility in Pd is total. transformations are present.

Investment Casting Behavior of Palladium-Based Alloys ��May 2008

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Figure 15 Au-Pd phase diagram. Figure 16 Pd-Ga phase diagram. Low temperature solid-state Many intermediate phases are present transformations are present. and a good Ga solubility in Pd is observed.

Figure 17 Pd-In. The Pd-In phase diagram reveals many intermediate phases, inter-metallic compounds and a good In solubility in Pd.

�� Investment Casting Behavior of Palladium-Based Alloys

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 18 The Pd-Sn phase diagram

shows many intermediate phases and a good Sn solubility in Pd.

Figure 19 Pd-Ru. The phase diagram Figure 20 The melting of Pd-Ru is a simple peritectic with range of 94.8Pd5.2Ru limited Ru and Pd-based solid solutions. alloy is about 70°C (158°F).


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Figure 21 Example of the influence of In and Ga on the microstructure of a Pd alloy containing 10% weight concentration of Cu. Mono-phasic

or biphasic alloys can be obtained as a function of the Ga and In content.13

Effects of Oxygen, Carbon, Hydrogen and Silicon On Palladium AlloysSince the early applications of palladium-based dental alloys, carbon, oxygen and silicon have always drawn the dental technicians’ attention. To understand the importance of these three elements the application procedures carried out by den-tal technicians are to be briefly summarized. Investment casting palladium-based dental alloys are used for metal-ceramic appliances. A metallic substructure, obtained by investment casting, is covered by a multilayer of ceramics to create the original tooth appearance (Figure 22). Ceramic layers are sintered at about 980°C (1796°F).


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 22 A metallic substructure, obtained by investment casting, is covered by a multilayer of ceramics to create the original tooth appearance.

To have a good bond between the ceramics and the alloy, adequate oxides are expected to form at the ceramics-alloy interface.14 A primary role is played by indium, tin and gallium in forming such oxides, which are produced during an oxidation treatment of the alloy, preliminary to the ceramics sintering. The stabil-ity of such oxides is crucial, in order that no gas evolution during the ceramics sintering occurs. If not, unaesthetic porosities in the ceramics as well as possible ceramics ruptures or detachment develop. Therefore, it is clear that if an alloy can release gases at the ceramics sintering temperature, the consequences can be most considerable. For this reason the relation between palladium and its ability to absorb and release gas has always been an important aspect in dental alloys applications.

OxygenDespite the use of palladium in a variety of fields, only sparse information is available in the literature about oxygen behavior in palladium. It was believed that palladium was able to dissolve great amounts of oxygen. This opinion came from early studies15 when it was noticed that the palladium weight increased at 900°C (1652°F) if oxygen was present. This was interpreted as oxygen absorption by the metal, but this was only partly true. In fact, oxygen absorption was due to the formation of oxides of metals present as impurities in palladium,16 and it was highlighted that palladium, if not pure enough, showed internal oxidation of base metal impurities. Both for dental and jewelry applications, internal oxidation

of palladium-based alloys is an important phenomenon. In the first case (Figure 23) it develops during both the oxidizing heat treatments prior to the ceramics sintering and the solidification process, similar to what happens to silver alloys (firestain).17 In 950 palladium goldsmith’s alloys, the phenomenon occurs if the material is exposed to high temperatures for long in an oxygen-containing atmo-sphere (Figure 24). It cannot be excluded that, depending on the chemical com-position, the palladium alloys for jewelry applications show internal oxidation during the post-casting cooling stages. Of course, for palladium jewelry alloys we expect a slower phenomenon.

Figure 23 Internal oxidation of Alloy 3 after heat treatment at 950°C (1742°F) for 10 minutes. A dispersion of oxides of the main alloy elements down to

a depth of about 30µm from the surface is visible.


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

of palladium-based alloys is an important phenomenon. In the first case (Figure 23) it develops during both the oxidizing heat treatments prior to the ceramics sintering and the solidification process, similar to what happens to silver alloys (firestain).17 In 950 palladium goldsmith’s alloys, the phenomenon occurs if the material is exposed to high temperatures for long in an oxygen-containing atmo-sphere (Figure 24). It cannot be excluded that, depending on the chemical com-position, the palladium alloys for jewelry applications show internal oxidation during the post-casting cooling stages. Of course, for palladium jewelry alloys we expect a slower phenomenon.

Figure 23 Internal oxidation of Alloy 3 after heat treatment at 950°C (1742°F) for 10 minutes. A dispersion of oxides of the main alloy elements down to

a depth of about 30µm from the surface is visible.


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 24 Internal oxidation of a jewelry 950 palladium alloy after heat treatment at 1000°C (1832°F) in air for 1 hour.

The thickness of the internal oxidation layer has been studied for palladium-based dental alloys and a higher depth in smaller grain alloys has been observed.18 The effect of alloy elements on internal oxidation has been studied as well, and it was shown that boron additions lower its thickness. This can be explained by the for-mation of B2O3, which slows down the oxygen diffusion.18

All in all, the observations indicate that oxygen preferentially diffuses along the grain boundaries, thanks to the interaction with some alloy elements that segre-gate there, also aided by high temperatures. This model has been recently verified on 98Pd2Cr and 87Pd13Cr by studying the microstructure of the internally-oxi-dized alloy by transmission electron microscopy, Auger Electron Spectroscopy and Thermogravimetry.19,20,21 Cr2O3–type oxides precipitate after alloy air oxida-tion at various temperatures.

It is then demonstrated that upon high-temperature exposure (>700°C/1292°F) of a Pd-M alloy to an oxidizing environment, oxygen dissociates at the surface and oxygen atoms diffuse to internally oxidize M. M is a solute metal more easily oxidizable than palladium, and produces oxide precipitates in a matrix of pure Pd (Figure 25).


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Figure 24 Internal oxidation of a jewelry 950 palladium alloy after heat treatment at 1000°C (1832°F) in air for 1 hour.

The thickness of the internal oxidation layer has been studied for palladium-based dental alloys and a higher depth in smaller grain alloys has been observed.18 The effect of alloy elements on internal oxidation has been studied as well, and it was shown that boron additions lower its thickness. This can be explained by the for-mation of B2O3, which slows down the oxygen diffusion.18

All in all, the observations indicate that oxygen preferentially diffuses along the grain boundaries, thanks to the interaction with some alloy elements that segre-gate there, also aided by high temperatures. This model has been recently verified on 98Pd2Cr and 87Pd13Cr by studying the microstructure of the internally-oxi-dized alloy by transmission electron microscopy, Auger Electron Spectroscopy and Thermogravimetry.19,20,21 Cr2O3–type oxides precipitate after alloy air oxida-tion at various temperatures.

It is then demonstrated that upon high-temperature exposure (>700°C/1292°F) of a Pd-M alloy to an oxidizing environment, oxygen dissociates at the surface and oxygen atoms diffuse to internally oxidize M. M is a solute metal more easily oxidizable than palladium, and produces oxide precipitates in a matrix of pure Pd (Figure 25).

Figure 25 Transmission electron micrograph illustrating the distribution of Cr2O3 particles in the internally-oxidized 98Pd2Cr alloy. Close to the

grain boundary the Cr2O3 precipitates are coarser in size and fewer in number as compared to those present in the grain interior. Arrows indicate some linear dislocations visible on the micrograph, and the letter L shows dislocation loops.

The studies performed so far show that palladium alloys, also including those which are developing for goldsmiths’ applications, can absorb significant amounts of oxygen at high temperatures, whereas the terminal solubility of oxygen in solid palladium is extremely low, below 1 atomic ppm22 and with the oxidation con-fined to the surface and near surface regions.23

Unfortunately, no precise information about oxygen solubility in liquid palla-dium is available. However, oxygen may be present in liquid-state palladium alloys, particularly if the alloy contains significant amounts of internal oxides of base metals before casting.

In fact, in the case of dental alloys the quality of scraps, which are used for the sub-sequent castings, is important. The grinding of surface scrap layers helps reduce gas porosity in the subsequent casting. Obviously, porosity may be due to other causes than oxygen. However, gas porosity is often present in palladium-based dental alloys (Figure 26). It is also observed in 950 palladium alloys for jewelry applications (Figure 27).


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 26 Gas porosity in palladium-based dental Alloy 2 - Table 2

Figure 27 Gas porosity in a 950 palladium jewelry alloy


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

CarbonSince the first applications of palladium-based dental alloys it has been known that palladium easily absorbs carbon. A high carbon concentration causes major problems in the ceramization phases of prostheses.

The absorbed carbon can be the source of the formation of carbon monoxide, which can cause bubbles in the ceramic during the firing process. Graphite cru-cibles are then to be avoided in the casting processes of palladium-based dental alloys. Experimental data shows that the alloy’s carbon absorption increases in proportion to its palladium content and melting time (Figure 28).

Figure 28 Carbon absorption by dental alloys containing different amounts of Pd during melting in a graphite crucible.24 The carbon content increases as a function of the alloy Pd content and the number of melting processes.

The alloy stays in the crucible for 3 to 4 minutes before casting.

In order to avoid porosity due to the evolution of carbon monoxide, the alloy’s carbon content must be lower than 60ppm.25 It follows that graphite-containing phosphate-bonded investments must not be applied to palladium-based dental alloys. These investments, however, show excellent performance in case of dental alloys where the palladium content is less than 40% in weight.

In principle, carbon contamination should not be so critical in the case of 950 Pd alloy for jewelry applications, since jewels do not undergo ceramization. On the contrary, the mechanical properties of these alloys are affected by carbon. Particularly, the ductility expressed as the total elongation at fracture was found to be considerably reduced by a small amount of carbon in the case of the high-palladium alloys.25

The solubility curve of carbon in palladium (Figure 29) was obtained by metal-lographic analyses of ad-hoc prepared specimens26 consisting of palladium sheets saturated with carbon. This study showed that palladium takes up more than


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

0.4% by weight of carbon as interstitial element at 1400°C (2552°F). Palladium- carbon alloys quenched from temperatures above the solubility line are simple solid solutions. Subsequent heat treatment at temperatures below the solubility line precipitated out carbon. The precipitated carbon appears as spher-oids. The same shape is shown by carbon absorbed by melted palladium in graphite crucibles, kept as a liquid for 5 minutes (Figure 30).

Figure 29 The solubility of carbon in solid palladium.26 The continuous line represents the solubility limit of carbon at different temperatures.

Figure 30 Microstructure of a carbon-palladium alloy produced by keeping melted palladium in a graphite crucible for 5 minutes.26


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

HydrogenIt is well known that palladium can absorb high amounts of hydrogen. At room temperature, the solubility of hydrogen in palladium is about 0.6 H/Pd (atomic ratio).1,27,28 Obviously, it is very high because it corresponds to 6 hydrogen atoms for every 10 palladium atoms. At a given temperature, the solubility of hydro-gen in palladium increases with the hydrogen pressure. Below 300°C (572°F) the homogeneous solid solution of hydrogen in palladium decomposes into an α phase with low hydrogen content and an expanded, hydrogen-rich β phase.28 The atom ratios (H/Pd) of α and β phases correspond to the dashed lines in both graphics of Figure 31.

Figure 31 shows the result of a study of hydrogen solubility in palladium. The isothermal curves represent the relation between the pressure of the hydrogen in equilibrium with palladium and the hydrogen concentration in the metal for each temperature.

Figure 31 Hydrogen concentration in palladium under equilibrium conditions.1,28 Each line represents the hydrogen concentration in palladium in equilibrium with the gas at

different pressures and specific temperature. At temperatures close to room temperature, the hydrogen concentration in palladium is very high even at low pressure. The graphic on the right shows a detail of the isothermal curves at low temperatures and pressures.

From the charts in Figure 31 it appears that the hydrogen concentration in pal-ladium is high even at hydrogen pressures as low as atmospheric, but only at low temperatures. This is important, because if palladium is annealed at high tem-peratures, for example higher than 600°C (1112ºF) in forming gas at atmospheric pressure, a high hydrogen absorption by the metal is not likely to occur. However, if the subsequent cooling process takes place in the same atmosphere, hydrogen could be absorbed by the metal when the temperature decreases. This phenom-enon is enhanced by the high diffusion rate of hydrogen in palladium.29


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

It should also be considered that if palladium or the palladium alloy contains internal oxides, the hydrogen is trapped by the oxygen atoms rather strongly and cannot be easily removed, for example by evacuation, until the temperature of about 500°C (932°F) is exceeded.

Furthermore, Figure 31 suggests that palladium could be charged with hydrogen from a hydrogen-containing gas mixture under conditions of temperature and pressure which favor high solubility of the gas in the metal.

It is also very well known that palladium and palladium alloys, when submitted to a high hydrogen charging, can form hydrides.30 Hydrides can cause internal cracks, particularly when internal oxidation is present.19

Hydrogen hardly ever enters the production cycles. So dental technicians only avoid gas flows containing hydrogen-nitrogen components when they have to protect the alloy during the melting stages. In these processes a reducing atmo-sphere may have other unwanted effects. The same is done by dental alloy manu-facturers during ingot furnace production and annealing.

SiliconThe embrittlement caused by silicon on palladium-based dental alloys has been recognized by some manufacturers since the 1980s.18 It is caused by the formation of a Pd - Pd3Si eutectic at 782°C (1439°F), usually located at grain boundaries (see Figure 32). However, experience in the dental alloy field shows that this type of embrittlement does not occur in all cases and that it depends on the chemical composition of the dental alloy. Dental technicians often use quartz crucibles with most alloys of Table 2 without any considerable negative effects. However, there is not enough experience about 950 palladium alloys to give advice about prevent-ing this type of embrittlement.


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Figure 32 Pd-Si phase diagram.1 The presence of the Pd – Pd3Si eutectic at 782°C (1439°F) can lead to alloy embrittlement, particularly during the solidification process.

The high palladium concentration suggests the use of silicon-free or low-silicon content crucibles, like the zirconium-based ones, whose silicon content is rather low (1.5 ÷ 1.7 weight %, Table 4). Alternatively, silicon-containing crucibles can be coated with zirconium oxide,31 which shields the liquid alloy. Silicon embrittle-ment is revealed by hot-cracking, which will be discussed more in the next para-graph. Reducing conditions must be avoided during the melting of palladium alloys. Actually, it should be remembered that under reducing conditions silicon dioxide can be reduced to elemental silicon and react with the molten palladium. Hence, as in the case of platinum, reducing conditions such as those occurring with a gas-forming protective atmosphere (25% H2 – 75% N2), must be avoided. The most appropriate protective gas is argon, widely used in the dental alloy field by means of induction casting machines.

Silicon in very low concentration may also affect the mechanical properties of palladium. The presence of silicon, in concentrations as low as a few parts per million, can have a pronounced yield point effect on palladium,32 a phenomenon usually associated with body-centered cubic metals such as iron.

Table 4 Typical chemical composition of zirconia crucibles (weight %)

SiO2 TiO2 Fe2O3 Al2O3 CaO MgO Na2O+K2 O ZrO2 + HfO2

Zirconia ZAM 1.5 0.2 <0.15 0.7 0.15 4.6 <0.15 92.5

Zirconia ZAL 1.7 0.2 <0.15 1.1 4.5 0.1 0.1 92.1


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Hot-Cracking Susceptibility of Palladium-based Dental AlloysAn important high-temperature property of a palladium-based dental alloy is its strength at a temperature just below its solidus temperature as regards its resist-ing shrinkage stresses.

This phenomenon can be better understood by considering the thermal expansion behavior of investment materials. As regards dental alloys, the commonly used phosphate-bonded investments contain the three silica forms, that is, quartz, cris-tobalite and tridymite, as refractory materials. These silicon oxides show a very limited expansion above 650°C/1202°F (see Figure 33). The total expansion of these investments depends on the blend of distilled water and the special liquid supplied with the powders. This liquid, provided by the manufacturer, is a form of silica sol in water. Therefore, the total investment expansion is the addition of setting expansion and thermal expansion (Figure 34).

Figure 33 The main constituents of phosphate-bonded investments used in the dental field do not show a significant expansion above 650°C (1202°F). Tridymite changes its crystal lattice between 105° and 160°C (221° and 320°F). Quartz undergoes a crystal lattice transformation at 573°C (1063°F) and cristobalite at 220°C (428°F). The three

silicon oxides transform into fused silica if heated above 1700°C (3092°F).


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Figure 34 The expansion of the phosphate-bonded investments used in lost-wax casting of palladium-based dental alloys is poor above 600°C (1112°F). The total expansion can

be controlled by adjusting the proportions in the blend of water and special liquid.

From the thermal expansion curves of the phosphate-bonded investments, it can be deduced that they do not show a significant shrinkage when the temperature decreases from the alloy solidus value to about 650°C/1202°F (Figure 34). On the contrary, the alloys generally tend to shrink almost linearly during cooling.

As a consequence, remarkable stresses are to be expected up to 650°C (1202°F) between the investment and the alloy during cooling, after casting. These stresses could cause hot-cracking ruptures. The hot-cracking susceptibility can be gener-ally defined as the sensitivity for an alloy to crack spontaneously at temperatures close to the solidus temperature. In fact, rupture can occur both slightly above the solidus temperature (supersolidus cracking) and at lower ones (subsolidus crack-ing). At temperatures lower than the liquidus temperature and still higher than the solidus one, solid growing nuclei are present. At decreasing temperatures these nuclei aggregate, giving rise to a coherent mass, though not completely solidified. The temperature at which this occurs is defined as the coherence temperature. At the coherence temperature the alloy can be said to have some mechanical strength, very low however. In fact, the alloy ductility is almost zero. When the alloy temperature decreases below the so-called nil-ductility temperature, ductil-ity starts to rise rapidly and the brittleness of the solidifying alloy decreases. The nil-ductility temperature is higher than the solidus one. The wider the interval between the coherence temperature and the nil-ductility temperature, the more likely supersolidus cracking is to occur, as the brittleness interval is wider. In this case the alloy nil-ductility may be due to the presence of a liquid inter-granular film that promotes hot-cracking as a consequence of the stresses related to the investment.


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

SEM investigations offer an experimental support to this model (Figure 35). Obviously, the wide melting range of palladium-based dental alloys promotes supersolidus hot-cracking.

Figure 35 Supersolidus hot-cracking in a palladium-based dental alloy of type 2, Table 2. The SEM image shows residue of solidified liquid film on the grain

boundary surfaces where they detached from the opposing grains.

The investments used with platinum are chemically much simpler than those used for dental alloys. They are basically about 99% fine silica with up to 1% additives. The silica is all quartz with no cristobalite.33 Hence, one of the most important dif-ferences with regard to phosphate-bonded investments used with dental alloys is their lower thermal expansion (see graphs in Figures 33 and 36).

Figure 36 Typical expansion curve of a Pt investment (adapted from Grierson33)


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

However, investment for platinum also shows small dimensional changes when temperature decreases from the typical solidus values of palladium or platinum alloys to about 600°C (1112°F).

The 950 palladium alloys for jewelry applications recently brought onto the mar-ket are used with the same investments as those studied for platinum. Therefore, no considerable differences with respect to palladium-based dental alloys are expected as regards the supersolidus hot-cracking which may occur in case of wide melting range. As a matter of fact, this kind of rupture has been observed in 950 palladium alloys for jewelry applications as well (Figures 37 and 38).

Figure 37 SEM image of inter-granular fracture surfaces originated by supersolidus hot-cracking in a 950 Pd alloy. The wavy borders of the liquid film covering the grain surfaces at rupture are visible. The arrows

indicate some points where the liquid film has created a bridge with the film present on the opposing surface at detaching. See details in Figure 38.


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 38 Detail of Figure 37. The connecting filaments of the liquid film on opposing separating surfaces are indicated by the arrows.

It has been demonstrated that the supersolidus hot-cracking resistance increases as the grain size decreases.18 As regards alloy elements, it is believed that indium and tin may negatively affect the supersolidus hot-cracking resistance, while gal-lium does not produce any negative effect.18

Generally, for palladium-based dental alloys a lower metal temperature and an investment mold temperature in the range of 700–750°C (1112°–1382°F) reduce supersolidus hot-cracking. Obviously, this is more difficult if this kind of rupture is caused by alloy contamination, for example by silicon, as discussed in the previ-ous paragraph.

Subsolidus hot-cracking is due to stresses that develop between the investment and the alloy during cooling as well. Also, in this case ruptures can be avoided by adopting low cooling rates. In fact, a considerable part of deformation that the alloy undergoes due to stresses is absorbed by high-temperature creep. Creep is a diffusion process of lattice defects within and on the boundaries of the crystal grains as a reaction to local stresses34 (Nabarro-Herring and Coble mechanisms – Figure 39). Small grains facilitate these diffusive phenomena and reduce the occurrence of subsolidus hot-cracking. Anyway, slow cooling is always recom-


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

mended. Figure 40 shows an example of subsolidus hot-cracking in a palladium-based dental alloy with coarse dendritic grains. If the alloy is subject to this kind of rupture, it must be cooled as slowly as possible so that the creep absorbs the stresses caused by the differential shrinkage of the metal and the investment.

Figure 39 Schematic representation of diffusion creep. Self-diffusion results in plastic flow if matter is carried from boundaries subject to compressive stress

(vertical boundaries) over to boundaries under tensile stress (horizontal boundaries).35 The diffusion of atoms occurs inside grains (Nabarro-Herring mechanism). The creep

is called “Coble creep” if the flow of atoms occurs only along the grain boundaries.


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

Figure 40 Subsolidus hot-cracking in a coarse-grain palladium-based dental alloy

ConclusionsA review of the palladium-based dental alloy investment casting behavior is use-ful to disclose possible problems with 950 palladium. Even though 950 palladium could be considered an easy-to-process material for people used to working with platinum, we may expect quite a different behavior due to the high reactivity of palladium alloys with oxygen, carbon, silicon and hydrogen. The casting proce-dure should also be reconsidered due to palladium’s lower density compared to that of platinum. Furthermore, the possible wider melting range of 950 palladium could give rise to a different behavior in solidification if compared to the 950 platinum, including hot-cracking susceptibility. Probably, investments especially developed for 950 palladium would be necessary to obtain better results.

References1. Smithells Metals Reference Book, 7th ed. (Boston: Butterworth – Heinemann,

1992).2. E. M. Savitski, Handbook of Precious Metals, (Hemisphere Publishing

Corporation, 1989).3. Edelmetall-Taschenbuch (Degussa, Hüthig, 1995).4. Paolo Battaini, “Investment casting for oral cavity applications: peculiarities

and problems,” La Metallurgia Italiana 10 (2004): 57–70.5. J.C. Wataha, “Alloys for prosthodontic restorations,” The Journal of Prosthetic

Dentistry (April 2002): 351–363.


Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

Battaini

6. D. Miller, T. Keraan, P. Park-Ross, V. Husemeyer and C. Lang “Casting Platinum Jewellery Alloys,” Platinum Metals Rev. 49, 3 (2005): 110–117.

7. E. Papazoglou, Q. Wu, W.A. Brantley, J.C. Mitchell and G. Meyrick, “Comparison of mechanical properties for equiaxed fin-grained and dendrit-ic high-palladium alloys,” Journal of Materials Science: Materials in Medicine 11 (2000): 601–608.

8. P.R. Mezger, A.L.H. Stols, M.M.A. Vrijhoef and E.H. Greener, “Metallurgical Aspects of High-palladium Alloys,” J.Dent. Res. 67, 10 (October 1988): 1307–1311.

9. W.A. Brantley, Zhuo Cai, D.W. Foreman, J.C. Mitchell, E. Papazoglou, A.B. Carr, “X-ray diffraction studies of as-cast high-palladium alloys” Dent. Mater 11 (May 1995): 154–160.

10. A. Odén, H. Herø, “The Relationship Between Hardness and Structure of Pd-Cu-Ga Alloys,” J. Dent. Res. 65 (January 1986) (1): 75–79.

11. A. Prince, G.V. Raynor and D.S. Evans, Phase Diagrams of Ternary Gold Alloys (London: The Institute of Metals, 1990).

12. H. Okamoto and T.B. Massalski, Phase Diagrams of Binary Gold Alloys (ASM International – 1987).

13. P.J. Cascone, “Phase relations of the palladium-base copper, gallium, indium alloy system,” J. Dent. Res. 63 (Special Issue): 233.

14. W.A. Brantley, Z. Cai, E. Papazoglou, J.C. Mitchell, S.J. Kerber, G.P. Mann and T.L. Barr, “X-ray Diffraction studies of oxidized high-palladium alloys,” Dent. Mater. 12 (November 1996): 333–341.

15. E. Raub and W. Plate, “Über da Verhalten der Hedelmetalle und hirer Legierungen zu Sauerstoff bei hoher Temperatur im festen Zustand.“ Zeit für Metallkunde 48 (1957): 529–539.

16. J.C. Chaston, “Reactions of Oxygen with the Platinum Metals- III - the oxidation of Palladium,” Platinum Metals Rev. 9, 4 (1965): 126–129.

17. Jörg Fischer-Bühner, “Improvement of Sterling Silver Investment Casting,” The Santa Fe Symposium on Jewelry Manufacturing Technology 2006, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2006).

18. J.M. van der Zel, High-Temperature Behavior of Palladium Based Dental Alloys, Academisch Proefschrift – Department of Dental Materials Science - ACTA - University of Amsterdam (1989).

19. G. Gouthama, R. Balasubramaniam, D. Wang and T.B. Flanagan, “Transmission electron microscopy of hydrogen effects in internally oxidized palladium-chromium alloys,” Journal of Alloys and Compounds, 404–406 (2005): 617–620.

20. D.L. Boyd and C. Vargas-Aburto, “Auger Electron Spectroscopy Study of Oxidation of a PdCr Alloy Used for High-Temperature Sensor,” NASA Technical Memorandum 106212 (March 1993).

21. D.L. Boyd, “Thermogravimetric Study of Oxidation of a PdCr Alloy Used for High-Temperature Sensor,” NASA Technical Memorandum 106473 (March 1994).


Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

iB

atta

ini

Bat

tain

i

22. Jon-Wan Park and C.J. Altstetter, “The diffusivity and solubility of oxygen in solid palladium,” Scripta Metallurgica, Vol.19 (1985): 1481–1485.

23. D. Wang and T.B. Flanagan, “The solubility of oxygen in Pd from its subse-quent trapping of hydrogen,” Scripta Materialia 49 (2003): 77–80.

24. J.M. van der Zel and M.M.A. Vrijhoef , “Carbon absorption of palladium-enriched dental alloys,” J. Oral Rehab. 15 (1988): 163–166.

25. H. Hero and M. Syverud, “Carbon impurities and properties of some palla-dium alloys for ceramic veneering,” Dent. Mater. 1 (1985): 106–110.

26. G.L. Selman, P.J. Ellison and A.S. Darling, “Carbon in Platinum and Palladium, solubility determinations and diffusion at high temperatures,” Platinum Metals Rev. 14, 1 (1970): 14–20.

27. R. Burch and F.A. LeWis, “The Form of the Interaction between Palladium and Hydrogen,” Platinum Metals Rev. 15, 1 (1971): 21–25.

28. G. Alefeld and B. Baranowski et al., Hydrogen in Metals II, Application-Oriented Properties, (Springer-Verlag, Berlin, Heidelberg, New York, 1978).

29. F.A. Lewis, “The Palladium-Hydrogen System. Part III Alloy Systems and Hydrogen Permeation,” Platinum Metals Rev. 26, 3 (1982): 121–128.

30. F.A. Lewis, “The Hydrides of Palladium and Palladium alloys,” Platinum Metals Rev. 5, 1 (1961): 21–25.

31. T. Fryé, “Palladium Casting: An Overview of Essential Considerations,” The Santa Fe Symposium on Jewelry Manufacturing Technology 2006, ed. Eddie Bell (Albuquerque: Met-Chem Research, 2006).

32. R.G. Hollister and A.S. Darling, “Yield Point Effects in Palladium,” Platinum Metals Rev. 3, 11 (1967): 94–99.

33. R. Grierson and M. Spasskiy, “Platinum Casting Investments,” 1988 Platinum Day Symposium - Platinum Guild International USA (1999).

34. R.E. Reed-Hill, Physical Metallurgy Principles, Third Ed. (Boston: PWS-Kent Publishing Company, 1992).

35. C. Herring, “Diffusional Viscosity of a Polycrystalline Solid,” Journal of Applied Physics 21 (May 1950): 437–445.

investment casting behavior of palladium-based alloys · investment casting behavior of...

Documents