The structure and oxidation of two palladium ceramic fusing alloys

The structure and oxidation of two palladium ceramic fusing alloys

The structure and oxidationof two palladiumceramicmIlg alloys Eero Suoninen*and H&on Her6 NIOM, Scandinavian institute of Dental Materials, Forsknings...

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The structure and oxidationof two palladiumceramicmIlg alloys Eero Suoninen*and H&on Her6 NIOM, Scandinavian institute of Dental Materials, Forskningsveien (Received 20 February 1384; revised 20 September 1384)

1. Oslo 3, Norway

The oxides formed at the surface of an alloy during preheating at about 1000°C vital for the formation constituent

prior to the firing of ceramic are

of a strong chemical bond between alloy and porcelain. Replacing Au used as the basic

in the ceramic fusing alloys with Pd seems to have a rather small effect on the behaviour of the

elements Sn and In which were found to oxidize internally. However, no external oxidation of the tmnsition metal component Co was found, in contmst to Ni in Au-based alloys. After cemmic firing a zone enriched with Co and In is found in the interaction area between alloy and cemmic. In another alloy Ga was observed to oxidize internally end much more easily than Cu due to its much lower free energy of formation. A solid solution was produced in both the alloys by ceramic firing as shown by SEM studies. Keywords:

Ceramics,

metal

alloy, indium.

tin, cobalt. gallium,

The recent general trend toward lower gold contents in dental casting alloys has created a number of new ceramic fusing alloys. The latitude in the design of new alloy configurations is, however, much narrower for such alloys than in the design of corresponding conventional casting alloys. Besides the obvious requirement of a sufficiently high melting point, the structure, composition and the kinetics of formation of the superficial oxide layer involved in the joint with the ceramic are of primary importance. The discolouration of the ceramic caused by Ag in such alloys limits the use of the ‘white gold’ alloys with a high silver content’ and complicates the design of low-gold ceramic fusing alloys. A characteristic feature of the composition of some of these alloys is high Pd content with additions of relatively small amounts of base metals for achieving a suitable oxide layer for fusing. Elements that are most frequently used for this purpose are Sn, In and Ga, but small amounts of transition metals, typically Fe, Co, etc. are also usually added. The surface layer obtained after the oxidizing heat treatment of the alloy has been studied by several researchers in the case of gold-base alloys3*4. The concept of the actual bonding mechanism and its relationship to the structure of the oxide layer on the metal to be fused to ceramic was originally proposed to Borom and Pask5. Later an excellent review on the bonding mechanism was presented by Pask’. Recently the theory has been further clarified by the studies of Ohno and coworkers4*5 who have pointed out the central role of the ‘bridging’ oxygen atoms in the bond formation. They have also established the essentially different surface oxide structure of goldbase alloys with the In but no Sn as compared to the structure of alloys containing both In and Sn. ‘On leave from: Department of Physical Sciences, SF-20500, Turku. 50, Finland @ 1985

Butterworth

Et Co (Publishers)

palladium,

bonding

No corresponding studies seem to have been reported for the Pd based ceramic fusing alloys, except for a recent rather brief report by Cascone on the oxidation of a PdCr+Ga alloyg. A study of the composition and microstructure of two palladium based ceramic fusing alloys was, therefore, performed. One of these” represents an alloy type with both In and Sn added together with Co whereas the other one contains Cu and Ga. These alloys represent two typical groups of Pd alloys and materials with completely different surface layers after oxidation.

MATERIALS

AND

METHODS

The alloy no. 1 was induction melted under argon atmosphere from elemental metals (purity better than 99.99 wt %). The alloy no. 2 was obtained commercially*. The bulk analyses of the alloys are given in Table 1. For alloy 1, the analysis is based on the weighed amounts of the component metals. The analysis of alloy 2 was provided by the producer. Roughly spherical pieces of the cast alloys were metallographically ground and polished with diamond paste down to 1 p and given the treatments corresponding to the different steps of the procedure of fusing opaque ceramic (Ceramco Paint-0-Pake) to the surface of the alloy (Tab/e 2). The oxidizing treatment of alloy 2 was according to the manufacturets instructions. Flat surfaces with the approximate area 20 mm2 were cut from the treated specimens using a diamond wheel saw and polished up to the 1 (urn grade diamond paste. The samples were then mounted in resin and studied with a combined SEM-electron microprobe instrument (Cameca, type Camebax Microbeam).

University of Turku. *‘Option’,

J.M.

Ney Co., Bloomfield,

CT, USA.

Ltd. 0142-9612/85/020133-05$03.00 Biomaterials

1985,

Vol 6 March

133

Palladium

Table 1

ceramic

alloy: Eero Suoninen

Microprobe

analyses

of the as-cast Structure

Phase

Alloy

Bulk4 ;Fig. 1 a)

and Hakon Hero

alloys (wt %) Pd’

In

Sn

78.2

8.9

7.7

co

Si

5.0

0.2

AU

cu

Ga

Designations in Fig. 3a

Light phase

fee

73.2

14.3

11.9

2.0

0.1

Grey phase

fee

79.0

8.9

7.7

5.0

0.2

79.0

2.0

10.0

9.0

fee’

79.2

2.4

12.0

9.2

80.9

2.4

12.1

7.8

b

77.7

1.5

10.2

13.9

C

74.1

1.8

16.0

12.0

d

Bulk3 Light grey grey

2

Medium

(Fig. 3a)

Dark grey

bee’

Dark ‘See Reference 11. 2Determined by difference. 3Comoosition oiven by the manufacturer 4By weighing. -

and includes only the elements

RESULTS Figure 1 shows the secondary electron pictures obtained for samples of alloy 1 after different treatments. Figure la shows that the as-cast sample consists of two different phases with the approximate microprobe analyses given in Table 1. The grey phase contains more Co and Pd than the light phase, but less In and Sn. The darker areas of the grey phase are slighly higher in Co (6.0 wt %) than the light grey parts (4.6 wt %). No oxide layerthick enough to be resolved by the microprobe beam of about 1 j&n exists on the surface of the as-cast sample. Heating the specimen in the ceramic firing furnace with the vacuum obtained by a rotating pump results in a 4-5 p thick surface layer consisting only of Co oxide with some Pd in it Figure lb. No appreciable oxidation of Sn or In can be detected. Heating in air Figure Ic produces a surface layer of mixed oxides containing predominantly Sn, In and Co together with some residual alloy. The distribution of Co in the

with concentrations

a

in excess of 1 wt %.

surface layer is shown on the micrograph of Co X-rays in Figure Id. It can be seen that in particular the dark oxides on the SEM micrograph in Figure lc are Co-rich. Figures 2a, 6, c show, for alloy 1, the distributions of the metallic elements in these oxides in the region near the surface of a sample treated in contact with the opaque ceramic at 1010°C for 2 min in air (Sample 1 d in Table 2). The boundary between the original metal substrate and the opaque ceramic is clearly visible as a roughly horizontal line in each map. All the above elements are seen to be enriched in the same regions consisting partly of oxide phases. However, it is interesting to note than a thin (< 1 p) layer at the boundary is particularly enriched with In and Co but has a rather low Sn content. Such a thin Co and In rich layer could not be observed after the oxidation process prior to the ceramic firing. The Sn signal from the opaque ceramic is probably due to its original Sn content Tab/e 3 shows the microprobe analyses of the oxide layer of alloy 1 in Figure 1c.

Figure I (a) As-cast sample: aircooled in the investment: SEM: back scattered electrons; (b) Heated 600 - 1010°C during 10 min in rarified aic SEM: back scattered electrons: (c) Heated at 7010°C for 10 min in air: SEM: back scanemd electrons; (d) X-ray micmgraph of Co-distribution in the area shown in Figure lc.

134

Biomaterials

1985,

Vol6

March

Palladium

Table 2

Treatment

of the samples

Alloy no.

Sample

Treatment

1

a

Air cooled after casting

b

Heated 650 rough vacuum

2

1010°C

during 10 min in a

c

Heated

d

Fusing opaque ceramic subsequent to preoxidation (Sample 1 b and 1 c). Heated 955 - 1010°C in a rough vacuum followed by 2 min in air

in air at lOlO”C,

10 min

a

Heated

b

Fusing opaque ceramic subsequent to preoxidation (Sample 2a). Heated 955 - 1010°C in a rough vacuum followed by 2 min in air

in air for 10 min at 1010°C

The firing of ceramic caused a solid solution of the two phases present in the as-cast structure judged by the SEM micrographs shown in Figures la and c. The as-cast bulk structure of alloy 2 is illustrated in Figure 3a. It is similar to the quenched structure, previously reported for an alloy, with almost the same composition, by Khabliyev et al.“. As has been shown by detailed X-ray di~ract~on studies”“2, the phase composition of these alloys varies greatly with the heat treatment and amount of cold work to which the sample has been subjected. In the present case, the X-ray diffraction pattern of the as-cast sample showed only the fee matrix found in the above studies, but the bee phase found has been shown to disappear during mechanical polishing due to cold working of the surface layer’2. It is, therefore, difficult to directly compare the phase compositions of the features seen in a SEM micrograph with the X-ray pattern. The light and medium grey areas probably represent the fee matrix and the dark grey and dark regions the bee phase found in References 11 and 12. This tentative conclusion is based mainly on the microprobe analyses of these regions and the estimated compositions of these phases. The bee phase is enriched with Ga but contains slightly less Au and Pd than the fee matrix. The overall distribution of Cu seems to be roughly even between the bee and the fee phases, but interphase segregation in both causes an uneven distribution within the regions of the original parent phases. The segregation in the fee phase is probably caused by coring during the solidification, but the inhomogeneity of the bee phase may reflect the precipitation of the (Pd, Cu)2Ga phase suggested by Khabliyev et al,“. Figure 36 shows the structure of alloy 2 after an oxidizing treatment in air at 1010°C. The heat treatment Table 3 Alloy

xig.

;ig.

tc)

3b)

Microprobe

analyses

of the surface layers formed by heating

ceramic

alloy: Eero Suoninen

and Hekon Hero

has removed almost entirely the structural inhomogeneities present in the alloy after casting as shown in Figure 3a. The surface of the metal is covered with a layer consisting of metal matrix mixed with the oxides of the main alloy components with a strong enrichment of Ga as shown by the X-ray micrograph in Figure 3c and the microprobe analyses of the oxide particles points (1, 2, 3 in Table 3). Some of the oxide particles contain Cu (points 2 and 3 in Table 3). A characteristic feature of these data is that the surface layer of both alloys is seen to consist mainly of a finedispersed rather dilute mixture of the oxide phases with the metal matrix (points 7,2,3 as compared with points4, 5, 6 in Tab/e 3). The thickness of the layer containing oxides is almost constant, about 15 pm, for both alloys. No thin zone of external oxides formed prior to ceramic veneering could be observed by SEM for alloy 2.

DISCUSSION The observations of the surface structure of alloy 1 with a fairly high content of In and Sn (Table I) show that the oxidation in air of these elements takes place in a similar fashion to that reported by Ohno et al. 7 for a Au-base fusing alloy with both In and Sn additions. No homogeneous oxide layer is formed, but internal oxidation which preferably occurs along grain boundaries creates a surface region consisting of remaining alloy and oxides of varying compositions (Figure 7~). This is reasonable because of the larger solubility of oxygen in Pd than in Au’~. The small concentration of oxygen in the residual atmosphere to which the roughly evacuated sample (1 b in Table 2) was subjected during the oxidation heating period caused a thin zone of internally oxidized material consisting mainly of Co0 (Figure Ibf. This oxidation turned out to be inadequate for the creation of a strong bond to the ceramic which tended to detach from the alloy substrate merely by handling. A better bonding in this respect was achieved by heating in air which consequently is to be preferred as an oxidation treatment for this alloy. Using this procedure (Sample lc in Table 2) no external oxidation of either Co (Figure Id), In or Sn was found, in contrast to the observations by Ohno et at.‘. They observed that NiO was preferentially formed on the Au alloy surface in the presence of In and Sn at lower concentrations than in the present Pd alloy. The reason for this difference in external oxidation behaviour cannot be found in the driving force for the formation of the oxides. This tendency can be estimated from Gibbs standard free energies of formation, AGo (Tab/e 4). in air fwt %)

Point

Pd

In

Sn

co

Si

1 2

35.0 37.6

28.5 27.2

17.7 13.5

1 I.8 9.7

1.2 2.8

5.7 9.2

4 3 5 6

70.4 12.9 25.1 32.0

24.3 6.5 31.6 28.7

25.3 5.9 22.1 20.2

26.4 15.1 16.1 15.8

0.5 1.6 0.6 0.8

10.0 0.4 4.4 2.4

1 2

26.8 40.6

0.8 0.8

3.0 14.7

48.3 25.7

21.7 18.2

4 3 5 6

38.8 82.9 84.4 79.1

1.1 1.9 2.4 2.2

16.1 9.3 10.6 lo.8

22.7 4.5 1.4 7.5

21.3 1.4 1.2 0.5

Au

Ga

cu

Biomaterials

1995,

Vol 6 March

0

735

Palladium ceramic alloy: Eem Suoninen and Hakon Hem

Figure 2 X-ray micmgmphs obtained by the microprobe showing distribution of elements in the oxide fayer after firing the ceramic in Sample Id in Tabie 2: alloy 1. (a) In: (b) Sn; (c) Co.

If the oxide of a metal M is formed according the reaction: uM +I0 2 or

2;M

2

*

to

M,O,

+ O2 --c $M,O,

the oxygen dissociation

pressure po, can be written:

AGo2 Inp02=

RT

*‘O’

+ In aM -

2t

where aM is the activity of the base metal equal to its concentration when the activity coefficient is assumed to be equal to one. Hence the more negative A$ is, the v&o” lower is po, which means an increased driving force for oxidation14. According to the AGo values in Tab/e 4r5-17, In and Sn should be more readily oxidized than Co and Ni. Furthermore, these data cannot explain why Co0 was formed in rarified air rather than SnOz and ln20s (Sample 1 b in Table 2). The metals Sn and In, however, have low melting points and near the surface they may possibly have evaporated in vacuum at 1010°C.

136

Biomaterials r365,

Vof 6 March

Figure 3 A/toy 2. {a) As-cast sample: air cooled in the investment: SEM: back scattered electrons. Compositions of areas a, 6, c and dare given in Table 1; (b) Sample 26 in Table 2 after preoxidation and ceramic firing: SEM: bsck scanemd electrons; (c) X-ray micrograph obtained by the micmpmbe showing distribution of Ga in the oxide layer in the area shown in Figure 36.

In alloy 2 as much as 9 wt % Ga is present but no preferential oxidation on the top surface was observed. Along grain boundaries Ga is more easily oxidized than Cu (Figure 3~: Tab/e 3). The obvious explanation for this is the low free energy of formation of Ga*Os (7’We 4). These thermodynamic data in Table 4 are, however, somewhat unreliable for several reasons: i) The data have been obtained using liquid instead of solid Sn and in; ii) the change in free energy due to dissolution of the alloying elements in Pd is not known and iii) some temperature extrapolations have been carried out (7’able 4). Nevertheless, the reason why Ni is oxidized externally in contrast to the otherelements must most likely be sought in kinetic factors; i.e., by the diffusion rates of the oxidizing atoms ratherthan by the AGovalues of the oxidation. The ratio of the molecular volumes of the oxides to the atomic volume of the parent alloy is possibly also an important factor. However, after ceramic firing a thin (< 1 p) layer of In and Co enriched oxides were created at the metalceramic interface (Figures 2a, b, c). Underneath this layer

Palladium

Table 4

Gibbs standard

free energies

of formation

of some metals

ceramic

alloy: Eero Suoninen

and Hakon Hero

REFERENCES

oxides’ 5 -

Reaction

AGO kcal

4

+ (0,)

= 2

36.6

2

+ (0,)

= 2

68.8

(Sn) -t (0,) $(ln)

=

+ (0,)

4 T(Ga) 2

-I (0,)

=G

=f





$ (0,) = 2

*Extrapolated ‘Extrapolated ‘Extrapolated

at 1273

1

German. R.M., national Metals

2

McLean, J.W., Physical and chemical charactensncs of alloy used for ceramic bonding, in Dental Porcelain. The State of the A~I7977, (Ed. H.N. Yamada). Univ. of Southern Cahf., Los Angeles, pp 79-84 Cascone, P.J., The theory of bondmg for porcelain-to-metal systems, Ibid. pp 109-l 17 Lugassy. A.A.. Characterization of surface properues of porcelainfused-to-metal alloys, /bid. pp 123-l 28 Borom. M.P. and Pask, J.A.. Role of ‘adherence oxides’ in the development of chemical bondmg at glass-metal Interfaces, J. Am. Ceram. Sot. 1966, 49, 1 Pask, J.A., Fundamentals of wettmg and blnding between ceramics and metals, in Alternatives to Gold Alloys in Dentistry. (Ed. Th.M. Valega), DHEW Publ. No. (NIH) 77-1227, Jan. 1977, pp 235-254 Ohno. H., Ichikawa.T., Shiokawa, N.. Ino, S., Iwasaki, H.J.. ESCA study on the mechanism of adherence of metal to silica glass, Materials Sci. 1981, 16, 1381-l 390 Ohno, H., Miyakawa, K., Watanabe, K.. Shiokawa, N., The structure of oxide formed by high temperature oxidation of commercial gold alloys for porcelatwmetal bonding, J. Dem. Res. 1982, 61, 1255-l 261 Cascone, P.J.. Oxide formation on palladium alloys and Its effect on porcelain adherence, AADR Absrract 7 72 1983 US Patent No. 4261744, 1981 Khabliyev, S.Z.8.. Sakhanskaya. I.N.. Cheremnykh, V.G. and Lrtvmov, V.S., Phase transformations in a high-duty alloy of palladium with copper and gallium, Phys. Met. Metal/. 1980,47, 187-l 89 Rltamaki, L., A diffraction study of the phase transformations in a Pd-Cu-Ga-Au alloy, Thesis (in Finnish), Department of Physical Sciences, University of Turku, Finland, 1984 Meijering, J.L., Internal oxidation in alloys, in Advances in Materials Research, (Ed. Herbert Hermann) Vol. 5, John Wiley b Sons, Inc. New York, 1971, p 22 Kubaschewski. 0. and Alcock, C.8.. Metallurgic Thermochemistry. (5th Edn) lnr. Ser. on Mater. Sci. and Technol, 24, Pergamon Press, Oxford, 1979, p 253 Kubaschewski. 0. and Alcock. C.8.. /bid. p 378 Chatterji, D. and West, R.W.. Thermodynamic properties of the system indrum-oxygen. J. Am. Ceram. Sot. 1972.. 55, 575 Smith. J.V. and Chatterji, D., EMF investigation of Ga-Ga,O, equilibrium, J. Am. Ceram. Sot. 1973, 56. 288 Lautenschlager, E.P.. Greener, E.H. and Elkington, W.E.. Microprobe analyses of gold-porcelain bonding, J. Dent. Res. 1969, 48.1206-1210 Anusavice, K.J., Horner, J.A. and Fairhurst, C.W., Adherence controlling elements in ceramicmetal systems. I. Precious alloys. J. Dent. Res. 977. 56, 1045-1052

K

74.6’ 82.1 +

106.7’ 56.9

from 980 K from 1123 K from 1073 K

a thin zone depleted in these oxides can be observed. This accumulation indicates a lowering of the free energy of the glass phase by the addition of In and Co oxides in the same way as for instance chromium carbides can be precipitated by certain heat treatments at grain boundaries in austenitic stainless steels creating Cr-depleted zones adjacent to the grain boundaries. Similaraccumulations of Fe, Sn and In in Au-based alloys have been observed previously by means of electron probe microanalyses”. It has been assumed that the glass in the ceramic contains S-0 network with unbridged oxygen atoms which makes it possible for the metal oxide on the substrate surface to dissolve in the glass and thus to create a strong, chemical bond between metal and ceramic3,8~1g. However, the possibility of embrittlement at the interface by such enrichment of alloying elements at the interface cannot be excluded. It is interesting to note that an enriched zone at the interface of Sn in alloy 1 and of Ga and Cu in alloy 2 could not be observed.

6

7

8

9 10 11

12

13

14

15 16 17

ACKNOWLEDGEMENTS The authors are grateful to J.S. Horst for assistance with the SEM investigations and to Dr P. Kofstad for reading the manuscript.

18

19

Precious-metal dental casting Review 1982, 5. 260-288

Biomaterials

1985,

alloys,

Vol 6 March

Inrer-

137