Silver-palladium alloys

The silver–palladium alloys contain little or no gold and as their name suggests contain primarily silver and palladium. There is generally a minimum of 25% palladium along with small quantities of copper, zinc and indium, in addition to gold which is sometimes present in small quantities.

The silver–palladium alloys have significantly lower density than gold alloys, a factor which may affect castability. For a given volume of casting, there is a lower force generated by the molten alloy during casting. In this regard, there appears to be a significant variation amongst brands, indicating that small variations in composition can affect castability. Some silver–palladium alloys have castability characteristics which compare favourably with gold alloys. Attention must be paid to details such as casting temperature and mould temperature if the mould is to be adequately filled by the alloy.

Alloys containing large quantities of palladium have a propensity for dissolving oxygen in the molten state which may lead to a porous casting. Care must be taken to avoid overheating or oxidation of the melt during casting.

The properties of silver–palladium alloys are similar to those of type 3 gold (Table 7.5). Materials claiming compliance with the ISO Standard for casting alloys having a noble metal content of greater than 25% must meet the specification limits set out in Table 7.4. There are significant variations in properties between individual brands within each of the two groups. Whereas most gold alloys have high ductility as demonstrated by an elongation value above 15%, some silver– palladium alloys have much lower ductility. The corrosion resistance is not as good as for gold alloys but, provided the palladium content is above 25%, the amount of corrosion expected during service is negligible.

Table 7.5 Properties of a typical Ag/Pd alloy compared with a type 3 gold alloy.

These alloys offer a suitable alternative to gold alloys provided that care is taken during casting. They offer a considerable saving in cost when compared with gold alloys.

Methods used to assess resistance to corrosion and tarnish are outlined in Section 2.7.

7.7 Soldering and brazing materials for noble metals

Some larger restorations/appliances produced from cast alloys are produced in two or more parts which are then joined together by soldering or brazing. This approach is used to overcome distortions that may occur during cooling for metal frameworks for long span bridge work, and also when joining metal components formed from different alloys, e.g. a type 4 gold veneer crown may be joined to a precious metal porcelain bonding alloy structure for bridgework. In dentistry the terms soldering and brazing are used interchangeably. However, in normal metallurgical terms the two processes, although trying to achieve the same goal, are carried out at different temperatures, brazing being a process carried out at higher temperatures.

The definition of a dental brazing material given in ISO Standards is ‘an alloy suitable for use as a filler material in operations in which dental alloy(s) parts are joined to form a dental restoration’.

The requirement of a brazing material for noble alloys is that it be capable of forming a strong, corrosion resistant joint between the two components being joined without changing their structure or properties. Hence, the tensile strength of the brazed joint is required to exceed the 0.2% proof stress for the weakest of the two components. There should be no visible signs of corrosion when a brazed joint is soaked in a solution of lactic acid and sodium chloride for 7 days. In order that the components being joined do not suffer from recrystallization, grain growth or distortion during the brazing procedure it is important that the brazing material has a flow temperature which is well below the melting point of the alloys being joined. The flow temperature is defined as the lowest temperature at which the filler material is fluid enough to flow into the gap and to wet the surface of the metallic parts. Solders for joining noble metal components are formulated from mixtures of gold, silver and copper designed to have low fusion temperatures. This is

achieved through the eutectic nature of certain silver/copper alloys and through the addition of tin and zinc such that the fusion temperature of the solder is generally 50–100ºC below that of the casting gold alloys.

Solders are susceptible to oxidation during the melting/softening procedure and the resulting oxides can weaken the soldered joint. Furthermore, the metal components being joined are often coated with a thin oxide film which can limit the ability to achieve proper jointing. Fluxes are employed to break down the surface oxide layers on metals and to prevent oxidation of the solder. Fluxes commonly used are fluoride salts and borax. Fluoride salts are particularly useful in breaking down the tenacious oxide film which forms on the surface of chromium containing alloys.

7.8 Noble alloys for metal-bonded ceramic restorations

Alloys of precious metals are commonly used for the fabrication of metal-bonded ceramic restorations. Their requirements are similar to those described in this chapter but with additional requirements related to the following:

(1)thermal stability during firing of the ceramic;

(2)bonding to the ceramic;

(3)compatibility with the ceramic;

(4)support for the ceramic.

For further information see Chapter 11.

7.9 Biocompatibility

Gold and other precious and semi-precious alloys are generally regarded as having good biocompatibility. However, the literature contains some disturbing reports of severe cases of allergy to gold alloys and high palladium content alloys. Though rare, these events serve to remind us of the need to assess and monitor all materials in the clinical situation on a case by case basis. For further details, see page 65.

7.10 Suggested further reading

Moller, H. (2002) Dental gold alloys and contact allergy. Contact Dermatitis 47, 63.

Wassell, R.W., Walls, A.W. & Steele, J.G. (2002) Crowns and extra-coronal restorations: materials selection. Br. Dent. J. 192, 199.

8.1 Introduction

Base metal alloys contain no gold, platinum or palladium. Their composition and properties are specified in three ISO standards for alloys. ISO 6871 specifies composition limits and requirements for dental base metal casting alloys used to construct removable dental appliances. There are two parts to this standard reflecting the two main groups of materials used, namely the Co/Cr alloys (part I) and the Ni/Cr alloys (part II). ISO 16744 specifies requirements for base metal alloys used to construct fixed dental restorations. There is no provision within this latter standard to further describe alloys according to composition (except for limiting the quantity of hazardous metals) but greater emphasis is placed upon classifying materials according to properties and aligning these properties to the four types of casting gold alloys described in the previous chapter.

8.2 Composition

The composition of alloys is described according to the limits given in the appropriate ISO standards.

Cobalt-chromium alloys

The chemical composition of these alloys specified in the ISO Standard for Dental Base Metal Casting Alloys (Part 1) is as follows:

A typical material would contain 35–65% cobalt, 25–35% chromium, 0–30% nickel, a little molyb-

denum and trace quantities of other elements such as beryllium, silicon and carbon. Cobalt and nickel are hard, strong metals. The main purpose of the chromium is to further harden the alloy by solution hardening and also to impart corrosion resistance by the passivating effect. Chromium exposed at the surface of the alloy rapidly becomes oxidized to form a thin, passive, surface layer of chromic oxide which prevents further attack on the bulk of the alloy. The concentrations of the minor constituents have a greater effect on the physical properties of the alloys than do the relative cobalt–chromium–nickel concentrations. The minor elements are generally added to improve casting and handling characteristics and modify mechanical properties. For example, silicon imparts good casting properties to a nickelcontaining alloy and increases its ductility. Likewise, molybdenum and beryllium are added to refine the grain structure and improve the behaviour of base metal alloys during casting. Carbon affects the hardness, strength and ductility of the alloys and the exact concentration of carbon is one of the major factors controlling alloy properties. The carbon forms carbides with any of the components and its concentration depends on both the amount added by the manufacturer and that which may be inadvertently introduced during casting if the alloy is melted with an oxyacetylene torch. The presence of too much carbon results in a brittle alloy with very low ductility and an increased danger of fracture. During crystallization the carbides become precipitated in the interdendritic regions which form the grain boundaries. The grains are generally much larger than those produced on casting gold alloys and it is possible for the precipitated carbides to form a continuous phase throughout the alloy. If this occurs the alloy becomes extremely hard and brittle as the carbide

phase acts as a barrier to slip. A discontinuous carbide phase is preferable since it allows some slip and reduces brittleness. Whether a continuous or discontinuous carbide phase is formed depends on the amount of carbon present and on the casting technique. High melting temperatures during casting favour discontinuous carbide phases but there is a limit to which this can be used to any advantage since the use of very high casting temperatures can cause interactions between the alloy and the mould.

Nickel-chromium alloys

The chemical composition of these alloys specified in the ISO Standard for Dental Base Metal Casting Alloys (Part 2) is as follows:

As for the Co/Cr alloys the concentrations of minor ingredients can have a profound effect on properties. The concentration of carbon and the nature of the grain boundaries are major factors in controlling the properties of these alloys.

Alloys for fixed restorations

These alloys are similar to those described within the previous two categories but the composition limits are less prescriptive. This approach used in ISO 16744 represents a trend in the development of standards in which more emphasis is put on meeting the requirements for certain key properties than on composition. These alloys consist of a variety of mixtures of Co, Cr, Ni and Mo but there are strict limits on the use of materials considered to be hazardous. For example, the limit for beryllium and cadmium is only 0.02% whilst alloys containing more than 0.1% nickel are required to carry a hazard warning.

8.3 Manipulation of base metal casting alloys

The fusion temperatures of the Ni/Cr and Co/Cr alloys vary with composition but are generally in the range 1200–1500ºC. This is considerably higher than for the casting gold alloys which

rarely have fusion temperatures above 950ºC. Melting of gold alloys can readily be achieved using a gas–air mixture. For base metal alloys, however, either an acetylene–oxygen flame or an electrical induction furnace is required. The latter method is to be favoured since it is carried out under more controlled conditions. When using oxyacetylene flames the ratio of oxygen to acetylene must be carefully controlled. Too much oxygen may cause oxidation of the alloy whilst an excess of acetylene produces an increase in the metal carbide content leading to embrittlement.

Investment moulds for base metal alloys must be capable of maintaining their integrity at the high casting temperatures used. Silica-bonded and phosphate-bonded materials are favoured with the latter product being most widely used. Gypsum bonded investments decompose above 1200ºC to form sulphur dioxide which may be absorbed by the casting, causing embrittlement. This effect can be reduced by the incorporation of oxalate in the investment, however the problem is generally avoided by choosing an investment which is more stable at elevated temperatures.

The density values of base metal alloys are approximately half those of the casting gold alloys. For this reason the thrust developed during casting may be somewhat lower, with the possibility that the casting may not adequately fill the mould. Casting machines used for base metal alloys must therefore be capable of producing extra thrust which overcomes this deficiency. The problem may be aggravated if the investment is not sufficiently porous to allow escape of trapped air and other gases. Careful use of vents and sprues of adequate size is normally sufficient to overcome such problems.

The greatest expense involved in producing a Co/Cr dental casting is in the time required for trimming and polishing. In the ‘as cast’ state, the alloy surface is normally quite rough, partially due to the coarse nature of some investment powders. Finer investments can be used to give a smoother surface requiring less finishing. One common technique involves painting the wax pattern with fine investment – this then forms the inner surface of the investment mould. The bulk of the mould is then formed from the coarser grade material.

Base metal alloys, and particularly the Co/Cr type, are very hard and consequently difficult to polish. After casting, it is usual to sandblast the metal to remove any surface roughness or adher-