Gold alloys using

Gold Casting and Wrought Alloys. Gold alloys useful ki dentistry may contaki gold, silver, platinum, palladium, iridium, kidium, copper, nickel, tin, kon, and zkic. Other metals occasionally are found ki minor amounts. The effect of each of the constituents is empirical, but some observations have been made. 

Bocking C (1994) High speed selective jet electrodeposition of gold and gold alloys using single circular jets. Trans Inst Met Finish 72 133-140... 

The solder alloy has a lower melting range than that of the parent metal or metals being joined, i.e the liqnidns of the solder is below the solidns temperatnre of the karat gold alloy used to make the parts to be soldered. 

In the electronics industry, gold is used as fine wires or thin film coatings and frequendy in the form of alloys to economize on gold consumption and to impart properties such as hardness. Gold has properties that satisfy specific requirements not achievable with less expensive metals (see Electrical connectors Electronics coatings Thin films). 

Electroforrning, which is used in the production of art objects or jewelry is a combination of electroless plating and electro deposition. A wax mold of the object to be produced is made conductive by electroless gold plating, a thick layer of gold or gold alloy is then electrodeposited and, finally, the wax is removed by melting (134). 

Sodium does not form alloys with aluminum but is used to modify the grain stmcture of aluminum—silicon alloys and aluminum—copper alloys for improved machinabiUty. Sodium—gold alloy is photoelectricaHy sensitive and may be used ia photoelectric cells. A sodium—2iac alloy, containing 2 wt % sodium and 98 wt % 2iac, is used to deoxidi2e other metals. 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

Copper [7440-50-8] Cu, produces a reddish color and reduces the melting pokit of the alloy. It produces heat-treatable compositions with gold, platinum, and palladium that result ki kicreased hardness, strength, and generally improved physical properties. The tarnish resistance of the alloy is usually decreased. The gold—copper, Au—Cu, system is the fundamental system of many dental gold alloys. Copper has a useful range of 0—20 wt %. 

GoldJilloys, Cast Types. Four types of gold alloys have been recognized for cast dental restorations (Table 5). They provide desired material for specific uses. The appropriate specifications for these alloys is ANSI/ADA specification no. 5. 

Type I, soft alloys (20—22-carat golds), are used for inlays of simpler non-stress-bearing types. Type I gold alloys can be burnished, and are not heat-treatable. They are composed essentially of gold—silver—copper with minor modifying additions, eg, zinc. 

Type II, medium-hard alloys, are harder, stronger, and have lower elongation than type I alloys. They are used for moderate stress appHcation, eg, three-quarter crowns, abutments, pontics, full crowns, and saddles. The type II gold alloys are difficult to burnish, and can usually be heat-treated. 

Table 7 gives the composition of gold alloys available for commercial use. The average coefficient of thermal expansion for the first six alloys Hsted is (14-15) X 10 j° C from room temperature to ca 1000°C two opaque porcelains used with them have thermal coefficient expansion of 6.45 and 7.88 X 10 from room temperature to 820°C (91). The HV values of these alloys are 109—193, and the tensile strengths are 464—509 MPa (67-74 X 10 psi). For the last four alloys in Table 7, the HV values are 102—216, and the tensile strengths are 358—662 MPa (52-96 x 10 psi), depending upon thermal history. 

Table 7. Composition of Gold-Based Alloys Used in Ceramo—Metallic Prostheses, wt %...

GoldJilloys, Wrought Type. Two types of wrought gold alloys were formerly recognized by the ADA specification no. 7 for the fabrication of orthodontic and prosthetic dental appHances, ie, type I, high-precious-metal alloys, and type II, low-precious-metal alloys (gold color). Alloys of this type are seldom used in the United States they have been replaced by stainless steels and nickel—titanium alloys. 

Alloys based on Ag—Pd have been used for a number of years and are available from most gold alloy manufacturers (148). The palladium content is 22—50 wt % silver content is from 35 to 66 wt %. Minor amounts of Zn, In, or Sn are often present to increase fluidity. Both In and Sn form intermetaUic compounds with both Pd and Ag and, therefore, some of the commercial alloys are susceptible to age hardening (149). These alloys are somewhat difficult to fabricate and require meticulous processing. They may also produce a greenish discoloration when they are fused with porcelain veneers. Nevertheless, clinical experience generally has been satisfactory, and cost is the primary criterion for use. 

In addition to the metals Hsted above, many alloys ate commercially electroplated brass, bronze, many gold alloys, lead—tin, nickel—iron, nickel—cobalt, nickel—phosphoms, tin—nickel, tin—zinc, ziac-nickel, ziac-cobalt, and ziac-iron. Electroplated alloys ia lesser use iaclude lead—iadium, nickel—manganese, nickel-tuagstea, palladium alloys, silver alloys, and zinc—manganese. Whereas tertiary and many other alloys can feasibly be electroplated, these have not found commercial appHcations. 

Pure ruthenium powder or mixed ruthenium-molybdenum powders have been found able to effect good joints between molybdenum and tungsten. A eutectic melting above 1 900°C is formed, and joints produced in hydrogen atmospheres at 2 100°C operate satisfactorily at 1 500°C. A cobalt-palladium-gold alloy has also been reported to be useful in brazing molybdenum. 

Shakudo. Shakudo is a copper alloy used in Japan since the third century b.c.e. for making decorative and ornamental objects. In addition to copper it also includes gold (1—4%), silver (about 2%), and lead (about 1%). Occasionally gold replaced part or even all of the silver. Exposed shakudo surfaces acquire a layer of patina consisting mainly of the mineral cuprite (composed of cuprous oxide), which exhibits a characteristic shine. Varying amounts of gold in the alloy are said to have a marked effect on the color and the shine (Notts 1988 Oguchi 1983). 

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