Metals and alloys used for crowns, bridges and porcelain which are made of metals and alloys resist the humidity degradation, pH variations and temperature changes. Scientists have done an effort for development new metal alloys in order to knowing their interaction with the human body as mechanical properties , corrosion behavior, bio-functionality and biocompatibility, that is because catastrophic results caused by non-biocompatible metals that are used in their fabrication.
Titanium, their alloys, cobalt based alloys, austenitic and martensitic steels are used for implants and medical instruments. Today in the dental industry are used cobalt and nickel based alloys as an economical alternative to precious gold and silver due to their mechanical properties and biocompatibility. Also cobalt-chromium alloys are widely used in prosthetic dentistry for fabrication of removable partial dentures and some fixed prosthetic appliances because it has low weight, great hardness and strength, resistance to tarnish and corrosion. Solid cobalt-chromium alloys contained about 64% cobalt and 29% chromium which they are major components with a minor as molybdenum, carbon, manganese, iron, silicon, beryllium, tungsten, boron and titanium which caused a great influence on mechanical and physical properties. Many alloys contained nickel are used in dental syringes and hand instruments but stainless steel equipment makes contact with the patient's tissues. Dental stainless steel contained chromium, nickel, carbon, iron and other trace materials, generally it contains less than 15% nickel.
I. Alloys used as porcelain fuse to metal restoration
Gold alloys (74% to 88% gold) with small amount of metals (indium, tin, iron and gallium) are added to promote both the strength and a porcelain bond to metal oxide. High nobility alloys have excellent corrosion resistance. Adding silver to (50% gold and 40% palladium) gold-palladium alloy, is improve its thermal expansion, which is incompatible with some of the higher expansion porcelains. Alloy from Gold- (30%) palladium – (15%) silver alloy has a tendency to a change the porcelain color because of their silver content. Alloys from palladium (70% to 80%) – copper (15%) – gallium (9%) are used for full- cast and metal ceramic restorations. Alloys from (50%) Palladium -silver have good corrosion resistance, solder well and lowest sag tendency of the precious metal alloys.
Intermetallic compounds, formed from aluminum and nickel metals, enhance strength and hardness of nickel-chromium alloys contained 70% nickel, 16% chromium, 2% aluminum and 0.5% beryllium but beryllium lowers the melting range, enhances fluidity and improves grain structure. Alloys from cobalt (60%) – chromium (25-30%) contained minor amount of metals as molybdenum, aluminum, iron, gallium, copper, silicon, carbon, platinum imparts corrosion resistance but manganese and silicon enhance fluidity of its molten alloys.
Titanium and their alloys differ from the gold-based alloys as use for full-cast restorations due to high melting temperature to accommodate temperatures required for porcelain application and its contained a little silver caused the porcelain to turn green, with a little copper caused the alloy to sag during porcelain firing.
II. Alloys used for removable partial dentures
Gold, chromium and cobalt based alloys are used in removable partial dentures. While the gold based alloys are strong enough to function adequately when use to make frameworks, the Cr and Co based alloys differ from metal ceramic counter parts due to contain a small amount (0.1-0.5%) of carbon for hardening.
III. Cobalt
Cobalt metal is used in many commercial industrial, military applications and aerospace to make parts for gas turbine aircraft engines. It is a hard, ferromagnetic, silver-white, lustrous, brittle element and is a member of group VIII at the periodic table. Cobalt can be magnetize and its physical properties are similar to iron and nickel. Cobalt is chemically active, slowly attacked by dilute acids, stable in air and unaffected by water. It is used as corrosion resistant alloys, high speed steels, cemented carbides, magnets and magnetic recording media, catalysts for the petroleum and chemical industries, drying agents for paints and inks. Cobalt blue is an important part of artists' palette and used by craft workers in porcelain, pottery, stained glass, tiles and enamel jewelry. But the radioactive cobalt-60 isotopes is used in medical treatment, irradiate food in order to preserve it and protect the consumer.
Cobalt- base alloys
Since 1929, removable partial dentures is used from cobalt-chromium alloys but cast metal prosthetic devices produce from gold alloys because their ductility, low corrosion and rability in the oral environment. Before 1970, the non-gold alloys is used are base metal alloys which contained cobalt, chromium and nickel. These alloys still being used in the construction of removable partial dentures as an alternative to gold alloys for economic reasons and low density compared to traditional gold alloys. The used elements with their weight percentage for some nickel and chromium based metal alloys are listed in Table (1.1a).
Table (1.1a):- composition of some alloys used in removable dentures
Alloys Elements
Cr Co Ni Fe Mo W Mn Si C Be Al
Co-Cr 30 62.5 – 1 5 – 0.5 0.5 0.3 – –
Ni-Cr 17 – 67 – 5 – 5 0.5 Trace – –
Co-Cr-Ni 26 54 14 0.1 4 – 0.8 0.6 0.2 – –
Fe-Cr 24 6 4 63 2.5 – – – Trace – –
Cobalt is found in cobaltite, smaltite, erythrite minerals and mainly in ore chromites. It is used to harden steel, manufacture of stainless steels and steel alloys. Cobalt and their alloys are non-ferrous metals with high strength and toughness, excellent corrosion and oxidation resistance and high-temperature strength. Cobalt alloy is ferromagnetic, non-ferrous composite contained cobalt as metallic element with a higher melting point than nickel or iron alloys. Cobalt and their alloys can absorb stress to higher absolute temperature than other metals, provide excellent thermal fatigue resistance and can used in electroplating. Metals contained high chromium content is used in gas-turbine atmospheres due to its resistance to corrosion at high temperatures.
The main source of molybdenum is ore molybdenite. Molybdenum is a silver-white metal and soft. It is used as alloying agent to increase the hardness and toughness of quenched tempered steels and some nickel-based alloys. Cobalt- chrome- molybdenum alloys resist abrasion and corrosion then often use in hip and knee replacement systems.
The development of the wear resistant for cobalt alloys started in the first decade of the 19th century with the development of the stellite alloys. Super cobalt-based alloys are suitable as use in turbine blades, gas turbines and jet aircraft engines because it's temperature stability. The stellite alloys are cobalt chromium alloys contain different ratio from tungsten and carbon content and they are very hard and wear resistant due to the formation of chromium and tungsten carbides. Special cobalt chromium molybdenum alloys, as Vitallium, are used for hip and knee replacements. Cobalt alloys are used for dental prosthetics, where they are useful to avoid allergies to nickel. Some high speed steels are also used cobalt to increase heat and wear resistance. Alnico is the special alloy makes from aluminum, nickel, cobalt and iron. Samarium cobalt alloys are used as permanent magnets.
In the beginning of 20th century, Elwood Haynes discovered the commercial ternaries cobalt-chromium-tungsten, cobalt-chromium-molybdenum alloys and the binary cobalt-chromium alloy with the high strength and stainless nature. Later Haynes identified tungsten and molybdenum as powerful strengthening agents within the cobalt-chromium system due to its high strength at elevated temperatures. He also promoted the use of Stellite alloys as cutting tool materials. Today cobalt-base wear alloys are little changed from the early alloys discovered by Elwood Haynes. Indeed, the most significant differences in the current Stellite alloy grades are carbon and tungsten contents, carbon content influences hardness, ductility and resistance to abrasive wear, and tungsten plays an important role in these properties.
Generally the cobalt base alloys is described by wear, corrosion and heat resistant. The softer and tougher compositions of these alloys are used for high temperature applications such as gas turbine vanes and buckets but, the harder alloys are used for resistance to wear. Cobalt-chrome is a metal alloy of them used in denture components and orthopedic implants. The principal metals are cobalt, chrome and molybdenum with remain traces as nickel, iron and carbon. The composition and physical properties of some cobalt- chromium alloys are listed in Table (1.1b).