Elemental Cobalt
Physical Properties: cobalt falls between iron and nickel on the periodic table with an atomic number of 27. Its thermal expansion coefficient lies between those of iron and nickel. The density of cobalt is 8.8 g/cm3. At temperatures below 417°C cobalt exhibits a hexagonal close-packed structure. Between 417°C and its melting point of 1493°C, cobalt has a face-centered cubic structure. The elastic modulus of cobalt is about 210 GPa (30 x 106 psi) in tension and about 183 GPa (26.5 x 106 psi) in compression.

Uses of Cobalt:
Cobalt is also an important ingredient in other materials:

• Paint pigments represent the single largest use of cobalt.
• Nickel-base super alloys
• Cemented carbides and tool steels
• Magnetic materials
• Artificial ?-ray sources

cobalt (which is present typically in the range 10 to 15 wt%) provides solid solution strengthening and decreases the solubility of aluminum and titanium, and increasing the volume fraction of gamma prime precipitate, In the nickel-base super alloys.

The commercially significant cemented carbides contain cobalt 3 to 25 wt%. The role of cobalt in cemented carbides is to provide a ductile bonding matrix for tungsten-carbide particles. As cutting tool materials, cemented carbides with 3 to 12 wt% are commonly used.

Cobalt, which is naturally ferromagnetic, provides resistance to demagnetization in several groups of permanent magnet materials. These include the aluminum-nickel-cobalt alloys (cobalt rang form 5 to 35 wt %), the iron-cobalt alloys (5 to 12 wt %). and the cobalt rare-earth intermetallics which have some of the highest magnetic properties of all known materials.

Cobalt-Base Alloys
Many of the properties of the alloys arise from the crystallographic nature of cobalt, the solid-solution-strengthening effects of chromium, tungsten, and molybdenum, the formation of metal carbides, and the corrosion resistance imparted by chromium. Generally the softer and tougher compositions are used for high-temperature applications such as gas-turbine vanes and buckets. The harder grades are used for resistance to wear.

In the beginning of 20th century Elwood Haynes investigated that many of the commercial cobalt-base alloys are derived from the cobalt-chromium-tungsten and cobalt-chromium-molybdenum. He discovered the high strength and stainless nature of the binary cobalt-chromium alloy. And he later identified tungsten and molybdenum as powerful strengthening agents within the cobalt-chromium system. He promoted the use of it as cutting tool materials because he discovered their high strength at elevated temperatures. He named them the stellite after the Italian star stella because of their star-like luster.

Cobalt-Base Wear-Resistant Alloys
There are some difference between the cobalt-base wear alloys today and the early alloys of Elwood Haynes. The main differences is in the current Stellite alloy grades are carbon and tungsten and it's contents relate to the control of carbon and silicon, Carbon content influences hardness, ductility, and resistance to abrasive wear. Tungsten also plays an important role in these properties.

Chemical composition of Stellite alloys is approximately:
Cr ~ 25-30%
Mo = 1% max
W = 2-15%
C ~ 0.25-3.3%
Fe = 3% max
Ni = 3% max
Si = 2% max
Mn = 1% max.
Co = rest of balance

Types of wear:
The type of wear encountered in a particular application is an important factor that influences the selection of a wear-resistant material. There are several distinct types of wear which generally fall into three main categories:

• Abrasive wear
• Sliding wear
• Erosive wear.

Abrasive wear:
Abrasive wear is encountered when hard particles or hard projections are forced against, and moved relative to a surface. High stress abrasion results from the entrapment of hard particles between metallic surfaces, while low stress abrasion is encountered when moving surfaces come into contact with packed abrasives, like soil and sand. If the abrasive medium is crushed, then the high stress condition is said to prevail. If the abrasive medium remains intact, the process is described as low stress abrasion.

Abrasion resistance is strongly influenced by the size and shape of the hard phase precipitates within the microstructure and the size and shape of the abrading species.

In alloys such as the cobalt-base wear alloys, which contain a hard phase, the abrasion resistance generally increases as the volume fraction of the hard phase increases.

Sliding Wear:
In the three major types of wear, sliding is perhaps the most complex, in the way different materials respond to sliding conditions.

Sliding wear is a possibility when two surfaces are forced together and moved relative to one another. The chances of damage are increased markedly if the two surfaces are metallic in nature, and if there is little or no lubrication present.

Cobalt-Base High-Temperature Alloys
The predominant user of high-temperature alloys was the gas turbine industry. In the case of aircraft gas turbines, the chief material requirements were elevated-temperature strength, resistance to thermal fatigue, and oxidation resistance. For land-base gas turbines, which burn lower grade fuels and operate at lower temperatures, sulfidation resistance was the major concern.

Today, the use of high-temperature alloys is more diversified, as new chemical processing techniques is developed, and more efficiency is sought from the burning of fossil fuels and waste.

Cobalt-base alloys are not widely used as nickel and nickel-iron alloys in high-temperature applications. but cobalt-base high-temperature alloys play an important role, by virtue of their excellent resistance to sulfidation and their strength at temperatures exceeding those at which the gamma-prime- and gamma-double-prime-precipitates in the nickel and nickel-iron alloys dissolve. Cobalt is also used as an alloying element in many nickel-base high-temperature alloys.

Cobalt-base high-temperature alloys have the following chemical composition:

Cr = 20-23%
W = 7-15%
Ni = 10-22%
Fe = 3% max
C = 0.1-0.6%
Co = rest of balance.

Cobalt-Base Corrosion-Resistant Alloys
The cobalt-base wear-resistant alloys are limited by grain boundary carbide precipitation although they possess some resistance to aqueous corrosion. The lack of vital alloying elements in the matrix, and in the case of the cast and weld overlay materials, by chemical segregation in the microstructure.

By virtue of their homogeneous microstructures and lower carbon contents, the wrought cobalt-base high-temperature alloys are more resistant to aqueous corrosion, but still can't be compared with the nickel-chromium-molybdenum alloys in corrosion performance.

To satisfy the industrial need for alloys which exhibit outstanding resistance to aqueous corrosion, yet share the attributes of cobalt as an alloy base several low-carbon, wrought cobalt-nickel-chromium-molybdenum alloys are produced.

Chemical composition of these alloys is:

Cr = 20-25%
W = 2%
Mo = 5-10%
Ni = 9-35%
Fe = 3% max
C = 0.8% max
N = 0.1% max
Co = rest of balance.