Wear Resistant Alloys & CoatingsCobalt is used in two ways to give hard, corrosion/erosion resistant, high temperature coatings. Firstly, carbide coatings containing up to 17% cobalt can be deposited by flame and plasma guns on to softer substrates to give the finish and hardness of carbide – work rolls, mixers, grinders, etc. The main interest in this section is the metallurgical alloys based on cobalt, whose primary aim is wear resistance and which may be applied by surface coating or used as castings and forgings.
Secondly, alloys used in this field are based on the Stellite® alloys developed in the early 1900’s, although coatings have moved on into cobalt-containing nickel based alloys as well, strictly for corrosion resistance.
The Stellites® were originally used as cutting tools and whilst this use has mainly been replaced by carbide, it does remain. More often now however, the CoCrW alloys are used to coat other metals or are used as castings wherever their unique erosion resistance and high temperature properties are needed. They also form the basis of the prosthetic alloys used to produce hip and knee replacement joints.
The spray alloys used for plasma or flame spray are in powder form and contain silicon and boron to form a low melting point eutectic which allows fusion with the substrate with minimum distortion.
In general, the cobalt-based alloys can be deposited by:
Welding – both rods and strip are available – MIG, TIG, submerged arc, oxy-acetylene, etc.
Plasma/flame spray – powders are available for both these processes or rod feed can be used
They can be cast and used as complete parts or as inserts – i.e. titanium hip joint with Co/Cr ball
Cobalt in Electroplating
Cobalt can easily be deposited from a number of electrolytes. However, it must be said that in the main, pure cobalt plated layers are not of great commercial interest. The interest lies in plating wear resistance coatings and those used in the magnetic recording industry.
Cobalt can be deposited from a bath containing non-metallics such as alumina, SiC, CrC, etc. These can be encased in the plated layer to a volume of 30% producing a hard, wear resistant coat, even at elevated temperatures. These find application in the aerospace, aircraft and automobile industries.
Coatings with Co/Ni and Co/W being simultaneously deposited and hardened by suspended non-metallics are also possible.
Nickel plating for decorative and industrial applications (electroformed moulds for glass and plastic) is perhaps more common than cobalt. Adding cobalt as chloride and sulphate produces a bright nickel deposit.
Figure 1 shows Ni/Co electrodeposited coats of varying cobalt levels showing peak hardness at circa 25/30% cobalt.