Cobalt Alloys 101: When Extreme Wear Resistance Really Counts

Most engineers encounter cobalt alloys later in their careers than they probably should. They'll appear on the material selection chart and usually trail titanium and the nickel alloys. They'll only really come into play once you've been unable to solve your design challenge using the more readily available materials.

That is something of a disservice to a family of materials with a genuinely distinctive set of properties — properties that, in the right application, no other metal class can match.

The Role of Cobalt in Extreme Wear Resistance

The starting point is wear resistance, but that term covers a broad range of failure mechanisms, so we'll want to be a bit more precise about how it works in this instance. There are several failure modes in which cobalt alloys excel, and these include: 

·         Abrasive wear - in which a harder surface removes material from a softer surface

·         Adhesive wear - in which bonding between surfaces occurs under load and causes material transfer from one surface to another… and then

·         Erosive wear - in which particle-containing fluids or gases remove surface material

These wear modes are successfully countered because of the microstructure of the cobalt alloy. While the metal itself possesses high intrinsic hardness, the real magic lies in the presence of carbides of chromium, tungsten, or mixed composition, which are precipitated throughout the alloy.

These particles resist wear, taking the strain of surface stresses. As the softer metal around them erodes through use, new particles come to the surface, maintaining a high level of hardness even after considerable use.

It's also worth mentioning a second effect that plays a part: cobalt undergoes a phase transition under mechanical loading. It changes from its high-temperature face-centred cubic form to a hexagonal close-packed form. This takes up the loading energy, contributing to work hardening of the surface and ensuring that the material resists further damage under service loads.

Hot Hardness: The Unique Properties of Cobalt Alloys

For many alloy systems, hardness and wear resistance drop off dramatically with rising temperature. This is a manageable problem in applications operating in ambient environments but becomes much more significant as the component begins to see elevated temperatures.

For the most common grades of cobalt alloys, that hot hardness is extremely pronounced. The Stellite family retain good hardness above 600°C, with some grades maintaining this property up to 800°C or more. In a way, this hot hardness property ensures that cobalt alloys are not just applicable to wear applications in general, but to hot wear applications: a tougher category altogether.

Applications Where Cobalt Excels

The types of applications which make use of cobalt alloys tend to have some common features: they involve repeated contact between surfaces under load, often at elevated temperatures, and are usually in a chemically aggressive environment, adding to the complexity of the design challenge.

Cobalt alloys feature in power generation applications as valve seats, turbine blade tip shields and wear pads where there is repeated metal to metal contact. Here, the combination of thermal resistance and wear resistance means that cobalt is the obvious choice.

In aerospace applications, cobalt alloys are often employed in bearing surfaces, bushings and fastener areas where galling is a concern. Galling, a type of adhesive wear, is particularly troublesome in expensive and complex assemblies, and here, cobalt alloys have superior resistance to most stainless and alloy steels.

Which Grade Do You Need?

The cobalt alloy family is not homogenous. There are big differences in the percentage of carbides present, in chromium or tungsten levels, and consequently there are variations in wear resistance, toughness and formability. An alloy optimised for hard-facing by welding will be different from one which will be supplied as wrought bar stock or casting. Getting the alloy right is key to successful component performance.

Dynamic Metals stocks cobalt alloys in a variety of forms and grades, all with complete traceability and material certification as standard. If you have requirements for a particular grade of cobalt alloy, contact our engineering team directly.