Cobalt-based alloy coatings can confer wear-resistance to a product.
Cobalt has long been associated with wear resistant materials, both as an alloying element and as a binder in cemented carbides. Cobalt can deliver wear resistance by being applied within castings and forgings or as a coating for a low cost substrate.
When selecting the appropriate wear resistant material, it is important to develop properties that just provides sufficient resistance. If too much wear resistance, the economic cost of producing the part will be too large.
Conditions of wear
There are three conditions of wear: abrasion, sliding and erosion.
Abrasion refers to when hard particles or projections are forced against and moved, relative to a surface. Material is lost from the surface by a ploughing or a chipping action depending on whether the surface is ductile or brittle. Abrasive wear can take the form of a high or low stress condition which relates to the state of the abrasive medium after interaction with the surface. In high stress abrasion, the hard particles or projections are crushed; during low stress abrasion they remain intact.
During sliding wear damage is either limited to the oxide films (which naturally occur on metallic surfaces) or the underlying substrates. Oxide control of the sliding wear process is common to systems which operate at low contact stresses and high surface temperatures. Damage to the underlying metallic substrate which is referred to as galling, can be caused either by oxide breakdown, leading to cold welding and shear of the weaker of the two mating materials or by sub-surface fatigue.
Erosion is associated with bubble formation and collapse in fluid streams. Bubbles are deformed and then destroyed by pressure changes in the streams resulting in surface damage caused by the formation of the liquid jets that occur during bubble collapse. It is believed that successive shock waves and high cycle fatigue on a fine scale are responsible for material being removed during the cavitational erosion process.
Cobalt-based wear resistant alloys
There are three main cobalt-based families within wear resistant alloys: Stellite, Tristelle and Tribaloy.
Developed in the early 1900s by Elwood Haynes, Stellite alloys were originally designed for use in the then new invention of automobiles. Stellite alloys usually contain around 50% cobalt (Co) with 30% of chromium (Cr). The Cr provides strength and corrosion resistance to the alloy matrix. Cr also forms carbides within the structure during alloy solidification. Molybdenum (Mo) and tungsten (W) provide the matrix with strengthening properties.
In 1985 a new range of stainless alloys were introduced with properties that provide exceptional protection from sliding wear and cavitational erosion. Tristelle alloys contain iron (Fe), Co and nickel (Ni). Co and silicon (Si) were added to increase resistance to galling and cavitational erosion whilst lowering the stacking fault energy. In this system Co also provides extra solubility for the silicon without being detrimental to the atomic structure of the stainless. Cr and carbon (C) are also introduced in the hard phase to increase abrasion resistance.
The Tribaloy range of high-performance alloys is composed of stable, non-reactive metals which contain Co and inclusions of Mo, Cr and Si. Such a composition provides outstanding resistance to wear and galling, good corrosion resistance and a suitability for applications where lubrication is a problem. Applications are mainly with powders for hard facing and as additions to the powders for the manufacture of sintered parts.
In general, the cobalt-based alloys can be deposited by:
Welding as both rods and strips
Plasma/flame spray which use powders or rod feed
Cast and used as complete parts or as inserts e.g titanium hip joint with Co/Cr ball
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.