Types of Dispersion Alloy Coating Systems

Dispersion alloy coatings combine the finite microstructure of a dispersed particle into a macrostructure. These coatings can be categorized as composites containing lubricant particles or those containing hard particles. These dispersed particles are incorporated into a metal matrix of nickel phosphorus or cobalt phosphorus. In most applications an undercoat is used to increase the corrosion resistance and improve the coverage of the dispersion coating. An overplate is also used to seal particles into the surface and produce a more consistent initial wear.

Nonelectrolytic solid-lubricant dispersion alloys are produced from solutions that use lubricant powders suspended in an aqueous electroless nickel or cobalt system. The powders are sensitized by cationic surface agents and dispersed by other surfactants.

There are commercial processes that use PTFE and CFx as well as fluorinated carbon and fluoride salts of lithium, calcium, and magnesium. Each of these lubricant particle systems has specific advantages. Generally PTFE is used where the loads are low. Fluorinated carbon powders can operate at higher temperatures and greater loads. Fluoride salts provide lubrication at temperatures near the melting point of the coating.

Nonelectrolytic Hard-Particle Dispersion Alloys. A wide range of hard particles can be incorporated into the deposits, including diamonds, silicon carbide, tungsten carbide, titanium carbide, silicon nitride, and others. The basic requirement for the process is that the particles remain passive while deposit is proceeding and at the same time come to rest on the surface and become incorporated into the coating. These particles are generally between 0.4 and 3 t'm in diameter and are loaded into the deposit from between 15% and 25% by weight. Coatings using these particles are generally designed to support moderate to high loads.

Silicon carbide in a nickel phosphorus matrix is the most common of the dispersed particle coating systems. These coatings provide abrasion resistance in mining and chemical processing applications and are used in internal combustion engines because the deposit provides for the retention of oil on the surface. Deposits containing dispersed diamond particles have provided significant improvement in the performance of textile-weaving equipment by reducing abrasive wear caused by textile fibers.

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