Plasma spray coatings are produced by passing powdered material through a specially designed gun which ionizes an inert gas to form a plasma. Flame temperatures of 10,000 to 30,000°F are reached. Powder is then injected into the plasma flame. This rapidly heated powder is propelled at speeds of 400 to 1000 feet per second onto the part being reconditioned. The resultant coating microstructure consists of thin lenticular particles, or “splats.”
The principal value of the high temperatures of the plasma process is that the melting point of the material being sprayed is reached very quickly. Unlike the oxyacetylene flame (6000°F), powder remains in the hot zone a much shorter time. There is little oxidation and little change in powder chemical composition. Also, the powder can be propelled through the plasma at higher speeds and reach the part being coated with greater impact. In addition, spraying may be done entirely within a protective atmosphere chamber in order to further protect the sprayed material. Minimizing oxides produces a more cohesive coating capable of being finished to a better surface condition. Numerous powder formulations are available to suit the particular application. Powders are available from suppliers such as Metco, Wall Colmonoy, Stellite Division (Cabot) and others.
• Low base material temperatures of 400 to 500°F (205 to 261°C) are maintained during application. No head affected zone is created.
• Minimal distortion or warping.
• No subsequent stress relief or heat treatment required.
• As applied, coatings are relatively smooth and require little grinding to achieve finished dimensions.
• Applicable to a wide variety of base materials.
• Good lubricant retention characteristics.
• Reasonably dense coating structure.
• Low oxide content.
• Low-moderate cost.
• Fair bond strength. Coatings are susceptible to spalling.
• Coatings are porous. Base metal corrosion protection is poor unless sealers are used.
• Thickness of coating is very limited (.006 inch). Excessive coating thickness increases susceptibility to chipping and spalling.
• Coatings may reduce base material fatigue life.
• High dependence on proper base material cleaning and surface preparation.
• Powder quality and application process parameters must be carefully adhered to.
• Coating quality can vary from shop to shop.
• Finish machining is required.
Piston rods usually require a grit blasting, grooving, or knurling operation to achieve an adequate bond between base metal and plasma coatings. It is mandatory that all parts in the process be clean and dry. Frequent in-process and product quality control checks are also necessary.
Because a plasma spray coating is relatively porous, it allows gas to penetrate. Upon release of the gas pressure, the coating may separate from the base metal if a secure bond has not been achieved. Peeling can result in considerable damage to packing and perhaps cylinder components.
To obtain a reasonable degree of corrosion protection, plasma coatings must be impregnated with suitable sealers to minimize porosity.