Model SG-100 Plasma Spray Gun
Operator’s Manual
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Issued: 12/11
Page 31
4.1 Plasma Spray Shop Notes
Factors that contribute to a quality plasma sprayed coating include
equipment choice, powder quality, surface preparation, gun operating
parameters, powder feed rates, part and gun manipulation, part cooling,
and overspray cleaning. Disregarding any of these factors causes poor
coating quality.
A. Choice of Proper Electrodes
Several operating regimes and powder injection schemes are necessary to
spray the variety of materials used for plasma spraying. This is because of
the variations in their physical and thermal properties. The relative gas
velocity for Praxair Surface Technologies’ (PST) plasma guns is given in
terms of subsonic, MACH I, and MACH II. Gas velocity dictates powder
particle velocity. PST guns are made in configurations for spraying both
external and internal surfaces. Additionally, internal and external powder
injection points are available, with a range of injection angles, to
accommodate the melting characteristics and particle sizes of the range of
powders to be sprayed. This flexibility enables proper heating of the
powder preventing the deposition of unmelted powder or the vaporization
of fine particulate.
Each coating material requires a different electrode set to obtain the
desired coating characteristics, e.g., density or adhesion.
B. Powder Sprayability
Most powders that have a definite melting point and a liquid phase can be
plasma sprayed. Some powders may require special processing. For
example, zirconium oxide powders must be partially stabilized to control
the phase changes during heating and cooling. Hydrides of reactive metals
are sometimes used to overcome oxidation problems.
Particle size (mesh size), distribution (mesh range), and tolerances must be
considered from both sprayability and economic standpoints. Powders
larger than 150 mesh are not normally recommended for plasma spraying.
Normally, coarser powders yield thicker coatings with lower residual
stress (higher bond strength). Finer powders produce coatings that are
smoother in the as-sprayed condition and have a finer microstructural
texture. However, residual stress is higher with finer powder, which limits
thickness buildup and decreases measured bond strength. Good cuts of
powder, whether coarse or fine, will produce dense coatings.