As the gas mixture burns, the powder particles are melted and accelerated
down the barrel. Temperatures above 6000 ºF are attained with the combustion chamber while the substrate
temperature is maintained below 350 ºF by a gas cooling system. Particle velocities of approximately 2,500
ft/s are produced. The kinetic energy released by impingement upon the substrate contributes additional heat and
promotes bonding, high density, and appreciable hardness values. The coating is built up to the specified thickness
while the workpiece is rotated or passed in front of the gun.
Surface Preparation is the most critical step in a thermal spraying operation. Coating adhesion quality is directly related to the cleanliness and
roughness of the substrate surface. The coating material and the substrate type are the major factors in determining
what surface preparation is necessary to achieve consistent bonding. Sprayed deposits do not add to the strength of
the substrate. The purposes of surface roughening are as follows: provide compressive surface stressesprovide
interlocking laminations (layers)increase the bond areadecontamination of the surfaceInternal stresses from shrinkage
develop in coatings and these stresses increase with increased coating thickness. The stresses are more severe in hard
metals or ceramics. Roughening is a method of reducing these stresses by dividing the internal stresses into smaller
components which will cancel each other out. As the layers are folded up and down, the coating strength is improved.
Surfaces subjected to low stress abrasion show that material has been removed by hard, sharp particles or
other hard, sharp surfaces plowing material out in the furrows.
Grinding with a surface grinder can be a controlled
from of low stress abrasion. The low stress qualifier means that the abradant is imposed on the surface with
relatively low normal forces. The operating forces must be low enough to prevent crushing the abradant. Low stress
abrasion rates are directly proportional to the sliding distance and the load on the particles or protuberances and
are significantly reduced by hard microconstituents within the surface microstructure.