Xtreme Coatings

The Process
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.

Component Preparation
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.

Wear Mode
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.