Structural Change in Plasma-Sprayed Alumina Coatings by Laser Melting (Part 1)

Abstract

Structural change in the plasma-sprayed alumina coatings by laser melting was investigated. Three layers were observed in the cross-section of laser-treated coatings, melted, heat-affected and as-sprayed zones. X-ray diffraction analysis revealed that both the melted and the heat-affected zones. X-ray diffraction analysis revealed that both the melted and the heat-affected zones were entirely alpha-alumina, while the assprayed zone consisted of mainly gamma-alumina. The melted zone, a portion apparently melted by laser with subsequent solidification, had a columnar structure with less porosity but with many cracks. The heataffected zone is probably a portion which did not melt but underwent the phase-transformation from gamma to alpha phase by laser heating. The relationship between the coating structure and laser parameters, such as power and beam traverse speed, was also investigated by one-pass treatment under various conditions. When treated under the conditions such that the beam speed is proportional to the laser power, the size of the heat-affected zone increased with the increase both in beam speed and laser power. This indicates that the size of the heat-affected zone depends on the energy density rather than the total energy input. Experiments under various beam traverse speeds revealed that the grain size in the melted zone was inversely proportional to the square root of the beam traverse speed.