LASER MELTING OF PLASMA-SPRAYED ALUMINA COATINGS

The effect of laser melting pure alumina plasma-deposited coatings (gamma-alumina) 100, 200 and 380 mum thick on a micro-alloyed steel was analysed. The alumina coatings were subjected to a multimode laser beam with an elliptical cross-section, with axes of 4 and 6 mm, and output power of 5, 2.5 and 1.5 kW Complete melting of the coating leads to the formation of alumina layers free of internal microdefects, and these exhibit macroscopic cracks and a loss of adherence to the base material. Partial melting of the coating leads to a reduction in the porosity in the alumina layer and microscopic cracks appear in a grid formation over the melted material. The melting of the alumina layer results in the transformation into alpha-alumina. Corrosion tests show that the laser-melted coating is less permeable than the plasma-deposited coating. In both cases, however, the corrosion behaviour of the alumina-covered metallic material is always controlled by the base material.