Structural Change in Plasma-Sprayed Alumina Coatings by Laser Melting (Part 2)

Abstract

Structural change in plasma-sprayed alumina coatings by laser melting was investigated particularly for pulsed mode treatment. Effects of various processing parameters on the structural change were discussed for one-pass treatment. A three-layer structure consisting of melted, heat-affected and as-sprayed zones was observed in the cross sections of the laser-treated coatings. The depths of both the melted and heat-affected zones depended significantly on the pulse parameters. The treatment increased the Vickers microhardness by 1.6-2.0 times, although defects such as cracks, humps and voids developed in the melted zone. The highest hardness was observed for the thin melted layer with narrow cracks and small humps that was obtained under the conditions of low frequency and small pulse width. These conditions lead to a large temperature gradient and rapid cooling in the layer, because the sample is heated by a high power density beam in a very short time and cooled appreciably during the subsequent interval between pulses.