Surface Modification and Characterization of Plasma Sprayed Alumina Coatings using Lasers

Abstract

A synergetic way of combining superior properties of ceramics with that of metals is to apply ceramic coatings over metallic substrates. Also, ceramic coatings provide better protection of the covered metal under concomitant corrosion and wear conditions. Of all the advanced ceramic materials, Al₂O₃ is widely used due to its low cost and availability in high purity forms. But the alumina coatings made through Air Plasma Spray suffers from major disadvantages such as presence of metastable γ-Al₂O₃ phase, high surface roughness and porosities. In this paper, we have shown the feasibility of transforming the γ-Al₂O₃ phase present in the as-sprayed coatings to a thermodynamically stable α-Al₂O₃ phase. A continuous wave CO₂ laser having maximum power of 500W was used to provide a surface specific heating without affecting the metallic substrate. In addition to XRD, Raman scattering and photoluminescence investigations were carried out using a Ar⁺ laser to confirm the restoration of alpha phase and the reduction in porosity respectively. Also, 1 mW He-Ne laser operating in TEM₀₀ mode was used to examine the reduction of surface roughness using laser scattering technique. The intensity of the specular reflection was used to quantify the surface roughness. Both reduction in surface roughness and porosity were evaluated using SEM micrographs taken on the surface and cross section of the coatings.