Fabrication and characterization of laser surface modified-plasma sprayed alumina coatings

Abstract

Thick alumina coatings fabricated using the air plasma spray (APS) process, predominantly contain metastable ?-Al₂O₃ phase with a porous microstructure. Presence of such a metastable phase and porosity are detrimental to long term performance of the coating. Such inherent disadvantages can be mitigated to a large extent through the use of a post-spray laser treatment. In the present work, a surface specific CO₂ laser heating method was used to achieve this objective. This heating procedure has the ability to transform the ?-Al₂O₂ phase to the thermodynamically stable ?-Al₂O₃ phase and remove porosities while retaining the metallic substrate intact. Phase analysis by XRD confirmed the transformation. Microstructural analysis by SEM revealed the absence of porosities. Using optimal laser power, an ?-Al₂O₃ coating, ~150 ?m thick, was obtained. The laser treated alumina coating showed an increase in Vickers microhardness from ~ 1100 to ~ 2300 HV. Feasibility of making thicker and harder ?-Al₂O₃ coatings by combining the APS process with laser surface modification was demonstrated.