Fabrication and characterisation of laser surface modified plasma sprayed alumina coatings

Thick alumina coatings fabricated using the air plasma spray (APS) process, predominantly contain metastable gamma-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 CO, laser heating method was used to achieve this objective. This heating procedure has the ability to transform the gamma-Al2O3 phase to the thermodynamically stable alpha-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 alpha-Al2O3 coating, similar to150 mum thick, was obtained. The laser treated alumina coating showed an increase in Vickers microhardness from similar to1100 to similar to2300 HV. Feasibility of making thicker and harder alpha-Al2O3 coatings by combining the APS process with laser surface modification was demonstrated.