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Deposition of silica coatings on Incoloy 800H substrates using a high-power laser

Using a cw 2 kW CO2 laser, thin silica coatings have been deposited on steeply inclined Incoloy 800H substrates traversed beneath the incident laser beam. The small angle of incidence (10 - 15 deg to the direction of beam propagation) of the beam on the substrate resulting from the large angle of incline gave reduced substrate heating and eliminated melting of the substrate surface but allowed melting of the silica powder injected into focus of the beam. The focus was positioned a known distance above the substrate and the molten powder fell onto the laser-heated substrate below. By making overlapping passes complete surface coverage was achieved over a large area, the coating thickness being 2 - 3 micrometers ; this overlay filled points of surface roughness to give the component a microscopically smooth outer appearance. The silica coating formed a good bond with the metallic substrate, adhesion appeared to be improved by having a slightly rough finish rather than a highly polished one. Resistance to sulphidation attack was assessed by placing coated samples into a furnace containing a mixture of gases as in a simulated coal-gasifier heat-exchanger environment at 450 and 750 degree(s)C. Sulphidation resistance was greatly improved compared with that of the uncoated alloy, and coating spallation and cracking did not occur during testing; the few sulphides observed were at small discontinuities in the coating which were eliminated by applying further silica overlayers and also by rastering the coating surface with the laser beam in the absence of additional powder to seal it.