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

Using a 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°C-15°C) 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 injection into the focus of the beam. The focus was positioned above the substrate and the molten powder was allowed to fall 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 μm; 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 a rough finish rather than a highly polished one. Resistance to sulphidation attack was assessed by placing coated samples in a furnace containing a mixture of gases as in a simulated coal gasifier heat exchanger atmosphere at 450 and 750°C. Sulphidation resistance was greatly improved from that of the untreated alloy and the coating did not spall or crack for the period of the test; the few sulphides observed were probably formed at small discontinuties in the coating which may be eliminated by applying a second silica coating.