Effect of periodic in situ deposition of silica layer on the long term operation of a micro-gas turbine

Tetraethylorthosilicate (TEOS) diluted in methanol was periodically supplied during operation of a 13 kgf (max. thrust)-class small gas turbine to deposit silica layers on the turbine blades and its effect on the degradation of the turbine blades upon the long term cyclic operation (1 cycle: 30/10 min run/pause at 20,000 rpm) was tested. The exhaust gas temperature (EGT) was set at 800 °C. For comparison, two other gas turbines were also tested without feeding TEOS at the EGT of 800 °C and 700 °C, respectively. The blades were inspected optically and metallographically before and after the test. The supply of TEOS produced a white silica layer indicating a very low level of the apparent density all around the surface of the blades, some of which became dense at the end to a thickness of 5–10 μm. The underlying substrate Inconel 713 was effectively protected, while the blades operated at the EGT of 800 °C, but not in situ coated, were severely degraded in terms of the surface oxidation and microstructural evolution of the substrate. The microstructure of the protected substrate was comparable with that of the unprotected but operated at the EGT of 700 °C. This result was discussed based on the effect of the formation of dense silica layer on the blade by comparing with the previously obtained results.