In situ deposition behavior of SiO2 on YSZ-TBC-coated IN738LC during a burner rig test

Abstract

This work was planned as a preliminary test to obtain an optimal condition for in situ deposition of SiO2 on zirconia-based thermal barrier coating (TBC)-coated IN738LC specimens using a burner rig. The effect of the in situ deposited SiO2 on the long-term reliability of TBCs upon cyclic burner rig operations will be tested in the next study. Tetraethylorthosilicate (TEOS), the precursor for the deposition, was fed admixed with methanol into the combustion chamber of the burner rig at an exhaust flame temperature of 1530°C. All five coating processes were selected by varying the concentration of TEOS in the mixture and applied to hollow type pin specimens infixed to the rotating carousel to face the downstream exhaust gas vertically. A total of 1500cc of the mixture at a given vol.% TEOS was fed evenly to the burner rig for 54min during the coating process.After the process, TBC spallation was observed on the front side of the specimens processed with 10, 5, and 2vol.% TEOS, while the specimen of 0.5vol.% TEOS was coated with a ~5μm thick SiO2 layer without any macroscopic damages. The surface of the coatings consisted of fractal-structured nodules at 10vol.% TEOS, and of smooth grains at 0.5vol.% TEOS. The spallation of SiO2-coated TBC was attributed to the thermal mismatch due to the difference in the thermal expansion coefficients of the two materials. Based on these results, the mixing ratio of TEOS to methanol was reduced to 0.3vol.%, which produced a 1–2μm thick, smooth and homogenous SiO2 coating without any spallation of TBC. This deposition condition was chosen for the planned test to evaluate on the effects of the in situ deposition of SiO2 on the long-term durability of TBC-coated IN738LC specimens upon cyclic burner rig operations.