IMPERMEABLE THIN Al2O3 OVERLAY FOR TBC PROTECTION FROM SULFATE AND VANADATE ATTACK IN GAS TURBINES

In order to improve the hot corrosion resistance of conventional YSZ TBC system, the Al{sub 2}O{sub 3} overlay coating has been successfully produced on the surface of YSZ by the Sol-gel route. The YSZ substrates were coated with boehmite sol by dip coating process, dried to form a gel film and calcined at 1200 C to form {alpha}-Al{sub 2}O{sub 3} overlay. The microstructures of TBC and Al{sub 2}O{sub 3} overlay were examined by scanning electron microscopy (SEM). The results showed that micro-pores ranged from 3 {micro}m to 20 {micro}m and micro-cracks could be clearly seen on the surface of APS YSZ coating. The thickness of alumina overlay increased with increasing the number of dip coating circles. The small microcracks (0.5-1.0 {micro}m width) on the YSZ surface could be filled and blocked by calcined alumina particles, whereas large pores remained empty and the alumina overlay was un-continuous after one time dip coating circle. Alumina overlay thicker than 5 m m obtained by five times dip coating circles largely cracked after calcinations. As a result, multiple dip coatings up to three times were ideal for getting high quality, crack- free and continuous overlay. The optimal thickness of alumina overlay was in the range of 2.5-3.5 {micro}m. In the next reporting period, we will study the hot corrosion behaviors of YSZ TBC with Al{sub 2}O{sub 3} overlay coating produced by sol gel route by exposure the samples to molten salts mixtures (Na{sub 2}SO{sub 4} + 5%V{sub 2}O{sub 5}) at 950 C