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Composites and Coatings Group

Department of Materials Science & Metallurgy

Gas turbines operate in harsh conditions of stress, temperature and corrosion. To raise working temperatures for increased efficiency, superalloys, cooling systems and thermal barrier coatings are necessary systems in gas turbines. Thermal barrier coatings made of yttria stabilized zirconia have been in use since the 1970s [Smialek & Miller] but are subject to corrosion induced stiffening leading to spalling, leaving superalloys unprotected.

ECP_sodium titante ...use argon.jpg

To avoid this stiffening to take place, a contaminant reducing agent such as titanium is plasma sprayed in combination with YSZ. The coatings are heat treated at 1000 C for several hours in presence of Na2SO4 , to provoke a chemical reaction between Na+ and the titanium. This reaction should lead to the formation of NaTi4O8 and evaporation of SO2, preventing liquid  Na2SO4 to stiffen the coating upon cooling. 


This reaction between Na2SO4 and titanium was found to take place in low oxygen atmospheres. In air, the titanium oxidizes to rutile TiO2 and the evaporation of Na2SO4 is favorised. In low oxygen atmospheres, NaTi3O7 are first formed, then further transform into NaTi4O8. This is illustrated in the XRD scan, which shows the different phases present in a YSZ 10%wt. Ti coating, covered with 3% wt. Na2SO4 and heat treated 1h, 2h and 5h in low oxygen atmosphere. TiO2 needles and sodium titanate crystals were also found on the samples, as displayed in the SEM photographs. 

ECP_titanium dioxid...use argon.png

ECP_YSZ 10Ti 1,5% n...ARGON XRD.png

Furtherwork would include the precise correlation between gas turbine working conditions and the optimal coating reactivity.