Improved Thermal Barrier Coating System Based on a Cathodically Deposited Alpha Alumina Sublayer

Abstract : This report was developed under a SBIR contract for Topic AF00-146. The primary objective of this Phase I research was to evaluate the feasibility of using an alpha alumina sublayer to reduce oxidation damage and improve the life of advanced thermal barrier coatings (TBC). A patented filtered cathodic arc deposition technique was used to deposit a 0.5-micrometers-thick alumina sublayer at 1000 deg C on a Pt-Al bond-coat deposited on a CMSX-4 single crystal. The unique coating method provides droplet-free coatings and high ionization (>80 percent) compared with standard physical vapor deposition (PVD) techniques (<20-percent ionization). An electron beam PVD technique was used to deposit the yttria stabilized zirconia (YSZ) layer on the alumina coating. Thermal cycling tests were conducted on companion samples between 170 and 1150 deg C. Excellent TBC life was obtained when the as-prepared Pt-Al bond-coat was grit blasted with alpha alumina prior to deposition of the alumina layer. Although the control grit-blasted sample, without the alumina layer, also exhibited good cyclic life, it indicated larger oxide thickness and more damage accumulation than the alumina-coated sample. Thus, the alumina sublayer is able to improve TBC performance. The primary mechanism of TBC damage was surface rumpling of the bond-coat, likely due to oxidation and nonuniform shrinkage of the bond-coat.