A comparative investigation of oxide formation on EQ (Equilibrium) and NiCoCrAlY bond coats under stepped thermal cycling

作者:Mercer C*; Kawagishi K; Tomimatsu T; Hovis D; Pollock T M
来源:Surface and Coatings Technology, 2011, 205(8-9): 3066-3072.


A comparative study of the characteristics of thermally grown oxide (TGO) formation under stepped (intermediate temperature dwell) thermal cycling on EQ (equilibrium) and NiCoCrAlY bond coats employed in thermal barrier systems, has been conducted. EQ bond coats are designed to be chemically compatible with the superalloy substrate, so that expansion mismatch and interdiffusion with the substrate are minimized. Stress mapping of the TGO was performed using luminescence spectroscopy. Regions of interest were cross-sectioned using focused ion beam techniques, to enable sub-surface examination by scanning electron microscopy. In the case of the NiCoCrAlY bond coat, stepped cycling resulted in severe surface undulations as a consequence of bond coat rumpling, a major problem in the application of TBCs, since it can lead to failure of the ceramic top coat. When the rumpling amplitude becomes sufficiently large, creep cracks develop in the TGO at the apex of the undulations, causing stress relaxation. The rumpling behavior arises due to creep displacements in the bond coat during the dwell period. No such rumpling was evident in the EQ bond coats. However, significant TGO spalling was observed in the EQ material with associated spinel formation. The evidence indicates that the spalls occurred at the TGO/bond coat interface. Spalling was less extensive in the NiCoCrAlY material, and the spalls were located entirely within the TGO. In both materials, regions of reduced TGO compressive stress coincided with the presence of yttria-rich particles.

  • 出版日期2011-1-25