FAILURE ANALYSIS OF PRE-OXIDIZED ReneN5/ β-(Ni,Pt)Al/7YSZ EB-PVD THERMAL BARRIER COATINGS

Abstract

Nowadays Pt-modified nickel aluminides, (Ni,Pt)Al, are highly used as alumina-forming alloys in the aerospace industry. These alloys have been studied to improve the lifetime of the thermal barrier coating systems (TBCs) when they are exposed to thermal-cyclic tests [1-3]. The use of (Ni,Pt)Al alloy as bond coat (BC) is to provide oxidation protection of the Ni-base superalloy (SA) used as a substrate of hot-section turbine blades. BCs act as an aluminum reservoir that leads to the formation of a thermally grown oxide (TGO). The TGO improves the adherence between the metal and a ceramic top coat (TC), which provides thermal insulation during the operation of these components. The aim of the present study is to evaluate the effect of the pre-oxidation treatments before TC deposition on the thermal-cyclic life of ReneN5/(Ni,Pt)Al/7YSZ TBCs. The effect of different oxygen partial pressures (pO2= 2.1x10-1, 1.05x10-1, 1x10-5 and 1x10-9 atm) pre-oxidation treatments were evaluated and compared to regular TBCs after EB-PVD TC deposition in “as-coated” conditions. A post-mortem analysis of the samples was also carried out after 1 h thermal-cyclic testing at 1100ºC to study the intrinsic mechanisms associated to the failure. Samples pre-oxidized with a combination of Argon-Gettered atmosphere (pO2=1x10-9 atm) showed the highest performance in average (1550 cycles to failure). For all the different pre-oxidation conditions, the failure occurred majority along the TC/TGO interface. Mapping of grain orientation determined by EBSD combined with nanoindentation tests were carried out to evaluate anisotropic effects and structural characterization with their mechanical properties after thermal-cyclic tests. Efforts are currently being done to link the failure of the samples to the microstructural characteristics. For instance a continuous layer of martensitic phase, L10, along the bond coat has been observed after failure for the thermal-cyclic tested samples. The reversible L10↔β phase transformation and the reaction β γ' are the main phenomena that take place during thermal cycling and results from the BC aluminum depletion due to TGO formation and its diffusion to the SA [4-6]. Moreover, cracks have been identified in the L10/γ' grains interface. Finally, it must be pointed out that the non-pre-oxidized TBCs have shown a mixed zone in the TGO, which consists on a region where zirconia particles are dispersed in the TGO matrix [7]. References [1] A.G. Evans, et.al., Progress in Materials Science 46 (2001) 505 - 553. [2] V.K. Tolpygo and D.R. Clarke, Surface & Coatings Technology 200 (2005) 1276 - 1281. [3] U. Schulz, et.al., Surface & Coatings Technology 203 (2008) 160 - 170. [4] M.W. Chen, et.al., Acta Materialia (2003), 4279 - 4294. [5] J.L. Smialek and R. Giballa; Metallurgical transactions A (1983), 2143 - 2161. [6] B. Gleeson, W. Wang, S. Hayashi, D.J. Sordelet; Mater Sci Forum 2004. [7] M.J. Stiger, et.al; Metallurgical and Materials Transitions A (2007) Vol.38A 848 - 857.