Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

Abstract

High-temperature coating systems, consisting of a Rene´N5 superalloy, a Ni-23Co-23Cr-19Al-0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal-barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl 2 O 4 spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl 2 O 4 /a-Al 2 O 3 interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl 2 O 4 , h-Al 2 O 3 , Y 3 Al 5 O 12 at the TGO surface after pre-oxidation was avoided. The a-Al 2 O 3 layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of h-Al 2 O 3 instead of a-Al 2 O 3 . Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.

TGO Growth and Crack Propagation in a Thermal Barrier Coating

“…This suggests that the VHT could, to a certain level, suppress the formation of mixed oxides during thermal exposure via developing a thin Al 2 O 3 layer in vacuum at high temperature. This is consistent with previous observations reported by many other researchers ( Ref 6,[11][12][13][14][15][16][17][18]. Similar to the as-sprayed TBC, cracking in the VHT TBC was very limited after up to twenty-five 100-h cycles (Fig.…”

“…Since oxidation of the bond coat has been recognized as the cause for separation of the ceramic layer from the substrate (Ref 2, 3, 7-10), the formation of the detrimental oxides may accelerate crack nucleation during thermal exposure, leading to premature TBC failure. Studies have shown that, when heat treated in low-pressure oxygen conditions, a continuous Al 2 O 3 layer may develop at the ceramic/bond coat interface in both thermally sprayed ( Ref 6,[11][12][13] and electron beam physical vapor deposited (EB-PVD) TBC systems (Ref [14][15][16][17][18]. Therefore, appropriate postspraying heat treatment(s) may suppress the formation of detrimental oxides by developing a TGO composed of a predominantly Al 2 O 3 layer, leading to improved TBC durability.…”

Effect of Y Distribution on the Oxidation Kinetics of NiCoCrAlY Bond Coat Alloys

“…This strain energy increases with increasing TGO thickness. This explains that the failure of a high-temperature system can often be correlated with a critical TGO thickness at failure [6][7][8]. Thus, knowledge about the rate at which the TGO grows during service is necessary to predict the life span of the entire coating system.…”

Mechanical Properties of Shark‐Skin Like Structured Surfaces for High‐Temperature Applications

“…The NiCoCrAlY coating shows a hightemperature resistance and oxidizes slowly. [14][15][16][17][18][19] Laser structuring was performed at the "Institute of Applied Material Physics--Applied Materials Physics" (IAM-AWP) 0 in Karlsruhe Institute of Technology (KIT), Germany. A micromachining workstation (PS450-TO, Optec, Belgium) was used which was equipped with an ultrafast Yb:YAG fiber laser (Tangerine, Amplitude Systems, France) operating at a maximum average power of 20 W and a maximum pulse energy of 100 mJ at 1 030 nm (TEM00 with M2 < 1.3).…”