Abstract
Durability of a thermal barrier coating (TBC) depends strongly on the type of mixed oxide in the thermally grown oxide (TGO) of a TBC. This study aims on discovering the effect of thermal stability in the TGO area containing mixed oxides. Two different bondcoats were studied using high-resolution transmission electron microscopy: high-velocity oxygen fuel (HVOF) and air-plasma spray (APS), under isothermal and thermal cyclic tests at 1400 °C. The HVOF bondcoats were intact until 1079 cycles. In comparison, APS failed at the early stage of thermal cycling at 10 cycles. The phase transformation of topcoat from tetragonal to the undesired monoclinic was observed, leading to TBC failure. The results showed that the presence of transient aluminas found in HVOF bondcoat helps in the slow growth of α-Al2O3. In contrast, the APS bondcoat does not contain transient aluminas and transforms quickly to α-Al2O3 along with spinel and other oxides. This fast growth of mixed oxides causes stress at the interface (topcoat and TGO) and severely affects the TBC durability leading to early failure. Therefore, the mixed oxide with transient aluminas slows down the quick transformation into alpha-aluminas, which provides high thermal stability for a high TBC durability.
Highlights
Thermal barrier coatings (TBCs) have been applied to the high-temperature section of engines to improve the performance of the gas turbine engine
scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) semi-quantitative analysis showed that the thermally grown oxide (TGO) of both bondcoats samples consisted of mixed oxide (Ni(Al, Cr)2 O4 ) and zirconia (Zr) with a low concentration of chromium (Cr)
Sample A100h (Figure 2a) shows that it started to fail by TBC spallation, whereas the high-velocity oxygen fuel (HVOF)-samples remained intact with slower growth of mixed oxide from samples H1h-H1000c, in which H1080c failed at 1080 cycles
Summary
Thermal barrier coatings (TBCs) have been applied to the high-temperature section of engines to improve the performance of the gas turbine engine. Typical TBCs contain a three-layered material system, consisting of (1) 6–8 wt.% of partially yttria-stabilized zirconia (PYSZ) topcoat deposited most commonly via an air-plasma spray (APS) process; (2) bondcoat to resist the oxidation of Ni-based superalloy substrates; and (3) a superalloy substrate (nickel or cobalt). TBCs exposed to temperatures above 1200 ◦ C are less resistant to oxidation and decrease their mechanical properties [3], they often face premature failure, undergoing a phase. The failure is due to the thermally grown oxide (TGO) layer which is formed between the metallic bondcoat and topcoat due to the oxidation of bondcoat at high temperatures
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