An investigation of oxidation, hot corrosion, and thermal shock behavior of atmospheric plasma-sprayed YSZ–Al₂O₃ composite thermal barrier coatings

Abstract

In the current study, different types of thermal barrier coatings with various mass fractions were investigated in terms of oxidation, hot corrosion, and thermal shock resistance. The thermal barrier coatings consisted of six different samples, which included the usual YSZ, Al2O3, hybrid composites with 65 wt.% YSZ and 35 wt.% Al2O3, 50 wt.% YSZ and 50 wt.% Al2O3, 35 wt.% YSZ and 65 wt.% Al2O3 and the final one, which was a double layer composite (Al2O3 and YSZ). High temperature isothermal oxidation behavior of the coatings was tested at 1050 °C, using an air furnace for 48 h, 80 h, and 120 h respectively. Hot corrosion tests were applied at 1050 °C using a 45 wt.% Na2SO4 and 55 wt.% V2O5 mixture of salts. The microstructure and phase stability of coatings were evaluated by means of scanning electron microscopy and X-ray diffraction techniques. The usual YSZ showed better hot corrosion and thermal shock resistance, while Al2O3 showed the lowest hot corrosion and oxidation resistance. Thermally grown oxide formation, thermal expansion coefficient mismatch and phase transformation in the thermal barrier coatings could be the main causes of degradation after thermal shock testing.

Isothermal oxidation characteristics of laser‐treated Al2O3 + Sm2SrAl2O7 composite thermal barrier coatings

“…Previous research reports reduced oxide growth with increasing alumina content in the top coat. [4] Generally, a large amount of spinel oxides such as NiOCr 2 O 3 which grows rapidly are formed at the interface in case of GdZ, YSZ, and LaZ systems. [6,32] In the developed system, no major spinels are noticed at the interface.…”

“…[33] Upon continuous exposure at high temperatures, the highpurity polycrystalline alumina formed at the interface offers higher oxygen shielding effect protecting the bond coat from further oxidation. [4] 3.4.4 | Oxide lumps at the interface The samples showed spallation after 150 h of exposure near the top coat bond coat interface region, within the proximity of the TGO formed. Interestingly, as shown in Figure 15, an oxide lump near the interface in the top coat region was observed in the as-coated samples after 150 h of exposure at 1100°C.…”

“…Al 2 O 3 + YSZ composites are highly resistant to hot corrosion and oxidation at high temperatures. [4] Similarly, the Al 2 O 3 + Sm 2 SrAl 2 O 7 composite system has shown high durability in high temperature corrosive and erosive environments. [12,13] The common failure mechanisms of TBCs in gas turbine components mainly include oxidation, hot corrosion, and solid particle erosion at high temperatures.…”

“…[ 3 ] Alumina is an economical, promising top coat material due to its high thermal expansion coefficient (9.6 × 10 −6 K −1 ), but it suffers from phase transformations at elevated temperatures. [ 4 ] Gadolinium zirconate (GdZ) and lanthanum zirconate (LZ) have been used as top coat materials due to their lower thermal conductivity. Recently, the rare‐earth‐based samarium strontium aluminate (Sm 2 SrAl 2 O 7 ) has been identified as a TBC material possessing high temperature stability and low thermal conductivity.…”

“…Al 2 O 3 + YSZ composites are highly resistant to hot corrosion and oxidation at high temperatures. [ 4 ] Similarly, the Al 2 O 3 + Sm 2 SrAl 2 O 7 composite system has shown high durability in high temperature corrosive and erosive environments. [ 12,13 ]…”