Crack Propagation and Effect of Mixed Oxides on TGO Growth in Thick La–Gd–YSZ Thermal Barrier Coating

Savisha Mahalingam,Rabiatul A Zainuddin,Salmi Mohd Yunus,Nurfanizan Mohd Afandi,Nurul F Kadir,Abreeza Manap

doi:10.3390/coatings9110719

Savisha Mahalingam, Rabiatul A Zainuddin + Show 4 more

Open Access

https://doi.org/10.3390/coatings9110719

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Abstract

Thick thermal barrier coatings (TBCs) are the main choice in the aviation industry due to their ability to handle elevated temperature exposure in turbines. However, the efficacy of thick TBCs has not been adequate. This study presents a highly durable, thick top-coat (TC) of Lanthanum–gadolinium–yttria stabilized zirconia (La–Gd–YSZ) on high-velocity oxygen fuel (HVOF)-bond coat (HVOF-BC). Crack propagation was quantitatively assessed using a three-dimensional (3D) measuring laser microscope due to higher reliability in calculating the actual crack length of TBC. The findings revealed the HVOF-BC is highly durable with intact structural composition, while the conventional TBC of atmospheric plasma spraying (APS) bond coat (APS-BC) of the same composition and thickness with identical TC was detached at a crack-susceptible zone. The significant enhancement in HVOF-BC is due to the low mixed-oxides growth rate in thermally grown oxide (TGO) with a uniform and dense protective layer of stable Al2O3 which reduces crack propagation. Meanwhile, the failure in APS-BC can be attributed to the high TGO growth rate and thickness with segmented and unstable Al2O3. Furthermore, detrimental mixed oxides such as spinel Ni(Cr,Al)2O4 and NiO lead to disastrous horizontal and compressive cracks. To that end, we study the effect of TGO growth and crack propagation on HVOF-BC TBCs using APS-BC TBCs as a reference.

Highlights

Sprayed coatings are often used to protect metallic components that suffer breakdown from wear, corrosion or excessive heat load during service in thermally drastic environments [1]
high-velocity oxygen fuel (HVOF)-bond coat (BC) exhibits a denser coating than atmospheric plasma spraying (APS)-BC due to a large amount of kinetic energy provided that it propels the molten material at greater speeds, whereas APS-BC has low particle velocity
The APS-BC contained globular pores and voids which reduced at longer exposures, while HVOF-BC

Summary

Introduction

Sprayed coatings are often used to protect metallic components that suffer breakdown from wear, corrosion or excessive heat load during service in thermally drastic environments [1]. These coatings are widely used as a thermal barrier coating (TBC) applied on gas turbines and in the aeronautical and automotive industries [2]. Performance of gas turbines is enhanced significantly by reducing the cooling or increasing the gas temperature during service. The low-thermal conductivity in TBC causes low heat transfer from TC and achieves effective cooling and reduces the surface temperature. A thickness of above 0.25 mm is considered as thick TC [4]

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