Failure of Multilayer Suspension Plasma Sprayed Thermal Barrier Coatings in the Presence of Na2SO4 and NaCl at 900\xa0\xb0C

Krishna Praveen Jonnalagadda,Ru Lin Peng,Pimin Zhang,Nicholas Curry,Stephanie Kramer,Xin-Hai Li,Satyapal Mahade

doi:10.1007/s11666-018-0780-5

Abstract

The current investigation focuses on understanding the influence of a columnar microstructure and a sealing layer on the corrosion behavior of suspension plasma sprayed thermal barrier coatings (TBCs). Two different TBC systems were studied in this work. First is a double layer made of a composite of gadolinium zirconate + yttria stabilized zirconia (YSZ) deposited on top of YSZ. Second is a triple layer made of dense gadolinium zirconate deposited on top of gadolinium zirconate + YSZ over YSZ. Cyclic corrosion tests were conducted between 25 and 900 °C with an exposure time of 8 h at 900 °C. 75 wt.% Na2SO4 + 25 wt.% NaCl were used as the corrosive salts at a concentration of 6 mg/cm2. Scanning electron microscopy analysis of the samples’ cross sections showed that severe bond coat degradation had taken place for both the TBC systems, and the extent of bond coat degradation was relatively higher in the triple-layer system. It is believed that the sealing layer in the triple-layer system reduced the number of infiltration channels for the molten salts which resulted in overflowing of the salts to the sample edges and caused damage to develop relatively more from the edge.

Highlights

Suspension plasma spray (SPS) is a recent advancement in thermal barrier coatings deposition (Ref [1, 2])
Yttria stabilized zirconia (YSZ), with its attractive properties, is still the industry standard for the topcoat material, the recent focus has been on other materials that can circumvent the limitations of conventional yttria stabilized zirconia (YSZ), for instance, the high-temperature phase stability of YSZ (Ref 12, 13)
The samples were held at the test temperature for 8 h after which they were removed from the furnace and allowed to cool in atmosphere (* 80 min) till they reached the room temperature (* 25 °C)

Summary

Introduction

Suspension plasma spray (SPS) is a recent advancement in thermal barrier coatings deposition (Ref [1, 2]). Thermal barrier coatings (TBCs), typically having a bi-layer structure with metallic layer and a ceramic top layer, are used in the hot sections of gas turbines and in diesel engines to provide components with resistance against high-temperature degradation (Ref [3,4,5,6,7,8]). The vertical columnar structure is believed to have a superior strain tolerance during thermal cycling than by other low-cost coating deposition techniques like atmospheric plasma spraying (Ref 11). Among all the materials being researched for the topcoat layer in TBCs,

Results

Discussion

Conclusion