Effect of CMAS viscosity on the infiltration depth in thermal barrier coatings of different microstructures

• Published in: Surface & coatings technology 2022-02, Vol.432 (C), p.128039, Article 128039
• Main Authors: Kumar, Rishi, Rommel, Sarshad, Jiang, Chen, Jordan, Eric H.
• Format: Article
• Language: English
• Subjects: Aluminum; Apatite; CMAS infiltration depth; CMAS viscosity; Cracks; Gadolinium; Gas turbines; Infiltration; Inter pass boundaries (IPBs); Magnesium; Magnesium aluminum silicates; Microscopy; Microstructure; Penetration; Solution precursor plasma spray; Stress relieving; TBC microstructure; Thermal barrier coatings; Time dependence; Transmission electron microscopy; Viscosity; Yttrium-aluminum garnet
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2021.128039

Calcium magnesium aluminum silicate (CMAS) is one of the leading concerns for the gas turbine industry. The effects of CMAS viscosity and the coating microstructure on CMAS infiltration depth were explored by conducting a time dependent interaction study. Three CMAS compositions were used from literature, and their viscosities predicted through FactSage viscosity module were drastically different. The interaction was carried out on three different TBCs synthesized using the solution precursor plasma spray process (SPPS): two of the TBCs were made of yttrium aluminum garnet (YAG) having different microstructures that promote different modes of CMAS infiltration, and one TBC was made of gadolinium zirconate (GZO). All samples had stress relieving vertical cracks and different intensities of horizontally banded porosity, (inter pass boundaries IPBs). A concentration of 100 mg/cm2 of CMAS was applied on the TBCs which were then subjected to a 5-minute interaction at 1300 °C. Samples were analyzed using scanning electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDXS), and transmission electron microscopy (TEM). Low viscosity CMAS readily penetrated the TBCs while more viscous CMAS showed less penetration. The depth of CMAS infiltration depended on the coating microstructure. In the YAG with IPBs, the CMAS spread horizontally in the IPBs before infiltrating deeper, resulting in reduced infiltration depth compared to other samples in spite of having wider vertical cracks. TEM and EDXS analysis were performed to investigate the phases present in the CMAS-TBC interaction region in YAG. Two regions were chosen, the top TBC surface in direct contact with the sea of CMAS, and the region at the CMAS penetration ended within the coating. The results showed that no secondary phases like apatite were observed in YAG, thus it can be concluded that the arrest of CMAS happened solely because of CMAS viscosity and the short infiltration time. •CMAS of three compositions with different melt viscosities were used to conduct a time dependent infiltration study in TBCs•SPPS technique was used to deposit YAG and GZO TBCs•YAG TBCs with two different microstructures were prepared, including low and high densities of horizontal porosity•CMAS infiltration depth was measured in each of the nine CMAS-TBC pairs•Both higher CMAS viscosity and high density of horizontal porosity reduced CMAS infiltration