Evaluation of erosion behavior of thermal barrier coatings

Thermal Barrier Coatings (TBCs) are advanced materials that play a pivotal role in in many fields associated with high temperatures. The role of TBCs is to isolate components exposed to sustained high heat flow.

Figure 1. Gas turbine.

These coatings are mostly applied in gas turbines and aerospace applications, with the task of protecting the underlying superalloy substrate from temperature-related damage and from the erosion brought by solid particles.

Figure 2.Thermal barrier coatings (TBCs) that protect the base alloy against heat and erosion.

The performance of TBCs can be evaluated high temperature erosion tests using Ducom Air Jet Erosion Tester. The Ducom Air Jet Erosion Tester is a unique instrument capable of testing bulk materials and coatings under erosion. Its capabilities range from subjecting sample to tightly controlled gentle erosion to some of the harshest erosion conditions seen on the field, with particles flowing at up to 100 m/s and 10 g/min, at temperatures as high as 1000 °C and more.

Ducom Air Jet Erosion Tester it is possible to map the characteristic curve of a specific coating and to compare it to the curve of known coatings for comparison. The main factors affecting the erosion behavior of TBCs are particles size and velocity, impingement angle and the microstructure of the coating.

TBCs usually show a columnar microstructure, that contributes to the coating’s resistance to crack propagation and strain. As shown in Figure X, there is a direct proportionality between TBCs’ column radius and erosion.

A similar trend is observed for the relationship between erosion and particle velocity, with the former being related to the latter by an exponential relationship. A precise control of particle velocity is therefore crucial for precisely evaluate its effect on the erosion behavior of advanced materials.

The influence of particle size on erosion is more complex. If the particle size is low, lateral cracking dominates the wear behavior and the erosion follows a near-exponential dependency on the particle size. With increasing particle size, compaction deformation becomes dominant and the rate at which increase particle size results in increased erosion is lower. Gross plastic deformation occurs when greater particles impacts on the coatings; the subsequent erosion is proportional to the particle size, but the coefficient of proportionality is even lower, resulting in an overall sigmoid relationship between erosion and particle size, overall.

The angle of impingement has a very different effect on the erosion of TBCs when compared to NiCr alloys. In the case of TBCs, the erosion increases with increasing impingement angle, albeit at a lower rate. NiCr alloys, on the other hand, follow the same trend until around 45°, then the relationship inverts and at a greater angle corresponds a lower erosion.

The characteristic erosion curves for TBCs can be used to validate, improve and benchmark TBCs. Ducom Air Jet Erosion Tester can precisely control velocity and angle of impingement of the erodent particles of different sizes, thus it is capable of generating the characteristic curves that enable researchers in studying and improving thermal barrier coatings.