Reliability Of Thermal Barrier Coatings Research Articles

Reliability evaluation of thermal barrier coatings for engine combustion chambers based on Monte-Carlo simulation

Due to their outstanding high temperature resistance, thermal barrier coatings are commonly applied in hot section such as aero-engine combustion chambers to protect components from high-temperature airflow. However, its material properties are insecure, the service environment is harsh, and the service life is uncertain. This paper uses a Monte-Carlo simulation-based dependability evaluation method to resolve this uncertainty. To begin with, the failure criteria for thermal barrier coatings were defined, as was the limit state equation. Second, using the Monte-Carlo simulation approach, the failure probability was calculated. Finally, the TBCs' dependability in a thermal gas flow environment was assessed and compared to actual measurement results. The thermal barrier coating in the area where the surface temperature of the combustion chamber is above 1100 °C begins to fail after 4000 h of service under the action of the hot air flow, with a failure rate of more than 60 %. The service life of the thermal barrier coating at surface temperatures of 1000, 1100, and 1197 °C was 6327, 3125, and 1642 h, respectively, based on the time it took to attain 60 % failure probability under the impact of hot air flow. The numerical relative error of the actual failure probability PT of the thermal barrier coating of the combustion chamber obtained by hole detection technology is less than 10 % (except for 3000 h), demonstrating the dependability and applicability of the reliability assessment technique. The reliability of thermal barrier coatings is influenced by temperature and TGO development.

Numerical Simulation of Initial Residual Stress in Thermal Barrier Coatings: Planar geometry model

Thermal barrier coatings (TBCs) are one of the key materials of turbines for propulsion and power generation. A typical TBCs system generally contains four layers, i.e. top ceramic coat (TC), thermally grown oxide (TGO), bond coat (BC) and substrate. The material parameters （such as Young’s modulus, Poisson’s ratio and thermal expansion coefficient） of each layer will result in the different cooling velocity of thermal barrier coatings (TBCs) system during the deposition process. Initial residual stresses in TBCs occur after cooling to the ambient, which play an important role for evaluating the failure and reliability of TBCs. In this paper, the initial residual stresses of a planar TBCs model were obtained by finite element method, the effects of cooling kinds and the thickness of TC and TGO on the fields of initial residual stresses were also considered. The results may be useful for the optimization of TBCs system design and preparation.

Modelling of alloying process of cold sprayed Ni/Al coating

Cold spray technology can reduce the oxidation of the as-sprayed coating and improve the reliability of thermal barrier coatings. Modelling the cold spray process can provide an approach to improve the process, and then increase the quality of the coatings. In this paper, a diffusion model with stress effect is introduced to analyze the alloying process of Ni/Al coating, and the effect of residual stress induced by cold spray is considered. The quantitative relationship between the thickness of ball milled Ni/Al splat and the alloying time is investigated. The presented model shows that tensile residual stress increases the concentration of Al in the Ni/Al coating, while compressive residual stress exhibits the opposite effects. The relationship between the thickness of ball milled Ni splat and the alloying time significantly deviates from the common parabolic law. And the reduced alloying time by ball milling Ni/Al powder is obtained quantitatively.

A LIFE PREDICTION MODEL OF THERMAL BARRIER COATINGS

The durability and reliability of thermal barrier coatings(TBCs) have become a major concern of hot-section components due to lack of a reliable life prediction model. In this paper, it is found that the failure location of TBCs is at the TBC/TGO interface by a sequence of crack propagation and coalescence process. The critical crack length of failure samples is 8.8mm. The crack propagation rate is 3-10µm/cycle at the beginning and increases largely to 40µm/cycle near coating failure. A life prediction model based a simple fracture mechanics approach is proposed.

Effect of thermal aging on the erosion resistance of air plasma sprayed zirconia thermal barrier coating

To increase the performance, efficiency and reliability of thermal barrier coatings, it is important to understand the influence of the microstructure on the erosion resistance. Therefore, the erosion behavior of air-plasma-sprayed 7 wt% yttria stabilized zirconia (7YSZ) thermal barrier coatings (TBC) at elevated temperatures was investigated. The paper focuses on both the porosity of the coating as well as the thermal pre-treatment of the ceramic. To simulate operating conditions on the ceramic samples, high-temperature erosion experiments at 1093°C were carried out on as-sprayed samples as well as on samples, thermally aged at different temperatures up to 1482°C in air prior to erosion testing. A significant dependence of erosion rate on porosity and thermal pre-treatment was found. Finally, a technique was developed to predict the erosion rates of air-plasma-sprayed 7YSZ TBCs independent of aging conditions or porosity levels. Erosion rates seem to be highly correlated to the micro-hardness of the zirconia TBC. A power-law model was empirically derived to estimate erosion rates of plasma-sprayed 7YSZ ceramic thermal barrier coatings.

Concept of Functionally Graded Materials for Advanced Thermal Barrier Coating Applications

This feature article explores the concept of creating functionally graded metal‐ceramic composite microstructures for thermal barrier coatings used in gas‐turbine applications. From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings. However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal‐ceramic composite microstructures. The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal‐ceramic microstructures for realistic application environments.