Thermal barrier coating systems (TBCs) are widely used in modern gas turbine engines in power generation, marine and aero engine applications to lower the metal surface temperature in combustor and turbine section hardware. The application of TBCs can provide increased engine performance/thrust by allowing higher gas temperatures or reduced cooling air flow, and/or increased lifetime of turbine blades by decreasing metal temperatures. TBC is a duplex material system consisting of an insulating ceramic topcoat layer and an inter metallic bondcoat layer. It is designed to serve the purpose of protecting gas turbine components from the severe thermal environment, thus improving the efficiency and at the same time decreasing unwanted emissions. Turbine entry gas temperatures can be higher than 1,500 K with TBCs providing a temperature drop of even higher than 200 K across them [1]. TBCs were first successfully tested in the turbine section of a research gas turbine engine in the mid-1970s [2].

Improvement in the performance of TBCs remains a key objective for further development of gas turbine applications. A key objective for such applications is to maximise the temperature drop across the topcoat, thus allowing higher turbine entry temperatures and thus higher engine efficiencies. This comes with the requirement that the thermal conductivity of the ceramic topcoat should be minimised and also that the value should remain low during prolonged exposure to service conditions. In addition to this, longer lifetime of TBCs compared to the state-of the art is of huge demand in the industry. In case of land-based gas turbines, a lifetime of around 40,000 h is desired. Therefore, substantial research efforts are made in these areas as TBCs have become an integral component of most gas turbines and are a major factor affecting engine efficiency and durability.

The coating microstructures in TBC applications are highly heterogeneous, consisting of defects such as pores and cracks of different sizes. The density, size and morphology of these defects determine the coating’s final thermal and mechanical properties and the service lives of the coatings [3–6]. To achieve a low thermal conductivity and high strain-tolerant TBC with a sufficiently long lifetime, an optimisation between the distribution of pores and cracks is required, thus making it essential to have a fundamental understanding of microstructure–property relationships in TBCs to produce a desired coating.

Failure in atmospheric plasma-sprayed (APS) TBCs during thermal cyclic loading is often within the topcoat near the interface, which is a result of thermo-mechanical stresses developing due to thermally grown oxide (TGO) layer growth and thermal expansion mismatch during thermal cycling. These stresses induce the propagation of pre-existing cracks in as-sprayed state near the interface finally leading to cracks long enough to cause spallation of the topcoat [7–14]. The interface roughness, although essential in plasma-sprayed TBCs for effective bonding between topcoat and bondcoat, creates locations of high stress concentration. Therefore, the understanding of fundamental relationships between interface roughness and induced stresses, as well as their influence on lifetime of TBCs, is of high relevance.

The traditional methodology to optimise the coating microstructure is by undertaking an experimental approach. In this approach, certain set spray parameters are chosen based on prior experience with the equipment and process, or as is the case quite often, spray parameters from a factorial design experiments are chosen. Thereafter, coatings are deposited using these parameters and evaluated by different testing methods. This procedure could be iterated until the specified performance parameters are obtained. As it might be apparent, this experimental approach is extremely time consuming and expensive, apart from the drawback that it does not enhance our knowledge of quantitative microstructure–property relationships.

Therefore, the derivation of microstructure–property relationships by simulation would be an advantage. Simulation approach, apart from being time saving and cost effective, is highly useful for the establishment of quantitative microstructure–property relationships. New coating designs can be developed and analysed with the help of simulation in a much easier manner compared to the experimental approach. Another advantage of using simulation is that the individual effects of microstructural features (such as defects and roughness profiles) could be artificially separated and analysed to study their effect on properties of TBCs independently which is not possible by experimental methods. However, it must be noted here that the relationships between process parameters and microstructure have to be established via an experimental approach [4, 6]. A simulation approach is schematically described in Fig. 1.1, showing the steps required to achieve a high performance TBC requiring low thermal conductivity and long lifetime. It should be noted that the microstructural parameters for modelling are obtained through the link with experimental spray parameters during the optimisation routine via process maps as described further in Sect. 2.4.

Fig. 1.1
figure 1

Block diagram showing the steps required to obtain a high performance TBC

Full size image

A two-step approach has been described in this book to design an optimised TBC by modelling:

  1. 1.

    Design microstructure defects in topcoat: As the thermal–mechanical properties of TBCs are highly dependent on coating microstructure, the microstructure defects can be designed to optimise the topcoat so as to achieve low thermal conductivity and high strain tolerance which would imply long lifetime.

  2. 2.

    Design topcoat–bondcoat interface: As the crack propagation in TBCs near topcoat–bondcoat interface is highly dependent on the interface topography, the interface can be designed to optimise the topography so as to reduce thermo-mechanical stresses leading to crack propagation and thus enhance lifetime.

1.1 Scope and Limitations

The present study is general in itself as it is based on analysis of microstructure images and roughness profiles and is not dependent on material or equipment used to fabricate them. However, other factors might have to be considered if this study is applied to other materials or coating applications.

The limitations in this work can be broadly divided into the following categories:

  1. 1.

    Process

    The study was limited to APS for topcoats and mainly high velocity oxy-fuel (HVOF) spraying for bondcoats using powder feedstock. The effect of varying bondcoat spray parameters on bondcoat microstructure and surface roughness was not considered.

  2. 2.

    Material

    Only one topcoat material, namely zirconia, was considered with different stabilisers such as yttria and dysprosia. NiCoCrAlY was the only bondcoat material considered in this study.

  3. 3.

    Experimental evaluation techniques

    The experimental analysis performed in this study was limited to the scope of the used technique. Microstructure was evaluated with light optical microscopy (LOM) and scanning electron microscope (SEM) which could be incapable of detecting all fine pores and cracks present in the microstructure. Lifetime testing was limited to thermal cyclic fatigue (TCF) testing.

  4. 4.

    Modelling

    The effect of changing chemistry was not considered in this study. The microstructure considered in this study consisted of only the defect morphology. The thermal–mechanical properties modelled in this study were thermal conductivity and Young’s modulus. Lifetime assessment considered in the modelling work was based on TCF testing.

Most of the material properties used in the modelling work were considered to be temperature independent even though the properties could change significantly over the wide range of temperatures considered. As the focus was set on qualitative analysis rather than on predicting exact values, this assumption was considered to be valid in this case. The effect of radiation was not considered when predicting thermal conductivity as it can be assumed to be scattered due to the porous microstructure.

Two-dimensional (2D) domain was considered in several models used to determine coating properties which could affect the final values, though it can be used effectively for comparative purposes. Virtually designed microstructures were limited by the scope of the software used. Focus was placed on the individual influence of microstructural features on the final properties of the coating.

The effect of coating thickness was not considered in the stress analysis model. The failure mechanism considered in the modelling work was limited to spallation of coating due to thermo-mechanical stresses; other failure mechanisms such as erosion were not considered.