Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor engine components from the hot gas stream, and improve the durability and energy efficiency of these engines. Improvements in TBCs will require a better understanding of the complex changes in their structure and properties that occur under operating conditions that lead to their failure. The structure, properties, and failure mechanisms of TBCs are herein reviewed, together with a discussion of current limitations and future opportunities.

http://science.sciencemag.org/content/sci/296/5566/280.full.pdf

Thermal Barrier Coatings for Gas-Turbine Engine Applications

1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

The Superalloys: Fundamentals and Applications

Mechanisms controlling the durability of thermal barrier coatings

The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.

Particulate reinforced metal matrix composites — a review

Thermal barrier systems have been the subject of vigorous research and development activities over the past few years, driven by the demands for enhanced reliability and substantially higher operating temperatures envisaged for the next generations of gas turbine engines. The menu of candidate materials and architectures has expanded considerably, including numerous concepts based on zirconia as well as radically different materials, multilayers and modulated distributions of porosity and chemical composition. Advances in deposition processes enable increased flexibility for tailoring composition and microstructure to local requirements within the coating system, e.g. for thermal insulation, control of interdiffusion, enhanced resistance against environmental degradation and condition monitoring. Many challenges remain but healthy and growing collaborations between the science and technology communities bode well for future progress in this area.

Emerging materials and processes for thermal barrier systems

Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings

Characterization of a cyclic displacement instability for a thermally grown oxide in a thermal barrier system

https://engagedscholarship.csuohio.edu/cgi/viewcontent.cgi?article=1148&context=enme_facpub

A numerical model for the cyclic instability of thermally grown oxides in thermal barrier systems

A thermal barrier system with plasma-sprayed thermal barrier coating (TBC) has been analyzed subject to thermal cycling. The thermally grown oxide (TGO) is allowed to grow at the peak temperature in the cycle, with both thickening and lateral components of the growth strain. The stresses in the TGO are also allowed to relax at high temperature when they attain a critical level. The bond coat is allowed to yield with temperature dependent yield strength. The stresses induced in the TBC and in the TGO have been calculated. The vertical component of the stress in the TBC is shown to have a large tensile domain with a maximum that increases systematically as the system cycles. There is a corresponding increase in the amplitude of the interface undulations. The stresses in the TBC have been used to calculate energy release rates, G , for cracks in the TBC extending parallel to the interface, just above the peaks of the undulations. This analysis reveals a minimum value, Gmin preceding instability. Equating this minimum to the TBC toughness identifies a delamination criticality. # 2002 Elsevier Science B.V. All rights reserved.

/pdf/simulation-of-stresses-and-delamination-in-a-plasma-sprayed-2htd53eaou.pdf

Simulation of stresses and delamination in a plasma-sprayed thermal barrier system upon thermal cycling

The effects of interface structure and microstructure on the fracture energy, Γi, of metal-ceramic interfaces are reviewed. Some systems exhibit a ductile fracture mechanism and others fail by brittle mechanisms. In the absence of either interphases or reaction products, Γi is dominated by plastic dissipation (for both fracture mechanisms), leading to important effects of metal thickness, h, and yield strength, σ0. Additionally, Γi is larger when fracture occurs by ductile void growth (for the same h and σ0). A fundamental understanding now exists for the ductile fracture mechanism. However, some basic issues remain to be understood when fracture occurs by brittle bond rupture, particularly with regard to the role of the work of adhesion, Wad. Interphases and reaction products have been shown to have an important effect on Γi. A general trend found by experiment is that Γi scales with the fracture energy of the interphase itself, wherein Γi tends to increase for the interphase sequence: amorphous oxides > crystalline oxides > intermetallics. However, there also appear to be important effects of the residual stresses in the interface (which influence the fracture mechanism and the layer thickness.

A.G. Evans

Papers

Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings

Characterization of a cyclic displacement instability for a thermally grown oxide in a thermal barrier system

A numerical model for the cyclic instability of thermally grown oxides in thermal barrier systems

Simulation of stresses and delamination in a plasma-sprayed thermal barrier system upon thermal cycling

The fracture resistance of metal-ceramic interfaces