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

Acknowledgments Preface Introduction 1. Methods of investigation 2. Thermodynamic fundamentals 3. Mechanisms of oxidation 4. Oxidation of pure metals 5. Oxidation of alloys 6. Oxidation by oxidants other than oxygen 7. Reactions of metals in mixed environments 8. Hot corrosion 9. Erosion-corrosion of metals in oxidizing atmospheres 10. Protective coatings 11. Atmosphere control for the protection of metals during production processes Appendix A. Solution to Fick's second law for a semi-infinite solid Appendix B. Rigorous derivation of the kinetics of internal oxidation Appendix C. Effects of impurities on oxide defect structure Index.

Introduction to the high-temperature oxidation of metals

Mechanisms controlling the durability of thermal barrier coatings

1. Introduction and overview 2. Film stress and substrate curvature 3. Stress in anisotropic and patterned films 4. Delamination and fracture 5. Film buckling, bulging and peeling 6. Dislocation formation in epitaxial systems 7. Dislocation interactions and strain relaxation 8. Equilibrium and stability of surfaces 9. The role of stress in mass transport.

Thin Film Materials: Stress, Defect Formation and Surface Evolution

High temperature corrosion in energy systems

The oxidation behavior of MoSi2 in three fabrication conditions has been studied in oxygen and in air The cast material was studied over the temperature range 500–1400°C, the hot isostatically pressed (HIP) material was studied at 500 and 1000°C and the single crystals were studied at 500°C The cast material exhibited three regimes of behavior Above 1000°C a continuous protective silica scale formed Between 600 and 1000°C a silica scale formed, but formation of silica within grain boundaries, which are believed to be cracked, was observed At temperatures near 500°C accelerated linear oxidation, involving the formation of both Mo and Si oxides was observed and the specimen fragmented into powder (“pested”) The HIP material exhibited two regimes At 1000°C a protective silica film formed At temperatures around 500°C the HIP material underwent accelerated oxidation but did not fragment The oxidation of the single crystal was qualitatively the same as that for the HIP material at 500°C It was concluded, therefore, that accelerated oxidation is a necessary, but not sufficient, condition for pesting to occur The pesting of the cast material was concluded to occur by oxidation along pre-existing microcracks in the MoSi2 Preoxidation at 1000°C was found to be only partially successful in limiting accelerated oxidation during subsequent exposure at 500°C

The oxidation of TaSi2 was observed to be qualitatively the same as that for MoSi2 However, the high thermodynamic stability and low volatility of Ta2O5 result in much higher temperatures being required for the occurrence of selective oxidation of Si

Oxidation of MoSi2 and comparison with other silicide materials

The cyclic oxidation of a variety of chromia-forming ferritic stainless steels has been studied in the temperature range 700–900°C in atmospheres relevant to solid-oxide fuel-cell operation The most detrimental environment at 800°C and 900°C was found to be air with 10% water vapor This resulted in excessive oxide spallation or rapid scale growth Impurities in the alloys, particularly Al and Si, were found to have a significant effect on the oxidation behavior Oxide growth was slow at 700°C but the higher-Cr-content alloys were observed to form sigma-phase at this temperature The sigma phase formation was accelerated by higher silicon contents, and remarkably, by the presence of water vapor in the exposure environment Alloys containing Mn were observed to form an outer layer of MnCr2O4 over the chromia scale The potential for this overlayer to suppress reactive evaporation of the chromia scales has been analyzed

The Oxidation of Ferritic Stainless Steels in Simulated Solid-Oxide Fuel-Cell Atmospheres

Oxidation studies were performed at 1100 °C in dry air and air containing fixed partial pressures of water vapor on a number of alloys and coatings that form α-Al2O3 scales under oxidizing conditions. The alloys investigated included RENE N5, PWA 1484, diffusion aluminide coatings (with and without Pt modification) on RENE N5, and a Ni-8 wt pct Cr-6 wt pct Al model alloy. The water vapor affected the oxidation of the alloys in three important ways: (1) The scales spalled more profusely during cyclic oxidation in wet air than in dry air, particularly for those alloys with alumina scales, which are only moderately adherent under dry conditions. The results were consistent with the mechanism previously proposed (Reference 1), whereby the water molecules decrease the fracture toughness of the alumina/alloy interface. (2) Thicker oxides are formed during oxidation in wet air than dry air. This effect comes primarily from accelerated transient oxidation during exposure in wet air. (3) Spinel was found to form on top of the alumina scales during long-term exposure. This phenomenon occurred in all atmospheres but was much more pronounced for exposures in wet atmospheres. Mechanisms for the preceding observations are proposed.

Frederick S. Pettit

Papers

Mechanisms controlling the durability of thermal barrier coatings

High temperature corrosion in energy systems

Oxidation of MoSi2 and comparison with other silicide materials

The Oxidation of Ferritic Stainless Steels in Simulated Solid-Oxide Fuel-Cell Atmospheres

Some water vapor effects during the oxidation of alloys that are α-Al2O3 formers