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

The oxidation behaviour of metals and alloys at high temperatures in atmospheres containing water vapour: A review

Binary Fe–Cr alloys containing 10 and 20 mass% Cr were studied with respect to isothermal oxidation behavior at 900 and 1,050 °C in Ar–20%O2, Ar–7%H2O and Ar–4%H2−7%H2O. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar–O2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy. This mechanism is an “easy” explanation of the frequently observed effect that Fe–Cr alloys with intermediate Cr contents (e.g. 10–20%, depending on temperature) exhibit protective chromia-rich scale formation in dry gases but breakaway type Fe-rich oxides in wet gases, provided the oxygen partial pressure is sufficiently high for Fe to become oxidized.

The Effect of Water Vapor on Selective Oxidation of Fe-Cr Alloys

A sintering model for plasma-sprayed zirconia TBCs. Part I: Free-standing coatings

A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the heat flow. Contributions from convective, conductive, and radiative heat transfer are considered, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions. In fact, convective heat flow within TBCs usually makes a negligible contribution to the overall heat transfer through the coating, although what might be described as convection can be important if there are gross through-thickness defects such as segmentation cracks. Radiative heat transfer, on the other hand, can be significant within TBCs, depending on temperature and radiation scattering lengths, which in turn are sensitive to the grain structure and the pore architecture. Under most conditions of current interest, conductive heat transfer is largely predominant. However, it is not only conduction through solid ceramic that is important. Depending on the pore architecture, conduction through gas in the pores can play a significant role, particularly at the high gas pressures typically acting in gas turbines (although rarely applied in laboratory measurements of conductivity). The durability of the pore structure under service conditions is also of importance, and this review covers some recent work on how the pore architecture, and hence the conductivity, is affected by sintering phenomena. Some information is presented concerning the areas in which research and development work needs to be focussed if improvements in coating performance are to be achieved.

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Heat Transfer Through Plasma-Sprayed Thermal Barrier Coatings in Gas Turbines: A Review of Recent Work

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.

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

Water vapor is present in most environments in which alloys are used at elevated temperatures and there are papers in the literature that show water vapor usually has adverse effects on the oxidation resistance of alloys. However, the exact effect of water vapor is dependent on the particular alloy under consideration. This paper is concerned with the oxidation of alloys that rely upon the development of α-Al2O3 scales for oxidation resistance. This paper describes two major deleterious effects of water vapor on the oxidation of such alloys. One effect involves increased spalling of α-Al2O3 which will be shown to be less significant in the case of alloys with extremely adherent α-Al2O3 scales. It is proposed that water vapor causes the α-Al2O3-alloy interfacial toughness to be decreased, however this effect is not sufficient to cause spalling of extremely adherent α-Al2O3 scales. Another effect of water vapor is that it causes more transient oxides to be formed during the selective oxidation of al...

Water vapor effects on the cyclic oxidation resistance of alumina forming alloys

Yttria-stabilized Zirconia (YSZ) coatings deposited by electron beam physical vapor deposition on platinum aluminide and NiCoCrAlY bond coats on single crystal superalloy substrates have been oxidized at temperatures between 1000 and 1200°C in air. The cyclic oxidation lives of the systems with platinum aluminide bond coats were substantially longer than those with NiCoCrAlY bond coats. The thermally grown oxide (TGO) that develops between the bond coat and the TBC during oxidation, as well as the bond coat and the TBC adjacent to the TGO, have been examined in detail using optical metallography, scanning electron microscopy (SEM), and cross-sectional transmission electron microscopy (XTEM). The YSZ is observed to undergo significant amounts of sintering. The TGO grows by the inward diffusion of oxygen and the outward diffusion of aluminum. In some cases, the outward growth component incorporates some of the TBC into the TGO. The depletion of aluminum results in phase changes in the bond coats. Failure of the TBCs occurs after fixed amounts of oxidation which result in increasing amounts of elastic energy being stored in the TGO and YSZ as well as degradation of the TGO-bond coat interface. The fracture path changed as a function of exposure time and temperature with larger amounts of separation occurring at the TGO/BC interface for higher temperatures and longer exposures in dry air. Failure can be accelerated in the presence of water vapor, particularly if spinel formation is induced. Fracture occurs primarily in the oxides, in this case. The fracture surface for systems with platinum aluminide bond coats often contains precipitates, which are rich in refractory metals. These features do not appear to be prevalent with NiCoCrAlY bond coats.

The Effects of High Temperature Exposure on the Durability of Thermal Barrier Coatings

The oxidation of superalloys that rely upon the development of Al2O3 or Cr2O3 scales for oxidation resistance in air containing water vapor are compared at temperatures from 700°C to 1100°C. It is shown that water vapor affects the oxidation of such alloys in different ways. Water vapor adversely affects the selective oxidation of aluminum in the -Al2O3-formers, and also affects the adherence of these oxide scales. The major effect of water vapor on the chromia-formers is enhanced vaporization of Cr2O3. Aluminide coated superalloys are also examined in these cyclic oxidation tests, and it will be shown that water vapor caused spalling of the -Al2O3 scales, but the effect is small for strongly adherent -Al2O3 scales.

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M.C. Maris-Sida

Papers

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

Water vapor effects on the cyclic oxidation resistance of alumina forming alloys

The Effects of High Temperature Exposure on the Durability of Thermal Barrier Coatings

The Effects of Water Vapor on the Oxidation of Nickel-Base Superalloys and Coatings at Temperatures from 700C to 1100C