There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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 recognition of the potential for enhanced fracture toughness that can be derived from controlled, stress-activated tetragonal (t) to monoclinic (m) transformation in ZrO2-based ceramics ushered in a new era in the development of the mechanical properties of engineering ceramics and provided a major impetus for broader-ranging research into the toughening mechanisms available to enhance the fracture properties of brittle-matrix materials. ZrO2-based systems have remained a major focal point for research as developments in understanding of the crystallography of the t→m transformation have led to more-complete descriptions of the origins of transformation toughening and definition of the features required of a transformation-toughening system. In parallel, there have been significant advances in the design and control of microstructure required to optimize mechanical properties in materials developed commercially. This review concentrates on the science of the t→m transformation in ZrO2 and its application in the modeling of transformation-toughening behavior, while also summarizing the microstructural control needed to use the benefits in ZrO2-toughened ceramics.

Transformation Toughening in Zirconia‐Containing Ceramics

Isothermal oxidation of NiAl + Zr has been performed over the temperature range of 800–1200°C and studied by TGA, XRD, and SEM. A discontinuous decrease in growth rate of two orders of magnitude was observed at 1000° C due to the formation of α-Al2O3 from θ-Al2O3. This transformation also resulted in a dramatic change in the surface morphology of the scales, as a whisker topography was changed into a weblike network of oxide ridges and radial transformation cracks. It is believed that the ridges are evidence for a shortcircuit outward aluminum diffusion growth mechanism that has been documented in a number of18O tracer studies.

Effect of the /theta/-/alpha/-Al/sub 2/O/sub 3/ transformation on the oxidation behavior of /beta/-NiAl + Zr

A high-pressure burner rig was developed to evaluate the response of chemical-vapor-deposited SiC material during exposure to simulated gas turbine combustor conditions. Linear weight loss and surface recession rates of SiC were observed in both fuel-lean and fuel-rich gas mixtures. This response was shown to result from SiO2 scale volatility. Arrhenius-type temperature dependence was demonstrated. In addition, the effects of pressure and gas velocity were defined in terms of a gaseous-diffusion-controlled process for volatile reaction products (such as SiO, Si(OH)4, and iO(OH)x). Accordingly, multiple linear regression was used to develop empirical recession relationships of the form exp(-DeltaQ/RT)Pxvyfor both lean and rich combustion conditions. Part II of this paper discusses the thermodynamics and gaseous-diffusion model of this recession. The empirical models discussed here enable prediction of SiC recession for any combination of T, P, and vin turbine environments. For typical combustion conditions, recession of 0.2-2 µm/h was predicted at 1200°-1400°C. Thus, long-term, high-temperature, high-velocity exposure may degrade silicon-based or SiO2-forming material by recession in combustion gas environments.

SiC Recession Caused by SiO2 Scale Volatility under Combustion Conditions: I, Experimental Results and Empirical Model

The transient oxidation of β-NiAl in air at 800 °C and 1100 °C has been studied using electron microscopy. The oxide scale consists predominatly of metastable Al2O3 phases. θ-Al2O3 is the major oxide phase within 10.0 hr of oxidation at 800 °C and 0.1 hr at 1100 °C. The scales form epitaxially on (001)β and (012)β specimens throughout the transient stage, whereas the degree of preferred oxide orientation decreases with oxidation time on (011)β and (111)β specimens. The orientation relationships reflect the small mismatch between parallel close-packed directions in the metal and in the cation sublattice of the oxides. The correlation of distinctive oxide surface morphologies with internal structural defects indicates the strong tendency of the Al2O3 scale to growvia short-circuit diffusion paths.

Transient oxidation of Single-Crystal β-NiAl

Phase stability in plasma-sprayed, partially stabilized zirconia-yttria

In combustion environments, volatilization of SiO2 to Si-O-H(g) species is a critical issue. Available thermochemical data for Si-O-H(g) species were used in the present study to calculate boundary-layer-controlled fluxes from SiO2. Calculated fluxes were compared to volatilization rates of SiO2 scales grown on SiC, which were measured in a high-pressure burner rig, as reported in Part I of this paper. Calculated volatilization rates also were compared to those measured in synthetic combustion gas furnace tests. Probable vapor species were identified in both fuel-lean and fuel-rich combustion environments, based on the observed pressure, temperature, and velocity dependencies, as well as on the magnitude of the volatility rate. Water vapor was responsible for the degradation of SiO2 in the fuel-lean environment. SiO2 volatility in fuel-lean combustion environments was attributed primarily to the formation of Si(OH)4(g), with a small contribution of SiO(OH)2(g). Reducing gases such as H2 and/or CO, in combination with water vapor, contributed to the degradation of SiO2 in the fuel-rich environment. The model to describe SiO2 volatility in a fuel-rich combustion environment gave a less satisfactory fit to the observed results. Nevertheless, it was concluded-given the known thermochemical data-that SiO2 volatility in a fuel-rich combustion environment is best described by the formation of SiO(g) at 1 atm total pressure and the formation of Si(OH)4(g), SiO(OH)2(g), and SiO(OH)(g) at higher pressures. Other Si-O-H(g) species, such as Si2(OH)6, may contribute to the volatility of SiO2 under fuel-rich conditions; however, complete thermochemical data are unavailable at this time.

James L. Smialek

Papers

Effect of the /theta/-/alpha/-Al/sub 2/O/sub 3/ transformation on the oxidation behavior of /beta/-NiAl + Zr

SiC Recession Caused by SiO2 Scale Volatility under Combustion Conditions: I, Experimental Results and Empirical Model

Transient oxidation of Single-Crystal β-NiAl

Phase stability in plasma-sprayed, partially stabilized zirconia-yttria

Sic Recession Due to Sio2 Scale Volatility Under Combustion Conditions: Part 2; Thermodynamics and Gaseous Diffusion Model