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

This paper summarizes the basic properties of ceramic materials for thermal barrier coatings. Ceramics, in contrast to metals, are often more resistant to oxidation, corrosion and wear, as well as being better thermal insulators. Except yttria stabilized zirconia, other materials such as lanthanum zirconate and rare earth oxides are also promising materials for thermal barrier coatings.

Ceramic materials for thermal barrier coatings

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot-zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high-grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness.

Silicon Nitride and Related Materials

Handbook of SiC properties for fuel performance modeling

The processes of passive oxidation, deposit-induced corrosion, active oxidation, scale/substance interactions, and scale volatility are presently studied in the case of high-purity SiC and Si3N4 in pure oxygen, giving attention to such secondary elements in the ceramics as water and CO2 oxidants, combustion environment impurities, and thermal cycling. Deposit-induced corrosion is discussed for the cases of NaSO4 as well as vanadate and oxide-slag deposits; issues associated with the active-to-passive oxidation transition are noted.

/pdf/corrosion-of-silicon-based-ceramics-in-combustion-2993joj3ia.pdf

Corrosion of Silicon-Based Ceramics in Combustion Environments

Oxidation microstructure studies of reinforced carbon/carbon

Mullite is promising as a protective coating for silicon-based ceramics in aggressive high-temperature environments. Conventionally plasma-sprayed mullite on SiC tends to crack and debond on thermal cycling. It is shown that this behavior is due to the presence of amorphous mullite in the conventionally sprayed mullite. Heating the SiC substrate during the plasma spraying eliminated the amorphous phase and produced coatings with dramatically improved properties. The new coating exhibits excellent adherence and crack resistance under thermal cycling between room temperature and 1000 to 1400 C. Preliminary tests showed good resistance to Na2CO3-induced hot corrosion.

/pdf/new-generation-of-plasma-sprayed-mullite-coatings-on-silicon-x2i4u99232.pdf

New generation of plasma-sprayed mullite coatings on silicon carbide

In this paper, we report the results of equilibrium pressure measurements designed to identify the volatile species in the Cr−O−H system and to resolve some of the discrepancies in existing experim...

Theoretical and experimental investigation of the thermochemistry of CrO2(OH)2(g).

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.

Nathan S. Jacobson

Papers

Corrosion of Silicon-Based Ceramics in Combustion Environments

Oxidation microstructure studies of reinforced carbon/carbon

New generation of plasma-sprayed mullite coatings on silicon carbide

Theoretical and experimental investigation of the thermochemistry of CrO2(OH)2(g).

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