Mechanisms controlling the durability of thermal barrier coatings

▪ Abstract The emphasis in this short review is to describe the materials issues involved in the development of present thermal barrier coatings and the advances necessary for the next generation, higher temperature capability coatings.

Materials design for the next generation thermal barrier coatings

The addition of reactive elements can have a significant effect on the oxidation behavior of alumina- and chromia-forming alloys. A model has been developed to explain the effects associated with the addition of reactive elements that is based on the segregation of reactive-element ions to scale grain boundaries and the metal-oxide interface. Reactive-element ions use these interaces as pathways for diffusion from the metal substrate to the gas interface of the scale. The driving force for this outward diffusion is the oxygen potential gradient across the scale. Doping of the scale grain boundaries results in scale growth primarily by inward oxygen diffusion, while doping at the metal-oxide interface slows the growth of interfacial voids and thus improves scale adhesion.

Experimental observations in support of the dynamic-segregation theory to explain the reactive-element effect

Some of the important principles that determine the establishment, growth and long-term maintenance of protective Cr2O3, Al2O3 and SiO2 scales on hightemperature iron-, nickel- and cobalt-base alloys are reviewed and discussed. Emphasis is placed on the effects of alloying elements and other additions, such as third elements and reactive elements or oxide dispersions, on each of these processes. Particular attention is paid to transport processes in the scales and the importance of short-circuit paths. Some of the important parameters that influence the long-term mechanical stability of such scales are considered and evaluated.

The influence of alloying elements on the development and maintenance of protective scales

A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects

Currentunderstanding of the complex interrelationships among growth kinetics, microstructure, and adhesion of protective Cr2O3 and Al2O3 scales is critically reviewed. Similarities and differences in the behavior of these two systems are highlighted. The morphology of the alloy-scale interface appears to be a critical factor. Recent ideas are advanced to interpret the effect of oxygen-active elements on the development of a tortuous interface conducive to improved scale tenacity.

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Microstructure, adhesion and growth kinetics of protective scales on metals and alloys

The depths of internal oxidation of Ni-Al (0.5–4 wt.%) alloys oxidized in the temperature range 800–1100°C in either Ni/NiO packs or in 1 atm oxygen are virtually independent of the alloy compositi...

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Enhanced diffusion of oxygen during internal oxidation of nickel-base alloys

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Peg Formation by Short‐Circuit Diffusion in Al2 O 3 Scales Containing Oxide Dispersions

The improvement in oxidation resistance produced by small additions of active elements to Al
2O3-forming CoCrAl alloys is primarily dependent on the formation of oxide pegs which grow into the alloy around the internal oxide particles of the active element; void formation at the alloy-scale interface is also suppressed. The distribution of these pegs is critical and this paper demonstrates that an internal oxidation pretreatment can be used to convert the active element to its oxide in a controlled manner, thereby optimizing the peg distribution. Al2O3-forming CoCrAl containing 1% Hf or Ce is internally oxidized in a sealed quartz capsule containing a 50/50 powder mixture of CoAl-Al2O3; it was not possible to oxidize internally Y-containing alloys. The isothermal and cyclic oxidation resistance of these alloys is superior to that of alloys not given a prior treatment. Detailed metallographic examination indicates that the prior internal oxidation treatment produces a finer, more uniform distribution of oxide pegs penetrating into the alloy which is more efficient in combatting scale spallation. Furthermore, the lower residual hafnium content in the alloy minimizes large HfO2 precipitates and the formation of gross Al2O3 intrusions, which can initiate scale failure. Thus, by internally oxidizing the alloy first, the advantages of a high alloy Hf content (1%) in producing sufficient oxide pegs, but of the right size, coupled with minimal thickening of the surface scale, can be achieved.

https://escholarship.org/content/qt6p445010/qt6p445010.pdf?t=p2asx3

D. P. Whittle

Papers

Microstructure, adhesion and growth kinetics of protective scales on metals and alloys

Enhanced diffusion of oxygen during internal oxidation of nickel-base alloys

Peg Formation by Short‐Circuit Diffusion in Al2 O 3 Scales Containing Oxide Dispersions

Improvements in oxidation resistance by dispersed oxide addition: Al2O3-forming alloys

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys