G. H. Meier

Mechanisms controlling the durability of thermal barrier coatings

Abstract The durability of thermal barrier coatings is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling. Because of differing manufacturing approaches and operating scenarios, several specific mechanisms are involved. These mechanisms have begun to be understood. This article reviews this understanding and presents relationships between the durability, the governing material properties and the salient morphological features. The failure is ultimately connected to the large residual compression in the thermally grown oxide through its roles in amplifying imperfections near the interface. This amplification induces an energy release rate at cracks emanating from the imperfections that eventually buckle and spall the TBC.

Oxidation of MoSi2 and comparison with other silicide materials

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.

Microstructural study of oxidized γ-tial

The microstructural development of oxidizedγ-TiAl is presented with a focus on oxidation inair. The investigations were carried out usingconventional, analytical, and, especially,energy-filtered transmission electron microscopy (EFTEM). Threeimportant points were studied in detail: (1) the“nitrogen effect,” (2) the“surface-finish effect,” and (3) thesubsurface zone. Nitrogen leads to the formation of TiN andTi2AlN at the metal-scale interfaceinterrupting alumina and thereby preventing thedevelopment of a continuous alumina layer. TheAl-depletion layer formed during the oxidation process develops from a single-phaselayer, consisting of a cubic phase, to a two-phaselayer, consisting of the cubic phase andα2-Ti3Al. The cubic phase isnot known in the system Ti-Al-O-N. Oxidation in oxygen depends on the surfacepreparation of the sample with rapid oxidation kineticsfor fine polishing and slow kinetics for a 600-gritSiC-paper finish. The rougher surface finish leads to the development of a recrystallization zonenear the surface and supports the formation of acontinuous alumina layer in the early stages ofoxidation. As for the oxidation in air, the cubic phaseis formed first underneath the oxide scale,followed by α2-Ti3Alformation.

The influence of superficially applied oxide powders on the high-temperature oxidation behavior of Cr2O3-forming alloys

The effects of superficially applied CeO2, mixed rare earth oxides, Co3O4, and Cr2O3 powders on the isothermal and cyclic oxidation of Ni-Cr alloys and the effects of CeO2 and MgO powders on the isothermal oxidation of Fe-25 wt.% Cr have been studied over the temperature range 940–1150°C in pure oxygen and dry air. The rates of oxidation of both the Ni- and Fe-base alloys were markedly reduced by the application of CeO2 powder. The presence of CeO2 also improved the scale adherence and resulted in marked changes in the oxidation morphology. The presence of Co3O4 or Cr2O3 powders on Ni-Cr alloys or MgO on Fe-Cr also produced changes in the oxidation morphology but did not decrease the rate of oxidation. These results are interpreted in terms of the influence of the oxide powders on the development of scale microstructure and their effectiveness in decreasing grain boundary transport in Cr2O3.

Formation of alumina on NbAl alloys

Abstract External alumina scales can be formed on Tial alloys with 21–30 wt.% Al in air at 1100–1400°C by increasing DAl through retention of a BCC (β-ti) structure as a major phase and simulataneously decreasing N0(S) and D0 in the beta phase through alloying additions of Cr and V. Chromium is more effective than V which has a “Cr equivalency” of %V/1.8. An alloy of Ti-30Al-12Cr-15V forms a single layer alumina scale in air at 1400°C with a parabolic rate constant the same as that for NiAl. The rate constant at 1100°C in oxygen also is the same as that of NiAl but in air at 1100°C the rate is somewhat greater. In either air or oxygen at 800°C, the rate constant is several orders of magnitude greater than that of NiAl as a result of increased transient oxidation. This is a result in part of reduced amounts of retained beta phase at lower temperatures. Increased amounts of Cr and/or V must be added to stabilize beta to lower temperatures in order to form protective alumina. Additions of V are detrimental in alloys which do not contain sufficient beta because of the accelerating effect of V on the formation of transient oxides.

Accelerated oxidation, internal oxidation, intergranular oxidation, and pesting of intermetallic compounds

The compounds MoSi2, NiAl, and NbAl3 all form protective oxide films, particularly at high temperatures where the diffusion of Si or Al is more rapid and, for the case of MoSi2, the transient oxides evaporate. However, at low temperatures, all three can undergo accelerated oxidation. The mechanisms of degradation are unique to the particular compound although there are some similarities. The accelerated oxidation of MoSi2 occurs at temperatures below 600°C by the rapid growth of Mo oxides which prevent development of a continuous silica film. Internal or intergranular oxidation does not occur. If the specimen contains cracks or pores, the rapid oxidation in these defects leads to fracture of the specimen or “pesting.” The accelerated oxidation of NiAl occurs at temperatures below 1000°C at reduced oxygen partial pressures as the result of internal oxidation and rapid intergranular oxidation. The intergranular oxidation does not lead to pesting. Special circumstances are required for the accelerated oxidation of NiAl as it does not appear to occur in flowing gases unless sulfur is present. The accelerated oxidation of NbAl3 also occurs at temperatures less than 1000°C and at reduced oxygen partial pressures and takes the form of intergranular oxidation of Al. The intergranular oxidation results in pesting of NbAl3. The phenomena of accelerated oxidation, internal oxidation, intergranular oxidation, and pesting have not been investigated in detail for most other intermetallic compounds but one or more of these phenomena seems to afflict most aluminides and silicides.

The influence of platinum on the failure of EBPVD YSZ TBCs on NiCoCrAlY bond coats

Abstract The failure of electron beam physical vapor deposition yttria stabilized zirconia thermal barrier coatings (TBCs) on NiCoCrAlY bond coats is described. It is shown that defects in the as-processed bond coats are responsible for TBC failures. By using a thin platinum layer deposited upon the NiCoCrAlY bond coat, it is shown that the lives of TBCs on such bond coats can be significantly extended since the as processed defects are removed.

The oxidation behavior of intermetallic compounds

Abstract The selective oxidation of intermetallic compounds is described. It is shown that the fundamental concepts developed for the oxidation of conventional alloys ( e . g . iron- and nickel-base alloys) pertain also to the oxidation of intermetallics with slight modification. The effects of temperature on the development of protective external scales and on the occurrence of “pesting” are considered. The effects of third element additions on the oxidation mechanisms and influence of gas composition on oxidation rates and morphologies and on the formation of volatile reaction products are described. Finally, the effects of interstitial embrittlement of intermetallic compounds are briefly considered. Selected data for compounds of primary interest as high temperature materials (aluminides, silicides) are used to illustrate the above-mentioned fundamentals.

The effect of nitrogen on the oxidation of γ-TiAl

Protective alumina scales are formed on TiAl, exposed in oxygen, up to temperatures near 1,000 C. However, the same exposures conducted in air result in the formation of TiO{sub 2}-rich scales which grow at rates orders of magnitude faster than the alumina scales. This paper describes the results of a study in which cross-section transmission electron microscopy was used to elucidate the effect of nitrogen on the development of the oxidation products on {gamma}-TiAl.

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

Abstract In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO2–30% H2O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO2 and Ar–H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(–H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.