Frederick S. Pettit

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.

Mechanisms for the hot corrosion of nickel-base alloys

The Na2SO4-induced accelerated oxidation of nickel-base alloys containing elements such as Cr, Al, Mo, W, and V has been studied in 1.0 atm O2 in the temperature range of 650° to 1000°C. It has been found that the hot corrosion behavior of these alloys can usually be characterized according to one of two types of attack: 1) Na2SO4-induced accelerated oxidation; 2) Na2SO4-induced catastrophic oxidation. In both types of hot corrosion, accelerated oxidation occurs as a result of the formation of a liquid flux based on Na2SC>4 which dissolves the normally protective oxide scales. Catastrophic, or self-sustaining rapid oxidation can occur in alloys which contain Mo, W, or V, because solutions of oxides of these elements with Na2SO4 decrease the oxide ion activity of the molten salts, producing melts which are acidic fluxes for oxide scales. The accelerated oxidation type of attack which was observed with most alloys which did not contain Mo, W, or V, was more severe than for normal oxidation, but much less severe than catastrophic oxidation. Na2SO4-induced accelerated oxidation occurs because the oxide ion activity of the Na2SO4 increases to the point where oxide scales can partially dissolve in the basic melt. Generally, this basic fluxing results from the diffusion of sulfur from the Na2SO4 into the alloy. In some alloys, the formation of sulfides during basic fluxing is a sufficient condition to cause accelerated oxidation. In other alloys, changes in the oxidation mechanism occur because of depletion of the alloy surface, concomitant with basic fluxing, of those elements needed for protective oxide scales, such as aluminum and chromium.

High temperature corrosion in energy systems

At high temperatures, particularly in response to the unique environments associated with the conversion or combustion of fossil fuels, further fundamental studies of alloy reactions with mixed gaseous oxidants are required. Thermodynamic, phase equilibria and diffusion data are lacking in part, and isotope and tracer studies have not been forthcoming. Corrosive thin films of salts and slags on the hardware of gas turbines, heat exchangers, fuel cells and batteries cause an accelerated “hot corrosion”. Thin film electrochemical studies for simple salts and alloys, and supporting thermodynamic studies (solubilities of solids and gases in salts), are required to understand the corrosion mechanism. The effects of several trace gaseous impurities (particularly chlorine) both on the growth and damaging of protective oxide scales and on the degradation of alloy mechanical properties should be studied. High resolution in situ scanning electron microscopy studies could prove fruitful in clarifying uncertain scale growth mechanisms. New protective coating compositions must be found for specific corrosive environments, and more reliable but less expensive coating methods are required. Factors critical to the adhesion of oxide scales (e.g. α-Al2O3 and Cr2O3) on alloys, and the effects of trace alloying elements or dispersed second phases on scale adherence, deserve further attention. The effect on gas-alloy attack of solid deposits, either reactive or relatively inert, should be examined. Electrochemical studies should be made of alloy corrosion in deep salt melts or slags, where the gaseous environment is remote from the alloy surface. The role of grain boundaries in corrosion product scales as short-circuit transport paths for the outward diffusion of metal and the inward ingress of oxygen, sulfur and carbon needs to be clarified. Erosion-corrosion interactions should be studied, with attempts to define the types of coatings that are most resistant to such conditions. Particularly in solar applications, the role of thermal cycling and cyclic stressing on high temperature scaling (corrosion fatigue) needs to be studied. New methods for the monitoring of the concentrations of corrosive components, particularly sulfur and chlorine, in gaseous and fused salt environments require development. The influences of temperature gradients and heat fluxes on material compatibility, redistribution of chemical components and properties of corrosion product layers need further study. High temperature corrosion-resistant alloys excluding the strategic metals chromium and cobalt need to be developed.

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.

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.

Mechanisms in the simultaneous erosion-oxidation attack of nickel and cobalt at high temperature

An apparatus is described by which a specimen may be exposed to a well-defined erosive gas stream containing 20 μm alumina particles. Samples of nickel and cobalt were exposed to conditions of erosion and high temperature oxidation simultaneously, using 90 deg impact angle, at velocities up to 140 m/s-1 and at temperatures up to 800 °C. From the results of erosion kinetics, surface morphologies, and cross section metallographic observations it is concluded that several definite regimes of interaction between erosion and oxidation exist. These regimes are described in qualitative terms, and it is shown how they arise as a result of the balance between the relative intensities of the erosion and oxidation processes.

The effect of the type of thermal exposure on the durability of thermal barrier coatings

Abstract The effect of cycle frequency on the spallation failure of thermal barrier coatings has been investigated. The exposure conditions affect the lifetimes of the coatings and can even change the relative performance of different bond coats. The very strong effect of exposure temperature is consistent with thermally grown oxide growth being a first order variable in scale failure.

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

Abstract 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 aluminum in alloys. This condition becomes more severe at lower temperatures. Possible mechanisms by which water vapor affects the selective oxidation of aluminum in alloys will be described.