Nureddin M. Abbas

Role of platinum in aluminide coatings

Abstract An investigation has been carried out to determine the role of platinum in aluminide coatings on selected nickel-based superalloys. Various microscopy, spectroscopy and diffraction techniques were used to characterize the microstructure. It is found that platinum improves the protective nature of the coating by (1) increasing its diffusional stability, (2) eliminating chromium-rich precipitates from the outer coating layer and (3) preventing refractory transition elements such as molybdenum, vanadium and tungsten from diffusing into the outer coating layer. However, the oxidation behaviour of the coating varies from one alloy substrate to another depending upon its chemical composition and, in particular, the rare earth elements.

Suppressed ion chromatographic analysis of anions in environmental waters containing high salt concentrations

Abstract The literature on the application of suppressed ion chromatography for the analysis of low concentrations of anions in the presence of a high concentration of matrix ions is critically reviewed. It is reported in the literature that suppressed ion chromatographic determinations of anions such as sulfate, arsenate, oxalate, etc., give erroneous results when low concentrations are quantified in the presence of high salt concentrations. In many cases this is believed to arise from overloading of the column by matrix ions, typically by matrix anions. However, as discussed in this review, the erroneous results in the suppressed ion chromatographic determination of small concentrations of anions such as sulfate, oxalate and molybdate in matrices having very high salt concentrations are not caused by column overloading but are due to some anion-proton interactions within the suppressor. Determination of anions of strong acids such as Cl − , Br − , and I − is not affected and they can be accurately quantified by suppressed ion chromatography in aqueous matrices containing high salt concentrations. Suppressed ion chromatography as a tool for the accurate analysis of small concentrations of anions in the presence of high ionic strength matrices, such as subsurface waters, well waters, and aqueous extracts of sandy soils has also been reported.

Role of Y during high temperature oxidation of an M-Cr-Al-Y coating on an Ni-base superalloy

Abstract A study was carried out to determine the role of Y during high temperature oxidation or an M-Cr-Al-Y-type coating on an Ni-base superalloy using various techniques of electron microscopy. Prior to oxidation exposures, Y was found to segregate within an external surface layer of the coating about 5 μm in thickness as well as within the interdiffusion zone. After exposure at 1000 or 1100°C in still air, Y exhibited a preferential tendency for segregation at grain boundaries of α-Al2O3 scale developed by the coating, however, a finite concentration of Y was also present in solid solution. In addition, some of the Y was found to oxidize into a Y-rich oxide “pegs” within the α-Al2O3 scale and extending into the coating. Experimental results showed that segregation of Y to grain boundaries could maintain a fine-grained scale after exposure for up to 1000 h at 1000 or 1100°C. It was concluded that, for the coating-substrate system studied, Y could improve the protective nature of α-Al2O3 scale by various mechanisms operating simultaneously.

Thermal stability of a platinum aluminide coating on nickel-based superalloys

An investigation was carried out to determine the thermal stability of a platinum aluminide coating on the directionally solidified alloy MAR M 002 and its single-crystal version alloy, SRR 99, at 800, 1000 and 1100°C. The morphology, structure and microchemical composition of the coating were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. In the as-deposited condition, the coating was found to consist of two layers. Most of the platinum was concentrated in the outer coating layer which consisted of a fine dispersion of PtAl2 in a matrix of β-(Ni, Pt)Al containing other elements in solid solution, such as cobalt and chromium. The inner coating layer was relatively free of platinum and consisted essentially of β-NiAl. Exposure at 800°C was found to have no significant effect on the structure and composition of the coating on each alloy. At temperatures ⩾1000°C, however, PtAl2 became thermodynamically unstable and significant interdiffusion occurred between the coating and alloy substrate. After exposure at 1000°C, the components of the outer coating layer were NiAl and Ni3Al. However, after exposure at 1100°C, the outer coating layer consisted only of Ni3Al. Also, after exposure at both temperatures, the composition of the outer coating layer approached that of the inner layer due to interdiffusion. Although the coating on both alloys exhibited similar structural stability at all temperatures investigated, the coating on alloy MAR M 002 was found to develop a more protective scale. This behaviour was correlated with differences in alloy substrate composition particularly rare-earth elements such as hafnium.

Effect of superalloy substrate composition on the performance of a thermal barrier coating system

An investigation was carried out to determine the performance of a thermal barrier coating system consisting of (ZrO2-8% Y2O3)/(Pt) on two single-crystal Ni-base superalloys. Coating/alloy behavior was studied with reference to: (i) initial microstructural features, (ii) oxidation properties, (iii) thermal stability characteristics, and (iv) failure mechanism. All thermal exposure tests were carried out at 1150°C in still air with a 24-h cycling period to room temperature. Failure of the coating system was indicated by macroscopic spallation of the ceramic top coat. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used to characterize the microstructure.Decohesion between the thermally grown oxide and bond coat was found to be the mode of failure of the coating system for both alloys. This was correlated with the formation of Ti-rich and/or Ti+Ta-rich oxide particles near the oxide-bond coat interface degrading the adherence of the thermally grown oxide. However, the thickening rate of the oxide had very little or no effect on the relative coating performance. It was concluded that the coating performance is critically dependent on alloy substrate composition particularly the concentration of elements, which could have adverse effects on oxidation resistance such as Ti.

Comparative thermal stability characteristics and isothermal oxidation behavior of an aluminized and a Pt-aluminized Ni-base superalloy

It is the objective of this paper to compare the thermal stability characteristics and isothermal oxidation behavior of an aluminide coating and a Pt-aluminide coating of the same Al content on a Ni-base superalloy. Addition of Pt to an aluminide coating was found to improve its thermal stability as well as its capability for selective oxidation of Al resulting in a purer scale of slower growth rate. This was correlated with the greater diffusional stability of the Pt-aluminide coating restricting the transport of substrate elements into the outer coating layers.

Failure mechanism of a thermal barrier coating system on a nickel-base superalloy

An investigation was carried out to determine the failure mechanism of a thermal barrier coating system on an Ni-base superalloy. The coating system consisted of an outer layer of yttria-stabilized zirconia (top coat), and an inner layer of Pt-aluminide (bond coat). Specimens were exposed at 1010 and 1150 °C with a 24-h cycling period to room temperature. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used in microstructural characterization. Spallation of the oxide scale developed by the bond coat was found to be the mode of failure. Experimental results indicated that the breakdown of oxide was affected by internal oxidation of Hf diffusing from the alloy substrate into the bond coat surface developing localized high levels of stress concentration at the oxide–bond coat interface. It was concluded that the cause of failure was degradation of thermal stability of the bond coat accelerating its oxidation rate and permitting outward diffusional transport of elements from the substrate.

Mechanism of carburization of high-temperature alloys

An investigation of the mechanism of gaseous carburization in a reducing environment was conducted for selected Fe- and Ni-base alloys. Carburization kinetics were measured as functions of temperature in the range 870–980 °C. Scanning electron microscopy, analytical electron microscopy and X-ray diffractometry were employed for microstructural characterization and microchemical analysis. Changes in mechanical strength produced by carburization were determined from microhardness and tensile property measurements. Kinetic studies indicated that the carburization reaction followed a parabolic rate law. Depending upon the nature of surface scale formed in the presence of a carburizing environment, the rate-determining step of the reaction varied from C diffusion into the alloy in the presence of a carbide scale to that in the presence of an oxide scale. Under reducing carburizing conditions, alloys inherently protected by Cr2O3-base scale were found to develop a surface carbide scale which allowed C to penetrate into the alloy with relative ease and, thus, the carburization kinetics was accelerated. In contrast, an alloy capable of forming Al2O3 developed and maintained a protective surface oxide scale which acted as an effective barrier to C diffusion into the alloy. Degradation of mechanical strength due to precipitation of carbides in the alloy was correlated with the rate of attack and consequently the nature of the surface scale.

Comparative performance of selected bond coats in advanced thermal barrier coating systems

An investigation was carried out to determine the comparative performance of selected bond coats representing the diffusion aluminides and overlays in thermal barrier coating systems. Emphasis was placed upon oxidation behavior, thermal stability, and failure mechanism. Isothermal oxidation tests were carried out attemperatures in the range of 1000 °C to 1150 °C. Scanning electron microscopy combined with energy dispersive x-ray spectroscopy, x-ray diffraction, and transmission electron microscopy were used to characterize the coating microstructure. Among the bond coats examined, overlays exhibited the best performance followed by Pt-aluminides and simple alunimides for a given alloy substrate. However, for all types of bond coats, failure of the coating system occurred by decohesion of the oxide scale at the oxide-bond coat interface. All bond coats examined were found to be degraded by oxidation and interdiffusion with the alloy substrate permitting the formation of non-protective oxide scale near the bond coat surface. Platinum as well as active elements such as Hf and Y were identified as key elements in improving the performance of thermal barrier coating systems.

Thermal stability of advanced Ni-base superalloys

Exposures consisting of 1 to 900 h at 1000 and 1100 °C after an ageing treatment of 16 h at 870 °C were used to study the thermal stability of selected γ′-strengthened Ni-based superalloys representing conventional, directional solidification, and single-crystal castings. Various techniques of microscopy, spectroscopy and diffraction were used to characterize the microstructure. Primary MC carbides in the alloys studied were found to be stable toward decomposition into lower carbides. In the aged condition, the strengthening γ′ phase assumed a cuboidal morphology; however, all alloys also contained varying proportions of coarse lamellar γ′ and hyperfine cooling γ′. On an atomic scale, the nature of the cuboidal γ′-matrix interface was found to vary from coherent to partially coherent. However, the overall lattice mismatch varied from one alloy to another depending upon its composition and the distribution of various elements in carbide phases and lamellar γ′ phase. Directional growth of the cuboidal γ′ phase upon exposure to higher temperatures was found to be accelerated by a large initial lattice mismatch leading to a considerable loss of coherency, as indicated by the observation of dislocation networks around the γ′ particles. Although the composition of the γ′ phase remained essentially unchanged, there was a marked change in matrix composition. Sigma phase was found to precipitate in all alloys, but its thermal stability was a function of alloy composition. The initial decrease in hardness followed by a hardening effect during exposure could be explained in terms of the partial dissolution of the γ′ phase and precipitation of sigma phase.