Pierre Josso

Effect of Pt Additions on the Sulfur Segregation, Void Formation and Oxide Scale Growth of Cast Nickel Aluminides

The addition of Pt in cast and diffusion nickel aluminide is known to improve the oxide scale adhesion, but the positive effects of this element remain still imprecise. This present study reports experimental results concerning the diminution of sulfur segregation in the presence of platinum (10% at.) in β-NiAl, at 750°C. However, Pt did not completely prevent sulfur from segregating to a free surface. It was also shown that the addition of Pt in the alloy decreased the formation of voids at the oxide/metal interface after isothermal oxidation at 900°C. The ratio of the oxide thickness to cavity depth strongly decreases in Pt modified nickel aluminide. Lastly, thermogravimetric investigations revealed that Pt increased the oxide growth rate at 1100°C.

Electroless deposition of pure nickel, palladium and platinum

Abstract Electroless deposition techniques are used for an increasing number of applications where thin metallic coating of complex shaped components is required. However, in most cases, the deposits obtained are not composed of pure metals but rather contaminated with undesirable elements released during the plating process (phosphorus, boron, sulphur, heavy metals). For specific applications involving a high temperature environment or requiring ductile coatings, electroless deposition of pure metals is necessary. For this purpose, a new process was designed using hydrazine as the reducing agent. Electrochemical measurements throughout a wide range of bath compositions were performed; several specific compositions were selected which provide high purity deposits of metals such as nickel, palladium and platinum. Attempts to rationalize the deposition processes in terms of electrochemical reactions are briefly reported. The optimization of several bath formulations is being discussed. Some physical and chemical characteristics of the pure metal deposits obtained are compared with those of classical electroless metallic coatings.

The Effect of Sulfur Segregation on the Adherence of the Thermally-Grown Oxide on NiAl—I: Sulfur Segregation on the Metallic Surface of NiAl(001) Single-Crystals and at NiAl(001)/Al2O3 Interfaces

The aim of this study was to improve the understanding of the deleterious effect of sulfur impurities on the adherence of the thermally-grown oxide on the boundary layer in thermal-barrier-coating systems. In Part I, the sulfur segregation on the free surface of NiAl(001) and at different interfaces between metal and transient alumina scales has been characterized by AES, XPS and LEED. The sulfur diffusion coefficient in the alloy has been determined (D = 0.15 exp(−218,000/RT) cm2/s). It is by three orders of magnitude larger than the nickel and aluminum self-diffusion coefficients. It has also been observed that the sulfur de-segregates upon Al enrichment of the metallic surface. The saturation of the metallic surface with an amorphous alumina layer formed at room temperature blocks the segregation of S. However, in the initial stages of oxidation where the transient θ-alumina grows by cationic transport and inject vacancies at the interface, S segregates at the interface between the alumina thin films and the metallic substrate.

STRUCTURE AND HIGH TEMPERATURE PERFORMANCE OF VARIOUS PALLADIUM-MODIFIED ALUMINIDE COATINGS: A LOW COST ALTERNATIVE TO PLATINUM ALUMINIDES

Abstract Of the protective coatings applied to turbine hot stage components (generally superalloys), platinum-modified aluminide coatings have proved to be satisfactorily hot corrosion resistant (type I hot corrosion). More recently, cost-saving measures have motivated studies on the replacement of platinum by a cheaper metal: palladium. The structures of three different types of palladium-modified aluminide coating on Inconel 100 (IN 100) substrates are described; it is shown that the integrity of the coatings critically relies on an adequate processing route (preventing hydrogen embrittlement). Cyclic oxidation tests at 1100 °C and 850 °C hot corrosion tests (in air and air plus 0.01% SO 2 ) have been performed on simple aluminides, commercial RT22 platinum-modified coatings and palladium-modified coatings applied to IN 100. Palladium not only stabilizes the protective alumina scale formed in cyclic oxidation, but also dramatically increases the hot corrosion resistance of the coatings. Indeed, the performance of palladium-modified coatings compares very well with that of commercial platinum-modified coatings in terms of hot corrosion resistance.

Zirconium Modified Aluminide by a Vapour Pack Cementation Process for Thermal Barrier Applications: Formation Mechanisms and Properties

The main mechanism of degradation of thermal barrier systems is the spallation of the thermally grown oxide (TGO) formed at the ceramic/metallic sub-layer. Present trends are the use of a platinum modified nickel aluminide layer as metallic sub-layer. But the effect of few p.p.m. or few % at. of zirconium on the oxidation of hipped or cast nickel aluminide was also studied for higher temperature applications. It has been shown that the system ZrO2-Y2O3 (8% mass.) EBPVD / cast NiAl(Zr) had at 1200°C a life durability three times longer than the classical system ZrO2-Y2O3 (8% mass.) EBPVD /NiAlPt coating/nickel base superalloy. Similar encouraging results were obtained with the system ZrO2-Y2O3 (8% mass.) EBPVD / NiAl(Zr) coating deposited by physical vapour deposition/ nickel base superalloy. In this work, a zirconium modified nickel aluminide coating by vapour pack cementation process was developed. The structure of this modified coating is discussed. Cyclic oxidation test results and microscopic investigations are also presented.

MCrAlY coating developed via a new electroless-like route: influence of deposition parameters

Abstract Improvement in efficiencies of gas turbines can be achieved by increasing the gas inlet turbine temperature. This leads to increased blade and vane temperatures and therefore to enhanced oxidation and corrosion attack of the coatings on these components. Coatings such as MCrAlY are typically used to improve components’ high temperature properties. Various processing routes have been studied to produce MCrAlY coatings including plasma spray and electrolysis methods. Plasma spray is convenient and permits a large range of MCrAlY composition to be obtained but the directionality of the process leads to non-uniformity of the coating thickness distribution. More recently, an electrolytic route has been developed using sulfamate or sulfate baths, which cause contamination of the coatings and has a detrimental effect on the adherence of the protective oxide scale. A potentially attractive processing route is the electroless autocatalytic process, since this has the advantage of permitting non-directional and sulfur-free deposition. We have investigated MCrAlY deposition by an electroless-like autocatalytic process. Coatings were deposited on a single crystal nickel-based alloy (MC-NG) and the differences in microstructure and phase composition according to the different process stages are reported. The influence of process parameters on the coating characteristics is discussed.

Development of a NiW in-situ diffusion barrier on a fourth generation nickel-base superalloy

A diffusion barrier based on a NiW electrolytic coating has been developed to limit interdiffusion between a Ni-base superalloy (MCNG) and a β-NiAl bondcoating. Isothermal oxidation tests of 50h at 1100°C confirmed that W-rich layer formed with NiW coating modifies the oxidation behaviour of the bondcoat and limits interdiffusion. The diffusion barrier reduced β-NiAl  γ’-Ni3Al transformation in the bondcoating and prevented SRZ formation.

Innovative Coating for Aggressive Environments: Land-Based Gas Turbine Applications

Severe demands on coatings for gas turbine engines that must operate in significantly more aggressive environments than previously required lead to the development of a new resistant system. Both conventional aluminide and MCrAlY coatings must maintain a reservoir of aluminium that forms a protective alumina scale on the surface to extend their life. The loss of aluminium occurs by a spallation of the alumina scale due to thermal cycling and by interdiffusion into the substrate. To have a high durability life, innovative coatings must ensure the aluminium concentration and activity in the coating is as high as possible, along with a high chromium concentration. Such coatings are usually of MCrAlY-type where M is equal to nickel, cobalt or a nickel-cobalt alloy. ONERA has developed a new MCrAlY coating via an innovative electrolesslike process. The aim of this work was to improve it in order to reach above coatings specifications. The aluminium concentration of the electrolesslike coating has been modified by using a modified aluminisation and/or by interposing a diffusion barrier. The aluminium activity has also been changed by us ing a platinum coating. The solubility of chromium has been altered by using a modified aluminide such as platinum modified aluminide. Processes including electrolytic, electroless-like and vapour pack cementation are described. The structures of these layers and of the whole system are discussed. Corrosion test results and microscopic investigations are also presented.

A Novel Duplex Re-NiW Based Diffusion Barrier on a Nickel-Base Superalloy for TBC Systems

A diffusion barrier between a 4th generation superalloy (MC-NG) and a β-(Ni, Pt)Al has been studied. The used coating process combines Re and NiW electrolytic deposits followed by thermal treatments. The diffusion barrier is composed of a continuous 3 &m thick ReWNi layer under a 10 &m thick β-(Ni, Pt)Al containing W rich precipitates. EDS analysis on as coated samples and on 50h-1100°C-Ar aged samples showed that the Re-NiW layer works as a diffusion barrier. The Al reservoir in the bond coat after aging is higher with the diffusion barrier than without. The concentrations of alloying elements are also lower in the bond coat with the diffusion barrier than without.

Influence of surface modification and long-term exposure on mechanical creep properties of γ-TiAl G4

Abstract An investigation of the corrosion processes were performed for coated and uncoated γ-TiAl G4, an alloy designed to work in the temperature range 750 – 800°C, where oxidation and corrosion phenomena occur. An aluminising pack cementation treatment was used to improve the oxidation resistance of this γ-TiAl G4 alloy. Cyclic corrosion tests were performed at 800°C in air for up to 800 1-hour cycles with a Na2SO4/NaCl mixture. The influence of both aluminisation and the corrosion phenomena on the creep behaviour was investigated. The cyclic corrosion resistance of the coated γ- TiAl G4 was shown to be improved by aluminising. The pack cementation treatment had no detrimental effect on the creep behaviour. Moreover, neither is creep affected by the corrosion of coated specimens. As corroded uncoated specimen exhibited good creep behaviour, it can be concluded that this alloy is suitable, even without coating, for turbine applications in hot corrosion atmospheres at least up to 800°C.