Marie Pierre Bacos

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.

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.

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 uf0e0 γ’-Ni3Al transformation in the bondcoating and prevented SRZ formation.

Short-Time Oxidation of a NiAl(Zr) Bond Coat Thermochemically Deposited on a Nickel-Based Superalloy

Thermal barrier systems, used for turbine blades, are made of a nickel-based superalloy, a nickel aluminide bond coat layer and a ceramic thermal barrier. The aim of the present work is to study the initial stages of oxidation of the AM1/NiAl(Zr) system. It is currently of prime importance to characterize the initial thin oxide layer that covers the bond coat prior to the topcoat deposition. Indeed, the adhesion of the thermal barrier layer and the lifetime of the system are partly influenced by the substrate pre-heating oxidizing treatment. In order to determine the contribution of zirconium during this intermediate temperature range oxidation, the AM1/NiAl(Zr) system was heat treated at 950°C, in two vacuum conditions, that were close to the industrial ones. The compositions of the extreme surface of the nickel aluminide and of the thermally grown oxide were investigated by Xray photoelectron spectroscopy. In particular, these experiments allowed us to detect zirconium at the surface of the system and to determine its oxidation state.

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.