Djar Oquab

Structural characterization of chromium carbide coatings deposited at low temperature by low pressure chemical vapour decomposition using dicumene chromium

Abstract The growth of chromium carbide coatings by pyrolysis of dicumene chromium in a hot-wall low pressure chemical vapour deposition reactor has been investigated between 300 and 550 °C. Amorphous chromium carbide films were obtained in the low temperature range 300 - 500 °C whereas a textured crystalline Cr 7 C 3 phase was grown above 500 °C. The total carbon content was independent of the deposition temperature and amounted to a carbon excess of about 30% compared with the Cr 7 C 3 stoichiometry. Electron spectroscopy for chemical analysis of both amorphous and crystalline coatings confirms the presence of this carbon excess since about 30% - 40% of free carbon was found. A heterogeneous structural model composed of the Cr 7 C 3 phase and free carbon is proposed both for crystalline and amorphous coatings.

Preliminary Results of the Isothermal Oxidation Study of Pt-Al-NiCoCrAlYTa Multi-Layered Coatings Prepared by Sparks Plasma Sintering (SPS)

MCrAlY coatings (where M = Co, Ni or Co/Ni) are widely used on turbine blades and vanes as oxidation and corrosion resistant overlays or as a bond-coating in thermal barrier coatings systems. MCrAlY are usually fabricated by Plasma Spraying, Physical Vapour Deposition, High Velocity Oxy-Fuel spraying or electrolytic techniques. The use of emergent Spark Plasma Sintering technique as a preparation method for NiCoCrAlYTa coatings has been presented previously [1]. SPS technique allows fast development of new coatings with a one-step fabrication of multilayered coatings. This work presents first results of the long term isothermal oxidation behaviour of Pt-Ni aluminide/NiCoCrAlYTa multi layered coatings. The obtained coating is dense and homogeneous. Isothermal oxidation up to 500 h at 1100°C leads to the formation of an adherent alumina scale with Y-rich precipitates and deep intergranular oxidation.

Thermal Barrier Systems and Multi-Layered Coatings Fabricated by Spark Plasma Sintering for the Protection of Ni-Base Superalloys

Aeronautic gas turbine blades, vanes and combustion chambers are protected against high temperature oxidation and corrosion by single or multilayered coatings. These include environmental coatings, generally based on Pt-modified Ni aluminides or MCrAlY overlays (where M = Ni and/or Co), thermal barrier coating (TBC) systems including a ceramic thermally insulating layer, and abradable seals. The present work shows the ability of the Spark Plasma Sintering technique to rapidly develop new coatings compositions and microstructures. This technique allows combining powders and metallic foils on a superalloy substrate in order to obtain multilayered coatings in a single short production step. Fabrication of MCrAlY overlays with local Pt and/or Al enrichments is shown, as well as fabrication of coatings made of -PtAl2, -PtAl, α-AlNiPt2, martensitic and (Ni,Pt)Al or Pt-rich ’ phases, including their doping with reactive elements. The fabrication of a complete TBC system with a porous and adherent Yttria Stabilized Zirconia (YSZ) layer on a bond-coating is also demonstrated, as well as the fabrication of a CoNiCrAlY-based cermet coating for abradable seal application. Difficulties of fabrication are reviewed, such as Y segregation, risks of carburization, local over-heating, or difficulty to coat complex shaped parts. Solutions are given to overcome these difficulties.

Cyclic Oxidation Kinetics Modeling of NiAl Single Crystal

For alumina forming nickel-based alloys, sulfur content is a key element as it modifies the onset and extent of spalling of the oxide scale. To get a better understanding of that effect, isothermal and cyclic oxidation of two NiAl single crystals (with two S contents) have been carried out at 1050 and 1150°C on (100) and (110) orientated surfaces. Continuous thermogravimetry in cyclic conditions allows to follow isothermal oxidation kinetics and spalling at each cycle. Oxidation kinetics are compared between the two compositions and the two crystallographic orientations. Low sulfur NiAl oxidizes with slightly lower kinetics for both (100) and (110) surface orientations, and, in both isothermal as well as in cyclic conditions at 1150°C in O2 . Experimental results of cyclic oxidation are compared to those obtained with simulations using a previously published simple statistical model and using a more complex Monte-Carlo code. Spalling is found to be an increasing function of the average oxide thickness and seems to depend on the square of the average oxide thickness.

On the Understanding of TGO Growth and Spallation in Nickel Aluminides

This paper describes various parameters influencing oxidation kinetics, oxide’s morphology and spallation phenomena encountered while studying nickel aluminides in high temperature oxidation. Questions are raised about the possibility of making precise lifetime or failure predictions of coatings and TBCs systems. Changes in growth mechanism due to the presence of elements other than Ni and Al in nickel-aluminide base coatings, the effect of atmosphere, particularly water vapor, and the effect of surface preparation and of crystallographic orientation are the main parameters discussed in this work.

Mechanical properties of alumina-metal-zirconia nano-micro hybrid composites

Al2O3 and Al2O3-2 wt% Fe0.8Cr0.2 nanocomposite powders were wet-mixed with different ZrO2 or Y-ZrO2 powders. The influence of the ball-milling medium and mixing duration on the microstructure and the amount of tetragonal zirconia retained after hot-pressing are discussed. The fracture strength and fracture toughness of the alumina-metal-zirconia specimens ave lower than that of the alumina-metal and alumina-zirconia composites. This could result from a partial annihilation of the different reinfor cement mechanisms involved and also from interactions between zirconia and the metallic phase. There is no correlation between the mechanical properties and the amount of tetragonal zirconia. However, the amount of tetragonal to monoclinic transformation at the surface of the specimens during grinding prior to the mechanical tests seems to be a key parameter

Characterization of TBC Systems with NiPtAl or NiCoCrAlYTa Bond Coatings after Thermal Cycling at 1100°C: A Comparative Study of Failure Mechanisms

During service, TBC can suffer degradation by CMAS, FOD, erosion or spallation. Whereas the first three are due to foreign particles, the last one is related to thermal cycling. When subjected to high temperature exposures followed by rapid coolings under oxidizing conditions, a TBC system undergoes morphological changes and stress development. This will initiate cracks which propagate and finally lead to failure by spallation. Consequently, the aim of the present study is to understand better the mechanisms responsible for such spallation events. Two kinds of TBC systems with different bond coatings (NiCoCrAlYTa or Pt-modified nickel aluminide bond coatings) are thermally cycled. Subsequently, SEM investigations on TBC systems after spallation concentrate on failure path, defect, morphological and microstructural changes to propose way for improving TBC system lifetime.

Toward the Improvement of the Microstructure of Chemical Vapor Deposited Aluminum on Silicon Carbide

This paper presents results on the chemical vapor deposition of aluminum on silicon carbide starting from triisobutylaluminum. The deposited films show important systematic trends in surface roughness and porosity. Considering literature information on the deposition of aluminum on silicon, pretreatments of the deposition surface with hydrofluoric acid and/or titanium tetrachloride have been tested. They lead to films with a smoother morphology and stronger adhesion with the substrate than in the case of an untreated surfacc. Different heteroelements are observed at the interface under these conditions. A short thermal activation of the nucleation process following these pretreatments does not unambiguously improve the characteristics of the aluminum films. The obtained results can be considered for the modification of surface characteristics of silicon carbide particles which are used in discontinuously reinforcement aluminum composite materials with the aim to improve their mechanical properties and corrosion resistance as well as their fabrication process.

Niobium Electrodeposition in Molten Fluorides Using Pulsed Electrolysis

that the nucleation pro-cess of niobium in molten LiF-NaF is instantaneous, three-dimen-sional, and closely controlled by linear diffusion. Under these con-ditions a columnar structure of coarse grains is expected. This situa-tion prevents obtaining a coating with a compact structure withoutdendrites if too high a current density is used. Consequently, indus-trial applications of the process would be hindered by the requiredslow deposition rates.However, pulsed electrolysis may be explored to improve theprocess, since pulsed electrodeposition of various metals in aqueousmedia have given good results in terms of high electrodepositionrates and smoothness of the coatings.

Shaping of Nanostructured Materials or Coatings through Spark Plasma Sintering

In the field of advanced ceramics, Spark Plasma Sintering (SPS) is known to be very efficient for superfast and full densification of ceramic nanopowders. This property is attributed to the simultaneous application of high density dc pulsed current and load, even though the sintering mechanisms involved remain unclear. In the first part of the paper, the mechanisms involved during SPS of two insulating oxide nanopowders (Al2O3 and Y2O3) are discussed while in the second part illustrations of the potential of SPS will be given for (i) Consolidation of mesoporous or unstable nanomaterials like SBA-15 or biomimetic apatite, respectively; (ii) Densification of core (BT or BST)/shell (SiO2 or Al2O3) nanoparticles with limited or controlled reaction at the interface. (iii) In-situ preparation of surface-tailored Fe–FeAl2O4–Al2O3 nanocomposites, and finally (iv) One-step preparation of multilayer materials like a complete thermal barrier system on single crystal Ni-based superalloy.