Materials Science Forum Vols. 595-598

Abstract: This paper examines the erosion and cyclic oxidation performance of novel thermal barrier coatings produced via the sol-gel route. The ceramic top coat, with a thickness of 5-80 m, was deposited via a sol-gel route onto standard MCrAlY and PtAl bond coats. In both the erosion and the cyclic oxidation tests it was found that the bond coat had a profound affect on the results. The erosion of the sol-gel coatings were compared to standard EB PVD and PS TBCs and were found to be significantly higher. The effect of aging (100 h at 1100°C) on the erosion rates was also evaluated and was found to increase the erosion rates. The information obtained from the erosion and cyclic oxidation tests have highlighted the need to develop and optimise the parameters for producing thicker coatings

Abstract: The oxide scale evolution with high temperature on CVD aluminide coatings deposited on a Directionally Solidified (DS) Ni-base superalloy is studied in this work. High temperature oxidation was carried out at 1100°C in air for 240 h under isothermal conditions and for 10 cycles (1 cycle = 24 h at 1100°C). The morphological and microstructural characterisation of the coatings has been performed using optical and electron microscopy as well as X-ray diffraction. Contrary to most of the results published in the literature, the rumpling phenomenon appears on the isothermally oxidised specimens whereas spallation, nodule formation and wrinkling of the oxide scale occur on the cyclically oxidised samples. The results are discussed in terms of the β-NiAl to γ’-Ni3Al phase transformation, the likely associated volume changes and of the growth stresses at high temperature.

Abstract: Gamma-TiAl samples were treated by phosphoric acid solutions at different concentrations. With 15 mol/dm3, a viscous deposit was left on the surface leading, after drying and high temperature oxidation, to a very heterogeneous surface aspect. Concentrations below 0.5 mol/dm3 were then used and allowed to greatly ameliorate the homogeneity of the superficial layer. After heating from ambient to oxidation temperature, however, cracks were always observed, due to the departure of gaseous molecules. Isothermal oxidation tests come out at 800°C and 900°C under reconstituted air showed that weight gains were strongly reduced when TiAl had been treated. A compound containing titanium, oxygen and phosphorus was detected at the end of the heating period, identified as titanium pyrophosphate, TiP2O7. This compound remained the only one detected for 100 h when oxidation was come out at 800°C, but evolved towards TiO2 when oxidation time was increased. In the case of oxidation at 900°C, the evolution from TiP2O7 to TiO2 also happened but appeared to go faster.

Abstract: 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.

Abstract: The present work, performed on nickel aluminides deposited on a single Ni-based superalloy AM1, focuses on the effect of the following several parameters on the microstructural and chemical changes occurring during isothermal heat treatment at 1100°C for 50h : -oxygen pressure by comparing heat treatment under ambient air (PO2 = 0.2 bar) and under secondary vacuum (PO2 = 0.2x10-6 bar). -cooling rate after isothermal heat treatment by comparing furnace cooling (3°C/min) and water quenching (500°C/min). -Pt addition in the coating by comparing NiAl and NiPtAl coatings. Characterizations were performed using SEM, analytical TEM and electron microprobe analyses. The results show that these parameters have a strong influence on both the microstructural evolution and the oxidation of the thermal barrier coating (TBC) system. Appropriate heat treatments are essential to improve interfacial resistance and increase the durability of TBC systems.

Abstract: Aluminide diffusion coatings are widely used in aeronautic domain. Nickel based superalloys present good mechanical properties and creep resistance at high temperature but their corrosion resistance is somewhat poor. In order to enhance their lifetime at high temperature, aluminide coatings can be applied on alloy surface. Aluminium present on material surface can form, at high temperature, alumina scale which is a protective oxide scale. In the present study, nickel aluminide coatings were carried out by aluminisation in a single step pack cementation process on nickel based superalloys 600 and AM1. Ni-Al intermetallic was obtained in each case. Oxidation tests were carried out at 900°C in air under atmospheric pressure. At this temperature, transient alumina as γ-Al2O3 and θ-Al2O3 appear on surface and are less protective than the stable α- Al2O3. Reactive elements can enhance α-Al2O3 nucleation and change diffusion mechanism during oxidation leading to a better adherence of alumina scale. In the present work, yttria was introduced by Metal-Organic Chemical Vapour Deposition (MOCVD) prior to the aluminisation. Yttria doped and undoped sample behaviours were compared under oxidising atmospheres.

Abstract: The evolution in surface morphology of platinum modified nickel aluminide (Ni,Pt)Al oxidation coatings during thermo-mechanical testing has been evaluated. One type of test consisted of cyclic oxidation between an upper temperature of 1150°C and a lower temperature varying from room temperature to 1050°C. The other type of test was cycling between 1000°C/1150°C under an applied compressive stress. Profilometry using optical interferometry was used to quantify the surface “rumpling”. First and second-order statistical parameters including RMS roughness and the auto-correlation function were calculated from the profilometry measurements. The results indicate that the grain structure of the aluminide coating plays a major role in the early stages of rumpling and set its wavelength. Also, the superimposed compressive stress during thermal cycling leads to an asymmetry in the rumpling pattern with respect with the loading axis as well as cracking along the applied stress direction.