Hans-Eberhard Zschau

The halogen effect for improving the oxidation resistance of TiAl-alloys

Abstract Alloys based on TiAl intermetallics are potential candidates for high temperature applications in e.g. aero engines or automotive engines because of their low specific weight and good high temperature strength. To improve their oxidation resistance at temperatures up to 1000°C the halogen effect offers an innovative and cost-effective way. The addition of small amounts of halogens into the surface leads to the preferential formation of gaseous aluminium halides which are oxidised to aluminium oxide during their outward migration forming a dense, protective and slowly growing alumina scale on the surface. In this paper two methods were used to apply halogens to the surface, ion implantation (F and Cl) and a liquid phase process (F). Ion beam analysis with detection limits in the ppm-range was applied to quantify the needed amount of halogens to achieve the halogen effect. Thermocyclic oxidation experiments at 900°C were performed in laboratory air and wet air. Depth concentration profiles of fluorine were measured by PIGE within the first 1.4 μm without destruction of the sample before and after oxidation. Furthermore, the loss of fluorine during heating up and oxidation was measured characterising the stability of the effect. Simultaneous RBS-measurements of the O-, Al- and Ti-depth profiles prove the formation and growth of an almost pure alumina scale. Correlation with the fluorine profiles validates the proposed model for the halogen effect. Furthermore, metallographic methods, REM, EPMA, AES and the proton micro beam (PIXE) were applied to study cross-sections. A virtually pure alumina scale was found after F-treatment and oxidation up to 1500 hours at 900–1000°C in air. The fluorine depth profiles after ion implantation and liquid phase treatment, respectively, show similar levels for both methods before and after oxidation. The development of the fluorine interfacial concentration underneath the oxide scale as a function of oxidation time and temperature was recorded. The results are discussed in the light of the existing model considerations on the halogen effect and with regards to differences in the behaviour between F- and Cl-doping.

A new concept of oxidation protection of Ni-base alloys by using the halogen effect

Abstract The Ni-base alloys with Al-contents of less than 10 wt% are widely used in high temperature technology due to their beneficial mechanical properties. However, their oxidation resistance may be insufficient at temperatures above 1000°C. Oxidation of these Ni-base alloys does not form a pure continuous Al2O3 protective scale on the surface, but rather a complex layer structure. This structure is characterized by inward growing oxides showing a discontinuous alumina scale. A new method for the formation of a dense protective alumina scale on the surface is now presented. The method is based on the halogen effect, which was successfully applied for TiAl-alloys. Thermodynamic calculations show the preferred formation of gaseous Al-halogenides within a certain region of fluorine partial pressures. The fluorine treatment is performed by ion implantation. The implantation parameters are defined by using Monte Carlo simulations. Following these results fluorine implantations of the Ni-base alloy IN 738 are performed to meet the required fluorine content near the surface. After oxidation at 1050°C a thin continuous external alumina scale is formed on the surface, whereas the untreated alloy shows a mixed oxide scale with significant inward growing oxides. The results offer a new and innovative way to protect the Ni-base alloys against oxidation.

Surface Modification by Ion Implantation to Improve the Oxidation Resistance of Materials for High Temperature Technology

The intermetallic Titaniumaluminides are expected to have a high potential as material in high temperature technology. Their high specific strength at temperatures above 700°C offers the possibility for manufacturing components of aerospace and automotive industries. With a specific weight of 50% of that of the widely used Ni-based superalloys TiAl is very suitable for fast rotating parts like turbine blades in aircraft engines and land based power stations, exhaust valves or turbocharger rotors. Thus lower mechanical stresses and a reduced fuel consumption are expected. In contrast to these benefits TiAl shows insufficient oxidation resistance at temperatures above 750°C (Rahmel et al., 1995). To reach higher service temperatures a protective alumina scale would be needed. A surface modification avoids any detrimental influence on the excellent mechanical properties of the material. By using the “halogen effect” a dense protective alumina scale was formed after doping the metal surface with small amounts of chlorine (Kumagai et al., 1996; Schütze & Hald, 1997; Donchev et al., 2003; Schumacher et al., 1999a; Schumacher et al., 1999b; Hornauer et al., 1999). The halogen effect can be explained by a thermodynamic model assuming the preferred formation and transport of volatile Alhalides through pores and microcracks within the metal/oxide interface and their conversion into alumina, forming a protective oxide scale on the surface (Donchev et al., 2003). However the alumina scale fails during thermocyclic loading of Cl-implanted TiAlsamples. Based on this model comprehensive calculations have been performed for fluorine and TiAl predicting a positive effect. The results of thermodynamical calculations have to be transformed into F-concentrations. The beam line ion implantation is chosen as a method of F-application because of its accuracy and reproducibility. By varying the implantation parameters optimal conditions have to be determined to meet the region of F-amounts necessary for a positive F-effect. The implanted F-depth profiles can be verified by the non-destructive ion beam analysis method PIGE (Proton Induced Gamma-ray Emission). The alumina scale formed via the F-effect is adherent even under thermocyclic conditions. However technical use is only possible if the fluorine effect can be stabilized for a time of at least 1000 hours. Hence after the alumina scale formation the stability of oxidation protection depends on the behaviour of the implanted fluorine vs.

The effect of moisture on the delayed spallation of thermal barrier coatings: VPS NiCoCrAlY bond coat+APS YSZ top coat

Abstract A fundamental understanding of failure mechanisms for thermal barrier coatings (TBC) is important for accurate life-time prediction and hence of much interest for industry. Failure (i.e. spallation or cracking) of the TBC usually occurs immediately upon cooling the specimen. However, in some cases spallation of the TBC is observed with a delay of several hours or even days after cooling, when the specimen is at ambient temperature and exposed to laboratory air. Because laboratory air contains water vapour, one hypothesis is that water plays a role in delayed failure of TBCs. This hypothesis is strongly supported by experiments in which the application of liquid water to a pre-oxidized TBC leads to spontaneous spallation/delamination at room temperature. The aim of this work is to study the effect of moisture on TBC systems in more detail. A series of experiments including acoustic emission techniques for in situ detection of cracking within the specimen and nuclear reaction analysis to determine hydrogen concentration depth profiles support the proposed hypothesis. Optical micrographs of APS TBCs isothermally oxidized at 1100°C show increased inward growing oxidation in cauliflower-like structures for specimens oxidized in moist atmospheres.

The Role of Fundamental Material Parameters for the Fluorine Effect in the Oxidation Protection of Titanium Aluminides

To improve the insufficient oxidation resistance of Titanium Aluminides at temperatures above 750°C the fluorine effect offers an innovative way. The focus of this paper is to define the fundamental material variables for the fluorine effect related to the macroscopic behaviour (oxidation resistance) and its long time stability. The thermodynamic model predicted the fluorine effect for the TiAl within a corridor of total fluorine amount in terms of partial pressures. To realize the fluorine effect the required F-concentration [in at.-%] within the near surface region had to be found. Using fluorine ion implantation several fluences within 5e15 and 5e17 F cm -2 were implanted with an energy of 20 keV. The implantation depth profiles were calculated by using the Monte Carlo simulation code T-DYN and verified experimentally by using the non-destructive PIGE - technique (Proton Induced Gamma-ray Emission). After oxidation tests at 800°C – 1000°C a value of 2e17 F cm-2 / 20 keV was determined as an optimal implantation parameter set. Following these results the maximal fluorine concentration was identified to be a fundamental material variable for starting the alumina formation with a required fluorine amount of about 40-45 at.-%. However this maximum fluorine concentration showed a rapid decrease to values less than 5 at.-% only after a few hours of oxidation (900°C and 1000°C) followed by a slow decrease. Therefore the maximum fluorine concentration Cmax – now located at the metal/oxide – interface – was identified to be a fundamental parameter for the long time stability. An exponential decay function containing a constant term of about 1 at.-% was found to describe the time behaviour of Cmax for isothermal and cyclic oxidation (900°C, 1000°C). Because the alumina scale on the surface acts as a diffusion barrier for fluorine, the stability of Cmax is strongly influenced by the F-diffusion into the metal. From the F-depth profiles the diffusion coefficient of fluorine into the TiAl at 900°C was determined as a fundamental parameter for the long-term stability of Cmax showing a value of 1.56e-15cm 2 /s.

STUDY OF CORROSION LAYERS USING BACKSCATTERING IONS

Abstract The backscattering technique was applied to materials of steam generator pipes of fossil fired plants in order to study the corrosion layers with respect to elemental amounts and depth structure. The samples were taken from plants and also from treatment in a laboratory device for the simulation of material stress. Using 7.6 MeV He-ions the oxide layer of a sample of a long time operated steam generator pipe was analyzed. Further three samples heated in a laboratory device in air show the initial state of corrosion. A damaged pipe of a coal fired plant was also investigated using 2 MeV He-ions in order to determine the elemental amounts in the oxide layer and in the ash layer at the fire side. The results allowed to draw conclusions for further plant operations.