See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/225001243 Fluorine treatment for improved adherence of EB-PVD thermal barrier coatings on TiAl alloys Conference Paper · January 2009 Source: DLR CITATIONS READS 3 70 3 authors, including: A. Donchev M. Schütze DECHEMA Forschungsinstitut DECHEMA Gesellschaft für Chemische Technik und Biotechnologie e.V. 64 PUBLICATIONS 691 CITATIONS 484 PUBLICATIONS 6,209 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Improvement of the oxidation resistance of TiAl alloys by ion implantation View project Oxidation protection of Ni-alloys by fluorine implantation View project All content following this page was uploaded by A. Donchev on 19 October 2015. The user has requested enhancement of the downloaded file. SEE PROFILE Supplemental Proceedings: Volume 1: Fabrication, Materials, Processing and Properties TMS (The Minerals, Metals & Materials Society), 2009 FLUORINE TREATMENT FOR IMPROVED ADHERENCE OF EB-PVD THERMAL BARRIER COATINGS ON TIAL ALLOYS A. Donchev1; R. Braun2, M. Schütze1 DECHEMA e.V. Karl-Winnacker-Institut, D-60486 Frankfurt/Main, Germany 2 German Aerospace Centre, Institute of Materials Research, D-51170 Cologne, Germany 1 Keywords: Titanium aluminides, Fluorine effect, Thermal barrier coatings, Oxidation Abstract TiAl-alloys are promising candidates for high temperature applications in e.g. aero turbines. The capability of TiAl to withstand high temperature environmental attack is limited to temperatures of about 800°C. The fluorine effect is one possibility to enhance the oxidation resistance of TiAl alloys by forming a protective alumina scale. This scale can work as a bond layer for thermal barrier coatings (TBCs). To investigate the potential of the fluorine treatment in combination with ceramic YSZ-coatings (yttria stabilised zirconia) disk-shape specimens were treated with fluorine and preoxidised in air to form an Al2O3-scale. On these samples a ceramic YSZ top coat was deposited by electron-beam physical vapour deposition (EB-PVD). The oxidation resistance of these samples was studied performing cyclic oxidation tests for up to 2400 1-h cycles between 60°C and 900°C, 950°C or 1000°C in air. The TBCs exhibited good adhesion to the pre-treated specimens. Failure only occurred on those areas which had not been protected by the F- treatment before the EB-PVD process. Introduction The oxidations resistance of TiAl-alloys is quite poor above temperatures of approximately 800°C limiting the use of these light weight materials in several high temperature applications e.g. aero or automotive engines despite of the promising mechanical high temperature properties [1, 2]. There are a number of efforts in research to improve the oxidation resistance of these alloys. The enrichment of the surface zone of TiAl-components with fluorine is a very successful treatment which does not interfere with the mechanical properties of the TiAl-components which is different if the bulk material is alloyed with e.g. Nb which does also improve the oxidation behaviour [3, 4]. The combination of TiAl and a TBC had only rather limited success so for if the substrate is untreated [5]. To get an alumina scale known as a good bond layer for Ni-base alloys and coatings the substrate has to be manipulated. The enrichment with Al by a powder pack process led to the formation of TiAl3 which forms Al2O3 during high temperature oxidation which is conducive to the adherence of the TBC [6]. However, these Al-rich phases are very brittle so that they tend to crack during thermocyclic load [7]. The fluorine effect is a more suitable technique to form a protective Al2O3-scale serving as a bond layer for a subsequent TBC deposition [8]. The fluorine effect is stable over more than one year under thermocyclic conditions at 900°C in air [9]. In this paper the results of high temperature exposure of TiAl-samples are presented which were treated with fluorine first, then an EB-PVD TBC was applied and finally the samples were thermally cycled in air at 900, 950 or 1000°C upper dwell temperature up to 2400 1h-cycles. The SEM-investigations revealed the formation of an alumina scale during high temperature oxidation and good adherence of the TBC to the sample in such cases where the fluorine effect was operating. 211 Experimental The material used was extruded γ-TiAl based alloy Ti-45Al-8Nb (nominal composition in at.%). The alloy exhibited a two-phase microstructure consisting of the γ-TiAl- and α2-Ti3Al- phases. From the extrusion disk-shaped specimens with 15 mm diameter and 1 mm thickness were machined. These samples were ground to a 1200-grit surface finish with SiC-paper, rinsed with ethanol and dried in air before further treatment. The samples were subsequently treated with fluorine via the liquid phase at room temperature (RT) by spraying of an organic fluorine containing compound. Then the samples were oxidised isothermally at 900°C in laboratory air for 24h to form an alumina scale. The samples were cooled down to RT slowly inside the furnace. These parameters had been evaluated during previous experiments [10]. On the preoxidised samples thermal barrier coatings of 7 wt.% yttria partially stabilised zirconia were deposited using electron-beam physical vapour deposition (EB-PVD) [11]. The thicknesses of the TBCs varied in the range between 150 and 180 µm. The oxidation behaviour of the untreated material, of samples treated only with fluorine and samples with TBC on top was studied in laboratory and synthetic air at 900, 950 and 1000°C under cyclic oxidation conditions up to 2400h on temperature. One cycle consisted of 10 min heating to temperature, 1 hour at high temperature and 10 cooling from 950 and 1000°C or 20 min cooling from 900°C down to about 60°C. After several cycles (20 or 100) the samples were removed from the hot furnace, cooled to RT, weighed, photographs were taken and then put back into the furnace after one hour so that the cycling was repeated. After oxidation the samples were documented by magnified photography. Finally metallographic cross-sections of the samples were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and electron probe micro analysis (EPMA). Sample-ID Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Sample 9 Sample 10 Sample 11 Treatment F + TBC F + TBC F + TBC F + TBC TBC F F + TBC F + TBC F F + TBC F + TBC Temperature 900°C 900°C 900°C 900°C 950°C 950°C 950°C 950°C 1000°C 1000°C 1000°C Table 1: Experimental parameters 212 Atmosphere Laboratory air Laboratory air Synthetic air Synthetic air Laboratory air Laboratory air Laboratory air Laboratory air Laboratory air Laboratory air Laboratory air Time 1200 h 1200 h 1200 h 2400 h 2000 h 2000 h 2000 h 1000 h 1000 h 1000 h 1000 h Results and discussion The mass change data of the samples at 900, 950 and 1000°C are presented in the figures 1 – 3. Figure 1 shows the results at 900°C. Two experiments were performed. One pair of samples was exposed in a closed tube furnace with a steady flow of dry synthetic air and the second pair was oxidised in an open furnace under laboratory air. 2,5 2,0 Mass change [mg/cm²] 1,5 1,0 0,5 0,0 -0,5 synth. air synth. air lab. air lab. air -1,0 -1,5 -2,0 -2,5 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Cycles (= time on temperature) Figure 1: Mass change behaviour of 4 TiAl samples treated with F and TBC on top at 900°C The mass change data of the samples in laboratory air differ more than those of the samples on synthetic air. This can be explained by the moisture in the laboratory air which causes enhanced oxidation [12]. The mass losses of the samples in laboratory air were due to spallation of oxide flakes at the unprotected area where the sample holder pin was removed. For handling during TBC deposition, the samples had a thin pin to fix a sample holder. After TBC deposition, the pin was cut off, leaving an unprotected spot on the circumference of the γ-TiAl sample because the protective alumina scale formed during preoxidation after fluorine treatment was removed. Similar results were achieved during testing at 950 and 1000°C (fig. 2, 3). Spallation of the TBC occurred only unprotected spots. 213 2.0 2 Mass Change [mg/cm ] 1.5 1.0 0.5 untreated TiAl fluorine fluorine + TBC 0.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Cycles (= time on temperature) Figure 2: Mass change data of fluorine treated γ-TiAl samples with and without TBC at 950°C in laboratory air. Data for untreated γ-TiAl with TBC are included. 2,0 fluorine fluorine + TBC 2 Mass Change [mg/cm ] 1,5 1,0 0,5 0,0 -0,5 0 200 400 600 800 1000 1h-Cycles Figure 3: Mass change data of fluorine treated γ-TiAl samples with and without TBC at 1000°C in laboratory air Figures 4 and 5 show photos of a sample exposed in laboratory air before and after oxidation. It can be clearly seen that the TBC and the oxide scale underneath have spalled at the uncoated area which is the “weak” point of the sample. The enhanced oxidation at this unprotected spot can influence the protective alumina scale and finally a mixed oxide scale is formed which becomes to thick and spalls during cooling. The TBC was well adherent to the sample where the fluorine effect was working. 214 Figures 4 + 5: Photos of the sample 2 (s. Table 1) before and after oxidation at 900°C in laboratory air for 1200 cycles The sample exposed for 2400 cycles in synthetic air shows also a slight mass loss after 1400 cycles but this was due to an “accident” where the sample fell on the floor after weighing and part of the TBC chipped off but after that the oxidation rate had not increased. The protective alumina scale had not been damaged. Figures 6 – 9 show photos of this sample before oxidation, before the “accident”, after the “accident” and after testing. Figures 6 + 7: Photos of the sample 4 (s. Table 1) in synthetic air before oxidation and before the “accident” (after 1300 cycles at 900°C) 215 Figures 8 + 9: Photos of the sample 4 (s. Table 1) in synthetic air after the “accident” (1400 cycles) and after 2400 cycles at 900°C The results of cyclic oxidation tests at 950 and 1000 °C indicate the formation of a protective Al2O3-layer after fluorine treatment which is also working well as a bond layer for the TBC. Mass losses are again caused by spallation of the TBC at the unprotected spots (figs. 2, 3). SEMinvestigations of a cross-section of a sample after exposure at 950°C for 2000 cycles show the bright TBC, a darker intermediate zone of some mixed oxide which was formed during the incubation time before the fluorine effect started to operate [13], the dark grey Al2O3-layer (roughly 2 µm) and the outer substrate with Nb-rich precipitations (fig.10). For the 950°C/2000 cycles specimen the line scan marked by the arrow starting from the TBC/oxide-interface into the substrate reveals the TBC, the mixed oxide with a higher quantity of TiO2, the Al2O3-layer, the Al-depletion zone [14] and a Nb-rich precipitate (fig.11). The Nb-content in the white precipitates can reach up to 30at.%. The Al-depletion zone has a higher amount of α2-phase and reaches up to about 10 µm in depth. TBC Mixed oxide Al2 O3 Nb-precipitate Al-depletion zone Figures 10 + 11: SEM-image and line scan of the elements of the sample after 2000 cycles at 950°C 216 The results from the post oxidation investigations after exposure at 1000°C differ slightly from those at 950°C. The Nb-rich precipitates are smaller and the oxide scale consisted predominantly of alumina. The concentration of Ti was very low, being slightly increased at the TBC/oxide scale interface. The thickness of the Al-depletion zone is again about 10 µm. TBC Al2O3 Al-depletion zone Figures 12 + 13: SEM-image and line scan of the elements of the sample after 1000 cycles at Al2O3 1000°C The TBC/oxide-interface has to be examined more closely. The grey layer which can be seen in the SEM-image of the sample after oxidation at 950°C is caused by the preoxidation. During preoxidation the organic residue of the fluorine compound evaporates and some deposits stay on the surface but as shown in the figures this has no detrimental effect on the performance of the samples i.e. the adherence of the TBC. Further investigations on the fluorine treatment and the TBC-performance are planned and will be executed in a new project. Conclusions The fluorine treatment provides a good protection against high temperature corrosion in oxidising environments even under thermocyclic conditions up to 1000°C. The fluorine effect works due to the formation of an alumina scale which can act as a bond layer for TBCs. The combination of the fluorine effect and a TBC showed good adherence of the ceramic coating on the alumina layer after preoxidation. The TBC stayed attached in the protected areas. Spallation does only occur at spots where the fluorine effect had been deteriorated. Removing of the TBC by mechanical damage without affecting the protective alumina scale does not have a detrimental effect. The fluorine effect is still working. Problems of other coatings like e.g. Al-depletion by inward diffusion play no role which shows the innovative aspect of the present work. Acknowledgements The material for this work was provided by GKSS which is gratefully acknowledged by the authors. 217 References 1. F. Appel, M. Oehring, “γ-Titanium Aluminide Alloys: Alloy Design and Properties,” Titanium and Titanium Alloys, ed. C. Leyens, M. Peters (Weinheim, WILEY-VCH 2003), 89-152 2. C. Leyens, “Oxidation and Protection of Titanium Alloys and Titanium Aluminides,” Titanium and Titanium Alloys, ed. C. 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