Nadine Laska

Oxidation and fatigue behaviour of gamma titanium aluminides coated with yttrium or zirconium containing intermetallic Ti–Al–Cr layers and thermal barrier coating

Abstract Intermetallic Ti–60Al–13Cr based coatings with additions of 1 at.-% yttrium or 4 at.-% zirconium were deposited on a γ-TiAl based Ti–45Al–8Nb (at.-%) TNB alloy using magnetron sputtering. The Zr containing layer was also used as bond coat for a thermal barrier coating (TBC) of yttria partially stabilised zirconia produced by electron beam physical vapour deposition. Cyclic oxidation tests at 850°C in laboratory air revealed excellent oxidation behaviour of the Y containing coating up to more than 2000 one hour cycles. Similarly high oxidation resistance was also observed for the Zr containing bond coat beneath the TBC. However, without ceramic topcoat, the Ti–60Al–13Cr–4Zr coating degraded after 400 one hour cycles due to the occurrence of cracks in the brittle intermetallic coating. Furthermore, fatigue tests of coated tension specimens were performed after an isothermal exposure at 850°C for 300 h. The deposition of the intermetallic Ti–Al–Cr based coatings reduced the fatigue strength by more than 50% in comparison to the bare substrate material; it was further deteriorated by the TBC.

Oxidation behaviour of an intermetallic Ti-Al-Cr-Zr bond coat on a γ-TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years. In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)2, the tetragonal Cr2Al phase and the cubic τ-TiAl3 phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α2-Ti3Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO2 precipitates.

The oxidation behaviour of aluminium-rich coatings on the TiAl alloy TNM-B1

Abstract To improve the oxidation resistance of a third generation Ti–Al alloy (TNM-B1) at high temperatures, aluminium rich coatings were deposited using pack cementation and magnetron sputtering. The coatings formed were analysed and compared with respect to phases formed and the containing elements. Oxidation tests were carried out on the additionally fluorine treated and preoxidised samples at 900 C in air. Compared to uncoated samples the beneficial effect of coating deposition was confirmed. Additionally, the concentration of incorporated oxygen in the subsurface zone and its influence on the mechanical properties were investigated using EPMA and microhardness measurements. Oxygen concentrations were determined for the three phases of the substrate material -TiAl, -TiAl, and -Ti(Al), and depended on the previous coating layer deposition.