Susanne Gebhard’s research while affiliated with Rolls-Royce and other places

Abradable coatings have been used in low- and high-pressure sections of jet engine compressors for more than 40 years. Today, they are also used in the high-pressure turbine of jet engines and are gaining more interest for applications in industrial gas turbines. They minimise the clearance between the rotating blade tips and the stationary liners. Aside from being abradable, the coatings have to be mechanically stable and withstand high thermo-mechanical loadings. A typical material used in engines today is yttria-stabilised zirconia (YSZ). This material advantageously combines a suitable thermal conductivity with a high thermal expansion coefficient, but shows a temperature capability limited to 1200 °C in long-term applications. Typical abradable coating thicknesses are above 1 mm. With increasing coating thickness and limited cooling efficiency leading to high surface temperatures, there is a risk of premature failure. As a result, new ceramic materials have been developed with better high-temperature capability. The present work investigates an atmospheric plasma sprayed ceramic double-layer coating system composed of 7YSZ as an intermediate layer and magnesia alumina spinel as a top layer.

Shroudless turbine designs offer the advantages of weight reduction and lower mechanical loads on the one hand but bear challenges as high gap sensitivity and high temperatures of the static parts on the other hand. In the last years, a lot of work was carried out in order to develop a sealing system for a shroudless design consisting of an abrasive blade tip coating and an abradable segment coating addressing all the requirements defined. Aside from being abradable, the segment coatings have to be mechanically stable, withstand high thermo-mechanical loadings and have to work for thicknesses larger than 1 mm. Due to the limited temperature capability of the currently used segment coating material yttria-stabilised zirconia, which combines advantageously a suitable thermal conductivity with a high thermal expansion coefficient, new ceramic materials for the segment coating had to be developed. A very promising sealing system combines an abrasive blade tip coating with an yttria-stabilised zirconia / magnesia alumina spinel double-layer abradable coating system with a 3D interface structure between the bond coat and the ceramic coatings. The present work gives an overview of the development and the performance of this sealing system.

The quasi-static and fatigue behavior after impact of the TiAl alloy TNBV3B produced via three different processing routes—cast, forged and extruded—has been studied on flat and airfoil-like shaped specimens making use of ballistic impact experiments. For impacts resulting in cracks the behaviour can be described using a linear-elastic fracture mechanics approach. The residual strength is described on the basis of the fracture toughness. The residual fatigue strength of impact-cracked specimens is estimated on the basis of the threshold for crack growth of the TNBV3B alloys. However, when there is no visible crack or when the crack length is below the size of the deformed impact area, residual stresses and micro-damage play a dominating role making the linear-elastic fracture mechanics approach invalid. The deformation hardened zone in TiAl has been studied making use of micro-hardness tests showing their extension and the degrees of deformation for different impact energies.

TiAl-Legierungen zählen zu den Hoffnungsträgern im Triebwerksbau. Aufgrund ihrer geringen Dichte bei gleichzeitig guter Hochtemperaturfestigkeit sind sie prädestiniert für Leit- oder Laufschaufeln im Triebwerk. Allerdings weisen sie eine geringe Schadenstoleranz auf, so dass sie besonders für typische Schäden im Triebwerk durch Partikeleinschlag (Impact) anfällig sind. Diese Eigenschaft hat sich als ein starkes Hindernis für ihren Einsatz erwiesen. Die vorliegende Arbeit soll daher das Schadensbild, verursacht durch Partikel-Impact, analysieren, die Einflussfaktoren auf eine solche triebwerkstypische Schädigung identifizieren und die Auswirkungen auf das mechanische Verhalten von TiAl-Legierungen beleuchten.

The impact resistance of the TiAl alloy TNBV3B produced via three processing routes – cast, forged and extruded – has been studied on flat and airfoil-like shaped specimens making use of ballistic impact experiments. Several factors influencing the damage behaviour were investigated. The evolution of centre and edge impact induced damage in flat specimens is characterized for different energy levels. Additionally, edge impact was studied for airfoil-like shaped specimens. The results indicate that it is necessary to differentiate between the properties influencing the impact crack initiation and the impact induced crack growth. For the former, strength and ductility appear to have an important influence. A dynamic fracture toughness is probably adequate to describe impact induced crack growth. As such a property was not available an analogy is sought with crack growth behaviour under monotonic and cyclic loading based on microstructural influences found investigating the cracked surfaces after impact.

The impact behaviour of the TiAl alloy TNBV3B produced on three different processing routes - cast, forged (with a relatively small degree of deformation) and extruded - has been studied making use of ballistic tests. The damage evolution due to centre and edge impacts on flat and airfoil-like specimens has been investigated with a focus on the influence of the microstructure. Apart from the influence of material properties, the impact location showed a strong effect on the damage evolution. The extension of impact cracks in the interior of the specimens has been studied making use of heat tinting experiments and computer tomography analyses.

Different microstructures were generated in the Ti–45Al–4.6Nb–0.2B–0.2C and Ti–45Al–1Cr alloys (at.%) by heat treatment. The microstructures were investigated using nanoindentation and atomic force microscopy whichwas compared with transmission electron microscopy. Topographic contrast is usually used for phase identification in the atomic force microscope. However, it was found that the topographic order of the phases changes with different microstructures and specimen preparations. Nanoindentation measurements provided local hardness values not obtainable by othermethods and enabled clear distinction of the phases. The hardness values can give information on surrounding microstructure and solid solution hardening. The mean lamellar spacing of the colonies was measured using both atomic force microscopy and transmission electron microscopy. Atomic force microscopy was found to be suitable to determine the spacing between 2/-interfaces offering the advantages of easier sample preparation and fewer specimens compared to evaluation by TEM analysis.

TiAl alloys show a low ductility, which limits their application especially in components prone to impact damage like turbine blades or vanes. Therefore, the influence of high-speed impact damage on the mechanical behaviour of a cast and a forged version of the intermetallic TiAl alloy TNBV3B has been investigated. Ballistic tests were performed with impact energies up to several Joules. In order to approach the impact situation with real blades or vanes, a blade-like geometry was chosen for the specimen edges. The caused damage has been evaluated as a function of the impact speed, the particle weight and the location of the impact. The location of the impact on the specimen plays an important role as the damage increases with increasing distance until a critical value is reached depending on the local thickness and the energy. Fatigue experiments (R=0.1) at different temperatures were performed to determine the threshold for crack growth as a function of the damage size of the material after impact. For these tests, specimens showing cracks with up to 2 mm length on the specimen back side and specimens showing blow-outs were chosen.

TiAl-alloys show a low ductility, which limits their application especially in components prone to impact damage like turbine blades or vanes. Therefore, the influence of impact damage on the mechanical behavior of a cast and a forged TiAl-alloy has been investigated. Ballistic tests were performed with impact energies up to several Joules. In order to approach the impact situation with real blades or vanes, a blade-like geometry was chosen for the specimen edges. The caused damage has been evaluated as a function of the impact speed, the particle weight and the impact location. Fatigue experiments at different temperatures were performed to determine the threshold for crack growth as a function of the damage size after impact. For these tests, specimens showing cracks with lengths up to some millimeters on the specimen back side as well as specimens with other damages like blow-outs were chosen.

One of the main problems representing a barrier for TiAl gas turbine applications is its low ductility, especially in reference to impact damage of turbine blades or vanes. For that reason the influence of high-speed impact damage on the mechanical behaviour of a cast TNBV3B TiAl alloy and a forged alloy with a similar composition but improved ductility has been investigated. Impact tests are performed making use of a ballistic gun. The caused damage has been evaluated as a function of the impact speed, the particle weight and, thereby, the impact energy level in relation to the specimen thickness. The location of the impact on the specimen plays an important role. Impact near the specimen edge appears to produce a severer damage as the depth of the damage zone close to the edge is larger and damage of a given size is more critical if it is located at the edge. This is shown with the aid of residual strength and fatigue experiments. The residual strength could be described on the basis of a linear elastic fracture mechanics analysis and a fracture toughness of 15 to 17MPa(m)1/2. Fatigue experiments were performed to determine the threshold for crack growth as a function of the impact damage size.