W.A. Kaysser

Functionally graded materials for sensor and energy applications

Principles, preparation, characterisation, and application of functional materials containing a gradient of their functional properties are surveyed, with main emphasis on thermoelectric (TE) materials for application in sensors and thermogenerators. Further examples of the implementation of functionally graded materials (FGM) presented are dielectric thin-film stacks for capacitors with low temperature coefficient, microwave-processed structural gradients in fuel cell electrodes, and zone-melted graded (Bi1 − xSbx)2Te3 materials for Peltier coolers. Preparation and properties of compositional gradients in TE solid solutions (FeSi2 doped by alloying, (Bi1 − xSbx)2Te3, Mg2(Si,Ge,Sn), PbTe) are analysed, as well as composites joining thermoelectrics of dissimilar chemistry and joints to metallic contacts and interlayers. Thermal spraying of doping-graded FeSi2 was developed as a preparation technique of TE silicide-based FGM. Design, preparation and test of a layered heat-flux sensor based on FeSi2 are described. A calibration test evidenced the feasibility of linearisation of thermal sensor characteristics. A theoretical design tool for functionally graded and segmented thermoelectric structures was based on a local selection criterion to identify the optimal spatial compositional distribution.

EB-PVD Thermal Barrier Coatings for Aeroengines and Gas Turbines

Ceramic thermal barrier coatings (TBCs) offer the potential to significantly improve efficiencies of aero engines as well as stationary gas turbines for power generation. On internally cooled turbine parts temperature gradients of the order of 100 to 150 °C can be achieved. Today, state-of-the-art TBCs, typically consisting of an yttria-stabilised zirconia top coat and a metallic bond coat deposited onto a superalloy substrate, are ntainly used to extend lifetime. Further efficiency improvements require TBCs being an integral part of the component which, in turn, requires reliable and predictable TBC performance. Presently, TBCs fabricated by electron beam physical vapor deposition are favoured for high performance applications. The paper highlights critical research and development needs for advanced TBC systems, such as reduced thermal conductivity, increased temperature capability, lifetime prediction modelling, process modelling, bond coat oxidation, and hot corrosion resistance as well as improved erosion behaviour.

Intermetallic Ti-Al coatings for protection of titanium alloys : oxidation and mechanical behavior

Abstract Intermetallic Ti-Al coatings were deposited onto near-α titanium alloy TIMETAL 1100 using magnetron co-sputtering technique. Two coating systems were investigated: gradient layers with increasing Al content towards the surface of the coatings and a multilayer system consisting of three single layers of Ti 3 Al, TiAl and TiAl 3 . The overall coating thickness was 4 μm and 16 μm for both systems. Isothermal oxidation tests at 750 °C revealed good oxidation resistance and effective oxygen prevention from the substrate by the coatings. Room temperature tensile tests after long-term exposure to air at 600 °C proved the beneficial influence of the coatings on ductility of the base material. The coatings are highly ductile under creep conditions, thus keeping oxygen away from the substrate alloy even at high straining. In some cases creep lifetime was considerably prolonged. No detrimental influence of the Ti-Al coatings on the fatigue properties of TIMETAL 1100 was found for the 4-μm multilayer coatings, whereas fatigue limit under repeated strain was slightly decreased for the 16-μm coatings.

Sputtered intermetallic Ti-Al-X coatings : phase formation and oxidation behavior

Intermetallic Ti–Al–X (X=Cr, Nb) coatings were deposited on conventional near-α titanium alloy TIMETAL 1100 and on γ-TiAl based Ti–48Al–2Cr–2Nb by magnetron sputtering for oxidation protection. Low temperature deposition process leads to a metastable coating structure which transforms into a two-phase microstructure during high temperature exposure. Oxidation behavior was tested in interrupted weight gain tests in laboratory air at 750°C for TIMETAL 1100 and at 900°C for Ti–48Al–2Cr–2Nb. Oxidation behavior of the coatings shows the same tendency for both substrates and oxidation temperatures. Nb-containing coatings exhibited only poor protection due to partial spallation of the coatings during cooling. Maximum oxidation resistance was achieved by Ti–63Al–7Cr coatings on both substrates. For the γ-TiAl based Ti–Al–Cr coatings oxidation resistance is improved with increasing Cr content. Despite high oxidation resistance of the Cr-containing coatings rutile was always found in addition to protective alumina in the oxide scale.

Oxide scale formation on an MCrAlY coating in various H2-H2O atmospheres

Abstract Virgin electron beam physically vapor deposited NiCoCrAlY coatings on Nimonic 75 sheets were glass bead peened, spark machined to coupons and variously heat treated in vacuum and in H 2 -H 2 O mixtures of different oxygen partial pressures at 1080 °C for 4 h and 1100 °C for 1 h and 28 h. Oxide scale formation was investigated by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, X-ray fluorescence and Auger electron spectroscopy. Continuous alumina layers were obtained by annealing in H 2 -H 2 O atmospheres whereas discontinuous oxides formed in a “technical” vacuum environment. The growth rates of the oxide films are compared with literature data on alumina forming alloys, and the distinctive changes are related to the influences of the specific atmosphere and to the interference of foreign elements with alumina. Mechanical properties of the scales also depend on growth conditions. Post-coating procedures such as glass bead peening and spark-erosion cutting have an effect on cleanness and scaling behavior.

Magnetron-sputtered Ti–Cr–Al coatings for oxidation protection of titanium alloys

Abstract Ti–Cr–Al coatings with a nominal composition of Ti–51Al–12Cr (in at.%) were deposited on the near-α titanium alloy TIMETAL 1100 (Ti–6Al–2.75Sn–4Zr–0.4Mo–0.45Si, in wt.%) by dual-source magnetron sputtering. Substrate bias voltage between 0 and 80 V was applied during coating deposition. The microstructure of the coating changes from fine columnar to a nearly structureless morphology by increasing the substrate bias voltage. Due to the specific shape of the electrical field around the samples the morphology of the as-coated samples is divided into three characteristic zones, the width of which are essentially dependent on the height of the bias voltage. All-around coated disk-shaped samples were isothermally tested in a thermobalance at 750°C in dry air to investigate their oxidation kinetics. Oxidation resistance of the coatings was maximal at zero bias voltage and slightly decreases up to 40 V. For those coatings deposited at 60 and 80 V strong spallation was observed, which is caused by bubble formation on heating due to argon incorporated during deposition.

Microstructure and Phase Stability of EB-PVD Alumina and Alumina/Zirconia for Thermal Barrier Coating Applications.

This paper describes recent progress on research aimed at improving the performance of EB-PVD thermal barrier coatings (TBCs) for gas turbine applications by incorporating alumina as an oxygen diffusion barrier between the bond coat and zirconia. Two approaches are being investigated, either a single, discrete alumina layer, or a graded alumina/zirconia layer. This paper reports on preliminary experiments in which alumina was evaporated under different conditions, and the phase content and morphology of the coatings were characterized. Deposition rate and chamber pressure had a significant effect on the microstructure of the coatings; however, phase formation was essentially unaffected. In agreement with previously published results, α-Al2O3 was produced either by in situ deposition on substrates heated to temperatures near 1000 °C, or by heat treatment of metastable as-deposited alumina coatings at temperatures above 1100 °C. By continuously changing the vaporization ratio of alumina and zirconia as a function of deposition time, TBCs exhibiting composition gradients through the film thickness were produced. In related work, suitable source materials for alumina deposition were produced using a new sinterless powder processing route involving bimodal powder mixtures.

Graded and stacked thermoelectric generators—numerical description and maximisation of output power

Large temperature differences applied to thermoelectric generators require that the variations of all material properties with temperature are included in a numerical description of their performance. A finite element algorithm is developed to calculate the temperature field in a thermoelectric device and concomitantly its thermoelectric performance under operation conditions. Spatially varying the composition of material or the doping concentration allows to enhance the power output or efficiency. The choice of the optimum concentration parameter profiles is shown not to be a function of the local temperature only, but to be dependent on a local criterion related to the entire temperature field. This criterion is included in an iterative calculus to find optimised concentration profiles. It is shown that the code developed has advantages over previously published solutions, since it can be applied to continuous and discontinuous material changes without any assumptions on the mutual dependence of the governing transport parameters or a need to cast their temperature dependence into analytical form. The performance is shown for some test situations and compared to literature.

Influence of long-term annealing on tensile properties and fracture of near-α titanium alloy Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si

Long-term exposure at 600 °C of the near-alpha titanium alloy Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si (TIMETAL 1100) influences tensile properties and fracture morphology due to microstructural changes by aging as well as environmentally induced effects. The influences of aging and oxygen penetration on the subsurface of the alloy were evaluated individually. Metallographic investigations, microhardness measurements, and Auger electron spectroscopy (AES) were performed to examine the oxygen-affected zone. Whereas ultimate tensile stress (UTS) and 0.2 pct yield stress (YS) are mainly influenced by aging at 600 °C up to 1000 hours in bimodal and lamellar microstructures, ductility sensitivity depends on oxygen embrittlement of the subsurface zone. Hereafter, little strain cracks are initiated which rapidly penetrate into the unaffected material. Fracture morphology which was investigated by scanning electron microscopy changes little during exposure. Ductile fracture exhibiting honeycomb morphology was observed on bimodal microstructure and interlamellar fracture with little ductile portion on lamellar microstructure. The oxygen-penetrated zone fails by brittle fracture.

Materials and design concepts for high performance compressor components

Abstract High-performance compressors with increased pressure ratios require sophisticated materials solutions and design concepts. Reduced weight and increased strength and stiffness are the major requirements for highly stressed fan blades in future aircraft engines. Two different materials and design approaches will be discussed in the paper, namely local reinforcement of the titanium fan blades by using titanium matrix composites (TMCs) and by partly replacing the bulk titanium fan blade by fiber reinforced plastics (hybrid concept). While the TMCs significantly improve the mechanical properties of the fan blades, the hybrid blades can contribute to considerable weight savings. Blings (=bladed rings) offer high potential in weight savings of the entire compressor and require TMCs in particular for elevated temperature applications that are relevant in the higher stages of the compressor.