Uwe Schulz

Graded coatings for thermal, wear and corrosion barriers☆

Abstract The present paper summarizes the main results generated in a German National Science Foundation (DFG) program on projects concerned with functionally graded materials applied to optimize the thermal, wear and corrosion properties of metallic and ceramic materials. Thermal barrier coatings deposited onto Cu substrates by pulsed laser deposition showed improved spallation behavior by a graded lamella microstructure with improved interface fracture toughness. A particle-hardened graded surface structure improved the wear resistance of plasma sprayed thermal barriers. By means of evaporation techniques a graded bonding area was manufactured with a high potential of lifetime improvement. For non-oxide ceramics graded coatings based on Si 3 N 4 and mullite led to improved oxidation resistance of the substrate material. Graded TiC–TiN thin films allowed to improve the wear resistance of cutting tool alloys with good adhesion to the substrate material. On light metal alloys, the limits of grading with respect to corrosion protection as well as wear were determined. Graded layers of arc-sprayed titanium with in situ produced particles or welded alloy gradients led to improved wear characteristics. Stress profiles in graded layers were analyzed with the help of a modified X-ray diffraction analysis.

Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems

EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline (SX) substrate alloys were thermally cycled at 1100°C. TBC spallation does not correlate solely to TGO thickness, but depends also very much on the substrate alloy. The longest lifetimes are achieved on Hf-containing alloys while SX alloys suffer from early TBC spallation. The formation of the thermally grown oxide was investigated in detail by TEM. A mixed layer of alumina and zirconia exists in the as-coated condition. After initial slight thickening, the thickness of this mixed layer remains constant over a long period of time. During thermal exposure, a continuous layer of pure α-alumina forms and grows underneath the mixed zone by oxygen inward diffusion.

Microstructure of ZrO2 thermal barrier coatings applied by EB-PVD

The electron beam physical vapor deposition process (EB-PVD) provides distinctive coatings of a unique columnar microstructure for gas turbine components. The main advantage of this microstructure is its superior tolerance against stresses, erosion and thermoshock, thus giving it a major edge in lifetime. This paper describes recent investigations of microstructure and texture of yttria-partially stabilized zirconia (Y-PSZ) thermal barrier coatings. Close to the substrate a thin zone of varying thickness consisting of equiaxed ZrO2 grains is observed which can be explained by a geometrical model. A fourfold texture is formed on rotated substrates via evolutionary selection of the fastest growth directions. Rotation causes a layered structure of bent columns. Texture and morphology of the coatings are drastically changed under stationary deposition conditions.

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.

Microstructure and texture of EB-PVD TBCs grown under different rotation modes

Abstract The role of vapor incidence pattern (VIP) on the microstructure and texture of thermal barrier coatings (TBCs) produced by electron-beam physical vapor deposition (EB-PVD) is examined. Two distinct VIPs are induced by proper design of the substrate rotation mode. One is the sunrise–sunset pattern typical of conventional deposition on the curved face of a rotating cylinder (mode A), and the other is a conical pattern resulting when deposition is done on the cylinder base at an offset distance from the plume axis (mode P). These geometries afford fundamental insight on the processes of microstructure and texture evolution and also have practical implications to the variability of properties that may be expected between the body and platform regions of a turbine airfoil. Comparable deposition rates and thickness uniformity can be achieved by proper selection of the experimental geometry. Both coatings exhibit the typical 〈100〉 texture normal to the substrate, but mode A also yields a preferred in-plane orientation which is absent in mode P. The ensuing differences in column packing and tip shadowing yield lower densities and larger pipe-like inter-columnar voids in mode P. The absence of an in-plane evolutionary selection mechanism in the latter also leads to narrower columns than in mode A.

EB-PVD Y2O3 and CeO2/Y2O3 Stabilized Zirconia Thermal Barrier Coatings- Crystal Habit and Phase Composition.

ZrO2-based ingot sources with stabilizing oxides of 6.5 and 20 wt.% Y2O3 and 252.5 wt.% CeO2Y2O3 respectively were used to deposit thermal barrier coatings (TBCs) on rotating cylindrical electron beam physical vapour deposition (EB-PVD) NiCoCrAlY-coated IN 100 substrates by reactive high rate EB-PVD. The TBCs were investigated by scanning electron microscopy, X-ray fluorescence and X-ray diffraction. The phases within the TBCs were t′ tetragonal for 6.5 wt.% Y2O3ZrO2 and cubic for 20 wt.% Y2O3ZrO2 TBCs. The phases found in ceria-stabilized TBCs were t′, cubic and monoclinic depending on the compositional fluctuations within the layers. Compositional variations were due to wide differences in the respective vapour pressures of the various oxides in ceria-stabilized ingot sources which become effective in single-source EB-PVD processing. The apparent crystal habits in the TBCs were correlated with process parameters, phases and chemistry and with regard to structural growth models.

Influence of bondcoat pre-treatment and surface topology on the lifetime of EB-PVD TBCs

Abstract In the present study two types of bondcoats (BC) were used, a NiCoCrAlY overlay coating produced by electron beam physical vapor deposition (EB-PVD) and a platinum aluminide diffusion coating (Ni,Pt)–Al. The coatings were deposited onto nickel-base IN100 and Rene142 substrates and were pre-treated in vacuum or Ar–H gas mixture prior to the deposition of an EB-PVD yttria partially stabilized zirconia top coat. The specimens were thermal cyclically tested in air at 1100 °C (50 min at 1100 °C/10 min forced air cooling). The microstructures of the thermally grown oxide (TGO) layers prior and after testing were examined by scanning electron microscopy and electron energy dispersive spectroscopy. Heat-treatment in Ar–H significantly improved the lifetime of the thermal barrier coatings (TBC) system with IN100+PtAl BC, whereas reduced lifetimes were found on IN100+NiCoCrAlY BC and Rene142+PtAl. High Y-contents of the NiCoCrAlY BC significantly reduced TBC lifetimes. Furthermore, the results for IN100+PtAl and Rene142+PtAl demonstrated that early TBC failure was associated with high TGO surface roughness of the metal/oxide interface, indicating that surface roughness has a significant effect on TBC system performance.

Two-source jumping beam evaporation for advanced EB-PVD TBC systems

Abstract The continuous increase of the turbine inlet temperature in gas turbines necessitates new TBCs with a temperature capability beyond the current partially yttria stabilized material coatings. The present paper focuses on two-source jumping EB-PVD processed novel candidate layers for future TBC applications. It is shown that mixtures of oxides with widely different vapor pressures can be manufactured by this technique. The microstructure of the layers depends strongly on deposition conditions and on materials properties as well. Partial yttria stabilized zirconia coatings show no differences in microstructure and phase formation if deposited either by two-source jumping-beam or by one-source one-beam evaporation, while for ceria stabilized zirconia coatings large differences mainly in chemistry are found; depending on the jumping frequency multilayers are formed. In mixed silica zirconia coatings, crystalline zircon (ZrSiO 4 ) was formed neither in the as coated condition nor after annealing. Finally, jumping beam experiments allowed a detailed understanding of the growth kinetics of EB-PVD TBCs and the formation of a feather-like structure within the columns.

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.

Influence of processing on microstructure and performance of electron beam physical vapor deposition (EB-PVD) thermal barrier coatings

The paper addresses the effect of processing parameters on microstructure and lifetime of electron beam physical vapor deposition, partially yttria-stabilized zirconia (EB-PVD PYSZ) coatings deposited onto NiCoCrAlY-coated Ni-base superalloys. In particular, the formation of a thermally grown oxide layer, an equi-axed zone, and various columnar arrangements of the highly textured PYSZ layers are discussed with respect to processing conditions. Three different microstructures were cyclically tested at 1100°C. The intermediate columnar structure was superior with respect to cyclic life times to a fine and to a coarse columnar structure which was mainly attributed to differences in the elastic properties. The effect of PYSZ microstructure on hot corrosion behavior of the thermal barrier coating (TBC) system at 950°C is briefly discussed.