ICMCTF2006 Session A3-1: Thermal Barrier Coatings
Wednesday, May 3, 2006 8:30 AM in Sunrise
A3-1-1 Effect of Surface Preparation on the Durability of Thermal Barrier Coatings with NiCoCrAlY and Pt-Modified Aluminide Bond Coats
N.M. Yanar, M.C. Maris-Sida, F.S. Pettit, G.H. Meier (The University of Pittsburgh)
A principal concern with ceramic thermal barrier coatings (TBCs) used in gas turbines is their loss of adhesion during service, leading to coating spallation. This presentation will describe the effects of bond coat surface preparation on the durability of TBCs deposited by electron beam physical vapor deposition (EBPVD) and exposed under thermal cycling conditions. State-of-the-art TBC systems with NiCoCrAlY overlay bond coats were found to fail as the result of defects which included transient oxides, defects in the bond coat surface, defects in the as-deposited microstructure of the TBC, and excessive oxidation of reactive element additions. The failures of the state-of-the-art systems with platinum-modified aluminide bond coats resulted from deformation of the bond coat by a mechanism known as ratcheting. This presentation will describe how a variety of surface treatments of the bond coats minimized the effects of both failure mechanisms and greatly improved the lives of systems with either platinum-modified aluminide or NiCoCrAlY bond coats. Suggestions for further improvements will be given.
A3-1-3 Synthesis by Sol-Gel Route and Characterization of Yttria Stabilized Zirconia Coatings for Thermal Barrier Applications
C. Viazzi (Université Paul Sabatier, France); J.-P. Bonino, F. Ansart (Université Toulouse III, France)
This paper deals with the development of new synthesis techniques for functional materials like Yttria Stabilized Zirconia (YSZ) used in the field of thermal barriers coatings. Currently, Thermal Barrier Coatings (TBCs) are fabricated by dry route technologies (EB-PVD or plasma spray) but these industrial processes often require costly investment, complicated operation and are very directional. Particular focus has recently been the development of sol-gel routes, which are nondirecting methods, to prepare, by suitable chemical modifications, nanocrystalline materials with a controlled morphology. Furthermore, wet chemistry routes provide interest because mixing of species occurs on the atomic scale. One of the advantages of this method is to decrease the crystallization temperature in comparison to the conventional ones, allowing the synthesis of reactive powders with nanometric particles size. In this study, several synthesis parameters to prepare YSZ powders have first been investigated (hydrolysis ratio, nature and concentration of precursors in the sol ...). Fine YSZ nanoparticles were obtained and are then dispersed in several media to obtain stable slurries. Several parameters of the slurry as powder content, nature of additives, incorporation of pore-formers have been studied. Superalloys are then immersed in the slurry and withdrawn at a controlled rate before being annealed at different temperatures. The microstructure and the crystallization process of the films have been investigated using scanning electron microscopy and X-ray diffraction. Properties of mono or multilayers as adhesion, roughness and thickness are then evaluated in order to establish correlations between experimental parameters (slurry composition, number of deposits...) and thermomechanical characteristics.
A3-1-4 Thick Porous and Dense Coatings Using The Solution Precursor Plasma Spray Process
M. Gell, E. Jordan (University of Connecticut); N Padture (Ohio State University); X. Ma (Inframat Corporation)
The Solution Precursor Plasma Spray (SPPS) process is a flexible process that can provide ceramic oxide coatings with highly variable thicknesses of 1 micron to greater than 4mm and porosity contents of less than 5 to greater than 40 volume percent. These broad ranges of coating variations will be related to SPPS processing parameters and in particular to the ability to control the volume percent of ultra-fine splats and through-thickness vertical cracks in the microstructure. Thick SPPS TBCs have been produced and show little variation of spallation resistance in coatings produced with thicknessess varying from 0.25 to 4mm. This insensitivity of spallation resistance with coating thickness is related to the through-thickness vertical crack density. As a result thick SPPS TBCs appear promising for applications to turbine vanes, combustors, turbine inlet and exhaust structures and as abradable tubine outer air seals. At the other extreme of coating density, thick dense deposits can be made consisting almost exclusively of ultra-fine splats and devoid of through-thickness vertical cracks. These SPPS coatings are nanostructured and, with the appropriate precursors, contain far-from-equilibrium microstructures. These deposits have the potential for superior wear resistance and when made thick to be used as bulk structural preforms.
A3-1-5 Influence of Ternary Additions of Dysprosia on the Phase Stability of EB-PVD TBCs
R.J.L. Steenbakker, R.G. Wellman, J.R. Nicholls (Cranfield University, United Kingdom)
Zirconia partially stabilised with 6-8 wt% yttria (PYSZ) is commonly used as a top coat material in a TBC system. Rare earth oxides like gadolinia, dysprosia, erbia or europia can be added to this composition to reduce the thermal conductivity of the coating. After deposition using electron beam physical vapour deposition (EB-PVD) the coating has a metastable tetragonal crystal structure denoted t’. Under long time ageing at temperatures above 1200°C, the t' phase tends to transform into a mixture of tetragonal (t) and cubic (c) phase. On cooling this t phase can transform to the monoclinic (m) phase. This phase transformation has to be avoided as it is associated with a volume increase between 4-6% building up stresses in the coating.@paragraph@ In this work the effect of dysprosia additions on the phase stability of EB-PVD TBCs has been determined. High purity single crystal alumina substrates were used to permit high temperature, up to 1550°C, heat treatments whilst minimising any influence from sintering aids normally present in polycrystalline aluminas. EB-PVD thermal barrier coatings deposited on this high purity substrate were then used to assess the influence of dysprosia additions on the phase stability of the TBC. PYSZ coatings doped with 0.3, 1 and 2 mol% of dysprosia were deposited. It was found that dopant additions decrease the tetragonality of the coating which means that a more "cubic-like" structure is obtained. The heat treatment of the sample at 1550°C revealed that the monoclinic phase was present in the standard coatings after 720 hours contrary to the doped coatings which remained in the t’. Thus dysprosia additions enhance the phase stability.
A3-1-7 Combined Pre-Annealing and Pre-Oxidation Treatment for Improved Service Life of Thermal Barrier Coatings on NiCoCrAlY Bond Coatings
T.J. Nijdam (Netherlands Institute for Metals Research, Netherlands); W.G. Sloof (Delft University of Technology, Netherlands)
The service life of High Temperature (HT) coating systems, comprising of a superalloy substrate, a Bond Coating (BC) and a ceramic Thermal Barrier Coating (TBC) on top, depends strongly on the processing method of the HT coasting system. The most critical processing steps are those that determine the microstructure and composition of the Thermally Grown Oxide (TGO) that develops in between the BC and the TBC, since failure of the HT coating systems normally occurs in the vicinity of this TGO. @paragraph@The composition and microstructure of the TGO can be controlled by executing a pre-annealing and/or pre-oxidation treatment on the BC prior to TBC deposition. The development of transient oxides (e.g. @theta@-Al@sub 2@O@sub 3@, Ni,Co(Cr,Al)@sub 2@O@sub 4@, YAG) next to the stable @alpha@-Al@sub 2@O@sub 3@ depends on the pre-treatment parameters, such as annealing atmosphere, oxidation time, temperature and partial oxygen pressure (pO@sub 2@). @paragraph@A combined pre-annealing and pre-oxidation treatment was developed for the processing of EB-PVD HT coating systems containing NiCoCrAlY bond coatings. To develop this combined pre-treatment, the influence of the ambient during pre-annealing and pre-oxidation on TGO microstructure evolution, HT coating system failure mechanisms and HT coating system life span upon thermal cycling was investigated. The results of this study showed that the longest life spans were obtained when the TGO after pre-annealing and pre-oxidation was initially composed of @theta@-Al@sub 2@O@sub 3@ and transformed into @alpha@-Al@sub 2@O@sub 3@ and when the Y was incorporated within a high density of pegs along the TGO/BC interface. Such an initial TGO resulted in a very low @alpha@-Al@sub 2@O@sub 3@ growth rate during service, due to a large @alpha@-Al@sub 2@O@sub 3@ grain size. In addition, the high density of Y-rich pegs provided a high resistance against failure along the TGO/BC interface, due to mechanical keying of the TGO to the BC.
A3-1-9 The NiAl/Yttria-Stabilized Zirconia Thermal Barrier Coating Deposited by Using a Hybrid Arc Ion Plating and Magnetron Sputtering System
J.T. Chang, K.J. Huang, J.L. He (Feng Chia University, Taiwan); A. Matthews, A. Leyland (The University of Sheffield, United Kingdom)
Aluminide/Zirconia thermal barrier coating (TBC) systems are widely used in the aerospace sector and for gas turbine components. It is considered that arc ion plating (AIP) may offer considerable benefits over current techniques used for the deposition of TBCs in terms of lower processing temperatures and the possibility to deposit both the bond and top in one continuous coating cycle. Nevertheless, a limitation of employing AIP lies in the successful doping of the stabilizing elements, such as yttrium, which necessitates the need for complex and expensive alloy materials. To overcome this shortfall, whilst still retaining the benefits of AIP, a hybrid arc/sputter deposition technique has been developed to synthesize the complete NiAl/YSZ TBC system on a Ni-based superalloy substrate. At a deposition temperature of 400@super o@C, the 60Ni-40Al at% bond coat forms a NiAl and Ni3Al structure and the 4Y-30Zr-66O at% top coat is almost entirely monoclinic. Before thermocyclic testing, the microstructure and morphology are seen to be of a dense columnar nature. The stresses induced during thermocyclic testing results in both intercolumnar cracks and cracks parallel to the substrate surface within the YSZ top coat. Furthermore, cubic phases form in the YSZ top coat. The initial absence of the cubic phases before thermocyclic testing, even with the high yttrium concentrations in the films, is attributed to the low deposition temperature and the deficiency of oxygen concentration. Only after 700 cycles, is there seen to be significant failure. This occurs at the interface between the substrate and bond coat through the growth of oxides in this region.
A3-1-10 The Thermal Cyclic Performance of NiAl/Alumina-Stabilized Zirconia Thermal Barrier Coatings Deposited Using a Hybrid Arc and Magnetron Sputtering System
K.J. Huang, J.T. Chang, K.C. Chen, J.L. He, C.K. Lin (Feng Chia University, Taiwan); A. Matthews, A. Leyland, A. Davison (The University of Sheffield, United Kingdom)
In the present work, a hybrid arc/sputter deposition system is used to deposit Yttria-Stabilized Zirconia (YSZ) and Alumina-Stabilized Zirconia (ASZ) films on a Ni-based superalloy substrate, with a pre-deposited NiAl bond coat. From X-ray diffractometry (XRD) it is found that tetragonal phases are found for the ASZ films. The YSZ films, however, exhibit an almost complete monoclinic structure. This is considered to result from the higher concentration of aluminum in the ASZ films of 10 at% compared to 4at% of yttrium in the YSZ coatings. The coatings have a dense columnar structure before thermal cyclic testing. Due to the tensile and compressive stresses during thermal cyclic testing, cracks occur in both the YSZ and ASZ coatings. A thermally grown oxide (TGO) between the top and bond coat is also formed during thermal testing. The ASZ coating is found to develop monoclinic phases during thermal cyclic testing. Conversely the YSZ coatings are found to form tetragonal phases. The ASZ treated samples can be observed to fail in the region just above the bond coat/ASZ interface after 220 cycles. This is considered to result from the dense coatings been unable to accommodate the thermal stresses parallel to the surface during the cyclic tests. The YSZ fail at the substrate/bond coat interface after approximately 700 cycles due to oxidation in this region.
A3-1-11 Potential of ZrO@sub 2@-YO@sub 1.5@-TaO@sub 2.5@ for TBC Applications
F.M. Pitek, S. Krämer, C. Levi (University of California, Santa Barbara)
Experiences with the ZrO@sub 2@-YO@sub 1.5@-TaO@sub 2.5@ system have demonstrated several beneficial characteristics for use in thermal barrier coating applications. The rather unique presence of a stable, non-transformable tetragonal region in this ternary oxide system, along with the associated stable, two-phase field(non-transformable tetragonal + cubic), allows for phase stability in this system to high temperatures(1500@super o@C). Selected compositions with high levels of yttria and tantala have shown superior resistance to vanadate corrosion than the commercially utilized 7YSZ. In addition, Y+Ta stabilized zirconia compositions within the non-transformable tetragonal phase field exhibit promising toughness, which is critical for TBC durability. These promising attributes will be discussed in this presentation in light of recent experimental work on this system.