ICMCTF2015 Session A2-2: Thermal and Environmental Barrier Coatings

The high temperature durability under combustion environment of the AM1/NiAlPt/EBPVD TBC coating system used for high temperature blades and vanes in several aeroengines has been investigated using creep, low cycle fatigue (LCF), dwell-fatigue (DF) and thermomechanical fatigue (TMF) in the 950-1100°C temperature range. This characterization has been performed using the MAATRE burner where massive or hollow samples can be mechanically loaded under hot gas flow conditions. This unique test rig allows for gas testing temperatures up to 1550°C and internal cooling of hollow samples.

In this presentation, the damage mechanisms under creep, LCF, DF and TMF conditions will be presented and the impact of a through thickness thermal gradient (induced by means of internal cooling) will be analyzed. It will also be shown how the microstructure evolutions in the bond coat could control the coating life. Finally, it will be shown that the temperature dependent mechanical behavior of the bond coat controls the critical interface of this TBC system and the subsequent damage mechanism.

Thermal barrier coatings from yttria partially stabilized zirconia (YPSZ) are widely used in gas turbines and optimizing their resistance to thermal cycling has always been a challenge. Searching ways to improve the ductility, an approach was investigated applying agglomerated nano-powder from 3mol% YSZ to modify the structure of a standard YPSZ thermal barrier coating.

Cylindrical specimens were prepared using a commercial Ni based alloy as substrate and a commercial MCrAlY bond coat deposited by LPPS. One specimen series was produced inserting an interlayer from agglomerated nano-powder from 3mol% yttria partially stabilized zirconia between the bond coat and the standard TBC from commercial YPSZ. Both layers were deposited by air plasma spraying (APS). A second specimen series was coated with a TBC solely using the agglomerated nano 3mol% YSZ. Two commercially available standard TBC systems with different thicknesses and from two different coaters were added to the experimentation program. The specimens were subjected to thermal cycling in air holding 23 hours at 1080°C and 1 hour at room temperature. After determination of the lifetimes, three samples of each specimen series were exposed for 25%, 50% and 75% of the total lifetime.

The Solution Precursor Plasma Spray (SPPS) process for making thermal barrier coatings (TBCs) provides unique microstructural features and improved properties. These microstructural features include: (a) through-coating-thickness cracks for improved strain tolerance and durability, (b) ultra-fine splats that provide improved inter-splat bonding and in-plane fracture toughness, (c) nano-and micron size pores that can be varied over a wide range from 5 to 40 volume percent for control of erosion resistance, abradability and thermal conductivity, and (d) planar arrays of porosity, called inter-pass boundaries, that reduce TBC thermal conductivity. These microstructural features have been used to develop two SPPS TBCs with improved performance: (a) a SPPS YSZ with IPBs that has a thermal conductivity of 0.6 watt/mK, about half that of air plasma spray YSZ, and (b) a high temperature SPPS yttrium aluminum garnet (YAG) TBC that provides more than a 200oC use temperature capability compared to YSZ. This presentation will describe the deposition process, microstructure and the engine critical properties for these SPPS TBCs. Comparisons will be made with the Suspension Plasma Spray (SPS) process.

New thermal barrier coatings (TBCs) that have the capability to perform at higher temperatures than yttria-stabilized zirconia (7YSZ) are integral to the development of more energy efficient turbine engines. Compositions in the TaO_2.5-YO_1.5-ZrO₂ (TaYZ) system have previously been investigated as potential TBC candidates due to numerous desirable high temperature properties. While more is known about the ZrO₂-rich region of the phase diagram, fundamental understanding of the phase relations in the ZrO₂-lean region is lacking. Precursor derived compacts of relevant TaYZ compositions were sintered at 1250°C and 1500°C to establish the phase equilibria of the various phase field regions. Samples were then investigated with x-ray diffraction and electron microscopy. By understanding the phase relations, especially within the ZrO₂-lean portion of the phase diagram, compositions with promising properties can be identified and investigated further. The fundamental understanding gained regarding the phase relations highlights the importance of clarifying issues in the TaYZ system relevant to TBC development.

Thermal barrier coatings (TBCs) for aeronautical applications are mostly based on yttria stabilized zirconia (YSZ) deposited by either thermal spray or electron-beam physical vapour deposition (EB-PVD) in the combustion chamber and the high pressure turbine components. The increase of the turbine inlet temperatures demands greater oxidation and thermal insulation to the turbine parts that are not currently TBcoated. This paper explores the mechanisms involved in the synthesis of full TBC systems in a single step process from slurries containing Al microspheres. The microspheres simultaneously deform and oxidise allowing Al to be supplied to the Ni-based superalloy substrate. The combustion synthesis mechanisms allow then fast aluminisation of the superalloy. Once Al is consumed by diffusion to the substrate, a bisque of hollow alumina microspheres is left on top that provides very low thermal conductivity (k) as measured by a laser flash method. While the erosion resistance is to be improved, this coating system appears to provide resistance to both molten salt in Na₂SO₄-V₂O₅ and to oxidation in wet and dry air.

Equimolar substitutions of YO_1.5 and TaO_2.5 into ZrO₂ form a single tetragonal phase that is stable up to 1500°C and non-transformable to monoclinic zirconia upon cooling, desirable characteristics for the tetragonal phase used in thermal barrier coatings (TBCs). However, exploiting such a phase to enhance the lifetime of TBCs has not been realized yet. In this study, TBCs made from Y/YbO_1.5 -TaO_2.5-ZrO₂ quaternary compositions within the tetragonal and cubic two-phase field were fabricated by electron beam physical vapor deposition and their phase evolution at 1250°C and 1500°C was studied using XRD, Raman, and SEM/TEM. The as-deposited coatings retain the columnar microstructure characteristic of conventional 7YSZ EB-PVD coatings but also contain a significant amount of compositional scatter not typically observed in 7YSZ TBCs. For both aging temperatures, the as-deposited cubic phase decomposes into equilibrium tetragonal and cubic phases with no evidence of monoclinic zirconia formation after the longest aging times (400 h). Additional indentation and three-point bend tests indicate that the toughness of this material is higher than that of cubic YSZ. This research was supported by Honeywell Aerospace.

Compositions in the ZrO₂-YbO_1.5-TaO_2.5 (YbTaZ) system offer lower thermal conductivity and improved phase stability up to 1500°C relative to the state of the art ZrO₂-8±1mol%YO_1.5, representing a promising candidate for next generation TBCs. It has recently been shown that ZrO₂-rich compositions in this system can be effectively deposited with strain tolerant columnar microstructures by electron-beam physical vapor deposition (EB-PVD), in spite of the expected differences in vapor pressures. This presentation discusses the effects of composition, specifically the Ta:Yb ratio, on the evolution of the columnar microstructure and the implications for the coating compliance. The behavior is also compared with observations on a ZrO₂-8%YbO_1.5 EB-PVD coating. In general, the as-deposited coatings are metastable in their phase constitution, and evolve at typical service temperatures into multiphase assemblages. The effects of heat treatment on the phase evolution and microstructural features, as well as the possible consequences to the coating properties, are discussed.. (Research supported by Honeywell Aerospace, Phoenix, AZ).