1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

The Superalloys: Fundamentals and Applications

Microstructural and compositional design of Ni-based single crystalline superalloys ― A review

The effect of the thermally grown oxide (TGO) formation on the lifetime of the thermal barrier coatings (TBC) with MCrAlY-bondcoats (BC) is reviewed. A number of factors affecting the TGO-formation and TBC-failure are discussed including the coating microstructure, geometrical (coating roughness and thickness) and processing parameters. Under given testing conditions for a specific EB-PVD-TBC-system forming a flat, uniform alumina TGO a critical TGO-thickness for TBC-failure can be defined. This TGO-morphology is, however, not necessarily optimum for obtaining long TBC-lifetime, which can be extended by formation of TGO’s with an uneven TGO/BC interface. In contrast, APS-TBC-systems are prone to formation of intrinsically inhomogeneous TGO-morphologies. This is attributed to non-uniform depletion of Y and Al underneath rough MCrAlY-surfaces as well as due to the commonly observed repeated-cracking/re-growth of the TGO during temperature cycling. The latter phenomenon depends on the exposure temperature and the mechanical properties of the APS-TBC. In both types of TBC-systems the TGO-formation and TBC-lifetime appear to be very sensitive to the manufacturing parameters, such as vacuum quality during bondcoat spraying and temperature regime of the bondcoat vacuum heat-treatment.

Failure mechanisms of thermal barrier coatings on MCrAlY-type bondcoats associated with the formation of the thermally grown oxide

https://www.research.manchester.ac.uk/portal/files/76397348/A_18_1526R1.pdf

Effect of microstructure on early oxidation of MCrAlY coatings

The specific permeability of oxygen in plasma sprayed zirconia has been measured to be approximately 10 −16 m 2 , with some dependence on microstructure, and hence on spraying conditions. Permeability rises with increasing temperature, as expected, and differences of a factor of approximately 4 were observed between room temperature and 600 °C. Maximum oxygen fluxes through the top coat via gas permeation, estimated for typical operating conditions of a thermal barrier coating (TBC) in a gas turbine, have been estimated and compared with those expected via ionic diffusion, obtained using published data. It is shown that gas permeation is normally expected to dominate. However, the rate of oxide growth at the bond coat/top coat interface of a TBC is controlled by diffusion through the oxide layer and either mechanism of oxygen transport through the top coat could generate fluxes sufficient to maintain this condition. This explains why oxide growth rates are similar whether or not the top coat is present. The permeability data, and the dependence of permeability on spraying conditions, may be of interest for other applications, such as permeable fuel cell membranes.

Oxygen transport by gas permeation through the zirconia layer in plasma sprayed thermal barrier coatings

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.

M Menzebach

Papers

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