Example Thermal barrier coatings

The turbine blade is commonly made of a nickel-base or cobalt-base super- 

Y203-stabilized Zr20a (YSZ), typically with a Y2O3 concentration of 7 to 8 wt%, is the common material of choice for the ceramic topcoat in light of its many desirable properties as a TBC (Padture et al. 2002)  

11 xlO /°C, which is closer to that of the metallic layer beneath it ( 14 X 10 /°C) than to that of most ceramic materials. Consequently, the stresses generated by thermal expansion mismatch with the underlying layers during the thermal cycles generated by the operation of the turbine engine would be minimized  

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Fig. 5.20. An example of an interface imperfection in the form of an undulation in a thermal-barrier coating (TBC) system comprising a Pt aluminize bondcoat on a Ni-based superalloy substrate. A thermally grown oxide (TGO) layer is formed between the bond coat and the zirconia TBC. Thermal cycling leads to interfacial fracture and cracking in the TBC in the region of high local tensile stress adjacent to the TCO layer. Such cracking and interfacial delamination ultimately lead to the onset of large scale buckling. Reproduced with permission from Evans et al. (2001).

Figure 7. Typical examples of specimens showing angles of crack propagation with respect to K /Ka of plasma sprayed Zr02 8wt%Y203 thermal barrier coatings (a) pure mode II K jKa 0, (b) KijKw 2.1, c) K jK i 5.5, and (d) pure mode I Ai/tfii °o.

The addition of cerium oxide, for example, to zirconia produces a material with exceptional toughness and good strength [29], Cerium oxide-doped zirconia is used also in thermal barrier spray coatings on metal surfaces(30]. 

The properties required of plasma sprayed coatings may vary considerably depending upon the application for example, low porosity is necessary for wear resistance and corrosion protection but very porous deposit may be more suitable for thermal barriers. The physical properties of a coating of a given material will largely depend upon the spraying conditions. 

Silicone adhesives and sealants are used in many advanced fields of technology where extreme environmental conditions are experienced, for example, in Aerospace applications, as coatings for firewalls, windshields and other thermal barriers, and for corrosion protection. 

Thermally sprayed coatings, for example, MCrAlY corrosion protection layers and ceramic-based thermal barriers... 

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