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NiAl coatings tends to suffer strongly from interdiffusion with the substrate, which results in formation of γ' at the expense of β.

The idea of introducing a diffusion barrier lead to the invention of the Pt-aluminide coatings, which are obtained by similar methods as conventional aluminide, but after electroplating the blade with Pt. The layer of Pt deposited is typically of 5-10 μm (Schütze, 2000).

Because the plating can increase the life of the blades by up to three times (Sivakumar, 1989), the cost of Pt plating is easily compensated.

Interestingly, it was found that Pt additions not only did not provide a diffusion barrier, but also enhanced Al diffusion (Chen, 1997). When applied on the second generation superalloy CMSX4, Pt formed TCP (topologically close-packed) phases with some elements of the substrate (Re, W, Mo, Cr).
The exact reasons for the beneficial effect of Pt are not fully understood, but it was found that Pt improves oxide adherence and also contributes to better hot corrosion resistance.

Pt appears to partially substitute Ni in β-NiAl, and also to form PtAl2 which is believed to act as an Al reservoir. It has also been proposed that Pt acts in a similar manner as Y in MCrAlY coatings. In these coatings, Y combines with S; this greatly increase the coating life as S is otherwise detrimental to the adherence of the oxide layer. There is nevertheless no evidence of a similar effect of Pt in β-NiAl coatings (Evans, 2001).

Further readings:

  1. Kim G. M. et al., Scripta Mater., 46:2002, 489-495, 'The effect of the type of thermal exposure on the durability of thermal barrier coatings'.
  2. Evans A. G. et al., Prog. Mater. Sci., 46:2001, 505-553, 'Mechanisms controlling the durability of thermal barrier coatings'.
  3. Chen J. H. et al., Surf. Coat. Techn., 92:1997, 69-77, 'Degradation of the platinum aluminide coating on CMSX4 at 1100 C'.
  4. Sivakumar R. et al., Surf. Sci. Techn., 37:1989, 139-160, 'High temperature coatings for gas turbine blades: a review'.
  5. M. Schütze ed., Corrosion and Environmental Degradation Vol. II, Wiley-vch, London, in Materials Science and Technology series.

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