A process for making superior diffusion platinum-aluminide bond coats for plasma-sprayed or physical-vapor-deposited thermal-barrier coats (TBCs) on superalloy substrates has been devised. The novel aspect of the process lies in the use of several relatively inexpensive pack diffusion steps to incorporate hafnium and silicon to increase resistance to oxidation.
TBCs are typically composed of yttria-stabilized zirconia and are typically used to protect superalloy turbine-engine components against high temperatures. A bond coat provides both mechanical and chemical bonding between the underlying superalloy and the overlying TBC. Diffusion-type aluminide bond coats offer advantages (including lower cost) over conventional MCrAlY (where "M" denotes Fe or Ni) bond coats, except that the diffusion-type aluminide coats exhibit lower resistance to oxidation. Any change in material or processing that increases the ability of bond coats to resist oxidation also increases the durability of TBC coats.
The incorporation of Pt into a diffusion-type aluminide bond coat increases its resistance to oxidation. Prior to the development of the present process, Hf and Si had been incorporated into MCrAlY bond coats to increase resistance to oxidation, but had not been incorporated into diffusion Pt-aluminides.
The basic version of the present process comprises the following steps:
- A hafnided surface layer with a thickness between 0.5 and 1 mil (between 13 and 25 μm) is formed on a superalloy substrate by pack diffusion of Hf at a temperature of 1,975 °F (1,079 °C) for 4 hours.
- A thin surface layer of Si is deposited by pack siliciding.
- The hafnided, silicided workpiece is plated with Pt to a thickness between 0.2 and 0.3 mils (between 5 and 8 μm).
- The workpiece is diffusion heat-treated at a temperature of 1,900 °F (1,038 °C) for 2 hours.
- The workpiece is vapor-phase or pack aluminized.
- The workpiece is given a post-aluminiding heat treatment to homogenize the coating.
The superior bond coats afforded by this process enable affected turbine components to withstand higher operating temperatures; higher operating temperatures result in greater energy-conversion efficiencies. Moreover, the superior bond coats provide some residual protection for parts from which TBCs have spalled.
This work was done by Bhupendra K. Gupta and Jon C. Schaeffer of General Electric Co. for Lewis Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
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Refer to LEW-16295.