Effects of pretreatments on structure and oxidation behaviour of d.c.-sputtered platinum aluminide coatings

Abstract

Pt aluminide coatings exhibit better high temperature oxidation and hot corrosion resistance than aluminide coatings. In this study the Pt layer was deposited by d.c. sputtering on Hastelloy-X superalloy instead of by an electroplating method. Laser remelting and diffusion heat treatment were applied to eliminate defects such as leaders existing in the as-deposited Pt layer and to effect the bonding between the Pt layer and the substrate before aluminization. The results of this study showed that the Pt aluminide coating pretreated by diffusion heat treatment and laser surface remelting consisted of a PtAl 2 + NiAl duplex-phase outer layer, an (Ni 2Pt)Al single-phase layer and an interdiffusion zone. The outer duplex-phase layer of the Pt aluminide coating pretreated by diffusion heat treatment consisted of a continuous PtAl 2 phase with NiAl as a second phase. In the laser-remelted Pt aluminide coating, by contrast, a minor PtAl 2 phase was embedded in NiAl. The results of oxidation tests at 1100°C showed that the diffusion-heat-treated Pt aluminide coating exhibited excellent oxidation resistance whereas the laser-remelted Pt aluminide coating failed to provide protection against high temperature oxidation. The latter is deemed to be inferior to a plain aluminide coating.