The formation of platinum aluminide coatings on IN-738 and their oxidation resistance
Platinum alumnide coatings have been produced by first plating a thin layer of platinum usinq a fused salt platinum plating technique and then pack aluminizing using powder packs containingAl, NH4 C1 and Al 2 0 3 or Ni 2A1 3 , NH4 C1 and Al 203 . The chemistry and morphology of these coatings on IN-738 superalloy both in the ascoated and in the subsequently heat treated condition have been studied. The coating morphology and chemistry are highly dependent upon the thickness of the platinum layer, pack activity and time of processing. A relatively thick platinum layer (l0 pm) produced a coating with an outer Pt2A13 layer above other narrow layers. The Pt concentration decreases towards zero as the diffusion zone is approached. A second type, usually formed with a thin (5 pm) Pt layer is characterised by a marked interaction with the substrate. An outer Pt/U 2 layer is followed by a layer of NiAl containing fine precipitates of a chromiumtungsten rich phase. A lamella-like layer hiqh in chromium and other refractory elements exists at the coating/substrate interface in most of the as-coated samples. A third type of coating has been produced by a post-platinising heat treatment process prior to aluminizing. This type of coating is characterised by an outer duplex layer of PtAl 2 and Ni/U. Heat treatment of the as-formed coating results in interdiffusion between Al , Ni and Pt to produce an overall thickening of the coating layer and a decrease in the coating Al concentration. Thus a (Pt,Ni) Al or (Ni,Pt) Al outer layer may develop after heat treating these types of coatings at 1000°C for up to 1200 hours. In addition to this Widmanstatten sigma phase plates extending into the substrate are normally found beneath the outer layer after several hours' of heat treatment. Diffusion paths on pseudo-ternary phase diagrams are made to represent the phase constitution of the as-formed coatings. Isothermal oxidation tests in an oxygen atmosphere between 800 - 1000°C of different Pt-Al surfaces have been studied and the result of tests showed that the incorporation of Pt into the aluminide coatings enhance the oxidation resistance (particularly at 1000°C). Furthermore, thermal cyclic oxidation tests showed a remarkable improvement in oxide adherence over the simple aluminides.