Effects of pretreatments on the oxidation behavior of Ni---Co---Cr---Al---Y coatings

Abstract

The influence of some pretreatments on the oxidation behavior of electron-beam-deposited Ni---Co---Cr---Al---Y-type coatings has been investigated in order to obtain a dense electrically insulating Al₂O₃ layer for subsequent deposition of Pt, Pt + Rh thin film thermocouples. The results of hardness, adhesion and conductivity measurements, X-ray diffraction studies, and scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDXA) investigations of surface topography are reported. The cross section and underside of the oxide were observed by SEM and those findings are also reported. X;-ray diffraction studies in conjunction with EDXA of the cross section and underside of the oxide show that, in samples pretreated at 1000 °C for 5 h in a vacuum or hydrogen environment and subsequently thermally oxidized in oxygen for 50 h, the chemical nature of the oxide varies from the top surface to the alloy matrix in the order, NiAl₂O₄, Al₂O₃ and Y₄Al₂O₉. Below the Y₄Al₂O₉, some enrichment of elemental chromium is observed. In samples thermally oxidized after an Al₂O₃ precoating, no NiAl₂O₄ and Y₄Al₂O₉ were detected. The sequence of the oxides was first Al₂O₃ and then Cr₂O₃. The sample thermally oxidized without any pretreatments showed NiAl₂O₄, Al₂O₃ and Cr₂O₃. Adhesion of the oxide scale on the pretreated samples was excellent. Although the oxide scale on the untreated sample did not spall off during the cool-down from the oxidation temperature, the adhesion test showed poor bonding with the base matrix. SEM studies of the underside of the oxide scale of all the samples did not show any yttrium-based stringers (or pegs) which could have been responsible for mechanical bonding of the scale with the base. A comparative study of all the samples reveals that an optimum combination of the hardness, adhesion, microstructure and electrical conductivity of the oxide scale for fabrication of thin film thermocouples can be obtained by depositing an Al₂O₃ precoating prior to thermal oxidation.