High-temperature oxidation behaviour of laser-clad NiCrAlY

Abstract

Summary To achieve greater efficiency, modern industrial plants are now required to be operated under more arduous process conditions, such as operation in harsher corrosive environments at elevated temperature.1 Plant construction materials require both good high-temperature mechanical properties and better oxidation resistance. One solution has been to use composite materials and most notably surface treatment techniques. Available surface treatment techniques include PTA (plasma transferred arc welding), CVD (chemical vapour deposition), PVD (physical vapour deposition), laser cladding, etc. Laser cladding has a high energy density enabling high melting point materials or even ceramics to be overlaid on the substrate and also offers good controllability together with the advantage of low dilution from the substrate. The oxidation behaviour of laser-clad NiCrAlY layers, however, has so far been poorly documented in the literature. This paper therefore describes an investigation of the oxidation behaviour of NiCr10AlY and NiCr20AlY clad layers formed in single layers in air using prepressed powder with regard for the effect of dilution from the substrate. The effects of the Al content and heat treatment are also examined. The results obtained may be summarised as follows. 1. The oxide formed on the NiCr1OAlY and NiCr20AlY clad layers is mainly α-Al2O3, and the growth rate of the oxide obeys a parabolic rate law. 2. The parabolic rate constant of the NiCr10AlY clad layer is of the same order as that of the bulk material, and the clad layer formed in a single layer by laser cladding has an oxidation resistance equivalent to that of the bulk material. 3. The parabolic rate constant of the NiCr20AlY clad layer is 1–2 orders of magnitude greater than that of the NiCr10AlY clad layer. This is due to the formation of unstable and faster growing 0-Al2O3 during oxidation. 4. Heat treatment of the clad layers produced by laser cladding is effective for improvement of the oxidation resistance. This is due to the fact that heat treatment makes the aluminium distribution in the clad layer more homogeneous, which enhances the nucleation of Al2O3 in the initial stage of oxidation, resulting in a lower parabolic rate constant.