Oxidation of Ferrous Alloys and Coatings Under Isothermal, Impulse Heating, and Diesel Engine Operation: Part II—MCrAlY Coatings for Protection of 4140 Steel

The use of nickel- and iron-based alloy coatings containing Cr, Al, and Y (NiCrAlY and FeCrAlY) were evaluated as potential oxidation barriers for low-alloy steels for the next generation of high-output diesel engines. Rapid oxidation at temperatures above 500 °C currently limits the use of the 4140 steels used to manufacture pistons crowns in this application. Isothermal furnace testing, a novel highly transient combustion-based laboratory test, and direct high temperature engine exposure were used to assess the efficacy of the coating-substrate systems. Both NiCrAlY and FeCrAlY coatings appear to protect the substrate from oxidation at isothermal temperatures up to 677 °C. On direct exposure to combustion, through Impulse Cyclic Heating Tests, the NiCrAlY coatings themselves also appear resistant to degradation at very high temperatures, while the FeCrAlY coatings degrade more rapidly for equivalent thermal loading. Engine tests further validated the efficacy of NiCrAlY coatings, with neither coatings nor 4140 steel pistons showing degradation at temperatures estimated to be more than 500 °C. From this work NiCrAlY coatings are estimated to have a potential upper limit of 677 °C, at which interdiffusion between the coating and substrate will likely complicate their use.