Development of Metal Based Thermal Barrier Coatings (MBTBCs) for Low Heat Rejection Diesel Engines

Abstract

Abstract In this work, metal-based thermal barrier coatings (MBTBCs) for use in low heat rejection diesel engines have been produced, using high frequency induction plasma spraying (IPS) of iron-based nanostructured alloy powders. Important advances have been made over recent years to the development of ceramic-based thermal barrier coatings (TBCs) for diesel engines, but they are not yet applied in mass production situations. Besides the important economic considerations, the reliability of ceramic TBCs is also an issue, being associated with the difficulty of predicting their “in-service” lifetime. Through engineering of the nano/amorphous structure of MBTBCs, their thermal conductivity can be made as low as those of ceramic-based TBCs, with reduced mean free paths of the electrons/phonons scattering. In this work, nano/amorphous structured coatings were deposited by IPS using the following spray parameters: spraying distance (200mm), plasma gas composition (Ar/N2-85/15, by volume %), IPS torch power (25kW), and powder feed-rate (16g/min.). The structure and properties of the deposited layers were characterized through SEM (Scanning Electron Microscopy) observations. The thermal diffusivity (α) properties of the MBTBCs were measured using a laser flash method. Density (ρ) and specific heat (Cρ) of the MBTBCs were also measured, and their thermal conductivity (k) calculated (k =αρCp). The thermal conductivity of MBTBCs, with 7.5% total porosity, was found to be 1.22 W/m/K. The heat treatment study showed that phase transformation started at 650oC, and grain size growth from nano- to micron- scales occurred at around 1000°C under static exposure conditions. Thermal expansion coefficient (TEC) of MBTBCs was 15E-6 /K, which is close to the TEC of cast iron and thus, closer to the TEC values of aluminium alloys than are conventional TBCs. Fracture toughness of MBTBCs has also been assessed by use of Vickers hardness tests, with a 100 g load for 15 s, and the results show that there are no measurable crack developments around “indented” areas on all samples of MBTBCs tested.