Fabrication of Oxide Dispersion Strengthened Bond Coats with Low Al 2 O 3 Content
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Fabrication of Oxide Dispersion Strengthened Bond Coats with Low Al2O3 Content Jan Bergholz1 • Bruce A. Pint2 • Kinga A. Unocic2 • Robert Vaßen1,3
Submitted: 21 October 2016 / in revised form: 7 March 2017 Ó ASM International 2017
Abstract Nanoscale oxide dispersions have long been used to increase the oxidation and wear resistance of alloys used as bond coatings in thermal barrier coatings. Their manufacturing via mechanical alloying is often accompanied by difficulties regarding their particle size, homogeneous distribution of the oxide dispersions inside the powder, involving considerable costs, due to cold welding of the powder during milling. A significant improvement in this process can be achieved by the use of process control agent (PCA) to achieve the critical balance between cold welding and fracturing, thereby enhancing the process efficiency. In this investigation, the influence of the organic additive stearic acid on the manufacturing process of Al2O3-doped CoNiCrAlY powder was investigated. Powders were fabricated via mechanical alloying at different milling times and PCA concentrations. The results showed a decrease in particle size, without hindering the homogeneous incorporation of the oxide dispersions. Two powders manufactured with 0.5 and 1.0 wt.% PCA were deposited by high velocity oxygen fuel (HVOF) spraying. Results showed that a higher content of elongated particles in the powder with the higher PCA content led to increased surface roughness, porosity and decreased coating thickness, with areas without embedded oxide particles.
& Jan Bergholz [email protected] 1
IEK-1, Forschungszentrum Ju¨lich GmbH, Ju¨lich, Germany
2
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak-Ridge, TN, USA
3
IWV1, Forschungszentrum Ju¨lich GmbH, Leo-Brandt-Starße, Ju¨lich 52425, Germany
Keywords bond coats mechanical alloying oxide dispersion strengthened process control agent thermal barrier coatings thermal spraying
Introduction Thermal barrier coating (TBC) systems are an essential feature of most modern Brayton cycle turbines that transform thermal energy into mechanical energy. Turbines have severe thermal and mechanical loading of the components for gaining higher efficiencies and TBCs are needed to reduce temperatures and improve oxidation (Ref 13). Current TBCs can lower the substrate’s surface temperature by 200-300 °C; however, higher temperatures are desired to further increase efficiency (Ref 3-6). TBCs consist of a ceramic top coating, typically made of yttria-stabilized zirconia (YSZ), which acts as the heat shield. A metallic oxidation-resistant bond coating (BC) is used below the ceramic layer. The BC’s primary purposes are to improve oxidation resistance of the superalloy substrate, as oxidizing species readily penetrate the porous outer layer, as well as to enable sufficient bonding of the ceramic to the substrate (Ref 1, 3, 7-9). Typical BCs include NiAl diffusion coatings as well as MCrAlY or MCrAlX alloys (M=Ni, Co). The latter are used for
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