Anisotropic Elasticity of APS and HVOF CoNiCrAlY Coatings Studied by Resonant Ultrasound Spectroscopy with Laser Doppler
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Anisotropic Elasticity of APS and HVOF CoNiCrAlY Coatings Studied by Resonant Ultrasound Spectroscopy with Laser Doppler Interferometry Kanta Adachi1 • Hiroyuki Waki1
Submitted: 18 February 2019 / in revised form: 7 May 2019 / Published online: 30 May 2019 ASM International 2019
Abstract The splat-based layered structure of thermal spray coatings leads to elastic anisotropy; however, elucidating the anisotropic elastic properties of such coatings is a challenging subject because of the limited sample size and microstructural factors such as high defect level. The objective of this study is to investigate the anisotropic elasticity of the as-sprayed and heat-treated CoNiCrAlY coatings prepared by atmospheric plasma spraying (APS) and high-velocity oxygen fuel (HVOF) spraying. For this purpose, we measure all five elastic constants accurately using resonant ultrasound spectroscopy with laser Doppler interferometry. This approach realizes the correct mode identification of the measured resonance frequencies, even for coatings with high internal friction by comparing the measured and calculated displacement distributions, providing a precise determination of the elastic constants with inverse calculation. We determined that an oriented array of defects causes a significantly low Young’s modulus along the spraying direction in both the as-sprayed coatings. Our measurements on the heat-treated coatings revealed that thermal treatment makes the APS coating stiffer than the HVOF coating, whereas the stiffness anisotropy of the APS coating remains stronger compared to the HVOF coating, even after heating. This phenomenon is consistently explained by focusing on the oxides in the coatings.
& Kanta Adachi [email protected] 1
Department of Systems Innovation Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan
Keywords anisotropic elasticity MCrAlY mechanical properties resonant ultrasound spectroscopy thermal spray coating Young’s modulus
Introduction MCrAlY alloys (M represents Co and/or Ni) are used widely in overlay coatings for hot section components because of their superior high-temperature oxidation and corrosion resistance. MCrAlY coatings also have a critical role as the bond coat of thermal barrier coatings (TBCs), which is a key technology in aircraft engines and gas turbine power plants. The bond coat enhances the adhesion of the ceramic top coat to a superalloy substrate and moderates the thermal expansion mismatch between them, improving the durability of the TBC. In-service hot section components are subjected to thermomechanical load as well as high-temperature oxidation and corrosion; hence, knowledge of the mechanical properties of MCrAlY coatings is necessary to manufacture a well-designed TBC system. Elastic moduli, among other properties, are of particular importance in increasing the lifetime of a TBC because they are essential to estimating the stress field in the TBC caused by the manufacturing process, thermal treatment, and high-temperature oper
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