Tensile Behavior of Air Plasma Spray MCrAlY Coatings: Role of High Temperature Agings and Process Defects
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NI-BASED superalloys used at high temperature need environment-protective coatings against high temperature oxidation and corrosion to limit detrimental impacts of the substrate oxidation on its long-term mechanical properties.[1–3] Metallic coatings, such as MCrAlY overlay coatings or aluminide diffusion coatings, are alternative solutions to provide a substantial reservoir of Al or Cr necessary to promote the formation of a dense and very low-growth-rate oxide layer at high temperatures, i:e:; a-Al2 O3 and/or Cr2 O3
DAMIEN TEXIER is with the Institut Clement Ader (ICA) UMR CNRS 5312, Universite´ de Toulouse, CNRS, INSA, UPS, Mines Albi, ISAE-SUPAERO, Campus Jarlard, 81013 Albi Cedex 09, France. Contact e-mail: [email protected] CLE´MENT CADET and VINCENT MAUREL are with Centre des Mate´riaux, Mines ParisTech - UMR CNRS 7633, BP 87, 91003 Evry Cedex, France. THOMAS STRAUB and CHRIS EBERL are with the Fraunhofer Institute for Mechanics of Materials IWM, 79108 Freiburg, Germany. Manuscript submitted September 8, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
depending on the operating temperature.[4–6] MCrAlY coatings (M = Ni, NiCo, CoNi, etc.) have demonstrated a very interesting trade-off between a low-cost process and excellent protection for industrial applications. Numerous line-of-sight and non-line-of-sight deposition processes were developed to manufacture coated components with complex geometries. Despite their environmental protection, coatings were reported to impair the mechanical integrity of the whole-coated components due to: (i) their brittle behavior below ductile-to-brittle transition temperature (DBTT) leading to premature cracks, (ii) their significant loss in mechanical strength but gain in ductility above the DBTT leading to non-bearing mechanical regions and thus additional weight, (iii) their interdiffusion with the Ni-based substrate affecting the mechanical properties of this latter structural material.[7–16] The DBTT of MCrAlY coatings, ranging from 500 °C to 800 °C, varies as a function of its composition with a high effect of the Cr content.[8,11,13,17,18] As a direct consequence, the low ductility of such coatings from room temperature up to 600 °C to 800 °C could be detrimental for coated Ni-based superalloys subjected to thermomechanical fatigue loading leading to tension in the brittle
range of temperature of the coating.[19–21] To obtain a robust identification of the mechanical response and the lifetime of coated materials, quantitative assessment of the mechanical properties of the overlay coating is needed. Based on the coating thickness deposited on industrial components, i.e., tens of micrometers, two main approaches are developed in the literature to assess the macroscopic mechanical properties of the coating: (i) macromechanical testing of coated Ni-based superalloy substrates paired with inverse identification methods and (ii) micromechanical testing of freestanding coating specimens. Tensile testing of coated Ni-based superalloys with either overlaid or diffusion
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