Phase evolution in an MCrAlY coating during high temperature exposure

Abstract

MCrAlY (M = Ni and/or Co) coating systems are often applied on gas turbine blades and vanes to withstand the challenges of severe conditions. During service MCrAlY coatings are subjected to microstructural transformations that can be an indication of components service temperatures. The development of indirect methods to measure this parameter is of great concern in the gas turbine “world” due to the impossibility of direct measurements. \ud In the present work the evolution of an MCrAlY coating applied on René80 by LPPS (Low Pressure Plasma Spray) technique has been studied in order to verify if it was possible to identify a microstructural indicator of the service temperature. The specimens were exposed for different lengths of time at test temperatures of 700 – 800 – 900 °C in order to characterize the phase evolution with time and temperature. Selective etching was employed for optical metallographic investigation. Scanning Electron Microscopy (SEM) observation combined with Electron Backscattered Diffraction (EBSD) and Energy Dispersive Spectroscopy (EDS) showed that the coating is composed of a γ- Co matrix, β-AlNi, σ-(Cr, Co), Cr carbide and Y-rich phases. Among these phases, the sigma phase resulted in a temperature – composition dependence that can be a useful tool for evaluating the local service temperature and modelling the residual lifetime

Bond strength and oxidation resistance of YSZ/(Ni, Al) composite coatings

“…It is known that the existence of Cr rich lower the coefficient of thermal expansion of composite coatings and reduces thermal stresses, thus resulting in a better adhesion. 16 On the other hand, it can also seen from Fig. 4 that Zr diffusion from the surface to the nickel substrate.…”

The Application of the Small Punch Tensile Test to Evaluate the Ductile to Brittle Transition of a Thermally Sprayed CoNiCrAlY Coating

“…Recently, MCrAlY (M=Ni,Co) overlay bond coats deposited by low pressure plasma spraying or by high velocity oxy-fuel thermal spraying have become more widely used because of advantages such as low cost, better control of composition and the possibility to employ complex MCrAlY alloys with tailored microstructures [7][8][9]. These can comprise, for example fcc γ-Ni, bcc B2-β-NiAl, ordered γ' (Ni 3 (Al,Ti)) and σ (Cr,Co) [10][11][12]. Through careful selection of alloy composition and phase fraction of constituents it is, in principle, possible to design specific combinations of chemical and mechanical properties for the bond coat.…”

The influence of microstructure on the ductile to brittle transition and fracture behaviour of HVOF NiCoCrAlY coatings determined via small punch tensile testing

“…The composition of HVOF MCrAlY bond coats can be adjusted without needing to alter the composition of the substrate; offering greater flexibility over traditional diffusion coatings which require a coating composition similar to the substrate composition [1,[7][8]]. MCrAlY's can comprise FCC γ-Ni, BCC B2-β-NiAl, ordered γ'-Ni 3 (Al,Ti) and σ-(Cr,Co) [9][10][11]. The B2-β-NiAl phase, commonly present in MCrAlY overlay coatings, is brittle at lower temperatures and shows increasing strain to fracture above a critical temperature termed the ductile to brittle transition temperature (DBTT); around 750 °C for NiAl.…”