Microstructural analysis of Ta-containing NiCoCrAlY bond coats deposited by HVOF on different Ni-based superalloys

Abstract The microstructural and chemical evolution after different processing steps of Ta-containing NiCoCrAlY bond coats (BC) sprayed by high velocity oxy-fuel on Ni-base superalloy (SA) substrates, top-coated with 7YSZ and furnace cycled at 1100 °C focusing along the BC/SA interfaces is presented. Three superalloys were selected considering their Ta content relative to that within the BC: a) no Ta in the superalloy composition (IN100) and either b) similar or c) higher Ta content with respect to the BC (M247LC SX and CMSX-4, respectively). The processing conditions were as-sprayed, as-annealed, after EB-PVD deposition of a 7YSZ top coat, and after furnace cyclic tests (1100 °C/120 h). The evolution of chemical composition after the different process steps are presented that include a normalization criteria to a Ni-Al-Cr system, elemental profiles as well as Weibull distribution plots of minor containing elements such as Ti, Ta, Mo, or W at the BC/SA interface. The effect of Ta was activated in some coated substrates by the presence of Ti and C in the superalloy consisting of inward Ta-diffusion, trapping of outward diffusing Ti and the subsequent formation of (Ta, Ti)-rich carbides in the BC avoiding possible segregation effects as titanium oxide in the oxide scale.