Interdiffusion in MCrAlY coated nickel-base superalloys

- Interdiffusion phenomena between different bond coats and René 80 were investigated.
- Thermodynamic simulations provide further understanding of observed characteristics.
- Results reveal a significant influence of coating composition and deposition process.
- Intense surface pre-treatment results in an increased amount of Kirkendall porosity.
- Matching the element activities minimizes detrimental interdiffusion effects.

In this work, interdiffusion between a polycrystalline nickel-base superalloy (René 80) and two MCrAlY bond coats, each with a different chemical composition, is investigated. Of primary interest is the evolution of Kirkendall porosity, which can form at the interface of substrate and bond coat and depends largely on the chemical composition of the coating and substrate, as well as on the coating deposition process. Severe formation of porosity at the interface causes a degradation of the bonding strength between substrate and coating, with functional breakdown of the coating system as worst result. To investigate the influence of the deposition process, the MCrAlY bond coats were applied using two different spraying processes, high velocity oxygen fuel spraying (HVOF) and low-pressure plasma spraying (LPPS). After coating deposition, the samples were annealed at 1050 °C for varying test periods up to 2000 h. Microstructural and compositional analyses were performed to determine and to evaluate the Kirkendall porosity. The results reveal a strong influence of both the spraying process and the chemical composition. Bond coats applied by means of LPPS show less Kirkendall porosity than bond coats applied by HVOF. Furthermore, the location of Kirkendall porosity formation shifts, depending on the spraying process used, from the substrate-side of the interface (LPPS) to the coating-side of the interface (HVOF). It is hypothesized that accelerated element diffusion, especially of aluminum, from coating to substrate is the main factor for these observations. Results indicate that matching the chromium activity in coating and substrate will result in lower amounts of Kirkendall porosity because of chromium accumulation at the interface.