Thermal expansion and microstructure evolution of atmospheric plasma sprayed NiCrAlY bond coat using in-situ high temperature X-ray diffraction

The paper focuses on in-situ high-temperature X-ray diffraction (HT-XRD) study on atmospheric plasma sprayed NiCrAlY coating. The sample was in-situ heated from 25 degrees C to 1150 degrees C in a controlled atmosphere (3 x 10-4 bar), and the corresponding X-ray diffraction patterns for different temperatures were recorded. The effect of tem-perature on crystallite size, lattice strain, and coefficient of linear thermal expansion was studied. Major phases identified are gamma-Ni, gamma'-Ni3Al, beta-NiAl, and alpha-Cr. The formation of stable alpha-Al2O3 and spinel was found above 1000 degrees C. The transformation of beta to gamma' and gamma phase was observed as a function of temperature. The equilibrium phases and the thermal expansion of disordered Face Centered Cubic (FCC) and Body Centered Cubic (BCC) phases were predicted and supported by Thermo-Calc prediction for the stable temperature range. Results showed that the non-equilibrium microstructure produced by thermal spray process did not alter the thermal expansion behaviour. In-situ treatment resulted in microstructure and elemental homogenization. The thermal expansion and mechanism of phase evolution were discussed.