Isothermal oxidation studies of $\beta\rm NiAl$ coatings for aeroengine applications

Nickel aluminide $(\beta\rm NiAl)$ coatings were prepared by $in~situ$ aluminising technique onto CMSX-$4$ superalloy substrates and isothermally oxidised at $1150^{\circ}$C for $100$ h. The factors influencing the spallation resistance of $\beta\rm NiAl$ coatings were examined. The thermally grown oxide formed during oxidation was found to be multiphase $(\rm TiO_2{,}~\rm Ni_2\rm AlO_4{,}$ and $\theta$-$\rm Al_2O_3)$ along with pure $\alpha$-$\rm AL_2\rm O_3$. Coatings and substrate underwent massive interdiffusion resulting in $(\rm i)$ segregation of alloy-precipitates at thermally grown oxide and coating interface, $(\rm ii)$ sulphur migrated from substrate was present in such precipitates and $(\rm iii)~\rm Ni$ influx and $\rm Al$ depletion rendering $\beta\rm NiAl \to \gamma'$-$\rm Ni_3\rm Al$ transformation. Moreover, the degree of spallation and thermally grown oxide residual stresses increased with the oxidation time.