Wetting and interfacial reactivity of Ni-Al alloys with Al2O3 and ZrO2 ceramics

The isothermal wetting, spreading behaviour and interfacial reactivity of NiAl1, NiAl10 and NiAl25 alloys (1, 10, 25 at% Al, respectively) on monocrystalline alumina (sapphire) and polycrystalline zirconia oxides stabilized with MgO (stoichiometric and non-stoichiometric ones) were studied by means of the sessile drop method at 1500 degrees C in vacuum. A detailed characterization of solidified couples was performed in order to analyse the final interfacial microstructure and microchemistry. The results were discussed in terms of chemical interactions and by means of ad-hoc calculated phase diagrams. The final contact angles varied with the content of Al leading to a slight wetting situation (contact angle theta < 90 degrees), for the NiAl25 alloy on both sapphire and zirconia. The contact of liquid Ni-Al alloys on sapphire was not accompanied by interfacial chemical reaction. The wetting of pure Ni and NiAl1 alloy was found to be worse for fully stoichiometric ZrO2 compared with under-stoichiometric ZrO2-x by virtue of the increased metallicity of the latter. On the other hand, the formation of interfacial alpha-Al2O3 for the NiAl10 and NiAl25 alloys on ZrO2, as foreseen by thermodynamic modelling and experimentally confirmed by transmission electron microscopy characterization, determined the kinetics of wetting and the final contact angle whatever the zirconia stoichiometry.