Enhanced reactive NiAl coatings by microwave-assisted SHS
Purpose - The paper aims to focus on microwave (2.45 GHz) assisted SHS, (MA-SHS) preparation of NiAl intermetallic coatings on titanium substrates conducted in single mode applicator in order to promote the formation of a complex Ni-Al-Ti interface. This enhances the NiAl coating adhesion to the Ti substrate and presents high hardness, high toughness and the capability of stopping the fracture propagation. Design/methodology/approach - Numerical modelling, coupling electromagnetic and heat transfer, allowed to demonstrate that the interface cooling rate can be controlled immediately after SHS using microwave heating, benefiting from the possibility of conveying energy to the newly formed intermetallic compounds, despite an adverse temperature gradient which would negatively affect conventional heating techniques, based exclusively on heat transfer. Experimental validation of the modelling results confirmed that by altering the synthesis conditions (load geometry, microwave power, auxiliary microwave absorbers) the thickness of the Ni-Al-Ti layer can be controlled. Findings - The growth of the interface layer can be ascribed to the formation of a liquid phase (ternary eutectic) which progressively consumes NiAl and Ti from the substrate. In case of MA-SHS, the liquid phase presence can be prolonged during cooling, thus explaining the formation of the thick interface layer. Practical implications - Microwave selective heating can be used to initiate the SHS without affecting the metallic substrate, which is only heated locally by the reaction products, thus preserving its properties. Originality/value - Coupling numerical simulation and experimental activity demonstrated that the different microstructures obtained by MA-SHS, are a result of the peculiar temperature profile, favoured by microwave volumetric and selective heating of the reacting powders.
coatings microwaves heat transfer simulation