In-situ synthesis of nanostructured NiAl-Al2O3 composite coatings on cast iron substrates by spark plasma sintering of mechanically activated powders

• Published in: Surface & coatings technology 2015-06, Vol.272, p.254-267
• Main Authors: Beyhaghi, Maryam, Kashefi, Mehrdad, Kiani-Rashid, Alireza, Vahdati Khaki, Jalil, Jonsson, Stefan
• Format: Article
• Language: English
• Subjects: Activated; Alumina; Aluminides; Cast iron; Coating; Coatings; Composite; FEM simulations; Intermetallics; Nickel aluminide; Nickel aluminides; Nickel base alloys; Pellets; Spark plasma sintering
• ISSN: 0257-8972; 1879-3347; 1879-3347
• DOI: 10.1016/j.surfcoat.2015.03.057

Bulk pellets and coatings of NiAl-Al2O3 composites on gray cast-iron substrates are fabricated by spark plasma sintering (SPS) at 700 and 1050°C using a highly reactive powder-mixture of “13Al+8Ni+3NiO” activated by 1hour ball milling. The reactions are complete in all cases, except for the coating produced at the lower temperature. At both temperatures, the pellets experienced internal explosions, due to the intense reactivity of the powder, producing inhomogeneous microstructures. At 1050°C, the heat absorption from the substrates resulted in damped reactions producing homogenous, dense, fully reacted NiAl-Al2O3 composite coatings with crystallite sizes of 73nm and 65nm, respectively. A bond layer forms by growing into the substrate and diffusion of Fe, Ni, Al and Si is found in the coating, the bond layer and the substrate. In all cases, the adherence of coatings to substrates is good with no signs of pores or cracks. The products are examined by LOM, SEM, EDS, XRD, Vickers hardness indentation and scratch testing. The SPS process is analyzed by FEM-simulations using a homogeneous reaction model where the properties are given by linear combinations of reactants and products. Melting enthalpies of all compounds are taken into consideration when calculating the maximum reaction temperatures for various combustion times and gap conductivities between powder and graphite parts of the SPS apparatus. The maximum reaction temperatures are calculated for pellets and for coatings on cast iron substrates and also for mixtures of activated and already reacted powder. The results are shown as isotherms. Comparison to experiments suggests a reaction time exceeding 1s and a gap conductivity of less than 10kW.m−2.K−1. For ignition at 500°C, the adiabatic temperature is estimated to 2056°C. •Un-damped reactions of activated 13Al+8Ni+3NiO powders produce poor bulk products.•Substrates serve as effective heat sinks during reaction synthesis of coatings.•A clean Fe-Si-Al-Ni bond layer with good adherence forms on nodular cast iron.•The bond layer grows solely into the substrate.•Simulations indicate a reaction time of ≥1s during SPS.