Riyadi, Tri Widodo Besar (2013) High performance intermetallic and ceramic formed by SHS process. (PhD thesis), Kingston University, .
Abstract
In this study, a new method to fabricate high performance intermetallic and ceramic materials and coatings was developed using the SHS process. Induction heating was used as an ignition source to initiate the SHS reaction of Ni/Al and preheat the steel substrate, while titanium was used as an underlayer to facilitate the adhesion between the coatings and the substrate. The reactions were performed in a reaction chamber with an atmosphere of argon gas, while the combustion temperatures were measured using type-K thermocouple and infrared pyrometer. The microstructure characterizations of synthesized products were conducted using SEM, XRD and laser Raman microspectroscopy. Vickers indentation tester was used to evaluate the microhardness and the adhesion strength of the coatings. In the fabrication of NiAl coating, the formation of NiAl was initiated by the melting of Al and subsequently reacted with Ni to form NiAl after receiving the heat generated by induction heating. The synthesized product formed a liquid phase during the reaction and partly diffused into the Ti underlayer. The heat released by Ni/Al reaction then promoted the melting of Ti and further reactions between Ti and Ni/Al to produce Ti3Al–Ti2Ni composites, whereas the unreacted Ti formed an alloy with the coating material. The heat released by Ni/Al reaction and that generated by induction heating were also responsible for the formation of metallurgical bonding in the interface between the underlayer and the substrate. The effects of current variations, underlayer thickness, compaction pressure and melting point of underlayer on the microstructure of synthesized products were investigated. The mechanical properties and thermal shock resistance of synthesized products were also studied. To investigate the effect of reactant compositions on the combustion temperature, microstructure and mechanical properties of synthesized products, NiAl–TiC–Al2O3 composites were fabricated using a low cost TiO2. The microstructure of synthesized product showed that the reaction of Ni/Al and TiO2/Al/C was complete. Two reaction stages were observed from the temperature profiles which represent the exothermic reaction of TiO2/Al/C system and the phase transformation of Ni/Al system. An increase in the TiO2/Al/C content increased the combustion temperature. However, the maximum temperature was still lower than the melting point of TiC and Al2O3 indicating that both products were formed in the solid state during the reaction. An increase in the TiO2/Al/C content produced a higher porosity in the product due to the increase of the solid TiC and Al2O3 particles compared with that of the liquid NiAl. The Ni/Al reactions acted as an exothermic agent to the ignition of TiO2/Al/C reaction, and increased the liquid content for improving the density of synthesized product. The microhardness of synthesized product also increased with an increase in the TiO2/Al/C content due to the increased content of the ceramic particles. In the fabrication of TiC intermetallic composite coating, induction heating was successfully used to initiate the exothermic reaction of the reactants composed of multilayer configurations of Ti/C, Ni/Al and Ti. The synthesized product was inhomogeneously composed of TiC/Ti3Al/Ti2Ni, while a small amount of TiO2 oxides and unreacted C were also observed. The hardness of TiC/Ti3Al/Ti2Ni was 1135.48 ± 139.58 HV, indicating that a hard TiC intermetallic composite coating was successfully synthesized using induction heating.
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