Innovative coating techniques using selective melting by induction heating of Al-Cr based compounds for enhanced corrosion resistance

This PhD research presents a novel methodology for fabricating high-temperature, corrosionresistant coatings through induction-assisted selective melting of Al–Cr composites, achieving the in-situ formation of Cr₂AlC MAX phases and aluminium-rich surface layers. The work introduces the first integrated use of Smoothed Particle Hydrodynamics (SPH) in MATLAB with ANSYS electromagnetic simulations to model phase change, heat conduction, and particle redistribution under non-equilibrium thermal gradients. Together, these innovations establish a scalable, energy-efficient pathway for fabricating multi-functional coatings suitable for extreme thermal and corrosive environments, with direct applications in aerospace, nuclear energy, and advanced power systems.

A customised induction coil operating at 500 kHz and 3 kW enabled precise control of heating profiles under an argon atmosphere. Simulations predicted peak magnetic flux density of 1.2 T and a central temperature of 1450 K, aligning closely with experimental values (1438 K, <0.8% error). Melting onset of aluminium occurred at 92.8 µs (simulation) versus 94 µs (experiment), confirming model accuracy. SPH simulations revealed buoyancy-driven transport of molten Al, forming a stratified Al-Cr interface. However, penetration depth was underestimated by 28% due to unmodelled intermetallic phase dynamics, highlighting a key area for AI-assisted model refinement.

Experimental trials confirmed the successful synthesis of Cr₂AlC MAX phase coatings directly from elemental aluminium and chromium powders, eliminating the need for pre-synthesised MAX phase precursors. The induction-assisted selective melting process enabled localised thermal control, facilitating in-situ phase formation under argon-shielded conditions. XRD and SEM analyses revealed a series of reactions initiating with the formation of intermediate binary compounds AlCr₂ and Al₈Cr₅ followed by diffusion-controlled transformation into the Cr₂AlC MAX phase. Notably, the formation of this complex layered structure occurred at subequilibrium conditions due to rapid heating and localised thermal gradients, distinguishing this method from conventional reactive sintering and hot pressing.

High-temperature oxidation testing at 1200 °C demonstrated that the Cr₂AlC coatings developed a protective α-Al₂O₃-rich scale, resulting in an oxidation rate of 0.031 mg/cm²/hr, significantly lower than that of Zircaloy (0.25 mg/cm²/hr). SEM cross-sections confirmed a dense and continuous oxide barrier with minimal spallation or porosity. In autoclave tests conducted at 250 °C and 250 bar for 5 hours, the coated samples exhibited minimal degradation, recording a weight gain of only 0.022 g, indicating excellent corrosion resistance in high-pressure steam environments. XRD analysis of post-autoclave samples revealed retained Cr₂AlC crystallinity with minor surface enrichment of protective alumina phases, validating structural integrity under reactor-simulated conditions. Furthermore, Vickers hardness testing yielded an average of 10.93 GPa, significantly outperforming uncoated Zircaloy samples, which averaged 6.5 GPa, thereby confirming both mechanical robustness and oxidation resistance.

Parallel investigations into ZrSi₂ coatings revealed a self-protective oxidation mechanism, where exposure to 1000–1200 °C led to the formation of a stable ZrSiO₄ surface layer. This transformation acts as a self-healing barrier, inhibiting oxygen diffusion and preventing further degradation of the underlying ZrSi₂. Thermogravimetric and XRD analysis confirmed a consistent parabolic oxidation rate of 0.03 mg/cm²/hr, placing it on par with Cr₂AlC and significantly below conventional Zircaloy. SEM imaging showed dense oxide morphology with minimal porosity, while EDS mapping confirmed silicon retention near the oxide-metal interface. Mechanical evaluation post-oxidation revealed a retained Vickers hardness of 10.93 GPa and a thermal expansion coefficient of 5 × 10⁻⁶ K⁻¹, closely matching that of SiCbased composites, ensuring dimensional compatibility under high thermal loads. Autoclave testing under reactor-like conditions (250 °C, 250 bar, 5 h) showed negligible structural or mass degradation. These results underscore ZrSi₂’s viability as an accident-tolerant cladding material, offering both thermal stability and long-term corrosion resistance in nuclear environments. This study addresses critical gaps in coating fabrication by eliminating the need for pre-synthesised MAX phases and enabling self-stratifying aluminium coatings through electromagnetic control. It establishes a validated, scalable, and energy-efficient pathway for producing functionally graded coatings for aerospace, nuclear, and power-generation applications.