The effect of electromagnetic heating frequencies on CaHPO4 coatings deposited on the C/C by induction heating deposition technology

Monetite (CaHPO4) coatings were deposited on H2O2-treated carbon/carbon (C/C) composites by induction heating deposition (IHD) at electromagnetic frequencies ranging from 289 kHz to 438 kHz. The composition, microstructure and bonding strength of the monetite coatings on the C/C composites were investigated by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and large load scratch tester (LLST). The effects of the electromagnetic frequencies on the deposition kinetics of the monetite coatings on the C/C were also analyzed. The results showed that when the electromagnetic heating frequencies were 289 kHz, 319 kHz and 381 kHz, the monetite coatings presented similar compact morphologies, and critical loads of 38 N, 64 N and 25 N. The deposition processes of these monetite coatings were controlled by a surface reaction and their apparent activation energies were 20.26, 22.17, and 31.07 kJ/mol at 289, 319 and 381 kHz. But at 438 kHz, it showed a loose morphology and could be very easily scratched off, presenting a critical load of 0 N. The changes in the microstructure and adhesiveness of the monetite coatings were discussed at the different electromagnetic frequencies.