Ideal coatings require three critical elements: hardness, toughness, and adhesion. Coatings of diamond-related materials are appealing owing to their high hardness but exhibit inadequate adhesion and toughness, especially on stainless steel substrates. Q-carbon, a newly discovered allotrope of carbon has the potential to improve hardness, toughness, and adhesion. The Q-carbon is formed on melting and quenching from super undercooled melt state. In this study, we have synthesized robust Q-carbon/α-carbon and Q-carbon/nanodiamond heterostructures on austenitic stainless steel substrates by laser annealing amorphous carbon films with nanosecond laser pulses above melt threshold (ED). Maximum melt regrowth velocity of 13 m/s corresponding to Q-carbon/nanodiamond composite was obtained by laser-solid melt interaction simulations. Subsequently, Q-carbon was used as seed layer to grow microdiamonds by HFCVD (hot filament chemical vapor deposition). The Q-carbon seed layer helped growth of better quality diamonds (50% less graphitic) with FWHM of 11.5 cm-1 and demonstrated higher nucleation density in comparision with amorphous carbon-coated and bare 316 SS substrates. Diamonds grown on Q-carbon displayed ballas type of microstructure indicative of high toughness. This study on Q-carbon nanocomposite coatings provides a new pathway for fabricating ultrahard carbon-based coatings on stainless steels for biomedical and tribocorrosive applications.