The aim of this work is to improve the mechanical properties of AISI 4140 steel substrates by using a
TiN[BCN/BN]n/c-BN multilayer system as a protective coating. TiN[BCN/BN]n/c-BN multilayered coatings
via reactive r.f. magnetron sputtering technique were grown, systematically varying the length period
() and the number of bilayers (n) because one bilayer (n = 1) represents two different layers (tBCN + tBN),
thus the total thickness of the coating and all other growth parameters were maintained constant. The
coatings were characterized by Fourier transform infrared spectroscopy showing bands associated with
h-BN bonds and c-BN stretching vibrations centered at 1400cm−1 and 1100cm−1, respectively. Coating
composition and multilayer modulation were studied via secondary ion mass spectroscopy. Atomic
force microscopy analysis revealed a reduction in grain size and roughness when the bilayer number (n)
increased and the bilayer period decreased. Finally, enhancement of mechanical properties was determined
via nanoindentation measurements. The best behavior was obtained when the bilayer period ()
was 80nm (n = 25), yielding the relative highest hardness (∼30 GPa) and elastic modulus (230 GPa). The
values for the hardness and elastic modulus are 1.5 and 1.7 times greater than the coating with n=1,
respectively. The enhancement effects in multilayered coatings could be attributed to different mechanisms
for layer formation with nanometric thickness due to the Hall–Petch effect; because this effect,
originally used to explain increased hardness with decreasing grain size in bulk polycrystalline metals, has
also been used to explain hardness enhancements in multilayered coatings taking into account the thickness
reduction at individual single layers that make up the multilayered system. The Hall–Petch model
based on dislocation motion within layered and across layer interfaces has been successfully applied to
multilayered coatings to explain this hardness enhancement.
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