Abstract
Chromium borides are promising candidates for several structural applications including protective coatings for materials exposed to corrosive and abrasive environments. In this paper the pulsed laser deposition of chromium diboride-rich thin films has been carried out in vacuum by using a frequency doubled Nd:glass laser with a pulse duration of 250 fs. The films have been deposited at different substrate temperatures and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. Lastly, the film’s hardness has been studied by Vickers indentation technique. The results indicate that only the films deposited at a substrate temperature of 500 °C are crystalline and formed by chromium diboride, together with a certain amount of boron and chromium, which suggests that, as main mechanism, a process taking place on the surface from atoms and ions from the gas phase. This hypothesis has been confirmed by the study of the plasma produced by the ablation process.
Highlights
Transition metal borides have received a large scientific and technological interest in the recent years, due to their peculiar chemical-physics and mechanical properties [1,2]
Chromium-Boron only B, B2O3 and Cr2O3, which indicate that, independently from the substrate temperature, no compounds are present in the films
It is possible to obtain thin films of chromium diboride by PLD performed by a laser with a pulse duration of 250 fs, but only when the ablation target contains an excess of boron with respect to the stoichiometric value
Summary
Transition metal borides have received a large scientific and technological interest in the recent years, due to their peculiar chemical-physics and mechanical properties [1,2]. Among these compounds, chromium borides are promising candidates for several structural applications such as protective coatings for materials exposed to corrosive and abrasive environments [3,4,5,6]. In the chromium-boron phase diagram there is evidence of the presence of six chromium borides, which include Cr2 B, Cr5 B3 , CrB, Cr3 B4 , CrB2 and CrB4 [7] Among these borides, chromium diboride (CrB2 ) is of particular interest, showing the higher melting temperature, a good electrical conductivity and a hardness of the order of. The results indicate that in some cases the films were boron-defective [9,11,14,15] while in others showed a brittle behavior [6,12,13] or Coatings 2019, 9, 777; doi:10.3390/coatings9120777 www.mdpi.com/journal/coatings
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